Complement activation is a central component of inflammation and sepsis and can lead to significant tissue injury. Complement factors are serum proteins that work through a cascade of proteolytic reactions to amplify proinflammatory signals. Inter-α-trypsin inhibitor (IaI) is an abundant serum protease inhibitor that contains potential complement-binding domains, and has been shown to improve survival in animal sepsis models. We hypothesized that IaI can bind complement and inhibit complement activation, thus ameliorating complement-dependent inflammation. We evaluated this hypothesis with in vitro complement activation assays and in vivo in a murine model of complement-dependent lung injury. We found that IaI inhibited complement activation through the classical and alternative pathways, inhibited complement-dependent phagocytosis in vitro, and reduced complement-dependent lung injury in vivo. This novel function of IaI provides a mechanistic explanation for its observed salutary effects in sepsis and opens new possibilities for its use as a treatment agent in inflammatory diseases.

LPS from bacteria is ubiquitous in the environment and can cause airway disease and modify allergic asthma. Identification of gene products that modulate the biologic response to inhaled LPS will improve our understanding of inflammatory airways disease. Previous work has identified quantitative trait loci for the response to inhaled LPS on chromosomes 2 and 11. In these regions, 28 genes had altered RNA expression after inhalation of LPS, including CD44, which was associated with differences in both TNF-␣ levels and neutrophil recruitment into the lung. It has previously been shown that CD44 can modulate macrophage recruitment in response to Mycobacterium tuberculosis, as well as clearance of neutrophils after lung injury with both bleomycin and live Escherichia coli bacteria. In this study, we demonstrate that the biologic response to inhaled LPS is modified by CD44. Macrophages failed to be recruited to the lungs of CD44-deficient animals at all time points after LPS exposure. CD44-deficient macrophages showed reduced motility in a Transwell migration assay, reduced ability to secrete the proinflammatory cytokine TNF-␣, reduced in vivo migration in response to monocyte chemotactic protein-1, and diminished adhesion to vascular endothelia in the presence of TNF-␣. In addition, CD44-deficient animals had 150% fewer neutrophils at 24 h and 50% greater neutrophils 48 h after LPS exposure. These results support the role of CD44 in modulating the biologic response to inhaled LPS.