Characterization of the surfactant protein A receptor: cell and ligand specificity

The cell and ligand specificity of the putative surfactant protein A (SP-A) receptor was investigated using a cell receptor assay in which SP-A-coated magnetic beads were incubated with 51Cr-labeled cells at 4 degrees C. Cells that bound to the SP-A-coated beads were isolated by magnetic separation. The recovery of alveolar macrophages was significantly higher than the recovery of peritoneal macrophages, alveolar type II cells, polymorphonuclear leukocytes, COS cells, and U937 cells. In "coincubation" experiments, in which the potential inhibitors were present during the cell recovery assay, SP-A, SP-D, and complement factor 1q (C1q) all inhibited recovery. In contrast, in "preincubation" experiments, in which cells were incubated with potential inhibitors and then washed before the cell recovery assay, only SP-A inhibited cell recovery. Because SP-A binds to SP-D and C1q, we speculate that inhibition of SP-A binding to macrophages by SP-D and C1q in coincubation assays is due to interaction of SP-A with SP-D or C1q, rather than competition for receptor binding. The recovery of alveolar macrophages was also inhibited by preincubation with heat-denatured SP-A, deglycosylated SP-A, and with the collagenase-resistant fragment of SP-A, but not by coincubation with mannan. These results suggest the presence of a receptor on alveolar macrophages with a high degree of specificity for SP-A.