Eosinophils express a functional receptor for interferon alpha: inhibitory role of interferon alpha on the release of mediators

Recent reports describe the beneficial use of alpha interferon (IFNalpha) for the treatment of idiopathic hypereosinophilic syndrome (HES) unresponsive to conventional therapy. A clinical improvement associated with a rapid decrease of peripheral blood eosinophilia suggested possible direct effects of IFNalpha on eosinophils through the presence of IFNalpha receptors (IFNalphaR). Reverse transcriptase-polymerase chain reaction (RT-PCR) and cytochemistry were used respectively to detect the presence and define the distribution of IFNalphaR on enriched eosinophil preparations purified from blood cells. IFNalphaR was found on eosinophils collected from patients with various eosinophilic disorders. In addition, IFNalpha inhibited the release of eosinophil granule proteins such as eosinophil cationic protein (ECP), neurotoxin (EDN, or interleukin-5 (IL-5). Moreover, antiparasite cytotoxicity was also strongly reduced in a dose-dependent manner by IFNalpha. These results provide the first evidence that human eosinophils express a functional receptor for IFNalpha and represent a potential basis for the beneficial effects of IFNalpha in patients with hypereosinophilic syndromes.

Interleukin 3, granulocyte-macrophage colony-stimulating factor, and interleukin 5 in eosinophilic gastroenteritis

BACKGROUND & AIMS

Eosinophilic gastroenteritis (EG) is characterized by an eosinophilic infiltration of the gastrointestinal tract. The mechanism for the intestinal recruitment of eosinophils in EG remains unknown. Eosinophil recruitment and activation is induced by three main cytokines: interleukin (IL) 3, granulocyte-macrophage colony-stimulating factor (GM-CSF), and IL-5. The aim of this study was to examine the immunoreactivity for IL-3, GM-CSF, and IL-5 within the duodenal and colonic mucosa of 10 patients with EG.

METHODS

Endoscopic biopsy specimens were obtained from 10 patients with EG and 10 controls. IL-3, GM-CSF, and IL-5 was evaluated by immunohistochemistry. Electron microscopy combined with immunogold staining was used to identify the labeled cells and to localize these growth factors ultrastructurally.

RESULTS

A significant increase in the number of eosinophils was found in both duodenal and colonic mucosa from all 10 patients with EG compared with controls. In the same tissue, immunohistochemistry detected IL-3, GM-CSF, and IL-5 in 9 of 10 patients with EG. The one exception had received treatment with steroids. These cytokines were not detected in the control group. Ultrastructurally, IL-3, GM-CSF, and IL-5 were localized in the granule matrix of eosinophils.

CONCLUSIONS

The release of these cytokines with autocrine and/or paracrine activities by eosinophils may be involved in the persistence of intestinal eosinophil infiltration.

Internalization of human lactoferrin by the Jurkat human lymphoblastic T-cell line

Binding of either iron-saturated or iron-free lactoferrin to the Jurkat human lymphoblastic T-cell line was saturable with a dissociation constant Kd of 40 nM. The total number of binding sites was estimated to be approximately 300,000. Non-specific binding did not exceed 30% of the total binding. Removal of the 4 clustered arginine residues of lactoferrin at position 2 to 5, which are involved in the interactions with heparan sulfate, did not modify the binding parameters. Therefore, the high number of low affinity binding sites previously described as responsible for the interaction of lactoferrin with either hepatocytes, enterocytes or the U937 monocytic cell line, is not involved in the binding of lactoferrin to Jurkat cells. After binding at 4 degrees C, a shift to 37 degrees C causes cell to internalize lactoferrin, with the maximum intracellular concentration found at 3 to 8 and 5 to 15 min for iron-saturated and iron-free forms, respectively. Addition of colchicine had no effect on binding or internalization. These results suggest that endocytosis of lactoferrin by Jurkat cells occurs through a receptor-mediated process. Jurkat cells internalize lactoferrin monophasically with a first-order endocytic constant K(in) of 0.060 min-1 at 37 degrees C. Confocal microscopic analysis, using fluorescein-carbohydrate-labeled lactoferrin showed that lactoferrin was mainly localized in intracellular vesicles. Following uptake, the endocytic path utilized by fluorescein-carbohydrate-labeled lactoferrin was shown to diverge from that of rhodamine-labeled serum transferrin; after internalization, lactoferrin and serum transferrin did not fully colocalize. Intracellular lactoferrin was found in endosome vesicles as assessed by electron microscopy. Raising the pH in endosomes using chloroquine led to the accumulation of lactoferrin into endosomes (acidic compartment). After internalization, Jurkat cells released both degraded and intact lactoferrin into the culture medium, suggesting that a fraction (30-40%) of the ligand is degraded at each round of endocytosis.

