Human HMC-1 mast cells exclusively express the Fc gamma RII subtype of IgG receptor.

Because of their localization at the interface of the internal and external environment mast cells play a crucial role in the immune response and in inflammatory reactions. Effects may be mediated not only by the high-affinity IgE receptor, but also by IgG receptors. Since in rodent mast cells signal transduction via the Fc gamma receptor family has been shown, we analysed the expression of surface receptors for IgG on the human mast cell line HMC-1. It was shown by flow cytometric analysis that HMC-1 constitutively expressed the Fc gamma RII/CD32 subtype whereas Fc gamma RI/CD64 and Fc gamma RIII/CD16 were not expressed. This exclusive expression of the Fc gamma RII subtype of IgG receptor is similar to the expression pattern of basophils, although concerning cell surface molecules HMC-1 rather seem to resemble monocytes. In contrast to monocytes the expression profile on HMC-1 did not change upon stimulation with IL-4, TNF alpha, IFN gamma, PMA or salbutamol. Moreover, the mast cell-activating cytokine SCF and the calcium ionophore A23187 did not modulate the Fc gamma R profile in this study. To assess the importance of the exclusive Fc gamma RIII expression on HMC-1, we investigated whether the production of the cytokine TNF alpha is modulated via Fc gamma RII activation or if an increase in intracellular calcium could be observed. No significant modulation of TNF alpha release or of intracellular free calcium after crosslinking of Fc gamma RII by heat-aggregated IgG or by a second antibody was observed. It remains to be clarified whether this low-affinity subtype for the IgG receptor is involved in antigen-dependent sensitization of human tissue mast cells resulting in secretion of immunoregulatory cytokines. This mechanism may be important for disease states associated with circulating or tissue-bound immune complexes.