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.