Dynamic differentiation of F4/80+ tumor-associated macrophage and its role in tumor vascularization in a syngeneic mouse model of colorectal liver metastasis

Tumor-associated macrophages (TAMs) are highly heterogeneous and play vital roles in tumor progression. Here we adopted a C57BL/6 mouse model imitating the late-stage colorectal liver metastasis (CRLM) by Mc38 colorectal cancer cell injection via the portal vein. With serial sections of CRLM biopsies, we defined 7–9 days post-injection as the critical period for tumor neovascularization, which was initiated from the innate liver vessels via vessel cooption and extended by vascular mimicry and thereof growth of CD34 + cells. In samples with increasing-sized liver metastases, the infiltrated Ly6C + CD11b + F4/80 − monocytes steadily gained the expression of F4/80, a Kupffer cell marker, before transformed into Ly6C − CD11b int F4/80 + cells, which, the same phenotype was also adapted by Ly6C − CD11b − F4/80 + Kupffer cells. F4/80 + TAMs showed proximity to neovascularization and tumor vessels, functionally angiogenic in vivo; and greatly promoted the activation of a few key angiogenic markers such as VEGFA, Ki67, etc. in endothelial cells in vitro. Depletion of macrophages or diversion of macrophage polarization during neovascularization impeded tumor growth and vascularization and resulted in greatly reduced F4/80 + TAMs, yet increased CD11b + cells due to inhibition of TAM differentiation. In summary, our results showed dynamic and spatial–temporal F4/80 + TAM transformation within the tumor microenvironment and strengthened its role as perivascular and angiogenic TAMs in CRLM.