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Protein-tyrosine phosphorylation by IgG1 subclass CD38 monoclonal antibodies is mediated through stimulation of the FcgammaII receptors in human myeloid cell lines.

The human surface Ag CD38 is a 46-kDa type II transmembrane glycoprotein, and its expression is dependent on the cell differentiation and activation of lymphocytes. Our previous work in human myeloid cells showed that ligation of CD38 with mAbs (HB-7 and T-16; IgG1 subclass) not only induced protein-tyrosine phosphorylation but also potentiated superoxide generation stimulated by G protein-coupled receptors. In the present study we analyzed the mechanisms of action of the agonistic mAbs. HB-7-induced tyrosine phosphorylation could be still observed in human myeloid cells expressing CD38 mutants, of which cytoplasmic and transmembrane domains had been deleted or replaced by those of another type II glycoprotein (PC-1). Moreover, N-linked glycosylation on the cell surface CD38 was not required for the HB-7-induced cell signaling. The profile of tyrosine-phosphorylated proteins by HB-7 was exactly the same as that induced by cross-linking of FcgammaII receptors (FcgammaRII/CD32), and FcgammaRII itself was tyrosine phosphorylated in the two stimulated cells. The HB-7-induced tyrosine phosphorylation was completely abolished after masking of FcgammaRII with its mAb. Finally, F(ab')2 of HB-7 failed to mimic the actions of the whole form of mAb. These results indicate that anti-CD38 mAb-induced tyrosine phosphorylation and its associated cell response are entirely mediated through the FcgammaRII-induced signaling pathway, possibly resulting from stimulation of the cell surface human FcgammaRII with the mouse Fc region (IgG1 subclass) of CD38-ligated mAbs.