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.

Conclusion

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.