Abstract 214: Engagement of Platelet Toll-like Receptor 9 by Classical and Novel Endogenous Ligands Promotes Platelet Hyperreactivity and Thrombosis

Abstract

A prothrombotic state and increased platelet reactivity are common in pathophysiological conditions associated with oxidative stress and infections. Such conditions are associated with an appearance in circulation of non-self or altered-self ligands that are commonly recognized by toll-like receptors (TLR). Platelets express a number of TLR, including TLR9, however, the role of TLR in platelet function and thrombosis is poorly understood. In this study, we demonstrate that carboxyalkylpyrrole protein adducts (CAPs), an altered-self ligand generated in oxidative stress, promotes platelet activation, granule secretion, and aggregation in vitro and thrombosis in vivo via the TLR9/MyD88 pathway. We demonstrate that CAPs represent a novel unconventional ligand for TLR9. Furthermore, we demonstrate that unmethylated CpG sequences, a canonical non-self ligand of TLR9, induce strong platelet activation via TLR9/MyD88. Platelet activation by TLR9 ligands induces IRAK1 and AKT phosphorylation, and is Src kinase dependent. Physiological platelet agonists act synergistically with TLR9 ligands by inducing TLR9 expression on the platelet surface. Thus, our study demonstrates that platelet TLR9 links oxidative stress, innate immunity, and thrombosis by serving as a sensor of canonical non-self and unconventional altered-self ligands.