Calreticulin binds preferentially with B cell linear epitopes of Ro60 kD autoantigen, enhancing recognition by anti-Ro60 kD autoantibodies

Detection of anti-calreticulin antibody in the sera of Chinese patients with primary Sjögren syndrome

BACKGROUND

Primary Sjögren syndrome (pSjS) is one of the most prevalent systemic autoimmune diseases and characterized with hyperactivation of B cell and the abundant presence of autoantibodies in sera. The salivary gland epithelial cells (SGECs) release autoantigens to evoke autoimmunity through releasing elevated apoptosis or secreting autoantigen-containing exosomes, thus identifying autoantibodies directly to SGECs might provide insights into disease related biomarkers as well as further elucidating pathogenesis mechanisms. The present study was undertaken to identify autoantibodies to SGECs and to evaluate its clinical values in Chinese pSjS.

METHODS

Cell-based indirect immunofluorescence and immunostaining, two-dimensional electrophoresis and liquid chromatograph-tandem mass spectrometry were conducted to identify the autoantibodies to human salivary gland cell line A253 in pSjS sera. Enzyme-linked immunosorbent assay (ELISA) was applied to identify autoantibody titer in pSjS cohort and healthy controls. The prevalence and clinical significance of the identified autoantibodies was further assessed in pSjS population.

RESULTS

Anti-calreticulin (CALR) antibody was identified as a new autoantibody directly to SGECs in sera from pSjS patients. Anti-CALR antibody were detected in 37 of 120 pSjS patients (30.83 %) and 1 of 54 healthy controls (1.85 %). It was found in 40.85 % pSjS with anti-SSA positive, 53.85 % with anti-SSB positive, and 14.7 % in sero-negative pSjS. Anti-CALR antibody was associated with clinical manifestations including weight loss(p = 0.045), vasculitis (p = 0.031), and laboratory parameters including erythrocyte sedimentation rate (ESR) (r = 0.056, p = 0.021), Krebs von den Lungen-6 (KL-6) (r = 0.121, p = 0.035), IgG (r = 0.097, p < 0.001), IgG2 (r = 0.142, p = 0.022), IgG3 (r = 0.287, p < 0.001), fibrinogen (r = 0.084, p = 0.016), D-Dimer (r = 0.086, p = 0.012) and fibrinogen degradation production (r = 0.150, p = 0.002). The expression of CALR in salivary glands was related to lymphocytes infiltration into salivary glands in pSjS patients (r = 0.7076, p = 0.0034).

CONCLUSION

To our knowledge, this was the first study to investigate the prevalence and clinical significance of anti-CALR antibody in Chinses pSjS patients. The present study identified an autoimmune antibody, anti-CALR antibody, as a good autoimmune biomarker for sero-negative pSjS.

Antibodies to calnexin and mutated calreticulin are common in human sera

PURPOSE OF THE STUDY

Calreticulin is an endoplasmic reticulum chaperone protein, which is involved in protein folding and in peptide loading of major histocompatibility complex class I molecules together with its homolog calnexin. Mutated calreticulin is associated with a group of hemopoietic disorders, especially myeloproliferative neoplasms. Currently only the cellular immune response to mutated calreticulin has been described, although preliminary findings have indicated that antibodies to mutated calreticulin are not specific for myeloproliferative disorders. These findings have prompted us to characterize the humoral immune response to mutated calreticulin and its chaperone homologue calnexin.

PATIENTS AND METHODS

We analyzed sera from myeloproliferative neoplasm patients, healthy donors and relapsing-remitting multiple sclerosis patients for the occurrence of autoantibodies to wild type and mutated calreticulin forms and to calnexin by enzyme-linked immunosorbent assay.

RESULTS

Antibodies to mutated calreticulin and calnexin were present at similar levels in serum samples of myeloproliferative neoplasm and multiple sclerosis patients as well as healthy donors. Moreover, a high correlation between antibodies to mutated calreticulin and calnexin was seen for all patient and control groups. Epitope binding studies indicated that cross-reactive antibodies bound to a three-dimensional epitope encompassing a short linear sequence in the C-terminal of mutated calreticulin and calnexin.

CONCLUSION

Collectively, these findings indicate that calreticulin mutations may be common and not necessarily lead to onset of myeloproliferative neoplasm, possibly due to elimination of cells with mutations. This, in turn, may suggest that additional molecular changes may be required for development of myeloproliferative neoplasm.

