Introduction
Aging precipitates intricate and multifaceted alterations in the immune system, collectively termed immunosenescence, which impair the body's defensive mechanisms against pathogens and amplify susceptibility to neurodegenerative conditions such as stroke. Stroke, a leading cause of disability and mortality in the elderly, is profoundly modulated by the immune system's status during the post-ischemic recovery phase. Recent investigations have illuminated the significant impact of age-related dysregulation of the peripheral immune system on post-stroke neuroinflammation and recuperative processes. This case study endeavors to elucidate the age-related modifications in peripheral immune cell dynamics and their implications on post-stroke neuropathology and recovery, employing advanced single-cell and spatial multi-omics techniques. Study Design
To meticulously explore the nexus between aging, immune alterations, and stroke recovery, an exhaustive experimental framework was implemented (Fig 1). C57BL/6 mice were utilized, with cohorts of young mice (2-3 months) and aged mice (18-20 months) representing distinct stages of immunosenescence.
Ischemic stroke was induced via the middle cerebral artery occlusion (MCAO) technique, a rigorous method that authentically replicates the pathophysiological conditions of human stroke. High-parameter flow cytometry was employed for immune cell phenotyping, enabling a meticulous characterization of immune cell subsets in peripheral blood, spleen, and lymph nodes. This sophisticated approach provided quantitative insights into age-associated shifts in immune cell populations, with a particular emphasis on lymphoid (T and B cells) and myeloid (monocytes and neutrophils) lineages. 
Key Findings
- The study unveiled substantial age-related modifications in the peripheral immune system, characterized by a pronounced reduction in T cell (CD4+ and CD8+) and B cell populations, coupled with an increase in myeloid cells (monocytes and neutrophils) in aged mice. This immunophenotypic shift indicates a diminished adaptive immune response and a predisposition towards a pro-inflammatory state in aged organisms.
- Following MCAO-induced stroke, aged mice exhibited a hyperactivated immune response, manifested by elevated levels of pro-inflammatory cytokines in peripheral blood and brain tissue. This heightened inflammatory milieu correlated with extensive cerebral infarct size, increased neuronal apoptosis, and exacerbated disruption of the blood-brain barrier.
- ScRNA-seq analysis disclosed upregulation of genes associated with inflammation and downregulation of genes involved in anti-inflammatory and reparative processes in immune cells from aged mice. Moreover, genes implicated in cellular senescence and impaired autophagy were markedly upregulated, signifying a compromised capacity to resolve inflammation and repair tissue damage.
- Histopathological examination of post-stroke brain tissue from aged mice demonstrated substantial infiltration of activated microglia and macrophages, with an elevated expression of M1-type markers (e.g., iNOS, CD86), indicative of a pro-inflammatory phenotype. Conversely, markers of M2-type macrophages (e.g., Arg1, CD206), which are associated with tissue repair and the resolution of inflammation, were significantly reduced.
- Behavioral assessments revealed that aged mice exhibited severe deficits in motor coordination, spatial memory, and sensorimotor integration post-stroke, with significantly slower and incomplete recovery compared to young mice. These functional impairments were directly correlated with the extent of neuroinflammation and neuronal loss observed histologically.
Conclusion
This study systematically revealed the age-related changes in the peripheral immune system and their significant impact on post-stroke outcomes. The findings indicate that dysfunction and excessive inflammatory responses in the peripheral immune system of aged individuals are critical factors leading to aggravated brain damage and poor neurological recovery after a stroke. The integration of high-parameter flow cytometry and scRNA-seq has facilitated an unprecedented dissection of immune cell heterogeneity and molecular dysfunctions associated with aging. These insights deepen our understanding of the mechanisms of immunosenescence and post-stroke brain injury, providing important theoretical foundations for developing new therapeutic strategies for the elderly. Future research could explore ways to modulate the immune response in elderly individuals to mitigate post-stroke inflammation and brain damage, thus improving neurological function recovery. Potential strategies could include developing new drugs or therapies targeting specific immune cells or inflammatory pathways to promote repair and regeneration processes after a stroke.
References
Lu, J., Li, H., Zhang, G., Yang, F., Zhang, X., Ping, A., Xu, Z., Gu, Y., Wang, R., Ying, D., Liu, J., Zhang, J., & Shi, L. (2023). Age-Related Alterations in Peripheral Immune Landscape with Magnified Impact on Post-Stroke Brain. Research (Washington, D.C.), 6, 0287. https://doi.org/10.34133/research.0287
