Ligand Binding and Phagocytosis by CD16 (Fc γ Receptor III) Isoforms: PHAGOCYTIC SIGNALING BY ASSOCIATED ζ AND γ SUBUNITS IN CHINESE HAMSTER OVARY CELLS

Abstract

CD16, the low affinity Fc gamma receptor III for IgG (Fc gamma RIII), exists as a polypeptide-anchored form (Fc gamma RIIIA or CD16A) in human natural killer cells and macrophages and as a glycosylphosphatidylinositol-anchored form (Fc gamma RIIIB or CD16B) in neutrophils. CD16A requires association of the gamma subunit of Fc epsilon RI or the zeta subunit of the TCR-CD3 complex for cell surface expression. The CD16B is polymorphic and the two alleles are termed NA1 and NA2. In this study, CD16A and the two alleles of CD16B have been expressed in Chinese hamster ovary (CHO) cells and their ligand binding and phagocytic properties analyzed. The two allelic forms of CD16B showed a similar affinity toward human IgG1. However, the NA1 allele showed approximately 2-fold higher affinity for the IgG3 than the NA2 allele. Although all three forms of CD16 efficiently bound rabbit IgG-coated erythrocytes (EA), only CD16A coexpressed with the gamma subunit phagocytosed EA. The phagocytosis mediated by CD16A expressed on CHO cells was independent of divalent cations but dependent on intact microfilaments. CHO cells expressing CD16A-gamma and CD16A-zeta chimeras also phagocytosed EA. The phagocytosis was specifically inhibited by tyrphostin-23, a tyrosine kinase inhibitor. In summary, our results demonstrate that glycosylphosphatidylinositol-anchored CD16B alleles differ from CD16A in their ability to mediate phagocytosis. Furthermore, since studies with other Fc gamma Rs have shown that CHO cells lack the phagocytic pathway mediated by the cytoplasmic domain of Fc gamma Rs, the phagocytosis of EA by CHO cells stably transfected with CD16A and CD16A-subunit chimera provides an ideal system to dissect the phagocytic signaling pathways mediated by these Fc gamma R-associated subunits.