Objective. Systemic sclerosis (SSc) is a severe connective tissue disease of unknown etiology, characterized by fibrosis of the skin and multiple internal organs. Recent findings suggested that the disease is driven by stimulatory autoantibodies to platelet-derived growth factor receptor (PDGFR), which stimulate the production of reactive oxygen species (ROS) and collagen by fibroblasts. These results opened novel avenues of research into the diagnosis and treatment of SSc. The present study was undertaken to confirm the presence of anti-PDGFR antibodies in patients with SSc.Methods. Immunoglobulins from 37 patients with SSc were purified by protein A/G chromatography. PDGFR activation was tested using 4 different sensitive bioassays, i.e., cell proliferation, ROS production, signal transduction, and receptor phosphorylation; the latter was also tested in a separate population of 7 patients with SSc from a different research center.Results. Purified IgG samples from patients with SSc were positive when tested for antinuclear autoantibodies, but did not specifically activate PDGFR␣ or PDGFR␤ in any of the tests. Cell stimulation with PDGF itself consistently produced a strong signal.Conclusion. The present results raise questions regarding the existence of agonistic autoantibodies to PDGFR in SSc.

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We have previously shown that a 30,000 Mr protein extracted from guinea pig inner ear tissue is recognized by autoantibodies present in the serum of patients suffering from autoimmune inner ear disease. This protein was localized in the modiolus and in the organ of Corti. We have now identified this protein by a combination of microsequencing and matrix-assisted laser desorption ionization time-of-flight mass spectrometry of its tryptic peptides. A partial sequence of the protein was thereby determined. These data and 2-dimensional gel electrophoresis followed by immunoblotting experiments showed that the 30,000 Mr inner ear antigen is the major peripheral myelin protein Po. This suggests that protein Po may be an important autoantigen in autoimmune inner ear disease.

It has been shown that sera from patients with autoimmune inner ear disease contain antibodies to several inner ear antigens. We report here the characterization of the 42-43 kDa protein against which a significant number of patients' sera react strongly. After separation of inner ear proteins from guinea-pig cochleas by SDS-PAGE, the band corresponding to the 42-43 kDa protein was digested with trypsin and the peptide fragments were separated by high-performance liquid chromatography. Two fractions were then subjected to amino acid sequencing by the classical automated Edman degradation. The sequence of a stretch of 15 amino acids of the first fragment was identical to that of amino acids 148-162 of beta-actin. The sequence of the 10 amino acids of the second fragment was also identical to beta-actin. On Western blots, monoclonal antibody directed against beta-actin reacted with the inner ear 42-43 kDa proteins. The serum samples from the patients and the monoclonal antibody reacted with the non-muscle actin used as antigen in Western blotting. Immunoblot analysis of inner ear proteins after two-dimensional gel electrophoresis showed a spot, corresponding to the region of the 43 kDa as compared to the protein standards. On the basis of these data it is concluded that the target 42-43 kDa protein for antibodies in sera of patients with autoimmune inner ear disease is beta-actin, a molecule, which has important and numerous functions inside cells. This is the first report to identify the cytoskeletal protein beta-actin as a candidate autoantigen in autoimmune inner ear disease.