Chapter 15 Neuroinflammation
Introduction
Perinatal Brain Damage
The consequences of perinatal injury include a spectrum of disorders, such as mental retardation, cerebral palsy, epilepsy, vision and hearing loss, learning difficulties, and school failure. Periventricular white matter damage, including periventricular leukomalacia, is most frequently observed in human preterm neonates [Volpe, 2009]. Full-term neonates with perinatal encephalopathy generally develop gray matter damage that most frequently affects the neocortex, basal ganglia, and hippocampus [Volpe, 2001]. Cerebrovascular occlusion leading to perinatal stroke may be arterial or venous, but excludes global injuries due to hypoxic-ischemic injury. Neurodevelopmental disability, including cerebral palsy, epilepsy, and behavior disorders, as well as impaired vision and language, is common after perinatal stroke [Kirton and deVeber, 2009].
The pathophysiology of perinatal brain damage (see Chapters 17 and 18) has proven to be multifactorial, with sensitizing factors occurring in utero that make the brain more vulnerable to secondary insults occurring around birth, such as hypoxia-ischemia (HI), and excess release of glutamate leading to excitotoxicity [Dammann et al., 2002; Mesples et al., 2005b; Nelson and Chang, 2008; Degos et al., 2008]. Systemic inflammation linked to chorioamnionitis has been recognized as a key sensitizing factor, while CNS inflammatory responses have been shown to play a key modulatory role in the amplitude of brain damage and subsequent adverse neurological outcome.
Systemic Inflammation and Perinatal Brain Damage
Epidemiological studies have shown a strong association between fetal infection/inflammation (chorioamnionitis) and brain damage in the newborn and/or neurological handicap in survivors [Dammann and Leviton, 2007]. Experimental studies have confirmed a sensitizing effect of systemic inflammation on perinatal brain lesions induced by hypoxic-ischemic or excitotoxic insults [Dommergues et al., 2000; Eklind et al., 2005]. In addition, some experimental data also suggest that perinatal exposure to infectious/inflammatory factors can alter, in a more or less subtle manner, the programs of brain development that will result in lasting neurological deficits. The relationship between this latter observation and human diseases remains to be fully demonstrated, although clinical evidence is supportive of this hypothesis [Volpe, 2009].
Sensitizing Effect
As mentioned above, perinatal brain damage may be caused by a combination of several insults. In this so-called multiple hit hypothesis, systemic infection/inflammation linked to chorioamnionitis can act as predisposing factors, making the brain more susceptible to a second stress (sensitization process). Indeed, systemic injection of low doses of lipopolysaccharide (LPS) to developing rats makes the newborn brain significantly more susceptible to hypoxic-ischemic insult [Eklind et al., 2005]. Similarly, systemic injection of interleukin-1-beta (IL-1β) or LPS to newborn mice or rats makes the brain much more sensitive to an excitotoxic insult [Dommergues et al., 2000]. The mechanisms by which sensitization is working are not yet fully understood, but could include changes in gene transcription and modifications of glutamate receptor activity.
Disruption of Brain Programming
Systemic infection/inflammatory factors can also alter brain development by themselves, even if they do not induce major clastic lesions. Accordingly, injection of Escherichia coli to pregnant rabbits induces diffuse white matter cell death [Debillon et al., 2000], and injection of Ureaplasma parvum, a pathogen frequently observed in chorioamnionitis, to pregnant mice induces myelin defects and loss of interneurons in the offspring [Normann et al., 2009]. Similarly, injection of LPS to pregnant rats induces transient central inflammation and myelination defects in the offspring [Rousset et al., 2006]. Of major concern, exposure of newborn mice to low doses of systemic IL-1β induces a moderate and transient inflammatory response during the neonatal period, that may be sufficient to disrupt oligodendrocyte maturation, myelin formation, and axonal development [Favrais and Gressens, personal communication]. These white matter abnormalities are moderate during the developmental period but persist until adulthood. They lead to permanent deficiencies in cognitive testing without detectable effects on motor function. The relevance of these abnormal behaviors to human pathology remains to be confirmed. The underlying molecular mechanisms include alterations of the transcription of genes implicated in oligodendrogenesis, myelin formation and axonal maturation.
