The effect of TRIM5 variants on the susceptibility to HIV-1 infection and disease progression in the Polish population

BACKGROUD

Tripartite motif containing 5α protein is a factor contributing to intracellular defense mechanisms against human immunodeficiency virus-1 (HIV-1) infection. The studies of TRIM5 variants effects on the risk of HIV-1 infection and the clinical course of disease provided inconclusive results in different ethnic groups. The aim of this study was to investigate the influence of TRIM5 variants on susceptibility to HIV-1 infection and clinical parameters among Polish HIV-1-infected patients.

MATERIALS & METHODS

In our study, we investigated 301 HIV-1-infected patients and 186 age-matched seronegative controls. Seven variants of the TRIM5 gene (rs7127617, rs3824949, rs3740996, rs11601507, rs10838525, rs11038628, and rs28381981) were genotyped using both sequencing and polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) techniques.

RESULTS AND CONCLUSIONS

The frequencies of rs7127617 TT genotype and T allele occurrence were lower in HIV-1-infected subjects compared to controls (0.14 vs. 0.26 for T/T genotype and 0.45 vs. 0.54 for T allele), suggesting their possible protective effect (p = 0.005 and p = 0.007, respectively). Heterozygosity and presence of the T allele at rs3740996 were enriched in controls compared to HIV-1-infected group (0.19 vs. 0.12 for C/T genotype and 0.11 vs. 0.07 for T allele; p = 0.03 and p = 0.02, respectively). Moreover, rs3824949 CC genotype carriers had a lower viral load than patients bearing rs3824949 GG/CG genotypes (4.0 vs. 4.6 log copies/mL; p = 0.049); however, none of the variants affected CD4+ cell count. In conclusion, our data confirm the role of TRIM5 variants in the HIV-1 transmission and the clinical course of HIV-1 infection. The presence of rs7127617 TT genotype and T allele seems to protect against HIV-1 transmission in examined population.

Patterns of Evolution of TRIM Genes Highlight the Evolutionary Plasticity of Antiviral Effectors in Mammals

The innate immune system of mammals is formed by a complex web of interacting proteins, which together constitute the first barrier of entry for infectious pathogens. Genes from the E3-ubiquitin ligase tripartite motif (TRIM) family have been shown to play an important role in the innate immune system by restricting the activity of different retrovirus species. For example, TRIM5 and TRIM22 have both been associated with HIV restriction and are regarded as crucial parts of the antiretroviral machinery of mammals. Our analyses of positive selection corroborate the great significance of these genes for some groups of mammals. However, we also show that many species lack TRIM5 and TRIM22 altogether. By analyzing a large number of mammalian genomes, here we provide the first comprehensive view of the evolution of these genes in eutherians, showcasing that the pattern of accumulation of TRIM genes has been dissimilar across mammalian orders. Our data suggest that these differences are caused by the evolutionary plasticity of the immune system of eutherians, which have adapted to use different strategies to combat retrovirus infections. Altogether, our results provide insights into the dissimilar evolution of a representative family of restriction factors, highlighting an example of adaptive and idiosyncratic evolution in the innate immune system.

The cyclophilin A-binding loop of the capsid regulates the human TRIM5α sensitivity of nonpandemic HIV-1

The rather few cases of humans infected by HIV-1 N, O, or P raise the question of their incomplete adaptation to humans. We hypothesized that early postentry restrictions may be relevant for the impaired spread of these HIVs. One of the best-characterized species-specific restriction factors is TRIM5α. HIV-1 M can escape human (hu) TRIM5α restriction by binding cyclophilin A (CYPA, also known as PPIA, peptidylprolyl isomerase A) to the so-called CYPA-binding loop of its capsid protein. How non-M HIV-1s interact with huTRIM5α is ill-defined. By testing full-length reporter viruses (Δ env) of HIV-1 N, O, P, and SIVgor (simian IV of gorillas), we found that in contrast to HIV-1 M, the nonpandemic HIVs and SIVgor showed restriction by huTRIM5α. Work to identify capsid residues that mediate susceptibility to huTRIM5α revealed that residue 88 in the capsid CYPA-binding loop was important for such differences. There, HIV-1 M uses alanine to resist, while non-M HIV-1s have either valine or methionine, which avail them for huTRIM5α. Capsid residue 88 determines the sensitivity to TRIM5α in an unknown way. Molecular simulations indicated that capsid residue 88 can affect trans-to-cis isomerization patterns on the capsids of the viruses we tested. These differential CYPA usages by pandemic and nonpandemic HIV-1 suggest that the enzymatic activity of CYPA on the viral core might be important for its protective function against huTRIM5α.

