Geographical, genetic and functional diversity of antiretroviral host factor TRIMCyp in cynomolgus macaque (Macaca fascicularis)

Development of a novel Macaque-Tropic HIV-1 adapted to cynomolgus macaques

Macaque-tropic HIV-1 (HIV-1mt) variants have been developed to establish preferable primate models that are advantageous in understanding HIV-1 infection pathogenesis and in assessing the preclinical efficacy of novel prevention/treatment strategies. We previously reported that a CXCR4-tropic HIV-1mt, MN4Rh-3, efficiently replicates in peripheral blood mononuclear cells (PBMCs) of cynomolgus macaques homozygous for TRIMCyp (CMs^TC). However, the CMs^TC challenged with MN4Rh-3 displayed low viral loads during the acute infection phase and subsequently exhibited short-term viremia. These virological phenotypes in vivo differed from those observed in most HIV-1-infected people. Therefore, further development of the HIV-1mt variant was needed. In this study, we first reconstructed the MN4Rh-3 clone to produce a CCR5-tropic HIV-1mt, AS38. In addition, serial in vivo passages allowed us to produce a highly adapted AS38-derived virus that exhibits high viral loads (up to approximately 10⁶ copies ml^-1) during the acute infection phase and prolonged periods of persistent viremia (lasting approximately 16 weeks postinfection) upon infection of CMs^TC. Whole-genome sequencing of the viral genomes demonstrated that the emergence of a unique 15-nt deletion within the vif gene was associated with in vivo adaptation. The deletion resulted in a significant increase in Vpr protein expression but did not affect Vif-mediated antagonism of antiretroviral APOBEC3s, suggesting that Vpr is important for HIV-1mt adaptation to CMs^TC. In summary, we developed a novel CCR5-tropic HIV-1mt that can induce high peak viral loads and long-term viremia and exhibits increased Vpr expression in CMs^TC.

HIV-1 capsid variability: viral exploitation and evasion of capsid-binding molecules

The HIV-1 capsid, a conical shell encasing viral nucleoprotein complexes, is involved in multiple post-entry processes during viral replication. Many host factors can directly bind to the HIV-1 capsid protein (CA) and either promote or prevent HIV-1 infection. The viral capsid is currently being explored as a novel target for therapeutic interventions. In the past few decades, significant progress has been made in our understanding of the capsid–host interactions and mechanisms of action of capsid-targeting antivirals. At the same time, a large number of different viral capsids, which derive from many HIV-1 mutants, naturally occurring variants, or diverse lentiviruses, have been characterized for their interactions with capsid-binding molecules in great detail utilizing various experimental techniques. This review provides an overview of how sequence variation in CA influences phenotypic properties of HIV-1. We will focus on sequence differences that alter capsid–host interactions and give a brief account of drug resistant mutations in CA and their mutational effects on viral phenotypes. Increased knowledge of the sequence-function relationship of CA helps us deepen our understanding of the adaptive potential of the viral capsid.

Multiple Pathways To Avoid Beta Interferon Sensitivity of HIV-1 by Mutations in Capsid

HIV-1 infection causes robust innate immune activation in virus-infected patients. This immune activation is characterized by elevated levels of type I interferons (IFNs), which can block HIV-1 replication. Recent studies suggest that the viral capsid protein (CA) is a determinant for the sensitivity of HIV-1 to IFN-mediated restriction. Specifically, it was reported that the loss of CA interactions with CPSF6 or CypA leads to higher IFN sensitivity. However, the molecular mechanism of CA adaptation to IFN sensitivity is largely unknown. Here, we experimentally evolved an IFN-β-hypersensitive CA mutant which showed decreased binding to CPSF6 and CypA in IFN-β-treated cells. The CA mutations that emerged from this adaptation indeed conferred IFN-β resistance. Our genetic assays suggest a limited contribution of known host factors to IFN-β resistance. Strikingly, one of these mutations accelerated the kinetics of reverse transcription and uncoating. Our findings suggest that HIV-1 selected multiple, known host factor-independent pathways to avoid IFN-β-mediated restriction.

