Human immunodeficiency virus

AIDS is an extraordinary kind of a crisis. AIDS is unique in human history in its rapid spread, its extent and the depth of its impact. Since the first AIDS case was diagnosed in 1981, the world has struggled to come to grips with its extraordinary dimensions. Early efforts to mount an effective response were fragmented, piecemeal and vastly underresourced. Few communities recognized the dangers ahead, and even fewer were able to mount an effective response. Now, more than 20 years later, 20 million people are dead and 37.8 million people (range: 34.6 - 42.3 million) worldwide are living with HIV. And still, AIDS expands relentlessly, destroying peoples lives and in many cases seriously damaging the fabric of societies.

AIDS was first recognized in the United States in 1981, following an increase in the incidence of usually rare opportunistic infections (such as the pneumonia caused by Pneumocystis carinii) in homosexual men that were caused by a general immune deficiency. Human immunodeficiency virus (HIV) was first isolated in 1983 and by the mid-1980s it was evident that two types of HIV, with slightly different genome structures, were circulating in human populations. Both viruses are characterized by extensive genetic diversity; HIV-1 is phylogenetically divided into three groups - M, N and O, with the M group further split into 9 subtypes and 15 circulating recombinant forms. Today, group M has a near global distribution, whereas groups N and O are restricted to individuals of West African origin. HIV-2 is also most common in individuals from West Africa and is composed of seven subtypes. Despite its initial association with homosexual men, it is clear that HIV-1 and HIV-2 are now primarily transmitted by heterosexual intercourse and from mother to infant.

Several independent isolates, called lymphadenopathy-associated virus or LAV, human T-cell lymphotropic virus type III, or HTLV-III, and AIDS-associated retrovirus or ARV by the laboratories of origin, are similar with respect to morphology, cytopathology, requirements for optimum reverse transcriptase activity, at least some antigenic properties, and some restriction endonuclease cleavage sites in viral DNA. Epidemiological studies show that infection by one of these viruses may be a necessary condition for the development of AIDS, although predisposing factors may contribute to the onset of the disease. Certain taxonomic issues need to be addressed with respect to the relationships among the human retroviruses at the nucleotide level. A probe representative of ARV-2 anneals under high stringency conditions to restriction enzyme DNA fragments from cells infected with LAV or with HTLV-III. Thus, these three retroviruses are closely related.

Human immunodeficiency virus type 1

HIV-1 is most closely related to simian immunodeficiency virus chimpanzee (SIVcpz), which is found in some sub-species of chimpanzee (Pan troglodytes troglodytes and Pan troglodytes schweinfurthii) that inhabit parts of equatorial Western and Central Africa, respectively. SIVcpz from P. t. troglodytes is of most interest because it shares the closest relationship with the abundant HIV-1 M group. The geographical range of P. t. troglodytes also encompasses the region in Africa that has the greatest genetic diversity of HIV-1, containing groups M, N and O; such a distribution is expected if this was where HIV-1 first emerged.

Human immunodeficiency virus type 2

HIV-2 is most closely related to SIVsm5, which is found at high prevalence in sooty mangabey monkeys (Cercocebus atys). As with HIV-1, sooty mangabeys are most frequent in the regions of West Africa where HIV-2 is likely to have emerged.

Human immunodeficiency virus type 3

Described is a new variety of retrovirus designated HIV-3, also known as HIV-1 subtype O, samples of which are deposited in the European Collection of Animal Cell Cultures (ECACC) under V88060301.

Human immunodeficiency virus type 1 (ARV2/SF2 ISOLATE)

The complete DNA sequence of ARV-2 reveals a fundamental genetic structure similar to that of other retroviruses. A distinct group of human retroviruses has been isolated from patients with the acquired immune deficiency syndrome (AIDS) and individuals with related conditions, such as persistent lymphadenopathy. Several independent isolates, called lymphadenopathy-associated virus or LAV, human T-cell lymphotropic virus type III, or HTLV-III, and AIDS-associated retrovirus or ARV by the laboratories of origin, are similar with respect to morphology, cytopathology, requirements for optimum reverse transcriptase activity, at least some antigenic properties, and some restriction endonuclease cleavage sites in viral DNA. Epidemiological studies show that infection by one of these viruses may be a necessary condition for the development of AIDS, although predisposing factors may contribute to the onset of the disease. Certain taxonomic issues need to be addressed with respect to the relationships among the human retroviruses at the nucleotide level. A probe representative of ARV-2 anneals under high stringency conditions to restriction enzyme DNA fragments from cells infected with LAV or with HTLV-III. Thus, these three retroviruses are closely related.

Approximately 17% divergence was observed between HXB-3 and ARV-2 sequences, which was most pronounced in the extracellular region of the envelope protein. The observed differences in sequence probably reflect divergent evolution of strains separated in time and geography. ARV-2 was recently isolated from the west coast of the United States. The HTLV-III isolates for which nucleotide sequences have been determined were all obtained from the east coast about one year earlier. Regardless of the reasons, the fact that such divergence can occur, and that it is more pronounced in the extracellular portion of the envelope protein, is an important consideration in the design of both diagnostic reagents and possible vaccines.

Human immunodeficiency virus type 1 (BH10 ISOLATE)

Clones BH10, BH8 and BH5 were derived from a library of SstI-digested DNA from HTLV-III-infected H9 cells cloned in lambda -gtWes_ lambda-B.

Human immunodeficiency virus type 1 (BH5 ISOLATE)

Clones BH10, BH8 and BH5 were derived from a library of SstI-digested DNA from HTLV-III-infected H9 cells cloned in lambda -gtWes_ lambda-B.

Human immunodeficiency virus type 1 (BH7 ISOLATE)

Human immunodeficiency virus type 1 (BH8 ISOLATE)

Clones BH10, BH8 and BH5 were derived from a library of SstI-digested DNA from HTLV-III-infected H9 cells cloned in lambda-gtWes_ lambda-B.

Human immunodeficiency virus type 1 (BRAIN ISOLATE)

HIV-1 BR (BRAIN) was isolated from autopsied brain tissue (frozen unfixed for 2 years) by cocultivating minced brain tissue (from frontal, occipital, and parietal lobes) with 3-day-old phytohemagglutin (PHA)-stimulated peripheral blood lymphocytes (PBLs). When compared for the rate of replication HIV-1 BR was up to 10-fold lower than HIV-1 451. In the extent of cytopathicity to CD4+ cells, HIV-1 BR was also lower than HIV-1 451. In addition, HIV-1 BR replicated in ROHA and U937 cell lines but to lesser extent than the prototype virus HIV-1 LAV.

Human immunodeficiency virus type 1 (BRU ISOLATE)

We designate our different AIDS virus isolates by three letters of patients names, lymphadenopathy-associated virus (LAV) BRU referring to the prototype AIDS virus isolated in 1983 from a French homosexual patient with lymphadenopathy syndrome (LAS), thought to have been previously infected in the USA.

Human immunodeficiency virus type 1 (CDC-451 ISOLATE)

The lymphotropic retrovirus designated as HIV (CDC-451) was isolated from a 16-year-old white male with severe hemophilia A. Throughout his life, he had been treated with lyophilized factor VIII concentrates. He had no other known risk factors. Just before his death in June 1984, the virus was isolated from his peripheral blood by previously described methods. After two passages in phytohemagglutinin-stimulated normal adult human T cells, the virus was further propagated in Hut-78 cells obtained from the American Type Culture Collection.

Human immunodeficiency virus type 1 (CLONE 12 ISOLATE)

Human immunodeficiency virus type 1 (ELI ISOLATE)

Both of the African patients originated from Zaire; lymphadenopathy-associated virus (LAV) ELI was recovered in 1983 from a 24 year old woman with AIDS

Human immunodeficiency virus type 1 (HXB2 ISOLATE)

Clone HXB2 was derived from a recombinant phage library of XbaI-digested DNA from HTLV-III-infected H9 cell cloned in lambda-J1.

Human immunodeficiency virus type 1 (HXB3 ISOLATE)

The integrated proviral genome of HTLV-III was recently cloned from the genomic DNA of H9 cells infected with HTLV-III. Since the HTLV-III provirus was found to lack Xba I restriction sites, a genomic library was constructed using Xba I digested H9/HTLV-III DNA. Screening of this library with HTLV-III cDNA probes yielded several clones, one of which, termed HXB-3 was used in the present study. Approximately 3% divergence was observed between the HTLV-III (HXB-3) and LAV amino acid sequences. However, among the three HTLV-III sequences (HXB-3, HB-10, BH-8), the divergence was 1.6%. Approximately 17% divergence was observed between HXB-3 and ARV-2 sequences, which was most pronounced in the extracellular region of the envelope protein. The observed differences in sequence probably reflect divergent evolution of strains separated in time and geography. ARV-2 was recently isolated from the west coast of the United States. The HTLV-III isolates for which nucleotide sequences have been determined were all obtained from the east coast about one year earlier. Regardless of the reasons, the fact that such divergence can occur, and that it is more pronounced in the extracellular portion of the envelope protein, is an important consideration in the design of both diagnostic reagents and possible vaccines.

Human immunodeficiency virus type 1 (JH3 ISOLATE)

A strain of HIV-1 JH3 was isolated from a Japanese male 10 years old with hemophilia who was a carrier of HIV at the time of virus isolation (August 1986). Peripheral lymphocytes were cocultured with a human T cell line CEM under the standard conditions, and the virus replicated in CEM cells was termed JH3.

Human immunodeficiency virus type 1 (JRCSF ISOLATE)

Patient J.R. died with Kaposis sarcoma and severe AIDS encephalopathy. The brain showed extensive leukoencephalopathy, and characteristic multinucleated giant cell syncytia were observed in pathologic specimens of frontal lobe brain tissue taken at autopsy. Virus was isolated from various tissue sources by infection of lectin-activated normal human peripheral blood lymphocytes (PBL). Two genotypically distinct viruses were obtained from the CNS of patient J.R. We will term these viruses HIV (JR-CSF) and HIV (JR-FL) for isolates derived from the cerebrospinal fluid and frontal lobe, respectively.

