Identification of candidate endothelial cell autoantigens in systemic lupus erythematosus using a molecular cloning strategy: a role for ribosomal P protein P0 as an endothelial cell autoantigen

Objective. To attempt to characterize the diversity and nature of antigens recognized by anti-endothelial cell antibodies (AECA) in patients with systemic lupus erythematosus (SLE) using a molecular cloning strategy. Methods. AECA in sera of 15 SLE patients were measured by ELISA and Western blot analysis was used to examine the diversity of autoantigen targets in two clinically active patients. A human umbilical vein endothelial cell cDNA expression library was immunoscreened with sera from these two patients to identify their autoantigen targets. An anti-ribosomal P peptide antibody ELISA was used to assess the clinical significance of anti-ribosomal P protein antibodies in the sera of one patient. Results. Significantly higher AECA levels were found in five patients with active disease and nephritis than in five patients with clinically inactive disease. Sera from two clinically active patients were found to recognize distinct spectra of autoantigens. The candidate autoantigens that were identified included (1) endothelial cell-specific plasminogen activator inhibitor, (2) the classical lupus antigen, i.e. ribosomal P protein P0; and (3) proteins never before described as putative autoantigens in SLE, including ribosomal protein L6, elongation factor 1 alpha, adenyl cyclase-associated protein, DNA replication licensing factor, profilin II and the novel proteins HEAPLA 1 and HEAPLA 2 (human endothelial associated putative lupus autoantigens 1 and 2). In one patient, antibodies against ribosomal P protein P0 were predominant and levels of these antibodies correlated with total AECA levels, anti-DNA antibody titres, overall clinical score and renal disease in a longitudinal study. Conclusions. A panel of candidate endothelial autoantigens in SLE, which includes previously described autoantigens and novel targets, has been identified by a molecular cloning strategy. This novel molecular approach could also be applied to the identification of autoantigens in other autoimmune vascular diseases.