Persistence of pathogenic challenge virus in macaques protected by simian immunodeficiency virus SIVmacDeltanef

Live attenuated simian immunodeficiency virus (SIV) is the most efficient vaccine yet developed in monkey models of human immunodeficiency virus infection. In all successful vaccine trials, attenuation was achieved by inactivating at least the nef gene. We investigated some virological and immunological characteristics of five rhesus macaques immunized with a nef-inactivated SIVmac251 molecular clone (SIVmac251Deltanef) and challenged 15 months later with the pathogenic SIVmac251 isolate. Three animals were killed 2 weeks postchallenge (p.c.) to search for the challenge virus and to assess immunological changes in various organs. The other two animals have been monitored up for 7 years p.c., with clinical and nef gene changes being noted. The animals killed showed no increase in viral load and no sign of a secondary immune response, although the challenged virus was occasionally detected by PCR. In one of the monkeys being monitored, the vaccine virus persisted and an additional deletion occurred in nef. In the other monkey that was monitored, the challenge and the vaccine (Deltanef) viruses were both detected by PCR until a virus with a hybrid nef allele was isolated 48 months p.c. This nef hybrid encodes a 245-amino-acid protein. Thus, our results show (i) that monkeys were not totally protected against homologous virus challenge but controlled the challenge very efficiently in the absence of a secondary immune response, and (ii) that the challenge and vaccine viruses may persist in a replication-competent form for long periods after the challenge, possibly resulting in recombination between the two viruses.     

The nef gene of simian immunodeficiency virus SIVmac1A11.

The role of the simian immunodeficiency virus (SIV) nef gene in viral replication was investigated in several tissue culture systems. SIVmac1A11 is a molecularly cloned virus which replicates in both peripheral blood mononuclear cells (PBMC) and macrophages, although no disease is observed in infected rhesus macaques. In this report, we demonstrate that SIVmac1A11 contains a full open reading frame for nef which specifies a 37-kDa protein. To investigate the effects of nef on viral replication, a 70-bp deletion was introduced into the nef gene of SIVmac1A11. Analysis of infected cell extracts by immunoblotting revealed that both SIVmac1A11 and nef deletion virus SIVmac1A11 delta nef produced the same viral proteins, except that Nef was absent in the mutant virus. The deletion mutation did not affect viral replication in PBMC, in monocyte-derived and alveolar macrophages obtained from rhesus macaques, and in human cell lines HUT-78 and CEMx-174. In addition, SIVmac1A11 and SIVmac1A11 delta nef exhibited similar patterns of cytopathologic changes and ultrastructural appearances in infected cells. SIVmac1A11 and SIVmac1A11 delta nef did not infect human tumor macrophage cell line U937, GCT, THP-1, or HL-60 cells, although virus was produced after these cells were transfected with either wild-type or nef mutant viral DNA. Similar levels of virus were recovered from U937 and THP-1 cells transfected with mutant and parental proviral DNAs. In transient expression assays in a T-cell line and a macrophage line, the nef protein of SIVmac1A11 did not significantly suppress or enhance expression of the chloramphenicol acetyltransferase reporter gene linked to the SIVmac long terminal repeat. Thus, abrogation of nef did not affect several in vitro properties of SIVmac1A11, including patterns of viral infection in rhesus PBMC, rhesus macrophages, or human T-cell lines.     

Induction of AIDS in Rhesus Monkeys by a Recombinant Simian Immunodeficiency Virus Expressing nef of Human Immunodeficiency Virus Type 1

A nef gene is present in all primate lentiviruses, including human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus of macaque monkeys (SIVmac). However, the nef genes of HIV-1 and SIVmac exhibit minimal sequence identity, and not all properties are shared by the two. Nef sequences of SIVmac239 were replaced by four independent nef alleles of HIV-1 in a context that was optimal for expression. The sources of the HIV-1 nef sequences included NL 4-3, a variant NL 4-3 gene derived from a recombinant-infected rhesus monkey, a patient nef allele, and a nef consensus sequence. Of 16 rhesus monkeys infected with these SHIVnef chimeras, 9 maintained high viral loads for prolonged periods, as observed with the parental SIVmac239, and 6 have died with AIDS 52 to 110 weeks postinfection. Persistent high loads were observed at similar frequencies with the four different SIV recombinants that expressed these independent HIV-1 nef alleles. Infection with other recombinant SHIVnef constructions resulted in sequence changes in infected monkeys that either created an open nef reading frame or optimized the HIV-1 nef translational context. The HIV-1 nef gene was uniformly retained in all SHIVnef-infected monkeys. These results demonstrate that HIV-1 nef can substitute for SIVmac nef in vivo to produce a pathogenic infection. However, the model suffers from an inability to consistently obtain persisting high viral loads in 100% of the infected animals, as is observed with the parental SIVmac239.     

