Ligation of TLR5 by Flagellin Converts Tolerogenic Dendritic Cells into Activating Dendritic Cells through Inhibition of IL-10 Production.

Antigen presentation by DCs differentiated in the presence of the anti-inflammatory cytokine IL-10 has been shown to result in the development of regulatory T cells (Tregs) and tolerance induction. Whether this outcome could be reverted -or not- through manipulation of the tolerogenic DCs (tol-DCs) has not been explored in detail. Here we report for the first time that tol-DCs differentiated in the presence of IL-10 display increased expression of TLR5 as compared with bone marrow-derived DCs differentiated in the presence of GM-CSF and IL-4 (BM-DCs). The increased expression of TLR5 in tol-DCs seems to be a unique characteristic given that expression of other TLRs such as TLR4 and TLR9 was not different in tol-DCs as compared to BM-DCs. In lieu of our previous observation that the TLR5 ligand, flagellin, inhibits IL-10 production in antigen-presenting cells (APCs)¹, and the unexpected finding that tol-DCs display higher expression of TLR5, we asked next whether flagellin could influence the functional properties of tol-DCs. First, unlike BM-DCs, tol-DCs stimulated with LPS (TLR4 ligand) produced high levels of IL-10. In sharp contrast, tol-DCs stimulated with flagellin do not produce IL-10 but produce significant levels of the pro-inflammatory cytokine IL-12. Second, while antigen presentation by LPS-stimulated tol-DCs results in the generation of IL-10 producing CD4⁺ T-cells, presentation of cognate antigen by flagellin-stimulated tol-DCs resulted in CD4⁺ T-cells that secrete IFN-gamma but not IL-10. This divergent T-cell outcome in response to antigen-presentation by tol-DCs stimulated with LPS (IL-10 producing T-cells) versus flagellin (IFN-gamma producing T-cells) suggests that TLR5 engagement might be a novel approach to convert tolerogenic DCs into activating DCs and effectively trigger productive immune responses in environments that are otherwise conducive to unresponsiveness due to the presence of IL-10.