Activated eosinophils and interleukin 5 expression in early recurrence of Crohn's disease

Endoscopic recurrences after radical surgery for Crohn's disease are useful for studying the pathogenesis of initial lesions of Crohn's disease. Factors predisposing to recurrence are poorly understood, but it has been shown that eosinophilic infiltration of the neoileum may occur within a few weeks of resection. The aim of this study was to compare, in nine patients having an ileocolectomy, the infiltration of eosinophils and their activation state in normal and diseased areas of the neoileum, three months after surgery. Tissue eosinophils were studied by histochemical methods and electron microscopy. Mucosal expression of interleukin 5 (IL 5), an important eosinophil activating factor was studied using in situ hybridisation. Sixty per cent of patients had endoscopic recurrence at three months. Eosinophil infiltration was more pronounced in diseased than in endoscopically normal areas and was associated with a high expression of IL 5 mRNA. Ultrastructural analysis showed features of eosinophil activation, but no cytotoxic lesions of surrounding inflammatory or epithelial cells. This study suggests that local synthesis of IL 5 associated with eosinophil activation in the tissues could participate in early mucosal damage in Crohn's disease.

Parallel interleukin 5 synthesis by eosinophils in duodenal and skin lesions of a patient with dermatitis herpetiformis

A 59 year old man is presented with a diagnosis of dermatitis herpetiformis. Duodenal and skin biopsy specimens from blisters of both recent and late onset were collected before treatment. Electron microscopy, immunohistochemistry, and in situ hybridisation were performed to analyse the presence of activated eosinophils and the local synthesis of interleukin 5 (IL5). Parallel state of eosinophil activation and IL5 synthesis was found in the duodenal mucosa with total flat mucosa and in skin vesicles of recent onset. It is suggested that duodenal and cutaneous eosinophils can synthesise IL5 and then participate in small bowel and skin lesions.

Eosinophils: from low- to high-affinity immunoglobulin E receptors

Several experimental approaches have been used to identify immunoglobulin (IgE) binding molecules expressed by human eosinophils. After the description that Fc epsilon RII/CD23 identified on eosinophils could participate in IgE binding and IgE-mediated cytotoxicity, Mac2/epsilon binding proteins belonging to the S-type lectin family were also detected on human eosinophils. Anti-Mac2 monoclonal antibodies inhibited eosinophil-dependent cytotoxicity towards parasitic targets. More recently, Fc epsilon RI was demonstrated on human eosinophils from hypereosinophilic patients. The 3 components of Fc epsilon RI, alpha, beta and gamma chains, were detected in eosinophils. The alpha chain of Fc epsilon RI was shown to be involved in IgE binding to eosinophils and in the selective release of eosinophil peroxidase. The participation of Fc epsilon RI-bearing eosinophils in a protective immune response against a parasitic infection indicates a so far unsuspected function of Fc epsilon RI. The interactions between the different types of IgE binding molecules are discussed.

Synthesis of cytokines by eosinophils and their regulation

In addition to cytotoxic and proinflammatory mediators, eosinophils can produce a variety of cytokines and growth factors. Besides interleukin (IL)-5, we show in the present work that human eosinophils can synthesize interferon (IFN)-gamma and IL-10, by RT-PCR, in situ hybridization and immunostaining. Double-labelling procedures revealed the coexpression of IL-5 and IL-10 but not IL-5 and IFN-gamma, indicating the existence of subpopulations of eosinophils expressing type 1 or type 2 cytokines. IFN-alpha efficiently used for the treatment of hypereosinophilic syndromes can significantly decrease eosinophil degranulation and IL-5 release by eosinophils, through binding to a receptor for IFN-alpha. Thus, eosinophils can represent major sources of cytokines with regulatory functions, especially in allergic diseases and parasitic infections.

