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

Abstract

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 Ca 2+ compared with Ca 2+ 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.

Calcium-sensing receptor autoantibody-mediated hypoparathyroidism associated with immune checkpoint inhibitor therapy: diagnosis and long-term follow-up

“…Patient serum samples post-ICI therapy were analyzed in ELISAs using CaSR peptides spanning amino acids 41-69, 75-115, 114-126, 138-170, 171-195, 214-238, 260-340, 344-358, and 374-391 as the target antigens. 13 The results indicated that the patient's CaSR antibodies were against the CaSR peptide 114-126, an epitope previously identified. 13 No other CaSR antibody-binding specificities were detected (figure 4A).…”

“…13 The results indicated that the patient's CaSR antibodies were against the CaSR peptide 114-126, an epitope previously identified. 13 No other CaSR antibody-binding specificities were detected (figure 4A). Subsequently, peptide affinity chromatography 13 was used to purify CaSR antibodies against CaSR peptide 114-126 from the patient's serum.…”

“…13 No other CaSR antibody-binding specificities were detected (figure 4A). Subsequently, peptide affinity chromatography 13 was used to purify CaSR antibodies against CaSR peptide 114-126 from the patient's serum.…”

“…The patient's serum samples were tested for: (1) Calcium-sensing receptor (CaSR) antibodies, as they can be present in patients with hypoparathyroidism. [12][13][14] (2) NACHT leucine-rich repeat protein 5 (NALP5) antibodies, as they can be indicative of hypoparathyroidism in the context of autoimmune polyendocrine syndrome type 1 (APS1). 15 (3) Cytokine antibodies to confirm or deny a diagnosis of APS1 of which hypoparathyroidism is a key manifestation.…”

A novel variant in AIRE causing a rare, non‑classical autoimmune polyendocrine syndrome type 1

“…a previous study demonstrated that AIRE variants in patients with hypoparathyroidism may induce autoantibodies to nacHT leucine-rich-repeat protein 5, which is selectively expressed in chief cells of the parathyroid glands (29). An autoimmune reaction targeting the calcium-sensing receptor was also found in parathyroid cells of patients with APS-1 (30). In addition, a loss of T-lymphocyte infiltration was observed surrounding follicles in female AIRE-knockout mice (22).…”