Bacterial infection can evokes endotoxin shock characterized by fever, myocardial dysfunction, acute respiratory failure, and multiple organ failure, resulting in a high mortality rate. LPS, a major component of the outer membrane of Gram-negative bacteria, is one of the major toxins that initiate the cascade of pathophysiological reactions called endotoxin shock. In the bloodstream LPS is carried by the serum protein LPS-binding protein (LBP). On the surface of neutrophils, monocytes, lymphocytes, and macrophages (1
), LPS binds to CD14 and Toll-like receptor 4 (TLR4),*
triggering the production of inflammatory cytokines and bioactive molecules which include TNF-α, IL-1 and IL-6, proteases, and NO (1
). Excessive production of cytokines is considered to be crucial to the initiation of the endotoxic shock cascade (1
). Neutrophil elastase and cathepsin G–deficient mice have been shown to be resistant to LPS-induced endotoxin shock (3
), thus demonstrating that both cytokines and proteases define the susceptibility to endotoxin shock.
Secretory leukoprotease inhibitor (SLPI) is an 11.7 kD (107 amino acid), nonglycosylated, single-chain serine protease inhibitor (4
). SLPI is produced by secretory cells in respiratory, genital and lacrimal glands, and by inflammatory cells that include macrophages, neutrophils, and B cells (5
). SLPI inhibits several serine proteases. Examples are elastase and cathepsin G secreted from neutrophils, trypsin and chymotyrpsin from pancreatic acinar cells, and chymase and tryptase from mast cells (4
). Therefore, a major physiological role of SLPI is considered to be the protection of tissue from these proteases at sites of inflammation (4
). However, recent studies have demonstrated that SLPI functions as more than just a protease inhibitor. SLPI suppresses bacterial growth (9
), inhibits infection of lymphocytes by human immunodeficiency virus-I (HIV-Ip; reference 10
), ameliorates bacterial arthritis (11
), and decreases production of prostaglandin (PG)E2 and matrix metalloproteinases (MMP)-1 and 9 (12
). SLPI is involved in normal cutaneous wound healing, a result previously shown in a study on SLPI−/−
). Moreover, SLPI modifies macrophage functions in mice, as shown by the fact that ectopic expression of SLPI cDNA in macrophages increases their resistance to LPS, and that SLPI suppresses macrophage response to LPS (7
). Consistently, serum SLPI levels are elevated in endotoxin shock in human (15
). These observations strongly suggest that SLPI is an important participant in innate immunity where it acts as an antiinflammatory molecule.
To clarify the function of SLPI in innate immunity, we generated a mouse strain lacking SLPI by gene targeting in embryonic stem cells and analyzed their response to experimental endotoxic shock induced by LPS. As expected, SLPI−/− mice were highly sensitive to LPS or cecal ligation and puncture (CLP)-induced sepsis compared with SLPI+/+ mice. Macrophages lacking SLPI were also highly responsive to LPS with increased IL-6 and HMG (high-mobility group)-1 production and nuclear factor (NF)-κB activation. These results indicated that endogenous SLPI inhibited the signaling pathways though LPS-CD14:TLR4 resulting in the protective function upon septic shock.