Human immunodeficiency virus type 1 (KB-1 ISOLATE)

HIV-1 strain KB-1gp41 was isolated from a Japanese male-hemophiliac by coculture of his peripheral blood mononuclear cells (PBMCs) with MT-2 cells.

Human immunodeficiency virus type 1 (Lai ISOLATE)

Human immunodeficiency virus type 1 (MAL ISOLATE)

Both of the African patients originated from Zaire; lymphadenopathy-associated virus (LAV) MAL in 1985 from a 7 year old boy with AIDS-related complex (ARC), probably infected in 1981 after a blood transfusion in Zaire, since his parents were LAV-seronegative.

Human immunodeficiency virus type 1 (MFA ISOLATE)

The primary virus isolate was obtained by cocultivation of peripheral blood lymphocytes from an HIV-1-seropositive asymptomatic hemophiliac with cells of the CD4+ cell line CEM.

Human immunodeficiency virus type 1 (MN ISOLATE)

The HIV-1 isolates HTLV-III(MN) and (SC) were obtained from a pediatric AIDS patient in the New York area in 1984 and a homosexual ARC (AIDS-related complex) patient from California in 1984, respectively. In the more constant regions of the env gene, HTLV-III(MN) and (SC), differ mostly by scattered point mutations, many of them resulting in conservative amino acid substitutions. A comparison of the inferred amino acid sequences of the env genes of other HIV-1 isolates for which DNA sequence data are available indicates that HTLV-III(MN) and (SC) differ over the entire length of their env genes from other US isolates to the same extent that they differ from HTLV-III (BH10) and to a significantly greater extent from several African isolates. They thus represent two new env genotypes of HIV-1.

Human immunodeficiency virus type 1 (NDK ISOLATE)

A highly cytopathic strain of HIV1, named HIV1-NDK, has been isolated from a Zairian patient affected with AIDS. This isolate is 10(4) times more cytopathic and infectious than the prototype. To correlate the high cytopathic properties of this strain with genetic variations, we have cloned and sequenced the genome of this isolate. The principal feature which could be drawn from the fine analysis of the HIV1-NDK sequence is that the variability is not clustered in one particular region but rather spread out all along the genome. Only minor differences seem to be responsible for the acute biological effect of HIV1-NDK.

Human immunodeficiency virus type 1 (NEW YORK-5 ISOLATE)

Peripheral blood lymphocytes (PBL) were obtained from five New York City patients in October 1984, from three Zairian patients in October 1983, and from a patient living in Birmingham, Alabama in January 1985. The New York 5 isolate was from a man with AIDS.

Human immunodeficiency virus type 1 (NIT-A ISOLATE)

We have previously described the isolation and molecular cloning of a family of HIV-1 isolates (HIV-1/N1T) from an individual with lymphadenopathy. Among these viruses, N1T-A behaves like prototypical HIV-1 isolates; its infection is rapid and cytopathic. N1T-E displays slow kinetics of infection and is not cytopathic, but like N1T-A, it is capable of efficient fusion and entry into target cells, expresses a transcriptionally active long terminal repeat (LTR) element, has functional tat and rev genes, and is highly productive in chronic infection in T lymphocytes and monocytes. Like other multiple isolates from one individual, N1T-E and N1T-A have similar restriction endonuclease maps and thus are closely related.

Human immunodeficiency virus type 1 (OYI ISOLATE)

An unusual HIV-1 strain has been isolated from the blood of a healthy Gabonese individual who presented an atypical western blot (WB). This virus, identified as isolated HIV-1OYl, grew to low titres of reverse transcriptase activity (less than 50,000 cpm/ml) and was not obviously cytopathic. Radioimmunoprecipitation and peptide ELISA studies indicated that the lack of env-specific reactivity was probably due to the absence of antibodies to the viral glycoproteins, rather than the virus encoding a highly divergent envelope protein.

Human immunodeficiency virus type 1 (PV22 ISOLATE)

Human immunodeficiency virus type 1 (RF/HAT ISOLATE)

A strain of HTLV-III was isolated from fresh mononuclear cells from the peripheral blood of a Haitian patient, R. F., who was diagnosed with AIDS acquired through heterosexual transmission.

Human immunodeficiency virus type 1 (SC ISOLATE)

Human immunodeficiency virus type 1 (SF162 ISOLATE)

HIV-1 SF162 was obtained by cocultivation of peripheral blood mononuclear cells (PMC) from seronegative donors with cerebrospinal cord fluid of a HIV-1-seropositive patient with toxoplasmosis.

Human immunodeficiency virus type 1 (SF33 ISOLATE)

HIV-l SF33 was obtained from the PMC of a patient with thrombocytopenia. The HIV-1 SF33 strain replicates rapidly and to high titers in the HUT 78 and CEM cell lines and productively infects HOS cells. This isolate is also very cytopathic and forms plaques in the MT-4 cell line. The molecular clones of HIV-l SF2 and HIV-l SF33 were found to be biologically active upon transfection into human rhabdomyosarcoma (RD-4) and HUT 78 cells. The biologic properties of the molecular clones showed characteristics identical to those of the parental viruses

Human immunodeficiency virus type 1 (STRAIN UGANDAN / ISOLATE U455)

U455 virus was isolated in December 1985 from peripheral blood lymphocytes (PBL) of a 35-year-old Ugandan male diagnosed to have slim disease and tuberculosis. We have made a large number of isolations of HIV-1 from seropositive asymptomatic individuals and from patients with slim disease, chiefly from the Kampala region of Uganda. We now describe the isolation and properties of a Ugandan virus.

Human immunodeficiency virus type 1 (WMJ1 ISOLATE)

Isolate WMJ-1 was similarly transmitted from the peripheral blood mononuclear cells of a Haitian infant with AIDS to an immortalized T cell line.

Human immunodeficiency virus type 1 (WMJ2 ISOLATE)

Human immunodeficiency virus type 1 (YU-2 ISOLATE)

The viral clones HIV-1YU-2, HIV-l Yu-1 , HIV-1YU 21, and HIV-1YU-32 analyzed in this study were obtained as described previously directly from uncultured brain tissue of a man who died in 1986 at the age of 40 with severe (stage 3) ADC. Although the date of initial HIV-1 infection of this individual was unknown, he first presented with an AIDS-defining illness (Kaposi's sarcoma) in 1983 and then developed extraoral candidiasis, cytomegalovirus retinitis, and Pneumocystis carnii pneumonia in 1985. He never received AZT or other antiretroviral therapy. Immunohistological examination of the brain described previously revealed severe multinucleated giant cell encephalitis and widespread HIV-1 infection of macrophages and microglia. Because the HIV-1 clones were derived from this tissue by recombinant lambda phage cloning of permuted circular viral DNA intermediate forms, the viral inserts were excised by using unique restriction endonucleases (SailI, SphIT, and EcoRI) and reconstructed in the plasmid vector pTZ so as to yield full-length nonpermuted viral genomes

Human immunodeficiency virus type 1 (Z-84 ISOLATE)

Peripheral mononuclear cells were isolated from a Zairian male with the acquired immunodeficiency syndrome (AIDS), age 52, phytohemagglutinin (PHA)-stimulated, and cocultivated with PHA-stimulated normal donor perhipheral mononuclear cells under standard conditions. H9 cells were infected with cell-free supernatant from the mononuclear cell cultures. This isolate of HIV-1, termed Z84, has been maintained in H9 cells.

Human immunodeficiency virus type 1 (Z2/CDC-Z34 ISOLATE)

Human immunodeficiency virus type 1 (ZAIRE 3 ISOLATE)

Human immunodeficiency virus type 1 (ZAIRE 6 ISOLATE)

We molecularly cloned the genome of HIV isolated from a Zairian AIDS patient. Restriction mapping of the recombinant clone, designated HIV-Zr6, revealed both common (as observed in other HIV isolates) and unique restriction sites. The DNA clone of HIV-Zr6, shown to give rise to infectious cytopathic virus after transfection of cultured lymphoid cells, was sequenced in several regions. The long terminal repeat (LTR), open reading frame 1 (ORF1), C-terminal envelope (env) gene domain, and ORF2 showed less than 6% difference in nucleotide sequence when compared to other HIV isolates including human T-lymphotropic virus-type III (HTLV-III) clone B10, lymphadenopathy-associated virus-1 (LAV-1), and AIDS-associated retrovirus-2 (ARV-2).

Human immunodeficiency virus type 1 (ZAIRE HZ321 ISOLATE)

Human immunodeficiency virus type 1 (Z321 designate, HIV-1Z321), the oldest known HIV, was isolated from a serum sample collected in Zaire in 1976 and was molecularly cloned.

Human immunodeficiency virus type 1 (lw12.3 ISOLATE)

Accidental infection of a laboratory worker (LW) with HIV-1 has been previously reported. LW was involved in high-volume production of concentrated HIV-1 (human T cell leukemia/lymphotropic virus [HTLV]-III). Although no obvious exposure incident could be documented and no symptoms of an acute HIV-1 seroconversion syndrome were apparent, seropositivity was noted during routine screening, and virus was subsequently isolated. HIV-1 isolates were established and grown at various times from September 1985 to February 1990 in three independent laboratories from the peripheral blood mononuclear cells (PBMCs) of LW.

Human immunodeficiency virus type 2 (BEN ISOLATE)

A HIV-2 strain named HIV-2ben was isolated from peripheral blood lymphocytes of a patient who, since 1984, had developed neurological symptoms such as Raynaud's syndrome, followed by paresthesia of extremities and ataxia, and finally paraparesis of the legs and incontinence. This new isolate could be distinguished from HIV-2rod by antibody-binding epitopes, peptide maps of core p24 and p18 polypeptides and restriction endonuclease cleavage pattern.

