Role of TIR8/SIGIRR in regulating TLR/IL1R dependent platelet activation

TIR8, also known as single Ig IL-1 receptor (IL-R)-related molecule, SIGIRR, is a member of the IL-1R like (ILR) family, which acts as a negative regulator of TLR/ILR signalling, playing nonredundant roles in avoiding detrimental and inappropriate inflammatory responses in different pathological conditions. Growing evidence shows that platelets express functional TLR4 and IL-1R, which are involved in neutrophils extracellular traps (NETs) production and platelet activation via P-selectin (CD62P) and active GPaIIIbII upregulation, respectively. These data prompted us to investigate the expression and function of ILR/TLR family members, and in particular of the negative regulator TIR8/SIGIRR, on platelets, to better understand their role in platelet activation in inflammatory conditions. We first investigated the expression of ILR/TLR family members and their function in response to specific ligands. Data obtained confirmed TLR4 and IL-1R expression and showed for the first time IL-18R and TIR8 expression by platelets. Functional studies showed that stimulation with LPS, IL1b and IL-18 induces upregulation of P-selectin (CD62P) and active GPaIIIbII and calcium mobilization. In addition, TIR8 expression on both platelets and megakaryocytic cell lines was downregulated upon LPS stimulation and this involved dense-like granule trafficking. Moreover, in agreement with in vitro experiments, TIR8 expression was downmodulated in patients with Systemic Inflammatory Response Syndrome (SIRS), as compared to age-matched healthy donors. Finally we used a genetic approach to investigate the role of TIR8 in platelet function. Data obtained so far suggest that the lack of TIR8 in mice is associated to platelet hyperactivity in basal conditions and upon stimulation with LPS, IL-18 and IL-1b. Together our results increase the evidence of the involvement of TLRs and IL-1R family members in platelet activation and show for the first time the role of TIR8 as key regulator of platelet activation in inflammatory conditions. In addition, the modulation of TIR8 expression by platelet in severe inflammatory conditions suggest that TIR8 might represent a novel diagnostic or therapeutic target in prothrombotic conditions associated to inflammatory diseases.