Clonal expansion of antigen-specific CD8+cytotoxic T lymphocytes is regulated by late exposure to serum to prevent apoptosis

Although serum-free media have been used to expand lymphokine-activated kil ler cells, antigen-specific CD8 T cell cytotoxicity does not develop in vit ro in the absence of serum. The immunodominant V(beta)17 response to an inf luenza A matrix protein epitope restricted by HLA A2.1 was used to study th e serum requirement for CTL activation. Serum acts directly on T cells and not indirectly by activating APCs. In the absence of serum, the initial ste ps of T cell activation, including expression of CD69 and CD25, are unimpai red and some antigen-specific cytotoxicity may be generated in the first fe w days after stimulation. However, expression of late activation markers, s uch as HLA-DR and CD38, and clonal expansion of class I-restricted antigen- specific CTL does not occur if CTL are not exposed to serum within 4 days o f antigen exposure. The antigen-specific CTL, but not unstimulated bystande r T cells, undergo apoptosis if they are not exposed to serum within a few days of activation. Apoptosis of TCR-activated CTL does not appear to be Fa s-mediated since it is not blocked by inhibiting the Fas pathway. Therefore , late exposure to an unidentified serum protein regulates the clonal expan sion of TCR-activated CD8 CTL. (C) 1999 Elsevier Science B.V. All rights re served.