Role of Perinatal Inflammation in Neonatal Arterial Ischemic Stroke

Abstract

Based on the review of the literature, perinatal inflammation often induced by infection is the only consistent independent risk factor of neonatal arterial ischemic stroke (NAIS). Preclinical studies show that acute inflammatory processes take place in placenta, cerebral arterial wall of NAIS-susceptible arteries and neonatal brain. A top research priority in NAIS is to further characterize the nature and spatiotemporal features of the inflammatory processes involved in multiple levels of the pathophysiology of NAIS, to adequately design randomized control trials using targeted anti-inflammatory vasculo- and neuroprotective agents.

Keywords: NAIS; chorioamnionitis; immunothrombosis; neuroprotection; physiopathology; risk factors; treatment; vasculitis.

Figure 1

Crosstalk between inflammation and thrombosis. C-reactive protein (CRP) and proinflammatory cytokines [interleukin (IL-β), IL-6, tumor necrosis factor (TNF)-α] are mediators implicated in the materno-fetal inflammation (–33). CRP increases tissue factor (TF) activity in vivo (34). TNF-α induces the endothelium activation: the production of TF and release of von Willebrand factor (vWF)-propeptide (35, 36). Microparticles (MPs), released from platelets, macrophages, and endothelial cells upon activation (37), trigger coagulation cascades via TF and vWF binding sites (37). Activated mononuclear cells attract and activate platelets via TF inducible expression (–38). Monocyte activation leads to TF-dependent thrombin generation and activation of coagulation (38, 39). Neutrophil extracellular traps (NET) regulate both inflammation and coagulation (40). Proinflammatory cytokines and activated monocyte/macrophages are present within the wall of NAIS-susceptible arteries (30). Glycosaminoglycan synthesis and anticoagulant activity is decreased under inflammation through TF pathway inhibitor and antithrombin interactions with their serine proteinases is impaired (37, 39, 41). Antithrombin activity is downregulated due to its consumption to counteract the mononuclear cells activation and thrombin generation (37). Thrombomodulin is downregulated upon TNF-α exposure (37, 39). IL-1β and TNF-α contribute to vasoconstriction via the upregulation of endothelin-1 (–44). Endothelin-1 increases superoxide anion production, cytokine release (45), and induces prothrombotic effect close to the endothelin-1-induced vasospasm. The ductus arteriosus closure is triggered by a vasospasm caused by the decreased plasma concentrations in PGE2 and the increased O₂ tension (46, 47). This mechanism could also happen in NAIS-susceptible fetal cerebral arteries. The thrombotic process is associated with the recruitment at the coagulation site of innate immune cells, this is the immunothrombosis. Thrombin induces the expression of proinflammatory cytokines and chemokines by the endothelial cells (37). Platelets are also involved in the trapping and clearing of bacterial agents (40, 48, 49). Activated platelets support neutrophils and monocytes recruitment (e.g., through CXCL1, CXCL4, CXCL5, CCL3, CCL5, CCL7 expression), adhesion (e.g., platelet P-selectin, CD40 ligand expression), and activation [e.g., receptor expressed on myeloid cell (TREM)-1 induced proinflammatory activity] (36, 37, 40).

Figure 2

Phases of injury occurring in neonatal arterial ischemic stroke (NAIS) and mechanistic pathways. (A) The first phase of injury in NAIS occurs between 0 and 6 h after the exposition to hypoxia–ischemia (HI) alone or infection/inflammation plus HI. This phase is characterized by different cell death types, including excitatory cell death, necrosis, and necroptosis. These primary cell deaths will induce several inflammatory cascades. Exposure to lipopolysaccharide (LPS) + HI releases DAMPs within neurons leading to an overexpression of IL-1β through inflammasome activation, which also leads to nuclear factor-κB (NFκB)-induced tumor necrosis factor (TNF)-α synthesis (73, 74). This will further result in the activation of the glial cells and the increase of the inflammation through the release of reactive oxygen species (ROS) and several inflammatory molecules by these cells. The secondary phase occurs between 24 and 72 h after NAIS and includes apoptosis, anoikis, and autophagy cell deaths. Overall, this will induce the activation of the endothelium of the brain vessels and can lead to the rupture of the blood–brain barrier (BBB) and the infiltration of leukocytes within the brain. (B) Cell death and inflammatory pathways at play within a neuron in the injured brain. Extrinsic apoptosis is induced by inflammatory molecules, such as Fas ligand (Fas-L) and TNF-α and further activation of their respective receptors FAS and TNFR-1. This leads to the recruitment of caspase-8. The activation of caspase-8 induces the recruitment of executioner caspases and subsequent cell death by apoptosis (75, 76). Activated caspase-8 negatively regulates necroptosis signaling by cleaving receptor interacting protein kinase (RIP)-1 (69, 77). In intrinsic apoptosis, caspase-8 can recruit and activate proapoptotic proteins, including Bax and Bak through the activation of t-Bid. The excess of proapoptotic protein as compared to antiapoptotic protein (Bcl2, Bcl-xL) results in an opening of the mitochondrial permeability transition pore, and the release of cytochrome-c (Cyt-c) into the cytoplasm. This leads to the apoptosome formation with the recruitment of apoptotic protease activating factor-1 (Apaf-1) and caspase-9, and the induction of the cell death by apoptosis. Necroptosis is induced by different signaling pathways, including Fas-L–Fas, TNF-α–TNFR-1, and LPS–TLR-4. This will induce the dimerization of RIP-1 and RIP-3, thus inducing the phosphorylation of RIP-3 (78). In the TLR-4-induced necroptosis, RIP-3 and MLKL are activated, but without RIP-1. Instead of RIP-1, TIR-domain-containing adapter-inducing interferon-β (TRIF) will associate with RIP-3 and subsequently induce necroptosis (79, 80). Upon RIP-3 activation, MLKL is recruited, phosphorylated, and translocated to the plasma membrane to initiate cell death through the disruption of the membrane integrity (80). Phosphorylated MLKL can also interact with the mitochondrial phosphatase PGAM5 and further induced ROS expression, and may activate dynamin-related protein 1 (Drp1) that could ultimately lead to cell death through mitochondrial fission (75, 80, 81). The release of mitochondrial ROS within the cytoplasm of the neuron, as well as the DAMPs can induce the activation of the inflammasome (82, 83). The activation of caspase-1 will induce the cleavage of the pro-IL-1β into IL-1β. The inflammation will pursue with the autocrine-paracrine loop of IL-1β activation and the activation of transcription factors, such as NFκB and P38/MAPK (83). Potential blocking agents (⊥) are as following: IL-1 blockers (e.g., IL-1Ra) (74), necrostatin-1 for RIP-1 (67), GSK’872 for RIP-3 (84), necrosulfonamide for MLKL (85), caspase inhibitors for apoptosis (–88), and apocynin for nicotinamide adenine dinucleotide phosphate (NADPH) oxidase targeting (89). Color codes: black: cell death; red: inflammation; purple: vascularization; blue: blockades.