IFNβ-producing CX3CR1 macrophages promote T-regulatory cell expansion and tumor growth in the APC / colon cancer model.

Increased T-regulatory cell activity drives tumor progression in the compound APC/enterotoxic colon cancer model. At the same time, how microbially-induced inflammation promotes T-regulatory cell expansion in the dysplastic intestine remains poorly described. Analysis of post-infection immune cell kinetics in the colon lamina propria revealed that CD4+ Foxp3+ cell numbers increased by 25-fold between days 3-14. Importantly, T-regulatory cell expansion was preceded by a 12-fold spike in lamina propria CD11b cell numbers between days 0-4; suggesting a link between the myeloid compartment and the T-regulatory cells. Consistent with this notion, co-culture studies utilizing sorted myeloid cell subsets and CD4 T-cells demonstrated that the CD11bCX3CR1 but not the CD11bCX3CR1 subset preferentially induced Foxp3 expression in CD4 T-cells. Phenotypic analysis revealed that the CD11bCX3CR1 subset represented a homogenous CD64CD24CD103a macrophage population. Global CX3CR1 knockout or conditional depletion of CX3CR1 myeloid cells resulted in diminished CD4Foxp3 cell expansion and a 3 to 6-fold reduction in tumor burden establishing CX3CR1 macrophages as a major driver of the T-regulatory cell-tumor axis. Quantitative analysis of CD11b myeloid cell subsets for IFNβ mRNA revealed that the CX3CR1 macrophages expressed 15-fold higher levels of IFNβ in comparison to the CX3CR1 myeloid subset. Antibody mediated neutralization of IFNβ resulted in the suppression of CD4Foxp3 cell induction and tumor growth, demonstrating the central role of IFNβ in mediating CX3CR1 macrophage-driven T-regulatory cell expansion. These studies shed new mechanistic light on the cellular ontogeny of pro-tumorigenic T-regulatory cells in the inflamed colon of the APC mouse.