TLR2 Plays a Key Role in Platelet Hyperreactivity and Accelerated Thrombosis Associated With Hyperlipidemia

Rationale: Platelet hyperreactivity, which is common in many pathological conditions, is associated with increased atherothrombotic risk. The mechanisms leading to platelet hyperreactivity are complex and not yet fully understood. Objective: Platelet hyperreactivity and accelerated thrombosis, specifically in dyslipidemia, have been mechanistically linked to the accumulation in the circulation of a specific group of oxidized phospholipids (oxPC(CD36)) that are ligands for the platelet pattern recognition receptor CD36. In the current article, we tested whether the platelet innate immune system contributes to responses to oxPC(CD36) and accelerated thrombosis observed in hyperlipidemia. Methods and Results: Using in vitro approaches, as well as platelets from mice with genetic deletion of MyD88 (myeloid differentiation factor 88) or TLRs (Toll-like receptors), we demonstrate that TLR2 and TLR6 are required for the activation of human and murine platelets by oxPC(CD36). oxPC(CD36) induce formation of CD36/TLR2/ TLR6 complex in platelets and activate downstream signaling via TIRAP (Toll-interleukin 1 receptor domain containing adaptor protein)-MyD88-IRAK (interleukin-1 receptor-associated kinase) 1/4-TRAF6 (TNF receptor-associated factor 6), leading to integrin activation via the SFK (Src family kinase)-Syk (spleen tyrosine kinase)PLC gamma 2 (phospholipase C gamma 2) pathway. Intravital thrombosis studies using ApoE(-/-) mice with genetic deficiency of TLR2 or TLR6 have demonstrated that oxPC(CD36) contribute to accelerated thrombosis specifically in the setting of hyperlipidemia. Conclusions: Our studies reveal that TLR2 plays a key role in platelet hyperreactivity and the prothrombotic state in the setting of hyperlipidemia by sensing a wide range of endogenous lipid peroxidation ligands and activating innate immune signaling cascade in platelets.