Analyzing the role of cancer‐associated fibroblast activation on macrophage polarization

Snail1 is a transcriptional factor required for cancer‐associated fibroblast (CAF) activation, and mainly detected in CAFs in human tumors. In the mouse mammary tumor virus‐polyoma middle tumor‐antigen (MMTV‐PyMT) model of murine mammary gland tumors, Snai1 gene deletion, besides increasing tumor‐free lifespan, altered macrophage differentiation, with fewer expressing low levels of MHC class II. Snail1 was not expressed in macrophages, and in vitro polarization with interleukin‐4 (IL4) or interferon‐γ (IFNγ) was not altered by Snai1 gene depletion. We verified that CAF activation modified polarization of naïve bone‐marrow‐derived macrophages (BMDMΦs). When BMDMΦs were incubated with Snail1‐expressing (active) CAFs or with conditioned medium derived from these cells, they exhibited a lower cytotoxic capability than when incubated with Snail1‐deleted (inactive) CAFs. Gene expression analysis of BMDMΦs polarized by conditioned medium from wild‐type or Snai1‐deleted CAFs revealed that active CAFs differentially stimulated a complex combination of genes comprising genes that are normally induced by IL4, downregulated by IFNγ, or not altered during the two canonical differentiations. Levels of RNAs relating to this CAF‐induced alternative polarization were sensitive to inhibitors of factors specifically released by active CAFs, such as prostaglandin E(2) and TGFβ. Finally, CAF‐polarized macrophages promoted the activation of the immunosuppressive regulatory T cells (T‐regs). Our results imply that an active CAF‐rich tumor microenvironment induces the polarization of macrophages to an immunosuppressive phenotype, preventing the macrophage cytotoxic activity on tumor cells and enhancing the activation of T‐reg cells.