Inflammation and Immune Response

Intravascular Events Leading to Inflammation

During the ischemic stroke, platelets become locally activated and adhere to the cerebral endothelium, thereby promoting thrombus formation. At the same time, brain microvascular endothelial cells are rapidly converted into a proinflammatory and prothrombotic state, which leads to immediate activation of the inflammatory cascade. Stagnant blood flow and altered rheology induce shear stress on the vascular endothelium and platelets, leading to the deployment of the adhesion molecule P-selectin to the cell surface from Weibel–Palade bodies in the endothelium or from α-granules in platelets within minutes after the ischemic stimulus ( Fig. 10.1 ). P-selectin mediates the initial recruitment and rolling of leukocytes at the capillary endothelium. In experimental stroke, deficiency of P-selectin in mice or inhibition of P-selectin with monoclonal antibodies in mice and non-human primates results in decreased polymorphonuclear leukocyte accumulation, decreased evidence of “no-reflow” phenomenon, and decreased infarct volume. Whereas inhibition of platelet adhesion to the vasculature by targeting glycoproteins (GP) Ib or GPVI has been shown to decrease ischemic injury in experimental stroke, targeting GPIIb/IIIa pathways to block irreversible platelet aggregation did not alter stroke size or functional outcome in the murine transient middle cerebral artery occlusion (MCAo) model but increased the incidence of intracerebral hemorrhages and mortality. ^,

Thromboinflammation has appeared as a novel pathophysiologic concept in ischemic stroke to describe the tightly regulated interplay between thrombotic (e.g., platelets, coagulation factors) and inflammatory (e.g., immune cells, endothelium) mechanisms taking place at the neurovascular unit. ^, Plasma kallikrein (PK) is a serine protease from the plasma contact-kinin system with a proinflammatory and prothrombotic dual action. Its proinflammatory effect is related to the release of the proinflammatory peptide bradykinin (BK) upon activation. A study performed by Gob et al. showed that PK-deficient mice or wild-type (WT) mice treated with a pharmacologic PK inhibitor and challenged to models of focal cerebral ischemia were protected from ischemic brain injury. Reduced intracerebral thrombosis, improved cerebral blood flow, and strong reduction of local inflammation were identified as underlying mechanisms. Jin et al. reported that the extracellular matrix metalloproteinase (MMP) inducer CD147 drives thrombotic and inflammatory responses after MCAo in mice. Functional blockage of CD147, broadly expressed on the surface of various cell types, including leukocytes, platelets, and endothelial cells, ameliorated acute ischemic stroke by reducing thromboinflammation. Thrombin induces expression of adhesion molecules on endothelial cells through activation of protease-activated receptors, acts as a chemotaxin for leukocytes, disrupts endothelial barrier function, and activates both C3 and C5 components of the complement system. The complement system—the humoral branch of innate immunity—has been consistently implicated in the pathobiology of stroke, and its activation was associated with unfavorable outcome in stroke patients. Bioactive products of the complement cascade implicated in stroke are opsonins (iC3b, C3dg, C3d) and anaphylatoxins (C3a, C5a). While intravascularly generated complement components might gain access to the brain parenchyma through a compromised BBB, there is also evidence of increased complement synthesis by microglia. Generated anaphylatoxins act on complement receptors found on most immune cells of myeloid origin to increase ROS production, secretion of proinflammatory cytokines, degranulation, and phagocytosis. The C3a receptor (C3aR) has been most consistently implicated in stroke pathobiology and C3 deficiency or treatment with C3aR antagonist have been shown to reduce ischemic brain injury and to improve functional outcome in mice. There is evidence that the lectin pathway—an alternative complement activating pathway that is independent of complement fixing antibodies—might contribute to ischemic brain injury and mice deficient in mannose binding-lectin, the major activator of this cascade, were protected after transient MCAo. Moreover, mannose binding-lectin deficiency in humans was correlated with better stroke outcome. ^, Complement has been shown to enhance neurotoxicity in the long-term outcome after stroke. It is believed that injured neurons get decorated by naturally occurring immunoglobulin M that recognize neoantigenic epitopes such as modified annexin IV. This triggers the activation of the complement cascade and C3d deposition that enhances the phagocytosis of stressed neurons by microglia/macrophages.

Parenchymal Events Leading to Inflammation

Brain cells, including microglia, astrocytes, oligodendrocytes, and pericytes vascular cells, maintain the homeostasis of the brain’s microenvironment, essential for the proper neuronal metabolism and function. Insufficient blood flow following ischemia results in hypoxia and glucose deprivation of the brain parenchyma, leading to cellular stress. Stressed cells produce ROS and inflammatory mediators such as tumor necrosis factor (TNF) and interleukin (IL)-1β, further increasing neuronal death. As a consequence of all these processes and increased cellular stress, the injured brain releases DAMPs that activate innate immunity receptors pattern recognition receptors (PPRs) on resident immune cells and the brain vasculature further enhancing inflammation (see Fig. 10.1 ). Some important DAMP activating the endothelium are high mobility group box 1 (HMGB1), S100B proteins, IL-1α, heparin sulphate, hyaluronan, adenosine triphosphate (ATP), peroxiredoxins (Prxs), DNA, and RNA. Activation of the endothelium through PPR and cytokine receptors induces the expression of adhesion molecules. Among them, E-selectin, intercellular adhesion molecule 1 (ICAM-1), and vascular cell adhesion molecule 1 (VCAM-1) play an important role in orchestrating blood leukocyte recruitment, firm adhesion, and transmigration. The involvement of DAMP and infiltrating leukocytes in the postischemic inflammatory response are described in detail in the following section.