Downregulation of the Syk Signaling Pathway in Intestinal Dendritic Cells Is Sufficient To Induce Dendritic Cells That Inhibit Colitis

Helminthic infections modulate host immunity and may protect people in less-developed countries from developing immunological diseases. In a murine colitis model, the helminth Heligmosomoides polygyrus bakeri prevents colitis via induction of regulatory dendritic cells (DCs). The mechanism driving the development of these regulatory DCs is unexplored. There is decreased expression of the intracellular signaling pathway spleen tyrosine kinase (Syk) in intestinal DCs from H. polygyrus bakeri-infected mice. To explore the importance of this observation, it was shown that intestinal DCs from DC-specific Syk(-/-) mice were powerful inhibitors of murine colitis, suggesting that loss of Syk was sufficient to convert these cells into their regulatory phenotype. DCs sense gut flora and damaged epithelium via expression of C-type lectin receptors, many of which signal through the Syk signaling pathway. It was observed that gut DCs express mRNA encoding for C-type lectin (CLEC) 7A, CLEC9A, CLEC12A, and CLEC4N. H. polygyrus bakeri infection downmodulated CLEC mRNA expression in these cells. Focusing on CLEC7A, which encodes for the dectin-1 receptor, flow analysis showed that H. polygyrus bakeri decreases dectin-1 expression on the intestinal DC subsets that drive Th1/Th17 development. DCs become unresponsive to the dectin-1 agonist curdlan and fail to phosphorylate Syk after agonist stimulation. Soluble worm products can block CLEC7A and Syk mRNA expression in gut DCs from uninfected mice after a brief in vitro exposure. Thus, downmodulation of Syk expression and phosphorylation in intestinal DCs could be important mechanisms through which helminths induce regulatory DCs that limit colitis.