Previous studies have attempted to segregate anti-60-kd Ro/SSA (anti-Ro 60) responses in systemic lupus erythematosus (SLE) and primary Sjögren's syndrome (SS) but have shown limited disease preference. The aim of the present study was to determine whether the presence of autoantibodies against an Ro 60 apotope (an epitope expressed on apoptotic cells) distinguishes anti-Ro 60 responses in SLE and primary SS.
Multiparameter flow cytometry was used to select early apoptotic cells and measure the simultaneous binding of annexin V, propidium iodide, and anti-Ro 60-positive IgG from SLE patients (n=21) and patients with primary SS (n=19). The specificity of the Ro 60 apotope was determined by inhibition experiments with recombinant and native Ro 60.
Autoantibodies against the Ro 60 apotope were prevalent in SLE patients (13 of 21, 62%) and were rarely observed in patients with primary SS (1 of 19, 5%) (P=0.0002). Further, within SLE patients, autoantibodies to the Ro 60 apotope strongly distinguished patients with anti-Ro 60 alone (12 of 13, 92%) from those with both anti-Ro 60 and anti-La (1 of 8, 13%) (P=0.0005). When we considered all patients with anti-Ro 60 alone, the presence of autoantibodies to the Ro 60 apotope had both high sensitivity (92.3%) and high specificity (85.7%) for SLE compared with primary SS (P=0.0012). The presence of autoantibodies to the Ro 60 apotope may therefore be of diagnostic value in patients with isolated anti-Ro 60 responses.
The preferential targeting of an Ro 60 apotope exposed on early apoptotic cells in a subset of SLE patients implies disease-specific pathways for the induction of anti-Ro 60 autoimmunity.