Mapping the determinants of human immunodeficiency virus 2 for infectivity, replication efficiency, and cytopathicity

APOBEC3F Constitutes a Barrier to Successful Cross-Species Transmission of Simian Immunodeficiency Virus SIVsmm to Humans

Simian immunodeficiency virus infecting sooty mangabeys (SIVsmm) has been transmitted to humans on at least nine occasions, giving rise to human immunodeficiency virus type 2 (HIV-2) groups A to I. SIVsmm isolates replicate in human T cells and seem capable of overcoming major human restriction factors without adaptation. However, only groups A and B are responsible for the HIV-2 epidemic in sub-Saharan Africa, and it is largely unclear whether adaptive changes were associated with spread in humans. To address this, we examined the sensitivity of infectious molecular clones (IMCs) of five HIV-2 strains and representatives of five different SIVsmm lineages to various APOBEC3 proteins. We confirmed that SIVsmm strains replicate in human T cells, albeit with more variable replication fitness and frequently lower efficiency than HIV-2 IMCs. Efficient viral propagation was generally dependent on intact vif genes, highlighting the need for counteraction of APOBEC3 proteins. On average, SIVsmm was more susceptible to inhibition by human APOBEC3D, -F, -G, and -H than HIV-2. For example, human APOBEC3F reduced infectious virus yield of SIVsmm by ∼80% but achieved only ∼40% reduction in the case of HIV-2. Functional and mutational analyses of human- and monkey-derived alleles revealed that an R128T polymorphism in APOBEC3F contributes to species-specific counteraction by HIV-2 and SIVsmm Vifs. In addition, a T84S substitution in SIVsmm Vif increased its ability to counteract human APOBEC3F. Altogether, our results confirm that SIVsmm Vif proteins show intrinsic activity against human APOBEC3 proteins but also demonstrate that epidemic HIV-2 strains evolved an increased ability to counteract this class of restriction factors during human adaptation. IMPORTANCE Viral zoonoses pose a significant threat to human health, and it is important to understand determining factors. SIVs infecting great apes gave rise to HIV-1. In contrast, SIVs infecting African monkey species have not been detected in humans, with one notable exception. SIVsmm from sooty mangabeys has crossed the species barrier to humans on at least nine independent occasions and seems capable of overcoming many innate defense mechanisms without adaptation. Here, we confirmed that SIVsmm Vif proteins show significant activity against human APOBEC3 proteins. Our analyses also revealed, however, that different lineages of SIVsmm are significantly more susceptible to inhibition by various human APOBEC3 proteins than HIV-2 strains. Mutational analyses suggest that an R128T substitution in APOBEC3F and a T84S change in Vif contribute to species-specific counteraction by HIV-2 and SIVsmm. Altogether, our results support that epidemic HIV-2 strains acquired increased activity against human APOBEC3 proteins to clear this restrictive barrier.

IFI16 Targets the Transcription Factor Sp1 to Suppress HIV-1 Transcription and Latency Reactivation

The interferon γ-inducible protein 16 (IFI16) is known as immune sensor of retroviral DNA intermediates. We show that IFI16 restricts HIV-1 independently of immune sensing by binding and inhibiting the host transcription factor Sp1 that drives viral gene expression. This antiretroviral activity and ability to bind Sp1 require the N-terminal pyrin domain and nuclear localization of IFI16, but not the HIN domains involved in DNA binding. Highly prevalent clade C HIV-1 strains are more resistant to IFI16 and less dependent on Sp1 than other HIV-1 subtypes. Furthermore, inhibition of Sp1 by IFI16 or pharmacologically by Mithramycin A suppresses reactivation of latent HIV-1 in CD4+ T cells. Finally, IFI16 also inhibits retrotransposition of LINE-1, known to engage Sp1, and murine IFI16 homologs restrict Friend retrovirus replication in mice. Thus, IFI16 restricts retroviruses and retrotransposons by interfering with Sp1-dependent gene expression, and evasion from this restriction may facilitate spread of HIV-1 subtype C.

