Characterization of IgE-reactive autoantigens in atopic dermatitis. 1. Subcellular distribution and tissue-specific expression

Autoreactive IgE: Pathogenic role and therapeutic target in autoimmune diseases

Autoimmunity is the break of tolerance to self-antigens that leads to organ-specific or systemic diseases often characterized by the presence of pathogenic autoreactive antibodies (AAb) produced by plasmablast and/or plasma cells. AAb are prevalent in the general population and not systematically associated with clinical symptoms. In contrast, in some individuals, these AAb are pathogenic and drive the development of signs and symptoms of antibody-mediated autoimmune diseases (AbAID). AAb production, isotype profiles, and glycosylations are promoted by pro-inflammatory triggers linked to genetic, environmental, and hormonal parameters. Recent evidence supports a role for pathogenic AAb of the IgE isotype in a number of AbAID. Autoreactive IgE can drive the activation of mast cells, basophils, and other types of FcεRI-bearing cells and may play a role in promoting autoantibody production and other pro-inflammatory pathways. In this review, we discuss the current knowledge on the pathogenicity of autoreactive IgE in AbAID and their status as therapeutic targets. We also highlight unresolved issues including the need for assays that reproducibly quantify IgE AAbs, to validate their diagnostic and prognostic value, and to further study their pathophysiological contributions to AbAID.

IgE Autoreactivity in Atopic Dermatitis: Paving the Road for Autoimmune Diseases?

Atopic dermatitis (AD) is a common skin disease affecting 20% of the population beginning usually before one year of age. It is associated with the emergence of allergen-specific IgE, but also with autoreactive IgE, whose function remain elusive. This review discusses current knowledge relevant to the mechanisms, which leads to the secretion of autoreactive IgE and to the potential function of these antibodies in AD. Multiple autoantigens have been described to elicit an IgE-dependent response in this context. This IgE autoimmunity starts in infancy and is associated with disease severity. Furthermore, the overall prevalence of autoreactive IgE to multiple auto-antigens is high in AD patients. IgE-antigen complexes can promote a facilitated antigen presentation, a skewing of the adaptive response toward type 2 immunity, and a chronic skin barrier dysfunction and inflammation in patients or AD models. In AD, skin barrier defects and the atopic immune environment facilitate allergen sensitization and the development of other IgE-mediated allergic diseases in a process called the atopic march. AD is also associated epidemiologically with several autoimmune diseases showing autoreactive IgE secretion. Thus, a potential outcome of IgE autoreactivity in AD could be the development of further autoimmune diseases.

IgE autoantibodies and autoreactive T cells and their role in children and adults with atopic dermatitis

The pathophysiology of atopic dermatitis (AD) is highly complex and understanding of disease endotypes may improve disease management. Immunoglobulins E (IgE) against human skin epitopes (IgE autoantibodies) are thought to play a role in disease progression and prolongation. These antibodies have been described in patients with severe and chronic AD, suggesting a progression from allergic inflammation to severe autoimmune processes against the skin. This review provides a summary of the current knowledge and gaps on IgE autoreactivity and self-reactive T cells in children and adults with AD based on a systematic search. Currently, the clinical relevance and the pathomechanism of IgE autoantibodies in AD needs to be further investigated. Additionally, it is unknown whether the presence of IgE autoantibodies in patients with AD is an epiphenomenon or a disease endotype. However, increased knowledge on the clinical relevance and the pathophysiologic role of IgE autoantibodies and self-reactive T cells in AD can have consequences for diagnosis and treatment. Responses to the current available treatments can be used for better understanding of the pathways and may shed new lights on the treatment options for patients with AD and autoreactivity against skin epitopes. To conclude, IgE autoantibodies and self-reactive T cells can contribute to the pathophysiology of AD based on the body of evidence in literature. However, many questions remain open. Future studies on autoreactivity in AD should especially focus on the clinical relevance, the contribution to the disease progression and chronicity on cellular level, the onset and therapeutic strategies.

