Role of tumor-associated macrophages in renal cell carcinoma pathogenesis

The role of tumor stroma in malignant tumor pathogenesis cannot be disputed. Macrophages are one of the crucial elements of tumor stroma. Tumor-associated macrophages (TAMs) are type 2-activated macrophages (M2). They were first described in 1992. They carry CD206, CD163, FXIIIa, βIG-H3, stabilin 1, YKL-39, SI-CLP, tenascin С, LOX-1, fibronectin, MARCO, interleukin 1 receptor antagonist (IL-1RA) and other markers. Unlike proinflammatory macrophages (M1), М2 display high anti-inflammatory activity and are responsible for inflammation reaction suppression and tissue recovery in inflamed area. TAMs significantly contribute to tumor progression by stimulating cell proliferation, angiogenesis, and suppression of antitumor immune response. Identification of macrophages in renal tumors involves a limited number of markers, which doesn’t allow making a conclusive answer about their function. However, a correlation between TAMs content and a negative disease prognosis can be considered proven. Studies of M1 and M2 using different markers have shown that renal tumors contain high levels of TAMs with mixed M1/M2 phenotype. TAMs in renal tumors are highly proangiogenic and immunosuppressive. TAMs density can be used as a prognostic marker, but development of an effective treatment strategy aimed at inhibition of TAMs antitumor activity requires systemic research involving a wide panel of M1 and M2 macrophage markers.