Cancer-associated fibroblasts: Vital suppressors of the immune response in the tumor microenvironment

Since the "seed and soil" hypothesis was proposed, the biological functions of the tumor microenvironment (TME), especially its stromal components, have received increasing attention. Cancer-associated fibroblasts (CAFs) are the major components of the stromal region, providing material support for tumor cell proliferation, migration, and invasion. Furthermore, CAFs are important mediators of suppressing immune responses by attracting the accumulation of immunosuppressive cells through cytokine/chemokine secretion. In this review, we summarized the major cytokines, chemokines and metabolites, including transforming growth factor-β (TGF-β), interleukin-6 (IL-6), C-X-C chemokine ligand (CXCL)12, C-C chemokine ligand (CCL) 2, prostaglandin E2 (PGE2), and other factors, by which CAFs suppress the immune systems in a variety of cancers. More importantly, we highlight potential therapeutic strategies to alleviate the immunosuppression produced by CAFs, thereby inhibiting tumor progression.