Tumor necrosis factor receptor (TNFR)-associated factors (TRAFs) are a family of intracellular signaling adaptors that associate with the cytoplasmic tails of a diverse range of lymphocyte receptors, including members of the TNFR superfamily, the Toll-like receptor (TLR)/interleukin-1 (IL-1) receptor superfamily, and the IL-6 receptor family that are major targets for therapeutic intervention for inflammatory diseases. TRAF5 is one of the seven family members of the TRAF family and is highly expressed by B- and T-lymphocytes. As compared to other family members, the biological and pathophysiological functions of TRAF5 have remained ambiguous since its discovery. TRAF5 promotes lymphocyte signaling for the TNFR family molecules such as glucocorticoid-induced TNFR family-related protein (GITR), CD27, and CD40. In contrast, TRAF5 limits the activity of the common signaling receptor subunit glycoprotein 130 kDa (gp130) in CD4⁺ T cells that requires signaling by IL-6 and IL-27. TRAF5 also restrains TLR signaling in B cells. Thus, TRAF5 regulates lymphocyte signaling in both positive and negative ways. This review will summarize the findings of recent studies of TRAF5 in terms of how TRAF5 regulates signaling in lymphocytes and other cell types and how TRAF5 expression contributes to inflammatory and autoimmune diseases in mice and humans.

Immune responses are orchestrated by controlling the initiation, magnitude, and duration of various signaling pathways. Adaptor proteins act as positive or negative regulators by targeting critical molecules of signaling cascades. Signal-transducing adaptor protein-2 (STAP-2) contains typical features of adaptor proteins, like a pleckstrin homology (PH) domain in the N-terminal region and a Src homology 2 (SH2) domain in the central region. STAP-2 binds to a variety of signaling or transcriptional molecules to control multiple steps of inflammatory/immune responses. STAP-2 enhances T-cell receptor (TCR)-mediated signaling via the association with TCR-proximal CD3ζ immunoreceptor tyrosine-based activation motifs (ITAMs) and lymphocyte-specific protein tyrosine kinase (Lck). STAP-2 decreases adherence of T-cells to fibronectin (FN) through an association with focal adhesion kinase (Fak) and Casitas B-lineage Lymphoma (c-Cbl), and increases chemotaxis of T-cells toward stromal cell-derived factor-1α (SDF-1α) through interactions with Vav1 and Ras-related C3 botulinum toxin substrate 1 (Rac1). STAP-2 positively regulates activation-induced cell deathrough the association with Fas and caspase-8. This review describes the current knowledge of the roles of STAP-2 in T-cell-dependent immune responses and the possible clinical utility of STAP-2-targeting therapies.