Fig. 2.

Activators and signaling pathways of MDSCs as well as the immunosuppressive mediators secreted by MDSCs. There are several activators of MDSC, mostly pro-inflammatory cytokines, complement factors, and many alarming factors. STAT, NF-κB, and C/EBPβ pathways are the key signaling mechanisms which activate the function of MDSCs. The major immunosuppressants include the cytokines IL-10 and TGF-β, the inducers of amino acid catabolism, i.e., ARG1 and IDO, and the activators of oxidative stress