Calcium-Sensing Receptor Autoantibodies in Patients with Autoimmune Polyendocrine Syndrome Type 1: Epitopes, Specificity, Functional Affinity, IgG Subclass, and Effects on Receptor Activity.

A major manifestation of autoimmune polyendocrine syndrome type 1 (APS1) is hypoparathyroidism, which is suggested to result from aberrant immune responses against the parathyroid glands. The calcium-sensing receptor (CaSR), which plays a pivotal role in maintaining calcium homeostasis by sensing blood calcium levels and regulating release of parathyroid hormone (PTH), is an autoantibody target in APS1. In this study, the aim was to characterize the binding sites, specificity, functional affinity, IgG subclass, and functional effects of CaSR autoantibodies using phage-display technology, ELISA, and bioassays. The results indicated that CaSR autoantibody binding sites were at aa 41-69, 114-126, 171-195, and 260-340 in the extracellular domain of the receptor. Autoantibodies against CaSR epitopes 41-69, 171-195, and 260-340 were exclusively of the IgG1 subclass. Autoantibody responses against CaSR epitope 114-126 were predominantly of the IgG1 with a minority of the IgG3 subclass. Only autoantibodies recognizing CaSR epitopes 114-126 and 171-195 affected receptor activity; inositol-phosphate accumulation was increased significantly in HEK293-CaSR cells, and PTH secretion from PTH-C1 cells was reduced significantly when either were incubated with purified Ab and Ca2+ compared with Ca2+ alone. In conclusion, although the majority of APS1 patients do not have CaSR-stimulating autoantibodies, the hypoparathyroid state in a small minority of patients is the result of functional suppression of the parathyroid glands.

Generation and Characterization of iPS Cells Derived from APECED Patients for Gene Correction.

APECED (Autoimmune-Polyendocrinopathy-Candidiasis-Ectodermal-Dystrophy) is a severe and incurable multiorgan autoimmune disease caused by mutations in the AIRE (autoimmune regulator) gene. Without functional AIRE, the development of central and peripheral immune tolerance is severely impaired allowing the accumulation of autoreactive immune cells in the periphery. This leads to multiple endocrine and non-endocrine autoimmune disorders and mucocutaneous candidiasis in APECED patients. Recent studies have suggested that AIRE also has novel functions in stem cells and contributes to the regulatory network of pluripotency. In preparation of therapeutic gene correction, we generated and assessed patient blood cell-derived iPSCs, potentially suitable for cell therapy in APECED. Here, we describe APECED-patient derived iPSCs's properties, expression of AIRE as well as classical stem cell markers by qPCR and immunocytochemistry. We further generated self-aggregated EBs of the iPSCs. We show that APECED patient-derived iPSCs and EBs do not have any major proliferative or apoptotic defects and that they express all the classical pluripotency markers similarly to healthy person iPSCs. The results suggest that the common AIRE R257X truncation mutation does not affect stem cell properties and that APECED iPSCs can be propagated in vitro and used for subsequent gene-correction. This first study on APECED patient-derived iPSCs validates their pluripotency and confirms their ability for differentiation and potential therapeutic use.

Interferon autoantibodies associated with AIRE deficiency decrease the expression of IFN-stimulated genes.

Neutralizing autoantibodies to type I, but not type II, interferons (IFNs) are found at high titers in almost every patient with autoimmune polyendocrinopathy candidiasis ectodermal dystrophy (APECED), a disease caused by AIRE gene mutations that lead to defects in thymic T-cell selection. Combining genome-wide expression array with real time RT-PCR assays, we here demonstrate that antibodies against IFN-alpha cause highly significant down-regulation of interferon-stimulated gene expression in cells from APECED patients' blood by blocking their highly dilute endogenous IFNs. This down-regulation was lost progressively as these APECED cells matured in cultures without neutralizing autoantibodies. Most interestingly, a rare APECED patient with autoantibodies to IFN-omega but not IFN-alpha showed a marked increase in expression of the same interferon-stimulated genes. We also report unexpected increases in serum CXCL10 levels in APECED. Our results argue that the breakdown of tolerance to IFNs in AIRE deficiency is associated with impaired responses to them in thymus, and highlight APECED as another autoimmune disease with associated dysregulation of IFN activity.

