Roles of signaling pathways in cancer cells and immune cells in generation of immunosuppressive tumor-associated microenvironments

Cancer cells trigger multiple immunosuppressive cascades and generate immunosuppressive tumor-associated microenvironments including tumor and sentinel lymph nodes. Constitutive activation of various signaling pathways (e.g., MAPK, STAT3, NF-κB, β-catenin) in human cancer cells was found to trigger the multiple immunosuppressive cascades through the production of immunosuppressive cytokines, such as TGF-β, IL-10, IL-6, and VEGF, and induction of immunosuppressive immune cells, such as regulatory T cells, tolerogenic dendritic cells, and myeloid derived suppressor cells. Some of these cancer-derived cytokines impair various immune cells through activation of their signaling molecules such as STAT3 and NF-κB. Inhibitors for these activated signals could inhibit the multiple immunosuppressive cascades by acting on both cancer cells and immune cells. Since common signaling mechanisms are often utilized for some of the hallmarks of cancer (e.g., cell proliferation/survival, invasion/metastasis, and immunosuppression), targeting these common signaling pathways may be an attractive strategy for cancer therapy, including immunotherapy.