4.4. Ischemic stroke
Under physiological conditions, brain barriers maintain homeostasis of the CNS by protecting the parenchyma from the constantly changing bloodstream (Mapunda, Tibar, Regragui & Engelhardt, 2022). As discussed previously, these barriers divide the CNS into different compartments that have varying levels of permeability to innate and adaptive immune cells. Another condition that alters the BBB is the ischemic stroke. During this event, an imbalance between proinflammatory and anti-inflammatory immune populations contributes to BBB disruption through several mechanisms. Activated proinflammatory T cells directly damage the BBB by producing cytokines. The secretion of IFNγ, IL-17 and IL-21 by Th1 and Th17 cells in the acute phase of ischemic stroke degrades tight junctions (TJ), leading to increased permeability and reduced integrity of the BBB. γδ T cells play a pathogenic role by producing IL-17, which decreases the expression of occludin and zonula occludens-1 (ZO-1), increasing BBB permeability. Monocytes and neutrophils are also recruited due to increased expression of CCL2 and CXCL1 in endothelial cells. (Wojkowska, Szpakowski & Glabinski, 2017). IL-17 also degrades endothelial TJ by boosting ROS generation through NADPH oxidase or xanthine oxidase (Huppert et al., 2010). However, Th17 cells produce IL-26, a BBB-protective cytokine by upregulating the expression of occludin, claudins and ZO-1 in endothelial cells. During oxygen-glucose deprivation, NK cells can increase the permeability of the brain microvascular endothelium through the production of IFNγ and reactive oxygen species. CD8+ T cells can also contribute to BBB permeability by inducing cytotoxicity, secreting granzyme B, FasL, TNFα, and IFNγ, and degrading claudin-5. After an ischemic event, Treg cells, which regulate the immune response, are decreased. However, Tregs and Th2 cells play a protective role in the ischemic hemisphere by preventing an excessive immune response that could cause secondary injury (Stubbe et al., 2013). The presence of Tregs after an acute ischemic stroke (AIS) controls the overactivity of resident microglia and infiltrated T cells. Through the upregulation of IL-10 and TGFβ, Tregs reduce the levels of proinflammatory factors such as TNFα, IFNγ and IL-1β. Furthermore, Tregs restrict the production of matrix metallopeptidase 9 in neutrophils through crosstalk of programmed death 1 (PD-1) and its ligand (PD-L1), leading to improved BBB integrity (Qiu et al., 2021).
The sequence of immune cell appearance and duration of persistence in the aftermath of the AIS has been characterized. Neutrophils are the first to swarm the tissue and reach their highest level of infiltration at 24 hours, followed by a peak in monocyte presence at 3-7 days. T cells begin to extravasate at 24 hours and reach their peak at 3 days, but remain persistent over time and can still be detected even one month after the stroke (Gill, Sivakumaran, Aravind, Tank, Dosh & Veltkamp, 2018). Double negative T cells (DNT) and CD8+ T lymphocytes can be observed within the first 24 hours with their peak at 3 days (Liesz et al., 2009), while CD4+ T cells are found at 24 hours and Tregs appear later but are more persistent over time (Chu et al., 2014; Stubbe et al., 2013). The recruitment of Treg and γδ T cells is governed by the CCL5/CCR5 and CCL6/CCR6 axis, respectively, while NK cells are dependent on the IP10/CXCR3 and CX3CL1/CX3CR1 axis, with the latter having a shorter duration over time (Gan et al., 2014). Clinical research has revealed that differentiation of T lymphocyte groups towards proinflammatory subpopulations occurs after stroke.
Following an AIS, the number of Treg cells decreases dramatically, leading to a weak immune response. This is due to a decrease in the number of CD39+ Treg cells, which results in a decline in the production of anti-inflammatory cytokines such as TGFβ and IL-10 (Ruhnau et al., 2016). Conversely, there is a rise in proinflammatory populations, including Th17 lymphocytes and γδ T cells. An increase in CD4+ CD28+ T cells has also been linked to increased tissue damage in the aftermath of an AIS (Dolati et al., 2018). Additionally, the number of T-cell immunoglobulin and mucin-domain containing-3 (TIM-3), mediated by a Th1 response, increases, along with TNFα, leading to a heightened inflammatory response (Dolati et al., 2018; Huppert et al., 2010; Zhao et al., 2011).
It is important to note that these immune responses are connected to the digestive system through the gut-brain axis. After an ischemic stroke, the gut microbiome can become imbalanced, leading to either amplification or reduction of the immune response. Therefore, controlling the gut microbiome may be a potential strategy for mitigating the impact of ischemic strokes (Li et al., 2019).