The Involvement of Alarmins in the Pathogenesis of Sjögren's Syndrome

Sjögren’s syndrome (SS) is a chronic autoimmune disease that affects exocrine glands, primarily the salivary and lachrymal glands. It is characterized by lymphoplasmacytic infiltration of the glandular tissues, ultimately leading to their dysfunction and destruction. Besides classic dry eyes and dry mouth defined as sicca syndrome, patients affected by the disease also typically display symptoms such as fatigue, pain and in more than 50% of cases, systemic manifestations such as arthritis, interstitial lung involvement, neurological involvement and an increased risk of lymphoma. The pathophysiological mechanisms underlying SS still remain elusive. The crucial role of innate immunity has been advocated in recent years regarding the pathogenesis of pSS, especially in the initiation and progression toward autoimmunity. Alarmins are endogenous molecules that belong to the large family of damage associated molecular pattern (DAMP). Alarmins are rapidly released, ensuing cell injury and interacting with pattern recognition receptors (PRR) such as toll-like receptors (TLR) to recruit and activate cells of the innate immune system and to promote adaptive immunity responses. This review highlights the current knowledge of various alarmins and their role in the pathogenesis of pSS.

Chaperones may cause the focus of diabetes autoimmunity on distinct (pro)insulin peptides

It is still an enigma why T cell autoreactivity in type 1 diabetes targets few beta cell antigens only. Among these, one primary autoantigen is pro(insulin). Autoimmune T cells preferentially recognise three epitopes on the proinsulin molecule, of which the peptide region B:11-23 is the dominant one. Interestingly, the three regions superimpose with binding sites of the chaperone hsp70, the region B:11-23 being the strongest binding one. Absence of an intact core region B:15-17 prevents autoimmune diabetes in NOD as well as binding of hsp70. A role of hsp70 in selecting autoimmune epitopes is supported by the ability of this and other chaperones to deliver bound peptides to MHC class I and II molecules for efficient antigen presentation. Binding of hsp70 to receptors on antigen presenting cells such as TLR4 results in costimulatory signals for T cell activation. Strongest effects are seen for the mixture of hsp70 with the peptide B:11-23. Thus, hsp70 may assist in proinsulin epitope selection and efficient presentation to autoreactive T cells. The concept of chaperone guided immune reactivity may also apply to other autoimmune diseases.

Ablation of the Chaperone Protein ERdj5 Results in a Sjögren's Syndrome-Like Phenotype in Mice, Consistent With an Upregulated Unfolded Protein Response in Human Patients

Objective: Sjögren's syndrome (SS) is a chronic autoimmune disorder that affects mainly the exocrine glands. Endoplasmic reticulum (ER) stress proteins have been suggested to participate in autoimmune and inflammatory responses, either acting as autoantigens, or by modulating factors of inflammation. The chaperone protein ERdj5 is an ER-resident disulfide reductase, required for the translocation of misfolded proteins during ER-associated protein degradation. In this study we investigated the role of ERdj5 in the salivary glands (SGs), in association with inflammation and autoimmunity.

Methods:In situ expression of ERdj5 and XBP1 activation were studied immunohistochemically in minor SG tissues from primary SS patients and non-SS sicca-complaining controls. We used the mouse model of ERdj5 ablation and characterized its features: Histopathological, serological (antinuclear antibodies and cytokine levels), and functional (saliva flow rate).

Results: ERdj5 was highly expressed in the minor SGs of SS patients, with stain intensity correlated to inflammatory lesion severity and anti-SSA/Ro positivity. Moreover, SS patients demonstrated higher XBP1 activation within the SGs. Remarkably, ablation of ERdj5 in mice conveyed many of the cardinal features of SS, like spontaneous inflammation in SGs with infiltrating T and B lymphocytes, distinct cytokine signature, excessive cell death, reduced saliva flow, and production of anti-SSA/Ro and anti-SSB/La autoantibodies. Notably, these features were more pronounced in female mice.

Conclusions: Our findings suggest a critical connection between the function of the ER chaperone protein ERdj5 and autoimmune inflammatory responses in the SGs and provide evidence for a new, potent animal model of SS.

Endoplasmic reticulum stress in autoimmune diseases: Can altered protein quality control and/or unfolded protein response contribute to autoimmunity? A critical review on Sjögren's syndrome

For many years, researchers in the field of autoimmunity have focused on the role of the immune components in the etiopathogenesis of autoimmune diseases. However, some studies have demonstrated the importance of target tissues in their pathogenesis and the breach of immune tolerance. The immune system as well as target tissue cells (plasmatic, β-pancreatic, fibroblast-like synoviocytes, thyroid follicular and epithelial cells of the lachrymal glands, salivary glands, intestine, bronchioles and renal tubules) share the characteristic of secretory cells with an extended endoplasmic reticulum (ER). The function of these cells depends considerably on a normal ER function and calcium homeostasis, so they can produce and secrete their main components, which include glycoproteins involved in antigenic presentation such as major histocompatibility complex (MHC) class I and II. All these proteins are synthesized and modified in the ER, and for this reason disturbances in the normal functions of this organelle such as protein folding, protein quality control, calcium homeostasis and redox balance, promote accumulation of unfolded or misfolded proteins, a condition known as ER stress. Autoimmune diseases are characterized by inflammation, which has been associated with an ER stress condition. Interestingly, patients with these diseases contain circulating auto-antibodies against chaperone proteins (such as Calnexin and GRP94), thus affecting the folding and assembly of MHC class I and II glycoproteins and their loading with peptide. The main purpose of this article is to review the involvement of the protein quality control and unfolded protein response (UPR) in the ER protein homeostasis (proteostasis) and their alterations in autoimmune diseases. In addition, we describe the interaction between ER stress and inflammation and evidences are shown of how autoimmune diseases are associated with an ER stress condition, with a special emphasis on the second most prevalent autoimmune rheumatic disease, Sjögren's syndrome.