STAT3-Dependent Gene TRIM5γ Interacts With HBx Through a Zinc Binding Site on the BBox Domain

Owing to its broad-spectrum antivirus activities, interferon (IFN) is an important alternative agent for use in the treatment of hepatitis B virus (HBV)-infected patients; however, the mechanism involved in the inhibition of HBV infection and replication by IFN remains unclear. We previously reported that the induction of TRIM5γ is important in the IFN treatment of HBV patients as it promotes the degradation of the HBx protein, while the manner in which TRIM5γ is induced by IFN and how TRIM5γ interacts with HBx remain unestablished until date. Our present findings confirmed the TRIM5γ-HBx-DDB1 interactions in the HBV-infected Primary human hepatocytes (PHH), and we further found that STAT3, and not STAT1, was responsible for the induction of TRIM5γ upon IFN stimulation and that the zinc binding site His123 on the BBOX domain was a decisive site in the interaction between TRIM5γ BBOX and HBx. In addition, based on the BBOX domain, we detected a 7-amino acid peptide with the potential of promoting HBx degradation and inhibiting HBV replication. On the other hand, we noted that the TRIM5γ expression was inhibited by HBV in chronically HBV infected patients. Thus, our study identified the crucial role of STAT3 in the induction of TRIM5γ, as well as proposed a 7-amino acid, small peptide as a potential candidate for the development of therapeutic agents targeting HBx.

[Increased TRIM5 is associated with a poor prognosis and immune infiltration in glioma patients]

Tripartite motif 5 (TRIM5) plays a significant function in autophagy and involves in immune and tumor processes. While the function of TRIM5 remains poorly understood in glioma. We purpose to evaluate the possible prognostic role of TRIM5 in glioma via bioinformatics analyses. The database clinical samples of glioma in this study included low grade glioma (LGG) and glioblastoma multiforme (GBM). TRIM5 expression in glioma tissues were explored in Oncomine, GEPIA and The Cancer Genome Atlas (TCGA) databases. Survival analysis and the multivariate Cox regression analysis of TRIM5 based on TCGA were used to evaluate the prognostic role of TRIM5. The protein networks of TRIM5 was detected by STRING database. KEGG enrichment analyses were performed to predict the potential molecular pathways of TRIM5 in glioma. In addition, immune infiltration analysis was conducted by CIBERSORT and TIMER databases. We found that TRIM5 was strongly increased in glioma samples compared with normal samples in Oncomine, GEPIA and TCGA databases. Higher TRIM5 was significantly contributed to worse overall survival (OS) in LGG+GBM patients and LGG patients, while was no correlated with OS of GBM patients. Interaction networks analysis identified that IRF3, IRF7, OAS1, OAS2, OAS3, OASL, GBP1, PML, BTBD1 and BTBD2 proteins were contacted with TRIM5. Moreover, KEGG revealed that apoptosis and cancer- and immune-related pathways were enriched with elevated TRIM5. Specifically, TRIM5 could influence the immune infiltration levels, such as activated NK cells, monocytes, activated mast cells and macrophages in glioma. In conclusion, our data indicated that TRIM5 was upregulated in glioma tissues and associated with poor prognosis and immune infiltration. TRIM5 may be acted as a biomarker in prognosis and immunotherapy guidance of glioma.

Delayed disease progression in HIV-2: the importance of TRIM5α and the retroviral capsid

HIV-2 is thought to have entered the human population in the 1930s through cross-species transmission of SIV from sooty mangabeys in West Africa. Unlike HIV-1, HIV-2 has not led to a global pandemic, and recent data suggest that HIV-2 prevalence is declining in some West African states where it was formerly endemic. Although many early isolates of HIV-2 were derived from patients presenting with AIDS-defining illnesses, it was noted that a much larger proportion of HIV-2-infected subjects behaved as long-term non-progressors (LTNP) than their HIV-1-infected counterparts. Many HIV-2-infected adults are asymptomatic, maintaining an undetectable viral load for over a decade. However, despite lower viral loads, HIV-2 progresses to clinical AIDS without therapeutic intervention in most patients. In addition, successful treatment with anti-retroviral therapy (ART) is more challenging than for HIV-1. HIV-2 is significantly more sensitive to restriction by host restriction factor tripartite motif TRIM5α than HIV-1, and this difference in sensitivity is linked to differences in capsid structure. In this review we discuss the determinants of HIV-2 disease progression and focus on the important interactions between TRIM5α and HIV-2 capsid in long-term viral control.

