Transient virus infection and pathogenesis of a new HIV type 2 isolate, UC12, in baboons

Impact of TRIM5α in vivo

HIV type 1 (HIV-1) has a very narrow host range that is limited to humans and chimpanzees. HIV-1 cannot replicate well in Old World monkey cells such as rhesus and cynomolgus monkeys. Tripartite motif (TRIM)5α is a key molecule that confers potent resistance against HIV-1 infection and is composed of really interesting new gene, B-box2, coiled-coil and PRYSPRY domains. Interaction between TRIM5α PRYSPRY domains and HIV-1 capsid core triggers the anti-HIV-1 activity of TRIM5α. Analysis of natural HIV variants and extensive mutational experiments has revealed the presence of critical amino acid residues in both the PRYSPRY domain and HIV capsid for potent HIV suppression by TRIM5α. Genetic manipulation of the human TRIM5 gene could establish human cells totally resistant to HIV-1, which may lead to a cure for HIV-1 infection in the future.

Electrostatic potential of human immunodeficiency virus type 2 and rhesus macaque simian immunodeficiency virus capsid proteins

Human immunodeficiency virus type 2 (HIV-2) and simian immunodeficiency virus isolated from a macaque monkey (SIVmac) are assumed to have originated from simian immunodeficiency virus isolated from sooty mangabey (SIVsm). Despite their close similarity in genome structure, HIV-2 and SIVmac show different sensitivities to TRIM5α, a host restriction factor against retroviruses. The replication of HIV-2 strains is potently restricted by rhesus (Rh) monkey TRIM5α, while that of SIVmac strain 239 (SIVmac239) is not. Viral capsid protein is the determinant of this differential sensitivity to TRIM5α, as the HIV-2 mutant carrying SIVmac239 capsid protein evaded Rh TRIM5α-mediated restriction. However, the molecular determinants of this restriction mechanism are unknown. Electrostatic potential on the protein-binding site is one of the properties regulating protein-protein interactions. In this study, we investigated the electrostatic potential on the interaction surface of capsid protein of HIV-2 strain GH123 and SIVmac239. Although HIV-2 GH123 and SIVmac239 capsid proteins share more than 87% amino acid identity, we observed a large difference between the two molecules with the HIV-2 GH123 molecule having predominantly positive and SIVmac239 predominantly negative electrostatic potential on the surface of the loop between α-helices 4 and 5 (L4/5). As L4/5 is one of the major determinants of Rh TRIM5α sensitivity of these viruses, the present results suggest that the binding site of the Rh TRIM5α may show complementarity to the HIV-2 GH123 capsid surface charge distribution.

Role of Human TRIM5α in Intrinsic Immunity

Human immunodeficiency virus (HIV) has a very narrow host range. HIV type 1 (HIV-1) does not infect Old World monkeys, such as the rhesus monkey (Rh). Rh TRIM5α was identified as a factor that confers resistance, intrinsic immunity, to HIV-1 infection. Unfortunately, human TRIM5α is almost powerless to restrict HIV-1. However, human TRIM5α potently restricts N-tropic murine leukemia viruses (MLV) but not B-tropic MLV, indicating that human TRIM5α represents the restriction factor previously designated as Ref1. African green monkey TRIM5α represents another restriction factor previously designated as Lv1, which restricts both HIV-1 and simian immunodeficiency virus isolated from macaque (SIVmac) infection. TRIM5 is a member of the tripartite motif family containing RING, B-box2, and coiled-coil domains. The RING domain is frequently found in E3 ubiquitin ligase, and TRIM5α is thought to degrade viral core via ubiquitin–proteasome-dependent and -independent pathways. The alpha isoform of TRIM5 has an additional C-terminal PRYSPRY domain, which is a determinant of species-specific retrovirus restriction by TRIM5α. On the other hand, the target regions of viral capsid protein (CA) are scattered on the surface of core. A single amino acid difference in the surface-exposed loop between α-helices 6 and 7 (L6/7) of HIV type 2 (HIV-2) CA affects viral sensitivity to human TRIM5α and was also shown to be associated with viral load in West African HIV-2 patients, indicating that human TRIM5α is a critical modulator of HIV-2 replication in vivo. Interestingly, L6/7 of CA corresponds to the MLV determinant of sensitivity to mouse factor Fv1, which potently restricts N-tropic MLV. In addition, human genetic polymorphisms also affect antiviral activity of human TRIM5α. Recently, human TRIM5α was shown to activate signaling pathways that lead to activation of NF-κB and AP-1 by interacting with TAK1 complex. TRIM5α is thus involved in control of viral infection in multiple ways.