The calcium-sensing receptor is a target of autoantibodies in patients with autoimmune polyendocrine syndrome type 1.

CONTEXT: Autoimmune polyendocrine syndrome type 1 (APS1) is an autosomal recessive disorder caused by mutations in the autoimmune regulator gene. Hypoparathyroidism occurs in 80% of patients with APS1 and has been suggested to result from an autoimmune reaction against the calcium-sensing receptor (CaSR) on parathyroid cells. However, the detection of CaSR antibodies in APS1 remains controversial, with some studies disputing the relevance of the receptor as an autoantigen. OBJECTIVE: The aim of this study was to analyze a defined set of APS1 patient sera for the presence of CaSR antibodies using different assay systems. RESULTS: APS1 patients and individuals with other autoimmune disorders along with healthy subjects were tested for antibody binding to the CaSR. In an immunoprecipitation assay with the CaSR expressed in human embryonic kidney 293 cells, 12 of 14 (85.7%) APS1 and two of 28 (7.1%) Graves' disease patients were considered positive for CaSR antibodies. The prevalence of receptor antibodies was significantly greater than that in the cohort of healthy individuals only in the APS1 patient group (P < 0.0001). In a flow cytometry assay, seven of 14 (50.0%) APS1 patient sera showed binding to the extracellular domain of the CaSR. The prevalence of receptor antibodies in the APS1 patient group was significantly greater than that in the group of healthy controls (P = 0.023). No CaSR antibodies could be detected in any patients or controls using a radiobinding assay. CONCLUSION: The CaSR is an autoantigen in APS1, but detection of antibodies against the receptor appears to be influenced by the assay system used.

Prevalence and clinical associations of calcium-sensing receptor and NALP5 autoantibodies in Finnish APECED patients.

CONTEXT: Previous studies have identified the calcium-sensing receptor (CaSR) and NALP5 as parathyroid autoantibody targets in patients with autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED). However, although NALP5 antibodies have been associated with the occurrence of hypoparathyroidism (HP) in APECED, it is unclear whether CaSR antibodies are a specific or sensitive marker for APECED-associated HP. OBJECTIVE: The objective of the study was to identify associations between the presence of CaSR and NALP5 antibodies and the disease manifestations and demographic characteristics of Finnish APECED patients. DESIGN, SUBJECTS, AND METHODS: This was a case-control study including 44 APECED patients and 38 age- and sex-matched healthy controls. Antibodies against the CaSR and NALP5 were detected using immunoprecipitation assays and radioligand binding assays, respectively. RESULTS: CaSR and NALP5 antibodies were detected in 16 of 44 (36%) and 13 of 44 (30%) patients, respectively. No statistically significant associations were found between the presence of CaSR or NALP5 antibodies and the disease manifestations of APECED including HP (P > .05). For the diagnosis of HP, CaSR and NALP5 antibodies had specificities of 83% and 50%, respectively, and sensitivities of 39% and 26%, respectively. A significant association between both a shorter APECED and HP duration (<10 y) and positivity for CaSR antibodies was noted (P = .019 and P = .0061, respectively). CONCLUSION: Neither CaSR nor NALP5 antibodies were found to be specific or sensitive markers for HP in APECED. Further investigations are required to determine the exact role of the autoimmune response against the CaSR and NALP5 in the pathogenesis of this autoimmune syndrome.

Mapping of human autoantibody binding sites on the calcium-sensing receptor.

