Dipeptidyl peptidase I (DPPI) cleaves surfactant protein D (SP-D): A mechanism for immune compromise in the lung

SP-D, a key orchestrator of innate immunity in the lung, has been found to be degraded in the airways of asthmatic patients. Chronic mast cell activation is also a prominent feature in asthmatic airways, and is associated with the release of mediators of inflammation including a series of proteases. Among these is the lysosomal cysteine-type peptidase DPPI. We have investigated the capacity of DPPI to cleave SP-D and alter its functionality. Recombinant human mast cell protease DPPI was incubated with recombinant fragment human surfactant protein D over a range of physiological conditions and time periods. The potential for SP-D cleavage was analysed by SDS polyacrylamide gel electrophoresis (PAGE), and Western blotting with antibodies specific for SP-D and DPPI was employed to confirm the identity of component proteins. Confirmatory studies were performed investigating native SP-D in bronchoalveolar lavage fluid. Functionality of SP-D was assessed by measuring inhibition of bacterial growth. DPPI induced a concentration-dependent degradation of SP-D with the appearance of new fragments of 8 and 10 kDa. The addition of selective protease inhibitors or heat-inactivation of DPPI resulted in substantially less degradation of SP-D, indicating dependence of an intact catalytic site. The ability of DPPI to alter the structural integrity of SP-D could contribute in important ways to inflammatory changes in the asthmatic lung.