TRIM5alpha mediates the postentry block to N-tropic murine leukemia viruses in human cells

Cellular restriction of retrovirus particle-mediated mRNA transfer

Analyzing cellular restriction mechanisms provides insight into viral replication strategies, identifies targets for antiviral drug design, and is crucial for the development of novel tools for experimental or therapeutic delivery of genetic information. We have previously shown that retroviral vector mutants that are unable to initiate reverse transcription mediate a transient expression of any sequence which replaces the gag-pol transcription unit, a process we call retrovirus particle-mediated mRNA transfer (RMT). Here, we further examined the mechanism of RMT by testing its sensitivity to cellular restriction factors and short hairpin RNAs (shRNAs). We found that both human TRIM5alpha and, to a lesser extent, Fv1 effectively restrict RMT if the RNA is delivered by a restriction-sensitive capsid. While TRIM5alpha restriction of RMT led to reduced levels of retroviral mRNA in target cells, restriction by Fv1 did not. Treatment with the proteasome inhibitor MG132 partially relieved TRIM5alpha-mediated restriction of RMT. Finally, cells expressing shRNAs specifically targeting the retroviral mRNA inhibited RMT particles, but not reverse-transcribing particles. Retroviral mRNA may thus serve as a translation template if not used as a template for reverse transcription. Our data imply that retroviral nucleic acids become accessible to host factors, including ribosomes, as a result of particle remodeling during cytoplasmic trafficking.

Comparison of anti-viral activity of rhesus monkey and cynomolgus monkey TRIM5alphas against human immunodeficiency virus type 2 infection

Human immunodeficiency virus type 2 (HIV-2) strains vary widely in their ability to grow in Old World monkey (OWM) cells. We previously evaluated several HIV-2 isolates for their sensitivity to cynomolgus monkey (CM) TRIM5alpha, an anti-HIV factor in OWM cells, and found that viruses carrying proline at the 120th position of the capsid protein were sensitive to CM TRIM5alpha, whereas those with either alanine or glutamine were resistant. In the study presented here, we tested these HIV-2 isolates for their sensitivity to rhesus monkey (Rh) TRIM5alpha and found that both CM TRIM5alpha-sensitive and -resistant viruses were restricted by Rh TRIM5alpha. The variable region 1 of the SPRY domain of Rh TRIM5alpha appeared to be the determinant of this difference. Furthermore, a mutagenesis study showed that three amino acid residues TFP at the 339th to 341st positions of Rh TRIM5alpha are important for restricting HIV-2 strains resistant to CM TRIM5alpha.

Independent genesis of chimeric TRIM5-cyclophilin proteins in two primate species

The host range of retroviruses is influenced by antiviral proteins such as TRIM5, a restriction factor that recognizes and inactivates incoming retroviral capsids. Remarkably, in Owl monkeys (omk), a cyclophilin A (CypA) cDNA has been transposed into the TRIM5 locus, resulting in the expression of a TRIM5-CypA fusion protein (TRIMCyp) that restricts retroviral infection based on the retroviral capsid-binding specificity of CypA. Here, we report that the seemingly improbable genesis of TRIMCyp has, in fact, occurred twice, and pigtailed macaques (pgt) express an independently generated TRIMCyp protein. The omkTRIMCyp and pgtTRIMCyp proteins restrict infection by several lentiviruses, but their specificities are distinguishable. Surprisingly, pgtTRIMCyp cannot bind to or restrict HIV-1 capsids as a consequence of a point mutation close to the Cyp:capsid-binding interface that was acquired during or after transposition of pgtCypA. However, the same mutation confers on pgtTRIMCyp the ability to restrict FIV in the presence of cyclosporin A, a drug that normally abolishes the interaction between pgtTRIMCyp or omkTRIMCyp and lentiviral capsids. Overall, an intuitively unlikely evolutionary event has, in fact, occurred at least twice in primates and represents a striking example of convergent evolution in divergent species.

Visualization of a proteasome-independent intermediate during restriction of HIV-1 by rhesus TRIM5alpha

TRIM5 proteins constitute a class of restriction factors that prevent host cell infection by retroviruses from different species. TRIM5alpha restricts retroviral infection early after viral entry, before the generation of viral reverse transcription products. However, the underlying restriction mechanism remains unclear. In this study, we show that during rhesus macaque TRIM5alpha (rhTRIM5alpha)-mediated restriction of HIV-1 infection, cytoplasmic HIV-1 viral complexes can associate with concentrations of TRIM5alpha protein termed cytoplasmic bodies. We observe a dynamic interaction between rhTRIM5alpha and cytoplasmic HIV-1 viral complexes, including the de novo formation of rhTRIM5alpha cytoplasmic body-like structures around viral complexes. We observe that proteasome inhibition allows HIV-1 to remain stably sequestered into large rhTRIM5alpha cytoplasmic bodies, preventing the clearance of HIV-1 viral complexes from the cytoplasm and revealing an intermediate in the restriction process. Furthermore, we can measure no loss of capsid protein from viral complexes arrested at this intermediate step in restriction, suggesting that any rhTRIM5alpha-mediated loss of capsid protein requires proteasome activity.

Interfering residues narrow the spectrum of MLV restriction by human TRIM5alpha

TRIM5α is a restriction factor that limits infection of human cells by so-called N- but not B- or NB-tropic strains of murine leukemia virus (MLV). Here, we performed a mutation-based functional analysis of TRIM5α-mediated MLV restriction. Our results reveal that changes at tyrosine³³⁶ of human TRIM5α, within the variable region 1 of its C-terminal PRYSPRY domain, can expand its activity to B-MLV and to the NB-tropic Moloney MLV. Conversely, we demonstrate that the escape of MLV from restriction by wild-type or mutant forms of huTRIM5α can be achieved through interdependent changes at positions 82, 109, 110, and 117 of the viral capsid. Together, our results support a model in which TRIM5α-mediated retroviral restriction results from the direct binding of the antiviral PRYSPRY domain to the viral capsid, and can be prevented by interferences exerted by critical residues on either one of these two partners.

Mammalian cells are endowed with intrinsic lines of defence against retroviruses, which notably contribute to limiting the cross-species transmission of these pathogens. TRIM5α is one such restriction factor, which acts by recognizing the capsid of incoming retroviruses through its C-terminal PRYSPRY domain. Human TRIM5α potently blocks the so-called N-tropic murine leukemia virus (MLV), but is ineffective against the closely related B-tropic and Moloney strains. In this study, we demonstrate that substitution of a single amino acid in the PRYSPRY domain of this protein expands its antiviral activity to these other MLV strains. Conversely, we show that protection of MLV from this restriction is governed by the negative influence of specific residues at a few critical positions of the retroviral capsid. These results support the model of a direct interaction between TRIM5α and retroviral capsids, shedding light on an important arm of innate antiretroviral immunity.

The presence of the Trim5alpha escape mutation H87Q in the capsid of late stage HIV-1 variants is preceded by a prolonged asymptomatic infection phase

BACKGROUND

Recently, the tripartite interaction motif 5alpha (Trim5alpha) has been identified as an inhibitory factor blocking infection of a broad range of retroviruses in a species-specific manner. In particular, HIV-1 replication can be efficiently blocked by Trim5alpha from Old World monkeys. The cyclophilin A binding region in the HIV-1 capsid is believed to be the viral determinant for Trim5alpha, and mutations in this region lift the restriction in simian cells. Human Trim5alpha is also able to inhibit HIV-1 replication in vitro, implying that Trim5alpha may contribute to host control of HIV-1 replication in vivo.

