title: C1D is a major autoantibody target in patients with the polymyositis-scleroderma overlap syndrome.
abstract: OBJECTIVE
  To assess whether the recently discovered exosome-associated proteins MPP6, C1D, KIAA0052/hMtr4, hSki2, and hSki8 are targeted by autoantibodies, and to determine whether these autoantibodies are accompanied by antibodies directed to the established exosome-associated autoantigens PM-Scl-75 and PM-Scl-100.
  
  
  METHODS
  Complementary DNAs encoding the recently identified human exosome-associated proteins were expressed as His-tagged fusion proteins in Escherichia coli cells. Sera obtained from patients with several different autoimmune diseases were analyzed for the presence of autoantibodies directed to these proteins, in an enzyme-linked immunosorbent assay (ELISA). The ELISA data obtained for C1D were confirmed by Western blot analysis, using recombinant C1D.
  
  
  RESULTS
  All exosome-associated proteins were found to be targeted by autoantibodies, although the frequency with which such antibodies occurred in patient sera was relatively low, with the exception of anti-C1D antibodies. Autoantibodies recognizing C1D were detected in 7 of 30 patients (23%) with the polymyositis (PM)-scleroderma overlap syndrome; this frequency was similar to the frequencies for the established autoantigens PM-Scl-75c (27%) and PM-Scl-100 (23%). Importantly, several patients with the PM-scleroderma overlap syndrome had anti-C1D antibodies but no anti-PM-Scl antibodies. Anti-C1D autoantibodies were observed in only 2 of 204 patients with other diseases, including PM, dermatomyositis, and scleroderma.
  
  
  CONCLUSIONS
  Our results demonstrate that the recently identified exosome-associated protein C1D is a major autoantigen in patients with the PM-scleroderma overlap syndrome and suggest that the use of recombinant C1D as an autoantibody target may aid in diagnosis of the PM-scleroderma overlap syndrome.
authors.affiliation: Nijmegen Center for Molecular Life Sciences, Institute for Molecules and Materials, Radboud University Nijmegen, Nijmegen, The Netherlands.
authors.forename: Geurt
authors.initials: G
authors.lastname: Schilders
authors.forename: Wilma Vree
authors.initials: WV
authors.lastname: Egberts
authors.forename: Reinout
authors.initials: R
authors.lastname: Raijmakers
authors.forename: Ger J M
authors.initials: GJ
authors.lastname: Pruijn
mesh_terms: D001323:Autoantibodies; D001324:Autoantigens / Q000276:immunology; D056970:Co-Repressor Proteins; D018076:DNA, Complementary / Q000235:genetics; D003882:Dermatomyositis / Q000235:genetics / Q000276:immunology; D006801:Humans; D017285:Polymyositis / Q000150:complications / Q000235:genetics* / Q000276:immunology; D012097:Repressor Proteins / Q000235:genetics* / Q000276:immunology; D012595:Scleroderma, Systemic / Q000150:complications / Q000235:genetics* / Q000276:immunology; D013577:Syndrome
chemical_list: D001323:Autoantibodies; D001324:Autoantigens; C117400:C1D protein, human; D056970:Co-Repressor Proteins; D018076:DNA, Complementary; D012097:Repressor Proteins