Simian immunodeficiency virus in which nef and U3 sequences do not overlap replicates efficiently in vitro and in vivo in rhesus macaques

The nef genes of human immunodeficiency virus and simian immunodeficiency virus (SIV) overlap about 80% of the U3 region of the 3' long terminal repeat (LTR) and contain several essential cis-acting elements (here referred to as the TPI region): a T-rich region, the polypurine tract, and attachment (att) sequences required for integration. We inactivated the TPI region in the nef reading frame of the pathogenic SIVmac239 clone (239wt) by 13 silent point mutations. To restore viral infectivity, intact cis-regulatory elements were inserted just downstream of the mutated nef gene. The resulting SIV genome contains U3 regions that are 384 bp shorter than the 517-bp 239wt U3 region. Overall, elimination of the duplicated Nef coding sequences truncates the proviral genome by 350 bp. Nonetheless, it contains all known coding sequences and cis-acting elements. The TPI mutant virus expressed functional Nef and replicated like 239wt in all cell culture assays and in vivo in rhesus macaques. Notably, these SIVmac constructs allow us to study Nef function in the context of replication-competent viruses without the restrictions of overlapping LTR sequences and important cis-acting elements. The genomes of all known primate lentiviruses contain a large overlap between nef and the U3 region. We demonstrate that this conserved genomic organization is not obligatory for efficient viral replication and pathogenicity.     

Simian immunodeficiency virus SIVmac239Deltanef vaccination elicits different Tat28-35SL8-specific CD8+ T-cell clonotypes compared to a DNA prime/adenovirus type 5 boost regimen in rhesus macaques

Different human immunodeficiency virus (HIV)/simian immunodeficiency virus (SIV) vaccine vectors expressing the same viral antigens can elicit disparate T-cell responses. Within this spectrum, replicating variable vaccines, like SIVmac239Δnef, appear to generate particularly efficacious CD8(+) T-cell responses. Here, we sequenced T-cell receptor β-chain (TRB) gene rearrangements from immunodominant Mamu-A 01-restricted Tat(28-35)SL8-specific CD8(+) T-cell populations together with the corresponding viral epitope in four rhesus macaques during acute SIVmac239Δnef infection. Ultradeep pyrosequencing showed that viral variants arose with identical kinetics in SIVmac239Δnef and pathogenic SIVmac239 infection. Furthermore, distinct Tat(28-35)SL8-specific T-cell receptor (TCR) repertoires were elicited by SIVmac239Δnef compared to those observed following a DNA/Ad5 prime-boost regimen, likely reflecting differences in antigen sequence stability.     

The human immunodeficiency virus type 1 nef gene can to a large extent replace simian immunodeficiency virus nef in vivo.

The nef gene of the pathogenic simian immunodeficiency virus (SIV) 239 clone was replaced with primary human immunodeficiency virus type 1 (HIV-1) nef alleles to investigate whether HIV-1 Nef can substitute for SIV Nef in vivo. Initially, two rhesus macaques were infected with the chimeric viruses (Nef-SHIVs). Most of the nef alleles obtained from both animals predicted intact open reading frames. Furthermore, forms containing upstream nucleotide substitutions that enhanced expression of the inserted gene became predominant. One animal maintained high viral loads and slowly progressed to immunodeficiency. nef long terminal repeat sequences amplified from this animal were used to generate a second generation of Nef-SHIVs. Two macaques, which were subsequently infected with a mixture of cloned chimeric viruses, showed high viral loads and progressed to fatal immunodeficiency. Five macaques received a single molecular clone, named SHIV-40K6. The SHIV-40K6 nef allele was active in CD4 and class I major histocompatibility complex downregulation and enhanced viral infectivity and replication. Notably, all of the macaques inoculated with SHIV-40K6 showed high levels of viral replication early in infection. During later stages, however, the course of infection was variable. Three animals maintained high viral loads and developed immunodeficiency. Of the remaining two macaques, which showed decreasing viral loads after the acute phase of infection, only one efficiently controlled viral replication and remained asymptomatic during 1.5 years of follow-up. The other animal showed an increasing viral load and developed signs of progressive infection during later stages. Our data demonstrate that HIV-1 nef can, to a large extent, functionally replace SIVmac nef in vivo.     

