In vitro macrophage tropism of pathogenic and nonpathogenic molecular clones of simian immunodeficiency virus (SIVmac)

Derivation and Characterization of a CD4-Independent, Non-CD4-Tropic Simian Immunodeficiency Virus

CD4 tropism is conserved among all primate lentiviruses and likely contributes to viral pathogenesis by targeting cells that are critical for adaptive antiviral immune responses. Although CD4-independent variants of human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) have been described that can utilize the coreceptor CCR5 or CXCR4 in the absence of CD4, these viruses typically retain their CD4 binding sites and still can interact with CD4. We describe the derivation of a novel CD4-independent variant of pathogenic SIVmac239, termed iMac239, that was used to derive an infectious R5-tropic SIV lacking a CD4 binding site. Of the seven mutations that differentiate iMac239 from wild-type SIVmac239, a single change (D178G) in the V1/V2 region was sufficient to confer CD4 independence in cell-cell fusion assays, although other mutations were required for replication competence. Like other CD4-independent viruses, iMac239 was highly neutralization sensitive, although mutations were identified that could confer CD4-independent infection without increasing its neutralization sensitivity. Strikingly, iMac239 retained the ability to replicate in cell lines and primary cells even when its CD4 binding site had been ablated by deletion of a highly conserved aspartic acid at position 385, which, for HIV-1, plays a critical role in CD4 binding. iMac239, with and without the D385 deletion, exhibited an expanded host range in primary rhesus peripheral blood mononuclear cells that included CCR5(+) CD8(+) T cells. As the first non-CD4-tropic SIV, iMac239-ΔD385 will afford the opportunity to directly assess the in vivo role of CD4 targeting on pathogenesis and host immune responses.

IMPORTANCE

CD4 tropism is an invariant feature of primate lentiviruses and likely plays a key role in pathogenesis by focusing viral infection onto cells that mediate adaptive immune responses and in protecting virions attached to cells from neutralizing antibodies. Although CD4-independent viruses are well described for HIV and SIV, these viruses characteristically retain their CD4 binding site and can engage CD4 if available. We derived a novel CD4-independent, CCR5-tropic variant of the pathogenic molecular clone SIVmac239, termed iMac239. The genetic determinants of iMac239's CD4 independence provide new insights into mechanisms that underlie this phenotype. This virus remained replication competent even after its CD4 binding site had been ablated by mutagenesis. As the first truly non-CD4-tropic SIV, lacking the capacity to interact with CD4, iMac239 will provide the unique opportunity to evaluate SIV pathogenesis and host immune responses in the absence of the immunomodulatory effects of CD4(+) T cell targeting and infection.

Intracellular nucleotide levels and the control of retroviral infections

Retroviruses consume cellular deoxynucleoside triphosphates (dNTPs) to convert their RNA genomes into proviral DNA through reverse transcription. While all retroviruses replicate in dividing cells, lentiviruses uniquely replicate in nondividing cells such as macrophages. Importantly, dNTP levels in nondividing cells are extremely low, compared to dividing cells. Indeed, a recently discovered anti-HIV/SIV restriction factor, SAMHD1, which is a dNTP triphosphohydrolase, is responsible for the limited dNTP pool of nondividing cells. Lentiviral reverse transcriptases (RT) uniquely stay functional even at the low dNTP concentrations in nondividing cells. Interestingly, Vpx of HIV-2/SIVsm proteosomally degrades SAMHD1, which elevates cellular dNTP pools and accelerates lentiviral replication in nondividing cells. These Vpx-encoding lentiviruses rapidly replicate in nondividing cells by encoding both highly functional RTs and Vpx. Here, we discuss a series of mechanistic and virological studies that have contributed to conceptually linking cellular dNTP levels and the adaptation of lentiviral replication in nondividing cells.

Selective expression of human immunodeficiency virus Nef in specific immune cell populations of transgenic mice is associated with distinct AIDS-like phenotypes

We previously reported that CD4C/human immunodeficiency virus (HIV)(Nef) transgenic (Tg) mice, expressing Nef in CD4(+) T cells and cells of the macrophage/dendritic cell (DC) lineage, develop a severe AIDS-like disease, characterized by depletion of CD4(+) T cells, as well as lung, heart, and kidney diseases. In order to determine the contribution of distinct populations of hematopoietic cells to the development of this AIDS-like disease, five additional Tg strains expressing Nef through restricted cell-specific regulatory elements were generated. These Tg strains express Nef in CD4(+) T cells, DCs, and macrophages (CD4E/HIV(Nef)); in CD4(+) T cells and DCs (mCD4/HIV(Nef) and CD4F/HIV(Nef)); in macrophages and DCs (CD68/HIV(Nef)); or mainly in DCs (CD11c/HIV(Nef)). None of these Tg strains developed significant lung and kidney diseases, suggesting the existence of as-yet-unidentified Nef-expressing cell subset(s) that are responsible for inducing organ disease in CD4C/HIV(Nef) Tg mice. Mice from all five strains developed persistent oral carriage of Candida albicans, suggesting an impaired immune function. Only strains expressing Nef in CD4(+) T cells showed CD4(+) T-cell depletion, activation, and apoptosis. These results demonstrate that expression of Nef in CD4(+) T cells is the primary determinant of their depletion. Therefore, the pattern of Nef expression in specific cell population(s) largely determines the nature of the resulting pathological changes.

Role of the long cytoplasmic domain of the SIV Env glycoprotein in early and late stages of infection

Background

The Env glycoproteins of retroviruses play an important role in the initial steps of infection involving the binding to cell surface receptors and entry by membrane fusion. The Env glycoprotein also plays an important role in viral assembly at a late step of infection. Although the Env glycoprotein interacts with viral matrix proteins and cellular proteins associated with lipid rafts, its possible role during the early replication events remains unclear. Truncation of the cytoplasmic tail (CT) of the Env glycoprotein is acquired by SIV in the course of adaptation to human cells, and is known to be a determinant of SIV pathogenicity.

Results

We compared SIV viruses with full length or truncated (T) Env glycoproteins to analyze possible differences in entry and post-entry events, and assembly of virions. We observed that early steps in replication of SIV with full length or T Env were similar in dividing and non-dividing cells. However, the proviral DNA of the pathogenic virus clone SIVmac239 with full length Env was imported to the nucleus about 20-fold more efficiently than proviral DNA of SIVmac239T with T Env, and 100-fold more efficiently than an SIVmac18T variant with a single mutation A239T in the SU subunit and with a truncated cytoplasmic tail (CT). In contrast, proviral DNA of SIVmac18 with a full length CT and with a single mutation A239T in the SU subunit was imported to the nucleus about 50-fold more efficiently than SIVmac18T. SIV particles with full length Env were released from rhesus monkey PBMC, whereas a restriction of release of virus particles was observed from human 293T, CEMx174, HUT78 or macrophages. In contrast, SIV with T Envs were able to overcome the inhibition of release in human HUT78, CEMx174, 293T or growth-arrested CEMx174 cells and macrophages resulting in production of infectious particles. We found that the long CT of the Env glycoprotein was required for association of Env with lipid rafts. An Env mutant C787S which eliminated palmitoylation did not abolish Env incorporation into lipid rafts, but prevented virus assembly.

