The innate immune roles of host factors TRIM5α and Cyclophilin A on HIV-1 replication

To TRIM the Immunity: From Innate to Adaptive Immunity

The tripartite motif (TRIM) proteins have been intensively studied as essential modulators in various biological processes, especially in regulating a wide range of signaling pathways involved in immune responses. Most TRIM proteins have E3 ubiquitin ligase activity, mediating polyubiquitination of target proteins. Emerging evidence demonstrates that TRIM proteins play important roles in innate immunity by regulating pattern recognition receptors, vital adaptor proteins, kinases, and transcription factors in innate immune signaling pathways. Additionally, the critical roles of TRIM proteins in adaptive immunity, especially in T cell development and activation, are increasingly appreciated. In this review, we aim to summarize the studies on TRIMs in both innate and adaptive immunity, focusing on their E3 ubiquitin ligase functions in pattern recognition receptor signaling pathways and T cell functions, shedding light on the developing new strategies for modulating innate and adaptive immune responses against invading pathogens and avoiding autoimmunity.

Activation of NF-κB induced by TRIMCyp showing a discrepancy between owl monkey and northern pig-tailed macaque

TRIMCyp generated by retrotransposition of a cyclophilin A inserting into TRIM5 locus, has been identified in owl monkey and most of Old World monkeys (OWM). Owl monkey TRIMCyp (omTRIMCyp) inhibits HIV-1 infection by direct interaction with viral capsid and indirect innate immune induction, whereas most of TRIMCyps from OWM cannot inhibit HIV-1, and the impact of which on immunoregulation is largely unknown. Here we reported that omTRIMCyp induces NF-κB, AP-1 and IFN-β activation in a dose-dependent manner, while TRIMCyp from northern pig-tailed macaque (npmTRIMCyp) does not activate NF-κB and moderately enhances AP-1 and IFN-β activities. The Cyclophilin A (CypA) domain plays an important role in omTRIMCyp-mediated NF-κB activation, and RBCC domains have a synergetic effect. We further indicated the mechanism by which npmTRIMCyp unable to activate NF-κB is that npmTRIMCyp hardly phosphorylates IκBα, different from omTRIMCyp which dramatically induces IκBα phosphorylation. Ubiquitination activity of omTRIMCyp was greater than npmTRIMCyp, although both could be ubiquitylated. Given that npmTRIMCyp neither interacts with viral capsid resulting in susceptibility to HIV-1 infection, nor activates NF-κB that is indispensable to HIV-1 provirus transcription, we proposed a model that npmTRIMCyp may play an important role in HIV-1 infected northern pig-tailed macaque with latency.

Novel host restriction factors implicated in HIV-1 replication

Human immunodeficiency virus-1 (HIV-1) is known to interact with multiple host cellular proteins during its replication in the target cell. While many of these host cellular proteins facilitate viral replication, a number of them are reported to inhibit HIV-1 replication at various stages of its life cycle. These host cellular proteins, which are known as restriction factors, constitute an integral part of the host's first line of defence against the viral pathogen. Since the discovery of apolipoprotein B mRNA-editing enzyme 3G (APOBEC3G) as an HIV-1 restriction factor, several human proteins have been identified that exhibit anti-HIV-1 restriction. While each restriction factor employs a distinct mechanism of inhibition, the HIV-1 virus has equally evolved complex counter strategies to neutralize their inhibitory effect. APOBEC3G, tetherin, sterile alpha motif and histidine-aspartate domain 1 (SAMHD1), and trim-5α are some of the best known HIV-1 restriction factors that have been studied in great detail. Recently, six novel restriction factors were discovered that exhibit significant antiviral activity: endoplasmic reticulum α1,2-mannosidase I (ERManI), translocator protein (TSPO), guanylate-binding protein 5 (GBP5), serine incorporator (SERINC3/5) and zinc-finger antiviral protein (ZAP). The focus of this review is to discuss the antiviral mechanism of action of these six restriction factors and provide insights into the probable counter-evasion strategies employed by the HIV-1 virus. The recent discovery of new restriction factors substantiates the complex host-pathogen interactions occurring during HIV-1 pathogenesis and makes it imperative that further investigations are conducted to elucidate the molecular basis of HIV-1 replication.

Rhesus monkey TRIM5α protein SPRY domain contributes to AP-1 activation

TRIM5α is an important host restriction factor that could potently block retrovirus infection. The SPRY domain of TRIM5α mediates post-entry restriction by recognition of and binding to the retroviral capsid. Human TRIM5α also functions as an innate immune sensor to activate AP-1 and NF-κB signaling, which subsequently restrict virus replication. Previous studies have shown that the AP-1 and NF-κB signaling activation relies on the RING motif of TRIM5α. In this study, we have demonstrated that the SPRY domain is essential for rhesus macaque TRIM5α to activate AP-1 but not NF-κB signaling. The AP-1 activation mainly depends on all of the β-sheet barrel on SPRY structure of TRIM5α. Furthermore, the SPRY-mediated auto-ubiquitination of TRIM5α is required for AP-1 activation. This study reports that rhesus macaque TRIM5α mainly undergoes Lys²⁷-linked and Met¹-linked auto-polyubiquitination. Finally, we found that the TRIM5α signaling function was positively correlated with its retroviral restriction activity. This study discovered an important role of the SPRY domain in immune signaling and antiviral activity and further expanded our knowledge of the antiviral mechanism of TRIM5α.