Type I interferons (IFNs), including alpha IFN (IFN-α) and IFN-β, potently suppress HIV-1 replication by upregulating IFN-stimulated genes (ISGs). The viral capsid protein (CA) partly determines the sensitivity of HIV-1 to IFNs. However, it remains to be determined whether CA-related functions, including utilization of known host factors, reverse transcription, and uncoating, affect the sensitivity of HIV-1 to IFN-mediated restriction. Recently, we identified an HIV-1 CA variant that is unusually sensitive to IFNs. This variant, called the RGDA/Q112D virus, contains multiple mutations in CA: H87R, A88G, P90D, P93A, and Q112D. To investigate how an IFN-hypersensitive virus can evolve to overcome IFN-β-mediated blocks targeting the viral capsid, we adapted the RGDA/Q112D virus in IFN-β-treated cells. We successfully isolated IFN-β-resistant viruses which contained either a single Q4R substitution or the double amino acid change G94D/G116R. These two IFN-β resistance mutations variably changed the sensitivity of CA binding to human myxovirus resistance B (MxB), cleavage and polyadenylation specificity factor 6 (CPSF6), and cyclophilin A (CypA), indicating that the observed loss of sensitivity was not due to interactions with these known host CA-interacting factors. In contrast, the two mutations apparently functioned through distinct mechanisms. The Q4R mutation dramatically accelerated the kinetics of reverse transcription and initiation of uncoating of the RGDA/Q112D virus in the presence or absence of IFN-β, whereas the G94D/G116R mutations affected reverse transcription only in the presence of IFN-β, most consistent with a mechanism of the disruption of binding to an unknown IFN-β-regulated host factor. These results suggest that HIV-1 can exploit multiple, known host factor-independent pathways to avoid IFN-β-mediated restriction by altering capsid sequences and subsequent biological properties.

IMPORTANCE HIV-1 infection causes robust innate immune activation in virus-infected patients. This immune activation is characterized by elevated levels of type I interferons (IFNs), which can block HIV-1 replication. Recent studies suggest that the viral capsid protein (CA) is a determinant for the sensitivity of HIV-1 to IFN-mediated restriction. Specifically, it was reported that the loss of CA interactions with CPSF6 or CypA leads to higher IFN sensitivity. However, the molecular mechanism of CA adaptation to IFN sensitivity is largely unknown. Here, we experimentally evolved an IFN-β-hypersensitive CA mutant which showed decreased binding to CPSF6 and CypA in IFN-β-treated cells. The CA mutations that emerged from this adaptation indeed conferred IFN-β resistance. Our genetic assays suggest a limited contribution of known host factors to IFN-β resistance. Strikingly, one of these mutations accelerated the kinetics of reverse transcription and uncoating. Our findings suggest that HIV-1 selected multiple, known host factor-independent pathways to avoid IFN-β-mediated restriction.

Simian immunodeficiency virus SIVmac239 infection and simian human immunodeficiency virus SHIV89.6P infection result in progression to AIDS in cynomolgus macaques of Asian origin

