Figure 1. Heterogeneity of tumor-associated macrophages. Ly6C^hi inflammatory or classical monocytes enter primary tumors and differentiate into MHCII^lo and MHCII^hi tumor-associated macrophages (TAMs). MHCII^lo and MHCII^hi TAMs express high levels of M2-associated (i.e., CD124, CD204, CD206, and arginase 1) or M1-associated (i.e., CD11c, iNOS) markers, respectively. Notably, MHCII^lo TAMs associate with hypoxic tumor regions, while MHCII^hi TAMs are located in close proximity of blood vessels. Consequently, ^99mTc-labeled anti-CD206 nanobodies that target MHCII^lo TAMs can be used as hypoxia tracers. In Egln1-haplodeficient mice, tumor oxygenation is improved as a result of vessel normalization. Data obtained in this model demonstrate that hypoxia does not alter the abundance of M2-like or M1-like TAMs, but increases the expression of angiogenic and metabolic proteins, including VEGFA, GLUT1, and GLUT3 specifically in hypoxic MHCII^lo TAMs.