Blood–Brain Barrier
At birth, the blood–brain barrier (BBB) in the neonate is substantially more mature than is commonly thought. The tight junctions are present early in embryonic development [Kniesel et al., 1996], restricting entrance of proteins into the brain in a controllable fashion, and by birth the BBB is functional with no fenestrations [Engelhardt, 2003]. The presence of the barrier substantially affects leukocyte passage but does not guarantee minimal leukocyte transmigration. The use of direct inflammatory challenge, such as intrastriatal injections of IL-1β or tumor necrosis factor alpha (TNFα) in rats of different ages, does not show a linear decline of leukocyte transmigration with age [Anthony et al., 1997], but rather that the newborn CNS is more resistant to inflammatory stress than the juvenile brain. The reported magnitude of BBB disturbance following HI or focal stroke in neonatal rodents varies and depends on the aspect of barrier studied [Svedin et al., 2007; Faustino et al., 2009]. Degradation of the extracellular matrix plays a role in neonatal ischemic injury. Excessive activation of matrix metalloproteinase-9 (MMP-9) early after HI is deleterious to the immature brain, as demonstrated by smaller injury size in MMP-9 knockout mice [Svedin et al., 2007] and following pharmacological inhibition of this protease [Leonardo et al., 2008].
Crosstalk between the Periphery and the CNS
The precise molecular mechanisms by which circulating mediators of inflammation have a deleterious effect on perinatal brain lesions remain a matter for debate [Hagberg and Mallard, 2005]. Circulating cytokines do not seem to cross the intact BBB easily, although this issue is contested. Different alternative pathways have been proposed to link serum cytokines with brain damage [Malaeb and Dammann, 2009]. Several major crosstalk mechanisms are outlined in Figure 15-1. Circulating cytokines could initially alter the permeability of the BBB to inflammatory mediators and cells. They could also act directly on parts of the brain lacking the BBB, such as the circumventricular organs, meninges, and choroid plexus, or, as demonstrated in the adult brain, indirectly through activation of the vagal nerve. Cytokine effects also could be mediated by cyclo-oxygenases (Cox) located in the BBB. In particular, cytokines could activate the inducible isoform Cox-2 to enhance the local production of prostaglandin E2 (PGE2), which could have deleterious effects on the developing brain. This latter mechanism has been demonstrated in a mouse model of perinatal excitotoxic brain damage [Favrais et al., 2007]. Some of these deleterious effects could involve an autocrine/paracrine loop, leading to excess production of inflammatory cytokines by brain cells.
Glial Cells
Macrophages are seen in abundance following neonatal HI [McRae et al., 1995; Ivacko et al., 1996] and focal stroke [Dingman et al., 2006; Denker et al., 2007], producing inflammatory cytokines, high levels of nitric oxide, MMPs, and complement molecules. The early postinjury macrophage population is predominantly comprised of resident microglia rather than invading monocytes [Denker et al., 2007]. The notion that microglia contribute to, rather than limit, acute ischemic injury in the immature brain comes from findings that reduction in injury is associated with diminished microglial activation/monocyte infiltration [Arvin et al., 2002; Dommergues et al., 2003]. At the same time, several studies have shown that anti-inflammatory drugs thought to protect adult brain by reducing macrophage accumulation after stroke protect neonatal brain without directly affecting inflammatory mechanisms associated with microglial activation [Tikka et al., 2001; Fox et al., 2005; van den Tweel et al., 2005; Dingman, et al. 2006]. Distinct steps of microglial maturation and differentiation (such as expression of class II histocompatibility complex [MHC], cathepsin, and other molecules), and the propensity of neurons to undergo apoptosis in the developing brain, may account for this age-dependence of the microglial response. Complement activation – C3 and C1q deposition in particular – is deleterious after HI, whereas sensitizing of neonatal rodents with cobra venom factor or deletion of the C1q gene confers protection [Cowell et al., 2003; Ten et al., 2005]. The mechanisms of protection are not completely understood but may include decreased C3 deposition and reduction in neutrophil activation [Ten et al., 2005].
The relative contribution of proinflammatory mechanisms in astrocytes, as opposed to other roles of these cells in ischemic injury, is not well understood but astrocytes express MHC and can upregulate inducible nitric oxide synthase (iNOS) and increase cytokine production. Mast cells have been shown to play an injurious role in neonatal HI [Jin et al., 2007] and focal stroke [Biran et al., 2008]. The injurious effects of these cells are thought to depend on TGF-β and IL-9 [Mesples et al., 2005a
