Human Prostate Tumor Antigen–Specific CD8+ Regulatory T Cells Are Inhibited by CTLA-4 or IL-35 Blockade

Abstract

Regulatory T cells play important roles in cancer development and progression by limiting the generation of innate and adaptive anti-tumor immunity. We hypothesized that in addition to natural CD4+CD25+ regulatory T cells (Tregs) and myeloid-derived suppressor cells, tumor Ag–specific Tregs interfere with the detection of anti-tumor immunity after immunotherapy. Using samples from prostate cancer patients immunized with a DNA vaccine encoding prostatic acid phosphatase (PAP) and a trans-vivo delayed-type hypersensitivity (tvDTH) assay, we found that the detection of PAP-specific effector responses after immunization was prevented by the activity of PAP-specific regulatory cells. These regulatory cells were CD8+CTLA-4+, and their suppression was relieved by blockade of CTLA-4, but not IL-10 or TGF-β. Moreover, Ag-specific CD8+ Tregs were detected prior to immunization in the absence of PAP-specific effector responses. These PAP-specific CD8+CTLA-4+ suppressor T cells expressed IL-35, which was decreased after blockade of CTLA-4, and inhibition of either CTLA-4 or IL-35 reversed PAP-specific suppression of tvDTH response. PAP-specific CD8+CTLA-4+ T cells also suppressed T cell proliferation in an IL-35–dependent, contact-independent fashion. Taken together, these findings suggest a novel population of CD8+CTLA-4+ IL-35–secreting tumor Ag–specific Tregs arise spontaneously in some prostate cancer patients, persist during immunization, and can prevent the detection of Ag-specific effector responses by an IL-35–dependent mechanism.