METHODS

HIV-1 variants from participants of the Amsterdam cohort studies were analysed for Trim5alpha escape mutations in the capsid. Patients who harboured HIV-1 variants with Trim5alpha escape mutations were compared with patients who lacked such variants in terms of clinical course of infection.

RESULTS

Trim5alpha escape mutants emerged in the late phase of infection and were ultimately present in 13.7% of HIV-1 infected individuals. Patients who developed Trim5alpha escape variants late in infection had a significantly lower set-point plasma viral RNA load and concomitantly a prolonged asymptomatic survival as compared to individuals who lacked Trim5alpha escape mutants. This protective effect was stronger in individuals who later developed X4 variants. In addition, X4-emergence was delayed in individuals who later developed Trim5alpha escape variants, compatible with suppression of viral replication.

CONCLUSION

Our data are compatible with Trim5alpha-mediated suppression of viral replication, resulting in prolonged asymptomatic survival and ultimately the selection of Trim5alpha escape variants.

Functional interplay between the B-box 2 and the B30.2(SPRY) domains of TRIM5alpha

The retroviral restriction factors, TRIM5alpha and TRIMCyp, consist of RING and B-box 2 domains separated by a coiled coil from carboxy-terminal domains. These carboxy-terminal domains (the B30.2(SPRY) domain in TRIM5alpha and the cyclophilin A domain in TRIMCyp) recognize the retroviral capsid. Here we show that some B-box 2 changes in TRIM5alpha, but not in TRIMCyp, resulted in decreased human immunodeficiency virus (HIV-1) capsid binding. The phenotypic effects of these B-box 2 changes on the restriction of retroviral infection depended on the potency of restriction and the affinity of the TRIM5alpha interaction with the viral capsid, two properties specified by the B30.2(SPRY) domain. Thus, some alterations in the TRIM5alpha B-box 2 domain apparently affect the orientation or conformation of the B30.2(SPRY) domain, influencing capsid recognition.

Alpha interferon enhances TRIM5alpha-mediated antiviral activities in human and rhesus monkey cells

Dominant, constitutively expressed antiretroviral factors, including TRIM5alpha and APOBEC3 proteins, are distinguished from the conventional innate immune systems and are classified as intrinsic immunity factors. Here, we demonstrate that interferon alpha (IFN-alpha) treatment upregulates TRIM5alpha mRNA in rhesus monkey cells, which correlates with the enhanced TRIM5alpha-mediated pre- and postintegration blocks of human immunodeficiency virus replication. In human cells, IFN-alpha increases the levels of TRIM5alpha mRNA, resulting in enhanced antiviral activity against N-tropic murine leukemia virus infection. These observations indicate that the TRIM5alpha-mediated antiviral effects can be orchestrated by the conventional innate immune response. It is conceivable that TRIM5alpha plays an essential role in controlling both the initial retroviral exposure and the subsequent viral dissemination in vivo.

An active TRIM5 protein in rabbits indicates a common antiviral ancestor for mammalian TRIM5 proteins

The recent identification of antiretroviral tripartite motif-bearing restriction factors that protect against retroviral infection has revealed a novel branch of innate immunity. The factors target the retroviral capsid and inhibit infectivity soon after the capsid has entered the cytoplasm by an incompletely characterized mechanism. Restriction is species specific. For example, TRIM5alpha from Old World monkeys, but not humans, restricts human immunodeficiency virus type 1 infection. Here, we identify an antiviral TRIM5 molecule in rabbits that is closely related to antiviral TRIM5 of both primates and cattle. We demonstrate that the rabbit TRIM5 protein is active against divergent retroviruses and leads to a strong block to viral DNA synthesis and infectivity. Furthermore, we show that antiviral activity is directed against the viral capsid and that human TRIM5 proteins are dominant negative to restriction in rabbit cells. We propose that the sequence and restriction characteristics conserved between restriction factors from primates, cattle, and rabbits indicate that these factors have evolved from a common ancestor with antiretroviral properties.

Analysis of human cell heterokaryons demonstrates that target cell restriction of cyclosporine-resistant human immunodeficiency virus type 1 mutants is genetically dominant

The host cell protein cyclophilin A (CypA) binds to CA of human immunodeficiency virus type 1 (HIV-1) and promotes HIV-1 infection of target cells. Disruption of the CypA-CA interaction, either by mutation of the CA residue at G89 or P90 or with the immunosuppressive drug cyclosporine (CsA), reduces HIV-1 infection. Two CA mutants, A92E and G94D, previously were identified by selection for growth of wild-type HIV-1 in cultures of CD4(+) HeLa cell cultures containing CsA. Interestingly, infection of some cell lines by these mutants is enhanced in the presence of CsA, while in other cell lines these mutants are minimally affected by the drug. Little is known about this cell-dependent phenotype of the A92E and G94D mutants, except that it is not dependent on expression of the host factor TRIM5alpha. Here, we show that infection by the A92E and G94D mutants is restricted at an early post-entry stage of the HIV-1 life cycle. Analysis of heterokaryons between CsA-dependent HeLa-P4 cells and CsA-independent 293T cells indicated that the CsA-dependent infection by A92E and G94D mutants is due to a dominant cellular restriction. We also show that addition of CsA to target cells inhibits infection by wild-type HIV-1 prior to reverse transcription. Collectively, these results support the existence of a cell-specific human cellular factor capable of restricting HIV-1 at an early post-entry step by a CypA-dependent mechanism.

Mechanisms of late restriction induced by an endogenous retrovirus

The host has developed during evolution a variety of "restriction factors" to fight retroviral infections. We investigated the mechanisms of a unique viral block acting at late stages of the retrovirus replication cycle. The sheep genome is colonized by several copies of endogenous retroviruses, known as enJSRVs, which are highly related to the oncogenic jaagsiekte sheep retrovirus (JSRV). enJS56A1, one of the enJSRV proviruses, can act as a restriction factor by blocking viral particles release of the exogenous JSRV. We show that in the absence of enJS56A1 expression, the JSRV Gag (the retroviral internal structural polyprotein) targets initially the pericentriolar region, in a dynein and microtubule-dependent fashion, and then colocalizes with the recycling endosomes. Indeed, by inhibiting the endocytosis and trafficking of recycling endosomes we hampered JSRV exit from the cell. Using a variety of approaches, we show that enJS56A1 and JSRV Gag interact soon after synthesis and before pericentriolar/recycling endosome targeting of the latter. The transdominant enJS56A1 induces intracellular Gag accumulation in microaggregates that colocalize with the aggresome marker GFP-250 but develop into bona fide aggresomes only when the proteasomal machinery is inhibited. The data argue that dominant-negative proteins can modify the overall structure of Gag multimers/viral particles hampering the interaction of the latter with the cellular trafficking machinery.

