CD16, the low affinity Fc ␥ receptor III for IgG (Fc␥RIII), exists as a polypeptide-anchored form (Fc␥RIIIA or CD16A) in human natural killer cells and macrophages and as a glycosylphosphatidylinositol-anchored form (Fc␥RIIIB or CD16B) in neutrophils. CD16A requires association of the ␥ subunit of Fc⑀RI or the 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 ␥ 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-␥ and CD16A-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␥Rs have shown that CHO cells lack the phagocytic pathway mediated by the cytoplasmic domain of Fc␥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␥R-associated subunits. In humans, CD16 is expressed as two distinct (CD16A and CD16B) forms (9 -15) which are products of two different highly homologous genes. CD16B is expressed on neutrophils in a glycosylphosphatidylinositol (GPI)-anchored form, whereas CD16A is expressed on NK cells, macrophages, and placental trophoblasts as a polypeptide-anchored transmembrane protein (8, 16 -18). The GPI-anchored CD16B exists as two allelic forms termed NA1 (CD16B NA1) and NA2 (CD16B NA2 ) (19). The polypeptide-anchored CD16A expressed on NK cells and macrophages is associated with subunits such as the chain of the 21) or the ␥ chain of . The NA1 and NA2 alleles of CD16B are 95% homologous to each other and are 95-97% homologous to CD16A in their extracellular domain (12). The functional significance of the existence of membrane isoforms and the polymorphism of CD16 are not clear. However, some studies have shown that CD16A differs from CD16B by triggering killing of tumor targets and signaling for IL-2 production (16,25,26).We established CHO cell lines expressing isoforms of CD16 and determined their ligand binding and phagocytic properties. The results show that the polypeptide-anchored CD16A is able to mediate phagocytosis of antibody-coated target cells, whereas under similar conditions the NA1 and NA2 alleles of GPI-anchored CD16B are not. ...

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