Adoptive cytotoxic T lymphocyte therapy triggers a counter-regulatory immunosuppressive mechanism via recruitment of myeloid-derived suppressor cells.

Complex interactions among multiple cell types contribute to the immunosuppressive milieu of the tumor microenvironment. Using a murine model of adoptive T-cell immunotherapy (ACT) for B16 melanoma, we investigated the impact of tumor infiltrating cells on this complex regulatory network in the tumor. Transgenic pmel-1-specific cytotoxic T lymphocytes (CTLs) were injected intravenously into tumor-bearing mice and could be detected in the tumor as early as on day 1, peaking on day 3. They produced IFN-γ, exerted anti-tumor activity and inhibited tumor growth. However, CTL infiltration into the tumor was accompanied by the accumulation of large numbers of cells, the majority of which were CD11b(+) Gr1(+) myeloid-derived suppressor cells (MDSCs). Notably, CD11b(+) Gr1(int) Ly6G(-) Ly6C(+) monocytic MDSCs outnumbered the CTLs by day 5. They produced nitric oxide, arginase I and reactive oxygen species, and inhibited the proliferation of antigen-specific CD8(+) T cells. The anti-tumor activity of the adoptively-transferred CTLs and the accumulation of MDSCs both depended on IFN-γ production on recognition of tumor antigens by the former. In CCR2(-/-) mice, monocytic MDSCs did not accumulate in the tumor, and inhibition of tumor growth by ACT was improved. Thus, ACT triggered counter-regulatory immunosuppressive mechanism via recruitment of MDSCs. Our results suggest that strategies to regulate the treatment-induced recruitment of these MDSCs would improve the efficacy of immunotherapy.