Restriction of an extinct retrovirus by the human TRIM5alpha antiviral protein

Primate genomes contain a large number of endogenous retroviruses and encode evolutionarily dynamic proteins that provide intrinsic immunity to retroviral infections. We report here the resurrection of the core protein of a 4-million-year-old endogenous virus from the chimpanzee genome and show that the human variant of the intrinsic immune protein TRIM5alpha can actively prevent infection by this virus. However, we suggest that selective changes that have occurred in the human lineage during the acquisition of resistance to this virus, and perhaps similar viruses, may have left our species more susceptible to infection by human immunodeficiency virus type 1 (HIV-1).

Two surface-exposed elements of the B30.2/SPRY domain as potency determinants of N-tropic murine leukemia virus restriction by human TRIM5alpha

Human TRIM5alpha (TRIM5alpha(hu)) potently restricts N-tropic (N-MLV), but not B-tropic, murine leukemia virus in a manner dependent upon residue 110 of the viral capsid. Rhesus monkey TRIM5alpha (TRIM5alpha(rh)) inhibits N-MLV only weakly. The study of human-monkey TRIM5alpha chimerae revealed that both the v1 and v3 variable regions of the B30.2/SPRY domain contain potency determinants for N-MLV restriction. These variable regions are predicted to be surface-exposed elements on one face of the B30.2 domain. Acidic residues in v3 complement basic residue 110 of the N-MLV capsid. The results support recognition of the retroviral capsid by the TRIM5alpha B30.2 domain.

Fusion of cyclophilin A to Fv1 enables cyclosporine-sensitive restriction of human and feline immunodeficiency viruses

TRIM5alpha is a potent intracellular antiviral restriction factor governing species-specific retroviral replication. In the New World species owl monkey the coding region for the viral binding B30.2 domain of TRIM5alpha has been replaced by a cyclophilin A (CypA) pseudogene by retrotransposition. The resultant TRIM5-CypA fusion protein restricts human immunodeficiency virus type 1 (HIV-1), as well as feline immunodeficiency virus (FIV), by recruitment of the CypA domain to the incoming viral capsids. Infectivity is rescued by agents such as cyclosporine that disrupt CypA binding to its substrates. Mice encode an antiviral restriction factor called Fv1 (for Friend virus susceptibility gene 1), which is active against murine leukemia virus and related to endogenous gag sequences. Here we show that fusing CypA to Fv1 generates a restriction factor with the antiviral specificity of TRIMCyp but the antiviral properties of Fv1. Like TRIMCyp, Fv1-Cyp restricts HIV-1 and FIV and is sensitive to inhibition by cyclosporine. TRIM5alpha is known to have a short half-life and block infectivity before viral reverse transcription. We show that Fv1-Cyp has a long half-life and blocks after reverse transcription, suggesting that its longer half-life gives the restricted virus the opportunity to synthesize DNA, leading to a later block to infection. This notion is supported by the observation that infectivity of Fv1-Cyp restricted virus can be rescued by cyclosporine for several hours after infection, whereas virus restricted by TRIMCyp is terminally restricted after around 40 min. Intriguingly, the Fv1-Cyp-restricted HIV-1 generates closed circular viral DNA, suggesting that the restricted virus complex enters the nucleus.

The ability of multimerized cyclophilin A to restrict retrovirus infection

In owl monkeys, the typical retroviral restriction factor of primates, TRIM5alpha, is replaced by TRIMCyp. TRIMCyp consists of the TRIM5 RING, B-box 2 and coiled-coil domains, as well as the intervening linker regions, fused with cyclophilin A. TRIMCyp restricts infection of retroviruses, such as human immunodeficiency virus (HIV-1) and feline immunodeficiency virus (FIV), with capsids that can bind cyclophilin A. The TRIM5 coiled coil promotes the trimerization of TRIMCyp. Here we show that cyclophilin A that is oligomeric as a result of fusion with a heterologous multimer exhibits substantial antiretroviral activity. The addition of the TRIM5 RING, B-box 2 and Linker 2 to oligomeric cyclophilin A generated a protein with antiretroviral activity approaching that of wild-type TRIMCyp. Multimerization increased the binding of cyclophilin A to the HIV-1 capsid, promoting accelerated uncoating of the capsid and restriction of infection.

The control of viral infection by tripartite motif proteins and cyclophilin A

The control of retroviral infection by antiviral factors referred to as restriction factors has become an exciting area in infectious disease research. TRIM5α has emerged as an important restriction factor impacting on retroviral replication including HIV-1 replication in primates. TRIM5α has a tripartite motif comprising RING, B-Box and coiled coil domains. The antiviral α splice variant additionally encodes a B30.2 domain which is recruited to incoming viral cores and determines antiviral specificity. TRIM5 is ubiquitinylated and rapidly turned over by the proteasome in a RING dependent way. Protecting restricted virus from degradation, by inhibiting the proteasome, rescues DNA synthesis, but not infectivity, indicating that restriction of infectivity by TRIM5α does not depend on the proteasome but the early block to DNA synthesis is likely to be mediated by rapid degradation of the restricted cores. The peptidyl prolyl isomerase enzyme cyclophilin A isomerises a peptide bond on the surface of the HIV-1 capsid and impacts on sensitivity to restriction by TRIM5α from Old World monkeys. This suggests that TRIM5α from Old World monkeys might have a preference for a particular capsid isomer and suggests a role for cyclophilin A in innate immunity in general. Whether there are more human antiviral TRIMs remains uncertain although the evidence for TRIM19's (PML) antiviral properties continues to grow. A TRIM5-like molecule with broad antiviral activity in cattle suggests that TRIM mediated innate immunity might be common in mammals. Certainly the continued study of restriction of viral infectivity by antiviral host factors will remain of interest to a broad audience and impact on a variety of areas including development of animal models for infection, development of viral vectors for gene therapy and the search for novel antiviral drug targets.

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.

Efficient Transduction of Simian Cells by HIV-1-based Lentiviral Vectors that Contain Mutations in the Capsid Protein

Recently, the cyclophilin A (CyPA)-binding region of the HIV-1 capsid protein was identified as a viral determinant involved in the post-entry restriction in Old World monkey cells. Here, we constructed a panel of HIV-1-based lentiviral vectors (LVs) that contain either mutations in the CyPA-binding region or the CyPA-binding region of the related viruses HIV-1 group O and HIV-2. We demonstrated that amino-acid changes in the CyPA-binding region of the capsid can alter the phenotype of the virus resulting in CyPA-independent infection in human cells and non-restricted infection in simian cells. Combining these data with the available structural data, we speculate that reduced affinity of the capsid for CyPA is associated with an unrestricted infection of simian cells. In addition, we observed that primary rhesus macaque peripheral blood mononuclear cells could be transduced efficiently by the LV that contained the CyPA-binding region of HIV-2. Therefore, this LV might be very useful for long-term safety studies in large animal models like rhesus macaques.