Conclusion

The results indicate that the long cytoplasmic tail of the SIV Env glycoprotein may govern post-entry replication events and plays a role in the assembly process.

Primary human immunodeficiency virus type 1 nef alleles show major differences in pathogenicity in transgenic mice

We previously reported that the human immunodeficiency virus type 1 NL4-3 Nef is necessary and sufficient to induce a severe AIDS-like disease in transgenic (Tg) mice when the protein is expressed under the regulatory sequences of the human CD4 gene. We have now assayed additional Nef alleles (SF2, JR-CSF, YU10x, and NL4-3 [T71R] Nef alleles), including some from long-term nonprogressors (AD-93, 032an, and 039nm alleles) in the same Tg system and compared their pathogenicities. All these Nef alleles downregulated cell surface CD4 in human cells in vitro and also, with the exception of Nef(YU10x), in Tg CD4(+) T cells. Depletion of double-positive and single-positive thymocytes occurred with all alleles but was less pronounced in Nef(YU10x) Tg mice. A loss of peripheral CD4(+) T cells was observed with all alleles but was minimal in Nef(YU10x) Tg mice. In Nef(032an) and Nef(SF2) Tg mice, T-cell loss was severe despite lower levels of Tg expression, suggesting a higher virulence of these alleles. All Nef alleles except the Nef(YU10x) and Nef(NL4-3(T71R)) alleles induced an enhanced activated memory (CD25(+) CD69(+) CD44(high) CD45RB(low) CD62L(low)) and apoptotic phenotype. Also, all could interact with and/or activate PAK2 except the Nef(JR-CSF) allele. Organ (lung and kidney) diseases were present in Nef(NL4-3(T71R)), Nef(032an), Nef(039nm), and Nef(SF2) Tg mice, despite very low levels of Tg expression for the last strain. However, no organ disease or minimal organ disease developed in Nef(YU10x) and Nef(AD-93) Tg mice and Nef(JR-CSF) Tg mice, respectively, despite high levels of Tg expression. Our data show that important differences in the pathogenicities of various Nef alleles can be scored in Tg mice. Interestingly, our results also revealed that some phenotypes can segregate independently, such as CD4(+) T-cell depletion and activation, as well as severe depletion of thymic CD4(+) T cells and peripheral CD4(+) T cells. Therefore, expression of Nef alleles in Tg mice under the CD4C regulatory elements represents a novel assay for measuring their pathogenicity. Because of the very high similarity of this murine AIDS-like disease to human AIDS, this assay may have a predictive value regarding the behavior of Nef in infected humans.

Role of CD8+ cells in controlling replication of nonpathogenic Simian Immunodeficiency Virus SIVmac1A11

Infection of macaques with the avirulent molecular clone SIVmac1A11 results in transient low viremia and no disease. To investigate if this low viremia is solely due to intrinsic poor replication fitness or is mediated by efficient immune-mediated control, 5 macaques were inoculated intravenously with SIVmac1A11. Three animals that were depleted of CD8+ cells at the start of infection had more prolonged viremia with peak virus levels 1 to 2 logs higher than those of 2 animals that received a non-depleting control antibody. Thus, CD8+ cell-mediated immune responses play an important role in controlling SIVmac1A11 replication during acute viremia.

A single amino acid change and truncated TM are sufficient for simian immunodeficiency virus to enter cells using CCR5 in a CD4-independent pathway

Entry of HIV and SIV into susceptible cells is mediated by CD4 and chemokine receptors, which act as coreceptors. To study cell entry of SIV, we constructed a cell line, xKLuSIV, derived from non-susceptible human K562 cells, that express the firefly luciferase reporter gene under control of a minimal SIV long terminal repeat (LTR). Using these susceptible cells, we studied the entry of a well-characterized molecularly cloned macrophage-tropic SIV. xKLuSIV cells that express rhesus macaque CD4 and/or the rhesus chemokine receptor CCR5 are susceptible to infection with the macrophage-tropic, neurovirulent strain SIV/17E-Fr, but only xKLuSIV cells expressing both CCR5 and CD4 were susceptible to infection by the macrophage-tropic, non-neurovirulent strain SIV/17E-Cl. CCR5-dependent, CD4-independent infection by SIV/17E-Fr was abrogated by pre-incubation of the cells with AOP-RANTES, a ligand for CCR5. In addition to viral entry occurring by a CD4-independent mechanism, neutralization of SIV/17E-Fr with rhesus mAbs from 3 different neutralization groups blocked entry into x KLuSIV cells by both CD4-dependent and -independent mechanisms. Triggering the env glycoprotein of SIV-17 EFr with soluble CD4 had no significant effect in infectivity, but triggering of the same glycoprotein of SIV/17E-Cl allowed it to enter cells in a CD4-independent fashion. Using mutant molecular clones, we studied the determinants for CD4 independence, all of which are confined to the env gene. We report here that truncation of the TM at amino acid 764 and changing a single amino acid (R751G) in the SIV envelope transmembrane protein (TM) conferred the observed CD4-independent phenotype. Our data suggest that the envelope from the neurovirulent SIV/17E-Fr interacts with CCR5 in a CD4-independent manner, and changes in the TM protein of this virus are important components that contribute to neurovirulence in SIV.

Multiple domains of the SIV Env protein determine virus replication efficiency and neutralization sensitivity

SIVmac239 and SIVmac1A11 are wild-type viruses encoding Env proteins with full-length or truncated cytoplasmic tails (CTs), respectively. A mutant designated SIVmac239T has a site-specific mutation which introduces a stop codon in the env gene resulting a truncated protein of similar length to SIVmac1A11 Env. To investigate the role of specific sequence differences in these Env proteins, we constructed SIV mutants encoding 1A11 or 239 Env proteins with reciprocal exchanges of the CT or exchanges of both the surface unit (SU) and CT sequences. A truncated CT in the context of the 1A11 SU subunit was found to significantly enhance replication in CEMx174 (human T-cell line) and rhesus PBMCs. However, similar Env CT truncation did not enhance replication of SIVmac239 in human or monkey cells. SIVmac1A11 with a full-length SIVmac239 CT did not replicate in human T-cell lines, but truncation of the CT by a stop codon resulted in replication. We also observed that these viruses differed significantly in sensitivity to neutralization by antibody. Taken together, the results indicated that the length of the CT domain as well as specific sequence differences in the SU domain affect viral replication capacity as well as sensitivity to neutralization.

