Macrophage cytotoxic activity and its role in the tumor pathogenesis

Macrophages, natural killers and T cells play the central role in tumor cells destruction. The purpose of this review is to summarize the state-of-the-art perspectives of the interplay between tumor cells and tumor stroma leading both to the formation of a macrophage population incapable of effective antitumor activity and to the selection of tumor cells resistant to macrophage cytotoxicity. Macrophages are highly versatile cells that can both stimulate the inflammatory response (type 1 macrophages, M1) and suppress it (type 2 macrophages, M2). Tumor-associated macrophages (TAMs) are considered the main regulator of the antitumor immune response and usually have anti-inflammatory properties, that is, they belong to M2 type. Tumor cells are able to affect macrophages, "reprogramming" them to perform an immunosuppressive function. In addition, TAMs stimulate angiogenesis and remodelling of the extracellular matrix necessary for metastasis. Recently, more and more studies have been published describing a mixed TAMs phenotype with characteristics of both M2 and M1. M1 is characterized by production of pro-inflammatory cytokines, reactive oxygen species, bactericidal and cytotoxic activity. M1 can destroy tumor cells both directly and indirectly by attracting other cells. Despite the mechanisms of direct cytotoxic activity are quite variable, their effectiveness is largely dependent on the properties of a particular tumor. The cytotoxic activity of macrophages is a powerful factor that inhibits tumor initiation and progression. However, in some cases, it is not sufficient to control the tumor process. Activation of the cytotoxic activity of TAMs is one of the strategies to use macrophages for cancer treatment. Understanding the mechanisms of macrophage cytotoxic activity and specific patterns of its manifestation in a tumor environment is of critical importance for better effectiveness of existing cancer treatments and development of promising methods for tumor immunotherapy.