SIV envelope acquires a nefarious habit

Deletion of the nef gene from macaque simian immunodeficiency virus (SIVmac) attenuates its ability to cause disease. Pathogenic viruses occasionally emerge in macaques infected with Nef-deleted SIVmac, with some genetic determinants mapping to the envelope (env) gene. An intriguing new study shows that these changes endow Env with a Nef-like ability to counteract tetherin/BST2 (Serra-Moreno et?al., 2011).     

Comprehensive analysis of nef functions selected in simian immunodeficiency virus-infected macaques

A variety of simian immunodeficiency virus (SIVmac) nef mutants have been investigated to clarify which in vitro Nef functions contribute to efficient viral replication and pathogenicity in rhesus macaques. Most of these nef alleles, however, were only functionally characterized for their ability to down-modulate CD4 and class I major histocompatibility complex (MHC-I) cell surface expression and to enhance SIV replication and infectivity. To obtain information on the in vivo relevance of more recently established Nef functions, we examined the ability of a large panel of constructed SIVmac Nef mutants and of variants that emerged in infected macaques to down-regulate CD3, CD28, and MHC-II and to up-regulate the MHC-II-associated invariant chain (Ii). We found that all these four Nef functions were restored in SIV-infected macaques. In most cases, however, the initial mutations and the changes selected in vivo affected several in vitro Nef functions. For example, truncated Nef proteins that emerged in animals infected with SIVmac239 containing a 152-bp deletion in nef efficiently modulated both CD3 and Ii surface expression. Overall, our results suggest that the effect of Nef on each of the six cellular receptors investigated contributes to viral fitness in the infected host but also indicate that modulation of CD3, MHC-I, MHC-II, or Ii surface expression alone is insufficient for SIV virulence.     

The nef gene products of both simian and human immunodeficiency viruses enhance virus infectivity and are functionally interchangeable.

Adult rhesus macaques infected with nef-defective simian immunodeficiency virus (SIV) exhibit extremely low levels of steady-state virus replication, do not succumb to immunodeficiency disease, and are protected from experimental challenge with pathogenic isolates of SIV. Similarly, rare humans found to be infected with nef-defective human immunodeficiency virus type 1 (HIV-1) variants display exceptionally low viral burdens and do not show evidence of disease progression after many years of infection. HIV-1 Nef induces the rapid endocytosis and lysosomal degradation of cell surface CD4 and enhances virus infectivity in primary human T cells and macrophages. Although expression of SIV Nef also leads to down-modulation of cell surface CD4 levels, no evidence for SIV Nef-induced enhancement of virus infectivity was observed in earlier studies. Thus, it remains unclear whether fundamental differences exist between the activities of HIV-1 and SIV Nef. To establish more clearly whether the SIV and HIV-1 nef gene products are functionally analogous, we compared the replication kinetics and infectivity of variants of SIVmac239 that either do (SIVnef+) or do not (SIV delta nef) encode intact nef gene products. SIVnef+ replicates more rapidly than nef-defective viruses in both human and rhesus peripheral blood mononuclear cells (PBMCs). As previously described for HIV-1 Nef, SIV Nef also enhances virus infectivity within each cycle of virus replication. As a strategy for evaluating the in vivo contribution of HIV-1 nef alleles and long terminal repeat regulatory sequences to the pathogenesis of immunodeficiency disease, we constructed SIV-HIV chimeras in which the nef coding and U3 regulatory regions of SIVmac239 were replaced by the corresponding regions from HIV-1/R73 (SIVR7nef+). SIVR7nef+ displays enhanced infectivity and accelerated replication kinetics in primary human and rhesus PBMC infections compared to its nef-defective counterpart. Converse chimeras, containing SIV Nef in an HIV-1 background (R7SIVnef+) also exhibit greater infectivity than matched nef-defective viruses (R7SIV delta nef). These data indicate that SIV Nef, like that of HIV-1, does enhance virus replication in primary cells in tissue culture and that HIV-1 and SIV Nef are functionally interchangeable in the context of both HIV-1 and SIV.     