Trivalent RING Assembly on Retroviral Capsids Activates TRIM5 Ubiquitination and Innate Immune Signaling

TRIM5 is a RING domain E3 ubiquitin ligase with potent antiretroviral function. TRIM5 assembles into a hexagonal lattice on retroviral capsids, causing envelopment of the infectious core. Concomitantly, TRIM5 initiates innate immune signaling and orchestrates disassembly of the viral particle, yet how these antiviral responses are regulated by capsid recognition is unclear. We show that hexagonal assembly triggers N-terminal polyubiquitination of TRIM5 that collectively drives antiviral responses. In uninfected cells, N-terminal monoubiquitination triggers non-productive TRIM5 turnover. Upon TRIM5 assembly on virus, a trivalent RING arrangement allows elongation of N-terminally anchored K63-linked ubiquitin chains (N-K63-Ub). N-K63-Ub drives TRIM5 innate immune stimulation and proteasomal degradation. Inducing ubiquitination before TRIM5 assembly triggers premature degradation and ablates antiviral restriction. Conversely, driving N-K63 ubiquitination after TRIM5 assembly enhances innate immune signaling. Thus, the hexagonal geometry of TRIM5's antiviral lattice converts a capsid-binding protein into a multifunctional antiviral platform.

The Human Immunodeficiency Virus Capsid Is More Than Just a Genome Package

Human immunodeficiency virus (HIV) is one of the most studied of all human pathogens. One strain-HIV-1 group M-is responsible for a global pandemic that has infected >60 million people and killed >20 million. Understanding the stages of HIV infection has led to highly effective therapeutics in the form of antiviral drugs that target the viral enzymes reverse transcriptase, integrase, and protease as well as biotechnological developments in the form of retroviral and lentiviral vectors for the transduction of cells in tissue culture and, potentially, gene therapy. However, despite considerable research focus in this area, there is much we still do not understand about the HIV replicative cycle, particularly the first steps that are crucial to establishing a productive infection. One especially enigmatic player has been the HIV capsid. In this review, we discuss three aspects of the HIV capsid: its function as a structural shell, its role in mediating host interactions, and its vulnerability to antiviral activity.

The Three-Fold Axis of the HIV-1 Capsid Lattice Is the Species-Specific Binding Interface for TRIM5α

Rhesus TRIM5α (rhTRIM5α) potently restricts replication of human immunodeficiency virus type 1 (HIV-1). Restriction is mediated through direct binding of the C-terminal B30.2 domain of TRIM5α to the assembled HIV-1 capsid core. This host-pathogen interaction involves multiple capsid molecules within the hexagonal HIV-1 capsid lattice. However, the molecular details of this interaction and the precise site at which the B30.2 domain binds remain largely unknown. The human orthologue of TRIM5α (hsTRIM5α) fails to block infection by HIV-1 both in vivo and in vitro. This is thought to be due to differences in binding to the capsid lattice. To map the species-specific binding surface on the HIV-1 capsid lattice, we used microscale thermophoresis and dual-focus fluorescence correlation spectroscopy to measure binding affinity of rhesus and human TRIM5α B30.2 domains to a series of HIV-1 capsid variants that mimic distinct capsid arrangements at each of the symmetry axes of the HIV-1 capsid lattice. These surrogates include previously characterized capsid oligomers, as well as a novel chemically cross-linked capsid trimer that contains cysteine substitutions near the 3-fold axis of symmetry. The results demonstrate that TRIM5α binding involves multiple capsid molecules along the 2-fold and 3-fold interfaces between hexamers and indicate that the binding interface at the 3-fold axis contributes to the well-established differences in restriction potency between TRIM5α orthologues.

IMPORTANCE TRIM5α is a cellular protein that fends off infection by retroviruses through binding to the viruses' protein shell surrounding its genetic material. This shell is composed of several hundred capsid proteins arranged in a honeycomb-like hexagonal pattern that is conserved across retroviruses. By binding to the complex lattice formed by multiple capsid proteins, rather than to a single capsid monomer, TRIM5α restriction activity persists despite the high mutation rate in retroviruses such as HIV-1. In rhesus monkeys, but not in humans, TRIM5α confers resistance to HIV-1. By measuring the binding of human and rhesus TRIM5α to a series of engineered HIV-1 capsid mimics of distinct capsid lattice interfaces, we reveal the HIV-1 capsid surface critical for species-specific binding by TRIM5α.

