Presence and primary sequence of a high-affinity IgG receptor on canine mastocytoma (CM-MC) cells

Abstract

Mast cells play a central role in IgE-dependent allergic responses. Although they have been reported to express only low-affinity IgG receptors and no high-affinity receptors (FcγRI), our recent study showed that canine mastocytoma CM-MC cells are activated by monomeric canine IgG, suggesting the presence of FcγRI on CM-MC cells. In the present study, we measured the affinity of canine IgG with CM-MC cells, determined the presence of the FcγRI protein and mRNA, and identified the cDNA sequence of it. The results showed that (7.5±3.1)×10⁴ receptor molecules are expressed on a CM-MC cell with a K _a of (9.1±1.6)×10⁷ M⁻¹ for binding to monomeric canine IgG. Canine IgG-conjugated beads precipitated an ~72-kDa surface protein, whose size is consistent with that of the FcγRI α subunit of humans and mouse. The expression of FcγRI mRNA was detected by reverse transcriptase polymerase chain reaction (RT-PCR), and the cDNA encoding the FcγRI α subunit was found to be 84% and 78% similar to that of humans and the mouse, respectively. The predicted amino acid sequence was 72% and 63% identical, respectively. Canine mastocytoma CM-MC cells are therefore very useful for studying FcγRI-mediated signal transduction in mast cells.