Accumulating evidence suggests that IgE-mediated activation of mast cells occurs even in the absence of antigen, which is referred to as "monomeric IgE" responses. Although monomeric IgE was found to induce a wide variety of responses, such as up-regulation of the FcεRI, survival, cytokine production, histamine synthesis, and adhesion to fibronectin, it remains to be clarified how mast cells are activated in the absence of antigen. It has been controversial whether monomeric IgE responses are mediated by a similar signaling mechanism to antigen stimulation, although recent studies suggest that IgE can induce the FcεRI aggregation even in the absence of antigen. In this study, we focused on the role of conventional protein kinase C (cPKC), since this response is suppressed by a specific inhibitor for cPKC. Monomeric IgE-induced Ca2+ influx was not observed in a mouse mastocytoma cell line, which lacks the expression of PKCβII, although Ca2+ influx induced by cross-linking of the FcεRI was intact. Transfection of PKCβII cDNA was found to restore the Ca2+ influx induced by monomeric IgE in this cell line. Furthermore, the dominant negative form of PKCβII (PKCβII/T500V) significantly suppressed the Ca2+ influx, histamine synthesis, and interleukin-6 production in another mouse mast cell line, which is highly sensitive to monomeric IgE. Expression of PKCβII/T500V was found not to affect the antigen-induced responses. These results suggest that PKCβII plays a critical role in monomeric IgE responses, but not in antigen responses.
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