Simian immunodeficiency virus (SIV) infection models in cynomolgus macaques are important for analysis of the pathogenesis of immunodeficiency virus and for studies on the efficacy of new vaccine candidates. However, very little is known about the pathogenesis of SIV or simian human immunodeficiency virus (SHIV) in cynomolgus macaques from different Asian countries. In the present study, we analysed the infectivity and pathogenicity of CCR5-tropic SIVmac and those of dual-tropic SHIV89.6P inoculated into cynomolgus macaques in Indonesian, Malaysian or Philippine origin. The plasma viral loads in macaques infected with either SIVmac239 or SHIV89.6P were maintained at high levels. CD4+ T cell levels in macaques infected with SIVmac239 gradually decreased. All of the macaques infected with SHIV89.6P showed greatly reduced CD4+ T-cell numbers within 6 weeks of infection. Eight of the 11 macaques infected with SIVmac239 were killed due to AIDS symptoms after 2-4.5 years, while four of the five macaques infected with SHIV89.6P were killed due to AIDS symptoms after 1-3.5 years. We also analysed cynomolgus macaques infected intrarectally with repeated low, medium or high doses of SIVmac239, SIVmac251 or SHIV89.6P. Infection was confirmed by quantitative RT-PCR at more than 5000, 300 and 500 TCID50 for SIVmac239, SIVmac251 and SHIV89.6P, respectively. The present study indicates that cynomolgus macaques of Asian origin are highly susceptible to SIVmac and SHIV infection by both intravenous and mucosal routes. These models will be useful for studies on virus pathogenesis, vaccination and therapeutics against human immunodeficiency virus/AIDS.

Phylogeny and History of the Lost SIV from Crab-Eating Macaques: SIVmfa

In the 20^th century, thirteen distinct human immunodeficiency viruses emerged following independent cross-species transmission events involving simian immunodeficiency viruses (SIV) from African primates. In the late 1900s, pathogenic SIV strains also emerged in the United Sates among captive Asian macaque species following their unintentional infection with SIV from African sooty mangabeys (SIVsmm). Since their discovery in the 1980s, SIVs from rhesus macaques (SIVmac) and pig-tailed macaques (SIVmne) have become invaluable models for studying HIV pathogenesis, vaccine design and the emergence of viruses. SIV isolates from captive crab-eating macaques (SIVmfa) were initially described but lost prior to any detailed molecular and genetic characterization. In order to infer the origins of the lost SIVmfa lineage, we located archived material and colony records, recovered its genomic sequence by PCR, and assessed its phylogenetic relationship to other SIV strains. We conclude that SIVmfa is the product of two cross-species transmission events. The first was the established transmission of SIVsmm to rhesus macaques, which occurred at the California National Primate Research Center in the late 1960s and the virus later emerged as SIVmac. In a second event, SIVmac was transmitted to crab-eating macaques, likely at the Laboratory for Experimental Medicine and Surgery in Primates in the early 1970s, and it was later spread to the New England Primate Research Center colony in 1973 and eventually isolated in 1986. Our analysis suggests that SIVmac had already emerged by the early 1970s and had begun to diverge into distinct lineages. Furthermore, our findings suggest that pathogenic SIV strains may have been more widely distributed than previously appreciated, raising the possibility that additional isolates may await discovery.

Novel mutant human immunodeficiency virus type 1 strains with high degree of resistance to cynomolgus macaque TRIMCyp generated by random mutagenesis

Old World monkey TRIM5α strongly suppresses human immunodeficiency virus type 1 (HIV-1) replication. A fusion protein comprising cynomolgus macaque (CM) TRIM5 and cyclophilin A (CM TRIMCyp) also potently suppresses HIV-1 replication. However, CM TRIMCyp fails to suppress a mutant HIV-1 that encodes a mutant capsid protein containing a SIVmac239-derived loop between α-helices 4 and 5 (L4/5). There are seven amino acid differences between L4/5 of HIV-1 and SIVmac239. Here, we investigated the minimum numbers of amino acid substitutions that would allow HIV-1 to evade CM TRIMCyp-mediated suppression. We performed random PCR mutagenesis to construct a library of HIV-1 variants containing mutations in L4/5, and then we recovered replication-competent viruses from CD4+ MT4 cells that expressed high levels of CM TRIMCyp. CM TRIMCyp-resistant viruses were obtained after three rounds of selection in MT4 cells expressing CM TRIMCyp and these were found to contain four amino acid substitutions (H87R, A88G, P90D and P93A) in L4/5. We then confirmed that these substitutions were sufficient to confer CM TRIMCyp resistance to HIV-1. In a separate experiment using a similar method, we obtained novel CM TRIM5α-resistant HIV-1 strains after six rounds of selection and rescue. Analysis of these mutants revealed that V86A and G116E mutations in the capsid region conferred partial resistance to CM TRIM5α without substantial fitness cost when propagated in MT4 cells expressing CM TRIM5α. These results confirmed and further extended the previous notion that CM TRIMCyp and CM TRIM5α recognize the HIV-1 capsid in different manners.