A template-dependent dislocation mechanism potentiates K65R reverse transcriptase mutation development in subtype C variants of HIV-1

Numerous studies have suggested that the K65R reverse transcriptase (RT) mutation develops more readily in subtype C than subtype B HIV-1. We recently showed that this discrepancy lies partly in the subtype C template coding sequence that predisposes RT to pause at the site of K65R mutagenesis. However, the mechanism underlying this observation and the elevated rates of K65R development remained unknown. Here, we report that DNA synthesis performed with subtype C templates consistently produced more K65R-containing transcripts than subtype B templates, regardless of the subtype-origin of the RT enzymes employed. These findings confirm that the mechanism involved is template-specific and RT-independent. In addition, a pattern of DNA synthesis characteristic of site-specific primer/template slippage and dislocation was only observed with the subtype C sequence. Analysis of RNA secondary structure suggested that the latter was unlikely to impact on K65R development between subtypes and that Streisinger strand slippage during DNA synthesis at the homopolymeric nucleotide stretch of the subtype C K65 region might occur, resulting in misalignment of the primer and template. Consequently, slippage would lead to a deletion of the middle adenine of codon K65 and the production of a -1 frameshift mutation, which upon dislocation and realignment of the primer and template, would lead to development of the K65R mutation. These findings provide additional mechanistic evidence for the facilitated development of the K65R mutation in subtype C HIV-1.

Factors Affecting Template Usage in the Development of K65R Resistance in Subtype C Variants of HIV Type-1

Background:

We have shown that the K65R resistance mutation in HIV type-1 (HIV-1) reverse transcriptase (RT) is selected more rapidly in subtype C than subtype B HIV-1 in biochemical, cell culture and clinical studies. Template-usage experiments demonstrated that subtype C nucleotide coding sequences caused RT to preferentially pause, leading to K65R acquisition. This new study now further establishes the basis for differential occurrence of both K65R and thymidine analogue mutations (TAMs) between subtypes.

Methods:

Gel-based nucleotide extension assays were used to study the homopolymeric sequence surrounding K65.

Results:

When positive double-stranded DNA synthesis was evaluated from a negative single-stranded DNA template, pausing at the 67 region, which is linked to occurrence of TAMs, was alleviated with both subtype B and C templates at high dCTP concentrations, but this alleviation was more pronounced with the subtype C template. By contrast, pausing at the 65 region on the subtype C but not subtype B template always occurred and was not alleviated at high levels of nucleotide triphosphates or by other means. Furthermore, templates containing repeats of the homopolymeric sequence spanning codons 64–66 of pol showed corresponding pausing repeats at the 65 region with the subtype C template only. Inverted RNA and DNA templates both displayed pausing at position K65 for the subtype C template and a ladder of pausing events culminating at codon 67 for the subtype B templates.

Conclusions:

These results further establish a mechanistic basis for the exclusion of both K65R and TAMs on single templates as well as the preferential acquisition of K65R in subtype C viruses.

Biological signature characteristics of primary isolates from human immunodeficiency virus type 1 group O in ex vivo human tonsil histocultures

Human immunodeficiency virus type 1 (HIV-1) group M viruses have achieved a global distribution, while HIV-1 group O viruses are endemic only in particular regions of Africa. Here, we evaluated biological characteristics of group O and group M viruses in ex vivo models of HIV-1 infection. The replicative capacity and ability to induce CD4 T-cell depletion of eight group O and seven group M primary isolates were monitored in cultures of human peripheral blood mononuclear cells and tonsil explants. Comparative and longitudinal infection studies revealed HIV-1 group-specific activity patterns: CCR5-using (R5) viruses from group M varied considerably in their replicative capacity but showed similar levels of cytopathicity. In contrast, R5 isolates from group O were relatively uniform in their replicative fitness but displayed a high and unprecedented variability in their potential to deplete CD4 T cells. Two R5 group O isolates were identified that cause massive depletion of CD4 T cells, to an extent comparable to CXCR4-using viruses and not documented for any R5 isolate from group M. Intergroup comparisons found a five- to eightfold lower replicative fitness of isolates from group O than for isolates from group M yet a similar overall intrinsic pathogenicity in tonsil cultures. This study establishes biological ex vivo characteristics of HIV-1 group O primary isolates. The current findings challenge the belief that a grossly reduced replicative fitness or inherently impaired cytopathicity of viruses from this group underlies their low global prevalence.

Is HIV-1 evolving to a less virulent form in humans?

During the rapid spread of HIV-1 in humans, the main (M) group of HIV-1 has evolved into ten distinct subtypes, undergone countless recombination events and diversified extensively. The impact of this extreme genetic diversity on the phenotype of HIV-1 has only recently become a research focus, but early findings indicate that the dominance of HIV-1 subtype C in the current epidemic might be related to the lower virulence of this subtype compared with other subtypes. Here, we explore whether HIV-1 has reached peak virulence or has already started the slow path to attenuation.

