Selective Cleavage of Nucleolar Autoantigen B23 by Granzyme B in Differentiated Vascular Smooth Muscle Cells: Insights into the Association of Specific Autoantibodies with Distinct Disease Phenotypes

Objective. To investigate the association of specific autoantibodies with distinct disease phenotypes. The association of autoantibodies to nucleophosmin/ B23 with pulmonary hypertension in scleroderma, and the susceptibility of autoantigens to cleavage by granzyme B (GB), provided a focus for these studies. Methods. Intact cells were subjected to cytotoxic lymphocyte granule-induced death, and the susceptibility of autoantigens to cleavage by GB was addressed by immunoblotting and/or by a novel immunofluorescence assay. Results. B23 was cleaved efficiently by GB in vitro, but was highly resistant to cleavage by GB during cytotoxic lymphocyte granule-mediated death of many intact cell types. In contrast, this molecule was highly susceptible to GB-mediated proteolysis exclusively in differentiated vascular smooth muscle cells. Topoisomerase I and several other GB substrates did not show this striking change in cleavage susceptibility in different cell types. Conclusion. These data demonstrate that the cleavage of B23 by GB in intact cells is dependent upon both cell type and phenotype. The susceptibility of this autoantigen (which is associated with a distinct pulmonary vascular phenotype in scleroderma) to GB-mediated proteolysis selectively in vascular smooth muscle cells suggests that the GB-cleavable conformation of autoantigens may occur selectively in the target tissue, and may play a role in shaping the phenotype-specific autoimmune response.