Effects of viral strain, transgene position, and target cell type on replication kinetics, genomic stability, and transgene expression of replication-competent murine leukemia virus-based vectors

The limited efficiency of in vivo gene transfer by replication-deficient retroviral vectors remains an obstacle to achieving effective gene therapy for solid tumors. One approach to circumvent this problem is the use of replication-competent retroviral vectors. However, the application of such vectors is at a comparatively early stage and the effects which virus strain, transgene cassette position, and target cell can exert on vector spread kinetics, genomic stability, and transgene expression levels remain to be fully elucidated. Thus, in this study a panel of vectors allowing the investigation of different design features on an otherwise genetically identical background were analyzed with respect to these readout parameters in cultures of both murine and human cells and in preformed tumors in nude mice. The obtained data revealed that (i) Moloney murine leukemia virus (Mo-MLV)-based vectors spread with faster kinetics, drive higher levels of transgene expression, and are more stable than equivalent Akv-MLV-based vectors; (ii) vectors containing the transgene cassette directly downstream of the envelope gene are genomically more stable than those containing it within the 3'-long terminal repeat U3 region; and (iii) the genomic stability of both strains seems to be cell line dependent.

Wild type and H43Y variant of human TRIM5alpha show similar anti-human immunodeficiency virus type 1 activity both in vivo and in vitro

Polymorphisms in human genes have been shown to affect the rate of disease progression to acquired immune deficiency syndrome in human immunodeficiency virus type 1 (HIV-1)-infected individuals. Recently, tripartite motif 5alpha (TRIM5alpha) was identified as a factor that confers resistance to HIV-1 infection in Old World monkey cells. Subsequently, Sawyer et al. (Curr Biol 16:95-100, 2006) reported a single nucleotide polymorphism (H43Y) in the human TRIM5alpha gene and TRIM5alpha protein with 43Y was found to lose its ability to restrict HIV-1. In the present study, we reevaluated effects of this allele on in vitro anti-HIV-1 activity as well as on HIV-1 disease progression in European and Asian cohorts of HIV-1-infected individuals. Our epidemiological and molecular biological findings clearly indicate H43Y has a very minor effect on anti-HIV-1 activity of TRIM5alpha, suggesting that this allele is immaterial, at least in HIV-1-infected Europeans and Asians.

Unique features of TRIM5α among closely related human TRIM family members

The tripartite motif (TRIM) protein, TRIM5alpha, restricts some retroviruses, including human immunodeficiency virus (HIV-1), from infecting the cells of particular species. TRIM proteins contain RING, B-box, coiled-coil and, in some cases, B30.2(SPRY) domains. We investigated the properties of human TRIM family members closely related to TRIM5. These TRIM proteins, like TRIM5alpha, assembled into homotrimers and co-localized in the cytoplasm with TRIM5alpha. TRIM5alpha turned over more rapidly than related TRIM proteins. TRIM5alpha, TRIM34 and TRIM6 associated with HIV-1 capsid-nucleocapsid complexes assembled in vitro; the TRIM5alpha and TRIM34 interactions with these complexes were dependent on their B30.2(SPRY) domains. Only TRIM5alpha potently restricted infection by the retroviruses studied; overexpression of TRIM34 resulted in modest inhibition of simian immunodeficiency virus (SIV(mac)) infection. In contrast to the other TRIM genes examined, TRIM5 exhibited evidence of positive selection. The unique features of TRIM5alpha among its TRIM relatives underscore its special status as an antiviral factor.

Structural basis for PRYSPRY-mediated tripartite motif (TRIM) protein function

The human tripartite motif (TRIM) family comprises 70 members, including HIV restriction factor TRIM5alpha and disease-associated proteins TRIM20 (pyrin) and TRIM21. TRIM proteins have conserved domain architecture but diverse cellular roles. Here, we describe how the C-terminal PRYSPRY domain mediates diverse TRIM functions. The crystal structure of TRIM21 PRYSPRY in complex with its target IgG Fc reveals a canonical binding interface comprised of two discrete pockets formed by antibody-like variable loops. Alanine scanning of this interface has identified the hot-spot residues that control TRIM21 binding to Fc; the same hot-spots control HIV/murine leukemia virus restriction by TRIM5alpha and mediate severe familial Mediterranean fever in TRIM20/pyrin. Characterization of the IgG binding site for TRIM21 PRYSPRY reveals TRIM21 as a superantigen analogous to bacterial protein A and suggests that an antibody bipolar bridging mechanism may contribute to the pathogenic accumulation of anti-TRIM21 autoantibody immune complex in autoimmune disease.

Resistance of monocyte to HIV-1 infection is not due to uncoating defect

Freshly isolated blood monocytes show an early postentry block to in vitro HIV-1 infection. Differentiation into monocyte-derived macrophages (MDMs) is required to allow HIV replication in this cell type. In this study, we investigated whether the resistance of monocyte to HIV infection stemmed from uncoating defect. Monocyte and MDM lysates induced HIV-1 core uncoating to a comparable degree. This suggests that monocyte lacks a factor(s) essential for viral reverse transcription and/or contains a factor(s) interfering with this step.

Complex determinants within the Moloney murine leukemia virus capsid modulate susceptibility of the virus to Fv1 and Ref1-mediated restriction

Two of the most well-known genetic mechanisms in mammalian cells which control the susceptibility of cells to productive infection by retroviruses and lentiviruses rely on the cellular Fv1 and Ref1 restriction factors, which act, after viral entry, to prevent productive infection through their interactions with viral capsid (CA) sequences. While previous studies of Fv1 restriction involving N- and B-tropic murine leukemia viruses (MLVs) had demonstrated that the identity of a single amino acid residue at CA110 (arginine vs. glutamic acid) determines whether the resulting virus is N (arg) or B-tropic (glu), analogous studies of dual-tropic MLVs, such as Moloney MLV (Mo-MLV), have shown that additional residues other than CA110 are also involved in the specification of dual-tropic host range. Here we have further studied the CA determinants of Mo-MLV host range, with an emphasis on identifying additional CA residues and unique combinations of CA residues which differentially influence the ability of the resulting virus to infect murine and human cells. First, we show that CA82, a residue previously identified to affect the pattern of Fv1 restriction of different MLV viruses in murine cells, is a particularly strong potentiator of B-tropism in an Mo-MLV background carrying a glutamic acid residue at CA110 (A110E substitution), and that interestingly, different residues at CA82 lead to distinct patterns of restriction in human but not in murine cells. We also identify another CA residue, CA214, as a similarly potent potentiator of B-tropism, in the context of the A110E substitution. While another substitution at CA110, A110R, leads to strong potentiation of N-tropism in murine cells, in the absence of additional mutations, we found that A110R alone was not sufficient to confer appreciable restriction in Ref1-expressing cells, despite the fact that authentic N-MLV shows strong restriction in those cells. In conjunction with the A110R substitution, substitutions at CA82, but not at CA214, do lead to significant restriction in human cells, thus demonstrating a distinction between the interactions between those two determinants of restriction and CA110. Finally, using cell lines engineered to express the TRIM5alpha(hu) gene product, recently identified as the Ref1 restriction factor, and RNAi technology to knock-down expression of TRIM5alpha(hu) in human cells, we directly demonstrate that the unique patterns of restriction observed in human cells with the different mutants are consistent with a TRIM5alpha(hu)-mediated restriction. These studies shed further light on the complex determinants within the viral CA gene product which control the susceptibility of murine and human cells to retroviral infection.

