Autoimmune response to proteins of proliferating cell nuclear antigen multiprotein complexes in patients with connective tissue diseases.OBJECTIVE:To analyze the autoimmune response to the proliferating cell nuclear antigen (PCNA) multiprotein complex in patients with connective tissue diseases (CTD).METHODS:The PCNA complex was purified by affinity chromatography using anti-PCNA monoclonal antibodies. Then 196 serum samples from patients with systemic lupus erythematosus (SLE) and 82 from patients with other CTD were tested for reactivity with the complex by immunoblotting.RESULTS:Of 196 SLE sera, 61 (31%) reacted with at least one component of the PCNA complex, and most reactive sera contained autoantibodies to several components of the complex. Autoantibodies to PCNA complex were less common in patients with other CTD, and most of their sera reacted only with one or a few proteins in the complex. Two out of 20 scleroderma sera reactive with 100, 85, and 70 kDa proteins in the PCNA complex also had autoantibodies to topoisomerase I (topo I) antibodies, which is an element of the complex. These findings suggest that the autoimmune response to the PCNA complex was specific for SLE. Anti-PCNA complex antibodies were associated with an increased serum level of PCNA detected by ELISA. The spreading of the autoimmune response to the elements of the complex was observed in parallel with the increased serum PCNA level when a series of sera from a lupus patient were tested longitudinally. In addition, anti-PCNA complex antibodies were significantly correlated with lupus erythematosus cells.CONCLUSION:The "antigen-drive" system may play a crucial role in inducing the autoimmune response to the PCNA complex in patients with SLE.