Fibronectin promotes binding but not ingestion of agarose beads by mouse macrophages adn human monocytes

We have examined to what extent human fibronectin associated with agarose beads with a 5- to 10-micron diameter mediates binding and uptake of the beads by mouse macrophages and human monocytes. Native agarose beads preincubated with 125I-fibronectin were neither associated with nor taken up by mouse macrophages after 30 min of incubation under serum-free conditions. When fibronectin was cross-linked to cyanogen bromide-activated agarose beads or incubated with gelatinized beads, this resulted in a significant increase in particle binding by macrophages and monocytes as compared with gelatinized beads, whereas the fraction of cells with ingested particles remained unaltered. Native agarose beads activated by cyanogen bromide and treated with ethanolamine were to a greater extent associated with and taken up by phagocytes than fibronectin- or gelatin-coated beads. Our results indicate that fibronectin acts as an adhesive glycoprotein and not as an opsonin. Since agarose beads are activators of the alternative pathway of complement, and fibronectin is reported to bind to factor C3, we speculate that cell-derived C3b is bound to the beads and fibronectin-coated beads are ingested by the phagocytes via complement C3b receptors on the cells.