Subcellular Redistribution and Surface Exposure of the Ro52, Ro60 and La48 Autoantigens During Apoptosis in Human Ductal Epithelial Cells: a Possible Mechanism in the Pathogenesis of Sjögren's Syndrome

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Abstract
The Ro52, Ro60 and La48 autoantigens are associated with Sjögren's syndrome (SS) and systemic lupus erythematosus (SLE). The mechanisms behind tolerance breakdown of these self‐peptides remain unclear; however, apoptosis has been proposed to cause their presentation to the immune system. We have examined the localization of transiently expressed enhanced green fluorescent protein (EGFP)‐tagged Ro52, Ro60 and La48 autoantigens in a human salivary gland (HSG) cell line by laser confocal microscopy under normal growth conditions and during apoptosis. Surface exposure of Ro52, Ro60 and La48 was demonstrated on nonfixed apoptotic cells with monoclonal antibodies (MoAbs) or with primary SS patient antisera. Laser scanning cytometry determined the apoptotic frequency. EGFP alone was studied as control. We found that Ro52 mainly is cytoplasmic, Ro60 both nuclear and cytoplasmic, while La48 only resides in the nucleus under normal conditions. During early apoptosis, La48 is dramatically redistributed to the cytoplasm, while the localization of Ro52 and Ro60 is maintained. All three autoantigens filled apoptotic blebs and covered TUNEL (terminal‐deoxynucleotidyl‐transferase‐mediated dUTP–digoxigenin nick end labelling)‐positive apoptotic bodies. Identical results were obtained in COS‐7 cells. We have developed a transfection system to study the intracellular localization of the three autoantigens Ro52, Ro60 and La48, without antibody detection. During apoptosis, there is an intracellular redistribution of endogenous and EGFP‐tagged Ro52, Ro60 and La48, leading to surface exposure. These findings may indicate a role for apoptosis in the induction and facilitation of humoral responses to Ro52, Ro60 and La48 in the autoimmune exocrinopathy of SS.