CD4+ T cells and monocytes elicited by immunization of rhesus monkeys with antigen-pulsed dendritic cells control SIV replication

Most HIV infections occur by transmission across mucosal surfaces, where dendritic cells (DCs) are the first cells to encounter the virus. Dendritic cells are specialized antigen-presenting cells critical for eliciting T cell-mediated immune responses. Delayed-type hypersensitivity (DTH) is a cellular immune response in some viral infections and it is mediated by CD4+ and/or CD8+ T cells. We hypothesized that a DTH response to HIV induced by antigen-pulsed DCs would protect against a mucosal exposure to the virus. In a small pilot experiment, six rhesus monkeys were immunized with autologous, antigen-pulsed DCs by the intradermal route and five of the monkeys were boosted with a second dose of DCs at 3 months. Antibody responses to SIV were detected in two of six vaccinated monkeys, lymphocyte-proliferative responses were detected in five of the six monkeys and cytotoxic T lymphocyte (CTL) responses were detected in four of the six monkeys. Using a novel in vitro assay of SIV replication in DCs cocultured with autologous CD4+ T cells and monocytes, suppression of viral replication was detected from five of the six monkeys at multiple time points before and after SIV challenge. Macaques were orally challenged with SIVmac239 at 1-3 months after the booster inoculation. Peak viral loads were similar to those of four naive animals but, compared with naive monkeys, declined at 6 months to levels 1 log(10) or more lower in monkeys that had been vaccinated and that had > or = 50% suppression of SIV replication in DCs. Optimizing this immunization strategy may result in a strong antiviral DTH response that could better control a mucosal lentiviral infection.

The simian immunodeficiency virus envelope glycoprotein contains multiple signals that regulate its cell surface expression and endocytosis

The cell surface expression of the envelope glycoproteins (Envs) of primate immunodeficiency viruses is, at least in part, regulated by endocytosis signal(s) located in the Env cytoplasmic domain. Here, we show that a membrane proximal signal that directs the simian immunodeficiency virus (SIV) Env to clathrin-coated pits, and is conserved in all SIV and human immunodeficiency virus Envs, conforms to a YxxØ motif (where x can be any amino acid and Ø represents a large hydrophobic residue). This motif is similar to that described for a number of cellular membrane proteins. By surface plasmon resonance we detected a high affinity interaction between peptides containing this membrane proximal signal and both AP1 and AP2 clathrin adaptor complexes. Mutation of the tyrosine in this membrane proximal motif in a SIV Env with a prematurely truncated cytoplasmic domain leads to a > or = 25-fold increase in Env expression on infected cells. By contrast, the same mutation in an Env with a full-length cytoplasmic domain increases cell surface expression only 4-fold. We show that this effect results from the presence of additional endocytosis signals in the full-length cytoplasmic domain. Chimeras containing CD4 ecto- and membrane spanning domains and a full-length SIV Env cytoplasmic domain showed rapid endocytosis even when the membrane proximal tyrosine-based signal was disrupted. Mapping experiments indicated that at least some of the additional endocytosis information is located between residues 743 and 812 of Env from the SIVmac239 molecular clone. Together, our findings indicate that the cytoplasmic domain of SIV Env contains multiple endocytosis and/or trafficking signals that modulate its surface expression on infected cells, and suggest an important role for this function in pathogenesis.

CCR5 signal transduction in macrophages by human immunodeficiency virus and simian immunodeficiency virus envelopes

The capacity of human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) envelopes to transduce signals through chemokine coreceptors on macrophages was examined by measuring the ability of recombinant envelope proteins to mobilize intracellular calcium stores. Both HIV and SIV envelopes mobilized calcium via interactions with CCR5. The kinetics of these responses were similar to those observed when macrophages were treated with MIP-1beta. Distinct differences in the capacity of envelopes to mediate calcium mobilization were observed. Envelopes derived from viruses capable of replicating in macrophages mobilized relatively high levels of calcium, while envelopes derived from viruses incapable of replicating in macrophages mobilized relatively low levels of calcium. The failure to efficiently mobilize calcium was not restricted to envelopes derived from CXCR4-utilizing isolates but also included envelopes derived from CCR5-utilizing isolates that fail to replicate in macrophages. We characterized one CCR5-utilizing isolate, 92MW959, which entered macrophages but failed to replicate. A recombinant envelope derived from this virus mobilized low levels of calcium. When macrophages were inoculated with 92MW959 in the presence of MIP-1alpha, viral replication was observed, indicating that a CC chemokine-mediated signal provided the necessary stimulus to allow the virus to complete its replication cycle. Although the role that envelope-CCR5 signal transduction plays in viral replication is not yet understood, it has been suggested that envelope-mediated signals facilitate early postfusion events in viral replication. The data presented here are consistent with this hypothesis and suggest that the differential capacity of viral envelopes to signal through CCR5 may influence their ability to replicate in macrophages.

SIV/HIV Nef recombinant virus (SHIVnef) produces simian AIDS in rhesus macaques

The simian immunodeficiency virus (SIV) nef gene is an important determinant of viral load and acquired immunodeficiency syndrome (AIDS) in macaques. A role(s) for the HIV-1 nef gene in infection and pathogenesis was investigated by constructing recombinant viruses in which the nef gene of the pathogenic molecular clone SIVmac239 nef was replaced with either HIV-1sf2nef or HIV-1sf33nef. These chimeras, designated SHIV-2nef and SHIV-33nef, expressed HIV-1 Nef protein and replicated efficiently in cultures of rhesus macaque lymphoid cells. In two SHIV-2nef-infected juvenile rhesus macaques and in one of two SHIV-33nef-infected juvenile macaques, virus loads remained at low levels in both peripheral blood and lymph nodes in acute and chronic phases of infection (for >83 weeks). In striking contrast, the second SHIV-33nef-infected macaque showed high virus loads during the chronic stage of infection (after 24 weeks). CD4+ T-cell numbers declined dramatically in this latter animal, which developed simian AIDS (SAIDS) at 47-53 weeks after inoculation; virus was recovered at necropsy at 53 weeks and designated SHIV-33Anef. Sequence analysis of the HIV-1sf33 nef gene in SHIV-33Anef revealed four consistent amino acid changes acquired during passage in vivo. Interestingly, one of these consensus mutations generated a tyr-x-x-leu (Y-X-X-L) motif in the HIV-1sf33 Nef protein. This motif is characteristic of certain endocytic targeting sequences and also resembles a src-homology region-2 (SH-2) motif found in many cellular signaling proteins. Four additional macaques infected with SHIV-33Anef contained high virus loads, and three of these animals progressed to fatal SAIDS. Several of the consensus amino acid changes in Nef, including Y-X-X-L motif, were retained in these recipient animals exhibiting high virus load and disease. In summary, these findings indicate that the SHIV-33Anef chimera is pathogenic in rhesus macaques and that this approach, i.e., construction of chimeric viruses, will be important for analyzing the function(s) of HIV-1 nef genes in immunodeficiency in vivo, testing antiviral therapies aimed at inhibiting AIDS, and investigating adaptation of this HIV-1 accessory gene to the macaque host.