TRIM5α and Species Tropism of HIV/SIV

Human immunodeficiency virus type 1 (HIV-1) infects humans and chimpanzees but not old world monkeys (OWMs) such as the rhesus monkey (Rh) and cynomolgus monkey (CM). HIV-1 efficiently enters cells of OWMs but encounters a block before reverse transcription. This narrow host range is attributed to a barrier in the host cell. In 2004, the screening of a Rh cDNA library identified tripartite motif 5α (TRIM5α) as a cellular antiviral factor. TRIM5α is one of splicing variants produced by TRIM5 gene and TRIM5 proteins are members of the TRIM family containing RING, B-box 2, and coiled-coil domains. The RING domain is frequently found in E3 ubiquitin ligase and TRIM5α is degraded via the ubiquitin–proteasome-dependent pathway. Among TRIM5 splicing variants, TRIM5α alone has an additional C-terminal PRYSPRY (B30.2) domain. Previous studies have shown that sequence variation in variable regions of the PRYSPRY domain among different monkey species affects species-specific retrovirus infection, while amino acid sequence differences in the viral capsid protein determine viral sensitivity to restriction. TRIM5α recognizes the multimerized capsid proteins (viral core) of an incoming virus by its PRYSPRY domain and is thus believed to control retroviral infection. There are significant intraspecies variations in the Rh-TRIM5 gene. It has also been reported that some Rh and CM individuals have retrotransposed cyclophilin A open reading frame in the TRIM5 gene, which produces TRIM5–cyclophilin A fusion protein (TRIMCyp). TRIMCyp, which was originally identified as an anti-HIV-1 factor of New World owl monkeys, is an interesting example of the gain of a new function by retrotransposition. As different TRIM5 genotypes of Rh showed different levels of simian immunodeficiency virus replication in vivo, the TRIM5 genotyping is thought to be important in acquired immunodeficiency syndrome monkey models.

Multiple sites in the N-terminal half of simian immunodeficiency virus capsid protein contribute to evasion from rhesus monkey TRIM5α-mediated restriction

BACKGROUND

We previously reported that cynomolgus monkey (CM) TRIM5α could restrict human immunodeficiency virus type 2 (HIV-2) strains carrying a proline at the 120th position of the capsid protein (CA), but it failed to restrict those with a glutamine or an alanine. In contrast, rhesus monkey (Rh) TRIM5α could restrict all HIV-2 strains tested but not simian immunodeficiency virus isolated from macaque (SIVmac), despite its genetic similarity to HIV-2.

RESULTS

We attempted to identify the viral determinant of SIVmac evasion from Rh TRIM5α-mediated restriction using chimeric viruses formed between SIVmac239 and HIV-2 GH123 strains. Consistent with a previous study, chimeric viruses carrying the loop between α-helices 4 and 5 (L4/5) (from the 82nd to 99th amino acid residues) of HIV-2 CA were efficiently restricted by Rh TRIM5α. However, the corresponding loop of SIVmac239 CA alone (from the 81st to 97th amino acid residues) was not sufficient to evade Rh TRIM5α restriction in the HIV-2 background. A single glutamine-to-proline substitution at the 118th amino acid of SIVmac239 CA, corresponding to the 120th amino acid of HIV-2 GH123, also increased susceptibility to Rh TRIM5α, indicating that glutamine at the 118th of SIVmac239 CA is necessary to evade Rh TRIM5α. In addition, the N-terminal portion (from the 5th to 12th amino acid residues) and the 107th and 109th amino acid residues in α-helix 6 of SIVmac CA are necessary for complete evasion from Rh TRIM5α-mediated restriction. A three-dimensional model of hexameric GH123 CA showed that these multiple regions are located on the CA surface, suggesting their direct interaction with TRIM5α.

