Proteomic profiling of antigens in circulating immune complexes associated with each of seven autoimmune diseases.

OBJECTIVE Immune complexes (ICs) trigger humoral immune responses. Therefore, the identification of constituent antigens within ICs would have very different clinical significance than identification of free antigens. DESIGN AND METHODS Here, we applied immune complexome analysis of serum to the study of seven major autoimmune diseases-anti-neutrophil cytoplasmic antibody-associated vasculitis, Takayasu's arteritis, mixed connective tissue disease, dermatomyositis, Sjögren's syndrome, systemic scleroderma, and systemic lupus erythematosus-and healthy donors to comprehensively identify antigens incorporated into circulating ICs and to find disease-specific antigens. RESULTS We identified 468 distinct IC-associated antigens using this method. Importantly, 62 of those antigens were disease-specific antigens, and there were at least three disease-specific antigens for each of the seven autoimmune diseases. Of the disease-specific antigens identified, coiled-coil domain-containing protein 158 and spectrin were identified as potential autoantigens important to SSc and SS pathogenesis, respectively; notable titin and spectrin autoantibodies are reportedly found in SSc and SS patients, respectively. CONCLUSION Immune complexome analysis may be generally applicable to the study of the relationship between ICs and autoimmune diseases in animals and humans.