Application of synthetic peptides for detection of anti-citrullinated peptide antibodies

Anti-citrullinated protein antibodies (ACPAs) are a hallmark of rheumatoid arthritis (RA) and represent an important tool for the serological diagnosis of RA. In this study, we describe ACPA reactivity to overlapping citrullinated Epstein-Barr virus nuclear antigen-1 (EBNA-1)-derived peptides and analyze their potential as substrates for ACPA detection by streptavidin capture enzyme-linked immunosorbent assay. Using systematically overlapping peptides, containing a 10 amino acid overlap, labelled with biotin C-terminally or N-terminally, sera from 160 individuals (RA sera (n=60), healthy controls (n=40), systemic lupus erythematosus (n=20), Sjögren's syndrome (n=40)) were screened for antibody reactivity. Antibodies to a panel of five citrullinated EBNA-1 peptides were found in 67% of RA sera, exclusively of the IgG isotype, while 53% of the patient sera reacted with a single peptide, ARGGSRERARGRGRG-Cit-GEKR, accounting for more than half of the ACPA reactivity alone. Moreover, these antibodies were detected in 10% of CCP2-negative RA sera. In addition, 47% of the RA sera reacted with two or three citrullinated EBNA-1 peptides from the selected peptide panel. Furthermore, a negative correlation between the biotin attachment site and the location of citrulline in the peptides was found, i.e. the closer the citrulline was located to biotin, the lower the antibody reactivity. Our data suggest that citrullinated EBNA-1 peptides may be considered a substrate for the detection of ACPAs and that the presence of Epstein-Barr virus may play a role in the induction of these autoantibodies.

Characterization of continuous monoclonal antibody epitopes in the N-terminus of Ro60

One of the major targets of the autoimmune response in the rheumatic autoimmune diseases, Systemic Lupus Erythematosus and Sjögrens Syndrome, is the protein Ro60. Ro60 is known to associate with small misfolded RNAs, and is involved in RNA quality control and in enhancing cell survival during cellular stress, e.g. after ultaviolet irradiation. In this study, six monoclonal antibodies to Ro60 were analyzed in order to identify antigenic regions and the nature of these. Preliminary analyses revealed that two of the antibodies recognized continuous epitopes, while the remaining antibodies most likely recognized conformational epitopes. The continuous epitopes of Ro60 were characterised by modified immunoassays employing resin-bound peptides and free peptides. Peptide screenings located the epitopes to the N-terminus of Ro60, and further analyses indicated that the epitopes of the monoclonal antibodies TROVE2 and SSI-HYB 358-02 were located to amino acids 8-17 and 34-49, respectively. Moreover, charged amino acids were found to be especially important for antibody reactivity, although antibody reactivity of the monoclonal antibody TROVE2 primarily was found to be epitope backbone-dependent.

Role of N- or C-terminal biotinylation in autoantibody recognition of citrullin containing filaggrin epitope peptides in rheumatoid arthritis

Here, we report on the synthesis, conformational analysis, and autoantibody binding properties of new sets of rheumatoid arthritis (RA) specific biotin-peptide conjugates derived from filaggrin epitope peptides. The biotin with or without a linker was attached to the Cit or Arg containing epitope core ((311)TXGRS(315)) or epitope region ((306)SHQESTXGXSXGRSGRSGS(324)) peptide (where X = Cit), through an amide bond at the N- or C-terminal of the epitopes. Antibody binding was detected by indirect enzyme-linked immunosorbent assay (ELISA) using sera from RA, Systemic lupus erythematosus (SLE) patients, as well as healthy individuals, and the secondary structure of conjugates was investigated by electronic circular dichroism (ECD). We found that autoantibodies from RA patients recognize specifically both filaggrin epitope region ((306)SHQESTXGXSXGRSGRSGS(324)) and short epitope core ((311)TXGRS(315)) peptides. Our data also indicate that the positioning of the biotin label within a peptide sequence can markedly influence the antibody binding, but the length of the linker incorporated has essentially no effect on the recognition. ECD experiments demonstrate that the Arg/Cit change does not influence the solution conformation of the peptide conjugates. However, the presence and position of the biotin moiety has a pronounced effect on the conformation of the 5-mer epitope core peptides, while it does not alter the secondary structure of the 19-mer epitope region peptides.