CONCLUSION

We found that multiple regions of the SIVmac CA are necessary for complete evasion from Rh TRIM5α restriction.

Anti-retroviral activity of TRIM5 alpha

Human immunodeficiency virus type 1 (HIV-1) shows a very narrow host range limited to humans and chimpanzees. Experimentally, HIV-1 does not infect Old World monkeys, such as rhesus (Rh) and cynomolgus (CM) monkeys, and fails to replicate in activated CD4 positive T lymphocytes obtained from these monkeys. In contrast, simian immunodeficiency virus isolated from a macaque monkey (SIVmac) can replicate well in both Rh and CM. In 2004, tripartite motif 5 alpha (TRIM5 alpha) was identified as a host factor which plays an important role in the restricted host range of HIV-1. Rh and CM TRIM5 alpha restrict HIV-1 infection but not SIVmac, while in comparison, anti-viral activity of human TRIM5 alpha against those viruses is very weak. TRIM5 alpha consists of the RING, B-box 2, coiled-coil and SPRY (B30.2) domains. The RING domain is frequently found in E3 ubiquitin ligase and TRIM5 alpha is degraded via the ubiquitin-proteasome pathway during HIV-1 restriction. TRIM5 alpha recognises the multimerised capsid (viral core) of an incoming virus by its alpha-isoform specific SPRY domain and is believed to be involved in innate immunity to control retroviral infection. Differences in amino acid sequences in the SPRY domain of TRIM5 alpha of different monkey species were found to affect species-specific restriction of retrovirus infection, while differences in amino acid sequences in the viral capsid protein determine viral sensitivity to restriction. Accurate structural analysis of the binding surface between the viral capsid protein and TRIM5 alpha SPRY is thus required for the development of new antiretroviral drugs that enhance anti-HIV-1 activity of human TRIM5 alpha.

A prime-boost immunisation regimen using recombinant BCG and Pr55(gag) virus-like particle vaccines based on HIV type 1 subtype C successfully elicits Gag-specific responses in baboons

Mycobacterium bovis BCG is considered an attractive live bacterial vaccine vector. In this study, we investigated the immune response of baboons to a primary vaccination with recombinant BCG (rBCG) constructs expressing the gag gene from a South African HIV-1 subtype C isolate, and a boost with HIV-1 subtype C Pr55(gag) virus-like particles (Gag VLPs). Using an interferon enzyme-linked immunospot assay, we show that although these rBCG induced only a weak or an undetectable HIV-1 Gag-specific response on their own, they efficiently primed for a Gag VLP boost, which strengthened and broadened the immune responses. These responses were predominantly CD8+ T cell-mediated and recognised similar epitopes as those targeted by humans with early HIV-1 subtype C infection. In addition, a Gag-specific humoral response was elicited. These data support the development of HIV-1 vaccines based on rBCG and Pr55(gag) VLPs.

A single amino acid of the human immunodeficiency virus type 2 capsid affects its replication in the presence of cynomolgus monkey and human TRIM5alphas

Human immunodeficiency virus type 2 (HIV-2) strains vary widely in their abilities to grow in Old World monkey (OWM) cells such as those of cynomolgus monkeys (CM). We evaluated eight HIV-2 isolates for their sensitivities to CM TRIM5alpha, an anti-HIV factor in OWM cells. We found that different HIV-2 isolates showed differences in their sensitivities to CM TRIM5alpha. Sequence analysis showed that TRIM5alpha-sensitive viruses had proline at the 120th position of the capsid protein (CA), whereas TRIM5alpha-resistant viruses had either alanine or glutamine. Mutagenesis studies indicated that the single amino acid at the 120th position indeed affected the sensitivity of the virus to CM TRIM5alpha.

