Caspase-8 Regulation by Direct Interaction with TRAF6 in T Cell Receptor-Induced NF-κB Activation

Triggering of lymphocyte antigen receptors is the critical first step in the adaptive immune response against pathogens. T cell receptor (TCR) ligation assembles a large membrane signalosome, culminating in NF-κB activation [1, 2]. Recently, caspase-8 was found to play a surprisingly prominent role in lymphocyte activation in addition to its well-known role in apoptosis [3]. Caspase-8 is activated after TCR stimulation and nucleates a complex with B cell lymphoma 10 (BCL10), paracaspase MALT1, and the inhibitors of κB kinase (IKK) complex [4]. We now report that the ubiquitin ligase TRAF6 binds to active caspase-8 upon TCR stimulation and facilitates its movement into lipid rafts. We identified in silico two putative TRAF6 binding motifs in the caspase-8 sequence [5] and found that mutation of critical residues within these sites abolished TRAF6 binding and diminished TCR-induced NF-κB activation. Moreover, RNAi-mediated silencing of TRAF6 abrogated caspase-8 recruitment to the lipid rafts. Protein kinase Cθ (PKCθ), CARMA1, and BCL10 are also required for TCR-induced caspase-8 relocation, but only PKCθ and BCL10 control caspase-8 activation. Our results suggest that PKCθ independently controls CARMA1 phosphorylation and BCL10-dependent caspase-8 activation and unveil an essential role for TRAF6 as a critical adaptor linking these two convergent signaling events.