The retroviral restriction factor TRIM5alpha

Retroviruses are obligate intracellular parasites that have coevolved with their hosts for millions of years. It is therefore not surprising that retroviruses take advantage of numerous host factors during their life cycle. In addition to positive cellular factors that are of use to the virus, host cells have also evolved intracellular proteins to antagonize the retroviral replication cycle. Such inhibitory cellular factors have been called retroviral restriction factors. Recently, several such restriction factors have been cloned, including Friend virus susceptibility factor 1, apolipoprotein B mRNA-editing enzyme catalytic proteins 3F and 3G, and ZAP. Here, we review the explosion of publications from the past 2 years concerning TRIM5, a host factor that potently inhibits HIV-1 and other retroviruses.

The human TRIM5alpha restriction factor mediates accelerated uncoating of the N-tropic murine leukemia virus capsid

The host cell factors TRIM5alpha(hu) and Fv-1 restrict N-tropic murine leukemia virus (N-MLV) infection at an early postentry step before or after reverse transcription, respectively. Interestingly, the identity of residue 110 of the MLV capsid determines susceptibility to both TRIM5alpha(hu) and Fv-1. In this study, we investigate the fate of the MLV capsid in cells expressing either the TRIM5alpha(hu) or Fv-1 restriction factor. The expression of TRIM5alpha(hu), but not Fv-1, specifically promoted the premature conversion of particulate N-MLV capsids within infected cells to soluble capsid proteins. The TRIM5alpha(hu)-mediated disassembly of particulate N-MLV capsids was dependent upon residue 110 of the viral capsid. Furthermore, the deletion or disruption of TRIM5alpha(hu) domains necessary for potent N-MLV restriction completely abrogated the disappearance of particulate N-MLV capsids observed with wild-type TRIM5alpha(hu). These results suggest that premature disassembly of the viral capsid contributes to the restriction of N-MLV infection by TRIM5alpha(hu), but not by Fv-1.

Interaction of moloney murine leukemia virus capsid with Ubc9 and PIASy mediates SUMO-1 addition required early in infection

Yeast two-hybrid screens led to the identification of Ubc9 and PIASy, the E2 and E3 small ubiquitin-like modifier (SUMO)-conjugating enzymes, as proteins interacting with the capsid (CA) protein of the Moloney murine leukemia virus. The binding site in CA for Ubc9 was mapped by deletion and alanine-scanning mutagenesis to a consensus motif for SUMOylation at residues 202 to 220, and the binding site for PIASy was mapped to residues 114 to 176, directly centered on the major homology region. Expression of CA and a tagged SUMO-1 protein resulted in covalent transfer of SUMO-1 to CA in vivo. Mutations of lysine residues to arginines near the Ubc9 binding site and mutations at the PIASy binding site reduced or eliminated CA SUMOylation. Introduction of these mutations into the complete viral genome blocked virus replication. The mutants exhibited no defects in the late stages of viral gene expression or virion assembly. Upon infection, the mutant viruses were able to carry out reverse transcription to synthesize normal levels of linear viral DNA but were unable to produce the circular viral DNAs or integrated provirus normally found in the nucleus. The results suggest that the SUMOylation of CA mediated by an interaction with Ubc9 and PIASy is required for early events of infection, after reverse transcription and before nuclear entry and viral DNA integration.

Role of cyclophilin A in HIV replication

More than a decade has passed since the discovery that the peptidyl prolyl isomerase cyclophilin A (CypA) specifically binds to a proline-rich sequence in HIV-1 capsid (CA) and is thereby incorporated into viral particles. Since then, a variety of possible functions of CypA in the HIV-1 replication cycle have been intensively investigated, but the biological function of this interaction remains to be determined. The binding of CypA to CA increases HIV-1 infectivity in human cells, but promotes an anti-HIV-1 restriction activity in cells from nonhuman primates. Numerous studies have been undertaken to understand the paradoxical effects of CypA and, along with the parallel discovery of the restriction factor tripartite motif 5α, our understanding of how CypA modulates HIV-1 infectivity has now been changed completely. However, 13 years after its discovery, the biological function of the specific interaction between HIV-1 CA and CypA is still not fully understood. Even though much insight has been provided to date, many questions remain unanswered.

A unified concept of HIV latency

The introduction of highly active antiretroviral therapy (HAART) combining potent drugs that can inhibit reverse transcriptase, integrase and protease activities has changed the natural history of the human immunodeficiency virus (HIV) type 1 disease. Unfortunately, poor penetrability into different anatomic compartments, toxicity and drug resistance are some of the problems related to their prolonged use. The ability of HIV to mutate and become resistant, along with the ongoing viral replication during HAART, can lead to the emergence of independently evolving viral strains in different anatomic compartments (i.e., brain, testes, lymph nodes, etc.). In addition, HAART predominantly effects the viral replication in the activated or differentiating CD(+) T lymphocytes, but appears to have a very limited effect on HIV-1 preintegration complexes in the latently infected cells. Existing drug therapies do not eliminate these viral reservoirs, nor do they prevent their formation. New strategies are needed for eliminating protected areas of HIV-1 in vivo. Therefore, the persistence of latent HIV-1 reservoirs is the principal barrier in the complete eradication of HIV-1 infection in patients by antiretroviral therapy at present. African non-human primates (NHPs) naturally infected with various simian immunodeficiency viruses (SIVs) appear not to develop immunodeficiency or AIDS, whereas Asian NHPs, which are unnatural hosts, infected with SIVs, as well humans infected with HIV-1, will nearly always develop progressive loss of CD(+) T lymphocytes and a gradual destruction of immune functions. Understanding the difference in the host responses between natural and unnatural hosts, and deciphering which host factors are responsible for the non-pathogenic course of natural SIV infections, would be valuable in developing more-effective treatment or prevention strategies for HIV/AIDS. A number of factors encoded by host cells have been identified that appear to play critical roles in the SIV infection process. Two of these factors, TRIM5alpha (a member of a large family of proteins known as the TRIM proteins) and cellular apolipoprotein B mRNA-editing enzyme-catalytic polypeptide-like-3G (APOBEC3G) have been recently identified. APOBEC3G genes belong to a family of primate genes that produce enzymes (in this case, APOBEC3G) that 'edit' RNA by replacing cytosine with guanine into viral particles as the virus undergoes reverse transcription in the cytoplasm of the host cell. HIV-1, in turn, counters with a protein called viral infectivity factor (Vif), which binds to the APOBEC3G enzyme that degrades it. Several other blocking factors have been described, including lentiviral blocking factor (Lv)1 and 2. These factors appear to block the infection at a postentry step; after reverse transcription has occurred, but before proviral integration. Thus, it is crucial to understand the molecular mechanisms involved in the establishment, maintenance and reactivation of lentiviral latency. This review presents various models of HIV-1 latency and forward a new unified model of lentiviral latency.

Balancing selection and the evolution of functional polymorphism in Old World monkey TRIM5alpha

Retroviral restriction factor TRIM5alpha exhibits a high degree of sequence variation among primate species. It has been proposed that this diversity is the cumulative result of ancient, lineage-specific episodes of positive selection. Here, we describe the contribution of within-species variation to the evolution of TRIM5alpha. Sampling within two geographically distinct Old World monkey species revealed extensive polymorphism, including individual polymorphisms that predate speciation (shared polymorphism). In some instances, alleles were more closely related to orthologues of other species than to one another. Both silent and nonsynonymous changes clustered in two domains. Functional assays revealed consequences of polymorphism, including differential restriction of a small panel of retroviruses by very similar alleles. Together, these features indicate that the primate TRIM5alpha locus has evolved under balancing selection. Except for the MHC there are few, if any, examples of long-term balancing selection in primates. Our results suggest a complex evolutionary scenario, in which fixation of lineage-specific adaptations is superimposed on a subset of critical polymorphisms that predate speciation events and have been maintained by balancing selection for millions of years.

