Human mononuclear phagocytes and their control by IL-10 in health and inflammatory bowel disease

PhDIntestinal macrophages, derived in part from monocytic precursors, are critical targets of IL-10, which prevents the development of colitis in mice. In humans, loss-of-function IL-10R mutations cause early-onset inflammatory bowel disease (IBD). We aimed to test the hypothesis that IL-10 responsiveness in monocyte/macrophages is suboptimal in adult IBD patients even in the presence of functional IL-10 signalling. A novel assay was developed to measure the ability of IL-10 to inhibit LPS-induced TNF production by monocyte from blood or extracted from colonic biopsies. In healthy donors, monocyte subsets differed in their sensitivity to IL-10: classical monocytes were most sensitive and non-classical monocytes least, most likely due to lower levels of STAT3 availability and phosphorylation in response to IL-10. In IBD, circulating numbers of classical and intermediate monocytes were comparable to controls but non-classical monocytes were reduced. IL-10 was significantly less effective at inhibiting TNFα production by classical monocytes from IBD patients than controls (p=0.026), despite increased expression of IL-10Rα and IL-10-induced STAT3 phosphorylation. To determine the significance of this suboptimal response to IL-10 for monocyte-derived cells in the intestine, colonic CD14+ cells were analysed. Two populations were identified: CD14hi (P1) and CD14lo (P2) the latter of which were significantly increased in active Crohn’s disease but not ulcerative colitis. Both populations produced TNFα in unstimulated cultures, and this response was significantly enhanced following LPS stimulation. The increase in TNFα production upon LPS stimulation was greater in P1 cells. Intestinal CD14+ cells were more sensitive to IL-10, in contrast to classical blood monocytes, TNF production by P1 and P2 cells, from non-IBD and inflamed IBD mucosa, was completely inhibited by 2ng/ml IL-10. Since inhibition was complete under these conditions it was not possible to draw conclusions about differences between health and disease; further studies with less IL-10 may be informative