Background: Inflammatory bowel disease (IBD), including Crohn's disease (CD) and ulcerative colitis (UC), is thought to be driven by an exacerbated immune response to the commensal microbiota. Macrophages (MΦ) are the most abundant mononuclear phagocytes in the gastrointestinal tract, where they are critical at shaping the type of immune elicited towards the microbiota. Hence, we decided to characterize human intestinal MΦ phenotype and function both in healthy controls and IBD patients.
Methods: Colonic biopsies were obtained from the inflamed and non-inflamed tissue from IBD patients (CD and UC), as well as from the non-inflamed tissue from quiescent patients and healthy controls. Biopsies were immediately processed and lamina propria mononuclear cells characterized by flow cytometry both in resting conditions and after overnight culture in the presence/absence of a pattern recognition receptor agonist (LPS).
Results: Human intestinal MΦ were identified within singlet viable cells as CD45+HLA-DR+CD14+CD64+ and further divided into CD11chigh, CD11cdim and CD11c- subsets. CD206 expression was higher compared with CD86 on total MΦ revealing a M2-biased MΦ profile in the healthy gut which however was not associated with any particular subset. However, the CD11c- subset had higher expression of HLA-DR, CD64 and PDL1 coupled with lower expression of SIRPα, CCR2 and CD40 compared with the CD11chigh/dim subsets. CD11c- MΦ had higher production of IL-10 and lower production of IL-6 and TNFα both in resting conditions and after LPS challenge compared with the CD11chigh/dim subsets. Finally, total MΦ numbers were increased in the inflamed tissue from IBD patients (both CD and UC), although not on the non-inflamed tissue or on quiescent patients, due to specifically higher numbers of the pro-inflammatory CD11chigh MΦ subset.
Conclusions: MΦ subsets are likely to represent transition stages from newly arrived monocytes (CD11chigh) into transient (CD11cdim) and resident (CD11c-) tolerogenic MΦ. The higher numbers of pro-inflammatory CD11chigh MΦ shown in the inflamed mucosa from IBD patients is probably reflecting the increased recruitment capacity of circulating monocytes elicited by the mucosa, hence exacerbating the immune response.