Human TRIM5alpha mediated restriction of different HIV-1 subtypes and Lv2 sensitive and insensitive HIV-2 variants

In order to characterize the antiviral activity of human TRIM5α in more detail human derived indicator cell lines over expressing wild type human TRIM5α were generated and challenged with HIV-1 and HIV-2 viruses pseudotyped with HIV envelope proteins in comparison to VSV-G pseudotyped particles. HIV envelope protein pseudotyped particles (HIV-1[NL4.3], HIV-1[BaL]) showed a similar restriction to infection (12 fold inhibition) compared to VSV-G pseudotyped viruses after challenging TZM-huTRIM5α cells. For HIV-2 a stronger restriction to infection was observed when the homologous envelope protein Env42S was pseudotyped onto these particles compared to VSV-G pseudotyped HIV-2 particles (8.6 fold inhibition versus 3.4 fold inhibition). It has been shown that HIV-2 is restricted by the restriction factor Lv2, acting on capsid like TRIM5α. A mutation of amino acid 73 (I73V) of HIV-2 capsid renders this virus Lv2-insensitive. Lv2-insensitive VSV-G pseudotyped HIV-2/I73V particles showed a similar restriction to infection as did HIV-2[VSV-G] particles (4 fold inhibition). HIV-2 envelope protein (Env42S)-pseudotyped HIV-2/I73V particles revealed a 9.3 fold increase in infection in TZM cells but remained restricted in TZM-huTRIM5α cells (80.6 fold inhibition) clearly indicating that at least two restriction factors, TRIM5α and Lv2, act on incoming HIV-2 particles. Further challenge experiments using primary isolates from different HIV-1 subtypes and from HIV-1 group O showed that wild type human TRIM5α restricted infection independent of coreceptor use of the infecting particle but to variable degrees (between 1.2 and 19.6 fold restriction).

Generation of HIV-1 derivatives that productively infect macaque monkey lymphoid cells

The narrow host range of human immunodeficiency virus type 1 (HIV-1) is caused in part by innate cellular factors such as apolipoprotein B mRNA-editing enzyme-catalytic polypeptide-like 3G (APOBEC3G) and TRIM5alpha, which restrict virus replication in monkey cells. Variant HIV-1 molecular clones containing both a 21-nucleotide simian immunodeficiency virus (SIV) Gag CA element, corresponding to the HIV-1 cyclophilin A-binding site, and the entire SIV vif gene were constructed. Long-term passage in a cynomolgus monkey lymphoid cell line resulted in the acquisition of two nonsynonymous changes in env, which conferred improved replication properties. A proviral molecular clone, derived from infected cells and designated NL-DT5R, was used to generate virus stocks capable of establishing spreading infections in the cynomolgus monkey T cell line and CD8-depleted peripheral blood mononuclear cells from five of five pig-tailed macaques and one of three rhesus monkeys. NL-DT5R, which genetically is >93% HIV-1, provides the opportunity, not possible with currently available SIV/HIV chimeric viruses, to analyze the function of multiple HIV-1 genes in a broad range of nonhuman primate species.

Proteasome inhibition reveals that a functional preintegration complex intermediate can be generated during restriction by diverse TRIM5 proteins

The primate TRIM5 proteins constitute a class of restriction factors that prevent host cell infection by retroviruses from different species. The TRIM5 proteins act early after virion entry and prevent viral reverse transcription products from accumulating. We recently found that proteasome inhibitors altered the rhesus monkey TRIM5alpha restriction of human immunodeficiency virus type 1 (HIV-1), allowing reverse transcription products to accumulate even though viral infection remained blocked. To assess whether sensitivity to proteasome inhibitors was a common feature of primate TRIM5 proteins, we conducted a similar analysis of restriction mediated by owl monkey TRIM-cyclophilin A (CypA) or human TRIM5alpha. Similar to rhesus monkey TRIM5alpha restriction, proteasome inhibition prevented owl monkey TRIM-CypA restriction of HIV-1 reverse transcription, even though HIV-1 infection and the output of 2-LTR circles remained impaired. Likewise, proteasome inhibition alleviated human TRIM5alpha restriction of N-tropic murine leukemia virus reverse transcription. Finally, HIV-1 reverse transcription products escaping rhesus TRIM5alpha restriction by proteasome inhibition were fully competent for integration in vitro, demonstrating that TRIM5alpha likely prevents the viral cDNA from accessing chromosomal target DNA. Collectively, these data indicate that the diverse TRIM5 proteins inhibit retroviral infection in multiple ways and that inhibition of reverse transcription products is not necessary for TRIM5-mediated restriction of retroviral infection.

Characterization of an amino-terminal dimerization domain from retroviral restriction factor Fv1

The Fv1 protein is an endogenous factor in mice that confers resistance to infection by certain classes of murine leukemia virus, a phenomenon referred to as restriction. The mechanism of restriction is not understood, and the low endogenous level of Fv1 in cells has prevented any biochemical or biophysical analysis of the protein. We have now purified recombinant Fv1(n) protein from a baculovirus system and demonstrate that Fv1 exists in a multimeric form. Furthermore, we have mapped the position of two domains within the protein using limited proteolysis. Biophysical characterization of the N-terminal domain reveals that it comprises a highly helical and extended dimeric structure. Based on these biochemical and biophysical data, we propose a model for the arrangement of domains in Fv1 and suggest that dimerization of the N-terminal domain is necessary for Fv1 function to allow the protein to interact with multiple capsid protomers in retroviral cores.

The systemic autoinflammatory diseases: inborn errors of the innate immune system

The autoinflammatory syndromes are a newly recognized group of immune disorders that lack the high titers of self-reactive antibodies and T cells characteristic of classic autoimmune disease. Nevertheless, patients with these illnesses experience unprovoked inflammatory disease in the absence of underlying infection. Here we discuss recent advances in eight Mendelian autoinflammatory diseases. The causative genes and the proteins they encode play a critical role in the regulation of innate immunity. Both pyrin and cryopyrin, the proteins mutated in familial Mediterranean fever and the cryopyrinopathies, respectively, are involved in regulation of the proinflammatory cytokine, IL-1beta, and may influence the activity of the transcription factor, NFkappaB. NOD2, the Blau syndrome protein, shares certain domains with cryopyrin and appears to be a sensor of intracellular bacteria. PSTPIP1, mutated in the syndrome of pyogenic arthritis with pyoderma gangrenosum and acne, interacts both with pyrin and a protein tyrosine phosphatase to regulate innate and adaptive immune responses. Somewhat unexpectedly, mutations in the p55 TNF receptor lead not to immunodeficiency but to dramatic inflammatory disease, the mechanisms of which are still under investigation. Finally, the discovery of the genetic basis of the hyperimmunoglobulinemia D with periodic fever syndrome has provided a fascinating but incompletely understood link between cholesterol biosynthesis and autoinflammation. In this manuscript, we summarize the current state of the art with regard to the diagnosis, pathogenesis, and treatment of these inborn errors of the innate immune system.