TRIM5 gene polymorphisms in HIV-1-infected patients and healthy controls from Northeastern Brazil

Humans show heterogeneity in vulnerability to HIV-1 infection, partially under control of genes involved in host immunity and virus replication. TRIM5α protein has restriction activity against replication of many retroviruses. Human TRIM5 gene single nucleotide polymorphisms have been reported as involved in susceptibility to HIV-1 infection. We recruited 213 HIV-1-positive patients and 234 healthy uninfected controls from Northeast Brazil; two non-synonymous variants at exon 2, rs3740996 (H43Y) and rs10838525 (R136Q), and one regulatory polymorphism (rs16934386) at 5'UTR region of TRIM5 were analyzed. The R136Q variation presented significant differences between HIV-1-positive patients and healthy controls. The 136Q allele and the 136QQ genotype were more frequent in healthy controls (32.7 and 10.2 %, respectively) than in HIV-1-positive patients (136Q allele: 24.4 %; OR 0.66; CI 95 % 0.49-0.90; p value = 0.008/136QQ genotype: 4.2 %; OR 0.33; CI 95 % 0.13-0.79, p = 0.008) also after adjusting for age and sex. We also stratified our findings according to the presence of CCR5Δ32 variation, but the results remained the same. We observed that rs10838525 (R136Q) and rs3740996 (H43Y) were in linkage disequilibrium (D' = 0.71), forming four possible haplotypes. The H43-136Q haplotype was significantly more frequent in healthy controls (28.2 %) than in HIV-positive patients (21.4 %; OR 0.69; CI 95 % 0.50-0.96; p = 0.022). An increased frequency of allele (136Q) and genotype (136QQ) of the non-synonymous rs10838525 (R136Q) variant and the haplotype (43H-136Q) was observed among healthy controls individuals. Being aware of the limitation of this study (unavailability of exposed but uninfected individuals), we hypothesize a potential role for TRIM5 variations in the protection against HIV-1 infection.

Allosteric HIV-1 Integrase Inhibitors Lead to Premature Degradation of the Viral RNA Genome and Integrase in Target Cells

Recent evidence indicates that inhibition of HIV-1 integrase (IN) binding to the viral RNA genome by allosteric integrase inhibitors (ALLINIs) or through mutations within IN yields aberrant particles in which the viral ribonucleoprotein complexes (vRNPs) are eccentrically localized outside the capsid lattice. These particles are noninfectious and are blocked at an early reverse transcription stage in target cells. However, the basis of this reverse transcription defect is unknown. Here, we show that the viral RNA genome and IN from ALLINI-treated virions are prematurely degraded in target cells, whereas reverse transcriptase remains active and stably associated with the capsid lattice. The aberrantly shaped cores in ALLINI-treated particles can efficiently saturate and be degraded by a restricting TRIM5 protein, indicating that they are still composed of capsid proteins arranged in a hexagonal lattice. Notably, the fates of viral core components follow a similar pattern in cells infected with eccentric particles generated by mutations within IN that inhibit its binding to the viral RNA genome. We propose that IN-RNA interactions allow packaging of both the viral RNA genome and IN within the protective capsid lattice to ensure subsequent reverse transcription and productive infection in target cells. Conversely, disruption of these interactions by ALLINIs or mutations in IN leads to premature degradation of both the viral RNA genome and IN, as well as the spatial separation of reverse transcriptase from the viral genome during early steps of infection.IMPORTANCE Recent evidence indicates that HIV-1 integrase (IN) plays a key role during particle maturation by binding to the viral RNA genome. Inhibition of IN-RNA interactions yields aberrant particles with the viral ribonucleoprotein complexes (vRNPs) eccentrically localized outside the conical capsid lattice. Although these particles contain all of the components necessary for reverse transcription, they are blocked at an early reverse transcription stage in target cells. To explain the basis of this defect, we tracked the fates of multiple viral components in infected cells. Here, we show that the viral RNA genome and IN in eccentric particles are prematurely degraded, whereas reverse transcriptase remains active and stably associated within the capsid lattice. We propose that IN-RNA interactions ensure the packaging of both vRNPs and IN within the protective capsid cores to facilitate subsequent reverse transcription and productive infection in target cells.

