Defective macrophage function in crohn's disease: role of alternatively activated macrophages in inflammation

Abstract

IntroductionThe aetiology of Crohn's disease (CD) involves a genetically determined dysregulated immune response to commensal intestinal microflora. In CD, viable E coli are found within lamina propria macrophages (M{Phi}s) and E coli intracellular survival is prolonged in CD-derived M{Phi}s in vitro. Different M{Phi} subpopulations exist, M1 cells are inflammatory cells, M2a cells are involved in tissue repair and M2c are regulatory cells that limit inflammation. The role these different M{Phi}s play in this abnormal handling of bacteria in CD is unclear. MethodsThe aim of this study was to examine in vitro M1 and M2 M{Phi} maturation in CD patients and healthy individuals and how these cells respond to challenge with CD-derived E coli. To do this we monitored M{Phi} morphology, surface markers and cytokine production and intracellular bacterial survival. Peripheral blood monocytes were isolated from CD patients and healthy controls and treated with cytokines to generate distinct M{Phi} subpopulations: IFN{gamma} for M1, IL4/IL13 for M2a and IL10 for M2c. M{Phi} morphology was assessed by H&E staining and surface marker expression of CD14, CD16 and CD33, chemokine receptor CCR2, scavenger receptor CD163, co stimulatory molecule CD40 and mannose receptor CD206 using flow cytometry. IL10 and TNF[α] production were measured by ELISA and intracellular E coli survival was measured using the gentamicin protection assay. ResultsIn contrast to M1 M{Phi}'s, both CD-derived M2 populations failed to develop the characteristic M{Phi} dendrite morphology seen in control macrophages. Surface expression of CD40 in M2 CD-derived M{Phi}s was 3.4-fold lower for M2a and 4.4-fold lower for M2c compared to controls after E coli challenge. CD163 was higher in M1 CD cells but reduced by 50% in M2 cells compared to healthy cells. After E coli challenge, TNF[α] production by M2 but not M1 M{Phi}s was significantly lower in CD than in controls (M2a 38%, M2c 27% respectively less than controls) but there were no differences in IL10 production. Prolonged intracellular survival of E coli was demonstrated in CD M2 cells but effective killing occurred in all M1 CD M{Phi}s and all control M{Phi}s. ConclusionIn CD, M2 (but not M1) M{Phi}s display abnormal morphological maturation, lower TNF[α] levels after E coli challenge, prolonged intracellular bacterial survival and differences in surface marker expression. The results are consistent with an innate M{Phi} defect in CD relating particularly to a failure of the normal role of M2 M{Phi}s to limit and control inflammation.