Partial CD4 depletion reduces regulatory T cells induced by multiple vaccinations and restores therapeutic efficacy.

PURPOSE: A single vaccination of intact or reconstituted-lymphopenic mice (RLM) with a granulocyte macrophage colony-stimulating factor-secreting B16BL6-D5 melanoma cell line induces protective antitumor immunity and T cells that mediate the regression of established melanoma in adoptive immunotherapy studies. We wanted to study if multiple vaccinations during immune reconstitution of the lymphopenic host would maintain a potent antitumor immune response. EXPERIMENTAL
DESIGN: RLM were vaccinated multiple times over a 40-day period. Spleens were isolated from these mice, activated in vitro, and adoptively transferred into mice bearing 3-day experimental pulmonary metastases.
RESULTS: Multiple vaccinations, rather than boosting the immune response, significantly reduced therapeutic efficacy of adoptive immunotherapy and were associated with an increased frequency and absolute number of CD3+CD4+Foxp3+ T regulatory (T(reg)) cells. Anti-CD4 administration reduced the absolute number of T(reg) cells 9-fold. Effector T-cells generated from anti-CD4-treated mice were significantly (P < 0.0001) more therapeutic in adoptive transfer studies than T cells from multiply vaccinated animals with a full complement of CD4+ cells.
CONCLUSION: These results suggest that CD4+ T(reg) cells limit the efficacy of multiple vaccinations and that timed partial depletion of CD4+ T cells may reduce suppression and "tip-the-balance" in favor of therapeutic antitumor immunity. The recent failure of large phase III cancer vaccine clinical trials, wherein patients received multiple vaccines, underscores the potential clinical relevance of these findings.