Emerging cytopathic and antigenic simian immunodeficiency virus variants influence AIDS progression

Genetic variants of human and simian immunodeficiency virus (HIV and SIV) that evolve during the course of infection and progression to AIDS are phenotypically and antigenically distinct from their progenitor viruses present at early stages of infection. However, it has been unclear how these late variants, which are typically T-cell tropic, cytopathic and resistant to neutralizing antibodies, influence the development of clinical AIDS. To address this, we infected macaques with cloned SIVs representing prototype variants from early-, intermediate- and late-stage infection having biological characteristics typical of viruses found at similar stages of HIV infection in humans. These studies demonstrate that sequential, phenotypic and antigenic variants represent viruses that have become increasingly fit for replication in the host, and our data support the hypothesis that emerging variants have increased pathogenicity and drive disease progression in SIV and HIV infection.

Lentivirus vectors using human and simian immunodeficiency virus elements

Lentivirus vectors based on human immunodeficiency virus (HIV) type 1 (HIV-1) constitute a recent development in the field of gene therapy. A key property of HIV-1-derived vectors is their ability to infect nondividing cells. Although high-titer HIV-1-derived vectors have been produced, concerns regarding safety still exist. Safety concerns arise mainly from the possibility of recombination between transfer and packaging vectors, which may give rise to replication-competent viruses with pathogenic potential. We describe a novel lentivirus vector which is based on HIV, simian immunodeficiency virus (SIV), and vesicular stomatitis virus (VSV) and which we refer to as HIV/SIVpack/G. In this system, an HIV-1-derived genome is encapsidated by SIVmac core particles. These core particles are pseudotyped with VSV glycoprotein G. Because the nucleotide homology between HIV-1 and SIVmac is low, the likelihood of recombination between vector elements should be reduced. In addition, the packaging construct (SIVpack) for this lentivirus system was derived from SIVmac1A11, a nonvirulent SIV strain. Thus, the potential for pathogenicity with this vector system is minimal. The transduction ability of HIV/SIVpack/G was demonstrated with immortalized human lymphocytes, human primary macrophages, human bone marrow-derived CD34(+) cells, and primary mouse neurons. To our knowledge, these experiments constitute the first demonstration that the HIV-1-derived genome can be packaged by an SIVmac capsid. We demonstrate that the lentivirus vector described here recapitulates the biological properties of HIV-1-derived vectors, although with increased potential for safety in humans.

Use of GPR1, GPR15, and STRL33 as coreceptors by diverse human immunodeficiency virus type 1 and simian immunodeficiency virus envelope proteins

Human and simian immunodeficiency viruses (HIV and SIV, respectively) use chemokine receptors as coreceptors along with CD4 to mediate viral entry. Several orphan receptors, including GPR1, GPR15, and STRL33, can also serve as coreceptors for a more limited number of HIV and SIV isolates. We investigated whether these orphan receptors could function as efficient coreceptors for a diverse group of HIV and SIV envelopes (Envs) in comparison with the principal coreceptors CCR5 and CXCR4. We found that a limited number of HIV-1 isolates could mediate inefficient cell-cell fusion with the orphan receptors relative to CCR5 and CXCR4; however, none of the orphan receptors tested could support pseudotype virus infection despite robust infection via CCR5 or CXCR4. All except one of the SIV Envs tested mediated some degree of cell-cell fusion and pseudotype infection, with target cells expressing at least one of these orphan receptors, although CCR5 proved to be the most efficient coreceptor for infection. Only one SIV Env protein, BK28, could mediate infection using GPR1 as a coreceptor, albeit much less efficiently than with CCR5. In addition, use of these coreceptors did not correlate with the published tropism of the SIV clones and was strictly CD4 dependent for both SIV and HIV. We also examined the expression of these molecules in cell lines and primary cells widely used for virus propagation and as targets for infection. All cells examined expressed STRL33, a more limited number expressed GPR15, and GPR1 was much more restricted in its expression pattern. Taken together, our results indicate that GPR15 and STRL33 are rarely used by HIV-1 but are more frequently used by SIV strains, although not in a manner that correlates with SIV tropism.

Role of the SH3-ligand domain of simian immunodeficiency virus Nef in interaction with Nef-associated kinase and simian AIDS in rhesus macaques

The nef gene of the human and simian immunodeficiency viruses (HIV and SIV) is dispensable for viral replication in T-cell lines; however, it is essential for high virus loads and progression to simian AIDS (SAIDS) in SIV-infected adult rhesus macaques. Nef proteins from HIV type 1 (HIV-1), HIV-2, and SIV contain a proline-Xaa-Xaa-proline (PxxP) motif. The region of Nef with this motif is similar to the Src homology region 3 (SH3) ligand domain found in many cell signaling proteins. In virus-infected lymphoid cells, Nef interacts with a cellular serine/threonine kinase, designated Nef-associated kinase (NAK). In this study, analysis of viral clones containing point mutations in the nef gene of the pathogenic clone SIVmac239 revealed that several strictly conserved residues in the PxxP region were essential for Nef-NAK interaction. The results of this analysis of Nef mutations in in vitro kinase assays indicated that the PxxP region in SIV Nef was strikingly similar to the consensus sequence for SH3 ligand domains possessing the minus orientation. To test the significance of the PxxP motif of Nef for viral pathogenesis, each proline was mutated to an alanine to produce the viral clone SIVmac239-P104A/P107A. This clone, expressing Nef that does not associate with NAK, was inoculated into seven juvenile rhesus macaques. In vitro kinase assays were performed on virus recovered from each animal; the ability of Nef to associate with NAK was restored in five of these animals as early as 8 weeks after infection. Analysis of nef genes from these viruses revealed patterns of genotypic reversion in the mutated PxxP motif. These revertant genotypes, which included a second-site suppressor mutation, restored the ability of Nef to interact with NAK. Additionally, the proportion of revertant viruses increased progressively during the course of infection in these animals, and two of these animals developed fatal SAIDS. Taken together, these results demonstrated that in vivo selection for the ability of SIV Nef to associate with NAK was correlated with the induction of SAIDS. Accordingly, these studies implicate a role for the conserved SH3 ligand domain for Nef function in virally induced immunodeficiency.

