The role of Mad, Mnt and c-Myc proteins in OX40 stimulated T cells (33.26)

Abstract

Abstract We sought to determine the molecular mechanisms by which OX40 mediates T cell function. Gene array analysis of OX40-activated Ag-specific T cells suggested a role for the transcription factor c-Myc and antagonist Mad family proteins. D011.10 T cells were adoptively transferred into BALB/c recipients and immunized with soluble OVA and an agonist anti-OX40 antibody, which enhances Ag-specific T cell proliferation and survival. Western blot analysis of Ag-specific T cells purified ex vivo revealed that Mad4, Mnt and c-Myc proteins were up-regulated after OX40 engagement. Mad4 and Mnt protein levels peaked at days 3-4 and decreased as the cells contracted in size. C-Myc protein levels remained unchanged over time after OX40 stimulation. We hypothesize that the up-regulation of c-Myc in activated T cells drives proliferation and consequently activation-induced cell death, which is counteracted by Mad4 and Mnt proteins after anti-OX40 engagement, allowing T cells to survive the "blast crisis" phase. Indeed, siRNA knockdown of Mad4 and Mnt proteins led to decreased survival of OX40-activated T cells. We also found that OX40 stabilized both Mad4 and Mnt proteins against degradation and we demonstrated the presence of a serine residue in Mad4 that was essential in mediating accelerated protein degradation. These data provide evidence that Mad4 and Mnt play a role to enhance memory T cell survival after OX40 engagement.