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The etiological agent of AIDS, LAV/HTLV-III, is common in Central Africa but is not endemic in other areas of that continent. A novel human retrovirus, distinct from LAV/HTLV-III, has now been isolated from two AIDS patients from West Africa. Partial characterization of this virus revealed that it has biological and morphological properties very similar to LAV but that it differs in some of its antigenic components. Although the core antigens may share some common epitopes, the West African AIDS retrovirus and LAV differ substantially in their envelope glycoproteins. The envelope antigen of the West African virus can be recognized by serum from a macaque with simian AIDS infected by the simian retrovirus termed STLV-IIImac, suggesting that the West African AIDS virus may be more closely related to this simian virus than to LAV. Hybridization experiments with LAV subgenomic probes further established that this new retrovirus, here referred to as LAV-II, is distantly related to LAV and distinct from STLV-IIImac.

Analysis of the nucleotide sequence of the human retrovirus associated with AIDS in West Africa, HIV-2, shows that it is evolutionarily distant from the previously characterized HIV-1. We suggest that these viruses existed long before the current AIDS epidemics. Their biological properties are conserved in spite of limited sequence homology; this may help the determination of the structure-function relationships of the different viral elements.

he use of combinations of antiretroviral drugs has proven remarkably effective in controlling the progression of human immunodeficiency virus (HIV) disease and prolonging survival, 1 but these benefits can be compromised by the development of drug resistance. 2,3 Resistance is the consequence of mutations that emerge in the viral proteins targeted by antiretroviral agents. In the United States, as many as 50 percent of patients receiving antiretroviral therapy are infected with viruses that express resistance to at least one of the available antiretroviral drugs. 4 Consequently, the transmission of drug-resistant strains is also a growing concern. 5-7 Because drug-resistant HIV often exhibits resistance to several classes of antiretroviral drugs 8 and because cross-resistance between drugs within a class is frequent, 9-12 the emergence of resistance always complicates further efforts to control viral replication. This review focuses on the mechanisms underlying the selection of drug-resistant HIV and on the consequences of viral resistance with respect to the evolution of HIV infection. The drugs currently used to treat HIV type 1 (HIV-1) infection (Table 1) belong to four distinct classes: nucleoside and nucleotide analogues, which act as DNA-chain terminators and inhibit reverse transcription of the viral RNA genome into DNA, a crucial event occurring at an early stage of the viral life cycle; nonnucleoside reverse-transcriptase inhibitors, which bind and inhibit reverse transcriptase, the viral enzyme that conducts reverse transcription; protease inhibitors, which target the viral protease, the enzyme required for the cleavage of precursor proteins (gag and gag-pol), permitting the final assembly of the inner core of viral particles; and entry inhibitors, which block the penetration of HIV virions into their target cells. Combinations of antiretroviral drugs are now used for the treatment of HIV infection-so-called highly active antiretroviral therapy (HAART). Current HAART regimens generally comprise three antiretroviral drugs, usually two nucleoside analogues and either a protease inhibitor or a nonnucleoside reverse-transcriptase inhibitor. 13 The use of agents from different classes is instrumental in controlling the development of resistance, but whereas some drug combinations have been shown to be antagonistic, there is no evidence that any combinations of currently available drugs are strongly synergistic in vitro. induced resistance Two concepts are important to an understanding of the development of drug resistance. First, HIV infection is characterized by high levels of virus production and turnover. In most untreated patients, the total number of productively infected cells in the lymphoid tissue has been estimated to be approximately 10 7 to 10 8 cells. 14 During the chronic phase of HIV infection, this number is relatively stable, reflecting the balance between t principles of drug therapy for hiv development of resistance

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Overall, resistance assays did not demonstrate benefit over standard of care. In patients with the most limited protease inhibitor experience, a significant benefit was observed in the genotyping arm.