Byeongwoon Song

Retrovirus Restriction by TRIM5α Variants from Old World and New World Primates

ABSTRACT The TRIM5α proteins of humans and some Old World monkeys have been shown to block infection of particular retroviruses following virus entry into the host cell. Infection of most New World monkey cells by the simian immunodeficiency virus of macaques (SIVmac) is restricted at a similar point. Here we examine the antiretroviral activity of TRIM5α orthologs from humans, apes, Old World monkeys, and New World monkeys. Chimpanzee and orangutan TRIM5α proteins functionally resembled human TRIM5α, potently restricting infection by N-tropic murine leukemia virus (N-MLV) and moderately restricting human immunodeficiency virus type 1 (HIV-1) infection. Notably, TRIM5α proteins from several New World monkey species restricted infection by SIVmac and the SIV of African green monkeys, SIVagm. Spider monkey TRIM5α, which has an expanded B30.2 domain v3 region due to a tandem triplication, potently blocked infection by a range of retroviruses, including SIVmac, SIVagm, HIV-1, and N-MLV. Tandem duplications in the TRIM5α B30.2 domain v1 region of African green monkeys are also associated with broader antiretroviral activity. Thus, variation in TRIM5α proteins among primate species accounts for the observed patterns of postentry restrictions in cells from these animals. The TRIM5α proteins of some monkey species exhibit dramatic lengthening of particular B30.2 variable regions and an expanded range of susceptible retroviruses.

The B30.2(SPRY) Domain of the Retroviral Restriction Factor TRIM5α Exhibits Lineage-Specific Length and Sequence Variation in Primates

ABSTRACT Tripartite motif (TRIM) proteins are composed of RING, B-box 2, and coiled coil domains. Some TRIM proteins, such as TRIM5α, also possess a carboxy-terminal B30.2(SPRY) domain and localize to cytoplasmic bodies. TRIM5α has recently been shown to mediate innate intracellular resistance to retroviruses, an activity dependent on the integrity of the B30.2 domain, in particular primate species. An examination of the sequences of several TRIM proteins related to TRIM5 revealed the existence of four variable regions (v1, v2, v3, and v4) in the B30.2 domain. Species-specific variation in TRIM5α was analyzed by amplifying, cloning, and sequencing nonhuman primate TRIM5 orthologs. Lineage-specific expansion and sequential duplication occurred in the TRIM5α B30.2 v1 region in Old World primates and in v3 in New World monkeys. We observed substitution patterns indicative of selection bordering these particular B30.2 domain variable elements. These results suggest that occasional, complex changes were incorporated into the TRIM5α B30.2 domain at discrete time points during the evolution of primates. Some of these time points correspond to periods during which primates were exposed to retroviral infections, based on the appearance of particular endogenous retroviruses in primate genomes. The results are consistent with a role for TRIM5α in innate immunity against retroviruses.

Retroviral Restriction Factor TRIM5α Is a Trimer

ABSTRACT The retrovirus restriction factor TRIM5α targets the viral capsid soon after entry. Here we show that the TRIM5α protein oligomerizes into trimers. The TRIM5α coiled-coil and B30.2(SPRY) domains make important contributions to the formation and/or stability of the trimers. A functionally defective TRIM5α mutant with the RING and B-box 2 domains deleted can form heterotrimers with wild-type TRIM5α, accounting for the observed dominant-negative activity of the mutant protein. Trimerization potentially allows TRIM5α to interact with threefold pseudosymmetrical structures on retroviral capsids.

The Human TRIM5α Restriction Factor Mediates Accelerated Uncoating of the N-Tropic Murine Leukemia Virus Capsid

ABSTRACT The host cell factors TRIM5αhu 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 TRIM5αhu and Fv-1. In this study, we investigate the fate of the MLV capsid in cells expressing either the TRIM5αhu or Fv-1 restriction factor. The expression of TRIM5αhu, but not Fv-1, specifically promoted the premature conversion of particulate N-MLV capsids within infected cells to soluble capsid proteins. The TRIM5αhu-mediated disassembly of particulate N-MLV capsids was dependent upon residue 110 of the viral capsid. Furthermore, the deletion or disruption of TRIM5αhu domains necessary for potent N-MLV restriction completely abrogated the disappearance of particulate N-MLV capsids observed with wild-type TRIM5αhu. These results suggest that premature disassembly of the viral capsid contributes to the restriction of N-MLV infection by TRIM5αhu, but not by Fv-1.

