| Summary: | Increased T-regulatory cell activity drives tumor progression in the compound APCmin/+/enterotoxic Bacteroides fragilis 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, in vitro co-culture studies utilizing sorted myeloid cell subsets and CD4+ T-cells demonstrated that the CD11b+CX3CR1+ but not the CD11b+CX3CR1− subset preferentially induced Foxp3 expression in CD4+ T-cells. Phenotypic analysis revealed that the CD11b+CX3CR1+ subset represented a homogenous CD64+CD24−CD103a− macrophage population. Global CX3CR1 knockout or conditional depletion of CX3CR1+ myeloid cells resulted in diminished CD4+Foxp3+ 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 CD4+Foxp3+ 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 APCmin/+ mouse.
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