Low-dose cyclophosphamide (CP) therapy induces immunogenic tumor cell death and decreases regulatory T cell (Treg) numbers in mice with transplantable tumors. Using the ret transgenic murine melanoma model that resembles human melanoma, we detected no beneficial antitumor effects with such treatment, despite a decrease in Tregs. On the contrary, low-dose CP enhanced the production of chronic inflammatory mediators in melanoma lesions associated with increased accumulation of Gr1+ CD11b+ myeloid-derived suppressor cells (MDSCs), which exhibit elevated suppressive activity and nitric oxide (NO) production as well as inhibition of T-cell proliferation. Moreover, the frequencies of CD8+ T cells in the tumors and their ability to produce perforin were decreased. To study whether the observed CP-induced MDSC expansion and activation also occurs under chronic inflammatory tumor-free conditions, mice exhibiting chronic inflammation were treated with CP. Similar to tumor-bearing mice, CP-treated inflamed mice displayed elevated levels of MDSCs with enhanced production of NO, reactive oxygen species, and a suppressed in vivo natural killer (NK) cell cytotoxic activity indicating CP effects on the host immune system independent of the tumor. We suggest that melanoma therapy with low-dose CP could be efficient only when combined with the neutralization of MDSC immunosuppressive function and chronic inflammatory microenvironment.
Bibliographical noteFunding Information:
We thank I Nakashima for initially providing ret transgenic mice, and K Frank for excellent technical assistance. This work was supported by the DKFZ-MOST Cooperation in Cancer Research CA128 (to VU and MB), Dr Mildred Scheel Foundation for Cancer Research 108992 (to VU and DS), and the Initiative and Networking Fund of the Helmholtz Association within the Helmholtz Alliance on Immunotherapy of Cancer (to VU and DS).