Microparticles in the blood of patients with systemic lupus erythematosus (SLE): phenotypic characterization and clinical associations.

Systemic lupus erythematosus (SLE) is a prototypic autoimmune disease characterized by circulating autoantibodies and the formation of immune complexes. In these responses, the selecting self-antigens likely derive from the remains of dead and dying cells, as well as from disturbances in clearance. During cell death/activation, microparticles (MPs) can be released to the circulation. Previous MP studies in SLE have been limited in size and differ regarding numbers and phenotypes. Therefore, to characterize MPs more completely, we investigated 280 SLE patients and 280 individually matched controls. MPs were measured with flow cytometry and phenotyped according to phosphatidylserine expression (PS+/PS-), cellular origin and inflammatory markers. MPs, regardless of phenotype, are 2-10 times more abundant in SLE blood compared to controls. PS- MPs predominated in SLE, but not in controls (66% vs. 42%). Selectively in SLE, PS- MPs were more numerous in females and smokers. MP numbers decreased with declining renal function, but no clear association with disease activity was observed. The striking abundance of MPs, especially PS- MPs, suggests a generalized disturbance in SLE. MPs may be regarded as "liquid biopsies" to assess the production and clearance of dead, dying and activated cells, i.e. pivotal events for SLE pathogenesis.