LIGAND-BINDING AND PHAGOCYTOSIS BY CD16 (FC-GAMMA RECEPTOR-III) ISOFORMS - PHAGOCYTIC SIGNALING BY ASSOCIATED ZETA-SUBUNITS AND GAMMA-SUBUNITS IN CHINESE-HAMSTER OVARY CELLS

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 hum an natural killer cells and macrophages and as a glycosylphosphatidyli nositol-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-CDS complex for cell surface expression. The CD16B is polymorphic and the two alleles are termed NA1 and NA2. In this st udy, CD16A and the two alleles of CD16B have been expressed in Chinese hamster ovary (CHO) cells and their Ligand binding and phagocytic pro perties analyzed. The two allelic forms of CD16B showed a similar affi nity toward human IgG1. However, the NA1 allele showed approximately S -fold higher affinity for the IgG3 than the NA2 allele. Although all t hree forms of CD16 efficiently bound rabbit IgG-coated erythrocytes (E A), 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 tyrosi ne kinase inhibitor. In summary, our results demonstrate that glycosyl phosphatidylinositol-anchored CD16B alleles differ from CD16A in their ability to mediate phagocytosis. Furthermore, since studies with othe r Fc gamma Rs have shown that CHO cells lack the phagocytic pathway me diated by the cytoplasmic domain of Fc gamma Rs, the phagocytosis of E A 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.