Immunization of patients with metastatic melanoma using both class I- and class II-restricted peptides from melanoma-associated antigens

Cancer vaccines targeting CD8(+) T cells have been successful in eliciting immunologic responses but disappointing in inducing clinical responses. Strong evidence supports the importance of CD4(+) T cells in helping cytotoxic CD8(+) cells in antitumor immunity. We report here on two consecutive clinical trials evaluating the impact of immunization with both human leukocyte antigen class I- and class II-restricted peptides from the gp100 melanoma antigen. In Protocol 1, 22 patients with metastatic melanoma were immunized with two modified class I A*0201-restricted peptides, gp100:209-217(210M) and MART-1:26-35(27L). In Protocol 2, 19 patients received the same class I-restricted peptides in combination with a class 11 DRB1*0401-restricted peptide, gp100:44-59. As assessed by in vitro sensitization assays using peripheral blood mononuclear cells (PBMC) against the native ap100:209-217 peptide, 95% of patients in Protocol I were successfully immunized after two vaccinations in contrast to 50% of patients in Protocol 2 (P-2 < 0.005). Furthermore, the degree of sensitization was significantly lower in patients in Protocol 2 (P = 0.01). Clinically, one patient in Protocol 2 had an objective response, and none did in Protocol 1. Thus, the addition of the class II-restricted peptide gp100:44-59 did not improve clinical response but might have diminished the immunologic response of circulating PBMC to the class I-restricted peptide gp100:209-217. The reasons for this decreased immune reactivity are unclear but may involve increased CD4(+)CD25(+) regulatory T-cell activity, increased apoptosis of activated CD8(+) T cells, or the trafficking of sensitized CD8(+) reactive cells out of the peripheral blood. Moreover, the sequential, nonrandomized nature of patient enrollment for the two trials may account for the differences in immunologic response.