Eosinophils and IgE Receptors : A Continuing Controversy

phils produce cytokines, such as interleukin-4 (IL-4) and D IL-5, which are potentially important in the recruitment of URING THE PAST 2 decades, considerable new information has been obtained about the functions of the eosinophils, thus causing chronic allergic inflammation. eosinophil and its roles in human disease. Presently, the Second, IgE bound to receptors on antigen-presenting cells, eosinophil is recognized as a proinflammatory granulocyte such as CD23 on B cells and to high-affinity IgE receptors implicated in protection against parasitic infections and be(FceRI) on Langerhans’ cells and monocytes, can enhance lieved to play a major role in allergic diseases, such as bronantigen internalization and presentation to T cells, resulting chial asthma, allergic rhinitis, and atopic dermatitis. The in continuous activation of the immune system. Finally, eosinophil is an important source of cytotoxic cationic proIgE may mediate killing of the invading helminth and host teins, such as major basic protein (MBP), eosinophil peroxicell damage by acting as a ligand for antibody-dependent dase, and eosinophil cationic protein. These proteins are pocell-mediated cytotoxicity (ADCC) by macrophages and tentially two-edged swords; on the one hand, they protect other immune cells. In fact, immunoepidemiological studthe host against overwhelming helminth infections, but on ies showed a significant correlation between the production the other hand, they damage the host’s tissues. Eosinophils of antischistosome IgE antibodies and the acquisition of imalso induce inflammation by releasing lipid mediators, oxymunity against reinfection to S haematobium. In allergic gen metabolites, and cytokines. Numerous studies have diseases such as bronchial asthma, there is a close correlation shown the association of eosinophils and various human between serum IgE levels and the prevalence and severity parasitic and allergic diseases. For example, at present, the of the diseases. Thus, there is now converging evidence to most common worldwide cause of eosinophilia is probably support roles for IgE in resistance to helminthic infections infection with helminths, and high eosinophil counts correand in the pathophysiology of allergic diseases in humans. late with lack of reinfection after treatment of Schistosoma Therefore, it is reasonable to speculate that IgE is involved haematobium infections. Analyses of patients infected with in the activation of eosinophils in these diseases. Onchocerca volvulus have shown striking deposition of the Early studies on the killing of schistosomula in vitro by eosinophil granule MBP around degenerating microfilaria. human eosinophils used cells purified from normal or In allergic asthma, eosinophilic and lymphocytic infiltration slightly eosinophilic individuals, together with heat-inactiin the epithelium and lamina propria of the airways are convated sera from individuals with schistosome infection. The sistently found even in mild and stable asthma. Indeed, results of these studies suggested that killing requires IgG correlations have been observed between the numbers of and is independent of complement. IgG1 and IgG3 subinfiltrating eosinophils and asthma disease severity. Pulmoclasses were effective in mediating ADCC by human eosinonary segmental allergen challenge in allergic individuals phils, whereas IgM, IgG2, and IgG4 were not only inactive, causes eosinophil recruitment into the airways; this is associbut blocked the effects of the active subclasses. A quite ated with the release of eosinophil granule proteins and the separate phenomenon was observed with low density, soincrease in vascular permeability. Despite the strong assocalled hypodense eosinophils that can be isolated from indiciations among eosinophils, their cytotoxic granule proteins, viduals with very high eosinophil counts. Receptors for IgE and human diseases, the mechanism(s) responsible for eosinwere identified on both rat and human hypodense eosinoophil activation in vivo is largely unknown. phils, and hypodense human eosinophils were shown to Helminth infections and allergic diseases are characteristikill schistosomula in the presence of IgE. Subsequently, cally associated not only with peripheral blood and tissue this human eosinophil IgE receptor was shown to be similar, eosinophilia, but also with high levels of both total and antigen-specific IgE antibodies. IgE antibodies may be involved in disease in three ways. First, the central feature in From the Departments of Immunology and Medicine, Mayo Clinic anaphylactic and immediate hypersensitivity reactions is and Mayo Foundation, Rochester, MN. IgE-dependent activation of mast cells and basophils leading Address reprint requests to Hirohito Kita, MD, Department of to the release of histamine and other inflammatory mediators, Immunology, Mayo Clinic, 200 First St SW, Rochester, MN 55905. such as prostaglandins and leukotrienes. Furthermore, upon q 1997 by The American Society of Hematology. 0006-4971/97/8910-0042$3.00/0 activation through IgE receptors, human mast cells and baso-


