FceRI receptor expression

Schroeder, J. T., Bieneman, A. P., Xiao, H. et al. 2005. TLR9- and FceRI-mediated responses oppose one another in plasmacytoid dendritic cells by down-regulating receptor expression, J Immunol 175 5724-5731. 

The next four factors partially determine how many molecules of antigen-specific IgE reside on fhe surface of the mast cell or basophil. Eor example, if a mast cell has 100,000 FceRI receptors, the total IgE concentration is 1000 ng/ mL, and the antigen specific IgE titer is 200 ng/mL, then 20,000 receptors will be occupied by the antigen-specific IgE (because the occupancy of the receptor is determined by the very high association constant between this receptor and IgE and when the IgE concentration is this high, it is simpler to state the numbers as if all receptors were occupied—the numbers will be valid within 2-3%). As will be explored in greater detail in the next section, the total IgE concentration also determines the expression of FceRI, so that a ratio of specific to total IgE of 20%, as in the example above, has a far different consequence if the total IgE concentration is 10 ng/mL vs. 1000 ng/mL. Since the IgE concentration determines receptor expression, the ability of IgE antibody to be present outside the vascular compartment also determines the ability of mast cells to respond to antigen. 

Surface FceRI expression on peripheral blood eosinophils has been variably observed in vivo since the first report of the high-affinity receptor on blood eosinophils (90) and remains a topic of controversy (91). The initial description involved blood eosinophils from subjects with hypereosinophilic syndromes (90), but subsequent studies have failed to reproduce these observations (85,92). Blood eosinophils express both mRNA and intracellular protein for the a and y subunits of the FceRI receptor complex but lack the P subunit protein. Eosinophils in culture with IL-5 release the a subunit protein into the culture supernatant, but the addition of IgE failed to induce expression of the receptor on the cell surface... 

It is also possible that IgE receptor expression would provide eosinophils with a means to exert their antitumoral activity through IgE-mediated ADCC. Indeed, it has been shown that eosinophils sometimes play a role in IL antim-moral activity (65) and that FcctR (66), FceRI (67,68), and FcyR (69) also participate to antitumoral activity. 

Mast cells express high-affinity IgE Fc receptors (FceRI) on their surface, contain cytoplasmic granules which are major sources of histamine and other inflammatory mediators, and are activated to release and generate these mediators by IgE-dependent and non-IgE-dependent mechanisms [1]. Disturbances either in the release of mast cell mediators or in mast cell proliferation are associated with clonal mast cell disorders including monoclonal mast cell activation syndrome (MMAS) and mastocytosis respectively, which are in turn associated with some cases of anaphylaxis [2], Molecular mechanisms have been identified which may link increased releasability of mast cell mediators and conditions leading to increased mast cell numbers [3]. Patients with mastocytosis have an increased risk to develop anaphylaxis [4, 5] and those with anaphylaxis may suffer from unrecognized mastocytosis or may display incomplete features of the disease [6-8]. 

Human basophils and mast cells are the only cells expressing the tetrameric high-affinity receptor of IgE (FceRI) and synthesizing histamine [26], Basophils and mast cells (FceRI + cells) play a prime role in the pathophysiology of allergic disorders through the elaboration and release of numerous proinflammatory and immunoregulatory molecules and the expression of a wide spectrum of receptors for cytokines and chemokines [27,28]. 

Shim, S. Y., Le, Q. T., Lee, S. H., and Kim, S. K. (2009b). Ecklonia cava extract suppresses the high-affinity IgE receptor, FceRI expression. Food Chem. Toxicol. 47, 555-560. 

The initial event in the activation of mast cells for mediator release is the binding of IgE antibodies to the high-affinity 10 M) FceRI IgE receptor abundantly expressed on the mast cell and basophil surfaces. 

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