Cyclophosphamide Augments Antitumor Immunity: Studies in an Autochthonous Prostate Cancer Model

Abstract

To study the immune response to prostate cancer, we developed an autochthonous animal model based on the transgenic adenocarcinoma of the mouse prostate (TRAMP) mouse in which spontaneously developing tumors express influenza hemagglutinin as a unique, tumor-associated antigen. Our prior studies in these animals showed immunologic tolerance to hemagglutinin, mirroring the clinical situation in patients with cancer who are generally nonresponsive to their disease. We used this physiologically relevant animal model to assess the immunomodulatory effects of cyclophosphamide when administered in combination with an allogeneic, cell-based granulocyte-macrophage colony-stimulating factor–secreting cancer immunotherapy. Through adoptive transfer of prostate/prostate cancer–specific CD8 T cells as well as through studies of the endogenous T-cell repertoire, we found that cyclophosphamide induced a marked augmentation of the antitumor immune response. This effect was strongly dependent on both the dose and the timing of cyclophosphamide administration. Mechanistic studies showed that immune augmentation by cyclophosphamide was associated with a transient depletion of regulatory T cells in the tumor draining lymph nodes but not in the peripheral circulation. Interestingly, we also noted effects on dendritic cell phenotype; low-dose cyclophosphamide was associated with increased expression of dendritic cell maturation markers. Taken together, these data clarify the dose, timing, and mechanism of action by which immunomodulatory cyclophosphamide can be translated to a clinical setting in a combinatorial cancer treatment strategy.

Selective Depletion of CD4+CD25+Foxp3+ Regulatory T Cells by Low-Dose Cyclophosphamide Is Explained by Reduced Intracellular ATP Levels

“…A commonly used chemotherapeutic agent CY can selectively deplete Treg cells when administered more frequenty or "metronomically" at a dose substantially lower than the maximum tolerated dose. Such low-dose CY is not only immunostimulatory but also avoids CY high-dose toxicity (16,42). In addition, the restoration of Treg numbers and function 10 days after low-dose CY administration implies that a limited period of Treg cell inhibition may avoid potential autoimmunity (15).…”

“…It is now clear that low-dose CY can selectively ablate CD4 + CD25 + Treg cells, leading to the enhancement of immune responses (14,15). Based on this principle, low-dose CY has been successfully tested in the treatment of various types of tumor (16)(17)(18). Moreover, our observations indicated that low-dose CY can effectively prevent the recurrence of condylomata acuminata caused by human papillomavirus (HPV) by selectively depleting Treg cells.…”

Immunogenic Chemotherapy Using Cyclophosphamide and Gemcitabine

“…Interestingly, several chemotherapeutic drugs have the potential to mitigate immunosuppression by Treg cells and MDSCs. Many studies have shown that low-dose CTX increases anti-tumor immune responses in tumor-bearing hosts by mitigating Treg cell-mediated immunosuppression [11][12][13][14][15][16]. CTX has multifaceted effects on immunity.…”

Breaking immunotolerance of tumors: A new perspective for dendritic cell therapy

“…The alkylating agent CYP, when given in low doses, selectively induces T reg cell depletion (Berd & Mastrangelo, 1987;Greten et al, 2010). Since the immunomodulatory properties of CYP include also DC activation and polarization of immune response to T-helper (T H )-1 cytokine secretion, the drug has been used in combination with both tumorcell vaccines and, more recently, with DC vaccines (Radojcic et al, 2010;Tzai et al, 1996;Wada et al 2009;Wersäll & Mellstedt, 1995). However, the efficacy of CYP in improving the effects of DC vaccines is still an open issue.…”