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J Immunol. 2010 Jan 15;184(2):975-83. doi: 10.4049/jimmunol.0900650. Epub 2009 Dec 9.

Surfactant protein B propeptide contains a saposin-like protein domain with antimicrobial activity at low pH.

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  • 1Division of Pulmonary Biology, Cincinnati Children's Hospital Medical Center and University of Cincinnati College of Medicine, Cincinnati, OH 45229-3039, USA.


Surfactant protein B (SP-B) proprotein contains three saposin-like protein (SAPLIP) domains: a SAPLIP domain corresponding to the mature SP-B peptide is essential for lung function and postnatal survival; the function of SAPLIP domains in the N-terminal (SP-BN) and C-terminal regions of the proprotein is not known. In the current study, SP-BN was detected in the supernatant of mouse bronchoalveolar lavage fluid (BALF) and in nonciliated bronchiolar cells, alveolar type II epithelial cells, and alveolar macrophages. rSP-BN indirectly promoted the uptake of bacteria by macrophage cell lines and directly killed bacteria at acidic pH, consistent with a lysosomal, antimicrobial function. Native SP-BN isolated from BALF also killed bacteria but only at acidic pH; the bactericidal activity of BALF at acidic pH was completely blocked by SP-BN Ab. Transgenic mice overexpressing SP-BN and mature SP-B peptide had significantly decreased bacterial burden and increased survival following intranasal inoculation with bacteria. These findings support the hypothesis that SP-BN contributes to innate host defense of the lung by supplementing the nonoxidant antimicrobial defenses of alveolar macrophages.

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