Filling the gaps on stroke research: Focus on inflammation and immunity

Abstract

For the last two decades, researchers have placed hopes in a new era in which a combination of reperfusion and neuroprotection would revolutionize the treatment of stroke. Nevertheless, despite the thousands of papers available in the literature showing positive results in preclinical stroke models, randomized clinical trials have failed to show efficacy. It seems clear now that the existing data obtained in preclinical research have depicted an incomplete picture of stroke pathophysiology. In order to ameliorate bench-to-bed translation, in this review we first describe the main actors on stroke inflammatory and immune responses based on the available preclinical data, highlighting the fact that the link between leukocyte infiltration, lesion volume and neurological outcome remains unclear. We then describe what is known on neuroinflammation and immune responses in stroke patients, and summarize the results of the clinical trials on immunomodulatory drugs. In order to understand the gap between clinical trials and preclinical results on stroke, we discuss in detail the experimental results that served as the basis for the summarized clinical trials on immunomodulatory drugs, focusing on (i) experimental stroke models, (ii) the timing and selection of outcome measuring, (iii) alternative entry routes for leukocytes into the ischemic region, and (iv) factors affecting stroke outcome such as gender differences, ageing, comorbidities like hypertension and diabetes, obesity, tobacco, alcohol consumption and previous infections like Covid-19. We can do better for stroke treatment, especially when targeting inflammation following stroke. We need to re-think the design of stroke experimental setups, notably by (i) using clinically relevant models of stroke, (ii) including both radiological and neurological outcomes, (iii) performing long-term follow-up studies, (iv) conducting large-scale preclinical stroke trials, and (v) including stroke comorbidities in preclinical research.

Keywords: Clinical trials; Experimental models; Immune response; Inflammation; Ischemic stroke; Translational research.

Fig. 1

Inflammatory/immune responses after ischemic stroke, and targets of the immunomodulatory drugs tested on clinical trials. In the healthy brain, three main barriers protect the parenchyma from external pathogens: the blood–brain barrier (BBB) around the cerebral vessels, the blood-meningeal barrier in the meninges, and the blood-CSF barrier of the choroid plexus. Immune cells circulate freely in the blood, and a few lymphocytes patrol the CSF to do immunosurveillance. In the brain parenchyma resting microglia survey the environment with their processes. After stroke, microglia switches from a resting form to an activated state, adopting a phagocytic phenotype and secreting pro-inflammatory factors. The BBB is disrupted, local ECs are activated and express CAMs. The tight junctions between ECs disappear. This allows leukocyte rolling and adhesion at the luminal side of the blood vessel and then transmigration from the vascular compartment to the brain parenchyma. Leukocytes can also invade the brain through blood-meningeal and blood-CSF barriers. Once infiltrated in the tissue, neutrophils secrete pro-inflammatory factors that will recruit monocytes/macrophages, and later lymphocytes to the parenchyma. Immunomodulatory drugs tested on clinical trials and discussed in this review include (i) Anakinra, an antagonist of the proinflammatory cytokine interleukin-1, (ii) Natalizumab, which acts by blocking the binding of integrin α4 to the adhesion molecule VCAM to reduce leukocyte infiltration. (iii) Enlimomab is an antibody targeting the adhesion molecule ICAM. (iv) Minocycline inhibits microglial activation among other anti-inflammatory properties. (v) Fingolimod is a high-affinity agonist for several of the sphingosine-1-phosphate receptors that prevents the egress of lymphocytes from lymph nodes, thus limiting the infiltration of lymphocytes to the brain. BBB, blood–brain barrier; CAM, cellular adhesion molecule; CSF: cerebrospinal fluid; EC, endothelial cell.

Fig. 2

Comparison of human stroke and experimental models of stroke in rodents. a) Schematic view of the human brain vasculature with one of the most frequent stroke subtypes, a thrombotic/embolic occlusion of the M2 segment of the middle cerebral artery (MCA), usually included in randomized clinical trials (RCT) on immunomodulatory/anti-inflammatory drugs for stroke treatment. b) Schematic view of the rodent brain vasculature (lateral view). In thromboembolic and electrocoagulation experimental stroke models, there is only one site of MCA occlusion, whereas in regional photothrombotic stroke there are multiple occlusion sites within the area illuminated by the laser. In both cases, lesions are limited to the brain cortex. The occlusion site in the monofilament experimental model is located at the origin of the MCA, leading to a bigger ischemic volume including both cortical and subcortical brain regions. c) Schematic view of the rodent brain vasculature (dorsal view). d) Schematic view of the rodent brain vasculature (ventral view). e) Comparison of ischemic lesions (delimitated area) visualized by MRI in human stroke (M1-M2 occlusion) and different experimental models of stroke.