Dectin-1/2-induced autocrine PGE2 signaling licenses dendritic cells to prime Th2 responses

The molecular mechanisms through which dendritic cells (DCs) prime T helper 2 (Th2) responses, including those elicited by parasitic helminths, remain incompletely understood. Here, we report that soluble egg antigen (SEA) from Schistosoma mansoni, which is well known to drive potent Th2 responses, triggers DCs to produce prostaglandin E2 (PGE2), which subsequently-in an autocrine manner-induces OX40 ligand (OX40L) expression to license these DCs to drive Th2 responses. Mechanistically, SEA was found to promote PGE2 synthesis through Dectin-1 and Dectin-2, and via a downstream signaling cascade involving spleen tyrosine kinase (Syk), extracellular signal-regulated kinase (ERK), cytosolic phospholipase A2 (cPLA2), and cyclooxygenase 1 and 2 (COX-1 and COX-2). In addition, this pathway was activated independently of the actions of omega-1 (ω-1), a previously described Th2-priming glycoprotein present in SEA. These findings were supported by in vivo murine data showing that ω-1-independent Th2 priming by SEA was mediated by Dectin-2 and Syk signaling in DCs. Finally, we found that Dectin-2-/-, and to a lesser extent Dectin-1-/- mice, displayed impaired Th2 responses and reduced egg-driven granuloma formation following S. mansoni infection, highlighting the physiological importance of this pathway in Th2 polarization during a helminth infection. In summary, we identified a novel pathway in DCs involving Dectin-1/2-Syk-PGE2-OX40L through which Th2 immune responses are induced.