Background Systemic lupus erythematosus (SLE) is a chronic autoimmune disease characterized by heterogeneous clinical manifestations and a complex pathogenesis. The combination of defective clearance of apoptotic cells and a sustained proinflammatory environment could contribute to the chronic inflammatory state. Circulating, heterogeneous subcellular microparticles (MPs) are released from cells and platelets constitutively and upon cellular activation or apoptosis. MPs may reflect the state of their parental cells, and could serve as markers of pathology. In SLE MPs may serve as carriers of autoantigens and constituents of immune complexes.

Objectives Aim of the study was to characterize the entire population of the MPs in plasma and urine of SLE patients and healthy controls (HC) and to correlate the presence of MPs with clinical and serological parameters. In addition, we evaluated the annexinV (AnxV)-binding capabilities of MPs.

Methods Consecutive SLE patients and sex- and age-matched HC were included in the study. MPs were isolated from citrate-treated plasma and urine and characterized by flow cytometry using AnxV (a probe that binds to the exposed phosphatidilserine - PS) and antibodies to platelet (CD31⁺CD41⁺), leukocytes (CD45⁺), endothelial cells (CD31⁺CD41^-) surface markers. MPs ultra-structure in plasma and urine samples was examined by Transmission electron microscopy in 2 HC and 2 SLE patients. Data were expressed as mean ± standard deviation; Chi squared, Mann-Whitney and Spearman correlation tests were used. A p value <0.05 was considered statistically significant.

Results Twenty-eight SLE patients (25F:3M, age 41.8±8.9 yrs, disease duration 159.3±121 months) and 10 HC were studied. The concentration of total MPs was significantly lower in the SLE patients compared to HC (p=0.015); in contrast, urinary Endothelial-MPs (EMPs) were significantly increased in SLE patients (p=0.013). Fig. 1 shows plasma and urinary concentrations of AnxV+ MPs. AnxV- MPs correlated with low C3 and C4 (p=0.039 and p=0.029, respectively) and with the use of immunosoppressants; urinary AnxV+ MPs correlated with hypertension. Total MPs positively correlated with SLEDAI (p=0.011). The concentration of both plasmatic and urinary Leucocytes-MPs correlated with skin involvement (discoid lupus p=0.029, photosensivity p=0.039, alopecia p=0.030); urinary EMPs correlate with the presence of antiphospholipid antibodies (p=0.002); Platelets-MPs, both plasmatic and urinary, correlate with serositis (p=0.009, p=0.005 respectively).

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Conclusions In conclusion, this study showed altered concentrations and distributions of MP subpopulations in SLE patients as compared with HC. In addition, the results of the study characterized for the first time the population of MPs in urine of SLE patients. AnxV- MPs correlated with low complement, and Leucocytes-MPs with cutaneous manifestations. In aPL positive patients an increased concentration of urinary EMPs was demonstrated suggesting a role of aPL in endothelial damage. Finally, as PS recognition is a fundamental step in phagocytosis of apoptotic bodies and particles, and a defective clearance of apoptosis has been linked to SLE pathogenesis, a lack of PS exposure on MPs could result in impaired MPs turnover and autoimmunity.

Disclosure of Interest None declared