An Independent Subset of TLR Expressing CCR2-Dependent Macrophages Promotes Colonic Inflammation

Abstract

Macrophages (Mϕs) in the large intestine are crucial effectors of inflammatory bowel disease, but are also essential for homeostasis. It is unclear if these reflect separate populations of Mϕs or if resident Mϕs change during inflammation. In this study, we identify two subsets of colonic Mϕs in mice, whose proportions differ in healthy and inflamed intestine. Under resting conditions, most F4/80+ Mϕs are TLR− CCR2− CX3CR1hi and do not produce TNF-α in response to stimulation. The lack of TLR expression is stable, affects all TLRs, and is determined both transcriptionally and posttranscriptionally. During experimental colitis, TLR2+ CCR2+ CX3CR1int Ly6Chi Gr-1+, TNF-α–producing Mϕs come to dominate, and some of these are also present in the normal colon. The TLR2+ and TLR2− subsets are phenotypically distinct and have different turnover kinetics in vivo, and these properties are not influenced by the presence of inflammation. There is preferential CCR2-dependent recruitment of the proinflammatory population during colitis, suggesting they are derived from independent myeloid precursors. CCR2 knockout mice show reduced susceptibility to colitis and lack the recruitment of TLR2+ CCR2+ Gr-1+, TNF-α–producing Mϕs. The balance between proinflammatory and resident Mϕs in the colon is controlled by CCR2-dependent recruitment mechanisms, which could prove useful as targets for therapy in inflammatory bowel disease.