Cancer is a form of non-resolving, persistent inflammation where varying numbers of tumor-associated macrophages (TAMs) infiltrate and adopt different activation states between anti-tumor M1 and pro-tumor M2 phenotypes. Here we resolve a cascade causing differential macrophage phenotypes in the tumor microenvironment. Reduction in TNF mRNA production or loss of Type I TNF receptor signaling resulted in a striking pattern of enhanced M2 mRNA expression. M2 gene expression was driven in part by IL-13 from eosinophils co-recruited with inflammatory monocytes, a pathway that was suppressed by TNF. Our data define regulatory nodes within the tumor microenvironment that balance M1 and M2 populations. Our results show macrophage polarization in cancer is dynamic and dependent on the balance between TNF and IL-13, thus providing a strategy for manipulating TAMs.

Summary Non-resolving inflammation expands a heterogeneous population of myeloid suppressor cells capable of inhibiting T cell function. This heterogeneity has confounded the functional dissection of individual myeloid subpopulations and presents an obstacle for anti-tumor immunity and immunotherapy. Using genetic manipulation of cell death pathways, we found the monocytic suppressor cell subset, but not the granulocytic subset requires continuous c-FLIP expression to prevent caspase-8-dependent, RIPK3-independent cell death. Development of the granulocyte subset requires MCL-1-mediated control of the intrinsic mitochondrial death pathway. Monocytic suppressors tolerate the absence of MCL-1 provided cytokines increase expression of the MCL-1-related protein A1. Monocytic suppressors mediate T cell suppression, while their granulocytic counterparts lack suppressive function. The loss of the granulocytic subset via conditional MCL-1 deletion did not alter tumor incidence implicating the monocytic compartment as the functionally immunosuppressive subset in vivo. Thus, death pathway modulation defines the development, survival and function of myeloid suppressor cells.

Current thinking suggests that despite the heterogeneity of myeloid-derived suppressor cells (MDSC), all Gr-1+CD11b+ cells can become suppressive when exposed to inflammatory stimuli. In vitro evaluation shows MDSC from multiple tissue sites have suppressive activity, and in vivo inhibition of MDSC function enhances T cell responses. However, the relative capacity of MDSC present at localized inflammatory sites or in peripheral tissues to suppress T cell responses in vivo has not been directly evaluated. We now demonstrate that during a tissue specific inflammatory response, MDSC inhibition of CD8 T cell proliferation and IFN-γ production is restricted to the inflammatory site. Using a prostate specific inflammatory model and a heterotopic prostate tumor model, we show that MDSC from inflammatory sites or from tumor tissue possess immediate capacity to inhibit T cell function, whereas those isolated from peripheral tissues (spleens and liver) are not suppressive without activation of iNOS by exposure to IFN-γ. These data show MDSC are important regulators of immune responses in the prostate during acute inflammation and the chronic inflammatory setting of tumor growth and that regulation of T cell function by MDSC during a localized inflammatory response is restricted in vivo to the site of an ongoing immune response.