Remote post-ischemic conditioning promotes stroke recovery by shifting circulating monocytes to CCR2+ pro-inflammatory subset

Brain injury from stroke is typically considered an event exclusive to the central nervous system, but injury progression and repair processes are profoundly influenced by peripheral immunity. Stroke stimulates an acute inflammatory response that results in a massive infiltration of peripheral immune cells into the ischemic area. While these cells contribute to the development of brain injury, their recruitment has been considered as a key step for tissue repair. The paradoxical role of inflammatory monocytes in stroke raises the possibility that the manipulation of peripheral immune cells prior to infiltration into the brain could influence stroke outcome. One such manipulation is remote ischemic limb conditioning (RLC), which triggers an endogenous tolerance mechanism. We observed that mice subjected to post-stroke RLC shifted circulating monocytes to a CCR2+ pro-inflammatory monocyte subset and had reduced acute brain injury, swelling, and improved motor/gait function in chronic stroke. The RLC benefits were observed regardless of injury severity, with a greater shift to a CCR2+ subset in severe stroke. Adoptive transfer of CCR2-deficient monocytes abolished RLC-mediated protection. The study demonstrates the importance of RLC-induced shift of monocytes to a CCR2+ pro-inflammatory subset in attenuating acute injury and promoting functional recovery in chronic stroke. The defined immune-mediated mechanism underlying RLC-benefits allows for an evidence-based framework for the development of immune-based therapeutic strategies for stroke patients.

SIGNIFICANCE STATEMENT

Stroke is the leading cause of physical disability worldwide, but has few treatment options for patients. Because remote ischemic limb conditioning (RLC) elicits endogenous tolerance in neither an organ- nor a tissue-specific manner, the immune system has been considered a mediator for an RLC-related benefit. Application of RLC after stroke increased a pro-inflammatory CCR2+ monocyte subset in the blood and the brain. RLC reduced acute stroke injury and promoted motor/gait function during the recovery phase. The RLC-benefits were absent in mice that received CCR2-deficient monocytes. This pre-clinical study shows the importance of CCR2+ pro-inflammatory monocytes in RLC-benefits in stroke and provides a therapeutic RLC platform as a novel immune strategy to improve outcomes in stroke patients.