Relationship of TRIM5 and TRIM22 polymorphisms with liver disease and HCV clearance after antiviral therapy in HIV/HCV coinfected patients

BACKGROUND AND AIMS

TRIM5 and TRIM22 are restriction factors involved in innate immune response and exhibit anti-viral activity. Single nucleotide polymorphisms (SNPs) at TRIM5 and TRIM22 genes have shown to influence several viral infections such as human immunodeficiency virus (HIV), hepatitis B, as well as measles and rubella vaccination. The aim of this study is to analyze whether TRIM5 and TRIM22 polymorphisms are associated with liver fibrosis inflammation-related biomarkers and response to pegylated-interferon-alpha plus ribavirin (pegIFNα/RBV) therapy in HIV/hepatitis C virus (HCV) coinfected patients.

METHODS

A retrospective study was performed in 319 patients who started pegIFNα/RBV therapy. Liver fibrosis stage was characterized in 288 patients. TRIM5 rs3824949 and TRIM22 polymorphisms (rs1063303, rs7935564, and rs7113258) were genotyped using the GoldenGate assay. The primary outcomes were: a) significant liver fibrosis (≥F2) evaluated by liver biopsy or transient elastography (liver stiffness values ≥7.1 Kpa); b) sustained virological response (SVR) defined as no detectable HCV viral load (<10 IU/mL) at week 24 after the end of the treatment. The secondary outcome variable was plasma chemokine levels.

RESULTS

Patients with TRIM5 rs3824949 GG genotype had higher SVR rate than patients with TRIM5 rs3824949 CC/CG genotypes (p = 0.013), and they had increased odds of achieving SVR (adjusted odds ratio (aOR = 2.58; p = 0.012). Patients with TRIM22 rs1063303 GG genotype had higher proportion of significant liver fibrosis than patients with rs1063303 CC/CG genotypes (p = 0.021), and they had increased odds of having significant hepatic fibrosis (aOR = 2.19; p = 0.034). Patients with TRIM22 rs7113258 AT/AA genotype had higher SVR rate than patients with rs7113258 TT genotypes (p = 0.013), and they had increased odds of achieving SVR (aOR = 1.88; p = 0.041). The TRIM22 haplotype conformed by rs1063303_C and rs7113258_A was more frequent in patients with SVR (p = 0.018) and was significantly associated with achieving SVR (aOR = 2.80; p = 0.013). The TRIM5 rs3824949 GG genotype was significantly associated with higher levels of GRO-α (adjusted arithmetic mean ratio ((aAMR) = 1.40; p = 0.011) and MCP-1 (aAMR = 1.61; p = 0.003).

CONCLUSIONS

TRIM5 and TRIM22 SNPs are associated to increased odds of significant liver fibrosis and SVR after pegIFNα/RBV therapy in HIV/HCV coinfected patients. Besides, TRIM5 SNP was associated to higher baseline levels of circulating biomarkers GRO and MCP-1.

Primate TRIM5 proteins form hexagonal nets on HIV-1 capsids

TRIM5 proteins are restriction factors that block retroviral infections by binding viral capsids and preventing reverse transcription. Capsid recognition is mediated by C-terminal domains on TRIM5α (SPRY) or TRIMCyp (cyclophilin A), which interact weakly with capsids. Efficient capsid recognition also requires the conserved N-terminal tripartite motifs (TRIM), which mediate oligomerization and create avidity effects. To characterize how TRIM5 proteins recognize viral capsids, we developed methods for isolating native recombinant TRIM5 proteins and purifying stable HIV-1 capsids. Biochemical and EM analyses revealed that TRIM5 proteins assembled into hexagonal nets, both alone and on capsid surfaces. These nets comprised open hexameric rings, with the SPRY domains centered on the edges and the B-box and RING domains at the vertices. Thus, the principles of hexagonal TRIM5 assembly and capsid pattern recognition are conserved across primates, allowing TRIM5 assemblies to maintain the conformational plasticity necessary to recognize divergent and pleomorphic retroviral capsids.