IgE, mast cells, and eosinophils in atopic dermatitis

Atopic dermatitis (AD) is a chronic inflammatory skin disease with specific immune and inflammatory mechanisms. Atopy is among the major features of the diagnosis criteria for AD but is not an essential feature. Thus, patients diagnosed with AD can be atopic or non-atopic. This review focuses on the role of IgE, mast cells, and eosinophils in the pathogenesis of AD. The known functions of IgE in allergic inflammation suggest that IgE and IgE-mediated mast cell and eosinophil activation contribute to AD, but direct evidence supporting this is scarce. The level of IgE (thus the degree of allergic sensitization) is associated with severity of AD and contributed by abnormality of skin barrier, a key feature of AD. The function of IgE in development of AD is supported by the beneficial effect of anti-IgE therapy in a number of clinical studies. The role of mast cells in AD is suggested by the increase in the mast cell number and mast cell activation in AD lesions and the association between mast cell activation and AD. It is further suggested by their role in mouse models of AD as well as by the effect of therapeutic agents for AD that can affect mast cells. The role of eosinophils in AD is suggested by the presence of eosinophilia in AD patients and eosinophil infiltrates in AD lesions. It is further supported by information that links AD to cytokines and chemokines associated with production, recruitment, and activation of eosinophils.

Does "autoreactivity" play a role in atopic dermatitis?

The role of autoimmunity in atopic dermatitis (AD) is unclear. We sought to critically examine the occurrence, correlation with severity, and possible causative role of autoreactivity in patients with AD. Our systematic review of studies identified from MEDLINE included 31 experiments that described autoreactivity in patients with AD. We defined autoreactivity as in vitro or in vivo evidence of immune response to autologous human, generic human, or recombinant human proteins or other tissue/cellular components. Autoreactivity prevalence in patients with AD ranged from 23% to 91% in 14 studies involving 2644 participants, although it did not appear to vary with age, sex, or disease duration. In contrast to studies of AD, IgE autoreactivity was not found in healthy subjects or in those with allergic rhinoconjunctivitis, psoriasis, systemic lupus erythematosus, or other inflammatory diseases (8 studies of 816 participants). Two reports found a positive correlation between autoreactivity and AD severity. We suggest that autoreactivity might be playing a causative role in AD based on the magnitude and specificity of the associations found; plausible mechanisms through IgE autoantibodies, IgG autoantibodies, and T(H)1 autoreactivity; and experimental elicitation of eczematous lesions after provocation. Whether autoantibodies contribute to AD chronicity now needs to be examined in longitudinal studies.

Serum and monoclonal immunoglobulin E antibodies from NC/Nga mice with severe atopic-like dermatitis recognize an auto-antigen, histone H3

BACKGROUND

NC/Nga mice are known to show a spontaneous outbreak of atopic-like dermatitis accompanied by a marked elevation in serum IgE levels when reared in a conventional environment. The specific effects of such a strong serum IgE response on the development of the dermatitis and specific antigens recognized by the IgE antibodies are still uncertain.

OBJECTIVE AND METHODS

To characterize the IgE of NC/Nga mice, we established IgE-secreting hybridoma clones from spleen cells of NC/Nga mice spontaneously developing dermatitis and identified variable-region genes and specific antigens of the IgE monoclonal antibodies (mAbs). Serum polyclonal IgE, as well as IgG1 and IgG2a, specific for the identified antigen were also analysed.

RESULTS

Four IgE-producing hybridoma clones were established. Variable-region nucleotide sequences of the IgE mAbs showed that these clones did not necessarily share common germline gene segments (V, D or J) for each variable region, and several somatic mutations had occurred in the V gene segments. Through antigen screening, histone H3 was identified to be an auto-antigen recognized by three of the four IgE mAbs. Serum IgE as well as IgG1 specific for histone H3 were almost undetectable in 6-week-old mice, but rapidly increased by 10-12 weeks of age. This age-dependent increase in the serum anti-histone H3 IgE was roughly in parallel with the onset of dermatitis, and slightly preceding total IgE elevation. The serum-specific IgE level correlated well with a dermatitis-severity score of each mouse at 12-16 weeks of age, and weakly with the severity of ear erosion of each mouse over 28 weeks of age. Furthermore, immunologically detectable histone-H3 antigens were observed in skin tissue sections from the dermatitis sites.

CONCLUSION

In NC/Nga mice, anti-histone H3 auto-antibodies may contribute, at least in part, to the considerably elevated serum IgE and might play some roles in the development and exacerbation of dermatitis.