Determinants of increased replicative capacity of serially passaged simian immunodeficiency virus with nef deleted in rhesus monkeys

Most rhesus macaques infected with simian immunodeficiency virus SIVmac239 with nef deleted (either Delta nef or Delta nef Delta vpr Delta US [Delta 3]) control viral replication and do not progress to AIDS. Some monkeys, however, develop moderate viral load set points and progress to AIDS. When simian immunodeficiency viruses (SIVs) recovered from two such animals (one Delta nef and the other Delta 3) were serially passaged in rhesus monkeys, the SIVs derived from both lineages were found to consistently induce moderate viral loads and disease progression. Analysis of viral sequences in the serially passaged derivatives revealed interesting changes in three regions: (i) an unusually high number of predicted amino acid changes (12 to 14) in the cytoplasmic domain of gp41, most of which were in regions that are usually conserved; these changes were observed in both lineages; (ii) an extreme shortening of nef sequences in the region of overlap with U3; these changes were observed in both lineages; and (iii) duplication of the NF-kappa B binding site in one lineage only. Neither the polymorphic gp41 changes alone nor the U3 deletion alone appeared to be responsible for increased replicative capacity because recombinant SIVmac239 Delta nef, engineered to contain either of these changes, induced moderate viral loads in only one of six monkeys. However, five of six monkeys infected with recombinant SIVmac239 Delta nef containing both TM and U3 changes did develop persisting moderate viral loads. These genetic changes did not increase lymphoid cell-activating properties in the monkey interleukin-2-dependent T-cell line 221, but the gp41 changes did increase the fusogenic activity of the SIV envelope two- to threefold. These results delineate sequence changes in SIV that can compensate for the loss of the nef gene to partially restore replicative and pathogenic potential in rhesus monkeys.     

Construction, replication, and immunogenic properties of a simian immunodeficiency virus expressing interleukin-2.

To study the effect of interleukin-2 (IL-2) on simian immunodeficiency virus (SIV) replication, pathogenesis, and immunogenicity, we replaced the nef gene of SIVmac239 by the IL-2 coding region. The virus, designated SIV-IL2, stably expressed high levels of IL-2 in cell culture. In comparison to SIVmac239, SIV-IL2 replicated more efficiently in peripheral blood mononuclear cells in the absence of exogenously added IL-2. To determine whether this growth advantage would be of relevance in vivo, four juvenile rhesus monkeys were infected with SIV-IL2 and four monkeys were infected with a nef deletion mutant of SIV (SIVdeltaNU). After a peak in the cell-associated viral load 2 weeks postinfection, the viruses could barely be isolated 3 to 7 months postinfection. Mean capsid antigen levels were higher in the SIV-IL2 group than in the nef deletion group 2 weeks postinfection. Viruses reisolated from the SIV-IL2-infected animals expressed high levels of IL-2 during the acute phase of infection. Deletions in the IL-2 coding region of SIV-IL2 were observed in two of the SIV-IL2-infected macaques 3 months postinfection. Urinary neopterin levels, a marker for unspecific immune stimulation, were higher in the SIV-IL2-infected macaques than in SIVdeltaNU-infected animals during the acute phase of infection. The SIV-specific T-cell-proliferative response and antibody titers were similar in both groups. Cytotoxic T cells directed against viral antigens were detected in all SIV-IL2-infected macaques and in two of the SIVdeltaNU-infected animals. Expression of IL-2 did not seem to alter the attenuated phenotype of nef deletion mutants fundamentally, although there might have been a slight increase in virus replication and immune stimulation during the acute phase of infection. Deletion of the viral IL-2 gene 3 months postinfection could be a consequence of a selective disadvantage due to local coexpression of viral antigen and IL-2 in the presence of an antiviral immune response.     

Construction and characterization of an infectious DNA clone and of mutants of simian immunodeficiency virus isolated from the African green monkey.

We constructed a full-length molecular clone of simian immunodeficiency virus from an African green monkey. Upon transfection, this clone directed the production of virus particles cytopathic and infectious to human CD4+ leukemia cell lines. Mutations were introduced by recombinant DNA techniques into eight open reading frames of simian immunodeficiency virus from the African green monkey thus far identified. The phenotypes of mutant viruses, i.e., infectivity, cytopathogenicity, transactivation of gene expression controlled by a long terminal repeat, and viral RNA and protein syntheses, were examined by transfection and infection experiments. Three structural (gag, pol, and env) and two regulatory (tat and rev) gene mutants were not infectious, whereas vif, vpx, and nef were dispensable for infectivity and mutant viruses were highly cytopathic. In transient transfection assays, a rev mutant produced mainly small mRNA species and no detectable virus protein and particles. The transactivation potential of a tat mutant was about 10-fold less than that of wild-type DNA, generating small amounts of virus.     