DOI:http://dx.doi.org/10.7554/eLife.16269.001

After infecting a cell, a virus reproduces by forcing the cell to produce new copies of the virus, which then spread to other cells. However, cells have evolved ways to fight back against these infections. For example, many mammalian cells contain proteins called restriction factors that prevent the virus from multiplying. The TRIM5 proteins form one common set of restriction factors that act against a class of viruses called retroviruses.

HIV-1 and related retroviruses have a protein shell known as a capsid that surrounds the genetic material of the virus. The capsid contains several hundred repeating units, each of which consists of a hexagonal ring of six CA proteins. Although this basic pattern is maintained across different retroviruses, the overall shape of the capsids can vary considerably. For instance, HIV-1 capsids are shaped like a cone, but other retroviruses can form cylinders or spheres.

Soon after the retrovirus enters a mammalian cell, TRIM5 proteins bind to the capsid. This causes the capsid to be destroyed, which prevents viral replication. Previous research has shown that several TRIM5 proteins must link up with each other via a region of their structure called the B-box 2 domain in order to efficiently recognize capsids. How this assembly process occurs, and why it enables the TRIM5 proteins to recognize different capsids was not fully understood. Now, Li, Chandrasekaran et al. (and independently Wagner et al.) have investigated these questions.

Using biochemical analyses and electron microscopy, Li, Chandrasekaran et al. found that TRIM5 proteins can bind directly to the surface of HIV-1 capsids. Several TRIM5 proteins link together to form large hexagonal nets, in which the B-box domains of the proteins are found at the points where three TRIM5 proteins meet. This arrangement mimics the pattern present in the HIV-1 capsid, and just a few TRIM5 rings can cover most of the capsid.

Li, Chandrasekaran et al. then analysed TRIM5 proteins from several primates, including rhesus macaques, African green monkeys and chimpanzees. In all cases analyzed, the TRIM5 proteins assembled into hexagonal nets, although the individual units within the net did not have strictly regular shapes. These results suggest that TRIM5 proteins assemble a scaffold that can deform to match the pattern of the proteins in the capsid. Further work is now needed to understand how capsid recognition is linked to the processes that disable the virus.

DOI:http://dx.doi.org/10.7554/eLife.16269.002

TRIM proteins regulate autophagy: TRIM5 is a selective autophagy receptor mediating HIV-1 restriction

The tripartite motif protein family (TRIM) constitutes a class of immune-regulated proteins with antiviral, immune, cancer, and other properties reminiscent of those ascribed to autophagy. We show that TRIMs have dual roles in autophagy: as regulators and as cargo receptors. As regulators, TRIMs nucleate the core autophagy machinery by acting as platforms that assemble ULK1 and BECN1 into a functional complex in preparation for autophagy. TRIMs also act as novel selective autophagy receptors as exemplified by TRIM5/TRIM5α, a known HIV-1 restriction factor with a hitherto poorly defined mode of action. TRIM5 recognizes and targets HIV-1 for autophagic destruction. TRIM5 interactions with mammalian Atg8 proteins are required for this effector function. This establishes TRIM family members as regulators of autophagy, explains the antiretroviral mechanism of TRIM5, and defines a new basis for selective autophagy.

An evolutionary screen highlights canonical and noncanonical candidate antiviral genes within the primate TRIM gene family

Recurrent viral pressure has acted on host-encoded antiviral genes during primate and mammalian evolution. This selective pressure has resulted in dramatic episodes of adaptation in host antiviral genes, often detected via positive selection. These evolutionary signatures of adaptation have the potential to highlight previously unrecognized antiviral genes (also called restriction factors). Although the TRIM multigene family is recognized for encoding several bona fide restriction factors (e.g., TRIM5alpha), most members of this expansive gene family remain uncharacterized. Here, we investigated the TRIM multigene family for signatures of positive selection to identify novel candidate antiviral genes. Our analysis reveals previously undocumented signatures of positive selection in 17 TRIM genes, 10 of which represent novel candidate restriction factors. These include the unusual TRIM52 gene, which has evolved under strong positive selection despite its encoded protein lacking a putative viral recognition (B30.2) domain. We show that TRIM52 arose via gene duplication from the TRIM41 gene. Both TRIM52 and TRIM41 have dramatically expanded RING domains compared with the rest of the TRIM multigene family, yet this domain has evolved under positive selection only in primate TRIM52, suggesting that it represents a novel host–virus interaction interface. Our evolutionary-based screen not only documents positive selection in known TRIM restriction factors but also highlights candidate novel restriction factors, providing insight into the interfaces of host–pathogen interactions mediated by the TRIM multigene family.

