An expanded peripheral T cell population to a cytotoxic T lymphocyte (CTL)-defined, melanocyte-specific antigen in metastatic melanoma patients impacts on generation of peptide-specific CTLs but does not overcome tumor escape from immune surveillance in metastatic lesions

It is not known if immune response to T cell-defined human histocompatibili ty leukocyte antigen (HLA) class I-restricted melanoma antigens leads to an expanded peripheral pool of T cells in all patients, affects cytotoxic T l ymphocyte (CTL) generation, and correlates with anti-tumor response in meta static lesions. To this end, a limiting dilution analysis technique was dev eloped that allowed us to evaluate the same frequency of peptide-specific T cells as by staining T cells with HLA-peptide tetrameric complexes. In fou r out of nine patients, Melan-A/Mart-1(27-35)-specific CTL precursors (CTLp ) were greater than or equal to 1/2,000 peripheral blood lymphocytes and fo und mostly or only in the CD45RO(+) memory T cell subset. In the remaining five patients, a low (<1/40,000) peptide-specific CTLp frequency was measur ed, and the precursors were only in the CD45RA(divided by) naive T cell sub set. Evaluation of CTL effector frequency after bulk culture indicated that peptide-specific CTLs could be activated in all patients by using professi onal antigen-presenting cells as dendritic cells, but CTLp frequency determ ined the kinetics of generation of specificity and the final number of effe ctors as evaluated by both limiting dilution analysis and staining with HLA -A*0201-Melan-A/Mart-1 tetrameric complexes. Immunohistochemical analysis o f 26 neoplastic lesions from the nine patients indicated absence of turner regression in most instances, even in patients with an expanded peripheral T cell pool to Melan-A/Mart-1 and whose neoplastic lesions contained a high frequency of tetramer-positive Melan-A/Mart-1-specific T cells. Furthermor e, frequent lack of a "brisk" or "nonbrisk" CD3(+)CD8(divided by) T cell in filtrate or reduced/absent Melan-A/Mart-1 expression in several lesions and lack of HLA class I antigens were found in some instances. Thus, expansion of peripheral immune repertoire to Melan-A/Mart-1 takes place in some meta static patients and leads to enhanced CTL induction after antigen-presentin g cell-mediated selection, but, in most metastatic lesions, it does not ove rcome tumor escape from immune surveillance.