Maintenance of white matter integrity in health and disease is critical for a variety of neural functions. Ischemic stroke in the white matter frequently results in degeneration of oligodendrocytes (OLs) and myelin. Previously, we found that toll-like receptor 2 (TLR2) expressed in OLs provides cell-autonomous protective effects on ischemic OL death and demyelination in white matter stroke. Here, we identified high-mobility group box-1 (HMGB1) as an endogenous TLR2 ligand that promotes survival of OLs under ischemic stress. HMGB1 rapidly accumulated in the culture medium of OLs exposed to oxygen-glucose deprivation (OGD). This conditioned medium exhibited a protective activity against ischemic OL death that was completely abolished by immunodepletion of HMGB1. Knockdown of HMGB1 or application of glycyrrhizin, a specific HMGB1 inhibitor, aggravated OGD-induced OL death, and recombinant HMGB1 application reduced the extent of OL death in a TLR2-dependent manner. We confirmed that cytosolic translocation of HMGB1 and activation of TLR2-mediated signaling pathways occurred in a focal white matter stroke model induced by endothelin-1 injection. Animals with glycyrrhizin coinjection showed an expansion of the demyelinating lesion in a TLR2-dependent manner, accompanied by aggravation of sensorimotor behavioral deficits. These results indicate that HMGB1/ TLR2 activates an autocrine trophic signaling pathways in OLs and myelin to maintain structural and functional integrity of the white matter under ischemic conditions. white matter stroke | oligodendrocyte | toll-like receptor 2 | HMGB1 | myelination W hite matter in the vertebrate brain is characterized by the presence of myelinated axonal fibers and glial cells, predominantly myelin-producing oligodendrocytes (OLs) (1). The myelin sheath enables rapid communication of neural signals at a millisecond timescale between different parts of the cerebral cortex or different regions of the CNS (2). Therefore, maintaining the integrity of white matter in health and disease, especially of OLs and the myelin sheath, is critical to the performance of a variety of brain functions. Furthermore, recent studies have shown that myelination and OL generation in adulthood are actively regulated in response to neural activities (3, 4), highlighting the potential contribution of white matter plasticity to learning and higher cognitive functions.Ischemic stroke that occurs in the white matter often results in degeneration of OLs and demyelination (5, 6). Consequently, patients with white matter stroke suffer from a wide range of neurological dysfunctions. For example, focal ischemic stroke in the internal capsule may result in severe hemiparesis (7). Furthermore, both cognitive deficits and gait disturbance are hallmarks of Binswanger's subcortical vascular dementia that is the most severe form of white matter stroke (8). How ischemia leads to OL degeneration and demyelination is poorly understood. We previously reported that toll-like receptor 2 (TLR2) expressed in OLs plays a protect...