Human immunodeficiency virus type 2 (CAM2 ISOLATE)

We report the complete nucleotide sequence of a human immunodeficiency virus type 2 (HIV-2) isolate from Guinea-Bissau (HIV-2CAM2). The genomic organization of HIV-2CAM2 is identical to that of other HIV-2 isolates but contains a stop codon in the pol gene. The deduced amino acid sequences of the viral proteins show variation of 20% in the gag, pol and vpx regions, and 25 to 45% in the tat, env and nef regions when compared to other isolates of HIV-2. This is greater than the variation observed between isolates of HIV-1.

Human immunodeficiency virus type 2 (D194 ISOLATE)

HIV-2 D194 was isolated in 1987 from the peripheral blood of a Gambian male who lived in Germany since May 1986. The patient suffered from severe neurological symptoms (Neuro-AIDS) and died at the end of May 1987. The virus isolate is characterized by its excellent growth in cultivated primary human macrophages.

Human immunodeficiency virus type 2 (D205,7 ISOLATE)

We have previously reported the molecular cloning of HIV-2 D205. This isolate from a healthy woman from Ghana was biologically characterized by its good growth on macrophages and by the absence of cytopathic effects on lymphocytes. It was originally classified as HIV-2 by western blot analysis. We have now determined the complete nucleotide sequence of the clone HIV-2 D205.7, which encompasses 7,817 base pairs (bp) of the proviral genome starting form the left lont terminal repeat (LTR).

Human immunodeficiency virus type 2 (GHANA-1 ISOLATE)

We isolated a retrovirus that hybridized with an HIV-2 ROD probe from an AIDS patient in Ghana and named it HIV-2[GH-1]. The restriction endonuclease patterns of the HIV-2[GH-1] were quite different from those of HIV-2[ROD].

Human immunodeficiency virus type 2 (KR ISOLATE)

Human immunodeficiency virus type 2 (NIH-Z ISOLATE)

Human immunodeficiency virus type 2 (ROD ISOLATE)

The ROD isolate of HIV-2 was obtained in 1985 from an AIDS patient from Cape Verde Islands (offshore Senegal).

Human immunodeficiency virus type 2 (SBL/ISY ISOLATE)

The comparison of the predicted amino acid sequence of the viral proteins of HIV-2 SBL/ISY revealed a divergence comparable to the divergence observed among HIV-1 Zairian isolates, which is higher than the divergence described among American isolates. The patients from whom the HIV-2 isolates were obtained originated from geographically neighboring African countries of Guinea Bissau (HIV-2 NIH-z), Cape Verde Islands (HIV-2 ROD), and Gambia (HIV- 2 SBL/ISY). The considerable degree of divergence among these HIV-2 isolates would suggest that HIV-2 has been present in the African population for a time similar to HIV-1.

In fact, the infectivity of HIV-2 SBL/ISY is not restricted to human cells, but we have shown in a parallel study that this virus infects and kills fresh peripheral blood T cells from rhesus macaques in vitro and infects the same animals in vivo.

Human immunodeficiency virus type 2 (ST ISOLATE)

To elucidate determinants of HIV pathogenicity, we have begun to molecularly dissect a previously reported, nonfusogenic and noncytopathic HIV-2 isolate, termed HIV-2/ST, that was obtained from a healthy Senegalese prostitute. Although this virus replicated to high titers in tissue culture, it infected cells at a slower rate than did cytopathic strains of HIV-1 and HIV-2 and caused little or no cell killing and fusion. This was the case despite the fact that its external envelope glycoprotein was cleaved correctly, transported to the cell surface, and shown to bind to a specific epitope on CD4, which was recognized by OKT4a but not OKT4 antibodies. HIV-2/ST therefore appeared to bind to the CD4 molecule analogous to other HIVs, but it failed to fuse with CD4-bearing target cells, suggesting that its infectivity was greatly retarded at the level of cell entry.

Human immunodeficiency virus type 2 (ST/24.1C#2 ISOLATE)

The present study was undertaken to investigate the glycoprotein regions responsible for differential syncytium formation by two genetically highly related but biologically distinct HIV-2 viruses. Recombinant vaccinia virus expresson of the env gene of HIV-2/ST (ST) (a previously described replication-competent but noncytopathic virus) indicated that the glycoprotein was deficient in syncytium formation. Compared with wild-type ST, the env protein of a molecularly cloned cytopathic ST variant, designated HIV-2/ST/24.1C#2, cased more efficient syncytium formation in the human T-cell line Sup T1, in which the variant was originally generated by repeated passage.

Human immunodeficiency virus type 2 (D205 ISOLATE)

It has been suggested that the human immunodeficiency virus type 2 (HIV-2) and the simian immunodeficiency virus from rhesus macaques (SIV-mac) evolved from the sooty mangabey virus (SIV-sm). We now describe an HIV-2-related isolated, HIV-2 D205 from a health Ghanaian woman that is genetically equidistant to the prototypic HIV-2 strains and to SIV-sm and SIV-mac. Supported by the observation that HIV-2 D205 differs in a step of envelope glycoprotein processing, our data indicate that it could represent an alternative HIV-2 subtype and that viruses of the HIV-2 / SIV-sm / SIV-mac group could have already infected humans before HIV-2 and SIV-sm / SIV-mac diverged.

Human immunodeficiency virus type 2 (EHOISOLATE)

HIV-2 EHO was initially isolated from peripheral blood lymphocytes of a patient from Ivory Coast with full-blown AIDS. This isolate manifested tropism for stimulated normal human lymphocytes and to neoplastic lymphoid or monocytoid (U937) cell lines. The affinity of HIV-2 EHO particles to bind soluble CD4 molecules was found to be severalfold higher compared to HIV-2 ROD.

100 nm in diameter

Budding particles are much more rigid than mature particles, which often have a wrinkled envelope. As the budding process proceeds, one sees progressively more and more of a thin zone of electron density situated under the portion of the cells unit membrane that is being usurped by the developing virus. Simultaneously with the budding, spikes (gp 120) are being inserted into the membrane and appear on the surface of the developing virion envelope. This budding process continues until the virus has formed a complete sphere, i.e., the immature particle

100 nm in diameter

Morphologically, HIV, like all of the Lentiviruses or slow viruses, has an asymmetric core, whose appearance depends on the plane of sectioning. The conical capsid in the mature HIV particle has an electron-dense broad end and a hollow tapered end. Depending on where the core is sectioned and its orientation in the field of view, one sees a hollow or dense spherical structure, when it is cut in cross section, or degrees of foreshortening of the conical core in tangential sections

Details on the sequence of events during HIV-1 assembly and maturation first emerged from electron microscopy studies. Large Gag complexes were visualized under the plasma membrane followed by the appearance of the spheres connected to the cell by a thin stalk. The nascent immature virions contained an electrondense layer under the lipid envelope and had a doughnut-shaped appearance. The maturation process changed the arrangment of the structural components inside the virion: the radially arranged Gag molecules were dismantled, and a conical core structurewas assembled in the center of the viral particle. It now appears that in immature HIV-1 particles Gag polyprotein is radially arranged under the virus membrane, the N termini lie at the membrane and the C termini extend toward the center of the particle. The MA domain is tightly associated with the inner face of the viral membrane and separated from the capsid layer by a low density region corresponding to its C terminus. Such organization of the MA layer stabilizes the viral membrane. The CA and NC domains form regular layers which contribute to the radial density to the immature particle. According to the generally accepted view, structural changes coupled with virus maturation are induced by proteolytic processing of Gag p55 by the viral protease during or after budding of the virus particles. As with all retroviruses, HIV-1 protease is an aspartic protease and is functional only as a dimer. Activation of protease is a critical step in HIV-1 morphogenesis. Cleavage of Gag and Gag-Pol precursors is necessary for virus maturation and infectivity, and mutation or inhibition of protease abolishes production of infectious virus. It is widely accepted that the cleavage of Gag p55 induces major structural rearrangements in the virus particle, and that the last step of cleavage between CA and p2 triggers the condensation of NC and the RNA genome into a cone-shape core whileCAforms the shell. This rearrangement is necessary for virus infectivity.

INTIAL SEQUENCE DESCRIPTION: A number of different recombinant DNA clones of HTLV-III/LAV have been obtained and analyzed by restriction enzyme digestion and/or nucleotide sequencing. This work has demonstrated that the viral genome is 9182-9213 nucleotides in length, with long terminal repeats (LTRs) of 636-637 nucleotides, and at least seven genes. Three replicative genes include gag, pol, and env, which are similar to those in other retroviruses, though env is longer than that of other retroviruses. A fourth gene, designated tat, is structurally distinct from that of other retroviruses, and encodes a trans-acting factor capable of enhancing virus expression in a positive feedback manner. A fifth gene has recently been identified, and has been named art or anti-repressor of transactivation or trs or trans-repressor of splicing. Two additional genes, designated short open reading frame (sor) and 3 open reading frame (3'orf) are also unique to HTLV-III/LAV, but the functions of their gene products are unknown. An additional open reading frame, designated R, is also presumed to encode a protein product based on the finding of antibodies in infected individuals reactive to these sequences expressed in E. coli.

9181bp

OVERVIEW: Most replication competent retroviruses depend on three genes: gag, pol and env : gag means "group-antigen", pol represents "polymerase" and env is for "envelope". The "classical" structural scheme of a retroviral genome is: 5'LTR-gag-pol-env-LTR 3'. The LTR ("long terminal repeat") regions represent the two end parts of the viral genome that are connected to the cellular DNA of the host cell after integration and do not encode for any viral proteins. The gag and env genes code for the nucleocapsid and the glycoproteins of the viral membrane; the pol gene codes for the reverse transcriptase and other enzymes. In addition, HIV-1 contains in its 9kB RNA six genes (vif, vpu, vpr, tat, rev and nef) that contribute to its genetic complexity. Nef, vif, vpr and vpu were classified as accessory genes in the past, as they are not absolutely required for replication in vitro. However, the regulation and function of these accessory genes and their proteins have been studied and characterized in more detail within the last years.