The replicative fitness of primary human immunodeficiency virus type 1 (HIV-1) group M, HIV-1 group O, and HIV-2 isolates

The main (M) group of human immunodeficiency virus type 1 (HIV-1) is responsible for the global AIDS epidemic while HIV-1 group O (outlier) and HIV type 2 are endemic only in west and central Africa. The failure of HIV-2 and especially HIV-1 group O to spread following the initial zoonotic jumps is not well understood. This study was designed to examine the relative replicative capacities between these human lentiviruses. A pairwise competition experiment was performed with peripheral blood mononuclear cells with eight HIV-2 isolates, 6 group O viruses, and 15 group M viruses of subtype A (2 viruses), B (5 viruses), C (4 viruses), D (2 viruses) and CRF01_AE (2 viruses). HIV-1 group M isolates of any subtype were typically 100-fold-more fit than group O or HIV-2 strains when competed in peripheral blood mononuclear cells from various humans. This order in replicative fitness was also observed when virus pairs were added to human dendritic cells and then cocultured with primary, quiescent T cells, which is the model for HIV-1 transmission. These results suggest that reduced replicative and transmission fitness may be contributing to the low prevalence and limited geographical spread of HIV-2 and group O HIV-1 in the human population.

Functional analysis of Vif protein shows less restriction of human immunodeficiency virus type 2 by APOBEC3G

Viral infectivity factor (Vif) is one of the human immunodeficiency virus (HIV) accessory proteins and is conserved in the primate lentivirus group. This protein is essential for viral replication in vivo and for productive infection of nonpermissive cells, such as peripheral blood mononuclear cells (PBMC). Vif counteracts an antiretroviral cellular factor in nonpermissive cells named CEM15/APOBEC3G. Although HIV type 1 (HIV-1) Vif protein (Vif1) can be functionally replaced by HIV-2 Vif protein (Vif2), its identity is very small. Most of the functional studies have been carried out with Vif1. Characterization of functional domains of Vif2 may elucidate its function, as well as differences between HIV-1 and HIV-2 infectivity. Our aim was to identify the permissivity of different cell lines for HIV-2 vif-minus viruses. By mutagenesis specific conserved motifs of HIV-2 Vif protein were analyzed, as well as in conserved motifs between Vif1 and Vif2 proteins. Vif2 mutants were examined for their stability, expression, and cellular localization in order to characterize essential domains of Vif2 proteins. Viral replication in various target cells (PBMC and H9, A3.01, U38, and Jurkat cells) and infectivity in single cycle assays in the presence of APOBEC3G were also analyzed. Our results of viral replication show that only PBMC have a nonpermissive phenotype in the absence of Vif2. Moreover, the HIV-1 vif-minus nonpermissive cell line H9 does not show a similar phenotype for vif-negative HIV-2. We also report a limited effect of APOBEC3G in a single-cycle infectivity assay, where only conserved domains between HIV-1 and HIV-2 Vif proteins influence viral infectivity. Taken together, these results allow us to speculate that viral inhibition by APOBEC3G is not the sole and most important determinant of antiviral activity against HIV-2.

Human immunodeficiency virus type 2 (HIV-2) vector-mediated in vivo gene transfer into adult rabbit retina

PURPOSE

To evaluate the potential usefulness of HIV-2 viral vector in in vivo retinal gene therapy.

METHODS

An HIV-2 virus based viral vector was constructed and administered subretinally and intravitreally into rabbit eyes. After viral vector administration, the eyes were closely monitored for any adverse effects by slit lamp, indirect ophthalmoscopy, and fundus photography. Eyes were enucleated at specified times after injection, and reporter gene expression was identified within cell types and graded by the pattern and distribution of staining cells using fluorescent microscopy.

RESULTS

The HIV-2 viral vector demonstrated efficient gene transfer into many types of retinal cells without apparent cytotoxicity. Notably with subretinal injection, the HIV-2 vector resulted in higher efficiency of transduction of photoreceptor cells than of the other cell types (p < 0.05). With the intravitreal administration of HIV-2 viral vectors, cellular transduction and transgene expression in the ganglion cell layer was the dominant finding.

