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

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 (FcgammaRI), our recent study showed that canine mastocytoma CM-MC cells are activated by monomeric canine IgG, suggesting the presence of FcgammaRI on CM-MC cells. In the present study, we measured the affinity of canine IgG with CM-MC cells, determined the presence of the FcgammaRI protein and mRNA, and identified the cDNA sequence of it. The results showed that (7.5+/-3.1)x10(4) receptor molecules are expressed on a CM-MC cell with a Ka of (9.1+/-1.6)x10(7)M(-1) for binding to monomeric canine IgG. Canine IgG-conjugated beads precipitated an approximately 72-kDa surface protein, whose size is consistent with that of the FcgammaRI alpha subunit of humans and mouse. The expression of FcgammaRI mRNA was detected by reverse transcriptase polymerase chain reaction (RT-PCR), and the cDNA encoding the FcgammaRI alpha 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 FcgammaRI-mediated signal transduction in mast cells.