Purpose: Immunotherapy is a promising antitumor strategy, which can be successfully combined with current anticancer treatments, as suggested by recent studies showing the paradoxical chemotherapy-induced enhancement of the immune response. The purpose of the present work is to dissect the biological events induced by chemotherapy that cooperate with immunotherapy in the success of the combined treatment against cancer. In particular, we focused on the following: (a) cyclophosphamide-induced modulation of several cytokines, (b) homeostatic proliferation of adoptively transferred lymphocytes, and (c) homing of transferred lymphocytes to secondary lymphoid organs and tumor mass.

Experimental Design: Here, we used the adoptive transfer of tumor-immune cells after cyclophosphamide treatment of tumor-bearing mice as a model to elucidate the mechanisms by which cyclophosphamide can render the immune lymphocytes competent to induce tumor rejection.

Results: The transfer of antitumor immunity was found to be dependent on CD4⁺ T cells and on the cooperation of adoptively transferred cells with the host immune system. Of note, tumor-immune lymphocytes migrated specifically to the tumor only in mice pretreated with cyclophosphamide. Cyclophosphamide treatment also promoted homeostatic proliferation/activation of transferred B and T lymphocytes. Optimal therapeutic responses to the transfer of immune cells were associated with the cyclophosphamide-mediated induction of a “cytokine storm” [including granulocyte macrophage colony-stimulating factor, interleukin (IL)-1β, IL-7, IL-15, IL-2, IL-21, and IFN-γ], occurring during the “rebound phase” after drug-induced lymphodepletion.

Conclusions: The ensemble of these data provides a new rationale for combining immunotherapy and chemotherapy to induce an effective antitumor response in cancer patients.