FceRI eosinophils

FceRI is a high-affinity receptor. Apart from mast cells and basophils, it may be found on Langerhans cells, eosinophils, and monocytes. 

FceRII (CD23) receptor plays an important role in regulation of IgE synthesis. In comparison to FceRI, it shows low affinity to IgE. It is found on dendritic cells, macrophages, monocytes, platelets, T and B lymphocytes, and eosinophils. In its soluble form, it sometimes activates B lymphocytes to produce IgE. 

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... 

Saini S, MacGlashan DW Jr., KUon A, Holland S, Hamilton R, Bochner B. Relationship between serum IgE and FceRI levels on human basophils, monocytes, and eosinophils. J Allergy Clin Immunol 1999 103 8168. 

Rajakulasingam K, Durham S, O Brien F, Humbert M, Batata L, Reece L, Kay A, Grant J. Enhanced expression of high-affinity IgE receptor (FceRI) a chain in human allergen-induced rhinitis with co-localization to mast cells, macrophages, eosinophils, and dendritic cells. J Allergy Clin Immunol 1997 100 78-86. 

As we observed for CD23, the status of FceRI on rat and mouse eosinophils is very different. Our recent results have demonstrated that, in rats, upon thiogly-collate elicitation, peritoneal eosinophils expressed a functional trimeric ay2 FceRI able to mediate IgE-dependent ADCC towards schistosomula (42). In mouse, countless studies have reported on eosinophilia as a hallmark of pulmonary inflammation and of granuloma formation. Since wild-type mice do not express FceR (27), this species does not appear to be the best suited model for the study of IgE-dependent, eosinophil-mediated reactions, as already mentioned by Jones et al. (43) after experiments on a model of parasitic infections. 

Finally, the expression levels of FceRI vary not only according to the pathology, but also probably according to the maturation stage and or tissue localization of eosinophils, as suggested by the fact that peritoneal or splenic eosinophils from IL-5 X hFceRIa Tg mice expressed more receptors at their surface than bone marrow or blood eosinophils (D. Dombrowicz et al., unpublished). 

The expression of both FceRI and CD23 by human eosinophils enables this cell type to act in two ways in the immune response as an effector and as a regulatory... 

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. 

As demonstrated for mast cells, eosinophils, which also synthesize several cytokines, are able to secrete cytokines upon FceRI engagement and therefore modulate the immune response. Indeed, it was first demonstrated that activation of eosinophils by IgE immune complexes induced IL-5 release (72). More recently, and similarly to IgA immune complexes (59), we have shown that EceRI or CD23 crosslinking induced the selective release of IL-10 by eosinophils. These cells would thus be able to polarize the immune response towards a type 2 profile. 

Since it has been shown that FceRI, expressed by professional antigen-presenting cells such as human epidermal Langerhans cells (73), dendritic cells (6), and monocytes (74), was able to very efficiently mediate antigen capture, processing, and presentation, eosinophils might possibly fulfill the same function. 

Seminario MC, Saini SS, MacGlashan DW, Jr., Bochner BS. Intracellular expression and release of FceRI alpha by human eosinophils. J Immunol 1999 162 6893-6900. 

Dombrowicz D, Quatannens B, Papin JP, Capron A, Capron M. Expression of a functional FceRI on rat eosinophils and macrophages. J Immunol 2000 165 1266-1271. 

Sihra BS, Kon OM, Grant JA, Kay AB. Expression of high-affinity IgE receptors (FceRI) on peripheral blood basophils, monocytes, and eosinophils in atopic and nonatopic subjects relationship to total serum IgE concentrations. J Allergy Chn Immunol 1997 99 699—706. 

K, Yoshimura K, Tanaka Y, et al. IL-4 upregulates FceRI a-chain messenger RNA in eosinophils. J Allergy Clin Immunol 1995 96 1161-1169. 

Tanaka Y, Takenaka M, Matsunaga Y, Okada S, Anan S, Yoshida H, Ra C. High affinity IgE receptor (FceRI) expression on eosinophils infiltrating the lesions and mite patch tested sites in atopic dermatitis. Arch Dermatol Res 1995 287 712-771. 

It may be important to note that there seems to be a dissociation between the synthesis of FceRI subunits and the cell surface expression of this receptor in human eosinophils. Indeed, Smith et al. (52) confirmed the presence of intracellular FceRla protein, using flow cytometry to analyze permeabilized cells as well as immunohistochemistry to directly visualize cytospin preparations, but negligible FceRla protein was detectable on the cell surface. Furthermore, Semi-... 

---