Characterization of macrophage subpopulations located in the peritoneal submesothelial space

Abstract

Based on preliminary data revealing the existence of a macrophage layer in the connective tissue beneath the peritoneal mesothelium, in the present work submesothelial macrophages (submes-MØs) have been characterized by wholemount immunofluorescence combined with confocal microscopy, as well as flow cytometry. Three main submes-MØ subpopulations were found in the submesothelial space of C57BL/6 mice in the steady state: Tim4+ MHCIIlow, Tim4+ MHCIIhigh and Tim4- MHCIIhigh submes-MØs. Differential expression of the resident peritoneal macrophage markers Tim4 and Gata6, and dependence on the chemokine receptor CCR2, support that Tim4+ MHCIIlow and Tim4+ MHCIIhigh submes-MØs belong to the resident macrophage family and therefore have an embryonic origin. In contrast, Tim4- MHCIIhigh submes-MØs would derive from adult bone marrow monocytes. Peritoneal E. coli infection promoted a transition from an elongated to a stellate morphology in submes-MØs, involving the formation of three-dimensional extensions that penetrated between mesothelial cells, suggesting that E. coli triggered submes-MØ activation. The contribution of the different submes-MØ subpopulations to defense against peritoneal infection remains to be investigated.