Quantitative assessment of Fc receptor expression and function during in vitro differentiation of human monocytes to macrophages.

The expression and function of IgG Fc receptors on peripheral blood monocytes and monocytes differentiating in vitro, within hydrophobic membranes, to macrophages have been determined. Three characteristic functional stages could be discerned, and these were reflected in the surface expression of specific IgG binding sites. Stage 1, represented by fresh monocytes, is characterized by efficient binding and phagocytosis of IgG-coated particles, although uptake was limited by the cell size. Within 1 day of culture, monocytes transform into Stage 2 cells. These bind and ingest particles with high IgG surface density only and these functions are exquisitely sensitive to inhibition by IgG in solution. The functional loss is paralleled by a decrease of the number of specific IgG binding sites, despite a gradual increase in cell size. Between Day 4 and Day 8, macrophages acquired the capacity to bind and ingest particles opsonized to a low degree, their phagocytic capacity is strongly enhanced, the number of IgG binding sites increases by a factor greater than 10, despite a moderate increase in cell size only. Mature human macrophages have about six times the Fc receptor number of blood monocytes, but the average binding affinity is decreased. It appears that the phagocytic capacity is related to the number of IgG binding sites, and the susceptibility to inhibition by IgG in solution is related to their surface density.