ER Stress Proteins in Autoimmune and Inflammatory Diseases

Over the past two decades, heat shock proteins (HSPs) have been implicated in inflammatory responses and autoimmunity. HSPs were originally believed to maintain protein quality control in the cytosol. However, they also exist extracellularly and appear to act as inflammatory factors. Recently, a growing body of evidence suggested that the other class of stress proteins such as, endoplasmic reticulum (ER) stress proteins, which originally act as protein quality control factors in the secretory pathway and are induced by ER stress in inflammatory lesions, also participate in inflammation and autoimmunity. The immunoglobulin heavy-chain binding protein (Bip)/glucose-regulated protein 78 (GRP78), calnexin, calreticulin, glucose-regulated protein 94 (GRP94)/gp96, oxygen regulated protein 150 (ORP150)/glucose-regulated protein 170 (GRP170), homocysteine-induced ER protein (Herp) and heat shock protein 47 (hsp47)/Serpin H1, which are expressed not only in the ER but also occasionally at the cell surface play pathophysiological roles in autoimmune and inflammatory diseases as pro- or anti-inflammatory factors. Here we describe the accumulating evidence of the participation of ER stress proteins in autoimmunity and inflammation and discuss the critical differences between the two classes of stress proteins.

Autoimmune response and target autoantigens in Sjogren's syndrome

BACKGROUND

Primary Sjogren's syndrome (pSS) is characterized by the presence of autoantibodies targeting mainly the Ro/La ribonucleoprotein complex. It is now appreciated that the production of autoantibodies is an antigen-driven immune response.

DESIGN

In this review, candidate mechanisms for autoantigen presentation and perpetuation of the autoimmune response within the autoimmune tissue lesion of pSS are discussed.

RESULTS

Several studies have shown that the epithelial cell in labial salivary glands of patients with Sjogren's syndrome is activated, bearing characteristics of an antigen-presenting cell, as suggested by inappropriate expression of class II HLA and co-stimulatory molecules. Other studies have confirmed that in salivary glands, there is an increased autoantigen presentation via apoptotic blebs and bodies, exosomes and heat shock protein-mediated cross-priming. There is also an increased expression of interferon (IFN)-induced genes, such as the autoantigen Ro52, which provide negative feedback regulation in inflammation. Ro60 and La autoantigens also appear to play a major role in the local autoimmune response in Sjogren's syndrome. In this regard, La and Ro60 the messenger RNA (mRNA) expression is upregulated in the affected salivary glands with different isoforms of La autoantigen mRNA to be expressed in patients with pSS. At the protein level, La/SSB in pSS salivary glands is found to be post-translationally modified.

CONCLUSIONS

 Autoantigen alterations in a microenvironment of local inflammation with increased in situ apoptosis, Toll-like receptor (TLR) signalling and antigen presentation may drive the autoimmune response and local autoantibody production in pSS.

Reactivity with dichotomous determinants of Ro 60 stratifies autoantibody responses in lupus and primary Sjögren's syndrome

OBJECTIVE

Analysis of B cell determinants of Ro 60 exposed on the surface of apoptotic cells (apotopes) or intracellular epitopes provides insight into the structural forms of the autoantigen that break immune tolerance. This study was initiated to compare anti-Ro 60 responses in systemic lupus erythematosus (SLE) and primary Sjögren's syndrome (SS) against membrane-bound and intracellular forms of Ro 60.

METHODS

The reactivity of autoantibodies from patients with SLE and primary SS to Ro 60 apotopes and epitopes was assessed by multiparameter flow cytometry and solid-phase immunoassay. Anti-Ro 60 IgG was eluted from early apoptotic cells or recombinant Ro 60 immobilized on nitrocellulose, and binding to membrane-bound and intracellular forms of Ro 60 was quantitated by flow cytometry.

RESULTS

An immunodominant apotope, which was recognized by IgG from a subset of SLE patients with anti-Ro, but not anti-La, autoantibodies, was mapped to a region forming a helix-loop-helix at the apical tip of the Ro 60 molecule. Immobilization of this region to the solid phase exposed an epitope that was recognized by IgG from primary SS and SLE patients whose sera had both anti-Ro and anti-La autoantibodies. Autoantibodies eluted from either the surface of apoptotic cells or the Ro 60 epitope on the solid phase were non-cross-reactive and specifically recognized membrane-bound or cytoplasmic forms of Ro 60.

CONCLUSION

This is the first example of a dichotomy of human autoantibody responses against mutually exclusive determinants linked to a single domain of a systemic autoantigen and supports a model in which tolerance is broken by different immunogenic forms of Ro 60.