Baboon as a nonhuman primate model for vaccine studies

Nonhuman primates are suitable and valid models to determine the safety, immunogenicity and efficacy of candidate vaccines that are being developed for human application. Among the nonhuman primates, baboons have been found to be ideal for vaccine studies because they are phylogenetically closer to humans, have a similar immune system and response and are available in large numbers. During the last two decades, numerous candidate vaccines have been evaluated in the baboon model.

Human immunodeficiency virus type 2 DNA vaccine provides partial protection from acute baboon infection

We determined if the genetic adjuvants, granulocyte-macrophage colony stimulating factor (GM-CSF) and B7-2, could improve the immunogenicity and efficacy of an HIV-2 DNA vaccine. The vaccine consisted of the HIV-2 tat, nef, gag, and env genes synthesized using optimized codons and formulated with cationic liposomes. Baboons (Papio cynocephalus hamadryas) were immunized by the intramuscular, intradermal, and intranasal routes with these expression constructs and challenged with HIV-2(UC2) by the intravaginal route. In the first month after HIV-2 vaginal challenge, the baboons receiving the HIV-2 DNA vaccine with or without the genetic adjuvants had significant reductions in the viral loads in the peripheral blood mononuclear cells (PBMC) (P = 0.028) while the reductions in their plasma viremia were suggestive of a protective effect (P = 0.1). These data demonstrate that partial protection against HIV-2 vaginal challenge, as measured by reduced viral load, can be achieved using only a DNA vaccine formulation.

Novel Kaposi's sarcoma-associated herpesvirus homolog in baboons

Kaposi's sarcoma (KS) and lymphoproliferative diseases induced by KS-associated herpesvirus (KSHV/human herpesvirus 8) cause substantial morbidity and mortality in human immunodeficiency virus-infected individuals. To understand KSHV biology it is useful to investigate closely related rhadinoviruses naturally occurring in nonhuman primates. Here we report evidence for a novel KSHV homolog in captive baboon species (Papio anubis and other). Using degenerate PCR we identified a novel rhadinovirus, PapRV2, that has substantial sequence identity to two essential KSHV genes, the viral polymerase and thymidylate synthase. A subset of animals exhibited detectable PapRV2 viral load in peripheral blood mononuclear cells. Extensive serological analysis of nearly 200 animals in the colony demonstrated that the majority carried cross-reacting antibodies that recognize KSHV or macaque rhadinovirus antigens. Seroreactivity increased with age, similar to the age-specific prevalence of KSHV in the human population. This establishes baboons as a novel resource to investigate rhadinovirus biology, which can be developed into an animal model system for KSHV-associated human diseases, vaccine development, and therapy evaluation.

Clearance of a productive lentivirus infection in calves experimentally inoculated with caprine arthritis-encephalitis virus

Lentiviruses have long been considered host-specific pathogens, but several recent observations demonstrated their capacity to conquer new hosts from different species, genera, and families. From these cross-species infections emerged new animal and human infectious diseases. The successful colonization and adaptation of a lentivirus to a nonnatural host depends on unspecific and specific host barriers. Some of those barriers exert a relative control of viral replication (i.e., cytotoxic T-lymphocyte response, viral inhibitory factors), but none of them was found able to totally clear the infection once the retrovirus is fully adapted in its host. In this study we examined the evolution of the host-lentivirus interactions occurring in an experimental animal model of cross-species infection in order to analyze the efficiency of those barriers in preventing the establishment of a persistent infection. Five newborn calves were inoculated with caprine arthritis-encephalitis virus (CAEV), and the evolution of infection was studied for more than 12 months. All the animals seroconverted in the first 0.75 to 1 month following the inoculation and remained seropositive for the remaining time of the experiment. Viral infection was productive during 4 months with isolation of replication competent virus from the blood cells and organs of the early euthanized animals. After 4 months of infection, neither replication-competent virus nor virus genome could be detected in blood cells or in the classical target organs, even after an experimental immunosuppression. No evidence of in vitro restriction of CAEV replication was observed in cells from tissues explanted from organs of these calves. These data provide the demonstration of a natural clearance of lentivirus infection following experimental inoculation of a nonnatural host, enabling perspectives of development of new potential vaccine strategies to fight against lentivirus infections.

