It is well known that naïve T cells transferred into lymphopenic hosts develop into memory like T cells and acquire some effector functions. We and others have been shown that transfer of naïve T cells following sublethal whole body irradiation augmented antitumor immunity and inhibited tumor progression. Further analyses showed that antitumor effector T cells were primed from transferred naïve T cells. Although the exact mechanism underlying this enhancement of antitumor immune responses remains unclear, possible explanations have been proposed; depletion of suppressor cells, improvement of tumor-antigen presentation, and elimination of lymphocytes competing activation cytokines. Combination of lymphodepletion by cytotoxic regimens, such as chemotherapy or radiotherapy, and transfer of naïve T cells seems to be a promising strategy. Although whole body irradiation induces lymphopenia to tumor-bearing animals and enhances antitumor immunity, it has not been routinely used in clinical settings. To test whether cytotoxic agents deplete lymphocytes and enhance antitumor immune responses, we infused cyclophosphamide (CPA), fludarabine, cisplatin and etoposide at the sublethal doses to mice. CPA treatment depleted 95% of lymphocytes in mice, and enhanced antitumor effects of transferred naïve T cells. Other cytotoxic drugs failed to augment antitumor immunity in combination with transfer of T cells. Previously, we have demonstrated that CD4+CD25+Foxp3+ regulatory T cells (Treg) and CD11b+Gr-1+ myeloid-derived suppressor cells (MDSC) inhibited priming of antitumor effector T cells. Depletion of these suppressor cells increased the number of tumor-specific T cells and augmented antitumor immune responses. To examine the effect of sublethal doses of cyclophosphamide on immune suppressor cells, we injected cyclophosphamide into mice and harvested lymph-nodes and spleens for FACS analyses. Unexpectedly, we found a significant increase in the frequency of Treg and MDSC after CPA treatment. Magnetically isolated Treg and MDSC from cyclophosphamide treated mice suppressed tumor-specific responses of effector T cells in vitro. Depletion of Treg with anti-CD25 monoclonal antibodies following CPA administration and transfer of naïve T cells increased the number of antitumor effector T cells. Further, the combination of Treg depletion, CPA treatment and transfer of naïve T cells succeeded to cure 20-day established skin tumors in mice. We have also been testing whether inhibition of MDSC after CPA treatment augments antitumor immunity. Our results showed that CPA treatment efficiently depletes lymphocytes and enhanced antitumor immune responses despite an increase of immune suppressor cells. Further inhibition of suppressor cells after CPA administration successfully treated advanced tumors.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1550. doi:1538-7445.AM2012-1550