Enhancement of mast cell IL-6 production by combined toll-like and nucleotide-binding oligomerization domain-like receptor activation

Background: Mast cells respond to bacterial infection by producing mediators that recruit and activate leukocytes, mediate vasodilation and induce bronchoconstriction. These mast cell-driven responses play a crucial role in protective immunity against bacterial infection, but may contribute to bacterial exacerbation of allergic diseases. Bacterial components including peptidoglycan (PGN) and lipopeptides are known to activate receptors such as Toll-like receptors (TLR) and nucleotide-binding oligomerization domain-like receptors (NLR). Since the consequences of mast cell activation by individual or combinations of bacterial components have not been fully characterized, we determined the effects of TLR2 and NLR activation, alone or in combination, on human mast cell mediator production. Methods: Cord blood-derived human mast cells were activated by bacterial PGN, the lipopeptide Pam₃CSK ₄ and NLR agonists alone or in combination. Mast cell degranulation, LTC₄ production and the production of cytokines were assessed. Results:PGN and the lipopeptide Pam₃CSK₄ induced human mast cells to produce the pro-inflammatory mediators IL-1β, IL-6, CXCL8 and LTC₄ in addition to anti-inflammatory IL-10. NLR agonists alone did not induce these responses, but significantly and selectively increased Pam ₃CSK₄-mediated mast cell IL-6 production. PGN- and Pam₃CSK₄-induced mast cell IL-6, but not IL-1β, production was dependent on adenylyl cyclase activity and could be partially inhibited by the cyclooxygenase inhibitor naproxen. Conclusions: Increased mast cell IL-6 production in response to combined TLR2 and NLR activation could play a role in the protection against bacterial infection, but potentially exacerbate inflammation-dependent conditions. In addition, mast cell IL-6 production is dependent on adenylyl cyclase activity.