CONCLUSIONS

HIV-2 viral vector may be a useful gene delivery vehicle for retinal photoreceptor cells and ganglion cells. It deserves further exploration to investigate its potential merit in long term gene therapy protocols and in other animal species.

The nef gene controls syncytium formation in primary human lymphocytes and macrophages infected by HIV type 1

nef, the 3'-most open reading frame of HIV, has been reported to enhance HIV replication in various host cell types and to promote in vivo replication and pathogenesis. The mechanism underlying the increased in vivo viral replication is still unclear. We have examined the effect of a nef deletion on the infection of primary human CD4+ T lymphocytes and macrophages, using clones with nef and env sequences derived, respectively, from T cell- and macrophage-tropic viruses. The deletion of nef enhanced the formation of syncytia in CD4+ T lymphocytes infected with macrophage-tropic clones, despite a severalfold reduced viral production. No such enhancement of syncytium formation was observed in CD4+ T lymphocytes infected with a T cell line-tropic clone, but in this clone, the deletion of nef imparted a more severe replication defect. A similar increase in syncytium formation was observed in primary human macrophages infected with nef-deleted clones compared with wild-type counterparts, except under conditions in which the deletion of nef markedly reduced viral replication. We could not demonstrate an enhanced cell surface expression of HIV-1 envelope in lymphocytes infected with nef-deficient clones to explain the increased syncytium formation. In enhancing the HIV-1 cytopathic effect, the deletion of nef might curtail virus production by infected cells, and thus explain in part the reduced viral load observed in vivo in hosts infected with nef-deficient viruses.

Identification of a human immunodeficiency virus type 2 (HIV-2) encapsidation determinant and transduction of nondividing human cells by HIV-2-based lentivirus vectors

Although previous lentivirus vector systems have used human immunodeficiency virus type 1 (HIV-1), HIV-2 is less pathogenic in humans and is amenable to pathogenicity testing in a primate model. In this study, an HIV-2 molecular clone that is infectious but apathogenic in macaques was used to first define cis-acting regions that can be deleted to prevent HIV-2 genomic encapsidation and replication without inhibiting viral gene expression. Lentivirus encapsidation determinants are complex and incompletely defined; for HIV-2, some deletions between the major 5' splice donor and the gag open reading frame have been shown to minimally affect encapsidation and replication. We find that a larger deletion (61 to 75 nucleotides) abrogates encapsidation and replication but does not diminish mRNA expression. This deletion was incorporated into a replication-defective, envelope-pseudotyped, three-plasmid HIV-2 lentivirus vector system that supplies HIV-2 Gag/Pol and accessory proteins in trans from an HIV-2 packaging plasmid. The HIV-2 vectors efficiently transduced marker genes into human T and monocytoid cell lines and, in contrast to a murine leukemia virus-based vector, into growth-arrested HeLa cells and terminally differentiated human macrophages and NTN2 neurons. Vector DNA could be detected in HIV-2 vector-transduced nondividing CD34(+) CD38(-) human hematopoietic progenitor cells but not in those cells transduced with murine vectors. However, stable integration and expression of the reporter gene could not be detected in these hematopoietic progenitors, leaving open the question of the accessibility of these cells to stable lentivirus transduction.

Development of HIV vectors for anti-HIV gene therapy

Current gene therapy protocols for HIV infection use transfection or murine retrovirus mediated transfer of antiviral genes into CD4+ T cells or CD34+ progenitor cells ex vivo, followed by infusion of the gene altered cells into autologous or syngeneic/allogeneic recipients. While these studies are essential for safety and feasibility testing, several limitations remain: long-term reconstitution of the immune system is not effected for lack of access to the macrophage reservoir or the pluripotent stem cell population, which is usually quiescent, and ex vivo manipulation of the target cells will be too expensive and impractical for global application. In these regards, the lentivirus-specific biologic properties of the HIVs, which underlie their pathogenetic mechanisms, are also advantageous as vectors for gene therapy. The ability of HIV to specifically target CD4+ cells, as well as non-cycling cells, makes it a promising candidate for in vivo gene transfer vector on one hand, and for transduction of non-cycling stem cells on the other. Here we report the use of replication-defective vectors and stable vector packaging cell lines derived from both HIV-1 and HIV-2. Both HIV envelopes and vesicular stomatitis virus glycoprotein G were effective in mediating high-titer gene transfer, and an HIV-2 vector could be cross-packaged by HIV-1. Both HIV-1 and HIV-2 vectors were able to transduce primary human macrophages, a property not shared by murine retroviruses. Vesicular stomatitis virus glycoprotein G-pseudotyped HIV vectors have the potential to mediate gene transfer into non-cycling hematopoietic stem cells. If so, HIV or other lentivirus-based vectors will have applications beyond HIV infection.