DESCRIPTION OF PROTEINS - gag: (336-1836) The gag region encodes Pr55 Gag. Pr55 Gag is myristylated at the 2Gly position. Proteolytic processing cleaves p17 MA, p24 CA, p7 NC and p2 from Pr55 Gag.

pol: (2102-4640) The pol ORF is in-frame with the pro ORF; pol gene products are synthesized as part of the Pr160 Gag-Pro-Pol polyprotein. RT is a p66/p51 heterodimer, the two subunits of which differ by the presence or absence of most of the RNase H domain. P32 IN is cleaved from the carboxy-terminal region of Pr160 Gag-Pro-Pol. Integration of HIV-1 DNA produces a 5-bp duplication of flanking cellular DNA.

vif: (4587-5163) The p23 Vif protein is translated from a singly spliced mRNA.

vpr: (5105-5339) The p15 Vpr protein is translated from a singly spliced mRNA. Vpr is found at the inner face of the cell membrane in infected cells. Large amounts of Vpr are packaged in virions, although it is not a structural protein.

tat: (5377-5991, 7925-7968) The p14 Tat protein is translated from multiply spliced mRNAs. Tat is localized in the nucleus of infected cells and activates viral transcription by binding to the TAR region of R. Tat is active as a homodimer and is phosphorylated in its carboxy-terminal region, although the functions of this modification is not known.

rev: (5516-5591, 7925-8197) The rev ORF spans exons 2 and 3 of a multiply spliced mRNA and encodes the Rev protein (p19). Rev localizes in the nucleus and is phosphorylated on serine residues. Rev binds the RRE (bases 7315-7548) present in intron-containing RNAs, facilitating mRNA transport to the cytoplasm.

vpu: (5608-5854) The vpu ORF is expressed from a weak initiation (AUG) codon upstream of the env AUG and encodes p16 Vpu. Vpu is phosphorylated on serine residues and localizes to the plasma cell membrane, but is not found in virions.

env: (5771-8339) The env ORF encodes the gPr160 Env polyprotein precursor that is processed by cellular proteases to generate mature gp120 SU and gp41 TM. The SU protein contains alternating conserved and variable (V) regions. SU mediates HIV-1 adsorption through the coordinate binding to host cell CD4 molecules and to one of several host cell coreceptors. Although the env genes of HIV1 have been extensively sequenced, the only known HIV env structure is the coiled-coil region of TM. The SU/TM pair is thought to form trimers on the virion membrane. There are 24 sites for N-linked glycosylation in SU. There are seven glycosylation sites in TM.

nef: (8343-8710) The nef ORF lies immedioately downstream from the env ORG and extends into the U3 region of the 3 LTR. P27 Nef is translated from a multiply spliced subgenomic mRNA. It undergoes posttranslational myristylation at the 2 Gly position which helps to localize Nef to the inner aspect of the cell membrane. Nef forms homodimers and is phosphorylated at 15Tyr.

Human immunodeficiency virus type 2 (HIV-2) shares many of the same biological properties with the well-studied HIV-1. Both are associated with acquired immunodeficiency syndrome (AIDS) in humans, are cytopathic in tissue culture, and have a tropism for CD4+ subset of T cells, macrophages, and other cells. Concordantly, their genome organization is quite similar and unique among retroviruses. The common feature of HIV genomes is the abundance of open reading frames (ORFs) not found in other retroviruses. In addition to three structural genes (gag, pol, env), there are several extra ORFs. ORFs designated as vif, vpr, tat, rev, and nef are common to HIV-1 and HIV-2. functional analyses of the HIV-1 genome have revealed the functions of these ORFs. In spite of the structural similarity of the genome, the nucleotide sequences and predicted amino acid sequences of these two viruses differ significantly, and furthermore, there is an unique ORF in each virus (vpu, unique to HIV-1, and vpx, unique to HIV-2).

Nucleotide sequence comparison between HIV-1, HIV-2 and SIV has revealed the presence of an open reading frame (ORF) in the central region of the genomes of HIV-2 and SIV that has no counterpart in HIV-1. This new ORF, called vpx, is highly conserved between HIV-2ROD and SIVmac. Using anti-peptide sera to the predicted protein and site-directed mutagenesis, we show that mutations in the vpx ORF eliminate the synthesis of a 16 kd protein in HIV-2 infected cells, confirming that this protein is the product of this gene. Full-length clones of HIV-2 containing these mutations are infectious in two permanent T lymphocytic cell lines and two monocytic cell lines. In contrast, we show that loss of VPX function results in a severe defect in the productive infection of human peripheral blood lymphocytes both in the amount of reverse transcriptase activity produced and in core protein expression. These findings suggest that the VPX protein plays an important role in the in vivo life cycle of the HIV-2/SIV viruses.

Antibody reactivity to the orf-x product was detected in 35 of 42 HIV-2 positive serum samples and 11 of 52 SIV seropositive monkeys. No such antibodies were detected in HIV-1 positive donors, blood donors seronegative for both HIV-2 and HIV-1, or SIV seronegative monkeys. This suggests that orf-x is dispensable for in vitro replication of SIVMAC and that the orf-x gene product of HIV-2 or its antibody can be used to distinguish HIV-2 from HIV-1 infection.

10359bp

Infectious molecular clones of the human immunodeficiency virus type 2 (HIV-2) will be valuable tools for the study of regulatory gene functions and the development of an animal model for the human acquired immunodeficiency syndrome (AIDS). To this end, we have cloned and sequenced a novel HIV-2 isolate, HIV-2BEN. One clone, designated MK6, is infectious for various human T-cell lines and for human and macaque peripheral blood lymphocytes (PBL), allowing molecular studies of HIV-2 infection and replication. Since MK6 is highly cytopathic in MT-2 and Molt-4 clone 8 cells, antiviral agents and neutralizing sera may be tested.

There appears to be at least two separate and unequal epidemics in the USA segregated by race and sexual orientation. The initial outbreak occurred among gay, predominantly white men, exploded rapidly across urban areas of the USA, and appears now to be decreasing. A later epidemic arose within the heterosexual, predominantly African-American community, initially fuelled by heterosexual injecting drug users, and concentrated in the southern USA. The increasing number and proportion of African-American men and women in the undetermined category may underestimate the heterosexual nature of the latter epidemic.

Heterosexual transmission of HIV and AIDS appears to be increasing in the USA and may be grossly underestimated by our present surveillance system. An insidious epidemic can be identified within the African-American heterosexual community that demands immediate attention. The increasing proportion of undetermined transmission may represent a large number of heterosexual cases that are not counted as such. The importance of recognising the true incidence and prevalence of HIV infection and AIDS among heterosexuals is a public health issue. The under-estimation of the heterosexual transmission rate falsely reassures the public and may actually increase the likelihood of HIV infection and AIDS in the general population over the next several decades.

Eighteen of the 20 counties with the largest proportional increases in the incidence rates of AIDS were located in the southeastern United States, compared with only 2 of the counties with the smallest increases. Compared with counties where the incidence of AIDS was increasing the least, counties where the incidence of AIDS was increasing the most had a higher proportion of low-income households, a higher proportion of households headed by a single mother, a larger proportion of the population that was African American, lower literacy levels, a higher proportion of persons with less than a 9th-grade education, a higher proportion of eligible voters who did not vote in the 2000 presidential election, and slightly more income inequality.

Phylogenetic analyses of globally circulating viral strains have identified three distinct groups of HIV-1 (M, N, and O), and nine genetic subtypes (A to D, F to H, J, and K) within major group (M). The vast majority of HIV-1 strains belong to group M (for Major), which is the pathogen responsible for the current pandemic. Group O (for Outlier) consists of a pool of highly divergent, genetically related strains with no defined clades. Group O infections are limited to people living in Central Africa (mainly Cameroon and some neighbouring countries), but even in this area they represent a small minority of HIV-1 infections. Only a few cases of group N (for New, or non-M/non-O) infections have been identified, and these were in patients from Cameroon.

On a global scale, the most prevalent HIV-1 genotypes are subtypes C (47%), A (27.2%), B (12.3%), D (5.3%) and CRF01_AE (3.2%). The greatest genetic diversity of HIV-1 was found in Central sub-Saharan Africa. Subtype A and C are the most common in these areas, but all groups and subtypes have been identified. This is consistent with the hypothesis that Africa is the source of the current pandemic. Subtype C is the predominant subtype in south and east Africa, which has the worst epidemic with more than 30% of the adult population infected with HIV. In West and West-central Africa, the majority of circulating strains is CRF02_AG. In North America and Europe, subtype B is predominant, showing a strong founder effect. In South America, subtype B is prevalent, while subtypes F and C, and CRF12_BF and the related B/F recombinants have been reported. In Asia, subtype C predominates in India and CRF01_AE is predominant in South-east Asia. Subtype B' (Thailand variant of subtype B) is a unique subtype B regional variant that spread primarily through injecting drug user (IDU) networks in South-east Asia. Two closely related CRFs, CRF07_BC and CRF08_BC are disseminating rapidly among IDU networks in North-western (Xinjiang Province) and South-eastern (Guangxi Province) China, respectively. Injecting drug use triggered a new HIV-1 epidemic in Eastern Europe: CRF03_AB was identified among IDUs in Kaliningrad, and in cities in Ukraine and Belarus.

FIRST PUBLISHED REPORT: In the period October 1980-May 1981, 5 young men, all active homosexuals, were treated for biopsy-confirmed Pneumocystis carinii pneumonia at 3 different hospitals in Los Angeles, California. Two of the patients died. All 5 patients had laboratory-confirmed previous or current cytomegalovirus (CMV) infection and candidal mucosal infection.

Pneumocystis pneumonia in the United States is almost exclusively limited to severely immunosuppressed patients. The occurrence of pneumocystosis in these 5 previously healthy individuals without a clinically apparent underlying immunodeficiency is unusual. The fact that these patients were all homosexuals suggests an association between some aspect of a homosexual lifestyle or disease acquired through sexual contact and Pneumocystis pneumonia in this population. All 5 patients described in this report had laboratory-confirmed CMV disease or virus shedding within 5 months of the diagnosis of Pneumocystis pneumonia.

