Interactions of pulmonary surfactant protein D (SP-D) with human blood leukocytes

Surfactant protein D (SP-D) is believed to contribute to nonimmune host defense within the alveoli and distal airways of the lung. SP-D molecules can bind to specific carbohydrates on the surface of bacterial, fungal, and viral organisms and can also interact with membrane glycoconjugates expressed by alveolar macrophages. Because neutrophils (PMN) and monocytes are recruited into the airspaces in association with many types of infection or lung injury, we examined the interactions of these cells with purified natural and recombinant SP-Ds, using a modified Boyden chamber assay and checkerboard analysis. Natural or recombinant rat SP-D (approximately 10(-9) to 10(-13) M) showed dose-dependent effects on human PMN and monocyte migration with a maximal response at a SP-D concentration of 5 ng/ml (approximately 10(-11) M). The migratory response was comparable to that obtained with the optimum concentration of FMLP (10(-8) M). HL-60 cells, after induction of differentiation with DMSO, responded to SP-D with the same dose-response as neutrophils. The effects of SP-D were abrogated by the simultaneous addition of SP-D to the upper chamber or by the addition of antibodies to the carboxy-terminal lectin domain. Migration toward SP-D was markedly inhibited (< 10% of controls) by 10 mM maltose but was not significantly inhibited by to 50 mM lactose. These studies establish that SP-D can bind to specific sites on neutrophils and monocytes and strongly suggest that these interactions involve the saccharide binding domains of SP-D.