Hypervariable epitope constructs representing variability in envelope glycoprotein of SIV induce a broad humoral immune response in rabbits and rhesus macaques

Using synthetic peptides, we developed an approach to account for protein epitope variability. We have prepared, in a single synthesis, a cocktail of peptides we have designated a hypervariable epitope construct (HEC), which collectively represents much of the in vivo variability seen in an epitope. Eight HECs representing the in vivo variability seen throughout the envelope glycoprotein of the simian immunodeficiency virus (SIV) were designed and synthesized. The constructs were collectively conjugated to KLH (HEC-KLH) or recombinant gp130 (HEC-rgp130) and used to immunize rabbits and rhesus macaques, respectively. Using sera collected from rabbits immunized with HEC-KLH, we demonstrated that individual components of the immunogen were recognized as antigen in ELISAs, and that the induced antibodies cross-reacted with several strains of SIV as well as with a strain of HIV-2. Following immunization of macaques with HEC-rgp130 antiviral antibodies were induced. These antibodies were still present 9.5 months after the last boost and were also capable of recognizing several different strains of SIV, including SIVmac239, SIVmac251, and SIVsmH3, as well as a strain of HIV-2 (HIV-2ROD). In addition, the antibodies were also capable of neutralizing SIV viral infectivity in vitro. Peripheral blood lymphocytes (PBLs) from immunized macaques proliferated in response to whole proteins and virus. Finally, sera from monkeys immunized with SIV, rgp130, and HIV-2 as well as sera from HIV-2-positive humans recognized HECs in ELISAs, demonstrating the relevance of these epitopes in vivo. This approach can be used as an effective method for generating a strong, broadly cross-reactive humoral response against HIV and can serve as an important component of combination vaccines against HIV and AIDS.

Differential association of uracil DNA glycosylase with SIVSM Vpr and Vpx proteins

The HIV-1 Vpr protein is a virion-associated protein which has been shown to facilitate infection of nondividing macrophages and additionally to alter cell cycle and proliferation status of the infected host cell. HIV-1 Vpr also was recently shown to associate with the DNA repair enzyme uracil DNA glycosylase (UDG). This association with a DNA repair enzyme is intriguing given that nonprimate lentiviruses encode a dUTPase, which, like UDG, minimizes the misincorporation of uracil into DNA and is important for virus replication in primary nondividing macrophages but not in dividing cells. This raises the possibility that the dependence upon Vpr for infection of nondividing macrophages may relate to its ability to interact with UDG. Members of the HIV-2/SIVSM group encode, in addition to Vpr, a related protein called Vpx. We previously demonstrated (Fletcher et al., 1996) that Vpx of HIV-2/SIVSM is necessary and sufficient for infection of primary macaque macrophages, while Vpr is not required for macrophage infection but governs cell cycle arrest. Here, we extend on these observations by demonstrating that Vpr, but not Vpx of HIV-2/SIVSM, associates with UDG, which suggests that Vpx facilitates infection of macrophages by a UDG-independent mechanism.

In vivo replication capacity rather than in vitro macrophage tropism predicts efficiency of vaginal transmission of simian immunodeficiency virus or simian/human immunodeficiency virus in rhesus macaques

We used the rhesus macaque model of heterosexual human immunodeficiency virus (HIV) transmission to test the hypothesis that in vitro measures of macrophage tropism predict the ability of a primate lentivirus to initiate a systemic infection after intravaginal inoculation. A single atraumatic intravaginal inoculation with a T-cell-tropic molecular clone of simian immunodeficiency virus (SIV), SIVmac239, or a dualtropic recombinant molecular clone of SIV, SIVmac239/1A11/239, or uncloned dualtropic SIVmac251 or uncloned dualtropic simian/human immunodeficiency virus (SHIV) 89.6-PD produced systemic infection in all rhesus macaques tested. However, vaginal inoculation with a dualtropic molecular clone of SIV, SIVmac1A11, resulted in transient viremia in one of two rhesus macaques. It has previously been shown that 12 intravaginal inoculations with SIVmac1A11 resulted in infection of one of five rhesus macaques (M. L. Marthas, C. J. Miller, S. Sutjipto, J. Higgins, J. Torten, B. L. Lohman, R. E. Unger, H. Kiyono, J. R. McGhee, P. A. Marx, and N. C. Pedersen, J. Med. Primatol. 21:99-107, 1992). In addition, SHIV HXBc2, which replicates in monkey macrophages, does not infect rhesus macaques following multiple vaginal inoculations, while T-cell-tropic SHIV 89.6 does (Y. Lu, P. B. Brosio, M. Lafaile, J. Li, R. G. Collman, J. Sodroski, and C. J. Miller, J. Virol. 70:3045-3050, 1996). These results demonstrate that in vitro measures of macrophage tropism do not predict if a SIV or SHIV will produce systemic infection after intravaginal inoculation of rhesus macaques. However, we did find that the level to which these viruses replicate in vivo after intravenous inoculation predicts the outcome of intravaginal inoculation with each virus.

A lymph node-derived cytopathic simian immunodeficiency virus Mne variant replicates in nonstimulated peripheral blood mononuclear cells

Lymph nodes (LNs) are sites of active human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) replication and disease at both early and late stages of infection. Consequently, variant viruses that replicate efficiently and subsequently cause immune dysfunction may be harbored in this tissue. To determine whether LN-associated SIVs have an increased capacity to replicate and induce cytopathology, a molecular clone of SIV was isolated directly from DNA extracted from unpassaged LN tissue of a pig-tailed macaque (Macaca nemestrina) infected with SIVMne. The animal had declining CD4+ T-lymphocyte counts at the time of the LN biopsy. In human CD4+ T-cell lines, the LN-derived virus, SIVMne027, replicated with relatively slow kinetics and was minimally cytopathic and non-syncytium inducing compared to other SIVMne clones. However, in phytohemagglutinin-stimulated pig-tailed macaque peripheral blood mononuclear cells (PBMCs), SIVMne027 replicated efficiently and was highly cytopathic for the CD4+ T-cell population. Interestingly, unlike other SIVMne clones, SIVMne027 also replicated to a high level in nonstimulated macaque PBMCs. High-level replication depended on the presence of both the T-cell and monocyte/macrophage populations and could be enhanced by interleukin-2 (IL-2). Finally, the primary determinant governing the ability of SIVMne027 to replicate in nonstimulated and IL-2-stimulated PBMCs mapped to gag-pol-vif. Together, these data demonstrate that LNs may harbor non-syncytium-inducing, cytopathic viruses that replicate efficiently and are highly responsive to the effects of cytokines such as IL-2.

