Binding of rat and human surfactant proteins A and D to Aspergillus fumigatus conidia.

Surfactant proteins A (SP-A) and D (SP-D) are thought to play important roles in pulmonary host defense. We investigated the interactions of rat and human SP-A and SP-D with Aspergillus fumigatus conidia. Rat SP-D but not rat SP-A bound the conidia, and the binding was inhibited by EDTA, mannose, glucose, maltose, and inositol. Binding studies using a mutant recombinant rat SP-D with altered carbohydrate recognition but normal structural organization clearly established a role for the carbohydrate recognition domain in binding to conidia. However, neither rat SP-A nor SP-D increased the association of fluorescein isothiocyanate-labeled conidia with rat alveolar macrophages as determined by flow cytometry. Both human SP-A (isolated from normal and alveolar proteinosis lungs) and SP-D (recombinant protein and protein isolated from alveolar proteinosis lungs) bound the conidia. These data indicate that important differences exist between rat and human SP-A in binding to certain fungi. Human SP-A and SP-D binding to conidia was also examined in the presence of hydrophobic surfactant components (HSC), containing both the phospholipid and hydrophobic proteins of surfactant. We found that HSC inhibited but did not eliminate human SP-A binding to Aspergillus conidia. In contrast, the SP-D binding to conidia was unaffected by HSC. These findings indicate that SP-D plays a major role in the recognition of Aspergillus conidia in alveolar fluid.