Animal models of AIDS

Animal models of AIDS are essential for understanding the pathogenesis of retrovirus-induced immune deficiency and encephalopathy and for development and testing of new therapies and vaccines. AIDS and related disorders are etiologically linked to members of the lentivirus subfamily of retroviruses; these lymphocytopathic lentiviruses are designated human immuno-deficiency virus type 1 (HIV-1) and human immuno-deficiency virus type 2 (HIV-2). The only animals susceptible to experimental HIV-1 infection are the chimpanzee, gibbon ape, and rabbit but AIDS-like disease has not yet been reported in these species. Macaques can be persistently infected with some strains of HIV-2 but no AIDS-like disease has resulted. It is not yet clear how suitable HIV-infected SCID-hu mice will be as a model for AIDS. Several subfamilies of naturally occurring cytopathic retroviruses cause immune suppression, including fatal immunodeficiency syndromes in chickens, mice, cats, and monkeys. Domestic cats suffer immunosuppression from both an onco-virus, feline leukemia virus, and a member of the lentivirus subfamily, feline immunodeficiency virus (FIV). Asian macaques are susceptible to fatal simian AIDS from a type D retrovirus, indigenous in macaques, and from a lentivirus, simian immunodeficiency virus (SIV), which is indigenous to healthy African monkeys. SIV is the animal lentivirus most closely related to HIV. Of these animal models, the lentivirus infections of cats (FIV) and macaques (SIV) appear to bear the closest similarity in their pathogenesis to HIV infection and AIDS. This review will summarize these various animal model systems for AIDS and illustrate their usefulness for antiviral therapy and vaccinology.     

Simian immunodeficiency virus vectors: replication and pseudotyping.

We studied a single round of replication of Simian immunodeficiency virus (SIV) through the use of a replication defective vector that expresses the hygromycin resistance gene. It was possible to pseudotype SIV particles by complementation with the env gene from a murine amphotropic retrovirus. Moreover, SIV RNA was packaged and propagated by core particles of the heterologous lentivirus, HIV-1. These results indicate that coinfection of cells with SIV and other retroviruses could lead to infection of new cell types in nature.     

A structured retroviral RNA element that mediates nucleocytoplasmic export of intron-containing RNA.

A common feature of gene expression in all retroviruses is that unspliced, intron-containing RNA is exported to the cytoplasm despite the fact that cellular RNAs which contain introns are usually restricted to the nucleus. In complex retroviruses, the export of intron-containing RNA is mediated by specific viral regulatory proteins (e.g., human immunodeficiency virus type 1 [HIV-1] Rev) that bind to elements in the viral RNA. However, simpler retroviruses do not encode such regulatory proteins. Here we show that the genome of the simpler retrovirus Mason-Pfizer monkey virus (MPMV) contains an element that serves as an autonomous nuclear export signal for intron-containing RNA. This element is essential for MPMV replication; however, its function can be complemented by HIV-1 Rev and the Rev-responsive element. The element can also facilitate the export of cellular intron-containing RNA. These results suggest that the MPMV element mimics cellular RNA transport signals and mediates RNA export through interaction with endogenous cellular factors.     

A Stretch of Unpaired Purines in the Leader Region of Simian Immunodeficiency Virus (SIV) Genomic RNA is Critical for its Packaging into Virions

Simian immunodeficiency virus (SIV) is an important lentivirus used as a non-human primate model to study HIV replication, and pathogenesis of human AIDS, as well as a potential vector for human gene therapy. This study investigated the role of single-stranded purines (ssPurines) as potential genomic RNA (gRNA) packaging determinants in SIV replication. Similar ssPurines have been implicated as important motifs for gRNA packaging in many retroviruses like, HIV-1, MPMV, and MMTV by serving as Gag binding sites during virion assembly. In examining the secondary structure of the SIV 5′ leader region, as recently deduced using SHAPE methodology, we identified four specific stretches of ssPurines (I-IV) in the region that harbors major packaging determinants of SIV. The significance of these ssPurine motifs were investigated by mutational analysis coupled with a biologically relevant single round of replication assay. These analyses revealed that while ssPurine II was essential, the others (ssPurines I, III, & IV) did not significantly contribute to SIV gRNA packaging. Any mutation in the ssPurine II, such as its deletion or substitution, or other mutations that caused base pairing of ssPurine II loop resulted in near abrogation of RNA packaging, further substantiating the crucial role of ssPurine II and its looped conformation in SIV gRNA packaging. Structure prediction analysis of these mutants further corroborated the biological results and further revealed that the unpaired nature of ssPurine II is critical for its function during SIV RNA packaging perhaps by enabling it to function as a specific binding site for SIV Gag.     

Simian immunodeficiency virus RNA is efficiently encapsidated by human immunodeficiency virus type 1 particles.

