In

addition to TCR signals, interactions Y-27632 datasheet between multiple ligands and their receptors are essential for the optimal activation of T cells. Several members of the TNFR superfamily, particularly OX40, 4-1BB, CD27, CD30 and HVEM, have been shown to provide signals both early and late after encounter with antigen 3, 4. We have shown that TNFR2 functions as one of the earliest members of the TNFR superfamily and plays a critical role in lowering the threshold for T-cell activation and in providing survival signals during the early phase of the T-cell response 6–8. Despite TNFR2′s role in providing crucial signals for initial T-cell activation, we found that it plays a critical role in limiting the duration RO4929097 in vivo of T-cell responses by promoting AICD. This study also provides novel insight regarding the mechanism by which TNFR1 and TNFR2 regulate AICD. AICD-resistant TNFR2−/− CD8+ T cells expressed high levels of intracellular TRAF2. Furthermore, blocking activated WT CD8+ T cells with anti-TNFR2

antibodies also increased intracellular TRAF2 levels with associated increase resistance to AICD (Figs. 1C and 3A). That TRAF2 provides pro-survival signals is supported by the observation that T cells expressing a dominant negative form of TRAF2 are much more susceptible to TNF-α-mediated cell death 16. However, in our retroviral transfection studies, the overexpression of TRAF2 in WT CD8+ T cells only increased the percentage of live cells without affecting the percentage of apoptotic cells (Fig. 3B). In this study, retroviral transfection may not lead to sufficiently high levels of intracellular TRAF2 to effectively block AICD. By contrast, silencing of TRAF2 in activated TNFR2−/− CD8+ T cells rendered them as sensitive to AICD as activated WT CD8+ T cells (Fig. 4) providing clear evidence that TRAF2 is directly involved in regulating cell death and apoptosis Aldol condensation in activated CD8+ T cells. Previous studies showed that the TRAF1 proteins could associate with TNFR1 and TNFR2 upon TNF-α binding 20 and the elevated levels of TRAF1 in activated CD8+ T cells could inhibit TNFR2-induced TRAF2 degradation 21. However, we found that silencing endogenous TRAF1

expression in either activated WT or TNFR2−/− CD8+ T cells did not affect the number of dead cells and apoptotic cells (data not shown) indicating that TRAF1 did not play a significant role in regulating cell death and apoptosis in activated CD8+ T cells. Our results support the hypothesis that TNFR1 functions as a pro-survival receptor in activated CD8+ T cells in the absence of TNFR2. This hypothesis is supported by the following observations: (i) activated WT and TNFR2−/− CD8+ T cells produced similar amounts of TNF-α, (ii) blocking of TNFR2 in WT CD8+ T cells rendered them more resistant to AICD and (iii) blocking antibodies to TNF-α increased susceptibility of activated TNFR2−/− CD8+ T cells to AICD. We propose the following model for these observations.