Initiation factor eIF4E binds to the 5-cap of eukaryotic mRNAs and plays a key role in the mechanism and regulation of translation. It may be regulated through its own phosphorylation and through inhibitory binding proteins (4E-BPs), which modulate its availability for initiation complex assembly. eIF4E phosphorylation is enhanced by phorbol esters. We show, using specific inhibitors, that this involves both the p38 mitogen-activated protein (MAP) kinase and Erk signaling pathways. Cell stresses such as arsenite and anisomycin and the cytokines tumor necrosis factor-␣ and interleukin-1␤ also cause increased phosphorylation of eIF4E, which is abolished by the specific p38 MAP kinase inhibitor, SB203580. These changes in eIF4E phosphorylation parallel the activity of the eIF4E kinase, Mnk1. However other stresses such as heat shock, sorbitol, and H 2 O 2 , which also stimulate p38 MAP kinase and increase Mnk1 activity, do not increase phosphorylation of eIF4E. The latter stresses increase the binding of eIF4E to 4E-BP1, and we show that this blocks the phosphorylation of eIF4E by Mnk1 in vitro, which may explain the absence of an increase in eIF4E phosphorylation under these conditions.

The antiretroviral restriction factor TRIM5 has recently emerged as an important mediator of innate immunity and species-specific inhibition of retroviral replication in mammals. Selection pressure from pathogenic infection has driven rapid evolution of TRIM5 genes, leading to the antiviral specificities we see today. Remarkably, the New World owl monkey (Aotus trivirgatus) encodes a TRIM5 protein in which the antiviral determinants in the B30.2 domain have been replaced by cyclophilin A (CypA) encoded by a retrotransposed cDNA. The owl monkey TRIMCyp protein restricts infection by a subset of lentiviruses that recruit CypA to their capsids, including HIV-1 and feline immunodeficiency virus. Here, we show that the Old World monkey, rhesus macaque (Macaca mulatta), also encodes a TRIMCyp protein that has arisen independently from that in owl monkeys. The rhesus TRIMCyp is encoded by a single, but common, allele (Mamu7) of the rhesus TRIM5 gene, among at least six further alleles that encode full-length TRIM5 proteins with no homology to CypA. The antiviral specificity of the rhesus TRIMCyp is distinct, restricting infection of HIV-2 and feline immunodeficiency virus but not HIV-1. Restriction by rhesus TRIMCyp is before reverse transcription and inhibited by blocking CypA binding, with cyclosporine A, or by mutation of the capsid CypA binding site. These observations suggest a mechanism of restriction that is conserved between TRIMCyp proteins. The lack of activity against HIV-1 suggests that Mamu7 homozygous animals will be null for TRIM5-mediated restriction of HIV-1 and could contribute to improved animal models for HIV/AIDS. cyclophilin ͉ lentivirus ͉ restriction ͉ TRIM5 ͉ zoonosis T RIM5 has recently been identified as a powerful restriction factor responsible for species-specific restriction of retroviral infectivity as part of the innate immune system (1-6). TRIM5 has a tripartite, or RBCC, motif consisting of RING, B Box 2, and coiled coil domains with the characteristic ordering and spacing that defines the TRIM family. Together with many other members of the TRIM family, TRIM5 has a C-terminal PRY/SPRY, or B30.2, domain. In the case of TRIM5, this domain defines antiviral specificity, probably by interacting directly with the incoming viral capsid (7-13). Antiretroviral TRIM5 variants have been described in primates, cattle, and rabbits (1-3, 14-16). These TRIM5 sequences form a monophyletic group, indicating that they are derived from a common ancestor that probably had antiviral properties (16). TRIM5 blocks retroviral infectivity by an incompletely characterized mechanism that involves the proteasome (6, 17, 18) and may involve capsid uncoating (13,19). In the New World owl monkey, the TRIM5 locus has been modified by insertion of a cyclophilin A (CypA) cDNA by retrotransposition into the seventh intron (20, 21), leading to the expression of a TRIM5 variant (TRIMCyp), in which CypA replaces the exon eight-encoded B30.2 domain. This change effectively replaces the antiviral specificity determinant, wi...

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Reactivation of lytic replication from viral latency is a defining property of all herpesviruses. Despite this, the authentic physiological cues for the latent-lytic switch are unclear. Such cues should ensure that viral lytic replication occurs under physiological conditions, predominantly in sites which facilitate transmission to permissive uninfected cells and new susceptible hosts. Kaposi's sarcoma-associated herpesvirus (KSHV) is associated with the B-cell neoplasm primary effusion lymphoma (PEL), in which the virus remains latent. We have previously shown that PEL cells have the gene expression profile and immunophenotype of cycling preplasma cells (plasmablasts). Here, we show that the highly active spliced isoform of plasma cell transcription factor X box binding protein 1 (XBP-1s) is a lytic switch for KSHV. XBP-1s is normally absent in PEL, but the induction of endoplasmic reticulum stress leads to XBP-1s generation, plasma cell-like differentiation, and lytic reactivation of KSHV. XBP-1s binds to and activates the KSHV immediate-early gene ORF50 and synergizes with the ORF50 gene product RTA to induce a full lytic cycle. These data suggest that KSHV remains latent until B-cell terminal differentiation into plasma cells, the transcriptional environment of which provides the physiological "lytic switch" through XBP-1s. This links B-cell terminal differentiation to KSHV lytic reactivation.

