Primates have evolved a variety of restriction factors that prevent retroviral replication. One such factor, TRIM5␣, mediates a postentry restriction in many Old World primates. Among New World primates, Aotus trivirgatus exerts a similar early restriction mediated by TRIMCyp, a TRIM5-cyclophilin A (CypA) chimera resulting from a CypA retrotransposition between exons 7 and 8 of the TRIM5 gene. convergent evolution ͉ cyclophilin A ͉ HIV ͉ retroviruses ͉ TRIM5␣ T he current HIV pandemic resulting from cross-species transmissions of simian immunodeficiency viruses SIVcpz or SIVsmm to humans has been well documented (1). However, the mechanisms enabling such transmissions are not yet fully understood. Mammals have evolved several restriction factors capable of inhibiting the replication of certain retroviruses in a species-specific manner. One of the best described primate host-restriction factors is TRIM5␣, which is expressed in most Old World primates.Macaca mulatta TRIM5␣ exerts an early, postentry block to HIV-1 replication (2). TRIM5␣ is a member of the tripartite motif family of proteins, characterized by the ordered Nterminal to C-terminal expression of a RING domain, B-Box, and coiled coil, also known as an RBCC domain (3). The TRIM5␣ isoform also expresses a B30.2 domain at the C terminus, which is required for recognition of the incoming retroviral capsid (4-7). Changes to the B30.2 domain have been shown to dramatically affect the breadth and potency of TRIM5␣-mediated anti-retroviral activity. For instance, the ability to restrict HIV-1 replication may be conferred to Homo sapiens TRIM5␣ by changing a single residue to the amino acid found in M. mulatta TRIM5␣, R332P (8). Similarly, site-directed mutagenesis studies have demonstrated that mutations around the cyclophilin A (CypA) binding loop of HIV-1 capsid effect the potency of TRIM5␣-mediated restriction, suggesting that the B30.2 domain interacts with or near the CypA binding loop (5, 9, 10).New World primates Aotus trivirgatus exert a postentry restriction to HIV-1 mediated by a TRIM5-CypA chimera. Sequencing the A. trivirgatus TRIM5 gene identified a LINE-1-mediated retrotransposition of CypA into intron 7, resulting in the expression of a fusion protein called TRIMCyp, which is unique to the Aotus genus (11-13). TRIMCyp retains the N-terminal tripartite motif of all TRIM family members, but the B30.2 domain of TRIM5␣ is replaced by the CypA domain. Functionally, TRIM5␣ and TRIMCyp are similar, preventing reverse transcription of incoming viruses at an early postentry stage. However, TRIMCyp exerts a more potent restriction to incoming retroviruses than TRIM5␣, and unlike TRIM5␣, TRIMCyp-mediated restriction is sensitive to cyclosporin A. A. trivirgatus has been shown to express only TRIMCyp,.We recently demonstrated that the Old World primates Macaca nemestrina do not express TRIM5␣. Instead, they transcribe novel isoforms TRIM5 and TRIM5 (14). These isoforms likely arise because of a single-nucleotide polymorphism (SNP) at the intron 6 splice acc...