Development of a vaccine strategy against human and bovine schistosomiasis. Background and update

Schistosomiasis is a chronic and debilitating parasitic disease that affects over 200 million people throughout the world and causes about 500,000 deaths annually. Two specific characteristics of schistosome infection are of primordial importance to the development of a vaccine: schistosomes do not multiply within the tissues of their definitive hosts (unlike protozoan parasites) and a partial non-sterilizing immunity can have a marked effect on the incidence of pathology and on disease transmission. Since viable eggs are the cause of disease pathology, a reduction in worm fecundity whether or not accompanied by a reduction in parasite burden is a sufficient goal for vaccine induced immunity. We originally showed that IgE antibodies played in experimental models a pivotal role for the development of protective immunity. These laboratory findings have been now confirmed in human populations. Following the molecular cloning and expression of a protein 28 kDa protein of Schistosoma mansoni and its identification as a glutathion S-transferase, immunization experiments have been undertaken in several animal species (rats, mice, baboons). Together with a significant reduction in parasite burden, vaccination with Sm28 GST was recently shown to reduce significantly parasite fecundity and egg viability leading to a decrease in liver pathology. Whereas IgE antibodies were shown to be correlated with protection against infection, IgA antibodies have been identified as one of the factors affecting egg laying and viability. In human populations, a close association was found between IgA antibody production to Sm28 GST and the decrease of egg output.(ABSTRACT TRUNCATED AT 250 WORDS)

Human eosinophils express a receptor for secretory component. Role in secretory IgA-dependent activation

The existence of a functional receptor for secretory component (SC) on the eosinophil membrane might explain the preferential degranulation induced by secretory IgA (sIgA) when compared to serum IgA. Indeed, flow cytometry analysis revealed that purified human SC could bind to a subpopulation (4-59%) of blood eosinophils purified from 19 patients with eosinophilia. Binding of radiolabeled human SC could be competitively inhibited using unlabeled SC or secretory IgA but not with serum IgA or IgG. Immunoprecipitation and immunosorbent chromatography using human SC revealed the presence of a major component at 15 kDa in eosinophil extracts as well as in culture supernatants but not in neutrophils. The 15-kDa protein eluted from the human SC immunosorbent was able to bind to SC or to sIgA but not to serum IgA. Eosinophils preincubated with human SC or sIgA released eosinophil cationic protein (ECP) and eosinophil peroxidase (EPO) after addition of anti-SC or anti-IgA monoclonal antibody as respective cross-linking reagents. These results indicated that binding of free or complexed SC to human eosinophils could induce eosinophil degranulation. Furthermore, the dose-dependent inhibition by SC of mediator release induced by sIgA but not by serum IgA, suggested that the receptor for SC could be involved in the preferential degranulation mediated by sIgA. These results indicate a novel pathway of eosinophil activation and its potential involvement in mucosal immunity, particularly in inflammatory diseases associated with infiltration of eosinophils and the enhanced production of sIgA.

Immunolocalization of the lactotransferrin receptor on the human T lymphoblastic cell line Jurkat

Monoclonal antibodies have been raised against the soluble lactotransferrin binding protein purified from the cell culture supernatant of Jurkat cell line, a human T-lymphoblastic cell. All monoclonal antibodies were able to specifically bind to the membrane of Jurkat cells. One of the monoclonal antibodies, DP5B3G10, recognized both the soluble lactotransferrin-binding protein and the membrane lymphocyte lactotransferrin receptor after SDS-PAGE in presence of 2-mercaptoethanol and electrotransfer on nitrocellulose. The monoclonal antibody DP5B3G10 inhibited the binding of lactotransferrin to Jurkat cells and human peripheral activated lymphocytes. In addition, lactotransferrin inhibited the binding of the monoclonal antibody to the cell surface. These results suggest that the 95 kDa lactotransferrin-binding protein isolated from the cell culture medium corresponds to the soluble form of the 105 kDa lymphocyte lactotransferrin receptor. Corresponding proteins of 105 kDa molecular mass were identified in Jurkat and CEM T-cells and Raji B-cells. Finally, the monoclonal antibody DP5B3G10 was used to immunolocalize the lactotransferrin receptor on the Jurkat cells. Using fluorescence and electron microscopy, the receptor was localized both inside and at the cell surface. The cell membrane receptor was associated into clusters. After permeabilization of the plasma membrane, the staining was positive in the peri-membrane area. The region near the nucleus was devoid of receptor.

