Effect of fibronectin fragments on macrophage phagocytosis of gelatinized particles

Rat plasma fibronectin enhances the binding and ingestion of gelatin-coated, formalin-fixed, or tanned sheep erythrocytes by elicited rat peritoneal macrophages. Fibronectin binding to the gelatinized erythrocytes is required for this enhancement, because macrophages preferentially recognize the surface bound molecule. This enhancement of particle uptake by fibronectin required the presence of a renewable, trypsin-sensitive component(s) on the macrophage surface (fibronectin receptor). When subjected to plasminolysis for 3 hr, fibronectin was degraded into gelatin-binding fragments (170 to 210 kd) and a 25-kd nongelatin binding fragment. The 170 to 210 kd gelatin binding fragments retained uptake-enhancing activity but were less active on a weight and molar basis than intact, dimeric fibronectin. The nongelatin binding 25 kd fragment alone did not enhance uptake. These results indicate that the sites for interaction with both the gelatinized erythrocyte surface and macrophages are retained on 170 to 210 kd fragments. However, the fibronectin dimeric structure is required for maximal expression of opsonic activity.