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IL-5, which are potentially important in the recruitment of URING THE PAST 2 decades, considerable new information has been obtained about the functions of the eosinophils, thus causing chronic allergic inflammation. 9eosinophil and its roles in human disease.Presently, the Second, IgE bound to receptors on antigen-presenting cells, eosinophil is recognized as a proinflammatory granulocyte such as CD23 on B cells and to high-affinity IgE receptors implicated in protection against parasitic infections and be- (Fc e RI) on Langerhans' cells and monocytes, can enhance lieved to play a major role in allergic diseases, such as bronantigen internalization and presentation to T cells, resulting chial asthma, allergic rhinitis, and atopic dermatitis. 1 The in continuous activation of the immune system. 10,11Finally, eosinophil is an important source of cytotoxic cationic pro-IgE may mediate killing of the invading helminth and host teins, such as major basic protein (MBP), eosinophil peroxicell damage by acting as a ligand for antibody-dependent dase, and eosinophil cationic protein.These proteins are pocell-mediated cytotoxicity (ADCC) by macrophages and tentially two-edged swords; on the one hand, they protect other immune cells. 12In fact, immunoepidemiological studthe host against overwhelming helminth infections, but on ies showed a significant correlation between the production the other hand, they damage the host's tissues. 2 Eosinophils of antischistosome IgE antibodies and the acquisition of imalso induce inflammation by releasing lipid mediators, oxymunity against reinfection to S haematobium. 13In allergic gen metabolites, and cytokines. 2Numerous studies have diseases such as bronchial asthma, there is a close correlation shown the association of eosinophils and various human between serum IgE levels and the prevalence and severity parasitic and allergic diseases.For example, at present, the of the diseases. 14Thus, there is now converging evidence to most common worldwide cause of eosinophilia is probably support roles for IgE in resistance to helminthic infections infection with helminths, and high eosinophil counts correand in the pathophysiology of allergic diseases in humans.late with lack of reinfection after treatment of Schistosoma Therefore, it is reasonable to speculate that IgE is involved haematobium infections. 3Analyses of patients infected with in the activation of eosinophils in these diseases.

Onchocerca volvulus have shown striking deposition of the
Early studies on the killing of schistosomula in vitro by eosinophil granule MBP around degenerating microfilaria. 4uman eosinophils used cells purified from normal or In allergic asthma, eosinophilic and lymphocytic infiltration slightly eosinophilic individuals, together with heat-inactiin the epithelium and lamina propria of the airways are convated sera from individuals with schistosome infection. 15The sistently found even in mild and stable asthma. 5Indeed, results of these studies suggested that killing requires IgG correlations have been observed between the numbers of and is independent of complement.IgG1 and IgG3 subinfiltrating eosinophils and asthma disease severity. 5Pulmoclasses were effective in mediating ADCC by human eosinonary segmental allergen challenge in allergic individuals phils, whereas IgM, IgG2, and IgG4 were not only inactive, causes eosinophil recruitment into the airways; this is associbut blocked the effects of the active subclasses. 16A quite ated with the release of eosinophil granule proteins and the separate phenomenon was observed with low density, soincrease in vascular permeability. 6,7Despite the strong assocalled hypodense eosinophils that can be isolated from indiciations among eosinophils, their cytotoxic granule proteins, viduals with very high eosinophil counts.Receptors for IgE and human diseases, the mechanism(s) responsible for eosinwere identified on both rat and human hypodense eosinoophil activation in vivo is largely unknown.phils, 17 and hypodense human eosinophils were shown to Helminth infections and allergic diseases are characteristikill schistosomula in the presence of IgE. 18Subsequently, cally associated not only with peripheral blood and tissue this human eosinophil IgE receptor was shown to be similar, eosinophilia, but also with high levels of both total and antigen-specific IgE antibodies.IgE antibodies may be involved in disease in three ways.First, the central feature in but not identical, to the low-affinity IgE receptor (Fc e RII) with the cestode, Mesocestoides corti.0][21][22][23] Eosinophils from patients with eosinophilia express another low-affinity IgE-binding to human eosinophils, expressed the type II Fc receptor for IgG (Fc g RII), but they were unable to detect any IgE binding molecule belonging to the S-type lectin family, called Mac-2/e-binding protein. 24The cytotoxic function of eosinophils to mouse eosinophils.More recently, Jones et al 33 thoroughly examined eosinophils obtained by bronchoalveolar lavage was abolished by the antibody against this molecule. 