Interleukin-5 messenger RNA and immunoreactive protein expression by activated eosinophils in lesional atopic dermatitis skin

The main cellular sources of interleukin-5 (IL-5) are T lymphocytes and mast cells. Recently, IL-5 mRNA has been identified in eosinophils from patients with celiac disease, eosinophilic heart diseases, and asthma. In an attempt to determine whether IL-5 is generated by eosinophils in atopic dermatitis we have used i) in situ hybridization with 35S-labeled IL-5 RNA probe combined with immunohistochemistry using a monoclonal antibody (MoAb) (EG2) directed against the activated form of Eosinophil Cationic Protein (ECP) and ii) double-immunostaining with anti-IL-5 MoAb and polyclonal anti-ECP antibody. We found that dermal eosinophils from lesional atopic dermatitis skin express IL-5 mRNA and protein. Moreover, highly purified blood eosinophils were also labeled with anti-IL-5 antibodies. The expression of IL-5 by eosinophils in atopic dermatitis might suggest an autocrine pathway of eosinophil differentiation and activation.

Interleukin 5 synthesis by eosinophils: association with granules and immunoglobulin-dependent secretion

Interleukin 5 (IL-5) is the main factor that promotes the terminal differentiation of eosinophil progenitors (as indicated by colony formation assays), and enhances the effector capacity of mature eosinophils. IL-5 is produced by T lymphocytes, CD4-/CD8- and mast cells and recently, messenger (m)RNA of this cytokine has been identified in eosinophils from patients with coeliac disease, asthma, or eosinophilic heart diseases. In this study, IL-5 mRNA and immunoreactive IL-5 protein were detected in tissue and blood eosinophils from patients with eosinophilic cystitis or hypereosinophilic syndromes but not in Crohn's disease. By electron microscopy associated to immunogold staining, immunoreactive IL-5 was identified in eosinophilic granules. After stimulation with IgA-, IgE-, or IgG-immune complexes, blood eosinophils were shown, by immunocytochemistry and by enzyme-linked immunosorbent assay, to secrete IL-5. These observations demonstrate that eosinophils, under physiological stimulation, can release significant amounts of IL-5, which may contribute to local eosinophil recruitment and activation.

High-affinity IgE receptor on eosinophils is involved in defence against parasites

Parasitic infections are often associated with eosinophilia and high levels of immunoglobulin E (IgE). This observation has led to speculation that eosinophils and IgE may act together in the immune response against parasites. In support of this hypothesis, IgE and eosinophils participate in cytotoxic reactions directed against Schistosoma mansoni larvae in vitro. Furthermore, epidemiological studies have shown an inverse correlation between levels of specific IgE and rates of infection with Schistosoma. The low-affinity IgE receptor (Fc epsilon RII/CD23) was first incriminated in eosinophil activation. The fact that the high-affinity IgE receptor (Fc epsilon RI) is not only expressed on mast cells and basophils but also on Langerhans cells led us to investigate the presence of Fc epsilon RI on eosinophils. Here we show that Fc epsilon RI is expressed on eosinophils from hypereosinophilic patients, is involved in eosinophil degranulation, and participates in eosinophil-mediated cytotoxicity against S. mansoni. Our results indicate that Fc epsilon RI may play a major part in immune defence against parasites.

Elevation of soluble CD23 in serum from patients with blood eosinophilia

The levels of soluble CD23 (sCD23) were evaluated by a two-site immunoradiometric assay in the sera of 41 patients with eosinophilia-associated disorders and 20 normal subjects. We observed that, in the absence of treatment, sCD23 levels were elevated in patients with eosinophilia-associated Gleich's syndrome, IgA deficiency, T lymphoma or hypereosinophilic syndrome (HES), but not in patients with a parasitic infection. A significant reduction in the sCD23 levels was found after treatment, with a parallel decrease in eosinophil counts and in sCD25 levels, a marker of disease activity in HES. The lack of increase in membrane CD23+ B cells in eosinophilic patients together with the detection of sCD23 in eosinophil supernatants suggest that activated eosinophils present in eosinophilia-associated disorders can release soluble molecules cross-reacting with CD23. In conclusion, our results suggest that eosinophils themselves can represent one cellular source of sCD23. These findings are not only basic but also of clinical interest.