We conclude that macrophages lack functional pattern recognition receptors for this virus and that HIV-1 tropism for macrophages helps to establish a foothold in the host without triggering innate immune cellular activation, which would otherwise block viral infection effectively.

Lv1/TRIM5␣ (tripartite motif 5␣) has recently emerged as an important factor influencing species-specific permissivity to retroviral infection in a range of primates, including humans. Old World monkey TRIM5␣ blocks human immunodeficiency virus type 1 (HIV-1) infectivity, and the human and New World monkey TRIM5␣ 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 TRIM5␣ ortholog uncertain, and we therefore refer to it as bovine Lv1. Bovine Lv1 is closely related to primate TRIM5␣ 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.Retroviruses rely on host cell biology to complete their life cycle, and their success throughout mammalian evolution and their ability to jump between divergent species imply the use of conserved pathways. The identification and characterization of these host-virus interactions are expected to enable improvement in animal models of infection, including human immunodeficiency virus (HIV)-AIDS, yield novel antiviral strategies, and facilitate the use of retroviruses for therapeutic gene delivery. A successful approach has been to characterize examples of species-specific or cell-type-specific retroviral infectivity. This has revealed that, in many cases, poor infection is due to the presence of dominant inhibitors of retroviral replication, as opposed to the nonpermissive cells lacking specific activities required by the virus (3,7,10,20,34,39,40,46). Evidence for dominant antiviral factors encouraged the genetic screens that led to the discovery of the restriction factors APOBEC3G (32), tripartite motif 5␣ (TRIM5␣) (34), and TRIM-Cyp (22, 30).The prototype restriction factor is the murine Fv1 protein, which is derived from an endogenous retroviral gag sequence (5, 18). The gag-like Fv1 protein targets incoming murine leukemia virus (MLV) capsids (CAs) and blocks infection after viral DNA synthesis but before the formation of a provirus (16). There is also evidence to suggest that there may be dominant antiviral factors that restrict the late stage of the viral life cycle. The HIV-1 vpu protein has been shown to overcome a dominant block to viral egress that exists in human but not simian cells (8,43). APOBEC...

TRIM5␣ is a potent barrier to cross-species retroviral transmission, and TRIM5␣s from different species have divergent antiretroviral specificities. Multiple TRIM5 alleles circulate within rhesus macaque populations. Here we show that they too have different antiretroviral specificities, highlighting how TRIM5 genotypes contribute to protection in an individual or a population.TRIM5␣ is an important mediator of antiretroviral innate immunity in mammals and represents a significant barrier to zoonotic transmission. It blocks retroviral infection in a species-specific manner; for example, human immunodeficiency virus type 1 (HIV-1) is restricted by Old World monkey TRIM5␣ but is not significantly restricted by human TRIM5␣ (12,26,31). TRIM5␣ consists of RING, B-box 2, and coiledcoil domains (RBCC), comprising a tripartite motif, as well as a C-terminal B30.2 domain, which determines antiviral specificity, and appears to interact directly with the incoming viral capsid (27). Recently, multiple TRIM5 alleles have been identified in an Old World monkey, the rhesus macaque (Macaca mulatta) (17). These alleles have surprisingly divergent B30.2 domains and are maintained at high frequencies in macaque populations. Because variation in the sequence of the B30.2 domain can have such profound effects on the antiretroviral specificity of TRIM5␣, these divergent macaque B30.2 domains have likely been selected to interact with different viral capsids. Remarkably, one of the TRIM5 alleles, Mamu-7, encodes a TRIM5-cyclophilin A (CypA) fusion protein with a different spectrum of antiretroviral activity to TRIM5␣ (4, 13, 18, 29, 30). In Mamu-7/TRIMCyp, exon 6 is joined to a downstream CypA cDNA sequence, leaving a vestigial B30.2 domain in the genome (Fig. 1A). Here we demonstrate the differential restriction of HIV-2 by rhesus TRIM5 alleles and map the determinant of restriction to a polymorphism in the B30.2 V1 region. Furthermore, we show that the different TRIM5 alleles have dominant negative properties against each other when exogenously expressed.To further characterize the degree of polymorphism in rhesus macaques, we sequenced TRIM5 exon 8 from DNA purified from 31 Indian and 38 Chinese Macaca mulatta monkeys from the Biomedical Primate Research Centre breeding colony in Rijswijk, The Netherlands (30). We identified the TRIM5 alleles 30). Predicted B30.2 domain amino acid sequences are shown in Fig. 1B. We also identified a mutation, G402D, in one animal. We are unsure whether this represents a mutation or a genuine polymorphism but have included it in our analyses. In order to explore the antiretroviral specificities of the various B30.2 domains, we generated a murine leukemia virus (MLV)-based vector (32) that expresses TRIM5␣ Mamu-1, driven by an internal cytomegalovirus promoter, with a silent SalI site at the V-301 and D-302 codons, facilitating the insertion of the entire exon 8 sequences from at this site. We then transduced CRFK cells with vectors encoding the different B30.2 domains appended to the hemagglutinin...

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