Autoantibodies in primary Sjögren's syndrome are directed against proteasomal subunits of the α and β type

Objective The proteasome subunit HC9 (α3) has recently been identified as a major target of the humoral autoimmune response in patients with autoimmune myositis and systemic lupus erythematosus. Since B cell hyperreactivity is a common feature of systemic autoimmune diseases, patients with primary Sjögren's syndrome (SS) and other control groups were investigated to evaluate the significance of autoantibodies against the proteasome. Methods Analyses of autoantibodies directed against the 20S proteasome were performed using enzyme‐linked immunosorbent assay, immunoblot, and 2‐dimensional electrophoresis. Forty‐three patients with primary SS, 47 patients with rheumatoid arthritis including 9 with secondary SS, 19 patients with gastrointestinal tumors, and 80 healthy controls were tested for antiproteasome antibodies. Results Antiproteasome antibodies were detected in 39% of patients (17 of 43) with primary SS. In contrast, only 1 of 47 patients with rheumatoid arthritis showed positive reactivity (P < 0.001). Serum samples from 19 tumor patients (P < 0.003) and 80 healthy controls (P < 0.001) were serologically negative. Moreover, immunoblotting and 2‐dimensional analysis of the antiproteasome response revealed a polyspecific recognition pattern in 7 patients with primary SS. Different proteasomal subunits of the α and β type, including subunits that carried the proteolytic active sites, were recognized by the patients' sera. Conclusion The humoral antiproteasome response in primary SS, in contrast to its secondary form, is characterized by an extensive recognition pattern of several subunits, indicating a polyspecific B cell activation against the 20S proteasome. Moreover, proteolytically active β‐type subunits, which are important for the generation of major histocompatibility complex class I–restricted antigens, appear to be targets of the autoimmune response. The data indicate that the proteasome itself may stand on a cross point of pathways that links mechanisms of the immune defense with features of systemic autoimmunity.