Surfactant protein A (SP-A) interacts with serum lipoproteins entering the airways

Serum lipoproteins may enter the airways and appear in the sputum (chylophtysis) when the lymphatic circulation is impaired by inflammation, neoplasm, or by an abnormal proliferation of smooth muscle cells. While analyzing the bronchoalveolar lavage fluid of a patient with chylophtysis, we noticed that surfactant could not be separated from contaminating lipoproteins, and we speculated that surfactant components may interact with lipoproteins entering the airways. To clarify this point, we immobilized SP-A on microtiter wells and incubated it with 125I-VLDL. We found that SP-A binds VLDL. The binding is time and concentration dependent, is inhibited by excess VLDL and by liposomes (dipalmitoyl phosphatidylcholine: egg phosphatidylcholine: phosphatidylglycerol: cholesterol, 50:25:15:10), is increased by 5 mM Ca++, is unaffected by surfactant proteins B and C, or by methyl mannoside, and is greatly decreased if SP-A is alkylated and reduced before immobilization. Furthermore, we found that soluble SP-A increases the degradation of VLDL by alveolar macrophages collected from normal subjects, and favors the binding of VLDL to surfactant membranes. We conclude that SP-A affects the catabolism of serum lipoproteins entering the airways.