Serum IgE autoantibodies target keratinocytes in patients with atopic dermatitis

Previous studies have shown that sera of patients with severe atopic dermatitis (AD) contain IgE specific for self-proteins, supporting the hypothesis of autoreactivity as a pathogenic factor in AD. In this study, we screened a large panel of AD patients (n=192) by western blotting (WB) for IgE reactivity not only against the human epithelial cell line A431 but also against primary keratinocytes (KCs). To investigate autoantigenic cell structures in detail, normal human skin and primary KCs were incubated with sera from both WB-reactive patients and, for control purposes, healthy individuals, and analyzed by immunohistology, confocal laser microscopy, and flow cytometry. Our analysis revealed that 28% of AD patients, but not healthy individuals, display serum IgE autoreactivity by WB analysis. The individual IgE reaction patterns of the sera pointed to the existence of unique as well as common specificities against epidermal or A431-derived proteins. Immunostainings identified cytoplasmic and, occasionally, also cell membrane-associated moieties as targets for autoreactive IgE antibodies. Interestingly, in certain autoreactive patients, the surface-staining pattern was accentuated at cellular contact sites. We conclude that IgE autoreactivity is common, particularly among severe AD patients, and that non-transformed primary cells are needed for characterization of the entire spectrum of IgE-defined autoantigens.

The IgE-reactive autoantigen Hom s 2 induces damage of respiratory epithelial cells and keratinocytes via induction of IFN-gamma

Hom s 2, the alpha-chain of the nascent polypeptide-associated complex, is an intracellular autoantigen that has been identified with IgE autoantibodies from atopic dermatitis patients. We investigated the humoral and cellular immune response to purified recombinant Hom s 2 (rHom s 2). rHom s 2 exhibited IgE reactivity comparable to exogenous allergens, but did not induce relevant basophil cell degranulation. The latter may be attributed to the fact that patients recognized single epitopes on Hom s 2 as revealed by IgE epitope mapping with rHom s 2 fragments. In contrast to exogenous allergens, rHom s 2 had the intrinsic ability to induce the release of IFN-gamma in cultured peripheral blood mononuclear cells from atopic as well as non-atopic individuals. IFN-gamma-containing culture supernatants from Hom s 2-stimulated peripheral blood mononuclear cells caused disintegration of respiratory epithelial cell layers and apoptosis of skin keratinocytes, which could be inhibited with a neutralizing anti-IFN-gamma antibody. Our data demonstrate that the Hom s 2 autoantigen can cause IFN-gamma-mediated cell damage.

Hom s 4, an IgE-Reactive Autoantigen Belonging to a New Subfamily of Calcium-Binding Proteins, Can Induce Th Cell Type 1-Mediated Autoreactivity

Skin inflammation in atopic dermatitis starts with Th2 and IgE-mediated responses against exogenous allergens and, for unknown reasons, resembles features of a Th1-driven reaction in the chronic stages. We report the characterization of a human protein, Hom s 4, recognized by IgE autoantibodies from atopic dermatitis patients. The complete Hom s 4 cDNA codes for a 54-kDa basic protein containing two typical calcium-binding domains separated by an unusually long alpha-helical domain. Therefore, Hom s 4 and homologous proteins found by sequence comparison in mice, fruit flies, and nematodes constitute a novel subfamily of calcium-binding proteins. Using Hom s 4-specific Abs, it is demonstrated that the protein is strongly expressed within epidermal keratinocytes and dermal endothelial cells. Purified Hom s 4 showed IgE cross-reactivity with exogenous calcium-binding allergens from plants and fish but, in contrast to the exogenous allergens, induced only weak histamine release from patient basophils. However, the analysis of Hom s 4-specific cytokine and humoral immune responses indicated that Hom s 4 strongly induces Th1 responses which are accompanied by the release of IFN-gamma, a cytokine implicated in epithelial cell damage. Hom s 4-induced IFN-gamma production was found in normal individuals, in patients with chronic inflammatory skin diseases and in Th2-prone atopic persons, suggesting that Hom s 4 represents a protein with an intrinsic property to induce Th1-mediated autoreactivity. It may thus contribute to chronic skin inflammation in atopic as well as in nonatopic persons.