Calreticulin is a B cell molecular target in some gastrointestinal malignancies

Calreticulin, upon translocation to the cell surface, plays a critical role in the recognition of tumour cells and in experimentally induced cellular anti-tumour immunity. However, less is known about anti-calreticulin antibodies and their role in malignancies. Using enzyme-linked immunosorbent assay (ELISA), we found immunoglobulin (Ig)A and/or IgG anti-calreticulin antibodies in sera of approximately 63% of patients with hepatocellular carcinoma (HCC), 57% of patients with colorectal adenocarcinoma (CRA) and 47% of patients with pancreatic adenocarcinoma (PACA), while healthy controls, patients with viral hepatitis C and with chronic pancreatitis reached only 2%, 20% and 31% seropositivity, respectively. We found significantly elevated mean levels of IgA anti-calreticulin antibodies (P < 0.001) in patients with HCC (78.7 +/- 52.3 AU, mean +/- standard deviation), PACA (66.5 +/- 30.9 AU) and CRA (61.8 +/- 25.8 AU) when compared to healthy controls (41.4 +/- 19.2 AU). Significantly elevated mean levels of IgG anti-calreticulin antibodies (P < 0.001) were detected in patients with HCC (121.9 +/- 94.2 AU), gall bladder adenocarcinoma (118.4 +/- 80.0 AU) and PACA (88.7 +/- 55.6 AU) when compared to healthy controls (56.7 +/- 22.9 AU). Pepscan analysis revealed a large number of antigenic epitopes of calreticulin recognized by both IgA and IgG antibodies of patients with HCC and PACA, indicating robust systemic immune response. Moreover, significantly elevated levels of antibodies against peptide KGEWKPRQIDNP (P < 0.001) in these patients, tested by ELISA, confirmed the distinct character of antibody reactivity against calreticulin. The high occurrence and specificity of serum anti-calreticulin autoantibodies in the majority of patients with some gastrointestinal malignancies provide the evidence for their possible clinical relevance.

Absence of high‐affinity calreticulin autoantibodies in patients with systemic rheumatic diseases and coeliac disease

Calreticulin has been reported to be an autoantigen in various autoimmune connective tissue diseases and in coeliac disease. Previous studies have used incubation buffers with low salt and low detergent concentrations (low stringency conditions) with serum albumin or other proteins as a blocking agent. Using these conditions we found a relatively high level of non-specific binding in many sera. Antibodies to proteins that are used as blocking reagents in ELISA (bovine serum albumin (BSA), ovalbumin, skimmed milk powder) are frequently present in sera, and these may cause false-positive results. Moreover, the low isoelectric point of calreticulin and its chaperone properties may give rise to false-positive results under low stringency conditions. We report that the use of a simple buffer without protein (50 mM Tris, pH 7.5, 1% Tween 20, 0.3 M NaCl) removes most of the problems with unwanted binding (high stringency conditions). Using the high stringency conditions, we screened sera from 107 patients with systemic lupus erythematosus, sera from patients with other systemic autoimmune diseases and from children with coeliac disease for the presence of high-affinity calreticulin autoantibodies by immunoblotting and ELISA. None of the sera contained high-affinity calreticulin antibodies. It is concluded that calreticulin is not a common autoantigen in patients with autoimmune connective tissue diseases or coeliac disease.

Calreticulin, a multi-process calcium-buffering chaperone of the endoplasmic reticulum

Calreticulin is an ER (endoplasmic reticulum) luminal Ca2+-buffering chaperone. The protein is involved in regulation of intracellular Ca2+ homoeostasis and ER Ca2+ capacity. The protein impacts on store-operated Ca2+ influx and influences Ca2+-dependent transcriptional pathways during embryonic development. Calreticulin is also involved in the folding of newly synthesized proteins and glycoproteins and, together with calnexin (an integral ER membrane chaperone similar to calreticulin) and ERp57 [ER protein of 57 kDa; a PDI (protein disulfide-isomerase)-like ER-resident protein], constitutes the ‘calreticulin/calnexin cycle’ that is responsible for folding and quality control of newly synthesized glycoproteins. In recent years, calreticulin has been implicated to play a role in many biological systems, including functions inside and outside the ER, indicating that the protein is a multi-process molecule. Regulation of Ca2+ homoeostasis and ER Ca2+ buffering by calreticulin might be the key to explain its multi-process property.

B cell apotopes of the 60-kDa Ro/SSA and La/SSB autoantigens

Apoptosis has been proposed to influence the initiation and diversification of autoimmunity to the Ro (SSA)/La (SSB) ribonucleoprotein (RNP) particle and serve as a target for autoantibody-mediated tissue injury. We have developed a new approach to B cell epitope mapping which identifies "apotopes," defined as epitopes expressed on the surface of apoptotic cells. Preliminary studies support a role for apotopes as diagnostic markers in systemic lupus erythematosus (SLE) and primary Sjögren's syndrome. For example, apotopes within the NH(2)-terminal and central regions of La react with the majority of sera from mothers of infants with congenital heart block. Furthermore, a Ro60 apotope is specific for a subset of SLE with isolated anti-Ro60 responses. The mapping of B cell apotopes may prove superior to standard epitope mapping by suggesting novel pathways of autoantibody production and identifying pathogenic species of autoantibodies.