Removal of arginine 332 allows human TRIM5alpha to bind human immunodeficiency virus capsids and to restrict infection

Human TRIM5alpha (TRIM5alpha(hu)) only modestly inhibits human immunodeficiency virus type 1 (HIV-1) and does not inhibit simian immunodeficiency virus (SIV(mac)). Alteration of arginine 332 in the TRIM5alpha(hu) B30.2 domain to proline, the residue found in rhesus monkey TRIM5alpha, has been shown to create a potent restricting factor for both HIV-1 and SIV(mac.) Here we demonstrate that the potentiation of HIV-1 inhibition results from the removal of a positively charged residue at position 332 of TRIM5alpha(hu.) The increase in restricting activity correlated with an increase in the ability of TRIM5alpha(hu) mutants lacking arginine 332 to bind HIV-1 capsid complexes. A change in the cyclophilin A-binding loop of the HIV-1 capsid decreased TRIM5alpha(hu) R332P binding and allowed escape from restriction. The ability of TRIM5alpha(hu) to restrict SIV(mac) could be disrupted by the presence of any charged residue at position 332. Thus, charged residues in the v1 region of the TRIM5alpha(hu) B30.2 domain can modulate capsid binding and restriction potency. Therapeutic strategies designed to neutralize arginine 332 of TRIM5alpha(hu) might potentiate the innate resistance of human cells to HIV-1 infection.

Pathogenesis of familial periodic fever syndromes or hereditary autoinflammatory syndromes

Familial periodic fever syndromes, otherwise known as hereditary autoinflammatory syndromes, are inherited disorders characterized by recurrent episodes of fever and inflammation. The general hypothesis is that the innate immune response in these patients is wrongly tuned, being either too sensitive to very minor stimuli or turned off too late. The genetic background of the major familial periodic fever syndromes has been unraveled, and through research into the pathophysiology, a clearer picture of the innate immune system is emerging. After an introduction on fever, interleukin-1beta and inflammasomes, which are involved in the majority of these diseases, this manuscript offers a detailed review of the pathophysiology of the cryopyrin-associated periodic syndromes, familial Mediterranean fever, the syndrome of pyogenic arthritis, pyoderma gangrenosum and acne, Blau syndrome, TNF-receptor-associated periodic syndrome and hyper-IgD and periodic fever syndrome. Despite recent major advances, there are still many questions to be answered regarding the pathogenesis of these disorders.

TRIM5alpharh expression restricts HIV-1 infection in lentiviral vector-transduced CD34+-cell-derived macrophages

Species-specific innate resistance against viral infections offers novel avenues for antiviral therapeutic and prophylactic approaches. The retroviral and lentiviral restriction factors Ref1 and Lv1 are variants of the tripartite motif protein TRIM5alpha, a component of cytoplasmic bodies. TRIM5alpha severely restricts productive retroviral infections at the postentry and preintegration steps by destabilizing the incoming viral capsid via ubiquitination. Using this approach, resistance to HIV-1 infection could be conferred by TRIM5alpha(rh) expression in otherwise susceptible cells. Here we show that stable expression of simian TRIM5alpha(rh) via a lentiviral vector in a permissive cell culture line, Magi-CXCR4, conferred resistance to HIV-1. To translate these findings into a stem cell gene therapy setting, the TRIM5alpha(rh) transgene was stably introduced into CD34(+) hematopoietic progenitor cells to derive transgenic macrophages. Upon viral challenge, TRIM5alpha(rh)-expressing macrophages were highly resistant to HIV-1 infection compared to control cells. Human macrophages expressing TRIM5alpha(rh) were also found to be phenotypically and functionally normal, expressing the characteristic surface markers CD14, CD4, CCR5, CXCR4, MHC II, and B7.1. These results demonstrate that the species-specific restriction factor TRIM5alpha(rh) is effective in conferring HIV-1 resistance in a stem cell setting, thus paving the way for its application in AIDS gene therapy.

Effects of human TRIM5alpha polymorphisms on antiretroviral function and susceptibility to human immunodeficiency virus infection

TRIM5alpha acts on several retroviruses, including human immunodeficiency virus (HIV-1), to restrict cross-species transmission. Using natural history cohorts and tissue culture systems, we examined the effect of polymorphism in human TRIM5alpha on HIV-1 infection. In African Americans, the frequencies of two non-coding SNP variant alleles in exon 1 and intron 1 of TRIM5 were elevated in HIV-1-infected persons compared with uninfected subjects. By contrast, the frequency of the variant allele encoding TRIM5alpha 136Q was relatively elevated in uninfected individuals, suggesting a possible protective effect. TRIM5alpha 136Q protein exhibited slightly better anti-HIV-1 activity in tissue culture than the TRIM5alpha R136 protein. The 43Y variant of TRIM5alpha was less efficient than the H43 variant at restricting HIV-1 and murine leukemia virus infections in cultured cells. The ancestral TRIM5 haplotype specifying no observed variant alleles appeared to be protective against infection, and the corresponding wild-type protein partially restricted HIV-1 replication in vitro. A single logistic regression model with a permutation test indicated the global corrected P value of <0.05 for both SNPs and haplotypes. Thus, polymorphism in human TRIM5 may influence susceptibility to HIV-1 infection, a possibility that merits additional evaluation in independent cohorts.

Isolation of an active Lv1 gene from cattle indicates that tripartite motif protein-mediated innate immunity to retroviral infection is widespread among mammals

Lv1/TRIM5alpha (tripartite motif 5alpha) has recently emerged as an important factor influencing species-specific permissivity to retroviral infection in a range of primates, including humans. Old World monkey TRIM5alpha blocks human immunodeficiency virus type 1 (HIV-1) infectivity, and the human and New World monkey TRIM5alpha proteins are inactive against HIV-1 but active against divergent murine (N-tropic murine leukemia virus [MLV-N]) and simian (simian immunodeficiency virus from rhesus macaque [SIVmac]) retroviruses, respectively. Here we demonstrate antiviral activity of the first nonprimate TRIM protein, from cattle, active against divergent retroviruses, including HIV-1. The number of closely related human TRIM sequences makes assignment of the bovine sequence as a TRIM5alpha ortholog uncertain, and we therefore refer to it as bovine Lv1. Bovine Lv1 is closely related to primate TRIM5alpha proteins in the N-terminal RING and B-box 2 domains but significantly less homologous in the C-terminal B30.2 domain, particularly in the region shown to influence antiviral specificity. Intriguingly, some viruses restricted by bovine Lv1, including HIV-1 and MLV-N, are unable to synthesize viral DNA by reverse transcription, whereas restricted HIV-2 makes normal amounts of DNA. The data support the conclusion that TRIM protein-mediated restriction of retroviral infection is a more common attribute of mammals than previously appreciated.

