Canine mast cell activation via human IgG1 and IgG4

We have reported that canine mastocytoma-derived CM-MC cells are activated via canine IgG and express a high-affinity IgG receptor (canine FcgammaRI). The predicted amino acid sequence of the canine FcgammaRI alpha subunit was found to be 72% similar to that of humans. These results suggest that canine FcgammaRI have binding activity with human IgG and led us to investigate CM-MC activation via canine FcgammaRI and human IgG.

The binding of human IgG to canine FcgammaRI was examined by flow cytometry using FITC-conjugated human IgG. [Ca2+]i increase or histamine release via canine FcgammaRI and the four human IgG subclasses was measured following aggregation of IgG-bound FcgammaRIs by anti-human IgG. To determine the binding activity of canine FcgammaRI with human IgG1 or IgG3, the displacement of 125I-labeled canine IgG from canine FcgammaRI was examined by unlabeled human IgG1 or IgG3.

The fluorescence intensity of CM-MC cells was markedly (about 50 times) elevated by incubation with FITC-human IgG compared with the fluorescence of the control cells. A significant (p < 0.01) calcium response and histamine release were observed following aggregation of canine FcgammaRIs bound with human IgG1 or IgG4. 125I-labeled canine IgG was displaced from canine FcgammaRI by preincubation with unlabeled total human IgG or human IgG1 dose-dependently, whereas no displacement was detected by preincubation with human IgG3.

Canine FcgammaRI possesses a significant binding activity with human IgG1 or IgG4, while IgG2 or IgG3 did not significantly react with canine FcgammaRI on CM-MC cells.