IFNβ-producing CX3CR1(+) macrophages promote T-regulatory cell expansion and tumor growth in the APC(min/+) / Bacteroides fragilis colon cancer model

Increased T-regulatory cell activity drives tumor progression in the compound APC(min/+)/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 APC(min/+) mouse.