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Abstract Macrophages are an essential component of innate immunity and play a vital role in inflammation and host defense. Based on immunological responses, the macrophages are classified into “activated” macrophage (M1 macrophages) participating in the responses of type I helper T (Th1) cells to pathogens and “alternatively activated” macrophages (M2 macrophages) in response to interleukin (IL)-4 and IL-13. In this review, we discuss the origin, classification and function of macrophages. We also discuss the mechanisms underlying polarization of different macrophage subtypes, including transcriptional, epigenetic and post-transcriptional regulation.

Abstract Background The macrophage, one of the several key immune cell types, is believed to be involved in tumorigenesis. However, the mechanism of macrophages promoting tumor progression is largely unknown. Methods The differentially secreted proteins of M1 and M2 macrophages were analyzed by mass spectrometry. We performed GST pull-down assay for the identification of cell-membrane receptors that interact with chitinase 3-like protein 1 (CHI3L1) protein. The mouse model was used to validate the function of CHI3L1 in cancer metastasis in vivo. Protein phosphorylation and gene expression were performed to study the signaling pathway activation of cancer cells after CHI3L1 treatment. Results M2 macrophage-secreted CHI3L1 promoted the metastasis of gastric and breast cancer cells in vitro and in vivo. The CHI3L1 protein functioned by interacting with interleukin-13 receptor α2 chain (IL-13Rα2) molecules on the plasma membranes of cancer cells. Activation of IL-13Rα2 by CHI3L1 triggered the activation of the mitogen-activated protein kinase signaling pathway, leading to the upregulated expression of matrix metalloproteinase genes, which promoted tumor metastasis. The results of this study indicated that the level of CHI3L1 protein in the sera of patients with gastric or breast cancer was significantly elevated compared with those of healthy donors. Conclusions Our study revealed a novel aspect of macrophages with respect to cancer metastasis and showed that CHI3L1 could be a marker of metastatic gastric and breast cancer in patients.