Human low-affinity IgG receptor Fc gamma RIIa (CD32) introduced into mouse fibroblasts mediates phagocytosis of sensitized erythrocytes.

Multiple isoforms of human (h) Fc gamma RII are currently recognized. We set up experiments to evaluate the significance of different hFc gamma RII (CD32) isoforms in the uptake of IgG-coated particles. Therefore, a panel of stable hFc gamma RII-transfected fibroblasts was constructed to study phagocytosis. It was observed that over 40% of 3T6 fibroblasts transfected with hFc gamma RIIa cDNA phagocytose human erythrocytes (E) coated with mouse (m) IgG1 or mIgG2a (EA). This was in contrast to cells expressing a tail-minus variant of Fc gamma RII (hFc gamma RIItail-). Fibroblasts transfected with hFc gamma RIIb1 cDNA infrequently internalized EA. There was no difference in EA binding by these transfectants; all three clones formed more than 80% EA-rosettes. Uptake of particles was dependent on the degree of E sensitization, and was inhibitable by CD32 monoclonal antibody AT10. Preincubation of cells with cytochalasin D blocked internalization completely, with no effect on binding. This shows that phagocytosis underlies the observed E internalization. Immunofluorescence studies using an anticathepsin D antiserum showed the maturation of phagosomes, containing antibody-coated E, into lysosomes. In conclusion, our studies show, for the first time, that fibroblasts possess the basic machinery for ingestion of erythrocytes, and are capable of phagocytosis only when equipped with an appropriate receptor. hFc gamma RIIa and, to a lesser extent hFc gamma RIIb1, are suited for this task, which seems dictated by the cytoplasmic domain.