Role of FcγRIIa (CD32) in IgG anti‐RhD‐mediated red cell phagocytosis in vitro

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Abstract
Summary. To test the role of FcγRIIa in IgG anti‐RhD‐mediated phagocytosis three IgGl and two IgG3 human monoclonal anti‐D antibodies were tested for ability to mediate binding/phagocytosis of cDE/cde and ‐D‐/‐D‐ red cells by FcγRIIa‐R131 and FcγRIIa‐H131 cDNA‐transfected 3T6 fibroblasts. Both IgG3 monoclonal antibodies brought about ‐D‐/‐D‐ cell interaction with IIa‐transfected fibroblasts, while only one of them, Og3, mediated binding of cDE/cde targets. Although FcγRIIa expression was three times greater on IIa‐R131 than on IIa‐H131 fibroblasts, the latter bound significantly more Og3‐coated cDE/cde‐ and IgG3 anti‐D‐sensitized ‐D‐/‐D‐ cells, respectively, than the former effectors and showed some phagocytosis of the ‐D‐/‐D‐targets. IgGl anti‐D antibodies were inactive in mediating red cell interaction with the fibroblasts. Moreover, monoclonal anti‐FcγRII IV.3 partially inhibited the phagocytosis by adult or fetal monocytes of Og3‐sensitized cDE/cde cells. FcγRIIa‐H/H131 monocytes exhibited higher phagocytic indices towards these targets than monocytes of other IIa allotypes. The results indicate that FcγRIIa can participate in the phagocytosis of red cells coated with IgG3 anti‐D; in this case the allotype of the receptor will modify the extent of red cell destruction.