The nucleotide-binding domain and leucine-rich repeat protein-3 inflammasome is not activated in airway smooth muscle upon toll-like receptor-2 ligation

Abstract

Inflammasomes have emerged as playing key roles in inflammation and innate immunity. A growing body of evidence has suggested that the nucleotide-binding domain and leucine-rich repeat protein-3 (NLRP3) inflammasomeisimportant inchronic airwaydiseases suchas asthma and chronic obstructive pulmonary disease. Inflammasome activation results, in part, in pro-IL-1β processing and the secretion of the proinflammatory cytokine IL-1β. Because asthma exacerbations are associated with elevated concentrations of secreted IL-1β, we addressed whether the NLRP3 inflammasome is activated under in vitro conditions that mimic infectious exacerbations in asthma. Primary cultures of airway smoothmuscle (ASM) cells were treated with infectious stimuli (mimicked using the Toll-like receptor-2 agonist Pam3CSK4, a synthetic bacterial lipopeptide).Whereas Pam3CSK4 robustlyup-regulatedASMcytokineexpressionin response toTNF-αand significantly enhanced IL-1β mRNA expression, we were unable to detect IL-1β in the cell supernatants. Thus, IL-1β was not secreted and therefore was unable to act in an autocrine manner to promote the amplification of ASMinflammatory responses.Moreover, Toll-like receptor-2 ligation did not enhanceNLRP3 or caspase-1 expression in ASM cells, and NLRP3 and caspase-1 protein were not present in the ASM layer of tracheal sections from human donors. In conclusion, these data demonstrate that the enhanced synthetic function of ASM cells, induced by infectious exacerbations of airway inflammation, is NLRP3 inflammasome-independent and IL-1β-independent. Activation of the NLRP3 inflammasome by invading pathogens may prove cell type-specific in exacerbations of airway inflammation in asthma. Copyright © 2013 by the American Thoracic Society