Positive and negative roles of myeloid cells in cancer immunotherapy

Myeloid cells are a diverse group of immune cell types with systemic and organ-specific functions. Myeloid cells are frequently found in the tumor microenvironment and their infiltration correlates with survival and response to treatment. High myeloid infiltration is typically a poor prognostic factor, and the immune suppressive and prometastatic roles of myeloid cells are well established. However, there is an increasing appreciation of the antitumor functions performed by myeloid cells, which include direct tumor cell killing, phagocytosis, antigen presentation and T and natural killer cell recruitment. Moreover, advances in immune phenotyping have uncovered myeloid subsets with positive prognostic significance, including subsets correlating with higher response rates to immunotherapy. This review summarizes recent progress in mapping and dissecting the opposing effects of myeloid cells on cancer progression and immunotherapy response. The overall impact of myeloid cells is context-dependent, and combination therapies are needed to leverage the antitumor potential of these cells.