Secretory Leukocyte Protease Inhibitor: A Secreted Pattern Recognition Receptor for Mycobacteria

Human secretory leukocyte protease inhibitor (SLPI) displays bactericidal activity against pathogens such as Escherichia coli and Streptococcus. Furthermore, it has been reported that murine SLPI shows potent antimycobacterial activity. The aim of the present study was to investigate whether human recombinant SLPI not only kills mycobacteria but also acts as a pattern recognition receptor for the host immune system. For the in vivo experiment, BALB/c mice were infected by intranasal instillation with Mycobacterium bovis BCG and viable BCG load in lung homogenates was later determined. For the in vitro experiments, SLPI was incubated overnight with a suspension of M. bovis BCG or the virulent strain Mycobacterium tuberculosis H37Rv, and the percentage survival as well as the binding of SLPI to mycobacteria was determined. Furthermore, bacteria phagocytosis was also determined by flow cytometry. Intranasal SLPI treatment decreased the number of colony-forming units recovered from lung homogenates, indicating that SLPI interfered with M. bovis BCG infection. Moreover, SLPI decreased the viability of both M. bovis BCG and H37Rv. We demonstrated that SLPI attached to the surface of the mycobacteria by binding to pathogen-associated molecular pattern mannan-capped lipoarabinomannans and phosphatidylinositol mannoside. Furthermore, we found that in the sputum of patients with tuberculosis, mycobacteria were coated with endogenous SLPI. Finally, we showed that phagocytosis of SLPI-coated mycobacteria was faster than that of uncoated bacteria. The present results demonstrate for the first time that human SLPI kills mycobacteria and is a new pattern recognition receptor for them.