24More recently, in 1994, Gounni et al 25 described Fc e RI on eosino-(BAL) from the lungs of CBA/J mice infected with Toxocara cani by flow cytometry.They found that murine eosinophils phils from patients with marked eosinophilia.The evidence to support this claim was comprehensive and included the are negative for surface IgM (sIgM), sIgA, sIgE, and Fc e RII, but are positive for sIgG1 and Fc g RII.Furthermore, they following: inhibition of [ 125 I] IgE binding to eosinophils by anti-Fc e RI a-chain monoclonal antibody (clone 15.1); sur-showed that culturing eosinophils for 24 or 48 hours with exogenous IgE and/or IL-4 did not induce IgE binding capac-face expression of Fc e RI a-chain by flow cytometry; immunostaining of tissue eosinophils with 15.1; the demonstration ity or Fc e RII expression.In this issue of Blood, de Andres et al 34 expand these studies considerably by including Fc e RI of Fc e RI a-, b-, and g-chain transcripts; release of eosinophil granule proteins after stimulation of eosinophils with 15.1; and Mac-2 and by examining mRNA transcripts and receptor-mediated cellular functions.de Andres et al 34 examine and inhibition of IgE-dependent eosinophil ADCC against schistosome targets by 15.1.Altogether, these findings sug-murine eosinophils from two sources, namely eosinophils isolated from liver granuloma of CBA mice infected with S gest that human eosinophils express three receptors for IgE, namely Fc e RI, Fc e RII, and Mac-2, and that IgE induces eo-mansoni and bone marrow cells isolated from BALB/c mice and cultured with a combination of eosinophil growth fac-sinophil mediator release and ADCC through these receptors.Thus, on the basis of these reports, IgE-mediated acti-tors.The results from these two different cell sources are virtually identical.Murine eosinophils lack IgE receptor ex-vation of eosinophils was implicated as an important mechanism for host defense and in the pathophysiology of human pression; neither surface expression of Fc e RII or Mac-2 nor binding of murine IgE to the cells could be detected.Reverse disease.
However, the seemingly strong association between the transcription polymerase chain reaction (RT-PCR) analyses did not detect mRNA transcripts for the a-chain of Fc e RI or eosinophil and disease becomes confusing and controversial in mice.For example, in helminth-infected animals, antibod-Fc e RII, but did detect Mac-2 mRNA.In vitro culture of granuloma eosinophils did not induce IgE-binding or expres-ies to IL-5 suppressed blood eosinophilia and eosinophil infiltration into the tissues.However, ablation of eosinophilia sion of IgE receptors.In contrast to the lack of IgE receptors, functioning IgG receptors, including Fc g RIIb and Fc g RIII, by anti-IL-5 was not associated with a diminution of resistance in mice infected with S mansoni 26 or with Trichinella were detected on granuloma eosinophils, consistent with previous observations by others.spiralis. 27Similarly, anti-IL-4 depletion of IgE responses failed to interfere with protective immunity to S mansoni. 26tudies of receptor expression have a number of potential pitfalls.For example, transcription of mRNA or even the These findings suggest that neither eosinophils nor IgE are critical for immunity to these parasites in the mouse.In presence of synthesized receptor protein within the cell does not necessarily indicate the expression of the receptor on contrast, mice infected with Trichuris muris showed the exact opposite: their resistance to infection was associated with cell surface. 35Another question concerns the potential discrepancies between receptor expression and actual function-the production of Th2 cytokines, such as IL-4 and IL-5, tissue eosinophilia, and intestinal IgA production. 28In mu-ing of the receptor.An antibody raised against an IgE receptor expressed on one cell type may not recognize the IgE rine models of asthma using BALB/c mice sensitized and challenged with ovalbumin (OVA), one study showed that receptor on another cell type, the best example being lack of binding of the antibody against human B-cell Fc e RII neither IL-5 nor eosinophils are required for airway hyperresponsiveness. 29 In contrast, another study in which the (CD23) to human eosinophils. 23Finally, precautions are needed to minimize or eliminate contamination by other cell C57BL/6 mice rendered IL-5 deficient by homologous gene recombination were sensitized and exposed to OVA, the types, particularly in highly sensitive RT-PCR analyses.The careful and well-designed study by de Andres et al 34 pub-animals failed to develop eosinophil infiltration into the lungs, airway hyperresponsiveness, and lung damage; their lished in this issue of Blood examined transcription of mRNA, surface receptor expression, IgE-binding capacity, littermate controls showed all these responses. 30Reconstitution of IL-5 production with recombinant vaccinia viruses and receptor-mediated cellular function; thus, they seem to address all of these potential problems.Their observations, completely restored antigen-induced eosinophilia and airway dysfunction in these IL-5-deficient mice, suggesting a cen-together with previous reports by others, provide convincing evidence that murine eosinophils, seemingly unlike human tral role for IL-5 and eosinophilia in the pathogenesis of allergic lung disease.The inconsistencies among these mu-eosinophils, lack IgE receptors.Therefore, murine eosinophils may use the Fc g R, complement receptors, and/or possi-rine disease models, as well as the inconsistencies between the human and mice findings, allows recognition of potential bly Fc a R in their antigen-dependent cellular functions; these may explain the differences between mouse and human ob-difference(s) between human and mouse eosinophilic leukocytes.
