Microparticles (MPs) are vesicles derived from the plasma membrane of different cells, are considered a source of circulating autoantigens, and can form immune complexes (MPs-ICs). The number of MPs and MPs-ICs increases in patients with systemic lupus erythematosus (SLE). MPs activate myeloid cells by inducing IL-6 and TNF-α in both SLE and other diseases. Therefore, we propose that the recognition of MPs-ICs by monocytes rather that MPs may define their phenotype and contribute to the inflammatory process in patients with SLE. Thus, the aims of this study were to evaluate the association among circulating MPs-ICs from different cell sources, alterations observed in monocyte subsets, and disease activity in patients with SLE and to establish whether monocytes bind and respond to MPs-ICs in vitro. Circulating MPs and monocyte subsets were characterized in 60 patients with SLE and 60 healthy controls (HCs) using multiparametric flow cytometry. Patients had higher MP counts and frequencies of MPs-CD41a + (platelet-derived) compared with HCs, regardless of disease activity. MPs from patients with SLE were C1q + and formed ICs with IgM and IgG. MPs-IgG + were positively correlated with active SLE (aSLE), whereas MPs-IgM + were negatively correlated. Most of the circulating total ICs-IgG + were located on MPs. The proportion and number of non-classical monocytes were significantly decreased in patients with SLE compared with HCs and in patients with aSLE compared with patients with the inactive disease. Non-classical monocytes obtained from patients with SLE exhibited increased levels of CD64 associated with MPs-IgG +, MPs-C1q +, total circulating ICs-IgG +, and disease activity. The direct effects of MPs and MPs-IgG + on monocytes were evaluated in cell culture. Monocytes from both HCs and patients bound to and internalized MPs and MPs-IgG + independent of CD64. These vesicles derived from platelets (PMPs), mainly PMPs-IgG +, activated monocytes in vitro and increased the expression of CD69, CD64, and pro-inflammatory cytokines such as IL-1β, TNF-α, and IFN-α. Therefore, MPs are one of the most representative sources of the total amount of circulating ICs-IgG + in patients with SLE. MPs-IgG + are associated with SLE activity, and PMPs-IgG + stimulate monocytes, changing their phenotype and promoting pro-inflammatory responses related to disease activity.

BackgroundHigh mobility group box protein 1 (HMGB1) is a nuclear DNA-binding protein that can function as an alarmin when is released from activated and dying cells. In association with nucleosomes, HMGB1 may contribute to the pathogenesis of systemic lupus erythematosus (SLE). Some previous reports have associated HMGB1 with the pathogenesis of cutaneous lupus and lupus nephritis (LN). HMGB1 may also be contained in microparticles (MPs). These vesicles have a wide spectrum of biological activities in intercellular communication, and they compete with apoptotic cells to bind mononuclear phagocytes.ObjectivesTo evaluate the association of MP-HMGB1+ circulating with LN and to correlate them with LN activity.MethodsBlood samples from 60 SLE patients were used to isolated MPs from platelet-poor plasma by centrifugation and their count, cell source and phenotype were characterized by flow cytometry. Renal pathology was reported using the standardized International Society of Nephrology/Renal Pathological Society classification. Inactive lupus nephritis (LN) was defined by the presence of one or more of the following criteria: 24 hrs proteinuria 500 mg/dl or inactive urine sediments (<5 red cells/HPF) and no red cell casts and no leucocyturia (<5 white cells/HPF) and stable serum creatinine.ResultsMean age of SLE patients was 31.9±10.8 years, and mean disease duration was 7.8±6.2 years. 73% patients had LN and 89% were female. Patients with LN had significantly higher frequency of MP-HMGB1+, no significant differences were found among patients with active versus inactive LN or among patients with proliferative vs non-proliferative LN; MP-HMGB1+ had a moderate positive correlation with disease activity (SLEDAI, r=0.367, p=0.020), anti-C1q antibodies titers (r=0.42, p=001) and 24 hours proteinuria (r=0,33, p=,032), but no correlation was found with activity or chronicity indexes on renal biopsies. A ROC curve for MP-HMGB1+ and renal involvement showed a good discriminative ability (AUC 0.706). A cutoff of 15.7% of MP-HMGB1+ showed the best discrimination threshold with a sensitivity of 63.3% and specificity of 83.3%.ConclusionsIn our cohort of patients with SLE, MP-HMGB1+ was significantly higher in patients with LN and in patients with active disease. Given the multiple implication of HMGB1 in SLE, including the active kidney recruitment of mononuclear phagocytes, we consider that MP-HMGB1+ could be considered as a potential biomarker for LN in SLE patients.References Harris, H. E. et al. HMGB1: A multifunctional alarmin driving autoimmune and inflammatory disease. Nat Rev Rheumatol. 8, 195–202 (2012).Zickert, A. et al. Renal expression and serum levels of high mobility group box 1 protein in lupus nephritis. Arthritis Res Ther. 20;14(1):R36 (2012). AcknowledgementsC. Burbano is recipient of a doctoral scholarship from COLCIENCIAS (call 617–2013). The authors are grateful to the grants “Sostenibilidad and Sistema Universitario de Investigaciones, CODI (2013–05–42869836) from Universidad de Antioquia, and to COLCIENCIAS (...