Surfactant Protein A Enhances Production of Secretory Leukoprotease Inhibitor and Protects It from Cleavage by Matrix Metalloproteinases

Mice lacking surfactant protein A (SP-A) are susceptible to bacterial infection associated with an excessive inflammatory response in the lung. To determine mechanisms by which SP-A is antiinflammatory in the lung during bacterial infection, SP-A regulation of secretory leukoprotease inhibitor (SLPI), an inhibitor of serine proteases, was assessed. SLPI protein expression and antineutrophil elastase activity were reduced in bronchoalveolar fluid of SP-A(-/-) compared with SP-A(+/+) mice. Intratracheal administration of SP-A to SP-A(-/-) mice enhanced SLPI protein expression and antineutrophil elastase activity in the lung. SLPI mRNA was similar in whole lung and alveolar type II cells; however, it was significantly reduced in alveolar macrophages from SP-A(-/-) compared with SP-A(+/+) mice. In vitro, SP-A enhanced SLPI production by macrophage THP-1 cells but not respiratory epithelial A549 cells. SP-A inhibited LPS induced IkappaB-alpha degradation in THP-1 cells, which was partially reversed with knockdown of SLPI. Matrix metalloproteinase (MMP)-12 cleaved SLPI and incubation with SP-A reduced MMP-12-mediated SLPI cleavage. The collagen-like region of SP-A conferred protection of SLPI against MMP mediated cleavage. SP-A plays an important role in the lung during bacterial infection regulating protease and antiprotease activity.