servations and the discrepancies among findings made in mice.Previously, Lopez et al 31,32  and human eosinophils deserve some comments and cau-individuals. 35In addition, transcription of mRNA for Fc e RI a-chain was enhanced by IL-4 in these human eosinophils, 35 tions.First, expression of IgE receptors on human eosinophils and their IgE-dependent functions are not phenomena similar to results with human mast cells. 43The expression of Fc e RII is tightly regulated in a tissue-specific manner, and commonly seen in eosinophils from all sources.Almost all the studies that showed the presence of IgE receptors on IL-4 again is the most potent inducer of Fc e RII expression for various types of cells. 44Therefore, those blood or tissue human eosinophils have used eosinophils from patients with marked eosinophilia, including the hypereosinophilic syn-conditions with abundant IL-4 may favor the expression of IgE receptors on eosinophils.Because they used eosinophils drome and diseases associated with skin disorders and lymphomas. 18,19,24,25There are no data to support expression isolated from an almost ideal source, namely liver granuloma of mice infected with S mansoni, the findings by de Andres of IgE receptors on eosinophilia from healthy donors and/ or subjects with mild to moderate eosinophilia due to more et al 34 are particularly informative.In addition to tissue and disease specificity, mice may also display another level of common conditions, such as allergy and helminth infection.Three studies that sought IgE receptors in these conditions complexity.Because strains of mice differ greatly in their capacities to produce IL-4 and high levels of serum IgE, 45 did not detect Fc e RII on peripheral blood eosinophils 36,37 and detected only minimal levels of Fc e RI expression on expression of IgE receptors on eosinophils may differ, depending on the strain.By using mice selected for heightened eosinophils infiltrating into the bronchial tissues of patients with asthma 38 and on blood eosinophils from patients with production of IgE, Eum et al 46 concluded that the recruitment of eosinophils to the airways and high IgE titers are both allergic rhinitis. 37Furthermore, IgE-dependent functions of eosinophils were not observed in blood eosinophils from required for lung pathology of allergic BP2 mice, and they speculated that IgE activation of eosinophils is important in normal individuals, whereas these eosinophils did respond vigorously to IgG1 and IgG3 through Fc g RII. 16,39,40Another this mouse model.It would be interesting if eosinophils from these animals were subjected to the rigorous analyses used level of complexity is added by the discrepancy between receptor expression and IgE-mediated cellular functions of by de Andres et al. 34 Furthermore, van der Vorst et al 47 reported the in situ localization of IgE cytophilic antibodies eosinophils.Normal eosinophils, which fail to mediate IgEdependent ADCC, do mediate such killing after activation on murine eosinophils in the gut tissues of Swiss albino mice infected with Hymenlepis diminuta, although this study with platelet-activating factor (PAF) even in the absence of the expression of IgE receptors. 41eeds to be interpreted with caution due to the ubiquitous expression of IgE binding proteins in murine tissues. 48Thus, Second, in association with the activation status issue described above, expression of IgE receptors on eosinophils as summarized in Table 1, IgE receptor expression may be disease, tissue, species, and/or strain specific, and the obser-may be tightly regulated by various environmental factors.For example, in humans the expression of Fc e RII was limited vations obtained in a certain condition should not be generalized.9 or Mac-2. 