Understanding restriction factors and intrinsic immunity: insights and lessons from the primate lentiviruses

Primate lentiviruses include the HIVs, HIV-1 and HIV-2; the SIVs, which are endemic to more than 40 species of nonhuman primates in Africa; and SIVmac, an AIDS-causing pathogen that emerged in US macaque colonies in the 1970s. Because of the worldwide spread of HIV and AIDS, primate lentiviruses have been intensively investigated for more than 30 years. Research on these viruses has played a leading role in the discovery and characterization of intrinsic immunity, and in particular the identification of several antiviral effectors (also known as restriction factors) including APOBEC3G, TRIM5α, BST-2/tetherin and SAMHD1. Comparative studies of the primate lentiviruses and their hosts have proven critical for understanding both the evolutionary significance and biological relevance of intrinsic immunity, and the role intrinsic immunity plays in governing viral host range and interspecies transmission of viruses in nature.

Binding of the rhesus TRIM5α PRYSPRY domain to capsid is necessary but not sufficient for HIV-1 restriction

The PRYSPRY domain of TRIM5α provides specificity and the capsid recognition motif to retroviral restriction. Restriction of HIV-1 by rhesus TRIM5α (TRIM5αrh) has been correlated to its ability to bind to the HIV-1 core, suggesting that capsid binding is required for restriction. This work explores whether the PRYSPRY domain of TRIM5αrh exhibits an additional function besides binding to the HIV-1 core. Using our recently described structure of the PRYSPRY domain, we performed an exhaustive structure-function study of the surface and interior residues of the PRYSPRY domain. Testing retroviral restriction and capsid binding of an extensive collection of 60 TRIM5αrh PRYSPRY variants revealed that binding is necessary but not sufficient for restriction. In support of this hypothesis, we showed that some human tripartite motif proteins bind the HIV-1 capsid but do not restrict HIV-1 infection, such as human TRIM6 and TRIM34. Overall this work suggested that the PRYSPRY domain serves an unknown function, distinct from the binding of TRIM5αrh to the HIV-1 core, to block HIV-1 infection.

TRIM5 acts as more than a retroviral restriction factor

The retrovirus restriction factor TRIM5α blocks post-entry infection of retroviruses in a species-specific manner. As a cellular E3 ubiquitin ligase, TRIM5α binds to the retroviral capsid lattice in the cytoplasm of an infected cell and accelerates the uncoating process of retroviral capsid, thus providing a potent restriction to HIV-1 and other retrovirus infections. The precise mechanism by which this restriction is imposed remains under scrutiny, and evidence is lacking to link the E3 ubiquitin ligase activity of TRIM5α to its ability to restrict retrovirus infection. In a recent study, Pertel and colleagues have uncovered the link between the two, providing compelling evidence to suggest that following the interaction with the retroviral capsid, TRIM5 triggers an antiviral innate immune response by functioning as a pattern recognition receptor [1]. This unique function of TRIM5 is dependent on its association with the E2 ubiquitin-conjugating enzyme complex UBC13-UEV1A and subsequent activation of the TAK1 kinase complex and downstream genes involved in innate immune responses. These findings have defined a novel function for TRIM5 as a pattern recognition receptor in innate immune recognition and provided valuable mechanistic insight into its role as a retroviral restriction factor. Here we discuss the significance of these new findings in understanding TRIM5-mediated HIV restriction.

Cellular restriction factors of feline immunodeficiency virus

Lentiviruses are known for their narrow cell- and species-tropisms, which are determined by cellular proteins whose absence or presence either support viral replication (dependency factors, cofactors) or inhibit viral replication (restriction factors). Similar to Human immunodeficiency virus type 1 (HIV-1), the cat lentivirus Feline immunodeficiency virus (FIV) is sensitive to recently discovered cellular restriction factors from non-host species that are able to stop viruses from replicating. Of particular importance are the cellular proteins APOBEC3, TRIM5α and tetherin/BST-2. In general, lentiviruses counteract or escape their species’ own variant of the restriction factor, but are targeted by the orthologous proteins of distantly related species. Most of the knowledge regarding lentiviral restriction factors has been obtained in the HIV-1 system; however, much less is known about their effects on other lentiviruses. We describe here the molecular mechanisms that explain how FIV maintains its replication in feline cells, but is largely prevented from cross-species infections by cellular restriction factors.