We have investigated the interactions between plasma fibronectin (Fn) and human peripheral blood phagocytic cells. As shown by studies of the binding of Fn-coated fluorescent microspheres (Fn-ms), both polymorphonuclear leukocytes (PMN) and monocytes had specific binding sites for Fn at the plasma membrane. However, as purified from blood, only monocytes were stimulated by Fn to become more actively phagocytic. This increase in phagocytosis was reflected by an Fn-induced increase in the ingestion of IgG-coated erythrocytes and, more dramatically by an Fn-dependent initiation of phagocytosis of C3b-coated erythrocytes. Despite this difference between PMN and monocytes in the functional consequences of Fn binding, the cell surface molecules responsible for Fn binding on the two cell types shared many characteristics. On both cells, binding of Fn-ms was inhibited by sufficient concentrations of fluid-phase Fn; both PMN and monocytes bound fewer Fn-ms at 4 degrees C than at 37 degrees C; both achieved maximal binding at similar Fn-ms/cell ratios; and phenylmethylsulfonyl fluoride did not inhibit Fn-ms binding to either cell type. Most dramatically, monoclonal anti-Fn antibodies that inhibited binding of Fn-ms to one cell type inhibited binding to both; conversely, monoclonal anti-Fn antibodies that did not inhibit Fn-ms binding to either cell type did not inhibit binding to the other. Fn will stimulate PMN to a more actively phagocytic state, like that induced in monocytes, if the PMN are first exposed to C5a or N-formyl-methionyl-leucylphenylalanine. This effect occurs without apparent change in the number of Fn receptors. We conclude that the PMN and monocyte receptors for Fn are very similar, but that their milieu is very different in the two cells as purified from peripheral blood. Whereas Fn induces increased phagocytosis in monocytes, PMN must be activated before the Fn can be effective.

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