EFFECTS OF HUMAN POLYMORPHONUCLEAR LEUKOCYTE ELASTASE UPON SURFACTANTPROTEINS IN-VITRO

Recent evidence has suggested that elastase is released by polymorphon uclear leukocytes (PMN) recruited from the pulmonary microcirculation into the alveoli during acute lung injury. This study was undertaken t o test the hypothesis that elastase from PMN (PMN elastase) damages or degrades one or more of the surfactant proteins (SP-A, SP-B and SP-C) of the lung, and thereby alters its function. We attempted to use amo unts of PMN elastase and quantities of surfactant that would be plausi ble in the lungs of patients with ARDS, Surfactant from normal dog lun gs (2 mg phospholipid, 200 mu g protein), and purified SP-A (20 mu g), SP-B (10 mu g) and SP-C (10 mu g) from the surfactant (identified by SDS-PAGE and N-terminal amino acid sequences) were incubated for 4-8 f i at 37 degrees C with various amounts (0.25-1.0 U) of human PMN elast ase purified by affinity chromatography. SDS-PAGE and amino acid compo sition analysis of the surfactant as well as of the purified SP-A, SP- B, and SP-C showed that degradation of these proteins progressed with incubation time and with the amount of PMN elastase, and was accompani ed by decreases in isopycnic density (g/cm(3)) and surface adsorption, and increase of surface tension of the surfactant. No effects were ob served with heat inactivated PMN elastase (95 degrees C, 30 min) or wi th PMN elastase in the presence of human alpha-1 protease inhibitor (2 mu g/mu g elastase). Phospholipid compositions of the surfactant afte r exposure to PMN elastase were not significantly different from those of the controls, suggesting that SP-A, SP-B, and SP-C play a major ro le in altering the surfactant properties. SP-A was also degraded by el astase acid trypsin from pancreas whereas SP-B and SP-C remained intac t, providing a natural surfactant without SP-A. Surface adsorption rat e of the SP-A deficient surfactant was lower than that of the control, but was much higher than that of the surfactant with completely degra ded SP-A, SP-B, and SP-C, suggesting that hydrophobic SP-B and SP-C ar e the essential components in enhancing adsorption. We conclude that p roteolytic degradation of SP-A, SP-B, and SP-C causes the decrease of surfactant isopycnic density, and is responsible for retarding adsorpt ion resulting in surfactant dysfunction.