An infectious chimeric human immunodeficiency virus type 2 (HIV-2) expressing the HIV-1 principal neutralizing determinant

The human immunodeficiency virus type 1 strain MN (HIV-1MN) principal neutralizing determinant (PND, V3 loop) was introduced into infectious molecular clones HIV-2KR and simian immunodeficiency virus mm239 (SIVmm239) by hybridization PCR, replacing the corresponding HIV-2 or SIV envelope cysteine loops with the HIV-1 coding sequence. The HIV-2 chimera (HIV-2KR-MNV3) was found to be capable of infecting a number of T-cell lymphoblastic cell lines as well as primary peripheral blood mononuclear cells. In contrast, the SIV chimera (SIV239MNV3) was not replication competent. Envelope produced by HIV-2KR-MNV3 but not the parental HIV-2KR was recognized by V3-specific and HIV-1-specific polyclonal antisera in radioimmunoprecipitation assays. HIV-2-specific antisera recognized both the chimeric and parental virus but not HIV-1MN. The chimeric HIV-2KR-MNV3 virus proved to be exquisitely susceptible to neutralization by HIV-1-specific and V3-specific antisera, suggesting the potential for use in animal models designed to test HIV-1 vaccine candidates which target the PND.

Localization of human immunodeficiency virus Rev in transfected and virus-infected cells

The rev gene product of human immunodeficiency virus (HIV) is obligatory for viral replication. Rev interacts specifically with a structured RNA sequence within the viral genome termed the REV response element (RRE). Although the importance of Rev for the expression of viral proteins is well documented, its functional mechanism remains unresolved. Previous studies identified Rev in the absence of RRE to be a nuclear protein localized primarily within the nucleoli. To extend our understanding of the role of Rev in viral replication, immunolocalization studies of Rev and other nuclear components were carried out in transfected cells expressing both the Rev protein and RRE-containing mRNA and in cells infected with HIV. In both types of cells, Rev-like immunoreactivity was distributed both in the nucleoplasm and cytoplasm. Within the nucleus, Rev immunoreactivity was not evenly distributed but was present within focal concentrations. In transfected cells that were double labeled for Rev and SC-35, which labels a known component of spliceosomes, the foci of Rev labeling were distinct from the "speckles" labeled by SC-35, although Rev foci and speckles were often juxtaposed. In addition, morphological changes in the three-dimensional network of speckles were observed in both transfected cells expressing both the Rev protein and RRE-containing mRNA and in cells infected with HIV-1 and HIV-2. Our observations are consistent with the proposed dual role of Rev in mRNA transport and splicing.

Intracellular expression of antibody fragments directed against HIV reverse transcriptase prevents HIV infection in vitro

We have tested a novel strategy of intracellular immunization to block human immunodeficiency virus (HIV) infection. The expression of a specific antibody within a cell was achieved by transduction of genes that encode for immunoglobulin chains with specificity to viral reverse transcriptase. We demonstrated that inhibition of this enzyme makes cells resistant to HIV infection by blocking an early stage of viral replication. If high efficiency transduction with a stable vector into lymphohaematopoietic stem cells or mature lymphocytes can be achieved, gene transfer-mediated intracellular immunization might be a feasible treatment strategy in AIDS.

Comparative analyses of human immunodeficiency virus type 1 (HIV-1) and HIV-2 Vif mutants

Virion infectivity factor (vif), a gene found in all lentiviruses, plays an essential role in virus replication in certain target cells. We examined the replication competence of the human immunodeficiency virus type 2 (HIV-2) vif mutant in different T-cell lines and primary cells in comparison with that of the HIV-1 vif mutant. Both mutant viruses were unable to replicate in peripheral blood-derived mononuclear cells but replicated with wild-type efficiency in certain T-cell lines, such as SupT1 and MOLT-4/8. These results confirm the importance of vif in the infection of relevant target cells and imply that some cellular factor(s) could compensate for vif function. However, HIV-1 and HIV-2 vif mutant viruses also show differential replications in other cell lines, suggesting either different threshold requirements for the same cellular factor(s) or the involvement of different factors to compensate for vif-1 and vif-2 functions. By cross complementation experiments, we showed that vif-1 and vif-2 have similar functions. Our studies further indicate the existence of two kinds of nonpermissive cells: H9 is unable to complement HIV-1 delta vif but is susceptible to a one-round infection with HIV-1 delta vif produced from permissive cells. In contrast, U937 is nonpermissive for HIV-2 delta vif produced from permissive cells but, once infected, is able to complement the delta vif function. In both types of nonpermissive cells, a step prior to proviral DNA synthesis is affected.

