Immune-Mediated Mechanisms Underlying Conditioning-Induced Stroke Recovery

Stroke is the leading cause of physical disability worldwide and is a global socioeconomic burden to human health. Brain injury from stroke is typically considered an exclusively CNS event, but injury progression and repair processes are profoundly influenced by peripheral immunity. Ischemic Limb Conditioning [LC] has been shown to trigger an endogenous protective phenomenon that provides tolerance and protection against stroke and cardiovascular diseases. Because LC is applied remotely from the injured brain, its beneficial effects may be mediated by peripheral immune cells, but there have been only a limited number of studies into interactions between the nervous and peripheral immune systems after stroke. To address the gap in our knowledge, this proposal will investigate if and how LC alters the peripheral immune system and whether LC improves acute outcome and chronic recovery in stroke. Our preliminary findings in C57 mice show that LC shifts circulating monocytes to a pro-inflammatory [MoPro] state. These findings also show that this MoPro shift does not occur in mice deficient for CD36, an inflammatory receptor highly expressed in monocytes. Moreover, post-stroke application of LC reduces infarct size, brain swelling, and profoundly enhances motor/gait function in chronic stroke. The improved function is associated with less neuronal loss in the substantia nigra, an area where secondary transneuronal degeneration has been observed in stroke involving the striatum. Despite the established detrimental role of MoPro in promoting acute tissue damage, our observations indicate that MoPro has a beneficial role in LC-induced functional recovery. Thus, we hypothesize that the LC-induced MoPro shift enhances resolution of inflammation at the primary injury site in a CD36-dependent manner, and promotes stroke recovery by counteracting secondary degeneration. Aim 1 will use loss- and gain-of-function studies to establish the critical role of MoPro for LC-induced benefits in stroke outcome. In Aim 2, we will identify and validate CD36 as an upstream mediator of the LC-induced monocyte shift. To address in vivo mechanism of LC-enhanced functional outcome, Aim 3 will investigate whether LC counteracts secondary degeneration in the substantia nigra and thalamus after stroke. In Aim 4, we will establish the frequency and time of secondary degeneration in the substantia nigra and thalamus from databases of stroke patients with basal ganglia injury. The successful completion of this project is expected to have an important and positive impact because it will provide an evidence-based framework for the subsequent development of translational strategies to use LC in order to improve functional recovery in chronic stroke.