The effects of TRIM5α polymorphism on HIV-2ROD and SIVmac239 replication in PBMCs from Chinese rhesus macaques and Vietnamese-origin cynomolgus macaques

Because of the difficulty of obtaining Indian-origin rhesus macaques, Chinese-origin rhesus macaques (CR) and Vietnamese-origin cynomolgus macaques (CM) are now used frequently in HIV/AIDS research. Nonetheless, the effects of TRIM5α polymorphism on viral replication in both CR and CM are unclear. To address these questions, we recruited 70 unrelated CR and 40 unrelated CM and studied the effect of TRIM5α polymorphism on HIV-2ROD and SIVmac239 replication in PBMCs. We found that 3 polymorphisms, located in the B30.2 domain of CR TRIM5α formed a haplotype and affected HIV-2ROD replication. In addition, we found that the variant Y178H, located in the Coiled-coil domain of CM TRIM5α, affected TRIM5α-mediated HIV-2ROD restriction. Finally, two polymorphisms, located in the Coiled-coil domain, altered anti-SIVmac239 activity in CR. We concluded that, CM TRIM5α polymorphism could alter HIV-2ROD infection; however, a different domain of CR TRIM5α was responsible for restricting different virus replication.

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.

Nonhuman primate models for HIV/AIDS vaccine development

The development of HIV vaccines has been hampered by the lack of an animal model that can accurately predict vaccine efficacy. Chimpanzees can be infected with HIV-1 but are not practical for research. However, several species of macaques are susceptible to the simian immunodeficiency viruses (SIVs) that cause disease in macaques, which also closely mimic HIV in humans. Thus, macaque-SIV models of HIV infection have become a critical foundation for AIDS vaccine development. Here we examine the multiple variables and considerations that must be taken into account in order to use this nonhuman primate (NHP) model effectively. These include the species and subspecies of macaques, virus strain, dose and route of administration, and macaque genetics, including the major histocompatibility complex molecules that affect immune responses, and other virus restriction factors. We illustrate how these NHP models can be used to carry out studies of immune responses in mucosal and other tissues that could not easily be performed on human volunteers. Furthermore, macaques are an ideal model system to optimize adjuvants, test vaccine platforms, and identify correlates of protection that can advance the HIV vaccine field. We also illustrate techniques used to identify different macaque lymphocyte populations and review some poxvirus vaccine candidates that are in various stages of clinical trials. Understanding how to effectively use this valuable model will greatly increase the likelihood of finding a successful vaccine for HIV.

TRIM5α polymorphism identification in cynomolgus macaques of Vietnamese origin and Chinese rhesus macaques

