Regulating the suppressors: apoptosis and inflammation govern the survival of tumor-induced myeloid-derived suppressor cells (MDSC)

Immune suppressive myeloid-derived suppressor cells (MDSC) are present in most cancer patients where they inhibit innate anti-tumor immunity and are a significant obstacle to cancer immunotherapy. Inflammation is a known inducer of Gr1⁺CD11b⁺ MDSC; however, the factors/conditions that regulate MDSC survival and half-life have not been identified. We have used mass spectrometry (MS) and proteomic analysis to identify proteins and pathways that regulate MDSC survival. This analysis revealed high expression of caspase family proteins and the Fas–FasL, p38 MAPK, and TGFβ pathways, suggesting that Fas–FasL apoptosis regulates MDSC survival. Flow cytometry, confocal microscopy, and western blot analyses confirmed the MS findings and demonstrated that Fas⁺ MDSC are susceptible to Fas-mediated killing in vitro. In vivo studies with FasL-deficient and Fas-deficient mice demonstrated that Fas–FasL interactions are essential for MDSC apoptosis and for rejection of established metastatic disease and survival and that FasL⁺ T cells are the effector population mediating MDSC apoptosis. MS findings validated by biological experiments demonstrated that inflammation increases MDSC levels by protecting MDSC from Fas-mediated apoptosis, possibly by activating p38 MAPK. These results demonstrate that MDSC half-life in vivo is regulated by FasL⁺ T cells and that inflammation increases MDSC levels by conferring resistance to Fas-mediated apoptosis and identifies T cells as the relevant effector cells causing MDSC apoptosis in vivo. This newly recognized mechanism for regulating MDSC levels identifies potential new targets for decreasing MDSC in cancer patients.