A new assay for fibronectin opsonic activity and its application to plasma and plasma fractions

A homologous assay system for human plasma fibronectin opsonic activity is presented. The system utilizes peripheral leukocytes purified from leukocyte concentrates and a metabolizable, radiolabeled lipid emulsion which has been gelatin-coated. The uptake of radiolabeled lipid was shown to depend on the presence of fibronectin and was stimulated by the addition of heparin. Both polymorphonuclear leukocytes (PMNs) and monocytes were active in this system. That at least a portion of the incorporated radiolabeled lipid emulsion was phagocytosed was indicated through the use of metabolic inhibitors, trypsin, and direct electron microscopic observation. This assay was applied to the analysis of donor plasma and plasma fractions. Opsonic activity levels measured with this assay system were shown to correlate (p = 0.05) with the level of fibronectin antigen in 17 samples of dialyzed normal donor plasma. Fibronectin opsonic activity was retained by outdated liquid plasma, cryo-depleted plasma, and cryoprecipitate; however, Cohn fraction I appeared to lose considerable fibronectin opsonic activity relative to fibronectin antigen.