INTERACTION OF LUNG SURFACTANT PROTEIN-A WITH ALVEOLAR MACROPHAGES

We analyzed the binding mechanism of human recombinant lung surfactant protein A (SP-A) to rat alveolar macrophages using anti-SP-A antiseru m and protein A coated onto gold particles. Results were compared with our recent data on binding and uptake of SP-A-coated colloidal gold p articles. The rationale for the current approach was to avoid any poss ible steric effects on SP-A binding to the cell surface. Binding of un labeled SP-A depends on the presence of calcium ions in the medium and involves a mannose-specific mechanism. Binding is partly inhibited by the collagenase-resistent fragment of SP-A, representing mainly the g lobular part of SP-A. Taken together, these facts indicate binding of SP-A via the carbohydrate binding site on the globular region of SP-A. On the other hand, a partial inhibition of SP-A binding by fragments of Clq (representing the collagenous region of Clq) indicates a second binding site for SP-A by the collagen-like portion to the Clq recepto r of macrophages. We conclude that two different mechanisms are probab ly involved in SP-A binding to alveolar macrophages. Specificity of th e binding was shown with fluorescein-labeled SP-A. Binding was inhibit ed by an excess of unlabeled SP-A. Binding and uptake of SP-A are seen only with alveolar macrophages and not with other macrophage populati ons isolated from rat, such as liver macrophages (Kupffer cells), resi dent peritoneal macrophages, and peritoneal macrophages activated by C orynebacterium parvum. Therefore, binding sites for SP-A occur exclusi vely on alveolar macrophages. In addition, the intracellular Ca2+ conc entration of the lung macrophages was determined by using the fluoresc ent dye fura-2/AM. Intracellular [Ca2+] increased immediately after ad dition of SP-A. This indicates immediate activation of macrophages by SP-A. (C) 1993 Wiley-Liss, Inc.