Binding and Susceptibility to Postentry Restriction Factors in Monkey Cells Are Specified by Distinct Regions of the Human Immunodeficiency Virus Type 1 Capsid

ABSTRACT In cells of Old World and some New World monkeys, dominant factors restrict human immunodeficiency virus type 1 (HIV-1) infections after virus entry. The simian immunodeficiency virus SIVmac is less susceptible to these restrictions, a property that is determined largely by the viral capsid protein. For this study, we altered exposed amino acid residues on the surface of the HIV-1 capsid, changing them to the corresponding residues found on the SIVmac capsid. We identified two distinct pathways of escape from early, postentry restriction in monkey cells. One set of mutants that were altered near the base of the cyclophilin A-binding loop of the N-terminal capsid domain or in the interdomain linker exhibited a decreased ability to bind the restricting factor(s). Consistent with the location of this putative factor-binding site, cyclophilin A and the restricting factor(s) cooperated to achieve the postentry block. A second set of mutants that were altered in the ridge formed by helices 3 and 6 of the N-terminal capsid domain efficiently bound the restricting factor(s) but were resistant to the consequences of factor binding. These results imply that binding of the simian restricting factor(s) is not sufficient to mediate the postentry block to HIV-1 and that SIVmac capsids escape the block by decreases in both factor binding and susceptibility to the effects of the factor(s).

Role for herpes simplex virus 1 ICP27 in the inhibition of type I interferon signaling

Host cells respond to viral infection by many mechanisms, including the production of type I interferons which act in a paracrine and autocrine manner to induce the expression of antiviral interferon-stimulated genes (ISGs). Viruses have evolved means to inhibit interferon signaling to avoid induction of the innate immune response. Herpes simplex virus 1 (HSV-1) has several mechanisms to inhibit type I interferon production, the activities of ISGs, and the interferon signaling pathway itself. We report that the inhibition of the Jak/STAT pathway by HSV-1 requires viral gene expression and that viral immediate-early protein ICP27 plays a role in downregulating STAT-1 phosphorylation and in preventing the accumulation of STAT-1 in the nucleus. We also show that expression of ICP27 by transfection causes an inhibition of IFN-induced STAT-1 nuclear accumulation. Therefore, ICP27 is necessary and sufficient for at least some of the effects of HSV infection on STAT-1.

Herpes Simplex Virus-Induced Keratitis: Evaluation of the Role of Molecular Mimicry in Lesion Pathogenesis

ABSTRACT Viruses are suspected but usually unproven triggering factors in autoimmunity. One favored mechanism to explain the role of viruses in the genesis of autoimmunity is molecular mimicry. An immunoinflammatory blinding lesion called herpetic stromal keratitis (HSK) that follows ocular infection with herpes simplex virus (HSV) is suggested to result from a CD4+ T-cell response to a UL6 peptide of HSV that cross-reacts with a corneal autopeptide shared with the immunoglobulin G2ab (IgG2ab) isotype. The present report reevaluates the molecular mimicry hypothesis to explain HSK pathogenesis. Our results failed to reveal cross-reactivity between the UL6 and IgG2ab peptides or between peptide reactive T cells and HSV antigens. More importantly, animals infected with HSV failed to develop responses that reacted with either peptide, and infection with a recombinant vaccinia UL6 vector failed to cause HSK, in spite of generating UL6 reactivity. Other lines of evidence also failed to support the molecular mimicry hypothesis, such as the failure to affect HSK severity upon tolerization of susceptible BALB/c and B-cell-deficient mice with IgG2ab or UL6 peptides. An additional study system revealed that HSK could be induced in mouse strains, such as the OT2 × RAG1−/− mice (T cell receptor transgenic recognizing OVA323–339) that were unable to produce CD4+ T-cell responses to any detectable HSV antigens. Our results cast doubt on the molecular mimicry hypothesis as an explanation for the pathogenesis of HSK and indicate that if autoimmunity is involved its likely proceeds via a bystander activation mechanism.

Simian TRIM5α proteins reduce replication of herpes simplex virus.

Old World monkey TRIM5alpha proteins are known to block the replication of human immunodeficiency virus and other retroviruses in a species-specific fashion. In this report, we show that specific forms of simian TRIM5alpha proteins can restrict herpes simplex virus (HSV) infection. To define the effect of TRIM5alpha on HSV replication, we examined HSV infection in HeLa cell lines that stably express simian and human orthologs of TRIM5alpha proteins. We demonstrated that several simian TRIM5alpha proteins can restrict HSV replication, with the TRIM5alpha protein of rhesus macaques showing the strongest inhibition of HSV infection. We also found that the level of the inhibition of virus replication was viral strain-specific. TRIM5alpha is likely to inhibit HSV at the early stage of infection; however, at later times of infection, the levels of TRIM5alpha are significantly decreased. Thus, some TRIM5alpha proteins exhibit antiviral effects that extend beyond retroviral infections, but HSV may be able to reduce this restriction by reducing TRIM5alpha levels during the later phases of virus replication. Our results also argue that TRIM5alpha is only part of the reduced level of HSV replication in rhesus macaques, which are known to be less susceptible to HSV infection than other primates.