Brain-immune interactions in perinatal hypoxic-ischemic brain injury

Abstract

Perinatal hypoxia-ischemia remains the primary cause of acute neonatal brain injury, leading to a high mortality rate and long-term neurological deficits, such as behavioral, social, attentional, cognitive and functional motor deficits. An ever-increasing body of evidence shows that the immune response to acute cerebral hypoxia-ischemia is a major contributor to the pathophysiology of neonatal brain injury. Hypoxia-ischemia provokes an intravascular inflammatory cascade that is further augmented by the activation of resident immune cells and the cerebral infiltration of peripheral immune cells response to cellular damages in the brain parenchyma. This prolonged and/or inappropriate neuroinflammation leads to secondary brain tissue injury. Yet, the long-term effects of immune activation, especially the adaptive immune response, on the hypoxic-ischemic brain still remain unclear. The focus of this review is to summarize recent advances in the understanding of post-hypoxic-ischemic neuroinflammation triggered by the innate and adaptive immune responses and to discuss how these mechanisms modulate the brain vulnerability to injury. A greater understanding of the reciprocal interactions between the hypoxic-ischemic brain and the immune system will open new avenues for potential immunomodulatory therapy in the treatment of neonatal brain injury.

Keywords: Adaptive immune response; Immunomodulatory therapy; Inflammatory mediators; Innate immune response; MicroRNAs; Neonatal brain injury; Neuroinflammation; Perinatal hypoxia-ischemia.

Figure 1. The pathogenesis of hypoxic-ischemic encephalopathy

Neonatal hypoxia-ischemia results in deprivation of energy substrates, oxygen and glucose, to the brain tissue, and transforms the cells to anaerobic metabolism. The reduction in ATP contributes to the failure of the energy-dependent cell membrane ion channels, which causes an acute intracellular influx of calcium and sodium and cell membrane depolarization, as well as accumulation of extracellular glutamate. This cascade consequently leads to cell swelling and necrotic cell death. If the initial insult is prolonged or severe, it may result in a secondary delayed energy failure within hours that most cell death occurs due to the apoptosis. During this phase, excitotoxicity, inflammation, and continual uptake of intracellular calcium as well as release of reactive oxygen species are observed. After the secondary phase of injury, another chronic phase of injury occurs within days and continues for months, maybe up to years. This phase includes astrogliosis, chronic inflammation, and tissue repair and remodeling, which further contribute to loss of brain cells and cerebral atrophy.

Figure 2. Early inflammatory response to neonatal hypoxia-ischemia in the developing brain

Neonatal hypoxia-ischemia leads to necrotic neuronal death that releases danger associated molecular pattern molecules (DAMPs). Resident immune effector cells (microglia and astrocytes) first sense these danger signals through pattern recognition receptors, such as TLRs and cytokine receptors, which results in inflammatory activation of microglia and astrocytes. Activated glia cells have a direct neurotoxic role in promoting the neuronal apoptosis by releasing a large amount of pro-inflammatory cytokines, nitric oxide (NO) and reactive oxygen species (ROS). In addition, synthesis and release of chemokines and matrix metalloproteinases (MMPs) by activated glia, together with DAMPs, increase blood-brain barrier (BBB) permeability, which contributes to the recruitment of peripheral inflammatory cells to injured brain, leading to further exacerbation of neuroinflammation and subsequent neuronal death.

Figure 3. Proposed miRNA pathways in modulating activation phenotypes of microglia/macrophages in the developing brain with HI injury

Following rapid activation of TLRs, NF-κB induces miR-210 expression in microglia/macrophages; however, miR-210 serves as a negative feedback regulator for the M1 activation by targeting NF-κB. Similarly, TLR-driven NF-kB activation upregulates the expression of miR-146a, which in turn down-regulates NF-kB activity by targeting two NF-kB upstream signaling transducers, TRAF6 and IRAK1. CNS enriched miRNA, miR-124, is down-regulated upon the LPS stimulation in microglia/macrophages. MiR-124 promotes the quiescent state of microglia/macrophages by direct repression of C/EBP-α and its downstream PU.1, two myeloid cell differentiation-associated transcription factors. Furthermore, miR-124 also contributes to the M2 phenotype of microglia/macrophages by increasing the production of IL-10 and TGF-β. In contrast, miR-155 expression is induced by TLR stimulation. This miRNA targets two negative regulators of M1 activation, SHIP-1 and SOCS1, thereby enhancing the M1 inflammatory responses. In addition, miR-155 is able to inhibit the M2 phenotype by targeting SMAD2, a protein involved in the TGF-β pathway, and C/EBP-β, a transcription factor important for the expression of M2 markers. Thus, dysregulation of these miRNA pathways may skew microglia/macrophage activation to classic M1 phenotype that induces secondary neuronal death. Accordingly, decreasing the levels of miR-155 or increasing the levels of miR-146a or miR-124 might switch the activation to alternative M2 phenotype, thereby protecting the neurons against apoptosis and promoting the resolution of inflammation and tissue repair in the developing brain with HI injury.