Absence of Functional Autoantibodies Targeting Angiotensin II Receptor Type 1 and Endothelin-1 Type A Receptor in Circulation and Purified IgG From Patients With Systemic Sclerosis

Objective: Systemic sclerosis (SSc) is a rare but severe autoimmune disease characterized by immune dysregulation, fibrosis, and vasculopathy. Although previous studies have highlighted the presence of functional autoantibodies targeting the angiotensin II receptor type 1 (AT) and endothelin-1 type A receptor (ETR), leading to autoantibody-mediated receptor stimulation and subsequent activation of endothelial cells (ECs), a comprehensive understanding of the direct interaction between these autoantibodies and their receptors is currently lacking. Moreover, existing data confirming the presence of these autoantibodies in SSc often rely on similar methodologies and assays. Our aim was to replicate previous findings and to investigate the functional effects of IgG derived from patients with SSc (SSc IgG) on AT and ETR signaling, the downstream EC response, and the presence of AT-binding autoantibodies in circulation.

Methods: Quantitative polymerase chain reaction and cytokine enzyme-linked immunosorbent assay, alongside a real-time cell analyzer, were used to assess receptor-specific functional characteristics of purified SSc IgG (n = 18). Additionally, a novel protein capture assay using solubilized epitope-tagged AT was developed to detect AT-binding autoantibodies in plasma samples from patients with SSc (n = 28) and healthy donors (n = 14).

Results: No evidence for EC activation in an AT- or ETR-dependent manner was revealed. Furthermore, stimulation with SSc IgG did not induce receptor activation or alter G protein-coupled receptor signaling on agonist stimulation in a model with receptor overexpression. Lastly, no AT-binding autoantibodies were detected in plasma samples from patients with SSc when using epitope-tagged solubilized AT.

Conclusion: Overall, our study did not provide evidence to support the presence of AT- or ETR-activating autoantibodies in purified SSc IgG or AT-binding autoantibodies in the circulation of patients with SSc.