Role of nitric oxide in heat shock protein induced apoptosis of γδT cells

Activation induced cell death (AICD) has been proposed to serve as a mechanism to limit T lymphocyte proliferation induced by antigenic stimulation. Heat shock proteins (hsp60 and hsp70) expressed on oral tumor cells serve as ligands for peripheral blood gamma delta T lymphocytes. Tumor cell lysis by gamma delta T lymphocytes is mediated via recognition of hsp expressed on tumor cells. In the present study, we report that upon stimulation with hsp, gamma delta T lymphocytes isolated from oral cancer patients undergo AICD as confirmed by DNA ploidy, annexin V staining and confocal microscopy. In co-cultures of gamma delta T lymphocytes and tumor cells, addition of antihsp60 and antihsp70 MAb, but not anti-Fas MAb (ZB4), inhibited DNA fragmentation of gamma delta T lymphocytes. Flow cytometric analysis revealed a down regulation of Fas expression on gamma delta T lymphocytes upon incubation with hsp60 and hsp70. Increased expression of iNOS was observed in hsp-stimulated gamma delta T lymphocytes. Addition of monomethyl L-arginine monoacetate, competitive inhibitor of NOS, inhibited nitric oxide (NO) production and apoptosis of gamma delta T lymphocytes induced by hsp60 and hsp70. The NO-induced apoptosis of gamma delta T lymphocytes involves activation of caspase-9 and loss of mitochondrial membrane potential. The present study explains a novel strategy adopted by tumor cells to evade immune recognition by gamma delta T lymphocytes. (c) 2006 Wiley-Liss, Inc.