Mechanisms and Functional Significance of Human Tumor-infiltrating Myeloid Cells

Myeloid cells as the major components of tumor-infiltrating leukocytes play critical roles in anti-tumor immunity. However, emerging evidences have revealed that soluble factors produced by tumor/stromal cells skew myeloid cells toward a tumor-promoting phenotype. Tumor-infiltrating myeloid cells (TIMs) including tumor-associated macrophages (TAMs), tumor-associated neutrophils (TANs), myeloid-derived suppressor cells (MDSCs), and tumor-associated dendritic cells (TADCs) are considered as the key mediators of tumor microenvironment (TME). TIMs have been shown to play important roles in various aspects of cancer biology and their presence is often linked to altered patient prognosis and survival. Regarding their critical role in TME, TIMs have been proposed as relevant targets of therapeutic strategies aimed at expanding immunostimulatory myeloid cell populations and depleting or modulating immunosuppressive ones. In this review, we briefly describe TIMs subsets and discuss the mechanisms by which TIMs induce immunosuppression, angiogenesis, and metastasis.