Recent Highlights in the Pathophysiology of Atopic Eczema

Atopic eczema (AE) belongs to the group of atopic diseases that have become increasingly prevalent over the last decades. The pathophysiology of AE has long remained enigmatic, but much scientific effort has been invested in elucidating the genetic background and the immunological mechanisms underlying AE, leading to a better understanding of this complex disease and to new therapeutic strategies. This review provides an update on the scientific progress that has been achieved in clarifying the pathophysiological mechanisms of AE.

Autoimmunity and atopic dermatitis

PURPOSE OF REVIEW

It has been demonstrated that a considerable percentage of patients suffering from atopic dermatitis mount IgE autoantibodies against a broad variety of human proteins. This review summarizes evidence for autoimmune mechanisms in atopic dermatitis and suggests novel pathomechanisms that may be involved in this disease.

RECENT FINDINGS

It has been shown that patients suffering from atopic dermatitis exhibit IgE autoreactivity to human proteins. These autoantigens are expressed in a variety of cell and tissue types. Complementary DNAs coding for IgE autoantigens have been identified, cloned and characterized at the molecular level. Using purified recombinant IgE autoantigens, it has been shown in paradigmatic models that IgE autoimmunity may be a pathogenetic mechanism in atopic dermatitis. Moreover, it has been shown that the levels of IgE autoantibodies are associated with severity of disease.

SUMMARY

Patients suffering from severe manifestations of atopy mount IgE autoantibodies against a variety of human proteins. The levels of IgE autoantibodies correspond with disease severity. Several mechanisms of IgE autoimmunity may contribute to the pathogenesis of atopic dermatitis.

IgG reactive to CTL-directed epitopes of self-antigens is either lacking or unbalanced in atopic dermatitis patients

We previously demonstrated that CTL-directed epitopes derived from non-mutated self-antigens elicit a type-I allergy in the majority of healthy donors (HD) as did the presence of IgE and IgG reactive to these peptides in the sera of the donors. We investigated in this study whether Igs reactive to eight types of CTL-directed peptides were elevated in the sera of 40 patients with atopic dermatitis (AD). Total IgE levels in the sera of AD patients were significantly higher than those of HD, however, no significant differences between the AD patients and the HD were observed in either the serum levels or the positive rates of IgE reactive to seven of the eight peptides. Total IgG levels were not different from each other, however, IgG reactive to the two peptides with no sequence similarity to other species and one peptide that had similarity to DNA helicase II of enterobacteria were not detectable in the sera of the AD patients. Although IgG reactive to the remaining five peptides, which had sequence similarity to other species, were detectable in both the AD patients and the HD, ratios of peptide-specific IgG1/IgG2 were mostly lower in the AD patients than in the HD. These results indicate that IgG reactive to CTL-directed epitopes of self-antigens is either lacking or unbalanced in AD patients. This information may provide new insight into the immune-mechanisms of elevated auto-reactivity of AD patients.

Caspase cleavage of the nuclear autoantigen LEDGF/p75 abrogates its pro-survival function: implications for autoimmunity in atopic disorders

Lens epithelium-derived growth factor p75 (LEDGF/p75) is a nuclear autoantigen in atopic disorders implicated in cellular protection against stress-induced apoptosis. We observed that LEDGF/p75 was cleaved during apoptosis into fragments of 65 and 58 kD generated by caspases-3 and -7 cleaving at three sites: DEVPD30/G, DAQD486/G and WEID85/N. Sequence analysis revealed that the DEVPD30/G and WEID85/N sites lie within the highly conserved HATH (homologous to amino terminus of hepatoma-derived growth factor) region, also known as PWWP domain. Alignment of proteins containing this domain failed to reveal conservation of the DEVPD30/G and WEID85/N sites, suggesting that the HATH/PWWP domain of LEDGF/p75 may be specifically targeted by caspases. Overexpression of LEDGF/p75 protected HepG2 cells from serum starvation-induced cell death, whereas expression of the 65 kD fragment failed to protect. The apoptotic cleavage of LEDGF/p75 may contribute to the pathogenesis of atopic disorders by abrogating its pro-survival function and enhancing its immunogenicity.