All the above observations suggest the possibility of a cellular-immune dysfunction related to a common exposure that predisposes individuals to opportunistic infections such as pneumocystosis and candidiasis. Although the role of CMV infection in the pathogenesis of pneumocystosis remains unknown, the possibility of P. carinii infection must be carefully considered in a differential diagnosis for previously healthy homosexual males with dyspnea and pneumonia.

PRISONS: According to a national survey of jails and prisons, compared to the genereal population, rates of human immunodeficiency virus (HIV) infection among incarcerated individuals are 8 to 10 times higher. In the 1980s and 1990s, the HIV and crack cocaine epidemics and high rates of incarceration associated with the war on drugs had a devastating and synergistic impact on the physical and mental health of families in urban neighborhoods in the United States. Within public health, the rapid expansion of the correctional system can also have unintended consequences. Initiating antibiotic treatment for tuberculosis, HIV, or other sexually transmitted diseases without adequate follow-up to ensure completion of treatment can lead to the development of drug resistance, a peril to the community as a whole.

On June 30, 1997, more than 6300 state/federal prison inmates and more than 2800 jail inmates had AIDS. Also, there were more than 2600 state/ federal prison releasees and more than 36 000 jail releasees with AIDS in 1997. Thus, almost 16% of the estimated total of 247 000 persons living with AIDS in the United States in 1997 passed through a correctional facility that year.

INJECTION DRUG USERS: Despite a rigorous seroepidemiological surveillance program, only 31 HIV-1-infected individuals were identified in Kaliningrad in the time period of 1988 to June 1996. The yearly incidence rate from 1988 - 1995 in the region was from 0 to 0.09 per 10 000 calculated for the total population. In July 1996, a rapid increase in monthly new HIV-1 infections was detected, which continued through the following year. Between 1 July 1996 and 30 June 1997, 1335 newly diagnosed HIV-1-infected individuals (plus 141 who were tested anonymously, total cumulative number 1507) were detected in Kaliningrad. This gives a yearly incidence rate of 6.07 per 10 000 in 1996 (an increase of 6744% compared with 1995) and the incidence for the first 6 months of 1997 was 8.79 per 10 000. Most of the infected individuals were young, over 65% were in the 14 to 25-year age-group, and 20% were aged 14 to 18 years. Amongst 1269 infected individuals who were asked about their possible risk factors for HIV infection, the majority (n = 1208) reported injecting drug use. In 47 cases the reported risk factor was sexual contact (homo-/heterosexual) and 14 infected individuals were children born to infected mothers. Individuals were asked about risk factors using a standardized questionnaire; 335 individuals were asked about their transmission risk before testing, and 934 were asked when positive test results were delivered. In addition to the 141 anonymous tests, risk factors could not be determined for 97 individuals. The drug that the majority of the interviewed individuals with the injecting drug use risk factor reported using (> 95%) was a home-made opiate extracted from locally produced or imported poppy stems and heads.

HEMOPHILIACS: In July l982, three heterosexual hemophilia A patients, who had developed Pneumocystis carinii pneumonia and other opportunistic infections, were reported. Each had in vitro evidence of lymphopenia and two patients who were specifically tested had evidence of T-lymphocyte abnormalities. All three have since died. In the intervening 4 months, four additional heterosexual hemophilia A patients have developed one or more opportunistic infections accompanied by in-vitro evidence of cellular immune deficiency. These additional cases of AIDS among hemophilia A patients share several features with the three previously reported cases. All but one are severe hemophiliacs, requiring large amounts of Factor VIII concentrate. None had experienced prior opportunistic infections. All have been profoundly lymphopenic (1000 lymphocytes/cubic mm) and have had irreversible deficiencies in T-lymphocytes. Clinical improvement of opportunistic infections with medical therapy has been short lived. Two of the five have died. In most instances, these patients have been the first AIDS cases in their cities, states, or regions. They have had no known common medications, occupations, habits, types of pets, or any uniform antecedent history of personal or family illnesses with immunological relevance.

WOMEN IN SOUTHERN US: In 2003, women constituted 28% of human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/ AIDS) cases in the United States; approximately 69% of those cases were among non-Hispanic black women. Heterosexual transmission is now the most commonly reported mode of HIV transmission among women. In North Carolina, black women make up a growing proportion of newly reported HIV infections and, in 2003, the HIV-infection rate for black women in North Carolina was 14 times higher than that for white women. Despite this disparity, few epidemiologic studies have examined HIV transmission among black women in the United States, particularly those residing in southern states. In August 2004, the North Carolina Department of Health (NCDOH) invited CDC to assist in an epidemiologic investigation of HIV transmission among black women in North Carolina. This report summarizes the results of that investigation, which indicated that the majority of HIV-positive and HIV-negative sexually active black women in North Carolina reported HIV sexual risk behaviors. These findings underscore the need for enhanced HIV-prevention strategies in this population.

ASIA: The extensive spread of HIV started late in Asia, compared with the rest of the world. The earliest cases of AIDS were reported from Asia in 1984 and 1985, but the potential for widespread epidemics was not appreciated until the more extensive spread of HIV in Cambodia, India, Burma, and Thailand in the early 1990s. Unfortunately, the lessons of devastation from AIDS in Africa and the Caribbean went unheeded in much of Asia, and success stories of disease prevention at the national level in the region remain few. National adult HIV prevalence varies from near 0% in several countries to almost 3% in Cambodia. But a static map cannot show the time evolution of these epidemicsand therein lie major hints about the future of HIV in the region. Temporally, the countries of Asia can be divided into three categories: (1) those where HIV hit early and hard, and where adult HIV prevalence now exceeds 1% eg, Cambodia, Burma, Thailand, and some states in India; (2) those currently in transition, with HIV epidemics growing noticeably in the past 5 years eg, China, Indonesia, Nepal, and Vietnam; and (3) those having very low levels of infection such as: Bangladesh, Laos, the Philippines, and South Korea. Adult HIV prevalence peaked at roughly 15% in Thailand in 1996 and at 33% in Cambodia in 1998. Prevalence is currently falling in both countries, rather than continuing a steady growth. The reason is that they undertook extensive and intensive prevention campaigns with good coverage, which were focused specifically on reducing risk related to sex work in both clients and sex workers. In both countries, condom use between sex workers and clients increased to more than 90%, and the number of men visiting sex workers was halved (from 20% to 10%). This result then raises the issue of what is likely to happen in the countries in transition such as China, Indonesia, and Vietnam. In the absence of extensive prevention programmes to reduce HIV transmission in at-risk populations (clients and sex workers, injecting drug users, and men who have sex with men), they might be expected to see steadily climbing HIV prevalence.

THAILAND: Thailand experienced its first case of AIDS in 1984. Approximately 800,000 Thais were infected with HIV in 1995 and 1 million Thais became infected by the year 2000. There have been 5 major epidemic waves: among male homosexuals (started 1984-5), intravenous drug users (started 1988), female commercial sex workers (started 1989), male clients (started 1990), and housewives and the newborn (started 1991). Approximately 96 per cent of HIV-1 infected Thais carried recombinant subtype A/E, the rest carried B'. Nine phase I/II HIV-1 vaccine trial protocols have been or are being tested. A phase III trial of gp120 subtype B/E (AIDSVAX, VaxGen) was started in 1999, a total of 2,500 volunteers will be enrolled, and interim analysis is planned for August 2002. Thai investigators are also participating in pre-clinical development of recombinant BCG and DNA vaccines. Multidisciplinary and multi-level approaches, both by the government and private sectors, have had a positive impact on the HIV epidemic as shown by the declining seroprevalence of HIV infection in Thai male conscripts, and of major sexually transmitted diseases in men. Nevertheless, more effort at the grass roots level is needed to ensure further success and sustainability of the control of the HIV epidemic in Thailand.

INDIA: The HIV pandemic is rapidly spreading in India and has infected more than 4 million individuals. Studies have shown a rapid rise in HIV seroprevalence in high-risk groups like commercial sex workers, patients having STDs and IV drug abusers. However, the Indian epidemic is not only restricted to persons with high-risk behaviour, but has also entered the general population. The first HIV positive Indian was reported in 1992 at the Centre. The occurrence of HIV infection increased from nil in 1986 to 19.9% in 2003. The HIV seropositivity was significantly higher (z = 6.79; P < 0.001) in males (6.9%) than in females (3.3%). It was observed that HIV prevalence was highest in the age group of 25 to 29 years in both sexes.

UGANDA: The timing and scale of HIV prevalence declines in Uganda were distinct. HIV prevalence nationally among pregnant women peaked in 1991 at 21.1% and by 1998 declined to 9.7%, a decline of 54% apparent in both rural and urban settings. By 2000, prevalence had declined further to 6%. In Kampala and other urban sites, where age-specific data were available, HIV declined most in younger age groups, best reflecting recent incidence, with declines of 75% in 15-to 19-year-olds and 60% in 20- to 24-year olds. We expected to see a similar HIV pattern in neighboring countries, but statistically significant HIV declines were absent in similar data in Kenya, Malawi, and Zambia overall or in the 15 to 24 age cohorts. Uganda provides the clearest example that human immunodeficiency virus (HIV) is preventable if populations are mobilized to avoid risk. Despite limited resources, Uganda has shown a 70% decline in HIV prevalence since the early 1990s, linked to a 60% reduction in casual sex. The response in Uganda appears to be distinctively associated with communication about acquired immunodeficiency syndrome (AIDS) through social networks. Despite substantial condom use and promotion of biomedical approaches, other African countries have shown neither similar behavioral responses nor HIV prevalence declines of the same scale. The Ugandan success is equivalent to a vaccine of 80% effectiveness. Its replication will require changes in global HIV/AIDS intervention policies and their evaluation.