Envelope glycoproteins from human immunodeficiency virus types 1 and 2 and simian immunodeficiency virus can use human CCR5 as a coreceptor for viral entry and make direct CD4-dependent interactions with this chemokine receptor

Several members of the chemokine receptor family have recently been identified as coreceptors, with CD4, for entry of human immunodeficiency virus type 1 (HIV-1) into target cells. In this report, we show that the envelope glycoproteins of several strains of HIV-2 and simian immunodeficiency virus (SIV) employ the same chemokine receptors for infection. Envelope glycoproteins from HIV-2 use CCR5 or CXCR4, while those from several strains of SIV use CCR5. Our data indicate also that some viral envelopes can use more than one coreceptor for entry and suggest that some of these coreceptors remain to be identified. To further understand how different envelope molecules use CCR5 as an entry cofactor, we show that soluble purified envelope glycoproteins (SU component) from CCR5-tropic HIV-1, HIV-2, and SIV can compete for binding of iodinated chemokine to CCR5. The competition is dependent on binding of the SU glycoprotein to cell surface CD4 and implies a direct interaction between envelope glycoproteins and CCR5. This interaction is specific since it is not observed with SU glycoprotein from a CXCR4-tropic virus or with a chemokine receptor that is not competent for viral entry (CCR1). For HIV-1, the interaction can be inhibited by antibodies specific for the V3 loop of SU. Soluble CD4 was found to potentiate binding of the HIV-2 ST and SIVmac239 envelope glycoproteins to CCR5, suggesting that a CD4-induced conformational change in SU is required for subsequent binding to CCR5. These data suggest a common fundamental mechanism by which structurally diverse HIV-1, HIV-2, and SIV envelope glycoproteins interact with CD4 and CCR5 to mediate viral entry.

Infected macaques that controlled replication of SIVmac or nonpathogenic SHIV developed sterilizing resistance against pathogenic SHIV(KU-1)

Twenty macaques were used to evaluate the ability of nonpathogenic SIV(mac) or nonpathogenic chimeric SIV-HIV (SHIV) to induce protection in macaques against superinfection with a pathogenic variant of SHIV (SHIV(KU-1)) originally containing the tat, rev, vpu, and env of HIV-1 (strain HXB2) in a genetic background of SIV(mac)239. Specifically, three macaques inoculated with molecularly cloned, macrophage-tropic SIV(mac)LG1 developed an early systemic infection but recovered with only traces of SIV(mac) DNA in visceral lymphoid tissues. These animals were then inoculated parenterally with pathogenic SHIV(KU-1). All three animals resisted infection with SHIV(KU-1), as indicated by lack of virus recovery and absence of SHIV-specific env and vpu sequences in the visceral lymphoid tissues and multiple regions in the CNS. We also examined the ability of five macaques that had been inoculated with nonpathogenic SHIV (NP-SHIV) to withstand challenge with the pathogenic SHIV(KU-1). Like the SIV(mac)LG1-inoculated macaques, these animals also resisted SHIV(KU-1) challenge as judged by the inability to recover infectious virus, normal CD4+ T cell counts, and the absence of SHIV(KU-1) signature sequences in the lymph node tissue. Thus, eight of eight animals that developed control over primary lentivirus infections had also developed resistance to infection with pathogenic SHIV(KU-1). Three groups of macaques were used as controls for this study. The first group consisted of six macaques inoculated with SHIV(KU-1) alone. All animals developed viremia, showed severe loss of CD4+ T cells within 4 weeks, and succumbed to AIDS within 6 months. The second group of three macaques was inoculated first with SHIV(KU-1) and inoculated later with uncloned, neurovirulent SIV(mac)7F-Lu. A third group of three macaques was inoculated with SIV(mac)7F-Lu followed by inoculation with SHIV(KU-1). PCR analyses using oligonucleotide primers specific for the SIV or HIV env revealed that macaques from the last two groups had widespread infection with both SHIV(KU-1) and SIV(mac), indicating that animals that failed to control productive replication of either SHIV(KU-1) or SIV(mac)7F-Lu could not resist superinfection with the other virus. These data indicate that sterilizing immunity against the virulent SHIV could be induced in animals that had experienced an immunizing infection. Moreover, the divergence of the envelope glycoprotein of the protective avirulent and virulent challenge virus suggests that a single vaccine could protect against infection with a virus containing a different envelope glycoprotein.

Molecular and biological characterization of a neurovirulent molecular clone of simian immunodeficiency virus

To identify the molecular determinants of neurovirulence, we constructed an infectious simian immunodeficiency virus (SIV) molecular clone, SIV/17E-Fr, that contained the 3' end of a neurovirulent strain of SIV, SIV/17E-Br, derived by in vivo virus passage. SIV/17E-Fr is macrophage tropic in vitro and neurovirulent in macaques. In contrast, a molecular clone, SIV/17E-Cl, that contains the SU and a portion of the TM sequences of SIV/17E-Br is macrophage tropic but not neurovirulent. To identify the amino acids that accounted for the replication differences between SIV/17E-Fr and SIV/17E-Cl in primary macaque cells in vitro, additional infectious molecular clones were constructed. Analysis of these recombinant viruses revealed that changes in the TM portion of the envelope protein were required for the highest level of replication in primary macaque macrophages and brain cells derived from the microvessel endothelium. In addition, a full-length Nef protein is necessary for optimum virus replication in both of these cell types. Finally, viruses expressing a full-length Nef protein in conjunction with the changes in the TM had the highest specific infectivity in a sMAGI assay. Thus, changes in the TM and nef genes between SIV/17E-Cl and SIV/17E-Fr account for replication differences in vitro and correlate with replication in the central nervous system in vivo.

Both SU and TM env proteins are responsible for monkey cell tropism of simian immunodeficiency virus SIV mac

While simian immunodeficiency virus (SIV) derived from an infectious molecular clone pMA239 is tropic and pathogenic for monkeys, the virus derived from another infectious clone pMA142 does not replicate in monkey cells. To determine genetic sequences responsible for this tropism, a series of recombinant clones were constructed from pMA142 and pMA239. The determinant in pMA239 was mapped within regions encompassing the env gene. Viruses, which carry the 239 env gene encoding surface and/or transmembrane proteins, were tropic for monkey cells.

