Immunomodulatory role of flagellin in antigen-presenting cells

Toll-like receptors (TLRs) expressed by cells of the immune system play a central role in the generation of immune responses against pathogens. Following TLR ligation, both pro-inflammatory and anti-inflammatory mediators are produced in order to elicit an immune response that controls the microbial infection while limiting tissue damage. Among these mediators, the proinflammatory cytokine IL-12 and the anti-inflammatory cytokine IL-10 are known to play major roles. Here, we show that in vitro or in vivo stimulation with flagellin, the TLR5 ligand, does not result in IL-10 production. Furthermore flagellin inhibits IL-10 production by other specific TLR ligands at the protein and mRNA levels while increasing IL-12p70 production. Several studies have linked the activation of extracellular signal regulated kinases (ERKs) with IL-10 induction by TLRs. Our findings that LPS-induced ERK activation is significantly decreased in flagellin-treated macrophages suggest that this pathway might play a role in the inhibition of IL-10 production by flagellin. Flagellin-mediated IL-10 inhibition was not observed in cells that do not express TLR5 supporting that this effect is TLR5-dependent.Flagellin used as an adjuvant is capable of priming antigen specific T cell responses in an in vivo model of tolerance using high dose peptide. Furthermore, DCs differentiated in tolerogenic conditions (tolerogenic-DCs) express higher levels of TLR5 mRNA than standard BM-DCs and respond more vigorously to flagellin stimulation. Antigen presentation by LPS-matured tolerogenic-DCs results in the differentiation of IL-10 producing T cells with a Tr1-like phenotype. On the contrary, antigen presentation by tolerogenic-DCs that have been stimulated with flagellin results in the differentiation of a typical Th1 responseThis study provides a new insight of the role of flagellin recognition by TLR5 in shaping the immune response elicited by flagellated microorganisms