Dry eye and designer ophthalmics

Expressed sequence tag (EST), proteomic, and antibody capture assays are revealing a level of tear film protein complexity far greater than previously appreciated. A systems biology approach will be needed to fully appreciate function as tear protein doses fluctuate in time through different conditions. Although consensus is growing on what fully constitutes the human tear proteome, questions remain about the source and significance of the approximately 256 tear proteins designated as "intracellular." Many of these may derive from normal cellular turnover and could therefore be informative. A further >183 are designated as "extracellular." Surprisingly, only 4 to 5% of these appear to be dysregulated in the three forms of dry eye preliminarily examined to date. Some differ and a couple overlap, suggesting that disease-specific signatures could be identified. Future dry eye treatment might include recombinant tear protein rescue as a personalized ophthalmic approach to ocular surface disease.

Anti-calreticulin immunoglobulin A (IgA) antibodies in refractory coeliac disease

Refractory coeliac disease (RCD) is a very rare and dangerous form of CD, in which gluten-free diet loses its therapeutic effect and the damage of intestinal mucosa persists. Because of the adherence to the diet, serological markers of CD [immunoglobulin A (IgA) antibodies against gliadin, tissue transglutaminase (tTG) and endomysium] are often missing in RCD patients. We found substantially elevated levels of IgA anti-calreticulin (CRT) antibodies in the sera of almost all RCD patients tested. These sera were negative for IgA antibodies to gliadin and tTG and only some of them showed IgA antibodies to enterocytes. Analysis of patients' IgA reactivity to CRT fragments (quarters and halves) by Western blotting revealed differences in the specificity of IgA antibodies between RCD and CD patients. We therefore used the Pepscan technique with synthetic overlapping decapeptides of CRT to characterize antigenic epitopes recognized by serum IgA antibodies of RCD patients. Employing this method we demonstrated several dominant antigenic epitopes recognized by IgA antibodies of RCD patients on the CRT molecule. Epitope GVTKAAEKQMKD was recognized predominantly by serum IgA of RCD patients. Our results suggest that testing for serum IgA antibodies against CRT and its selected peptide could be a very useful tool in RCD differential diagnosis.

A B cell apotope of Ro 60 in systemic lupus erythematosus

OBJECTIVE

Previous studies have attempted to segregate anti-60-kd Ro/SSA (anti-Ro 60) responses in systemic lupus erythematosus (SLE) and primary Sjögren's syndrome (SS) but have shown limited disease preference. The aim of the present study was to determine whether the presence of autoantibodies against an Ro 60 apotope (an epitope expressed on apoptotic cells) distinguishes anti-Ro 60 responses in SLE and primary SS.

METHODS

Multiparameter flow cytometry was used to select early apoptotic cells and measure the simultaneous binding of annexin V, propidium iodide, and anti-Ro 60-positive IgG from SLE patients (n=21) and patients with primary SS (n=19). The specificity of the Ro 60 apotope was determined by inhibition experiments with recombinant and native Ro 60.

RESULTS

Autoantibodies against the Ro 60 apotope were prevalent in SLE patients (13 of 21, 62%) and were rarely observed in patients with primary SS (1 of 19, 5%) (P=0.0002). Further, within SLE patients, autoantibodies to the Ro 60 apotope strongly distinguished patients with anti-Ro 60 alone (12 of 13, 92%) from those with both anti-Ro 60 and anti-La (1 of 8, 13%) (P=0.0005). When we considered all patients with anti-Ro 60 alone, the presence of autoantibodies to the Ro 60 apotope had both high sensitivity (92.3%) and high specificity (85.7%) for SLE compared with primary SS (P=0.0012). The presence of autoantibodies to the Ro 60 apotope may therefore be of diagnostic value in patients with isolated anti-Ro 60 responses.

CONCLUSION

The preferential targeting of an Ro 60 apotope exposed on early apoptotic cells in a subset of SLE patients implies disease-specific pathways for the induction of anti-Ro 60 autoimmunity.

The role of chaperone proteins in autoimmunity

Heat-shock or stress proteins (HSPs) are intracellular molecules that are expressed under cellular stress and have housekeeping and cytoprotective functions. Many of them act also as molecular chaperones, assisting the correct folding, stabilization, and translocation of proteins. In pathological situations, such as necrotic cell death, they can be released into the extracellular environment complexed with intact or fragmented cellular proteins. Evidence is now accumulating to indicate that, under certain circumstances, these complexes can contribute to induction of autoimmunity by receptor-mediated activation of the innate immune response (signaling the "danger") and by participation in the presentation of autoantigens for the adaptive immune response (acting as natural adjuvants). In addition, the conservation of HSPs through prokaryotes and eukaryotes, together with the increased production of host and microbial HSPs at the site of infection, has led to the proposition that these proteins may provide a link between infection and autoimmunity. This review outlines the mechanisms for the potential involvement of chaperones in the induction of autoimmune disease.