Possible role of bovine immunodeficiency virus in bovine paraplegic syndrome: evidence from immunochemical, virological and seroprevalence studies

Bovine paraplegic syndrome (BPS) is a debilitating cattle disease of unknown origin that is characterized by leukocytosis, lymphocytopenia and monocytopenia. The major clinical signs are difficulties in locomotion affecting hind limbs, hypoalgesia in the hind quarters, posterior paralysis and death within 72 to 96 hours after recumbency. To investigate the aetiological basis of BPS, we examined a possible association of the syndrome with infection by bovine immunodeficiency virus (BIV), a lentivirus implicated in immune system dysfunction and central nervous system lesions in cattle. Serum samples (n = 1,278) were collected from both healthy and BPS-prevalent cattle herds in Venezuela, and organ extracts were prepared from euthanized animals (n = 11) suspected of having BPS. Sera were analysed for reactivity to recombinant BIV and bovine leukaemia virus gag precursor proteins by immunoblot procedures. Serum reactivity to BIV ranged from 12 to 66% between groups of BPS prevalent herds. The percentage of samples reactive to BLV antigen was much lower (2 to 17%). Rabbits inoculated with extracts from BPS-afflicted animals exhibited an anamnestic immune response to BIV antigens as well as the presence of BIV gag antigens in their tissues. We present evidence for a possible association between BPS disease and a viral agent related to BIV. The role of BIV, in combination with malnutrition, in BPS is discussed.

Characterization of simian immunodeficiency virus (SIV) infected AA-2 cells by SEM and immunoelectron microscopy

The ultrastructural features of AA-2 cells infected with either of two strains of simian immunodeficiency virus (SIVMne-E11S or SIVSMM-PBj) were examined by scanning electron microscopy (SEM). Transformed CD4+ human B lymphocytes (AA-2) were inoculated with SIV and observed at 2, 4, and 7 days post-inoculation (dPI). Infected AA-2 cells were distinguished by the progressive loss of microvilli, and variable numbers of free or protruding spherical particles measuring 90-120nm in diameter along the cell surface. Syncytial cell formation (complexes of fused cells) and necrotic cells were evident at each time point with the most numerous observations at 7 dPI. While the distribution and severity of the viral induced changes increased with time and affected virtually all cells by 7 dPI, the alterations were detected sooner and were more pronounced in SIVSMM-PBj infected cells. This finding is consistent with the in vivo data from primate studies using the same strains of SIV. Syncytial cells exhibited slight to moderate indentations which appeared to coincide with the boundaries of individual cells forming the complex. The plasma membrane of syncytial cells was relatively smooth and lacked microvilli. Spherical particles and buds protruding from the plasma membrane were predominate features of syncytial cell surfaces. By the employment of antisera generated against whole SIVMne-E11S, both transmission and scanning immunoelectron microscopy confirmed the identity of the spherical structures as free and budding SIV virions.

Mucosal immunization with a live, virulence-attenuated simian immunodeficiency virus (SIV) vaccine elicits antiviral cytotoxic T lymphocytes and antibodies in rhesus macaques

An effective AIDS vaccine must protect against sexual transmission of human immunodeficiency virus (HIV). Therefore, vaccine regimens which stimulate antiviral immunity in the genital tract as well as in peripheral blood and systemic lymphoid tissues are needed. Here, we describe a method of immunization by direct inoculation of the vaginal submucosa with a live attenuated SIV, SIVmac1A11. Immunization by this route generated low levels of SIV-specific IgG and IgA antibodies in serum and vaginal secretions and viral specific cytotoxic T lymphocyte (CTL) activity in peripheral blood.

Primary cultures of rhesus placental syncytiotrophoblasts are permissive for SIV infection

Primary cultures of rhesus syncytiotrophoblasts incubated with SIVdeltaB670, SIVmac251, or SIVmac239 produced readily detectable virus in the supernatant for up to three weeks after infection. At four weeks, cells generally failed to release virus but placental cell lysates and placental cells cocultured for 24 hours with uninfected CEM x 174 cells were able to transmit infection. The presence of virus was confirmed by electron microscopy and PCR amplification of viral sequences from trophoblast genomic DNA. SIV p27 antigen was localized by immunostaining primarily in syncytiotrophoblasts.

Viral determinants of simian immunodeficiency virus (SIV) virulence in rhesus macaques assessed by using attenuated and pathogenic molecular clones of SIVmac

To identify viral determinants of simian immunodeficiency virus (SIV) virulence, two pairs of reciprocal recombinants constructed from a pathogenic (SIVmac239) and a nonpathogenic (SIVmac1A11) molecular clone of SIV were tested in rhesus macaques. A large 6.2-kb fragment containing gag, pol, env, and the regulatory genes from each of the cloned (parental) viruses was exchanged to produce one pair of recombinant viruses (designated SIVmac1A11/239gag-env/1A11 and SIVmac239/1A11gag-env/239 to indicate the genetic origins of the 5'/internal/3' regions, respectively, of the virus). A smaller 1.4-kb fragment containing the external env domain of each of the parental viruses was exchanged to create the second pair (SIVmac1A11/239env/1A11 and SIVmac239/1A11env/239) of recombinant viruses. Each of the two parental and four recombinant viruses was inoculated intravenously into four rhesus macaques, and all 24 animals were viremic by 4 weeks postinoculation (p.i.). Virus could not be isolated from peripheral blood mononuclear cells (PBMC) of any animals infected with SIVmac1A11 after 6 weeks p.i. but was consistently isolated from all macaques inoculated with SIVmac239 for 92 weeks p.i. Virus isolation was variable from animals infected with recombinant viruses; SIVmac1A11/239gag-env/1A11 and SIVmac239/1A11env/239 were isolated most frequently. Animals inoculated with SIVmac239 had 10 to 100 times more virus-infected PBMC than those infected with recombinant viruses. Three animals infected with SIVmac239 died with simian AIDS (SAIDS) during the 2-year observation period after inoculation, and the fourth SIVmac239-infected animal had clinical signs of SAIDS. Two animals infected with recombinant viruses died with SAIDS; one was infected with SIVmac239/1A11gag-env/239, and the other was infected with SIVmac1A11/239gag-env/1A11. The remaining 18 macaques remained healthy by 2 years p.i., and 13 were aviremic. One year after inoculation, peripheral lymph nodes of some of these healthy, aviremic animals harbored infected cells. All animals seroconverted within the first few weeks of infection, and the magnitude of antibody response to SIV was proportional to the levels and duration of viremia. Virus-suppressive PBMC were detected within 2 to 4 weeks p.i. in all animals but tended to decline as viremia disappeared. There was no association of levels of cell-mediated virus-suppressive activity and either virus load or disease progression. Taken together, these results indicate that differences in more than one region of the viral genome are responsible for the lack of virulence of SIVmac1A11.

The NF-kappa B binding site is necessary for efficient replication of simian immunodeficiency virus of macaques in primary macrophages but not in T cells in vitro

We demonstrate here that the nuclear factor-kappa B (NF-kappa B) binding site in the simian immunodeficiency virus (SIVmac) long terminal repeat is essential for efficient virus replication in primary alveolar macrophages but dispensable for efficient replication in primary T cells. Mutation of the NF-kappa B site does not seriously impair replication of a T-cell-tropic SIVmac239 or a macrophagetropic SIVmacEm* in peripheral blood lymphocytes or established CD4+ cell lines; however, mutation of the NF-kappa B site prevents efficient SIVmacEm* replication in primary alveolar macrophages. These data suggest that efficient replication in primary macrophages requires both envelope and long terminal repeat determinants.

