Expression of a Functional High-Affinity IgG Receptor, FcγRI, on Human Mast Cells: Up-Regulation by IFN-γ

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Abstract
Biologically relevant activation of human mast cells through Fc receptors is believed to occur primarily through the high-affinity IgE receptor FcεRI. However, the demonstration in animal models that allergic reactions do not necessarily require Ag-specific IgE, nor the presence of a functional IgE receptor, and the clinical occurrence of some allergic reactions in situations where Ag-specific IgE appears to be lacking, led us to examine the hypothesis that human mast cells might express the high-affinity IgG receptor FcγRI and in turn be activated through aggregation of this receptor. We thus first determined by RT-PCR that resting human mast cells exhibit minimal message for FcγRI. We next found that IFN-γ up-regulated the expression of FcγRI. This was confirmed by flow cytometry, where FcγRI expression on human mast cells was increased from ∼2 to 44% by IFN-γ exposure. FcεRI, FcγRII, and FcγRIII expression was not affected. Scatchard plots were consisted with these data where the average binding sites for monomeric IgG1 (Ka = 4–5 × 108 M−1) increased from ∼2,400 to 12,100–17,300 per cell. Aggregation of FcγRI on human mast cells, and only after IFN-γ exposure, led to significant degranulation as evidenced by histamine release (24.5 ± 4.4%): and up-regulation of mRNA expression for specific cytokines including TNF-α, GM-CSF, IL-3 and IL-13. These findings thus suggest another mechanism by which human mast cells may be recruited into the inflammatory processes associated with some immunologic and infectious diseases.