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Abstract

Introduction: Surfactant protein A (SP-A) is the most abundant surfactant protein and a member of the collectin family of innate immune molecules. SP-A gene targeted mice (SP-A-/-) are susceptible to Pseudomonas aeruginosa (PA) infection. PA flagella are known virulence factors that mediate IL-1β expression by interacting extracellularly with toll-like receptor-5 (TLR-5) and intracellularly with a caspase-1 activating inflammasome. Hypothesis: SP-A interacting with flagellin may be a mechanism by which SP-A reduces pulmonary inflammation following PA infection and facilitates bacterial clearance. Methods: SP-A+/+ and SP-A-/- mice were intratracheally infected with flagellated PA (PAK) or a mutant strain lacking flagella (PAfliC). Bronchoalveolar lavage fluid was assessed for inflammatory cells and cytokines, and alveolar macrophage phagocytosis of bacteria was determined by flow cytometry. In vitro studies were performed on a mouse alveolar macrophage cell line (MH-s) to evaluate signaling pathways. Results: In vivo, increased inflammatory cells and impaired phagocytosis of PAK were observed in the lungs of SP-A-/- mice but not when infected with the flagella deficient PAfliC mutant. SP-A-/- mice expressed increased proinflammatory cytokines IL-6 and TNF-α, and decreased levels of IL-1β with PAK infection but there were no differences with PAfliC infection. SP-A bound flagellin directly in a concentration dependent manner and SP-A-/- alveolar macrophage phagocytosis of flagellin was reduced by 50% compared to wild type. In vitro, SP-A enhanced flagellin induced IL-1β production by MH-s cells (51.0 ± 5 pg/ml* versus 19.0 ± 2 pg/ml with flagella stimulation alone, *p<0.05). Transfection of MH-s cells with caspase-1 siRNA reduced the SP-A enhanced production of IL-1β to undetectable levels. Conclusions: SP-A plays an important role in PA pulmonary infections by directly binding and enhancing phagocytosis of flagellin and augmenting the production of IL-1β mediated by the inflammasome.