Early cytokine and chemokine gene expression in lymph nodes of macaques infected with simian immunodeficiency virus is predictive of disease outcome and vaccine efficacy.

Competitive PCR was used to evaluate the expression of cytokine, granzyme B, and chemokine genes in lymph nodes of macaques recently infected with the simian immunodeficiency virus (SIV) pathogenic molecular clone SIVmac239 (n = 16), the nonpathogenic vaccine strain SIVmac239 delta nef (n = 8), and the nonpathogenic molecular clone SIVmac1A11 (n = 8). For both SIVmac239 and its nef-deleted derivative, strong expression was observed as early as 7 days postinfection for interleukin 1beta (IL-1beta), IL-6, tumor necrosis factor alpha, gamma interferon, and IL-13. The levels of gene induction were equally intense for both viruses despite a lower viral load for SIVmac239 deltanef compared with that for SIVmac239. However, the nature of the cytokine network activation varied with the viral inocula. Primary infection with SIVmac239 was characterized by a higher level of IL-4, IL-10, MIP-1alpha, MIP-1beta, MCP-1, and RANTES gene expression and a lower level of IL-12 and granzyme B gene expression compared with infection with SIVmac239 delta nef. Thus, infection with nef-deleted SIV was associated with a preferential Th1 versus Th2 pattern of cytokine production. Infection with SIVmac1A11 was characterized by a delayed immune response for all markers tested. The unique patterns of cytokine and chemokine gene expression in lymph nodes correlated nicely with the pathogenic potential of the SIV strains used as well as with differences in their ability to serve as protective vaccines.     

In vivo attenuation of simian immunodeficiency virus by disruption of a tyrosine-dependent sorting signal in the envelope glycoprotein cytoplasmic tail

Attenuated simian immunodeficiency viruses (SIVs) have been described that produce low levels of plasma virion RNA and exhibit a reduced capacity to cause disease. These viruses are particularly useful in identifying viral determinants of pathogenesis. In the present study, we show that mutation of a highly conserved tyrosine (Tyr)-containing motif (Yxxphi) in the envelope glycoprotein (Env) cytoplasmic tail (amino acids YRPV at positions 721 to 724) can profoundly reduce the in vivo pathogenicity of SIVmac239. This domain constitutes both a potent endocytosis signal that reduces Env expression on infected cells and a sorting signal that directs Env expression to the basolateral surface of polarized cells. Rhesus macaques were inoculated with SIVmac239 control or SIVmac239 containing either a Tyr-721-to-Ile mutation (SIVmac239Y/I) or a deletion of Tyr-721 and the preceding glycine (DeltaGY). To assess the in vivo replication competence, all viruses contained a stop codon in nef that has been shown to revert during in vivo but not in vitro replication. All three control animals developed high viral loads and disease. One of two animals that received SIVmac239Y/I and two of three animals that received SIVmac239DeltaGY remained healthy for up to 140 weeks with low to undetectable plasma viral RNA levels and normal CD4(+) T-cell percentages. These animals exhibited ongoing viral replication as determined by detection of viral sequences and culturing of mutant viruses from peripheral blood mononuclear cells and persistent anti-SIV antibody titers. In one animal that received SIVmac239Y/I, the Ile reverted to a Tyr and was associated with a high plasma RNA level and disease, while one animal that received SIVmac239DeltaGY also developed a high viral load that was associated with novel and possibly compensatory mutations in the TM cytoplasmic domain. In all control and experimental animals, the nef stop codon reverted to an open reading frame within the first 2 months of inoculation, indicating that the mutant viruses had replicated well enough to repair this mutation. These findings indicate that the Yxxphi signal plays an important role in SIV pathogenesis. Moreover, because mutations in this motif may attenuate SIV through mechanisms that are distinct from those caused by mutations in nef, this Tyr-based sorting signal represents a novel target for future models of SIV and human immunodeficiency virus attenuation that could be useful in new vaccine strategies.     

