Remote ischemic conditioning attenuates transneuronal degeneration and promotes stroke recovery via CD36-mediated efferocytosis.
BACKGROUNDRemote ischemic limb conditioning (RIC) has been implicated in cross-organ protection in cerebrovascular disease, including stroke. However, the lack of a consensus protocol and controversy over the clinical therapeutic outcomes of RIC suggest inadequate mechanistic understanding of RIC. The current study identifies RIC-induced molecular and cellular events in the blood that enhance long-term functional recovery in experimental cerebral ischemia METHODSNaive mice or mice subjected to transient ischemic stroke were randomly selected to receive sham conditioning or RIC in the hind limb at 2 h post-stroke. At 3d post-stroke, monocyte composition in the blood was analyzed, and brain tissue was examined for monocyte-derived macrophages (M{varphi}), levels of efferocytosis, and CD36 expression. Mouse with conditional deletion of CD36 in M{varphi} (cKOMM{varphi}) was used to establish the role of CD36 in RIC-mediated modulation of efferocytosis, transneuronal degeneration, and recovery following stroke. RESULTSRIC applied 2h after stroke increased entry of monocytes into the injured brain. In the post-ischemic brain, M{varphi} had increased levels of CD36 expression and efferocytosis. These changes in brain M{varphi} were derived from RIC-induced changes in circulating monocytes. In the blood, RIC increased CD36 expression in circulating monocytes and shifted monocytes to a proinflammatory LY6CHigh state. Conditional deletion of CD36 in M{varphi} abrogated the RIC-induced monocyte shift in the blood and efferocytosis in the brain. During the recovery phase of stroke, RIC rescued the loss of the volume and of tyrosine hydroxylase+ neurons in substantia nigra (SN) as well as behavioral deficits in WT mice, but not in cKOMM{varphi} mice. CONCLUSIONSRIC induces a shift in monocytes to a proinflammatory state with elevated CD36 levels, and this is associated with CD36-dependent efferocytosis in M{varphi}s that rescues delayed transneuronal degeneration in the post-ischemic brain and promotes stroke recovery. Together, these findings provide novel insight into our mechanistic understanding of how RIC improves in post-stroke recovery. Novelty and SignificanceO_ST_ABSWhat Is Known?C_ST_ABSO_LIInfiltrated monocyte-derived macrophages (M{varphi}) into the post-ischemic brain cause neural inflammation, but they also engage in efferocytosis that promotes tissue repair in the injured CNS. C_LIO_LIRemote ischemic limb conditioning (RIC) changes monocyte composition and enhances functional recovery in experimental brain ischemia. C_LIO_LIThe application of RIC is safe, feasible, and tolerable in stroke patients, but clinical outcomes remain inconsistent. C_LI What New Information Does This Article Contribute?O_LIWe provide experimental evidence that RIC modifies peripheral monocyte composition and molecular expression, and leads to favorable changes in debris clearance, structure integrity, transneuronal degeneration, and behavior following stroke. C_LIO_LIProtective effects of RIC disappear in the absence of CD36 in M{varphi}, suggesting an essential mechanistic role for CD36 in RIC-induced endogenous protective outcomes. C_LIO_LIThe current study demonstrates that immune-mediated RIC mechanisms facilitate inflammatory and recovery processes in the injured CNS. Given the challenges in directly manipulating the brain after stroke, the study suggests that RIC is a promising alternative strategy by inducing changes in peripheral monocytes that can influence injury progression and recovery. Moreover, RIC-induced peripheral changes uncovered by this study may serve as biomarkers to establish an optimal RIC protocol. C_LI