DOP82 Macrophages in Crohn’s disease mesentery are predominantly inflammatory and produce calprotectin

Background Alterations in the mesentery of Crohn’s disease (CD) patients have long been described, although the functional importance of this tissue is less clear. Some studies hypothesise an anti-inflammatory role for enhanced mesentery, whereas others consider a more pathologic function, given its close proximity to the inflamed intestine. Better phenotypic and functional evaluation of the cells present in this tissue is warranted to improve our understanding of its role in disease. We have previously shown a disproportionate presence of macrophages in CD mesorectum. In this study, we aimed to better characterise the phenotype and function of the macrophages in the mesentery in CD. Methods We collected mesenteric specimens from Crohn’s disease, ulcerative colitis (UC) and non-inflammatory bowel disease (IBD) patients, undergoing surgical bowel resections. In IBD patients, mesentery was collected contiguous to the inflamed region of the intestine, in non-IBD patients, mesentery adjacent to the resection margin. Macrophages were sorted by flow cytometry as CD45+ CD66b-CD14+CD11b+ cells. Sorted cells were analysed by expression profiling and functional analysis. Results Macrophages were present in all analysed tissues. Within the population, a discriminating expression pattern for CD11b was present, dividing CD14+ macrophages in a CD11bhigh and a CD11bdim subset. Transcriptional profiling and gene set enrichment showed the CD11bhigh population to be consistent with IFN-induced pro-inflammatory macrophages, while the CD11bdim population was most consistent with IL4-induced regulatory macrophages. Among the top upregulated genes in CD11bhigh macrophages were S100A8 and S100A9, the two heterodimeric subunits for calprotectin. Flow cytometry confirmed higher expression of calprotectin in the CD11bhigh population also on the protein level. Functionally, CD11bdim macrophages showed higher phagocytic capacity, again consistent a role in tissue repair. Strikingly, the CD11bdim subset was diminished significantly in the mesentery of CD patients, with a near absence in some patients, while the profile in UC was comparable to non-IBD patients. Conclusion Mesenteric macrophages contain two populations, CD11bhigh with a pro-inflammatory profile and CD11bdim with a regulatory profile. In the mesentery of CD patients, the CD11bdim population was strongly decreased, consistent with a pro-inflammatory role for this tissue.