Tumor-specific immune response: current in vitro analyses may not reflect the in vivo immune status

Although our knowledge and understanding of tumor-specific cytotoxic T lymphocytes (CTL) have expanded considerably, the long-term work needed to assay CTL has precluded their analysis in large numbers of patients. Moreover, in vitro culture steps may introduce major biases. New approaches to identify tumor-specific CTL clones would be helpful. As a means to describe the in situ immune status by T-cell repertoire analysis, we developed the Immunoscope approach, a PCR-based method that allows us to determine the spectra of CDR3 lengths of the TCR chains displayed by complex populations of T cells. We review here some of our data about melanoma. Tumor-infiltrating lymphocytes of a melanoma patient were analyzed by different means and melanoma-specific T-cell clones were derived. Two categories of tumor-specific CD8+ CTL clones were derived from the infiltrate of a tumor-proximal invaded lymph node. The majority of T-cell clones specifically lyse the autologous tumor cell lines and predominantly recognize the HLA-A2/MART-1(27-35) peptide complex. The in vivo representativity of such CTL was assessed by the immunoscope technology. Among three MART-1-specific clones, none was detectable in situ. The other kind of tumor-specific CD8+ CTL did not lyse autologous melanoma cell lines but lysed the "fresh" autologous tumor cells in a MHC class I dependent manner. The immunoscope approach revealed that one of the latter was detectable in situ among tumor-infiltrating lymphocytes although not among PBMC. These data indicate that melanoma-specific lymphocytes that could not have been selected through conventional screening procedures may be important in tumor rejection. Our results suggest that a better characterization of tumor-specific immune responses will be important for the optimization of specific immunotherapy strategies and the long-term follow-up of patients.