SOUTH AFRICA: South Africa is in the midst of one of the fastest growing HIV epidemics in the world. In the year 2000, an estimated 40% of deaths in adults aged 15-49 were attributable to AIDS, making it the single highest cause of death in South Africa. In the year 2002, there were more people living with HIV in South Africa than in any other country in the world. Simultaneously, South Africa has been the site of growing alarm at the high levels of rape reported from various sources, and the issues of sexual violence and violence against women in general have gained considerable political importance and visibility. Recent initiatives such as the Global Fund to Fight AIDS, Tuberculosis and Malaria, are making resources to address HIV/AIDS available on an unprecedented scale, and donor enthusiasm for introducing antiretroviral drugs must be met with an equal commitment to strengthening the systems that enable such drugs to be accessible, and which give them broader meaning.

ORPHANS: According to a United Nations report published in October 2003, half the new cases of HIV infection that occur across the world each year are among 15-24 year olds. This group, constituting two and a half million people, are the next generation of parents. The situation is particularly catastrophic in sub-Saharan Africa where widespread poverty and underdevelopment already undermine childrens health and well being. It is estimated that 10 million people in this region between the ages of 15 and 24, and up to 45% of pregnant women, are infected. With this recognition of the high prevalence of HIV in pregnant women in parts of sub-Saharan Africa, major efforts have been directed at developing and implementing interventions to prevent mother-to-child transmission. These efforts have been largely successful: antiretroviral medication, caesarean section, and locally appropriate feeding practice can now reduce transmission from 40% to below 10%.4 In sub-Saharan Africa these interventions are unfortunately not widely available, but nonetheless, it is still the minority of children who are infected. Orphaning is increasing in sub-Saharan Africa as rates of adult mortality have started to accelerate. However, by far the largest group of vulnerable young children are those living with an HIV infected mother. In sub-Saharan Africa approximately 70% of infected mothers survive for at least the first five years of their childrens lives, and this number will increase with the rollout of antiretroviral medication. It is known that the early years of life are crucial for a childs development, and it is likely that maternal HIV disrupts the rearing environment, thereby putting these children at risk.

LATIN AMERICA: By May 2000, 306 536 AIDS cases were reported to the Pan American Health Organization (PAHO/WHO) from countries in Latin America and Caribbean (LAC). Only 1.8% were paediatric cases and 1.5% were perinatal. Since 1993 the male-to-female ratio has significantly decreased from 3.3 to 2.3 in 1999, but the sex rate varies from 1.5 in the Caribbean to 3.5 in the Andean region. There are important differences in the mode of transmission in the different sub-regions. Whereas 42% of total cumulative AIDS cases were transmitted by homosexual or bisexual contact in the Andean region, only 9 and 11% reported this mode of transmission in the Caribbean and Central America. Transmission by injecting drug use varies widely, with higher rates reported in Brazil (19%) and Southern Cone countries (up to 33.4%) and much lower numbers in other subregions. The dominant mode of transmission varies from country to country, but it is mainly through homosexual and bisexual contacts and injecting drug use in most of Mexico, South America and the Andean countries. In Central America and Brazil, heterosexual transmission plays an increasing and important role for HIV dissemination. Brazil being the largest country and most populated country presents a very different state of epidemics among its population. In the Caribbean basin, the main mode of HIV transmission is by heterosexual sex.

GENERAL: The variability observed among and within routes of HIV exposure depends partly on the viral dose and also on whether the virus is transmitted directly into the blood or onto a mucous membrane. In addition, these differences are influenced by a variety of host factors, including both factors common to all routes of exposure and those unique to sexual transmission.

Host susceptibility depends on viral entry into cells through CD4 and chemokine surface receptors. These cells include CD4 T lymphocytes, T Langerhans cells, and other macrophages. In macaques, virus appears in dendritic cells of the vaginal lamina propria soon after vaginal inoculation with simian immunodeficiency virus (SIV). HIV-receptive cells have been found in the lamina propria of oral, cervicovaginal, foreskin, urethral, and rectal epithelia in other primate models.

A late stage of infection is a strong predictor of infectiousness according to both epidemiologic and biologic data. When the index partner has more advanced HIV infection indicated by symptoms of HIV disease, a diagnosis of the acquired immunodeficiency syndrome (AIDS), CD4 counts below 200 cells per cubic millimeter, or p24 antigenemia sexual partners are at a much higher risk of acquiring infection (relative-risk range, 6.1 to 17.6). Host infectiousness is likely to increase as a function of the concentration of virus in the genital tract. Higher viral loads in the blood have been associated with the transmission of HIV to sexual partners of people with transfusion-acquired infections. Data on viral concentration in blood and semen generally support the epidemiologic inferences about the importance of the stage of infection in the transmission of HIV

The presence of reproductive tract infections is strongly associated with susceptibility to HIV, even after adjustment for sexual behavior. The prevalence of genital ulcer disease (chancroid, syphilis, or herpes) is associated with an increased relative risk of HIV infection, ranging from 1.5 to 7.0 in both men and women.

The properties of HIV itself may also influence transmission. Subtype E, the most common subtype in Thailand, is reported to have a greater tropism for Langerhans cells than subtype B. This tropism may contribute to the rapid epidemic spread of HIV through Thailand and the high percontact transmission rate observed there. High concentrations of HIV in semen specimens from sub-Saharan Africa may reflect differences among HIV clades in the ability to replicate in vivo. There appear to be phenotypic differences between isolates in blood and those in semen. Nonsyncytia-inducing viral isolates that are macrophagetropic are found early in HIV disease and may be better adapted to spreading than lymphocytotropic organisms. Particular viral-envelope genetic sequences are required for vaginal transmission of chimeric simian-human immunodeficiency viruses. Genotypic differences in the viral envelope in blood as compared with genital specimens have been reported in women. In addition, other phenotypic differences between HIV harvested from blood plasma and that harvested from genital secretions may affect the efficiency of transmission. Antiretroviral drug resistance, for example, appears in cell-free and cell-associated virus in the blood and semen at different times.

SEXUAL: Transmission through sexual contact accounts for 75 to 85 percent of the nearly 28 million infections with the human immunodeficiency virus (HIV) that have occurred so far.

On a world-wide scale, the vast majority of new infections are acquired through heterosexual contact. The likelihood of transmission from male to female has been estimated to be as high as 8-fold more likely that from female to male, although the biological basis for this is not fully understood.

VAGINAL (MALE to FEMALE): Using logistic regression analysis, including gender and controlling for condom use, frequency of intercourse, anal sex, partner's CD4+ cell count and clinical stage, sexually transmitted diseases, genital infections, and contraceptive use, we found that the efficiency of male-to-female transmission was 2.3 (95% confidence interval = 1.1-4.8) times greater than that of female-to-male transmission.

The risk per episode of receptive vaginal exposure is estimated at 0.1% to 0.2%

VAGINAL (FEMALE to MALE): The overall probability of female-to-male HIV-1 transmission per sex act was 0.0063, several times higher than has been estimated from studies of HIV-1 serodiscordant couples. Infectivity for uncircumcised men was significantly higher than for circumcised men (0.0128 vs. 0.0051;P=0.04 ). Overall transmission probability estimates ranged from 0.0038 to 0.0122 when different HIV-1 prevalences among sex partners were assumed, and a ~23-fold greater infectivity was seen for uncircumcised men compared with circumcised men, across all HIV-1 prevalences

ANAL: U.S. survey and other data suggest that, in terms of absolute numbers, approximately seven times more women than homosexual men engage in unprotected receptive anal intercourse. Research among higher risk subpopulations, including bisexual men, injecting drug users, female sex workers, inner-city adolescents, and serodiscordant heterosexual couples, indicates that persons particularly at risk of being infected by or transmitting HIV are also more likely to practice anal sex. Considering this finding, along with the much greater efficiency for HIV infection as well as lower rates of condom usage, a significant proportion of heterosexual transmission in some populations is due to anal intercourse.

The major behavioral risk factors in the transmission of the Human Immunodeficiency Virus (HIV) between homosexual/bisexual men include receptive anogenital exposure to infected ejaculate and a large number of sexual partners. Among the 129 men with infected study partners, only the number of receptive anal exposures, truncated at 100, was significantly associated with infection at the 0.05 level. The risk of infection per outside partner was estimated to be 0.075 (+/- 0.018) when only participants with uninfected primary partners were included in the analysis, but 0.095(+/- 0.018) when the full cohort was used.

The risk for HIV transmission per episode of receptive penile-anal sexual exposure is estimated at 0.1% to 3%

ORAL: Although casual contact with saliva remains an insignificant factor, oral sexual contact may now be of increasing importance in the transmission of HIV. Oral sex may be less efficient than needle-sharing or anal intercourse for the transmission of HIV, but its increased use by men who have sex with men (MSM) and its prominence in the sexual activity of crack smokers may increase its contribution to HIV transmission. Based on several different models, the per-partner infectivity of receptive oral intercourse (ROI) was about 1% (range, 0.85% - 2.3%), where per-partner refers to the risk with a given partner, uncontrolled for activity level, and should be distinguished from the risk of a single sex act (per-contact risk). In comparison, the per-partner infectivity of anal receptive intercourse was about 10% (range, 4.2% - 12%). In a modeling study that incorporated the stage of infection, it was estimated that oral sex acts imposed a transmission risk that was one-sixth that of anal sex acts. In an assessment of per-contact risk for transmission associated with four types of homosexual contact, Vittinghoff et al. estimated that the risk for unprotected receptive anal sex (0.24%; 95% CI, 0.05 - 0.43) was eight times the risk for unprotected receptive oral sex (0.03%; 95% CI, 0.01 - 0.18).

ROLE OF VIRAL LOAD: The mean serum HIV-1 RNA level was significantly higher among HIV-1-positive subjects whose partners seroconverted than among those whose partners did not seroconvert (90,254 copies per milliliter vs. 38,029 copies per milliliter, P=0.01). There were no instances of transmission among the 51 subjects with serum HIV-1 RNA levels of less than 1500 copies per milliliter; there was a significant dose-response relation of increased transmission with increasing viral load. In multivariate analyses of log-transformed HIV-1 RNA levels, each log increment in the viral load was associated with a rate ratio of 2.45 for seroconversion.

