Monocytes exhibit dual roles in tumor immunity, shifting from immunosurveillance to immune evasion.

The tumor microenvironment (TME) is a complex ecosystem where immune cells significantly influence cancer progression and therapeutic outcomes. Among these, monocytes, as key innate immune cells of the mononuclear phagocyte system, are extensively involved in immune and inflammatory responses. However, their precise regulatory role in tumor development and progression is multifaceted, often presenting a paradox where they can either promote or suppress tumor growth. Understanding this intricate plasticity and the mechanisms driving monocyte polarization is crucial for overcoming current limitations in cancer immunotherapy.

This review synthesizes current understanding of monocyte biology within the tumor microenvironment (TME), focusing on their origin, migration, polarization, and transformation. It systematically examines the mechanisms by which different monocyte subsets regulate both antitumor immune responses and immune evasion. The authors also discuss recent advances in monocyte-based therapeutic applications, including monocyte-mediated vaccines and combination strategies with immune checkpoint inhibitors, highlighting key research focuses in cancer immunotherapy.

Monocytes demonstrate a profound duality, capable of exerting both pro-tumor and anti-tumor functions by modulating immune responses. Their role is largely determined by their subset and the specific cues within the tumor microenvironment. Through the secretion of various cytokines and chemokines, monocytes actively regulate local immune activity. Crucially, tumor cells exploit these signaling pathways to influence monocyte polarization, often inducing their transformation into immunosuppressive phenotypes that facilitate immune evasion. This transformation can lead to the suppression of effector T-cells and promotion of angiogenesis, contributing to tumor growth. Conversely, certain monocyte subsets contribute to immunosurveillance by presenting antigens and initiating anti-tumor responses. > Precise regulation of monocyte function, particularly their polarization and transformation, holds significant promise for developing novel strategies in tumor immunotherapy. Current therapeutic approaches leveraging this understanding include monocyte-mediated vaccines and combination therapies with immune checkpoint inhibitors, aiming to re-educate monocytes towards an anti-tumor phenotype.