Thrombo-inflammation in acute ischaemic stroke — implications for treatment

Ischemic stroke leads to significant morbidity and mortality in the Western world. Early reperfusion strategies remain the treatment of choice but can initiate and augment an inflammatory response causing secondary brain damage. The understanding of postischemic inflammation is very limited. The objectives of this study were to define the temporal and spatial infiltration of immune cell populations and their activation patterns in a murine cerebral ischemia-reperfusion injury model. Transient middle cerebral artery occlusion was induced for 1 hour followed by 12-hour to 7-day reperfusion in C57/BL6 mice. Immunohistochemistry and flow cytometry were used to quantify the infiltrating immune cell subsets. Accumulation of microglia and infiltration of the ischemic hemisphere by macrophages, lymphocytes, and dendritic cells (DCs) preceded the neutrophilic influx. DCs were found to increase 20-fold and constituted a substantial proportion of infiltrating cells. DCs exhibited a significant upregulation of major histocompatibility complex II and major histocompatibility complex II high-expressing DCs were found 100 times more abundant than in sham conditions. Upregulation of the costimulatory molecule CD80 was observed in DCs and microglial cells but did not further increase in major histocompatibility complex II high-expressing DCs. No lymphocyte activation was observed. Additionally, regulatory immune cells (natural killer T-cells, CD4(-)/CD8(-)T lymphocytes) cumulated in the ischemic hemisphere. This study provides a detailed analysis of the temporal dynamics of immune cell accumulation in a rodent stroke model. The peculiar activation pattern and massive increase of antigen-presenting cells in temporal conjunction with regulatory cells might provide additional insight into poststroke immune regulation.

Under current guidelines, intravenous thrombolysis is used to treat acute stroke only if it can be ascertained that the time since the onset of symptoms was less than 4.5 hours. We sought to determine whether patients with stroke with an unknown time of onset and features suggesting recent cerebral infarction on magnetic resonance imaging (MRI) would benefit from thrombolysis with the use of intravenous alteplase.

Lymphocyte recruitment and activation have been implicated in the progression of cerebral ischemia-reperfusion (I/R) injury, but the roles of specific lymphocyte subpopulations and cytokines during stroke remain to be clarified. Here we demonstrate that the infiltration of T cells into the brain, as well as the cytokines interleukin-23 (IL-23) and IL-17, have pivotal roles in the evolution of brain infarction and accompanying neurological deficits. Blockade of T cell infiltration into the brain by the immunosuppressant FTY720 reduced I/R-induced brain damage. The expression of IL-23, which was derived mostly from infiltrated macrophages, increased on day 1 after I/R, whereas IL-17 levels were elevated after day 3, and this induction of IL-17 was dependent on IL-23. These data, together with analysis of mice genetically disrupted for IL-17 and IL-23, suggest that IL-23 functions in the immediate stage of I/R brain injury, whereas IL-17 has an important role in the delayed phase of I/R injury during which apoptotic neuronal death occurs in the penumbra. Intracellular cytokine staining revealed that gammadeltaT lymphocytes, but not CD4(+) helper T cells, were a major source of IL-17. Moreover, depletion of gammadeltaT lymphocytes ameliorated the I/R injury. We propose that T lymphocytes, including gammadeltaT lymphocytes, could be a therapeutic target for mitigating the inflammatory events that amplify the initial damage in cerebral ischemia.