Dispensable role of the human immunodeficiency virus type 2 Vpx protein in viral replication

Human immunodeficiency virus type 2 (HIV-2) is similar in genetic organization to HIV-1 but contains a unique gene (vpx) that encodes a 16-kDa protein. A replication-competent molecular clone of HIV-2 (HIV-2sbl/isy) that infects human primary cells in vitro and rhesus monkeys was used to generate three mutations in the vpx gene. In the first mutant, the vpx open reading frame was truncated at amino acid 20; the second mutant was tailored to eliminate the proline-rich carboxyl terminus of the protein; and the third mutant was obtained by addition of four amino acids (KDEL) to the carboxyl terminus of the protein to provide a retention signal in the endoplasmic reticulum. The viral infection kinetics of the three mutant viruses and isogeneic HIV-2sbl/isy in the SupT1 cell line were similar. Slight impairment in the early phases of viral replication was observed during infection of primary human peripheral blood mononuclear cells with the vpx mutant viruses. All of the vpx mutant viruses readily infected macrophages, indicating that vpx expression is dispensable for HIV-2 infection and replication in human macrophages.

The human immunodeficiency virus type 2 vpr gene is essential for productive infection of human macrophages

The human immunodeficiency virus (HIV) genetic determinant(s) responsible for tropism in human T cells or macrophages are not well defined. We studied the role of the HIV type 2 (HIV-2) nef and vpr genes in viral tropism. HIV-2 mutants, lacking either vpr or nef genes, or both vpr and nef, were obtained by site-specific mutagenesis of a biologically active HIV-2 proviral clone (HIV-2sbl/isy), which is infectious in both human T cells and macrophages. Viral progeny carrying mutations of nef, vpr, or of both nef and vpr genes replicated more efficiently than the parental virus in primary human peripheral blood cells and in the human Hut 78 T-cell line. In contrast, the HIV-2 nef- mutant infected human macrophages as efficiently as the parental virus, whereas viruses lacking the vpr gene either alone or in conjunction with the lack of the nef gene did not replicate in macrophages. Thus, some lack of nef in HIV-2 enhances viral replication in T cells and does not interfere with viral replication in primary macrophages, whereas vpr is essential for replication of HIV-2 in human macrophages. Because the parental HIV-2sbl/isy cloned virus also infects rhesus macaques, the use in animal studies of these HIV-2 mutants with differences in cell tropism and rates of replication will be highly useful in understanding the mechanism of viral infectivity and possibly pathogenicity in vivo.

Persistent infection of rhesus macaques with a molecular clone of human immunodeficiency virus type 2: evidence of minimal genetic drift and low pathogenetic effects

In an attempt to generate a suitable animal model to study the infectivity and possible pathogenicity of human immunodeficiency viruses, we intravenously inoculated juvenile rhesus macaques and African green monkeys with a molecularly cloned virus, human immunodeficiency virus type 2 HIV-2sbl/isy, as well as with the uncloned HIV-2nih-z virus. Infection was monitored by virus recovery from the peripheral blood cells and by seroconversion against HIV-2 antigens measured by Western immunoblot, radioimmunoprecipitation, and enzyme-linked immunosorbent assay. We successfully infected two out of two macaques with the molecularly cloned virus and one macaque out of two with the HIV-2nih-z. No evidence of infection was seen in the African green monkeys with either virus. We followed the infected animals for 2 years. The animals remained healthy, although we observed intermittent lymphadenopathy and a transient decrease in the absolute number of circulating CD4+ T lymphocytes in both animals infected with the molecularly cloned virus. Virus isolation from the peripheral blood cells of the infected animals was successful only within the first few months after inoculation. Evidence of persistent infection was provided by the detection of proviral DNA by polymerase chain reaction analysis of the blood cells of the inoculated animals and by the stability of antiviral antibody titers. To evaluate the genetic drift of the proviral DNA, we molecularly cloned viruses which were reisolated 1 and 5 months postinoculation from one of these animals. Comparison of the DNA sequences of the envelope genes of both these isolates indicated that a low degree of variation (0.2%) in the envelope protein had occurred in vivo during the 5-month period. These data suggest that the use of HIV-2sbl/isy in rhesus macaques may represent a good animal model system to study prevention of viral infection. In particular, molecularly cloned virus can be manipulated for functional studies of viral genes in the pathogenesis of acquired immune deficiency syndrome and provides a reproducible source of virus for vaccine studies.

