Quiescent phenotype of tumor-specific CD8+ T cells following immunization.

In a human melanoma model of tumor antigen (TA)-based immunization, we tested the functional status of TA-specific CD8+ cytotoxic T lymphocytes. A "quiescent" phenotype lacking direct ex vivo cytotoxic and proliferative potential was identified that was further characterized by comparing its transcriptional profile to that of TA-specific T cells sensitized in vitro by exposure to the same TA and the T-cell growth factor interleukin 2 (IL-2). Quiescent circulating tumor-specific CD8+ T cells were deficient in expression of genes associated with T-cell activation, proliferation, and effector function. This quiescent status may explain the observed lack of correlation between the presence of circulating immunization-induced lymphocytes and tumor regression. In addition, the activation of TA-specific T cells by in vitro antigen recall and IL-2 suggests that a complete effector phenotype might be reinstated in vivo to fulfill the potential of anticancer vaccine protocols.