Tissue-Specific Expression of the Low-Affinity IgG Receptor, FcγRIIb, on Human Mast Cells

Abstract

Immediate hypersensitivity reactions are induced by the interaction of allergens with specific IgE antibodies bound via FcεRI to mast cells and basophils. While these specific IgE antibodies are needed to trigger such reactions, not all individuals harboring IgE exhibit symptoms of allergy. The lack of responsiveness seen in some subjects correlates with the presence of IgG antibodies of the same specificity. In cell culture studies and in vivo animal models of food allergy and anaphylaxis such IgG antibodies have been shown to exert suppression via FcγRIIb. However, the reported absence of this inhibitory receptor on primary mast cells derived from human skin has raised questions about the role of IgG-mediated inhibition of immediate hypersensitivity in human subjects. Here, we tested the hypothesis that mast cell FcγRIIb expression might be tissue specific. Utilizing a combination of flow cytometry, quantitative PCR, and immunofluorescence staining of mast cells derived from the tissues of humanized mice, human skin, or in fixed paraffin-embedded sections of human tissues, we confirm that FcγRIIb is absent from dermal mast cells but is expressed by mast cells throughout the gastrointestinal tract. IgE-induced systemic anaphylaxis in humanized mice is strongly inhibited by antigen-specific IgG. These findings support the concept that IgG, signaling via FcγRIIb, plays a physiological role in suppressing hypersensitivity reactions.

Keywords: Fc receptors; FcγRIIb; IgE; IgG; anaphylaxis; food allergy; immediate hypersensitivity; mast cells.

Figure 1

Fcγ receptor (FcγR)IIb expression by mast cells and basophils from humanized mice, human blood and human skin. Cells from human samples or humanized mouse organs were assessed for FcgRIIa and FcgRIIb expression by flow cytometry. Each experiment was performed at least three times and a representative histogram is presented.

Figure 2

Quantitative PCR analysis of FCGR2A and FCGR2B transcripts in human FceRI⁺ cells. Data are from 5 to 7 samples per cell type.

Figure 3

Fcγ receptor (FcγR)IIb expression by mast cells in human intestine and skin. Immunofluorescent staining for mast cell tryptase (green) and FcgRIIb (red) on human intestinal or skin tissue sections in tissue arrays. Mast cells are indicated by green arrows. Larger fields, showing more cells, are presented in Figure S3 in Supplementary Material.

Figure 4

Fcγ receptor II-dependent inhibition of human mast cells and anaphylaxis. (A) In vitro degranulation of anti-PN IgE-sensitized mast cells from humanized mice and inhibition by IgG anti-PN. Anti-CD32 or an isotype control mouse IgG was added 2 h prior to IgG anti-PN. (B) In vitro degranulation of human skin mast cells under the conditions used in (A). (C) Core temperature loss (anaphylaxis) in humanized mice passively sensitized with IgE anti-PN and challenged by PN injection. (D) Serum tryptase levels 1 h post-challenge in humanized mice.