The TRIM5 family of proteins contains a RING domain, one or two B boxes, and a coiled-coil domain. The TRIM5␣ isoform also encodes a C-terminal B30.2(SPRY) domain, differences within which define the breadth and potency of TRIM5␣-mediated retroviral restriction. Because Macaca nemestrina animals are susceptible to some human immunodeficiency virus (HIV) isolates, we sought to determine if differences exist in the TRIM5 gene and transcripts of these animals. We identified a two-nucleotide deletion (⌬2) in the transcript at the 5 terminus of exon 7 in all M. nemestrina TRIM5 cDNA clones examined. This frameshift results in a truncated protein of 300 amino acids lacking the B30.2(SPRY) domain, which we have named TRIM5. This deletion is likely due to a single nucleotide polymorphism that alters the 3 splice site between intron 6 and exon 7. In some clones, a deletion of the entire 27-nucleotide exon 7 (⌬exon7) resulted in the restoration of the TRIM5 open reading frame and the generation of another novel isoform, TRIM5. There are 18 amino acid differences between M. nemestrina TRIM5 and Macaca mulatta TRIM5␣, some of which are at or near locations previously shown to affect the breadth and potency of TRIM5␣-mediated restriction. Infectivity assays performed on permissive CrFK cells stably transduced with TRIM5 or TRIM5 show that these isoforms are incapable of restricting either HIV type 1 (HIV-1) or simian immunodeficiency virus infection. The expression of TRIM5 alleles incapable of restricting HIV-1 infection may contribute to the previously reported increased susceptibility of M. nemestrina to HIV-1 infection in vivo. The human immunodeficiency virus (HIV) pandemic is the result of cross-species transmissions of simian immunodeficiency virus (SIV) from non-human primates to humans (28).The recent discovery of human T-cell leukemia virus types 3 and 4 suggests that cross-species transmission of retroviruses is not an infrequent occurrence (33). However, to establish a productive infection in a new species, it is necessary for retroviruses to evade host-specific restriction factors. A broad range of antiretroviral host restriction factors has been identified in mammalian species. The best-described primate host restriction factors are APOBEC3F/G and TRIM5␣. APOBEC3F/G are cytidine deaminases which exert a late block to retroviral replication, inhibiting the virus in target cells rather than producer cells (29), whereas TRIM5␣ exerts a dominant block to infection immediately after viral entry into the cell through the inhibition of reverse transcription (30).TRIM5␣ is a member of the tripartite motif family of proteins, also called RBCC proteins, because of the presence of a RING domain (C3HC4 type), one or two B boxes, and a coiled-coil region in an ordered arrangement from N terminus to C terminus (23). Six isoforms of TRIM5 have been identified in mammals (26). These proteins have been shown to form homo-and heteromultimers, although only homomultimers of TRIM5␣ have been shown to restrict lentiviruses (3,8,15,...

HIV-1 establishes persistent infection in part due to its ability to evade host immune responses. Occlusion by glycans contributes to masking conserved sites that are targets for some broadly neutralizing antibodies (bNAbs). Previous work has shown that removal of a highly conserved potential N-linked glycan (PNLG) site at amino acid residue 197 (N7) on the surface antigen gp120 of HIV-1 increases neutralization sensitivity of the mutant virus to CD4 binding site (CD4bs)-directed antibodies compared to its wild-type (WT) counterpart. However, it is not clear if the role of the N7 glycan is conserved among diverse HIV-1 isolates and if other glycans in the conserved regions of HIV-1 Env display similar functions. In this work, we examined the role of PNLGs in the conserved region of HIV-1 Env, particularly the role of the N7 glycan in a panel of HIV-1 strains representing different clades, tissue origins, coreceptor usages, and neutralization sensitivities. We demonstrate that the absence of the N7 glycan increases the sensitivity of diverse HIV-1 isolates to CD4bs-and V3 loop-directed antibodies, indicating that the N7 glycan plays a conserved role masking these conserved epitopes. However, the effect of the N7 glycan on virus sensitivity to neutralizing antibodies directed against the V2 loop epitope is isolate dependent. These findings indicate that the N7 glycan plays an important and conserved role modulating the structure, stability, or accessibility of bNAb epitopes in the CD4bs and coreceptor binding region, thus representing a potential target for the design of immunogens and therapeutics. A lthough the role of neutralizing antibodies has yet to be determined in the only clinical trial of human immunodeficiency virus type 1 (HIV-1) vaccines that has shown a modest degree of protection, it is generally believed that it would be advantageous for a vaccine to elicit broadly neutralizing antibodies (bNAbs) against diverse primary isolates. Passive administration of neutralizing monoclonal antibodies (MAbs) or bNAbs derived from HIV-1-infected patients has been shown to protect macaques from simian-human immunodeficiency virus (SHIV) infection (1-5). A fraction of HIV-1-infected individuals (ϳ20 to 30%) generate bNAbs within 2 to 4 years of initial infection (6-10). However, generation of bNAbs by active immunization has been a challenge, as no HIV-1 vaccine candidate has successfully elicited antibodies with a similar neutralizing breadth (8, 11).Nevertheless, broadly neutralizing monoclonal antibodies isolated from selected individuals have helped define the targets of bNAbs. These bNAbs are directed against one of five conserved epitopes on HIV-1 envelope glycoprotein (Env); the CD4 binding site (CD4bs), the membrane-proximal ectodomain region (MPER), carbohydrates on the outer domain, a quaternary structure in the V1 and V2 loops, and a newly described epitope present only in cleaved envelope trimers (7,(11)(12)(13). However, HIV-1 has evolved many protective mechanisms to evade host immune respon...