Human fibronectin is an opsonin for IgG antibody-coated sheep erythrocytes

We studied the effect of human fibronectin on the binding and phagocytosis of IgG antibody-coated sheep erythrocytes (EA) by human monocytes. EA were preincubated with or without fibronectin (100 micrograms/ml) at 37 degrees C for 3 hours and both EA and EA-fibronectin were subsequently exposed to monocyte monolayers. A significantly higher percent of monocytes formed rosettes and phagocytosed EA-fibronectin than EA. To determine if the enhancing effect of fibronectin was on the EA or on the monocytes, several experiments were performed. EA were preincubated with or without fibronectin for 3 hours, washed, and added to monocyte monolayers. Percent rosettes and phagocytosis was significantly higher in monocytes incubated with EA-fibronectin than with EA. By contrast, preincubation of monocytes with soluble fibronectin at 37 degrees C for 15 minutes had no significant effect on their ability to bind or phagocytose EA. Thus fibronectin affects EA and enhances its binding to the monocytes. Both monocyte-EA and EA-fibronectin rosettes were completely inhibited by preincubation of monocytes with large concentrations of heat-aggregated IgG. However, below this maximal inhibitory concentration, heat-aggregated IgG produced significantly less inhibition of EA-fibronectin than EA rosettes. Our results suggest that EA-fibronectin binds exclusively to the monocyte Fc receptor and that fibronectin enhances the affinity of EA for the receptor. We did not demonstrate any effect of fibronectin on the interaction of monocytes with IgM antibody-coated or heat-damaged sheep erythrocytes. In conclusion under our test conditions, fibronectin acts as a opsonin for EA.