Analysis of parotid glands of primary Sjögren's syndrome patients using proteomic technology reveals altered autoantigen composition and novel antigenic targets

Sjögren's syndrome (SS) is an autoimmune disease characterized by lymphocytic infiltration, destruction of the salivary and lacrimal glands and production of autoantibodies against a variety of cellular proteins. The aberrant immune response against these autoantigens may begin or extend to other proteins that are not yet defined. Several studies have shown that autoantibody production is taking place in the affected salivary glands. In the present study, using proteomic approaches, we aimed to: (a) identify new autoantigens in the salivary glands of primary SS (pSS) patients and (b) evaluate the epigenetic changes of known autoantigens. Total parotid gland extracts of pSS patients were analysed using two-dimensional gel electrophoresis, sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblot with pSS patients' sera or purified autoantibodies and immunoprecipitation using homologous IgG. Identification of the unknown proteins was performed using mass spectrometry (MS). Immunoblot analysis on two-dimensional gels using purified anti-La/SSB antibodies revealed that pSS salivary glands contain high levels of post-translationally modified La/SSB autoantigen, while the native form of the protein is recognized faintly, in contrast to normal controls. Moreover, salivary glands of pSS patients contain post-translationally modified actin that becomes immunogenic in the microenviroment of the affected tissue. The alteration of the physicochemical properties of self-proteins could thus contribute to the break of immune tolerance against them.

Autoantibodies to intracellular autoantigens and their B-cell epitopes: molecular probes to study the autoimmune response

A common laboratory finding in systemic autoimmune diseases is the presence of autoantibodies against intracellular autoantigens. Although their pathogenesis is not fully understood, autoantibodies are important tools for establishing diagnosis, classification, and prognosis of autoimmune diseases. Autoantibodies mainly target multicomponent complexes containing both protein antigens and (ribo)-nucleic acid(s), such as the spliceosome or Ro/La RNPs. In this review, we address the main characteristics and the clinical value of the main autoantibody types with respect to their disease association, and we describe the corresponding autoantigens, their biologic function, and their B-cell antigenic determinants (epitopes). The structural characteristics and clinical associations of these epitopes, and their utility as tools to investigate the autoimmune response, are discussed in detail. New insights into the pathogenetic role of epitopes in systemic autoimmunity are also examined. In this regard, using the defined structures of the B-cell antigenic epitopes, complementary epitopes can be designed according to the "molecular recognition" theory. These complementary epitopes can be used as probes to study pathogenetic and immunoregulatory aspects of the anti-idiotypic response. The origin of humoral autoimmunity and the spreading of the epitopes in systemic lupus erythematosus are also discussed. Finally, the ability of post-translational modifications to induce autoreactive immune attack via the generation of neo-epitopes is summarized.

Association of MBP peptides with Hsp70 in normal appearing human white matter

Multiple Sclerosis is an autoimmune disease directed against myelin proteins. The etiology of MS is poorly defined though, with no definitive causative agent yet identified. It has been hypothesized that MS may be a multifactorial disease resulting in the same end product: the destruction of myelin by the immune system. In this report we describe a potential role for heat shock proteins in the pathogenesis of MS. We isolated Hsp70 from the normal appearing white matter of both MS and normal human brain and found this was actively associated with, among other things, immunodominant MBP peptides. Hsp70-MBP peptide complexes prepared in vitro were shown to be highly immunogenic, with adjuvant-like effects stimulating MBP peptide-specific T cell lines to respond to normally sub-optimal concentrations of peptide. This demonstration of a specific interaction between Hsp70 and different MBP peptides, coupled with the adjuvanticity of this association is suggestive of a possible role for Hsp70 in the immunopathology associated with MS.

Cross-reaction between antibodies to the major epitope of Ro60 kD autoantigen and a homologous peptide of Coxsackie virus 2B protein

Coxsackie virus RNA has recently been detected in biopsy specimens of minor salivary glands from patients with primary Sjögren's Syndrome (pSS). A peptide derived from Coxsackie virus 2B protein (pepCoxs) presents 87% sequence homology with the 222-229 region of the major linear B-cell epitope of Ro60 kD autoantigen (pep216-232). Synthetic peptides corresponding to pep216-232: (216)KALSVETEKLLKYLEAV(232) and pepCoxs: (31)MVTSTITEKL LKNLVKI(47), were prepared. Sera from 42 patients with pSS and 43 patients with systemic lupus erythematosus (SLE) as well as sera from 27 healthy individuals (normal controls) and sera from 30 patients with rheumatoid arthritis (disease controls) were tested against the two homologous peptides. Twenty-five percent of SLE sera and 33.3% of pSS sera reacted against pep216-232, whereas 28% of SLE sera and 37% of pSS sera recognized the pepCoxs. The sera reacting with pep216-232 were apparently the same as those reacting with pepCoxs. Normal sera and disease control sera presented only a limited reactivity against both peptides (ranging from 3.7% to 10%). Both peptides reacted more prominently with anti-Ro/La (+) sera from pSS patients. Thus, pep216-232 was recognized by 17% of the anti-Ro (+) sera and by 42% of the anti-Ro/La (+) sera, whereas pepCoxs was recognized by 28.5% and 38% of the a-Ro(+) and a-Ro/La(+) sera, respectively. Purified anti-pep216-232 antibodies readily reacted with both peptides while inhibition experiments revealed the specificity of this reaction. These results suggest a possible cross-reaction between antibodies to the major linear B-cell epitope of Ro60 kD autoantigen and the homologous pepCoxs in pSS patients. This cross-reaction might potentially play a role in autoantibody formation and the perpetuation of the autoimmune response against Ro/SSA and La/SSB.

