fig2

Figure 2. The immunosuppressive role of macrophages and myeloid-derived suppressor cells (MDSCs) in tumors. (A) The two phenotypes of TAMs and their respective markers and functions. (B) TAM2 and MDSCs suppress tumor killing through five immunosuppressive mechanisms: (i) via cell-to-cell contact, e.g., programmed cell death ligand-1(PD-L1) and cluster of differentiation 80 (CD80); (ii) secreted factors interleukin-10 (IL-10) and transforming growth factor-β (TGFβ); (iii) expression of enzymes arginase-1 (ARG1), inducible nitric oxide synthase (iNOS), and indoleamine 2,3 deoxygenase (IDO); (iv) engaging Tregs to aid in suppression by expression of factors that stimulate differentiation (e.g., IDO, prostaglandin E2 (PGE2), IL-10, and TGFβ) or recruitment (e.g., CCL22); and (v) interfering with TAM activity by suppressing the expression of IFN-γ and IL-12 which impact direct tumor killing and activation of killer T cells^[8,15,20-23].