INJECTION DRUG USE: The AIDS epidemic among injection drug users (IDUs) in the United States was first recognized in 1981. Through case surveillance conducted by the Centers for Disease Control and Prevention (CDC), IDUs with opportunistic infections (OIs) and poor immune response were identified and classified as cases of what was then known as gay-related immune disease. These early cases among IDUs had significant epidemiologic impact. They provided the first evidence that the disease was not restricted to men who have sex with men (MSM), and raised the possibility that the causal agent was blood-borne and likely transmitted through the reuse of contaminated injection equipment.

The risk for HIV transmission per episode of intravenous needle or syringe exposure is estimated at 0.67%

BREASTFEEDING: In this trial, the estimated rate of breast milk HIV-1 transmission was 16.2% during the first 2 years of life. Given an HIV-1 infection rate of 36.7% in the breastfeeding arm, breast milk transmission accounted for 44% of all infant infections among those exposed to breast milk. Because more than one quarter of women in the formula arm admitted to noncompliance with feeding modality, our estimated breast milk transmission rate is an underestimate. Because cumulative HIV-1 infection rates in the 2 arms were significantly different as early as 6 weeks of life, our data suggest that substantial transmission occurs early during breastfeeding. By 6 months, an estimated 75% of all breast milk transmission had occurred, despite ongoing exposure for an average of 1 additional year.

BLOOD TRANSFUSION: The general availability of a safe blood supply is of considerable concern to the bood-banking community, public health practitioners, clinicians, and the public. Approximately 1.5 percent of the U.S. population receives transfusions of blood products donated by an estimated 4 million persons each year, and the transfusion rate in persons over 65 years of age is three to four times this figure. The screening of blood donors for HIV-1 antibodies that now takes place has greatly reduced but not eliminated the risk of HIV-1 transmission by transfusion. Our estimate of this risk was 0.003 percent.

The risk of viral infection associated with blood transfusion is lower than ever before because of aggressive screening and testing practices. The risks were recalculated based on 2001 repeat donor incidence data collected by the American Red Cross. The calculated risk of HIV infection per 1,000,000 RBC units transfused is 0.82 (1 / 1.2 million units transfused).

HIV can be transmitted efficiently through blood transfusions: an estimated 95% of recipients become infected from transfusion of a single unit of infected whole blood.

OCCUPATIONAL: Of 57 healthcare workers with documented occupationally acquired HIV infection, most (86%) were exposed to blood, and most (88%) had percutaneous injuries. The circumstances varied among 51 percutaneous injuries, with the largest proportion (41%) occurring after a procedure, 35% occurring during a procedure, and 20% occurring during disposal of sharp objects. Unexpected circumstances difficult to anticipate during or after procedures accounted for 20% of all injuries. Of 55 known source patients, most (69%) had acquired immunodeficiency syndrome (AIDS) at the time of occupational exposure, but some (11%) had asymptomatic HIV infection. Eight (14%) of the healthcare workers were infected despite receiving postexposure prophylaxis (PEP).

The risk per episode of percutaneous exposure (e.g., a needlestick) to HIV-infected blood is estimated at 0.4% (upper limit of 95% confidence interval [CI] = 0.8%)

SELF-INOCULATION: We identified seven cases of intentional self-inoculation with HIV-positive material. Of these, six are known to have died, and the median time from HIV diagnosis to death was 71 months. All were white and four were women. Five worked in healthcare settings with access to contaminated sharps; one was a caregiver for family members with AIDS, and one had access to HIV-contaminated material through an HIV-positive friend. Five out of the seven had been diagnosed with depression before the inoculation event, and most had other diagnosed psychiatric disorders based on medical record review or patient self-report.

ARTIFICIAL INSEMINATION: A 35-year-old female health worker with some occupational risk for acquisition of HIV-1 infection had an acute infectious mononucleosis-like syndrome. She was in hospital for several days. 3 weeks later, she was positive for HIV-1 antibodies. She initially denied any risk factors in her private life, but later disclosed that 3 weeks before her illness she had unsuccessfully undergone artificial insemination with fresh sperm. Upon notification of the patient's illness, the gynaecologist concerned retested the donor, who had meanwhile seroconverted. Viral RNA from the recipient's serum (collected 12 weeks after the date of insemination) was reverse transcribed and the V3 region was amplified by nested PCR. Nucleotide sequence determination of the amplification product revealed 100% identity with viral sequences from the donor on the nucleotide level which was amplified from the DNA of peripheral blood mononuclear cells.

AIDS is unique in human history in its rapid spread, its extent and the depth of its impact. Since the first AIDS case was diagnosed in 1981, the world has struggled to come to grips with its extraordinary dimensions. Now, more than 20 years later, 20 million people are dead and 37.8 million people (range: 34.6 - 42.3 million) worldwide are living with HIV. And still, AIDS expands relentlessly, destroying peoples lives and in many cases seriously damaging the fabric of societies.

Two viruses cause AIDS in humans, HIV-1 and HIV-2. In the light of recent confirmation that SIVcpz is the progenitor of HIV-1, it is not surprising that HIV-1 can readily be transmitted to common chimpanzees (Pan troglodytes). HIV-1 infection of the other species of great apes (gorilla, bonobo, orangutan) has not been reported, though HIV-1 infection of gibbons has been reported. In concordance with the generalization that the primate lentiviruses do not cause disease in their natural hosts, chimpanzees infected with viruses of the HIV-1/SIVcpz group do not develop significant CD4+ T-cell loss or AIDS.

SIVsmm is the progenitor of HIV-2 and SIVmac, which appear to have arisen due to cross-species transmission to humans and macaques, respectively. SIVmac infection in macaque monkeys is a well-characterized model of AIDS.

Both HIV-1 and HIV-2 are of zoonotic origin. The closest simian relatives of HIV-1 and HIV-2 have been found in the common chimpanzee (Pan troglodytes) and the sooty mangabey (Cercocebus atys), respectively, and phylogenetic evidence indicates that lentiviruses from these species (SIVcpz and SIVsm, respectively) have been transmitted to humans on at least eight occasions. The current HIV-1 pandemic provides compelling evidence for the rapidity, stealth, and clinical impact that can be associated with even a single primate lentiviral zoonotic transmission event. We document for the first time that humans are exposed to a plethora of primate lentiviruses through hunting and handling of bushmeat in Cameroon, a country at the center of HIV-1 groups M, N, and O endemicity that is home to a diverse set of SIV-infected nonhuman primates. To what extent wild monkey populations in other parts of Africa are also infected with diverse SIVs is unknown. A complete and accurate assessment of all SIV-infected nonhuman primate species is needed, as well as a determination of the virus lineage(s) present in each species. Studies are also needed to determine whether zoonotic transmissions of SIVs from primates other than chimpanzees and mangabeys have already occurred and what clinical outcomes were associated with these infections. Results from these studies will yield critical insights into the circumstances and factors that govern SIV cross-species transmission and thus allow determination of human zoonotic risk for acquiring these viruses.

IN HUMAN: The median survival [of HIV-1 infected patients] from seroconversion to death was 9.8 years, which is considerably longer than has been expected in African populations. This is also comparable to survival times of around 10 years (ranging from 8.3 to approximately 13 years) reported by cohort studies in industrialized countries prior to the widespread use of antiretroviral therapy.

IN SYRINGE: At 4C, 50% of all syringes contained viable HIV-1 at 42 days storage, the longest storage duration tested. At room temperature (20C), the last day that syringes with 2uL of infected blood were positive was Day 21, and viable HIV-1 was recovered from 8% of syringes. Above room temperature (27, 32, and 37C), the likelihood of encountering syringes with viable HIV-1 when periods of storage exceeded 1 week decreased to less than 1%. The temperature at which drug injectors are likely to store their used syringes will vary according to climate, season, and circumstances faced by the injector. The survival of HIV-1 in contaminated syringes varied over a range of temperatures, and this may be a factor influencing the syringe-borne transmission of HIV-1.

LAB STORAGE: A recently developed assay for measuring infectious HIV-1 particles was used to determine the stability of the virus under various storage conditions as well as the effect of commonly used disinfectants. At the optimum pH of 7.1 the half life of the virus ranged from approx. twenty-four hours at 37C to no significant loss over 6 months at -75C. Drying the virus on a glass surface or freezing caused a 5-12 fold and 4-5 fold decrease of activity, respectively. The dried preparations, however, were about as stable as when stored in a buffered solution. A solution of iodine and detergent (2% Jodopax) was the only disinfectant examined which removed all detectable HIV-1 activity. Isopropanol and ethanol were more potent than acetone; however, all three solvents left some viable particles after a 30 min treatment with 70% solutions.

Transmission of human immunodeficiency virus type 1 (HIV-1) from a male accused of rape and deliberate transmission of HIV-1 was investigated by sequencing of the HIV-1 pol and gag genes from virus obtained from the male and from the female victim. We found that the male and female shared two distinct genetic variants of HIV-1. In pl7gag the major variant had an unusual, out-of frame deletion of 3 nucleotides which the minor variant lacked. These results indicated that the male had transmitted more than one infectious unit to the female. From this study we concluded that it was highly likely that the HIV-1 strains carried by the male and the female were closely epidemiologically linked.