Identification of the fusion peptide of primate immunodeficiency viruses

Membrane fusion induced by the envelope glycoproteins of human and simian immunodeficiency viruses (HIV and SIVmac) is a necessary step for the infection of CD4 cells and for the formation of syncytia after infection. Identification of the region in these molecules that mediates the fusion events is important for understanding and possibly interfering with HIV/SIVmac infection and pathogenesis. Amino acid substitutions were made in the 15 NH2-terminal residues of the SIVmac gp32 transmembrane glycoprotein, and the mutants were expressed in recombinant vaccinia viruses, which were then used to infect CD4-expressing T cell lines. Mutations that increased the overall hydrophobicity of the gp32 NH2-terminus increased the ability of the viral envelope to induce syncytia formation, whereas introduction of polar or charged amino acids in the same region abolished the fusogenic function of the viral envelope. Hydrophobicity in the NH2-terminal region of gp32 may therefore be an important correlate of viral virulence in vivo and could perhaps be exploited to generate a more effective animal model for the study of acquired immunodeficiency syndrome.

Genetic variability between isolates of human immunodeficiency virus (HIV) type 2 is comparable to the variability among HIV type 1

The isolation from macaques of retroviruses related to human immunodeficiency virus (HIV) led to the identification of a second group of human retroviruses (termed HIV-2), which are prevalent in West Africa and closely related to the simian immunodeficiency virus (SIV). We have cloned and determined the complete nucleotide sequence of the human West African retrovirus HIV-2NIH-Z and compared it to that of a previously described strain of HIV-2 (HIV-2ROD) as well as to SIV and HIV-1. We have reached the following conclusions: (i) The HIV-2 isolates are (slightly) more closely related to each other than to SIV, compatible with their isolation from different species. (ii) The variability between HIV-2 isolates is similar in degree and kind to that found among HIV-1 isolates. The equivalent degrees of intragroup divergence suggest that HIV-1 and HIV-2 have existed in their present ranges in Africa for approximately equal lengths of time. The fact that acquired immunodeficiency syndrome is widespread in regions where HIV-1 is prevalent but not in regions where HIV-2 is prevalent suggests a substantial difference in the morbidity rates associated with HIV-1 vs. HIV-2 infection. (iii) HIV-2 and SIV are related to each other more closely than they are to HIV-1.

The simian immunodeficiency virus envelope open reading frame located after the termination codon is expressed in vivo in infected animals

Genetic comparison of SIVmac to the human retroviruses generally associated with AIDS revealed a closer relationship to HIV-2 than to HIV-1. A common feature differentiating SIV and HIV-2 from HIV-1 is the size of the transmembrane portion of the envelope, which is smaller (gp32) in SIVmac and HIV-2 than in HIV-1 (gp41). The presence of this truncated form of the transmembrane glycoprotein in SIVmac and HIV-2 virions is apparently related to the presence of a translation termination codon in the env gene of all SIV proviruses analyzed as well as in one HIV-2 provirus. Since the carboxy terminus of the envelope transmembrane protein has been implicated in the cytopathic effect of HIV-1 in vitro, we decided to investigate whether putative expression of the open reading frame located after the termination codon correlates with the pathogenicity of SIVmac in vivo. We generated two synthetic peptides from the inferred amino acid sequence of SIVmac and tested their reactivity by Western blot against the sera of naturally and experimentally infected monkeys as well as against sera of HIV-2-infected individuals. Our results indicate that the protein synthesized from this open reading frame is expressed in vivo, since an immunoresponse can be detected against the synthetic peptides in two of three experimentally SIVmac-infected animals. However, no correlation can be found between its expression and disease progression at this time. Furthermore, a rabbit immune serum raised against the synthetic peptide failed to identify any specific protein in SIVmac-infected cells.