Secretory Leukocyte Protease Inhibitor: More than Just A Protease Inhibitor

Secretory leukocyte protease inhibitor (SLPI) was originally discovered in various secretory fluids with broad inhibitory spectra against serine proteases. Its major physiological function was proposed to protect host from proteasemediated tissue damage at sites of injury. Subsequent investigation revealed that SLPI could influence and regulate host immune responses in a variety of ways: SLPI possesses anti-microbial activities against both gram-negative and grampositive bacteria, serves as the major component of saliva that suppresses infections of immunodeficiency virus 1 (HIV- 1), and inhibits monocyte/macrophage inflammatory responses to host-derived or microbial stimuli. Additionally, SLPI has been reported to be involved in tumorigenesis and metastasis. Inflammation induced by either infection or injury often leads to an elevated level of endogenous SLPI at the sites of inflammation, which underscores the physiological importance of this protein. This review outlines the multiple modulatory roles of SLPI in the host and summarizes therapeutic implications of this molecule.