Deep Immunophenotyping in Aneurysmal Subarachnoid Hemorrhage: A Prospective and Controlled Exploratory Clinical Study

Introduction: Inflammatory response plays a major role in the development of post hemorrhagic complications after aneurysmal subarachnoid hemorrhage (aSAH). Monocytes and T cells play a decisive role here, but the exact mechanism of polarization and role of their subpopulations in aSAH is still unclear. Therefore, this prospective controlled study aims to investigate monocyte and T cell polarization after aSAH exploring their involvement in acute post hemorrhagic complications and clinical outcome with overarching goal to achieve better insight on the role of inflammation in clinical setting and to explore potential novel therapeutic targets.

Methods: We conducted a prospective and controlled clinical study involving 75 patients with aneurysmal subarachnoid hemorrhage (aSAH) and 20 healthy controls). Monocyte subpopulations (classically activated (M1), intermediate, alternatively activated (M2)) and T cell subpopulations (Th1, Th2, Th17, T-regulatory cells) were analyzed using multi-parametric flow cytometry of single cells on days 1, 4, 7, and 11 post-admission. Clinically relevant events such as macrovasospasm, clinical and radiological delayed cerebral ischemia (DCI) were analyzed and the clinical outcome (mRS after 6 months 0-2 vs. 3-5 vs. 6) were recorded prospectively.

Results: Compared to healthy controls, the patients with aSAH exhibited a significant decrease in anti-inflammatory alternatively activated monocytes (6.3% vs 3.0%; p=0.04) within 24 hours after bleeding. T-cell subpopulations also showed significant differences, with increased pro-inflammatory Th17 T cells (36.3% vs 6.2%; p< 0.001) and decreased anti-inflammatory Th2 cells (45.5% vs 75.2%; p< 0.001). Notably, a significant loss of anti-inflammatory alternatively activated monocytes was observed both prior to the onset of macrovasospasm (3.8% vs 2.7%; p=0.031) and prior to the occurrence of clinical or radiological DCI (4.2% vs 2.9%; p=0.006).

Conclusion : This study demonstrates a correlation between loss of alternatively activated monocytes just before occurrence of macrovasospasm and clinical or radiological delayed cerebral ischemia (DCI) providing a novel cellular point-of-care diagnostic tool. Future research should explore if rescuing alternatively activated monocytes is an effective strategy to reduce morbidity and mortality in aSAH.