Toll-like receptor expression and functional behavior in platelets from Systemic Lupus Erythematosus patients

Abstract

Abstract Background: Multiple blood cell abnormalities participate in the development of inflammation in systemic lupus erythematosus (SLE). Although platelets were pointed as one of these contributors through the release of their content during activation, there is scarce specific data about their role as immune players in SLE. Materials and Methods: Thirteen SLE patients were included. Toll-like receptor (TLR) 2, 4 and 9 were evaluated in resting platelets, while platelet-activation markers (PAC-1 binding, P-selectin, CD63 and CD40 ligand -L) and platelet-leukocyte aggregates were measured before and after specific TLR stimulation, by flow cytometry. Soluble CD40L and von Willebrand factor (vWf) release from stimulated platelets was measured by ELISA. Results: In resting conditions, TLR levels were normally expressed in SLE platelets. Platelet surface activation markers, soluble CD40L and vWf release were normal at baseline and after TLR stimulation. In contrast to normal samples behavior, platelet-monocyte aggregates in SLE were elevated in resting conditions and barely increased after TLR stimulation, while basal and stimulated platelet-neutrophil and platelet-lymphocyte aggregates in SLE samples were normal. C-reactive protein levels positively correlated with platelet-monocyte aggregates, at baseline and after stimulation with the TLR-2 agonist PAM3CSK4, suggesting these complexes could reflect the inflammatory activity in SLE. In this cohort 12 from 13 patients were under hydroxychloroquine (HCQ) treatment, a known inhibitor of endosomal activity and a potential inhibitor of platelet activation. The fact that SLE platelets responded normally to TLR agonists suggest that, in spite of this treatment, they are able to respond to increased damage-associated molecular patters (DAMPs) present in circulation from SLE patients. Interestingly, increased plasmatic levels of High Mobility Group Box 1 (HMGB1), a known DAMP, correlated with vWf release from TLR-stimulated platelets, suggesting HMGB1 could be also released by platelets, generating a positive platelet activation feedback loop that contribute to inflammation. Conclusion: This study shows normal platelet TLR expression and function, increased circulating platelet-monocyte aggregates and a direct correlation between plasmatic HMGB1 levels and platelet vWf release after TLR2 stimulation. This platelet behavior in a group of patients who are under HCQ, suggests platelets participate in the inflammatory state of SLE.