Early CCR2-dependent monocyte recruitment defines a critical neuroinflammatory window after subarachnoid hemorrhage, and early but not delayed CCR2 inhibition reduces inflammatory amplification, secondary neuronal injury, and improves neurological recovery.
Key Findings
Results
CCR2 deletion or early pharmacologic CCR2 inhibition significantly reduced Ly6Chi monocyte infiltration during the early brain injury (EBI) period following experimental SAH.
A murine endovascular perforation model was used to induce SAH in CCR2-deficient mice and wild-type mice treated with a pharmacologic CCR2 antagonist.
The EBI period was defined as the first 72 hours after SAH.
Both genetic deletion of CCR2 and pharmacologic antagonism reduced infiltration of Ly6Chi monocytes into the CNS compartment during this acute phase.
Resident microglial responses have been extensively investigated previously, but CCR2-dependent infiltrating monocyte contributions during EBI were incompletely defined prior to this study.
Results
Early CCR2 inhibition attenuated CSF levels of TNFα and IL-6 following SAH.
Cerebrospinal fluid (CSF) concentrations of TNFα and IL-6 were measured as markers of neuroinflammatory amplification.
Both CCR2-deficient mice and pharmacologically treated wild-type mice showed reduced CSF TNFα and IL-6 compared to controls.
These cytokines are implicated in inflammatory amplification within the CSF and meningeal compartments during EBI.
Reduction in these cytokines was associated with attenuation of the broader neuroinflammatory response during the acute post-SAH phase.
Results
Early CCR2 inhibition reduced delayed neuronal injury within the motor cortex and improved neurological recovery after SAH.
Neuronal injury was assessed in the motor cortex as a relevant functional brain region.
Both CCR2-deficient mice and early pharmacologically treated mice showed reduced delayed neuronal injury compared to controls.
Neurological recovery was improved in animals receiving early CCR2 inhibition.
The study characterized this as 'secondary neuronal injury,' consistent with delayed pathology following the initial hemorrhagic insult.
Results
The therapeutic benefit of CCR2 inhibition was strongly time-dependent, as delayed CCR2 inhibition after the EBI period failed to improve long-term motor outcomes.
CCR2 antagonist was administered at distinct post-SAH timepoints to distinguish early versus delayed treatment effects.
Delayed pharmacologic CCR2 inhibition initiated after the 72-hour EBI window did not improve long-term motor outcomes.
This finding defines a critical therapeutic window corresponding to the EBI period for CCR2-targeted intervention.
The time-dependence of benefit was described as 'strongly time dependent' by the authors.
Results
Monocyte co-culture with human cortical neurons under hemin-induced hemorrhagic stress exacerbated neuronal injury and increased TNFα production.
Human cortical neurons were exposed to hemin-induced hemorrhagic stress as an in vitro model of SAH-related injury.
THP-1 monocytes were co-cultured with stressed neurons to assess monocyte-associated neurotoxicity.
Co-culture with monocytes worsened neuronal injury compared to neurons exposed to hemin stress alone.
Monocyte co-culture increased TNFα production in this hemorrhagic stress model.
Results
TNFα neutralization partially restored neuronal viability under severe hemorrhagic stress conditions in the monocyte co-culture model.
TNFα neutralization was applied in the context of hemin-stressed human cortical neurons co-cultured with THP-1 monocytes.
Neutralization of TNFα resulted in partial, not complete, restoration of neuronal viability.
The partial rescue effect was specifically noted under 'severe hemorrhagic stress conditions,' suggesting additional mechanisms of monocyte-associated neurotoxicity beyond TNFα.
These findings implicate TNFα as one mechanistic mediator linking monocyte recruitment to secondary neuronal injury.
What This Means
Subarachnoid hemorrhage (SAH) — bleeding around the brain — causes both immediate and delayed brain damage. This research focused on the first 72 hours after bleeding, a period called 'early brain injury,' during which the brain's immune response can spiral into harmful inflammation. The study found that a specific type of immune cell called CCR2-dependent monocytes rapidly infiltrates the brain after SAH, amplifies inflammation by boosting levels of inflammatory proteins (TNFα and IL-6) in the fluid around the brain, and contributes to damage in the motor cortex — the brain region controlling movement.
Using mice genetically engineered to lack CCR2 (a protein monocytes use to migrate into the brain) and mice treated with a drug that blocks CCR2, the researchers showed that stopping this monocyte influx early reduced inflammation and brain cell death and improved neurological recovery. Critically, when the blocking drug was given after the 72-hour early injury window had passed, it no longer helped — suggesting there is a narrow, time-sensitive opportunity for this type of treatment. Laboratory experiments using human brain cells confirmed that monocytes make hemorrhagic brain injury worse partly by releasing TNFα, and that neutralizing TNFα offered some — but not complete — protection.
This research suggests that targeting CCR2-dependent monocyte recruitment very early after subarachnoid hemorrhage could be a viable strategy to limit secondary brain damage and improve recovery. The finding that timing is critical has practical implications for how and when such treatments might need to be delivered in a clinical setting, emphasizing the importance of rapid intervention after hemorrhagic stroke.
Kang D, Seok S, Lee S, Na Y. (2026). Early CCR2-dependent monocyte recruitment defines a critical neuroinflammatory window after subarachnoid hemorrhage.. Frontiers in immunology. https://doi.org/10.3389/fimmu.2026.1888359