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Objective: Chronic autoimmune diseases are associated with increased risk of cardiovascular death. Endothelial dysfunction represents the first stage of subclinical atherosclerosis and multiple factors contribute to endothelial injury. Among these, an altered balance between endothelial microparticle (EMP) release and endothelial progenitor cell (EPC) generation promotes endothelial dysfunction. The role of EMPs and EPCs in promoting endothelial damage in primary SS (pSS) has never been investigated. Our aim was to evaluate the role of EMPs and EPCs as markers of endothelial damage in pSS and their correlation with disease clinical and immunological features., Methods: Circulating EMPs (CD31(+)/CD42(-)), true EPCs (CD34(+)/KDR(+)/CD133(+)) and mature EPCs (CD34(+)/KDR(+)/CD133(-)) were quantified by FACS analysis in 34 pSS patients and 18 age- and sex-matched controls. Correlation between EMP and EPC levels and parameters of disease activity and damage, clinical features and markers of immunological dysfunction was performed., Results: Patients displayed higher EMP numbers with respect to healthy controls [HCs; mean 450 n/μl (S.D. 155) vs 231 (110), P < 0.0001]. EPC and mature EPC levels were higher in patients compared with HCs [mean 226 n/ml (S.D. 181) vs 69 (53), P < 0.001 and 166 (161) vs 36 (32), P < 0.0001, respectively). EMP levels directly correlated with disease duration from symptoms and diagnosis (ρ = 0.5, P < 0.01). Early EPCs inversely correlated with disease duration from symptoms (ρ = -0.5, P < 0.01) and diagnosis (ρ = -0.4, P < 0.05)., Conclusion: This is the first demonstration of chronic endothelial fragmentation characterizing pSS. The reparative potentiality of the endothelial layer appears to be preserved in the earliest stages of disease. During the course of the disease, progressive exhaustion of the precursor endothelial pool may be hypothesized, leading to defective vascular layer restoration and endothelial dysfunction., (© The Author 2014. Published by Oxford University Press on behalf of the British Society for Rheumatology. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)