Antinuclear autoantibodies in systemic lupus erythematosus

PURPOSE OF REVIEW

The production of autoantibodies against nuclear antigens is the hallmark of systemic lupus erythematosus. Among the large number of autoantibodies known, only a limited number appear to be clinically important. The various autoantibodies have different clinical significance in lupus patients. In this review, we will discuss the various antinuclear autoantibodies detected in lupus patients, their potential pathogenic role, and their usefulness in clinical practice.

RECENT FINDINGS

Recent advances include the clear demonstration of autoantibody transport into living cells, a process that clearly includes interactions with a number of cellular components that may play a role in cellular dysfunction and disease. Also, the anti-Sm B/B' response originates from a single antigenic epitope that appears to be the same structure in different patients, before spreading to other epitopes and becoming the typically mature, complex humoral autoimmune anti-Sm autoantibody response.

SUMMARY

The existing data strongly support a central role of autoantibodies in the pathogenesis of lupus. Better characterization of autoantibodies, their mechanisms of production, and their interactions with various cellular constituents will clarify the pathogenesis of this disease.

Role of calreticulin from parasites in its interaction with vertebrate hosts

Although parasites range from protozoan to complex, evolutionary advanced arthropods, in general, a hallmark of parasite life cycles is their ability to adapt to changes in temperature, pH and host defense strategies. Calreticulin, a calcium-binding protein, highly conserved and multifunctional, is present in every cell of higher organisms, except erythrocytes. The surprising array of calreticulin-associated functions include lectin-like chaperoning, calcium storage and signaling, modulation of gene expression, cell adhesion, enhancement of phagocytosis of C1q or collectin opsonized apoptotic cells, inhibition of angiogenesis and tumoral growth, inhibition of perforin pore formation in T and NK cells, and inhibition of C1q-dependent complement activation. Likewise, calreticulin is present in a wide spectrum of sub cellular compartments. Parasite calreticulin shows a surprisingly high degree of conservation within the framework of its functional domains. Its role within the parasite/host relationship needs to be assessed further, in particular with regard to its impact on parasite infectivity, by helping to evade from its hosts' immune response. With special emphasis on calreticulin from Trypanosoma cruzi, the intracellular protozoan agent of American trypanosomiasis (Chagas' disease), we wish to exemplify and highlight the various implications of parasite calreticulin, within the pathophysiology of parasite-mediated human and animal disease.

The clinical value of intracellular autoantigens B-cell epitopes in systemic rheumatic diseases

A hallmark of autoimmune diseases is the production of autoantibodies against intracellular autoantigens. Although their pathogenetic and their etiologic relationship are not fully understood, these autoantibodies are important tools for establishing the diagnosis, classification and prognosis of autoimmune diseases. Systemic rheumatic diseases are among the most complex disorders because their clinical presentation and constellation of findings are in part reflected by the wide spectrum of autoantibodies found in the sera of patients suffering from these disorders. These autoantibodies usually target large complexes consisting of protein antigens noncovalently associated with (ribo)-nucleic acid(s), like the spliceosome or Ro/La-RNPs. In this review, we first address the main characteristics and the clinical value of several autoantibodies, with respect to their diagnostic sensitivity and specificity. Subsequently, we provide a brief overview of the antigenic determinant types that have been identified on the corresponding autoantigens. The antibody targets of autontigens include primary, secondary, tertiary and quarternary structure epitopes, as well as cryptotopes, neoepitopes and mimotopes. We next focus on antigenic structures corresponding to B-cell epitopes with high disease specificity and sensitivity for all the major autoantigens in systemic autoimmunity including the Ro/La and U1 ribonucleoprotein complexes and the Ku70/80, ribosomal P, DNA topoisomerase I, filaggrin, Jo-1 and PM/SCl-100 autoantigens. These epitopes, defined at the peptide level, can be chemically synthesized and engineered for the development of new inexpensive and easier to perform assays and the improvement of the methods for autoantibody detection. Specific examples of newly developed assays that incorporate (i) epitopes with high disease specificity and sensitivity, (ii) modified epitopes, (iii) conformational epitopes and (iv) complementary epitopes are discussed in detail. Finally, we examine the potential of combining these synthetic epitopes for future development of multiplex diagnostic tests based on miniaturized autoantigen arrays.