Transmembrane TNF-α promotes activation-induced cell death by forward and reverse signaling.

Secretory tumor necrosis factor-alpha (sTNF-α) is known to mediate activation- induced cell death (AICD). However, the role of tmTNF-α in AICD is still obscure. Here, we demonstrated that tmTNF-α expression significantly increased accompanied with enhanced apoptosis during AICD in Jurkat and primary human T cells. Knockdown or enhancement of tmTNF-α expression in activated T cells suppressed or promoted AICD, respectively. Treatment of activated T cells with exogenous tmTNF-α significantly augmented AICD, indicating that tmTNF-α as an effector molecule mediates AICD. As tmTNF-α can function as a receptor, an anti-TNF-α polyclonal antibody was used to trigger reverse signaling of tmTNF-α. This antibody treatment upregulated the expression of Fas ligand, TNF-related apoptosis-inducing ligand and tmTNF-α to amplify AICD, and promoted activated T cells expressing death receptor 4, TNF receptor (TNFR) 1 and TNFR2 to enhance their sensitivity to AICD. Knockdown of TNFR1 or TNFR2 expression totally blocked tmTNF-α reverse signaling increased sensitivity to sTNF-α- or tmTNF-α-mediated AICD, respectively. Our results indicate that tmTNF-α functions as a death ligand in mediation of AICD and as a receptor in sensitization of activated T cells to AICD. Targeting tmTNF-α in activated T cells may be helpful in facilitating AICD for treatment of autoimmune diseases.