An epitope on the surface envelope glycoprotein (gp130) of simian immunodeficiency virus (SIVmac) involved in viral neutralization and T cell activation

SIVmac infection of macaques is an important animal model for HIV infection and AIDS; this model is being utilized for development of antiviral therapies and vaccines. In the present article, we sought to identify neutralization epitopes of SIVmac envelope surface glycoprotein (gp130). Algorithms were used to predict antigenicity of specific regions. Four regions from the primary amino acid sequence of the viral surface glycoprotein were selected. A synthetic peptide representing one of these regions (414-434) induced virus-neutralizing antibodies in mice; in addition, this peptide induced T cell-proliferative responses in macaques. To address the in vivo relevance of these observations, we demonstrated that experimentally infected macaques produce antibodies to the neutralization epitope. In addition, rhesus macaques protected against infection by an inactivated SIV vaccine develop antibodies that bind to peptide 414-434. These observations demonstrate that the region that includes the sequence 414-434 in the fourth variable domain (V4) of SIVmac gp130 contains both a linear neutralization epitope and a T cell epitope.

Correlation of specific virus-astrocyte interactions and cytopathic effects induced by ts1, a neurovirulent mutant of Moloney murine leukemia virus

ts1 is a highly neuropathogenic and lymphocytopathic mutant of Moloney murine leukemia virus TB (MoMuLV-TB). We previously reported that the primary neuropathogenic determinant of ts1 maps to a single amino acid substitution, Val-25-->Ile, in precursor envelope protein gPr80env. This Val-25-->Ile substitution apparently renders gPr80env inefficient for transport from the endoplasmic reticulum to the Golgi apparatus. These findings suggest that the cytopathic effect of ts1 in neural cells might be due to the accumulation of gPr80env in the endoplasmic reticulum. Since endothelial and glial cells are targets of ts1 infection in the central nervous system, we established primary endothelial and astrocyte cultures to investigate the mechanism of cell killing caused by ts1. A continuous cell line, TB, was used as a control. Our results showed that both ts1 and MoMuLV-TB replicated and induced a cytopathic effect in astrocyte cultures, albeit to different degrees; ts1 appeared to be more lethal than MoMuLV-TB. On the other hand, ts1 and MoMuLV-TB infections of endothelial or TB cells were not cytopathic. The cytopathic effect in infected astrocytes correlated with the inefficiency of gPr80env transport and the intracellular accumulation of gPr80env as well as aberrant virus particles.

Pathologic features of SIV-induced disease and the association of macrophage infection with disease evolution

Since the original isolation of simian immunodeficiency virus (SIV) from a macaque with an AIDS-like disease, numerous studies have demonstrated the close biologic and genetic relationship of the SIVs to the HIVs. Probably most important, the clinical spectrum of disease associated with SIVmac/SIVsmm infection in rhesus monkeys is strikingly similar to AIDS in HIV-1-infected human beings. Herein are summarized the pathologic features of SIVmac-induced disease in a cohort of rhesus monkeys, with special reference to the role of infected macrophages in the development of AIDS-related manifestations.

Genetic and biological comparisons of pathogenic and nonpathogenic molecular clones of simian immunodeficiency virus (SIVmac)

Simian immunodeficiency virus (SIV) is a designation for a group of related but unique lentiviruses identified in several primate species. A viral isolate from a rhesus macaque (i.e., SIVmac) causes a fatal AIDS-like disease in experimentally infected macaques, and several infectious molecular clones of this virus have been characterized. This report presents the complete nucleotide sequence of molecularly cloned SIVmac1A11, and comparisons are made with the sequence of molecularly cloned SIVmac239. SIVmac1A11 has delayed replication kinetics in lymphoid cells but replicates as well as uncloned SIVmac in macrophage cultures. Macaques infected with virus from the SIVmac1A11 clone develop antiviral antibodies, but virus does not persist in peripheral blood mononuclear cells and no disease signs are observed. SIVmac239 infects lymphoid cells, shows restricted replication in cultured macrophages, and establishes a persistent infection in animals that leads to a fatal AIDS-like disease. Both viruses are about 98% homologous at the nucleotide sequence level. In SIVmac1A11, the vpr gene as well as the transmembrane domain of env are prematurely truncated, whereas the nef gene of SIVmac239 is prematurely truncated. Sequence differences are also noted in variable region 1 (V1) in the surface domain of the env gene. The potential implications of these and other sequence differences are discussed with respect to the phenotypes of both viruses. This animal model is critically important for investigating the roles of specific viral genes in viral/host interactions that cannot be studied in individuals infected with the human immunodeficiency virus (HIV).

Identification of viral determinants of macrophage tropism for simian immunodeficiency virus SIVmac

Simian immunodeficiency virus (SIV), a lymphocytopathic lentivirus, induces an AIDS-like disease in rhesus macaques (Macaca mulatta). A pathogenic molecular clone of rhesus macaque SIV (SIVmac), SIVmac-239, replicates and induces cytopathology in T lymphocytes but is restricted for replication in macrophages. In contrast, a nonpathogenic molecular clone of SIVmac, SIVmac-1A11, replicates and induces syncytia (multinucleated giant cells) in cultures of both T lymphocytes and macrophages. SIVmac-1A11 does not cause disease in macaques. To map the viral determinants of macrophage tropism, reciprocal recombinant genomes were constructed between molecular clones of SIVmac-239 and SIVmac-1A11. Infectious recombinant viruses were rescued by transfection of cloned viral genomes into permissive lymphoid cells. Analysis of one pair of reciprocal recombinants revealed that an internal 6.2-kb DNA fragment of SIVmac-1A11 was necessary and sufficient for both syncytium formation and efficient replication in macrophages. This region includes the coding sequences for a portion of the gag gene, all of the pol, vif, vpr, and vpx genes, the first coding exons of tat and rev, and the external env glycoprotein gp130. Thus, the transmembrane glycoprotein of env, the nef gene, the second coding exons of tat and rev, and the long terminal repeats are not essential for in vitro macrophage tropism. Analysis of additional recombinants revealed that syncytium formation, but not virus production, was controlled by a 1.4-kb viral DNA fragment in SIVmac-1A11 encoding only the external env glycoprotein gp130. Thus, gp130 env of SIVmac-1A11 is necessary for entry of virus into macrophages but is not sufficient for a complete viral replication cycle in this cell type. We therefore conclude that gp130 env and one or more genetic elements (exclusive of the long terminal repeats, transmembrane glycoprotein of env, and second coding exons of tat and rev, and nef) are essential for a complete replication cycle of SIVmac in rhesus macaque macrophages.