Evidence for eosinophil activation in eosinophilic cystitis

Eosinophilic cystitis (EC) is a rare condition. Recent studies have shown that activated eosinophils release cytotoxic cationic proteins which can induce tissue damage. Moreover, in vitro studies have shown that interleukin-5 (IL-5) is a cytokine able to attract and activate eosinophils. The goal of this study was to detect a possible activation of eosinophils in EC using electron microscopy, in situ hybridization with an IL-5 RNA probe and immunochemistry with a specific anti-IL-5 antibody. Using these combined methods in a typical case of EC, we found numerous activated eosinophils synthesizing and secreting IL-5 protein. IL-5 could enhance the activation of eosinophils and their cytotoxic potential in bladder tissues. This mechanism might explain the chronicity of the lesions in EC.

IgE-binding molecules (Mac-2/epsilon BP) expressed by human eosinophils. Implication in IgE-dependent eosinophil cytotoxicity

Macrophage cell-surface protein 2 (Mac-2), a galactose specific S-type lectin identified in inflammatory macrophages, presents a high degree of homology with the rat IgE-binding protein (epsilon BP). In the present study, we show by different experimental approaches that human eosinophils can express Mac-2/epsilon BP. Flow cytometry analysis revealed that a large proportion of eosinophilic patients expressing binding sites for IgE on their eosinophil membrane, were able to bind anti-Mac-2 monoclonal antibody (mAb). Northern blot performed with eosinophil RNA hybridized with the human Mac-2 or epsilon BP cDNA probes revealed that eosinophils presented a unique transcript at 1.2 kb. Immunoprecipitation of eosinophil extracts with anti-Mac-2 mAb revealed the presence of a molecule of 29 kDa corresponding to Mac-2 protein, as well as one additional molecule of 15 kDa, absent from control alveolar macrophages. The function of these molecules was investigated in a radiolabeled IgE binding assay. Anti-Mac-2 mAb as well as galactose and lactose saccharides significantly inhibited the binding of radiolabeled human myeloma IgE protein to eosinophils. Moreover, the dose-dependent inhibition by anti-Mac-2 mAb of IgE-dependent eosinophil-mediated cytotoxicity towards parasite targets indicated the role of these IgE-binding molecules in the function of human eosinophils. These results suggest that in addition to transmembrane receptors, lectin-type molecules can participate in the IgE-dependent effector function of eosinophils.

Synthesis of interleukin-5 by activated eosinophils in patients with eosinophilic heart diseases

Eosinophilic endomyocardial disease represents a major evolutive risk in chronic eosinophilia-associated disorders. Eosinophil granule proteins appear to be involved in cardiac injury, but the mechanisms leading to eosinophil infiltration and degranulation are not clear. Interleukin-5 (IL-5) has been recently shown to be produced by eosinophils and might play a role in both chemoattraction and degranulation of eosinophils. In four cases of eosinophilic diseases with severe cardiac failure, we evaluated the proportion of eosinophil phenotypes and the serum levels of eosinophil cationic protein (ECP) and soluble IL-2 receptor (sIL-2R), markers of disease activity in the hypereosinophilic syndromes. All four patients showed a markedly increased proportion of hypodense eosinophils with elevated serum ECP and sIL-2R levels. In all four patients, extracellular deposition of eosinophil granule proteins and features of eosinophil activation were observed in cardiac tissues. The synthesis of IL-5 by eosinophils was detected in myocardial sections and blood cells by in situ hybridization and by immunostaining with a monoclonal antibody against human IL-5. Sixty percent to 90% of tissue eosinophils expressed IL-5 mRNA and IL-5 protein. These data suggest that IL-5 can be produced by eosinophils at the sites of myocardial tissue damage and might participate in local eosinophil activation.

