Fibronectin binds to complement-coated agarose beads and increases their association to mouse macrophages

We have studied the binding of fibronectin to complement (C3b, C3bi, C3d)-coated agarose beads and its effect on cell association of such beads to mouse macrophages. Fibronectin bound to agarose beads preincubated in human serum, whereas no binding occurred after preincubation of the beads with complement-inactivated (50 degrees C for 20 min or ethylenediaminetetraacetic acid) sera. The binding of iodine-labelled fibronectin to beads preincubated in fibronectin-depleted serum (HS-FIB) was about twice that of beads preincubated in normal serum. Unlabelled fibronectin inhibited the following binding of labelled fibronectin to beads pretreated in HS-FIB. A similar amount of fibronectin bound to agarose beads coated with equimolar amounts of C3b, C3bi, or C3d, suggesting that the common domain C3d carries the main binding site(s) for fibronectin. Preincubation of serum-treated and trypsinized agarose beads with fibronectin led to an increased association (22%) of such beads to mouse macrophages. The results indicate that fibronectin promotes binding of complement-coated agarose beads to mouse macrophages, whereas the ingestion of the beads is mediated via complement C3 receptors.