Understanding how TRAF6 controls T lymphocyte activation

The expression of TRAF6 (TNF Receptor-Associated Factor 6) is essential for many biological processes, which include signaling networks of the innate immune system, the production of osteoclasts and bone formation, lymph node organogenesis, and the development of hair follicles, sweat and sebaceous glands. TRAF6 is also needed for dendritic cell, B cell and T-cell function.
It is a tenet of the field of innate immunity that the essential role of TRAF6, an E3 ubiquitin ligase activity, is to generate Lys63-linked ubiquitin chains, which then activate TAK1, the "master kinase" of the system. Recently, one of our laboratories (PC) found that this long-held view is incorrect, since innate immune signaling could be restored by the re-expression of E3 ligase-inactive mutants of TRAF6 in TRAF6 knock-out (KO) cells. These studies also revealed that the E3 ligase activity of TRAF6 was not essential because two other E3 ligases, Pellino1 and Pellino2, are also able to generate the Lys63-linked ubiquitin chains required for signaling. An important conclusion from these studies was that the essential roles of TRAF6 in innate immunity are independent of its E3 ligase activity (1).
The PC lab also generated knock-in mice in which TRAF6 was replaced by the E3 ligase-inactive TRAF6[L74H] mutant. In contrast, to TRAF6 KO mice that have deformed bones and no teeth, the TRAF6[L74H] mice have normal bones and teeth, because the RANKL signaling pathway required for osteoclast formation is unimpaired in TRAF6[L74H] cells (1).
These remarkable findings have raised the question of what the physiological role of the TRAF6 E3 ligase might be? Previous studies showed that TRAF6 has a crucial role to restrain T cell effector function (2). Hence T cell selective deletion of TRAF6 results in deregulation of T cell function and the development of T cell mediated autoimmunity. Analysis of TRAF6[L74H] mice by the PC laboratory indicates that they have hyperactive T cells in lymph nodes and spleens. Moreover, these mice develop autoimmune inflammatory diseases indicating that the E3 ubiquitin ligase activity of TRAF6 has a critical role in T cells to restraint cell effector function. Since DC's lab is pre-eminent in the field of T cell signaling, the PC and DC labs propose to jointly supervise the PhD student who will elucidate the molecular mechanism by which TRAF6 restricts T cell activation.
The first part of the project will explore the development of effector and regulatory T cells in the secondary lymphoid tissues, gut and lung of TRAF6[L74H] mice. It will also explore the ability of TRAF6[L74H] T cells to mediate in vivo immune responses to pathogens and cancer and their role in autoimmunity. The second part will investigate the mechanisms by which TRAF6 activation suppresses T cell function which, in principle, could occur at any step in T cell signaling pathways. Pellino isoforms may function redundantly with TRAF6 and the project may benefit from the availability of Pellino knock-in mice developed in PC's laboratory, in which all three Pellinos have been replaced by E3 ligase-inactive mutants, alone and in combination. It has been described that the TRAF6 expression is required for IL-18 signaling in CD4 T cells and inhibits TGF-mediated suppression of IL-2 (3). We will therefore focus initially on how TRAF6 contributes to the signalling pathways used by these cytokines to control T cell biology. A fundamental strategy will be to use high resolution mass spectrometry to precisely map how IL-18 regulates the proteome, phosphoproteome and ubiquitome of wild type and TRAF6[L74H] CD4 T cells. How the signaling pathways controlled by these cytokines are integrated to control the cellular response of CD4 T cells is poorly understood at the molecular level and the complementary expertise of the DC and PC labs may enable important aspects of this problem to be cracked.