Packaging of retroviral RNA is attained through the specific recognition of a cis-acting encapsidation site (located near the 5' end of the viral RNA) by components of the Gag precursor protein. Human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) are two lentiviruses that lack apparent sequence similarity in their putative encapsidation regions. We used SIV vectors to determine whether HIV-1 particles can recognize the SIV encapsidation site and functionally propagate SIV nucleic acid. SIV nucleic acid was replicated by HIV-1 proteins. Thus, efficient lentivirus pseudotyping can take place at the RNA level. Direct examination of the RNA contents of virus particles indicated that encapsidation of this heterologous RNA is efficient. Characterization of deletion mutants in the untranslated leader region of SIV RNA indicates that only a very short region at the 5' end of the SIV RNA is needed for packaging. Comparison of this region with the corresponding region of HIV-1 reveals that both are marked by secondary structures that are likely to be similar. Thus, it is likely that a similar higher-order RNA structure is required for encapsidation.     

A distinct African lentivirus from Sykes' monkeys.

Asymptomatic infection with simian immunodeficiency virus (SIV) has been demonstrated in African Sykes' monkeys (Cercopithecus mitis albogularis), and virus isolation confirmed infection with a novel SIV from Sykes' monkeys (SIVsyk). Macaques inoculated with SIVsyk became persistently infected but remained clinically healthy. We utilized polymerase chain reaction amplification to generate a full-length, infectious molecular clone of SIVsyk. The genome organization of SIVsyk is similar to that of the other primate lentiviruses, consisting of gag, pol, vif, vpr, tat, rev, env, and nef. A unique feature is the absence of the highly conserved NF-kappa B binding site in the long terminal repeat. SIVsyk is genetically equidistant from other primate lentiviruses. Thus, SIVsyk represents a new group that is distinct from the four previously recognized primate lentivirus groups: human immunodeficiency virus type 1 (HIV-1), SIV from sooty mangabeys (SIVsmm) and HIV-2, SIV from African green monkeys (SIVagm), and SIV from mandrills (SIVmnd). The genetic differences between SIVsyk and SIVagm, isolates derived from monkeys of the same genus, underscore the potential for other distinct SIVs which have yet to be isolated and characterized.     

Vectors derived from simian immunodeficiency virus (SIV)

In contrast to other retroviruses, lentiviruses have the unique property of infecting non-proliferating cells. Thus vectors derived from lentiviruses are promising tools for in vivo gene delivery applications. Vectors derived from human primate and non-primate lentiviruses have recently been described and, unlike retroviral vectors derived from murine leukemia viruses, lead to stable integration of the transgene into quiescent cells in various organs. Despite all the safety safeguards that have been progressively introduced in lentiviral vectors, the clinical acceptance of vectors derived from pathogenic lentiviruses is subject to debate. It is therefore essential to design vectors derived from a wide range of lentivirus types and to comparatively examine their properties in terms of transduction efficiency and bio-safety. Here, we review the properties of lentiviral vectors derived from simian immunodeficiency virus (SIV).     

Serological definition of the lentivirus group of retroviruses.

The major polypeptides of visna viruses and other lentiviruses have been isolated and shown to be closely related if not identical in radioimmunoassays. By this criterion the lentiviruses form a distinct group of retroviruses unrelated to spuma viruses, mammalian and avian retroviruses that cause tumors, and unclassified retroviruses of cattle and horses. Two sera obtained from goats immunized with Mason-Pfizer monkey virus or squirrel monkey virus reacted with visna p30. Additional data suggest that this reaction represents infection of goats with a lentivirus or a new retrovirus closely related to the lentiviruses.     

Xenoinfection of nonhuman primates by feline immunodeficiency virus

New viral infections in humans usually result from viruses that have been transmitted from other species as zoonoses. For example, it is accepted widely that human immunodeficiency virus (HIV) is the result of the propagation and adaptation of a simian immunodeficiency virus (SIV) from nonhuman primates to man [1]. Previously, we reported productive infection of primary human cells in vitro by feline immunodeficiency virus (FIV) [2], a lentivirus that causes an immunodeficiency syndrome in cats similar to HIV in humans [3]. The present study extends these findings by demonstrating that cynomolgus macaques (Macaca fasicularis) infected with FIV exhibited clinical signs, including depletion of CD4+ cells and weight loss, that are consistent with FIV infection. The development of an antibody response to FIV gag-encoded proteins and detection of virus-specific sequences in sera, blood-derived cells, and necropsied tissue accompanied these changes. Moreover, the reactivation of FIV replication from latently infected cells was observed after stimulation in vitro with phorbol esters and in vivo with tetanus toxoid. The proposed use of lentiviruses in human gene therapy [4, 5] and of nonhuman cells and organs in xenotransplantation [6] has raised concerns about zoonoses as potential sources of new human pathogens. Therefore, the study of FIV infection of primate cells may provide insight into the principles underlying retroviral xenoinfections.     