Acute-Phase CD8 T Cell Responses That Select for Escape Variants Are Needed to Control Live Attenuated Simian Immunodeficiency Virus

The overall CD8 T cell response to human/simian immunodeficiency virus (HIV/SIV) targets a collection of discrete epitope specificities. Some of these epitope-specific CD8 T cells emerge in the weeks and months following infection and rapidly select for sequence variants, whereas other CD8 T cell responses develop during the chronic infection phase and rarely select for sequence variants. In this study, we tested the hypothesis that acute-phase CD8 T cell responses that do not rapidly select for escape variants are unable to control viral replication in vivo as well as those that do rapidly select for escape variants. We created a derivative of live attenuated SIV (SIVmac239Δnef) in which we ablated five epitopes that elicit early CD8 T cell responses and rapidly accumulate sequence variants in SIVmac239-infected Mauritian cynomolgus macaques (MCMs) that are homozygous for the M3 major histocompatibility complex (MHC) haplotype. This live attenuated SIV variant was called m3KOΔnef. Viremia was significantly higher in M3 homozygous MCMs infected with m3KOΔnef than in either MHC-mismatched MCMs infected with m3KOΔnef or MCMs infected with SIVmac239Δnef. Three CD8 T cell responses, including two that do not rapidly select for escape variants, predominated during early m3KOΔnef infection in the M3 homozygous MCMs, but these animals were unable to control viral replication. These results provide evidence that acute-phase CD8 T cell responses that have the potential to rapidly select for escape variants in the early phase of infection are needed to establish viral control in vivo.     

Evidence for recombination of live, attenuated immunodeficiency virus vaccine with challenge virus to a more virulent strain

Live, attenuated immunodeficiency virus vaccines, such as nef deletion mutants, are the most effective vaccines tested in the simian immunodeficiency virus (SIV) macaque model. In two independent studies designed to determine the breadth of protection induced by live, attenuated SIV vaccines, we noticed that three of the vaccinated macaques developed higher set point viral load levels than unvaccinated control monkeys. Two of these vaccinated monkeys developed AIDS, while the control monkeys infected in parallel remained asymptomatic. Concomitant with an increase in viral load, a recombinant of the vaccine virus and the challenge virus could be detected. Therefore, the emergence of more-virulent recombinants of live, attenuated immunodeficiency viruses and less-aggressive wild-type viruses seems to be an additional risk of live, attenuated immunodeficiency virus vaccines.     

Retroviral recombination in vivo: viral replication patterns and genetic structure of simian immunodeficiency virus (SIV) populations in rhesus macaques after simultaneous or sequential intravaginal inoculation with SIVmac239Deltavpx/Deltavpr and SIVmac239Deltanef

To characterize the occurrence, frequency, and kinetics of retroviral recombination in vivo, we intravaginally inoculated rhesus macaques, either simultaneously or sequentially, with attenuated simian immunodeficiency virus (SIV) strains having complementary deletions in their accessory genes and various degrees of replication impairment. In monkeys inoculated simultaneously with SIVmac239Deltavpx/Deltavpr and SIVmac239Deltanef, recombinant wild-type (wt) virus and wild-type levels of plasma viral RNA (vRNA) were detected in blood by 2 weeks postinoculation. In monkeys inoculated first with SIVmac239Deltavpx/Deltavpr and then with SIVmac239Deltanef, recombination occurred but was associated with lower plasma vRNA levels than plasma vRNA levels seen for monkeys inoculated intravaginally with wt SIVmac239. In one monkey, recombination occurred 6 weeks after the challenge with SIVmac239Deltanef when plasma SIVmac239Deltavpx/Deltavpr RNA levels were undetectable. In monkeys inoculated first with the more highly replicating strain, SIVmac239Deltanef, and then with SIVmac239Deltavpx/Deltavpr, wild-type recombinant virus was not detected in blood or tissues. Instead, a virus that had repaired the deletion in the nef gene by a compensatory mutation was found in one animal. Overall, recombinant SIV was eventually found in four of six animals intravaginally inoculated with the two SIVmac239 deletion mutants. These findings show that recombination can occur readily in vivo after mucosal SIV exposure and thus contributes to the generation of viral genetic diversity and enhancement of viral fitness.     

Utilization of C-C chemokine receptor 5 by the envelope glycoproteins of a pathogenic simian immunodeficiency virus, SIVmac239.

We examined chemokine receptors for the ability to facilitate the infection of CD4-expressing cells by viruses containing the envelope glycoproteins of a pathogenic simian immunodeficiency virus, SIVmac239. Expression of either human or simian C-C chemokine receptor CCR5 allowed the SIVmac239 envelope glycoproteins to mediate virus entry and cell-to-cell fusion. Thus, distantly related immunodeficiency viruses such as SIV and the primary human immunodeficiency virus type 1 isolates can utilize CCR5 as an entry cofactor.