PULMONARY SURFACTANT PROTEIN-A AND PROTEIN-D ENHANCE NEUTROPHIL UPTAKE OF BACTERIA

The collectins are a class of collagenous lectin proteins present in s erum and pulmonary secretions [pulmonary surfactant protein (SP) A and SP-D] that are believed to participate in innate immune responses to various pathogens. With the use of flow cytometric and fluorescent-mic roscopic assays, SP-A and SP-D were shown to increase calcium-dependen t neutrophil uptake of Escherichia coli, Streptococcus pneumoniae, and Staphylococcus aureus. Evidence is provided that the collectins enhan ced bacterial uptake through a mechanism that involved both bacterial aggregation and direct actions on neutrophils. The degree of multimeri zation of SP-D preparations was a critical determinant of both aggrega ting activity and potency in enhancing bacterial uptake. The mechanism s of opsonizing activity of SP-D and SP-A differed in important respec ts from those of opsonizing antibodies. These results provide the firs t evidence that surfactant collectins may promote neutrophil-mediated clearance of bacteria in the lung independently of opsonizing antibody .