24Hence, earlier studies showed evidence 4, -5, and -6 and tumor necrosis factor-a in normal and asthmatic for the expression of the low-affinity IgE receptor without airways: Evidence for the human mast cell as a source of these any evidence for the high-affinity IgE receptor, Fc e RI.Howcytokines.Am J Respir Cell Mol Biol 10:471, 1994 ever, later in 1994, the same investigators reported the pres- 10. Mudde GC, Bheekha R, Bruijnzeel-Koomen CAFM: IgEence of Fc e RI on human eosinophils, and all of the IgEmediated antigen presentation.Allergy 50:193, 1995 dependent functions of human eosinophils were explained 11.Maurer D, Fiebiger E, Ebner C, Reininger B, Fischer GF, Wichlas S, Jouvin MH, Schmittegenolf M, Kraft D, Kinet JP, Stingl by this receptor. 25Again, IgE-dependent ADCC for schisto-G: Peripheral blood dendritic cells express Fc e RI as a complex comsomula was essentially completely inhibited by antibody to posed of Fc e RIa-and Fc e RIg-chains and can use this receptor for Fc e RI, raising an obvious question as to how the IgE binding IgE-mediated allergen presentation.J Immunol 157:607, 1996 and ADCC can be totally inhibited by antibodies against 12. Capron A, Dessaint JP, Haque A, Capron M: Antibody-dethree distinct IgE receptors.Thus, more confirmatory work pendent cell-mediated cytotoxicity against parasites.Prog Allergy is needed to finally resolve the presence or absence of IgE 31:234, 1982 receptors on human eosinophils, as well as on mouse eosino-13.Hagan P, Blumenthal UJ, Dunn D, Simpson AJG, Wilkins phils.HA: Human IgE, IgG4 and resistance to reinfection with Schisto-In conclusion, with the recent development of techniques soma haematobium.Nature 349:243, 1991 to manipulate genes and with the increased availability of a 14.Sears MR, Burrows B, Flannery EM, Herbison GP, Hewitt CJ, Holdaway MD: Relation between airway responsiveness and wide variety of immunologic reagents for mice, the numbers serum IgE in children with asthma and in apparently normal children. of murine models of human immunity and disease have strik-N Engl J Med 325:1067, 1991 ingly increased.The studies by de Andres et al 34 in this issue 15.Butterworth AE, Sturrock RF, Houba V, Mahmoud AA, Sher of Blood warn us of potential cellular differences between A, Rees PH: Eosinophils as mediators of antibody-dependent dammouse and human immunologic responses and encourage us age to schistosomula.Nature 256:727, 1975 to reexamine the suitability of mouse models for human 16.Khalife J, Dunne DW, Richardson BA, Mazza G, Thorne diseases.At the same time, their report raises unanswered KJI, Capron A, Butterworth AE: Functional role of human IgG questions regarding the expression of IgE receptors on husubclasses in eosinophil-mediated killing of schistosomula of Schisman eosinophils.Further studies on IgE receptors on mouse tosoma mansoni.J Immunol 142:4422, 1989 and human eosinophils may solve the existing controversies 17.Capron M, Capron A, Dessaint JP, Torpier G, Johansson SGO, Prin L: Fc receptors for IgE on human and rat eosinophils.J and help to interpret and to understand the pathophysiologic Immunol 126:2087, 1981 mechanisms of human eosinophilic disorders, the roles of 18. Capron M, Spiegelberg HL, Prin L, Bennich H, Butterworth eosinophils in human immunity, and their murine models.AE, Pierce RJ, Ouaissi MA, Capron A: Role of IgE receptors in effector function of human eosinophils.J Immunol 132:462, 1984

Table 1 . Reports on the Expression of IgE Receptors by Eosinophils
have described the distribution of Fc receptors on eosinophils isolated from mice infected The issues regarding expression of IgE receptors on mouse AID Blood 0059 / 5H35$$1162 04-25-97 17:21:13 blda WBS: Blood * IgE-dependent function after stimulation with PAF.
Expression of the mRNA transcript for Fc e RI achain was detected in peripheral blood eosinophils from pa-ciled.In 1988, the eosinophil IgE receptor was originally found to have a low affinity (K D of 10 07 mol/L) and to tients with allergic rhinitis, but not in those from normal .Galli SJ, Gordon JR, Wershil BK: Cytokine production by possess a molecular weight corresponding to that of mast cells and basophils.Curr OpinImmunol 3:865, 1991Fc e RII.20,42IgE-binding and IgE-dependent ADCC to schis-9.Bradding P, Roberts JA, Britten KM, Montefort S, Djukanovic tosomula were completely inhibited by antibody to either R, Mueller R, Heusser CH, Howarth PH, Holgate ST: Interleukin-Fc e RII 8