Previously, we have demonstrated the presence of anti-calcium-sensing receptor (CaSR) antibodies in patients with autoimmune polyglandular syndrome type 1 (APS1), a disease that is characterized in part by hypoparathyroidism involving hypocalcemia, hyperphosphatemia, and low serum levels of parathyroid hormone. The aim of this study was to define the binding domains on the CaSR of anti-CaSR antibodies found in APS1 patients and in one patient suspected of having autoimmune hypocalciuric hypercalcemia (AHH). A phage-display library of CaSR peptides was constructed and used in biopanning experiments with patient sera. Selectively enriched IgG-binding peptides were identified by DNA sequencing, and subsequently, immunoreactivity to these peptides was confirmed in ELISA. Anti-CaSR antibody binding sites were mapped to amino acid residues 41-69, 114-126, and 171-195 at the N-terminal of the extracellular domain of the receptor. The major autoepitope was localized in the 41-69 amino acid sequence of the CaSR with antibody reactivity demonstrated in 12 of 12 (100%) APS1 patients with anti-CaSR antibodies and in 1 AHH patient with anti-CaSR antibodies. Minor epitopes were located in the 114-126 and 171-195 amino acid domains, with antibody reactivity shown in 5 of 12 (42%) and 4 of 12 (33%) APS1 patients, respectively. The results indicate that epitopes for anti-CaSR antibodies in the AHH patient and in the APS1 patients who were studied are localized in the N-terminal of the extracellular domain of the receptor. The present work has demonstrated the successful use of phage-display technology in the discovery of CaSR-specific epitopes targeted by human anti-CaSR antibodies.

Chronic mucocutaneous candidiasis in APECED or thymoma patients correlates with autoimmunity to Th17-associated cytokines.

Chronic mucocutaneous candidiasis (CMC) is frequently associated with T cell immunodeficiencies. Specifically, the proinflammatory IL-17A-producing Th17 subset is implicated in protection against fungi at epithelial surfaces. In autoimmune polyendocrinopathy candidiasis ectodermal dystrophy (APECED, or autoimmune polyendocrine syndrome 1), CMC is often the first sign, but the underlying immunodeficiency is a long-standing puzzle. In contrast, the subsequent endocrine features are clearly autoimmune, resulting from defects in thymic self-tolerance induction caused by mutations in the autoimmune regulator (AIRE). We report severely reduced IL-17F and IL-22 responses to both Candida albicans antigens and polyclonal stimulation in APECED patients with CMC. Surprisingly, these reductions are strongly associated with neutralizing autoantibodies to IL-17F and IL-22, whereas responses were normal and autoantibodies infrequent in APECED patients without CMC. Our multicenter survey revealed neutralizing autoantibodies against IL-17A (41%), IL-17F (75%), and/ or IL-22 (91%) in >150 APECED patients, especially those with CMC. We independently found autoantibodies against these Th17-produced cytokines in rare thymoma patients with CMC. The autoantibodies preceded the CMC in all informative cases. We conclude that IL-22 and IL-17F are key natural defenders against CMC and that the immunodeficiency underlying CMC in both patient groups has an autoimmune basis.

Activating autoantibodies against the calcium-sensing receptor detected in two patients with autoimmune polyendocrine syndrome type 1.

CONTEXT: Autoimmune polyendocrine syndrome type 1 (APS1) is an autosomal recessive disorder caused by mutations in the autoimmune regulator (AIRE) gene. Hypoparathyroidism occurs in 80% of patients with APS1 and has been suggested to result from an autoimmune reaction against the calcium-sensing receptor (CaSR) in parathyroid cells. Anti-CaSR binding antibodies have previously been detected in patients with APS1. OBJECTIVE: The aim of this study was to determine whether anti-CaSR antibodies present in APS1 patients could modulate the response of the CaSR to stimulation by Ca(2+). RESULTS: The results indicated that two of the 14 APS1 patients included in the study had anti-CaSR antibodies that stimulated the receptor. These antibodies were detected by their ability to increase both Ca(2+)-dependent extracellular signal-regulated kinase phosphorylation and inositol phosphate accumulation in human embryonic kidney 293 cells expressing the CaSR. CONCLUSION: An important implication of the present results is that although the majority of APS1 patients do not have CaSR-stimulating antibodies, there may be a small but substantial minority of patients in whom the hypoparathyroid state is the result of functional suppression of the parathyroid glands rather than their irreversible destruction.