Tat-independent replication of human immunodeficiency viruses

The replication of human immunodeficiency retroviruses involves a complex series of events that is regulated at both transcriptional and posttranscriptional levels. The tat gene product is a potent trans-activator of viral transcription and therefore an attractive target for the development of antiviral drugs. Tat-defective HIV-1 proviral DNA clones have been shown previously to be replication defective. In this study, we report that tat-defective HIV-1 and HIV-2 viral DNA transfected into U937 cells can direct efficient viral replication in the presence of transcriptional stimulators such as TNF-alpha and PMA. In MT-4 cells, tat-defective HIV-1 can replicate without any stimulation. The viruses recovered from MT-4 cells remained tat defective defined by their inability to infect T cell lines (e.g., Molt 4/8) although replication could be rescued with cytokines. Limited replication was observed in primary mononuclear cells. Furthermore, we showed that Ro 24-7429, a potent tat antagonist and antiviral compound, failed to suppress HIV-1 replication in TNF-alpha-stimulated T cells. These results have important implications for targeting tat as a therapeutic strategy for AIDS.

Genetic diversity of human immunodeficiency virus type 2: evidence for distinct sequence subtypes with differences in virus biology

The virulence properties of human immunodeficiency virus type 2 (HIV-2) are known to vary significantly and to range from relative attenuation in certain individuals to high-level pathogenicity in others. These differences in clinical manifestations may, at least in part, be determined by genetic differences among infecting virus strains. Evaluation of the full spectrum of HIV-2 genetic diversity is thus a necessary first step towards understanding its molecular epidemiology, natural history of infection, and biological diversity. In this study, we have used nested PCR techniques to amplify viral sequences from the DNA of uncultured peripheral blood mononuclear cells from 12 patients with HIV-2 seroreactivity. Sequence analysis of four nonoverlapping genomic regions allowed a comprehensive analysis of HIV-2 phylogeny. The results revealed (i) the existence of five distinct and roughly equidistant evolutionary lineages of HIV-2 which, by analogy with HIV-1, have been termed sequence subtypes A to E; (ii) evidence for a mosaic HIV-2 genome, indicating that coinfection with genetically divergent strains and recombination can occur in HIV-2-infected individuals; and (iii) evidence supporting the conclusion that some of the HIV-2 subtypes may have arisen from independent introductions of genetically diverse sooty mangabey viruses into the human population. Importantly, only a subset of HIV-2 strains replicated in culture: all subtype A viruses grew to high titers, but attempts to isolate representatives of subtypes C, D, and E, as well as the majority of subtype B viruses, remained unsuccessful. Infection with all five viral subtypes was detectable by commercially available serological (Western immunoblot) assays, despite intersubtype sequence differences of up to 25% in the gag, pol, and env regions. These results indicate that the genetic and biological diversity of HIV-2 is far greater than previously appreciated and suggest that there may be subtype-specific differences in virus biology. Systematic natural history studies are needed to determine whether this heterogeneity has clinical relevance and whether the various HIV-2 subtypes differ in their in vivo pathogenicity.

Viral genetic determinants in SIVsmmPBj pathogenesis

A variant simian immunodeficiency virus (SIV) from sooty mangabeys, SIVsmmPBj, induces an acutely lethal disease in pigtailed macaques (Macaca nemestrina). This study further characterizes the viral genetic determinants involved in this acutely lethal disease. We have generated chimeric molecular clones constructed between SIVsmmPBj and either SIVsmH4 or SIVsmm9 to analyze the role of the 5' half of the genome and the envelope gene in the induction of acute disease. These studies suggest that the gag and gp40 of SIVsmmPBj are required for the development of lethal disease, and an additional determinant in the central regulatory gene region of the SIVsmmPBj genome is also required.