TRIM5α is a retroviral restriction factor, in which the B30.2 (SPRY) and coiled-coil domains cooperate to determine the specificity of TRIM5α-mediated capture of retroviral capsids. Here, all exons of TRIM5α were analyzed in 39 Vietnamese cynomolgus macaques (VCE) and 29 Chinese rhesus macaques (CR). Our results revealed the presence of 22 alleles using the PHASE 2.0 software package (PHylogenetics And Sequence Evolution), including two novel species-specific alleles with a low frequency in VCE in exons 4 and 8, which encoded the coiled-coil and B30.2 (SPRY) domains, respectively. Nine alleles were detected in both VCE and CR, while four alleles were likely shared between the species. Of these alleles, the highest frequencies of 38% and 26% occurred in VCE and CR, respectively. Importantly, we found that some alleles encoded the same coiled-coil domain, but not the SPRY domain. In contrast, other alleles encoded the same SPRY domain, but not the coiled-coil domain. Our findings will contribute to the understanding of the interaction between the two domains and the determination of the specificity of TRIM5α-mediated capture of retroviral capsids. Our results from the phylogenetic trees constructed for VCE and CR suggested that the macaques' ability to inhibit SIV replication became gradually stronger if they carried corresponding alleles in four clades (clades4-7). More interesting, in clade3, both novel allele pairs (4E100a, 10E147a) and allele pairs (7R17b and 13R11b), which had the strong ability to inhibit SIV replication, originated from the same ancestral allele, suggesting that the novel alleles might play a key role to determine an animal's ability to inhibit SIV/HIV replication. However, further studies are needed to increase our understanding of the genetic background of TRIM5α in these two macaque species.

Macaque-tropic human immunodeficiency virus type 1: breaking out of the host restriction factors

Macaque monkeys serve as important animal models for understanding the pathogenesis of lentiviral infections. Since human immunodeficiency virus type 1 (HIV-1) hardly replicates in macaque cells, simian immunodeficiency virus (SIV) or chimeric viruses between HIV-1 and SIV (SHIV) have been used as challenge viruses in this research field. These viruses, however, are genetically distant from HIV-1. Therefore, in order to evaluate the efficacy of anti-HIV-1 drugs and vaccines in macaques, the development of a macaque-tropic HIV-1 (HIV-1mt) having the ability to replicate efficiently in macaques has long been desired. Recent studies have demonstrated that host restriction factors, such as APOBEC3 family and TRIM5, impose a strong barrier against HIV-1 replication in macaque cells. By evading these restriction factors, others and we have succeeded in developing an HIV-1mt that is able to replicate in macaques. In this review, we have attempted to shed light on the role of host factors that affect the susceptibility of macaques to HIV-1mt infection, especially by focusing on TRIM5-related factors.

Macaques as model hosts for studies of HIV-1 infection

Increasing evidence indicates that the host range of primate lentiviruses is in part determined by their ability to counteract innate restriction factors that are effectors of the type 1 interferon (IFN-1) response. For human immunodeficiency virus type 1 (HIV-1), in vitro experiments have shown that its tropism may be narrow and limited to humans and chimpanzees because its replication in other non-human primate species is hindered by factors such as TRIM5α (tripartite motif 5 alpha), APOBEC3G (apolipoprotein B mRNA-editing, enzyme-catalytic, polypeptide-like 3), and tetherin. Based on these data, it has been hypothesized that primate lentiviruses will infect and replicate in a new species if they are able to counteract and evade suppression by the IFN-1 response. Several studies have tested whether engineering HIV-1 recombinants with minimal amounts of simian immunodeficiency virus sequences would enable replication in CD4⁺ T cells of non-natural hosts such as Asian macaques and proposed that infection of these macaque species could be used to study transmission and pathogenesis. Indeed, infection of macaques with these viruses revealed that Vif-mediated counteraction of APOBEC3G function is central to cross-species tropism but that other IFN-induced factors may also play important roles in controlling replication. Further studies of these macaque models of infection with HIV-1 derivatives could provide valuable insights into the interaction of lentiviruses and the innate immune response and how lentiviruses adapt and cause disease.

TRIM5 genotypes in cynomolgus monkeys primarily influence inter-individual diversity in susceptibility to monkey-tropic human immunodeficiency virus type 1

TRIM5α restricts human immunodeficiency virus type 1 (HIV-1) infection in cynomolgus monkey (CM) cells. We previously reported that a TRIMCyp allele expressing TRIM5-cyclophilin A fusion protein was frequently found in CMs. Here, we examined the influence of TRIM5 gene variation on the susceptibility of CMs to a monkey-tropic HIV-1 derivative (HIV-1mt) and found that TRIMCyp homozygotes were highly susceptible to HIV-1mt not only in vitro but also in vivo. These results provide important insights into the inter-individual differences in susceptibility of macaques to HIV-1mt.

