Phagocytosis-promoting activity of avian plasma and fibroblastic cell surface fibronectins

The purpose of this investigation was to determine whether fibronectin preparations from both chicken plasma and cell surface of fibroblasts can promote phagocytosis of gelatin-coated latex particles. Chicken plasma fibronectin was isolated (a) by ammonium sulfate fractionation, chromatography on Sepharose-4B followed by purification on a Sepharose-4B-heparin column; (b) by affinity chromatography on a Sepharose-4B-rat-antifibronectin column; (c) by affinity chromatography on Sepharose-4B-gelatin followed by molecular sieve separation on Sepharose-CL4B; (d) by a dual affinity chromatographic method using a Sepharose-4B-gelatin column and a Sepharose-4B-heparin column. Chicken cell surface fibronectin from fibroblast cultures was purified by ammonium sulfate precipitation followed by chromatography on Sepharose-CL4B. The purity of preparations was examined by polyacrylamide gel electrophoresis in the presence of sodium dodecylsulfate; all samples showing high purity. The opsonic activities of the preparations were measured by the uptake of 125I-labeled gelatin coated latex particles in conjunction with rat liver slice, and peritoneal macrophage monolayer systems. Both the plasma fibronectin and cell surface fibronectin preparations showed substantial opsonic activities in the test systems. Fresh chicken plasma did not reveal any phagocytosis promoting activity due to the presence of some unidentified inhibitor(s). The results showed that an opsonically active protein can be isolated from chicken plasma or serum and this protein is identical to plasma fibronectin. Furthermore, it could be concluded that cell surface fibronectin from chicken fibroblasts also can serve as an opsonin for gelatin coated particles.