Interaction of Pneumocystis carinii surface glycoprotein-A with surfactant protein-D

P. carinii induced pneumonia is paralleled by increased surfactant protein-D (SP-D) production and accumulation in alveolar spaces. Previous results have shown that collectin SP-D interacts with the microorganism, mainly by binding to the surface glycoprotein (gpA) complex on P. carinii, leading to enhanced organism binding to alveolar macrophages. Because of its significance in pathogenesis, we attempted to define the chemical nature of this interaction using a solid phase binding assay. We hypothesized that the carbohydrate recognition domain (CRD) of SP-D interacts with glucose and mannose sugars on the gpA complex. The results showed that native rat SP-D bound to plastic-adsorbed gpA in a concentration-dependent manner in the presence of 2mM calcium. This binding was almost completely abolished with 10 mM EDTA. Addition of increasing concentrations of calcium to cation-free binding medium resulted in increased binding affinity, whereas manganese and magnesium cations had no significant effect on binding. As determined by saccharide competition assay, the SP-D binding preference was maltose>glucose>mannose>N-acetyl-glucosamine. The optimal pH range for SP-D/gpA interaction was between pH 5 and 9, with strong inhibition at pH 4. Comparison of different molecular forms of SP-D showed that the number of CRDs in a polymer determines the relative binding affinity of SP-D. Maximum binding was achieved with native SP-D (dodecamers and multimers of dodecamers), followed by recombinant dodecamers. Recombinant full length trimers showed a weaker binding capacity which was similar to the binding achieved with the recombinant truncated molecule, consisting of CRD domains assembled as trimers or smaller oligomers. These findings support our hypothesis that the carbohydrate recognition domain of SP-D mediates interaction with the P. carinii gpA surface complex.