Ligation of CD23 triggers cyclic AMP generation in human B lymphocytes

The low affinity IgE receptor CD23 may play a role in several B lymphocyte functions, such as cell activation and multiplication, Ag presentation, and IgE production. We have previously reported that ligation of the CD23 molecule with anti-CD23 mAb, or IgE-anti-IgE complexes, leads to phosphoinositide hydrolysis and calcium mobilization through the generation of Inositol (1,4,5) trisphosphate via a process involving a Pertussis toxin insensitive GTP-binding protein. In our work, we show that anti-CD23 mAb elicit an increase in cAMP concentration in human peripheral blood-derived B lymphocytes. This effect was detected both in resting and in IL-4-stimulated B cells displaying, respectively, low and high levels of CD23. Maximum cAMP accumulation was reached about 20 min after addition of the mAb. Involvement of Fc gamma RII in this process could be excluded because cAMP increase was also triggered by mAb anti-CD23 F(ab')2 fragments. Accumulation of cAMP was also observed when IgE-sensitized activated B lymphocytes were challenged with the specific hapten. Several lines of evidence indicate that the cAMP increase after CD23 ligation may result, in part, from the stimulation of phosphoinositidase C, inasmuch as it was markedly impaired by treatment with TMB-8, an inhibitor of InsP3-induced calcium release from intracytoplasmic stores and with BAPTA, an intracellular calcium chelator. Addition of GTP-gamma S to permeabilized B cells or to membrane preparations did not potentiate the effect of the mAb, suggesting that a Gs protein is not directly implicated in the generation of cAMP. Besides, cAMP accumulation is not due to the production of PG because it is not modified by indomethacin, an inhibitor of the cyclooxygenase pathway. Pretreatment of B lymphocytes with either anti-CD23 mAb or IL-4 led to autologous as well as heterologous desensitization. This negative cross-talk, at the level of cAMP, between the signaling pathways triggered by ligation of CD23 and of the IL-4 receptor, could contribute to the inhibitory effect of anti-CD23 mAb on IL-4-dependent B cell activation and differentiation.

Ligation of CD23 triggers cyclic AMP generation in human B lymphocytes

The low affinity IgE receptor CD23 may play a role in several B lymphocyte functions, such as cell activation and multiplication, Ag presentation, and IgE production. We have previously reported that ligation of the CD23 molecule with anti-CD23 mAb, or IgE-anti-IgE complexes, leads to phosphoinositide hydrolysis and calcium mobilization through the generation of Inositol (1,4,5) trisphosphate via a process involving a Pertussis toxin insensitive GTP-binding protein. In our work, we show that anti-CD23 mAb elicit an increase in cAMP concentration in human peripheral blood-derived B lymphocytes. This effect was detected both in resting and in IL-4-stimulated B cells displaying, respectively, low and high levels of CD23. Maximum cAMP accumulation was reached about 20 min after addition of the mAb. Involvement of Fc gamma RII in this process could be excluded because cAMP increase was also triggered by mAb anti-CD23 F(ab')2 fragments. Accumulation of cAMP was also observed when IgE-sensitized activated B lymphocytes were challenged with the specific hapten. Several lines of evidence indicate that the cAMP increase after CD23 ligation may result, in part, from the stimulation of phosphoinositidase C, inasmuch as it was markedly impaired by treatment with TMB-8, an inhibitor of InsP3-induced calcium release from intracytoplasmic stores and with BAPTA, an intracellular calcium chelator. Addition of GTP-gamma S to permeabilized B cells or to membrane preparations did not potentiate the effect of the mAb, suggesting that a Gs protein is not directly implicated in the generation of cAMP. Besides, cAMP accumulation is not due to the production of PG because it is not modified by indomethacin, an inhibitor of the cyclooxygenase pathway. Pretreatment of B lymphocytes with either anti-CD23 mAb or IL-4 led to autologous as well as heterologous desensitization. This negative cross-talk, at the level of cAMP, between the signaling pathways triggered by ligation of CD23 and of the IL-4 receptor, could contribute to the inhibitory effect of anti-CD23 mAb on IL-4-dependent B cell activation and differentiation.

Protective immunity induced in rat schistosomiasis by a single dose of the Sm28GST recombinant antigen: effector mechanisms involving IgE and IgA antibodies

Rats immunized by a single dose of the recombinant Sm28GST antigen, using either aluminium hydroxide or Bacillus Calmette-Guérin adjuvant, were significantly protected (up to 59% reduction in worm burden) against a challenge infection with Schistosoma mansoni cercariae. A follow-up study of the humoral response revealed the presence of high levels of IgE and IgA antibodies together with specific IgG. Sera from once Sm28GST-immunized rats induced a cytotoxic response for schistosomula targets in the presence of normal rat eosinophils, similar to the one induced by sera from twice immunized rats. Depletion or competition studies indicated the participation of both IgE and IgA antibodies in eosinophil-dependent cytotoxicity mechanisms. These results suggest the existence, in immunized rats exhibiting protection against schistosomiasis, of an original effector mechanism implying eosinophils and IgA antibodies, together with documented effector mechanisms involving IgE and eosinophils. In addition, they raise questions concerning the role of IgA antibodies in schistosomiasis.