Limited proteolysis of surfactant protein D causes a loss of its calcium-dependent lectin functions.

Surfactant protein D (SP-D) is a multimeric collagenous lectin that mediates the clearance of pathogens and modulates immune cell functions via its C-terminal carbohydrate recognition domain (CRD). We hypothesized that extracellular proteolysis of SP-D may result in a loss of its functional properties. Multimeric SP-D was partially digested by human leukocyte elastase (HLE) dose- and time-dependently. Physiologic concentrations of calcium slowed, but did not protect from degradation. In solution, both native and degraded SP-D had an apparent molecular weight of 650 to >1000 kDa. Under reducing conditions, the degraded SP-D monomers run at 10 kDa less than native SP-D. Amino acid sequencing located all major cleavage sites into the CRD. Functional studies showed that degraded SP-D had lost its calcium-dependent lectin properties, i.e. neither bound to mannose nor agglutinated bacteria. These studies demonstrate that elastase results in the limited proteolysis of SP-D with loss of its CRD-dependent activities and suggest that proteases at concentrations observed in various lung diseases may impair the antimicrobial and immunomodulatory roles of SP-D.