Restriction of feline immunodeficiency virus by Ref1, Lv1, and primate TRIM5alpha proteins

The Ref1 and Lv1 postentry restrictions in human and monkey cells have been analyzed for lentiviruses in the primate and ungulate groups, but no data exist for the third (feline) group. We compared feline immunodeficiency virus (FIV) to other restricted (human immunodeficiency virus type 1 [HIV-1], equine infectious anemia virus [EIAV]) and unrestricted (NB-tropic murine leukemia virus [NB-MLV]) retroviruses across wide ranges of viral inputs in cells from multiple primate and nonprimate species. We also characterized restrictions conferred to permissive feline and canine cells engineered to express rhesus and human TRIM5alpha proteins and performed RNA interference (RNAi) against endogenous TRIM5alpha. We find that expression of rhesus or human TRIM5alpha proteins in feline cells restricts FIV, impairing pseudotyped vector transduction and viral replication, but rhesus TRIM5alpha is more restricting than human TRIM5alpha. Notably, however, canine cells did not support restriction by human TRIM5alpha and supported minimal restriction by rhesus TRIM5alpha, suggesting that these proteins may not function autonomously or that a canine factor interferes. Stable RNAi knockdown of endogenous rhesus TRIM5alpha resulted in marked increases in FIV and HIV-1 infectivities while having no effect on NB-MLV. A panel of nonprimate cell lines varied widely in susceptibility to lentiviral vector transduction, but normalized FIV and HIV-1 vectors varied concordantly. In contrast, in human and monkey cells, relative restriction of FIV compared to HIV-1 varied from none to substantial, with the greatest relative infectivity deficit for FIV vectors observed in human T-cell lines. Endogenous and introduced TRIM5alpha restrictions of FIV could be titrated by coinfections with FIV, HIV-1, or EIAV virus-like particles. Arsenic trioxide had complex and TRIM5alpha-independent enhancing effects on lentiviral but not NB-MLV infection. Implications for human gene therapy are discussed.     

Differential restriction of human immunodeficiency virus type 2 and simian immunodeficiency virus SIVmac by TRIM5alpha alleles

Primate lentiviruses have narrow host ranges, due in part to their sensitivities to mammalian intracellular antiviral factors such as APOBEC3G and TRIM5alpha. Despite the protection provided by this innate immune system, retroviruses are able to transfer between species where they can cause disease. This is true for sooty mangabey simian immunodeficiency virus, which has transferred to humans as HIV-2 and to rhesus macaques as SIVmac, where it causes AIDS. Here we examine the sensitivities of the closely related HIV-2 and SIVmac to restriction by TRIM5alpha. We show that rhesus TRIM5alpha can restrict HIV-2 but not the closely related SIVmac. SIVmac has not completely escaped TRIM5alpha, as shown by its sensitivity to distantly related TRIM5alpha from the New World squirrel monkey. Squirrel monkey TRIM5alpha blocks SIVmac infection after DNA synthesis and is not saturable with restriction-sensitive virus-like particles. We map the determinant for TRIM5alpha sensitivity to the structure in the capsid protein that recruits CypA into HIV-1 virions. We also make an SIV, mutated at this site, which bypasses restriction in all cells tested.     

Characterization of a Novel Simian Immunodeficiency Virus (SIV) from L’Hoest Monkeys (Cercopithecus l’hoesti): Implications for the Origins of SIVmnd and Other Primate Lentiviruses

The human immunodeficiency virus types 1 and 2 (HIV-1 and HIV-2) appear to have originated by cross-species transmission of simian immunodeficiency virus (SIV) from asymptomatically infected African primates. Few of the SIVs characterized to date efficiently infect human primary lymphocytes. Interesting, two of the three identified to infect such cultures (SIVsm and SIVcpz) have appeared in human populations as genetically related HIVs. In the present study, we characterized a novel SIV isolate from an East African monkey of the Cercopithecus genus, the l’hoest monkey (C. l’hoesti), which we designated SIVlhoest. This SIV isolate efficiently infected both human and macaque lymphocytes and resulted in a persistent infection of macaques, characterized by high primary virus load and a progressive decline in circulating CD4 lymphocytes, consistent with progression to AIDS. Phylogenetic analyses showed that SIVlhoest is genetically distinct from other previously characterized primate lentiviruses but clusters in the same major lineage as SIV from mandrills (SIVmnd), a West African primate species. Given the geographic distance between the ranges of l’hoest monkeys and mandrills, this may indicate that SIVmnd arose through cross-species transmission from close relatives of l’hoest monkeys that are sympatric with mandrills. These observations lend support to the hypothesis that the primate lentiviruses originated and coevolved within monkeys of the Cercopithecus genus. Regarded in this light, lentivirus infections of primates not belonging to the Cercopithecus genus may have resulted from cross-species transmission in the not-too-distant past.     

Evolution of lentiviruses and receptor specificity

Lentiviruses consist of primate lentiviruses, ungulate lentiviruses and feline immunodeficiency virus (FIV). The primate lentiviruses utilize CD4 and chemokine receptors as a primary receptor and coreceptors, respectively. Recently we found that FIV utilizes CD134 and CXCR4 as a primary receptor and a coreceptor, respectively. FIV utilizes feline CD134 but not human CD134, whereas it can utilize both feline and human CXCR4. Exceptionally an FIV laboratory strain can infect human cells via CXCR4 only by the CD134-independent manner. Similarly several strains of primate lentiviruses also infect cells by the CD4-independent manner. In this review, the evolution of the lentiviruses and possible mechanism for lentiviral cross-species transmission is discussed.