Fibronectin Promotes Binding But Not Ingestion of Agarose Beads by Mouse Macrophages and Human Monocytes

We have examined to what extent human fibronectin associated with agarose beads with a 5- to 10-μm diameter mediates binding and uptake of the heads 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 heads or incubated with gelatinized heads, 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 heads 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 heads. Our results indicate thai 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 fibroneetin-coaled beads arc ingested by the phagocytes via complement C3b receptors on the cells.