Antiretroviral potential of human tripartite motif-5 and related proteins

TRIM5alpha is a potent inhibitor of infection by diverse retroviruses and is encoded by one of a large family of TRIM genes. We found that several TRIM motifs among a panel of selected human TRIM proteins (TRIM1, 5, 6, 18, 19, 21 22, 34) could inhibit infection when artificially targeted to an incoming HIV-1 capsid. Conversely, when ectopically expressed as authentic full-length proteins, most lacked activity against a panel of retroviruses. The exceptions were TRIM1, TRIM5 and TRIM34 proteins. Weak but specific inhibition of HIV-2/SIV(MAC) and EIAV by TRIM34 was noted, and human TRIM5alpha modestly, but specifically, inhibited an HIV-1 strain carrying a mutation in the cyclophilin binding loop (G89V). Restriction activity observed in ectopic expression assays was sometimes not detectable in corresponding RNAi-based knockdown experiments. However, endogenous owl monkey TRIMCyp potently inhibited an SIV(AGM) strain. Overall, sporadic examples of intrinsic antiretroviral activity exist in this panel of TRIM proteins.

Cyclophilin, TRIM5, and innate immunity to HIV-1

The peptidyl-prolyl isomerase cyclophilin A (CypA) binds a proline-rich loop on the surface of HIV-1 capsid (CA). This interaction increases HIV-1 infectivity in humans but promotes an anti-HIV-1 restriction activity in non-human primates. Efforts to understand these paradoxical effects of cyclophilin, along with more targeted approaches to uncover the genetic basis for HIV-1 restriction, led to the discovery of TRIM5 (tripartite motif protein 5), a CA-specific receptor for the retroviral core. The ensuing TRIM5 publication flurry established a paradigm of innate immunity in which the protein lattice of an invading retroviral core, rather than double-stranded RNA or lipopolysaccharide, is recognized by a multimeric, cytoplasmic receptor. CypA modulates HIV-1 virion core detection by this class of innate pattern recognition molecule, apparently by inducing subtle shifts in CA conformation.

Functional replacement of the RING, B-box 2, and coiled-coil domains of tripartite motif 5alpha (TRIM5alpha) by heterologous TRIM domains

Tripartite motif 5alpha (TRIM5alpha) restricts some retroviruses, including human immunodeficiency virus type 1 (HIV-1), from infecting the cells of particular species. TRIM5alpha is a member of the TRIM family of proteins, which contain RING, B-box, coiled-coil (CC), and, in some cases, B30.2(SPRY) domains. Here we investigated the abilities of domains from TRIM proteins (TRIM6, TRIM34, and TRIM21) that do not restrict HIV-1 infection to substitute for the domains of rhesus monkey TRIM5alpha (TRIM5alpha(rh)). The RING, B-box 2, and CC domains of the paralogous TRIM6 and TRIM34 proteins functionally replaced the corresponding TRIM5alpha(rh) domains, allowing HIV-1 restriction. By contrast, similar chimeras containing the components of TRIM21, a slightly more distant relative of TRIM5, did not restrict HIV-1 infection. The TRIM21 B-box 2 domain and its flanking linker regions contributed to the functional defectiveness of these chimeras. All of the chimeric proteins formed trimers. All of the chimeras that restricted HIV-1 infection bound the assembled HIV-1 capsid complexes. These results indicate that heterologous RING, B-box 2, and CC domains from related TRIM proteins can functionally substitute for TRIM5alpha(rh) domains.

Specific interaction of CXCR4 with CD4 and CD8alpha: functional analysis of the CD4/CXCR4 interaction in the context of HIV-1 envelope glycoprotein-mediated membrane fusion

We investigated possible interactions between HIV-1 receptor (CD4) and the main coreceptors CXCR4 and CCR5. We found that CD4 and CXCR4 coexpressed in 293T cells form a complex that can be immunoprecipitated with antibodies directed against the extracellular domain of either protein. Mutagenesis revealed that the CD4/CXCR4 interaction maps to two previously uncharacterized basic motifs in the cytoplasmic domain of CD4. HIV-1 envelope glycoprotein-mediated membrane fusion was found to be independent of the ability of CD4 and CXCR4 to interact, whether fusion was studied in a virus-cell or a cell-cell model. However, this interaction might explain the adaptation of HIV-1 to CXCR4 as an alternative to CCR5. We found that CXCR4 also interacts with the cytoplasmic domain of CD8alpha in a way that is similar to the CD4/CXCR4 interaction. The CD4/CXCR4 and CD8alpha/CXCR4 interactions may thus be involved in cellular signaling pathways shared by the CD4 and CD8alpha molecules.

Characterization of TRIM5alpha trimerization and its contribution to human immunodeficiency virus capsid binding

The coiled-coil domain of the tripartite motif (TRIM) family protein TRIM5alpha is required for trimerization and function as an antiretroviral restriction factor. Unlike the coiled-coil regions of other related TRIM proteins, the coiled coil of TRIM5alpha is not sufficient for multimerization. The linker region between the coiled-coil and B30.2 domains is necessary for efficient TRIM5alpha trimerization. Most of the hydrophilic residues predicted to be located on the surface-exposed face of the coiled coil can be altered without compromising TRIM5alpha antiviral activity against human immunodeficiency virus (HIV-1). However, changes that disrupt TRIM5alpha trimerization proportionately affect the ability of TRIM5alpha to bind HIV-1 capsid complexes. Therefore, TRIM5alpha trimerization makes a major contribution to its avidity for the retroviral capsid, and to the ability to restrict virus infection.

A dominant-negative effect of cynomolgus monkey tripartite motif protein TRIM5α on anti-simian immunodeficiency virus SIVmac activity of an African green monkey orthologue

African green monkey (AGM) tripartite motif protein (TRIM) 5alpha can inhibit both human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus SIVmac, whereas cynomolgus monkey (CM) TRIM5alpha can inhibit HIV-1, but not SIVmac. We previously reported that the 17-amino-acid region and an adjacent 20-amino-acid duplication in the SPRY(B30.2) domain of AGM TRIM5alpha determined the species specificity. In the present study, we demonstrated that CM TRIM5alpha had a dominant-negative effect on the anti-SIVmac activity of AGM TRIM5alpha. In contrast, mutant TRIM5alphas lacking the 20-amino-acid duplication did not have the dominant-negative effect, even though they failed to restrict SIVmac. These results indicated that oligomerization of the SPRY domain is required for anti-SIVmac activity and suggest that tight interaction between the viral capsid and all three molecules in one TRIM5alpha trimer may not be necessary for restriction activity.

The pericentriolar recycling endosome plays a key role in Vpu-mediated enhancement of HIV-1 particle release

The HIV-1 accessory gene product Vpu is required for efficient viral particle release from infected human cells. The mechanism by which Vpu enhances particle assembly or release is not yet defined. Here, we identify an intracellular site that is critical for Vpu-mediated enhancement of particle release. Vpu was found to co-localize with markers for the pericentriolar recycling endosome. Expression of dominant negative mutants of Rab11a and myosin Vb that disrupt protein sorting through the recycling endosome abrogated the ability of Vpu to augment particle release. Remarkably, the effects of blocking recycling endosome function on HIV particle release were demonstrable only in human cell lines known to be responsive to Vpu, while no effect on particle release was seen in African green monkey cells. Inhibition of recycling endosome function in human cells also blocked the ability of HIV-2 envelope to enhance particle release. These studies indicate that Vpu and HIV-2 envelope glycoprotein enhance particle release via a common mechanism that requires the activity of the pericentriolar recycling endosome.