Human

Homo sapiens

The earliest documented case of infection with HIV-1 was identified in a sample stored in 1959 from the city now known as Kinshasa in the Democratic Republic of Congo (DRC). The lack of other good-quality material from early time points in the history of HIV makes it hard to be sure about the precise timing of the human epidemic. An important feature of HIV is its inherent variability, which is a function of both the lack of a proof-reading mechanism in the viral reverse transcriptase enzyme and the rapid replication rate of the virus. Together, these features allow HIV to mutate rapidly and they enable variants that are resistant to drugs or immune responses to emerge under selection pressure. Assuming that the evolutionary behaviour of the virus has been consistent over time, it is possible to use the wealth of available HIV-1 sequence data from viruses with known dates of sampling to model virus diversification and to estimate the timing of important events in the virus past. In this way, Korber et al. proposed that the main (M) group of HIV-1 strains that are currently infecting the human population began to diversify around 1930. This gives us some idea of the minimum length of time for which HIV has infected humans, but it does not tell us exactly how or when the first ancestral virus infected a human. It seems probable that much of the early evolution of the virus took place in central Africa, particularly in the DRC, as this is the country from which the most diverse range of virus sequences has been recovered. The most probable source of HIV in humans is the inadvertent introduction of a simian immunodeficiency virus (SIV) from a closely related primate species. In 1986, a second retrovirus strain, known as HIV-2 and having approximately 4060% homology with HIV-1, was isolated from patients with AIDS from West Africa. Careful studies of HIV-2 genomes showed that there is a remarkably close relationship between HIV-2 and SIV strains that infect sooty mangabeys (SIVsm) in the same parts of West Africa, and it is now thought that there have been at least three separate entries of SIVsm into the human population. For some time, the origin of HIV-1 was less clear the most closely related viruses were those found occasionally in captive chimpanzees, but these SIVcpz strains had much less homology with HIV-1 than SIVsm did with HIV-2. The mystery was at least partly solved when SIVcpz strains with greater similarity to HIV-1 were discovered in a chimpanzee species whose habitat had marked geographical overlap with the regions of central Africa where the M group of HIV-1 is believed to have originated. It is still not clear to what extent chimpanzees are infected with these SIV strains in the wild or exactly how humans first became infected.

The variability observed among and within routes of HIV exposure depends partly on the viral dose and also on whether the virus is transmitted directly into the blood or onto a mucous membrane. In addition, these differences are influenced by a variety of host factors, including both factors common to all routes of exposure and those unique to sexual transmission.

Serum levels of HIV-1 RNA of more than 1500 copies per milliliter. There was a significant dose-response effect with respect to both male-to-female transmission and female-to-male transmission (P<0.001). The rate of transmission was zero among the 51 couples in which the HIV-1 positive partner had undetectable serum levels of HIV-1 RNA or less than 1500 copies per milliliter. Among HIV-1 positive partners with serum HIV-1 RNA levels of less than 3500 copies per milliliter, the rate of transmission was 2.2 per 100 person-years, and the rates progressively increased with increasing viral loads, to a maximum of 23.0 per 100 person-years at a level of 50,000 or more copies per milliliter. It is noteworthy that among the 90 instances of transmission, 5.6 percent occurred among couples in which the HIV-1 positive partner had serum HIV-1 RNA levels of 400 to 3499 copies per milliliter, 17.7 percent among couples in which the seropositive partner had levels of 3500 to 9999 copies per milliliter, 40.0 percent among couples in which the seropositive partner had levels of 10,000 to 49,999 copies per milliliter, and 36.7 percent among couples in which the seropositive partner had levels of 50,000 or more copies per milliliter. There was no significant difference between male-to-female and female-to-male transmission rates after the results were adjusted for viral load (P=0.76), and there were no consistent differences between male-to-female or female-to-male transmission rates within strata of viral load.

Findings from several trials in breast-feeding settings support the efficacy of ultrashort (intrapartum and 1 week postpartum) courses of infant antiretroviral prophylaxis using 3 different regimens (NVP, ZDV alone, ZDV/3TC) in reducing the risk of MTCT. However, these peripartum interventions do not address prevention of late breast-feeding transmission after the first weeks of life.

~40%

Male circumcision consistently shows a protective effect against HIV infection. This may be due to the abundance of Langerhans cells in the foreskin or to a receptive environment for HIV in the sulcus between the foreskin and glans. The prevalence of HIV infection is 1.7 to 8.2 times as high in men with foreskins as in circumcised men, and the incidence of infection is 8 times as high. A greater proportion of the sex partners of uncircumcised men than of circumcised men are infected with HIV, which suggests that the presence of the foreskin may also increase infectiousness.

Circumcised men have a reduced risk of HIV-1 infection compared with uncircumcised men. Some investigators have argued that circumcision is simply an epidemiological marker of reduced behaviours related to risk of HIV-1 infection, including religious or cultural factors. Others have suggested that circumcision reduces the risk of other sexually transmitted diseases associated with genital ulceration or mucosal inflammation, which secondarily reduces the risk of HIV-1. After adjusting for sociodemographic and behavioural risk factors in the proportional hazards model, circumcision had no significant protective effect on incident HSV-2, syphilis, or gonococcal urethritis. Circumcision was strongly protective against HIV-1 acquisition, with a 67-fold reduction in risk of HIV-1 infection among circumcised men. A unique and important finding from this study was a highly significant and specific protective effect of male circumcision on the risk of HIV-1 acquisition. Our data failed to show a significant protective effect of circumcision on the risk of the other STIs. These epidemiological data lend support to the hypothesis that male circumcision protects against HIV-1 infection primarily due to removal of the foreskin, which contains a high density of HIV-1-specific cellular targets, including CD4+ T-lymphocytes and Langerhans cells, which are easily accessible to the virus through the thin layer of keratin overlying the inner mucosa.

The exhaustive Cochrane review of the evidence for possible protection from female-to-male sexual transmission of HIV-1 by circumcision concluded that insufficient evidence exists to support an interventional effect of male circumcision in heterosexual males.

8-fold reduction in incidence

Lifetime

CULTURAL ACCEPTANCE: Circumcision would not be culturally acceptable to Hindu men. In addition, there are other factors to consider before taking any decision to introduce circumcision. These include potential adverse medical and psychosexual effects, as well as legal, ethical, and human rights issues.

Although the Centers for Disease Control and Prevention (CDC) recommend routine HIV counseling, testing, and referral (HIVCTR) in settings with at least a 1 percent prevalence of HIV, roughly 280,000 Americans are unaware of their human immunodeficiency virus (HIV) infection. In the high-risk population, the addition of one-time screening for HIV antibodies with an enzyme-linked immunosorbent assay (ELISA) to current practice was associated with earlier diagnosis of HIV (mean CD4 cell count at diagnosis, 210 vs. 154 per cubic millimeter). One-time screening also improved average survival time among HIVinfected patients (quality-adjusted survival, 220.7 months vs. 219.8 months). The incremental cost-effectiveness was $36,000 per quality-adjusted life-year gained. Testing every five years cost $50,000 per quality-adjusted life-year gained, and testing every three years cost $63,000 per quality-adjusted life-year gained. In the CDC threshold population, the cost-effectiveness ratio for one-time screening with ELISA was $38,000 per quality-adjusted life-year gained, whereas testing every five years cost $71,000 per quality-adjusted life-year gained, and testing every three years cost $85,000 per qualityadjusted life-year gained. In the U.S. general population, one-time screening cost $113,000 per quality-adjusted life-year gained. Under current screening practices in the high-risk population, we expect to observe 44,000 to 60,000 secondary transmissions per 100,000 participants in the screening program. A single ELISA could avert up to 300 of these secondary transmissions. Repeated testing every five years, three years, and one year could avert 2700, 3600, and 5100 infections, respectively. In all but the lowest-risk populations, routine, voluntary screening for HIV once every three to five years is justified on both clinical and cost-effectiveness grounds. One-time screening in the general population may also be cost-effective.

The pathogenesis of HIV disease, from a virological and immunological standpoint, has been studied intensively and defined progressively over the past 20 years. The pathogenic mechanisms of HIV disease are extremely complex and multifactorial. Even before HIV was identified, it was recognized that an apparent paradox existed whereby the immune system was aberrantly activated at the same time that the individual was experiencing immune deficiency. This was later shown to be due to a combination of the aberrant secretion of various cytokines, many of which could upregulate virus expression, and the intensive cell signaling induced by the viral envelope. Depletion of CD4 T cells was recognized as a hallmark of disease early on, even before the classic demonstration in 1984 that the CD4 molecule was the primary receptor for the virus on a subset of T cells and monocytes. In the mid-1990s, a number of diverse areas of investigation elucidated the roles of the chemokine receptors CXCR4 and CCR5 in the efficient binding and entry of two different strains of HIV-1 called X4 and R5, respectively. Indeed, RANTES, MIP-1 and MIP-1 , the ligands for CCR5, were shown to potently inhibit the binding of virus to its target cell. This recognition that HIV could use different coreceptors also helped to explain the occurrence of syncytial (CXCR4-using) and nonsyncytial (CCR5-using) variants of HIV. The importance of the CCR5 coreceptor in the pathogenesis of HIV infection was proven by the finding that cells from individuals homozygous for a deletion of 32 base pairs in the CCR5 gene could not be infected in vitro with R5 viruses and that such individuals, who comprise about 1% of white populations, are extremely resistant to HIV infection even when repetitively exposed to virus. Studies of lymphoid tissue in individuals infected with HIV revealed the disseminated nature of HIV infection and the fact that lymphoid tissue is indeed the chief target and reservoir of HIV infection. In addition, it became clear that HIV continually replicates at varying degrees in lymphoid tissue despite the fact that the individual might appear to be clinically well. Although the clinical course varied widely among individuals, the inexorably progressive nature of disease in most individuals became clear. An important advance in HIV research has been the development of highly sensitive techniques for the precise quantification of small amounts of nucleic acids. The measurement of serum or plasma levels of HIV RNA is now an essential component of the monitoring of individuals with HIV infection and, together with CD4 T cell counts, guides therapeutic decisions. Assays such as RT-PCR and the bDNA (branched DNA) technique for directly detecting HIV RNA have helped to clarify the direct relationship between amounts of virus and rates of disease progression, rates of viral turnover, the relationship between immune system activation and viral replication, and responsiveness to therapy. The ability to measure plasma viremia precisely led to the classic viral dynamics studies of Ho and Shaw in 1995, which characterized the enormous turnover of virus in HIV disease and the delicate balance between virus production and T cell dynamics. These studies led to a cascade of insights into HIV pathogenesis, among them an appreciation of the direct relationship betw