Stuge et al. report a detailed analysis of the fine specificity of CD8 T cells against tumor-associated antigen in melanoma
patients . They compared peptide-
vaccination-driven with naturally arising T cell responses against the HLA-A0201 restricted melanoma peptide antigens M26 (derived from Melan-A/MART-1) and G209-2M (derived from gp100 protein). A major endpoint of this study was in vitro tumor cell
recognition by T cells. Fortunately, this is increasingly used as a golden standard in the assessment of tumor-specific T cells. The authors suggest that spontaneously arising antigen-specific T cell populations are qualitatively different from those induced by vaccination with heteroclitic peptides (which are altered for increased HLA binding): tumor cell recognition was found in nearly all T cells from the former, but only in a minority from the latter. As reported previously, these results correlated with recognition efficiency of antigenic peptides. We agree that this has considerable implications for immunotherapy and congratulate the authors for analyzing T cell recognition in great detail. However, in one point our own studies lead to different results: we repetitively found that the majority of T cells generated with the heteroclitic Melan-A M26 peptide were tumor reactive. This was the case for Melan-A-specific T cell populations generated in HLA-A0201 transgenic mice , in vitro , and in melanoma patients . The latter studies also assessed T cells from vaccination-site sentinel lymph nodes, containing T cells that are very likely selected and activated by vaccination and not by the tumor.
The authors point out correctly that tetramer T cells comprise many cells unable to recognize and kill tumor cells in an antigen-specific manner, presumably owing to low T cell receptor avidity to cognate antigen. An extreme case is nave T cell populations, of which the majority are unable to recognize tumor cells, despite their specific binding to MHC/ peptide tetramers . Therefore, it is crucial to exclude nave T cells from studies analyzing tumor recognition. HLA-A0201 humans (healthy individuals and melanoma patients) have 0.07 0.05 nave Melan-A tetramer cells within peripheral blood CD8 T cells ,. The three patients studied by Stuge et al. had 0.23, 0.12, and 0.50 Melan-A tetramer cells. Thus, one can estimate that the studied populations from the three patients contained approximately 30, 60, and 15 nave Melan-A-specific T cells, respectively. This is only a rough estimatetetramer analysis before vaccination and assessment of CD45RA/ CCR7 expression would give more insight. Nevertheless, it remains likely that the first two patients had considerably more nave cells than the third patient (i.e., the one without immunotherapy). In addition, nave-derived CD8 T cells have a higher clonogenic potential than activated Melan-A-specific T cells from melanoma patients (unpublished data). This means that overrepresentation of clones derived from nave CD8 T cells is likely to occur when both nave and activated antigen-specific CD8 T cells co-exist in a given lymphocyte population. As mentioned, Stuge et al. found unexpected high frequencies of T cell clones not recognizing tumor cells in the two vaccinated patients. It is conceivable that this was due to the presumably high percentages of nave Melan-A-specific cells present in the populations used for generating the clones, which would provide an explanation for the discrepancy with the results of our studies ,,.
Ethical considerations limit vaccination studies in healthy humans. In patients, candidate antigens should therefore be tested with strong adjuvants , to increase the likelihood that the studied responses are predominantly vaccination-driven, with only minor contribution of spontaneous T cell activation . It would be desirable to directly compare vaccination with heteroclitic peptide versus vaccination with natural peptide. However, this is hampered by the lack of ex v