Blocking CCR2 ⁺ Monocyte Infiltration Enhances Neurological Recovery after Subarachnoid Hemorrhage

BACKGROUND

Subarachnoid hemorrhage (SAH) patient’s mortality rate has been decreasing, but improving survivor’s prognosis outcome, which impacts patient’s quality of life, is still limited. An Early Brain Injury (EBI), which is a period of first 72 hours after SAH, is critical phase when resident and infiltrating innate immune cells induce neuroinflammation, deciding the prognosis outcome. While the roles of microglia have been studied, little is known about the role of infiltrating immune cells, especially that of monocytes. We investigated how the absence of monocyte infiltration during EBI may affect SAH outcomes.

METHODS

Using Middle cerebral artery (MCA) perforation method, we established Wild type (WT) and C-C Chemokine Receptor type 2 (CCR2) knockout transgenic mice (CCR2 ^-/- ) SAH model. Using flow cytometry, we determined differences in immune cell infiltration population. Also, through Cytometric bead array (CBA), we measured inflammatory cytokine level. We performed different behavioral experiments to assess neurological and behavioral recovery and prognosis of SAH. Furthermore, to confirm neuronal cell death severity, immunohistochemistry was performed. We also tested CCR2 antagonists in the early EBI period to see if it impacted WT mice’s neurological outcome.

RESULTS

SAH model showed increased infiltration of CD45 ^hi immune cells, mainly Ly6C ⁺ monocyte at 24 hours after SAH, which were nearly abolished in CCR2 ^-/- SAH mice. Increased IL-6 and TNF-α cytokine levels in cerebrospinal fluid was significantly reduced as well. Also, better neurological and motor recovery was observed from CCR2 ^-/- SAH mice. Overall neuronal cell death by late EBI period was significantly decreased. Finally, WT SAH mice treated with CCR2 antagonists showed improved neurological outcomes 24 hours after SAH, while showing reduced total infiltrating immune cells and monocytes.

CONCLUSION

Classical monocyte infiltration, which occurs via CCR2 signaling, is detrimental for neuroinflammation during EBI of SAH that leads to poor prognosis. CCR2 inhibition could be a potential target for interventional therapeutic strategy.

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