Expression patterns of phenotypic markers on lymphocytes from human immunodeficiency virus type 2-infected baboons

The development of AIDS in HIV-1-infected humans is associated with profound changes in the expression patterns of lymphocyte phenotypic markers associated with increased immune activation and with decreased recall immune responses. In assessing these immunologic changes in an animal model, we characterized the expression patterns of immune activation markers on lymphocyte subsets during the acute, chronic, and end stages of HIV-2 infection in baboons. Using flow cytometry, we identified 21 human-specific monoclonal antibodies that were cross-reactive with baboon lymphocytes; however, expression of only 2 of these markers was altered significantly after HIV-2 infection. We found an increase in baboon class II antigen (as measured by anti-HLA-DR) in the CD4(+) T cell subset within 8 weeks of infection (p = 0.045). Moreover, after 1 year of infection, CD11b was downregulated on CD8(+) T lymphocytes (p = 0.027). This downregulation of CD11b was consistently observed in all of the groups of baboons that were chronically infected with three different HIV-2 isolates. In addition, we found substantial downregulation of the interleukin 2 receptor (CD25) and upregulation of class II antigen on CD8(+) lymphocytes in a baboon with an AIDS-like disease. These and other phenotypic markers of immune activation may facilitate characterization of the immunopathogenesis of AIDS in nonhuman primate animal models.

Increased virus replication and virulence after serial passage of human immunodeficiency virus type 2 in baboons

Similar to human immunodeficiency virus type 1 (HIV-1) infection of humans, the natural history of HIV-2 infection in baboons (Papio cynocephalus) is a slow and chronic disease that generally takes several years before an AIDS-like condition develops. To shorten the amount of time to the development of disease, we performed five serial passages of HIV-2(UC2) in baboons by using blood and bone marrow samples during the acute phase of infection when viral loads were at high levels. After these serial passages, virus levels in plasma, peripheral blood mononuclear cells (PBMC) and lymphatic tissues in the acutely infected baboons were increased. Within 1 year of the HIV-2 infection, all of the inoculated baboons showed specific signs of AIDS-related disease progression within the lymphatic tissues, such as vascular proliferation and lymphoid depletion. The HIV-2(UC2) recovered after four serial passages showed increased kinetics of viral replication in baboon PBMC and cytopathicity. This study suggests that the HIV-2 isolate recovered after several serial passages in baboons will be useful in future studies of AIDS pathogenesis and vaccine development by using this animal model.

Feline immunodeficiency virus clade C mucosal transmission and disease courses

The transmissibility and pathogenicity of a clade C feline immunodeficiency virus (FIV-C) was examined via the oral-nasal, vaginal, or rectal mucosa. FIV-C was transmissible by all three mucosal routes. Vaginal transmission was most efficient (100%), oral exposure resulted in a 80% infection rate, and rectal transmission was least effective (44%). In contrast to previous intravenous passage studies, a broader range of host-virus relationships was observed after mucosal exposure. Three categories of FIV-C infection were defined: (1) rapidly progressive infection marked by high virus burdens and rapid CD4+ cell depletion (43% of vaginally exposed animals); (2) conventional (typical) infection featuring slowly progressive CD4+ cell decline (61% of all exposed animals); and (3) regressive (transient) infection marked by low and then barely detectable virus burdens and no CD4+ cell alterations (22% of rectally inoculated cats). These disease courses appear to have parallels in mucosal HIV and SIV infections, emphasizing the importance of the virus-mucosa interface in lentiviral pathogenesis.