Non-mutated tumor-rejection antigen peptides elicit type-I allergy in the majority of healthy individuals

IgE-mediated type-I allergy is generally considered to be a hypersensitivity reaction to foreign antigens, and it is believed that self-antigens do not evoke this type of allergy. We report here, for the first time, that non-mutated self-antigen peptides identified as tumor-rejection antigen peptides recognized by HLA class I-restricted and tumor-specific cytotoxic T lymphocytes (CTLs) elicited a type-I allergy in the majority of healthy individuals. Peptide-specific IgE was detectable in sera from certain cases, although the levels did not always correlate with those of type-I allergy. Repeated vaccinations of nonallergic peptides derived from the same antigens possessing allergic peptides resulted in the suppression of both allergic peptide-specific IgE responses and type-I allergy, providing evidence for a new approach to the development of peptide-based desensitization.

Autoantibodies in atopic dermatitis

Recent progress in the autoimmune aspects of atopic dermatitis has clarified that patients with atopic dermatitis develop IgE-class autoantibodies mainly against intracellular proteins. The cloning of cDNAs encoding autoallergens with human expression cDNA libraries and serum IgE from atopic dermatitis patients has unraveled the molecular characteristics of IgE-binding autoantigens. Some patients with atopic dermatitis also have IgG-class autoantibodies, especially anti-nuclear antibodies. One of the nuclear autoantigens was identified as DFS70/transcription coactivator p75. In addition, p80-coilin in nuclear coiled bodies is also targetted. Several lines of evidence suggest that a subset of atopic dermatitis may be associated with an autoimmune response.

What are the most promising strategies for the therapeutic immunomodulation of allergic diseases?

Specific immunotherapy and other immunomodulatory strategies have long been a stronghold in the management of allergic diseases. In particular, "immunodeviation-therapy" or "vaccination for allergies", i.e. the redirection of Th2-type immune responses towards a Th1-response pattern, has become an ever more popular concept. The present feature of CONTROVERSIES complements our previous discussion of atopy (Röcken et al., Exp Dermatol 7: 97--104, 1998), and is dedicated to a critical analysis of the general problems and limitations one faces with the main immunomodulatory strategies traditionally considered in this context. We also explore alternative approaches that appear promising in order to achieve both a more effective and/or a more specific immunotherapy of allergic diseases. Given that the mast cell remains a key protagonist in the pathogenesis of allergic diseases finally, this feature examines how innovative, more selectively mast cell-targeted strategies may be developed for the management of allergic diseases.

Atopic diseases and upper respiratory infections

This article reviews information on the topics of asthma, atopic dermatitis, food allergy, and upper respiratory infections. The asthma section provides an in-depth look at sociodemographic factors contributing to asthma morbidity and the barriers to asthma control. New findings on the triggers and therapies of atopic dermatitis and new articles on formula allergy and peanut allergy are presented. Recent publications in the areas of sinusitis and upper respiratory infections are also reviewed.

Autoallergy: a pathogenetic factor in atopic dermatitis?

Long before the discovery of IgE it was reported that human dander extract can elicit immediate-type skin reactions in patients with severe atopy and that this skin sensitivity can be passively transferred with serum. Several recent findings have rekindled the interest in this phenomenon and led to the concept that IgE autoreactivity may play a pathogenetic role in severe and chronic forms of atopy. The elucidation of the nature of several environmental allergens has revealed striking structural and immunologic similarities with human proteins. It was also reported that patients predominantly with severe and chronic manifestations of atopy (eg, atopic dermatitis) contain IgE autoantibodies against a wide variety of proteins expressed in histogenetically unrelated human cell types and tissue specimens. Last, complementary DNAs coding for autoallergens were isolated from human expression complementary DNA libraries and recombinant autoallergens were produced. The autoallergens characterized to date represent mainly intracellular proteins, but some of them could be detected as IgE immune complexes in sera of sensitized patients. We suggest that at least two pathomechanisms could play a role in autoallergy. First, autoallergens may cross-link effector cell-bound IgE autoantibodies and, by release of inflammatory mediators, lead to immediate-type symptoms. Second, IgE-mediated presentation of autoallergens may activate autoreactive T cells to release proinflammatory cytokines, contributing to the magnitude of the allergic tissue reaction.