The TRIMCyp genotype in four species of macaques in China

The tripartite motif protein (TRIM)5α/CypA fusion protein TRIMCyp in Old World monkeys is generally considered unable to restrict HIV-1 replication. Monkeys with TRIMCyp can serve as a unique animal model for studies of HIV-1 infection. The present study investigated the distribution and expression status of TRIMCyp in four species of macaques originating from China and its borderlands: pigtail macaques (Macaca nemestrina), rhesus macaques (Macaca mulatta), long-tailed macaques (Macaca fascicularis), and Tibetan macaques (Macaca thibetana). The results revealed that the frequencies of the TRIMCyp genotype were significantly different among different species and even within different populations of the same species. Interestingly, the TRIMCyp genotype was more prevalent among macaques originating from Yunnan and surrounding regions than those from other regions of China. Importantly, TRIMCyp individuals were first identified in Chinese M. mulatta originating from Yunnan, although multiple earlier studies failed to find CypA retrotransposition in this subspecies. Furthermore, TRIMe7-CypA, one of the splicing isoforms of the TRIMCyp transcript was expressed in M. nemestrina and M. mulatta but not M. fascicularis. The intra- and interspecies polymorphisms in the deduced TRIMCyp amino acid sequences of these macaques were also analyzed. Taken together, the data in this study provide important information about the genomic background of TRIMCyp among major species of Chinese macaques.

Upregulation of TRIM5α gene expression after live-attenuated simian immunodeficiency virus vaccination in Mauritian cynomolgus macaques, but TRIM5α genotype has no impact on virus acquisition or vaccination outcome

Polymorphism in the TRIM5α/TRIMcyp gene, which interacts with the lentiviral capsid, has been shown to impact on simian immunodeficiency virus (SIV) replication in certain macaque species. Here, in the context of a live-attenuated SIV vaccine study conducted in Mauritian-origin cynomolgus macaques (MCM), we demonstrate upregulation of TRIM5α expression in multiple lymphoid tissues immediately following vaccination. Despite this, the restricted range of TRIM5α genotypes and lack of TRIMcyp variants had no or only limited impact on the replication kinetics in vivo of either the SIVmac viral vaccine or wild-type SIVsmE660 challenge. Additionally, there appeared to be no impact of TRIM5α genotype on the outcome of homologous or heterologous vaccination/challenge studies. The limited spectrum of TRIM5α polymorphism in MCM appears to minimize host bias to provide consistency of replication for SIVmac/SIVsm viruses in vivo, and therefore on vaccination and pathogenesis studies conducted in this species.

Allele frequency of antiretroviral host factor TRIMCyp in wild-caught cynomolgus macaques (Macaca fascicularis)

A recent study showed that the frequency of an antiretroviral factor TRIM5 gene-derived isoform, TRIMCyp, in cynomolgus macaques (Macaca fascicularis) varies widely according to the particular habitat examined. However, whether the findings actually reflect the prevalence of TRIMCyp in wild cynomolgus macaques is still uncertain because the previous data were obtained with captive monkeys in breeding and rearing facilities. Here, we characterized the TRIM5 gene in cynomolgus macaques captured in the wild, and found that the frequency of the TRIMCyp allele was comparable to those in captive monkeys. This suggests that the previous results with captive monkeys do indeed reflect the natural allele frequency and that breeding and rearing facilities may not affect the frequency of TRIM5 alleles. Interestingly, the prevalence of a minor haplotype of TRIMCyp in wild macaques from the Philippines was significantly lower than in captive ones, suggesting that it is advantageous for wild monkeys to possess the major haplotype of TRIMCyp. Overall, our results add to our understanding of the geographic and genetic prevalence of cynomolgus macaque TRIMCyp.

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.