TLR7 agonist modulation of postasphyxial neurophysiological and cardiovascular adaptations in preterm fetal sheep

Induction of Tertiary Phase Epileptiform Discharges after Postasphyxial Infusion of a Toll-Like Receptor 7 Agonist in Preterm Fetal Sheep

Background: Toll-like receptor (TLR) agonists are key immunomodulatory factors that can markedly ameliorate or exacerbate hypoxic–ischemic brain injury. We recently demonstrated that central infusion of the TLR7 agonist Gardiquimod (GDQ) following asphyxia was highly neuroprotective after 3 days but not 7 days of recovery. We hypothesize that this apparent transient neuroprotection is associated with modulation of seizure-genic processes and hemodynamic control. Methods: Fetuses received sham asphyxia or asphyxia induced by umbilical cord occlusion (20.9 ± 0.5 min) and were monitored continuously for 7 days. GDQ 3.34 mg or vehicle were infused intracerebroventricularly from 1 to 4 h after asphyxia. Results: GDQ infusion was associated with sustained moderate hypertension that resolved after 72 h recovery. Electrophysiologically, GDQ infusion was associated with reduced number and burden of postasphyxial seizures in the first 18 h of recovery (p < 0.05). Subsequently, GDQ was associated with induction of slow rhythmic epileptiform discharges (EDs) from 72 to 96 h of recovery (p < 0.05 vs asphyxia + vehicle). The total burden of EDs was associated with reduced numbers of neurons in the caudate nucleus (r² = 0.61, p < 0.05) and CA1/2 hippocampal region (r² = 0.66, p < 0.05). Conclusion: These data demonstrate that TLR7 activation by GDQ modulated blood pressure and suppressed seizures in the early phase of postasphyxial recovery, with subsequent prolonged induction of epileptiform activity. Speculatively, this may reflect delayed loss of early protection or contribute to differential neuronal survival in subcortical regions.

Adverse neural effects of delayed, intermittent treatment with rEPO after asphyxia in preterm fetal sheep

KEY POINTS

We have previously shown that high-dose constant infusion of recombinant human erythropoietin (rEPO) from 30 min to 72 h after asphyxia in preterm fetal sheep reduced histological injury and improved electrophysiological recovery. This study shows that a high-dose infusion of rEPO from 6 to 72 h after asphyxia did not improve EEG recovery, oligodendrocyte and neuronal survival at 1 week post-asphyxia. Of concern, intermittent rEPO boluses started 6 h after asphyxia were associated with impaired EEG recovery and bilateral cystic injury of temporal lobe intragyral white matter. Intermittent boluses of rEPO were associated with significantly increased cerebral vascular resistance and hypoperfusion, particularly after the first dose, but did not affect seizures, suggesting mismatch between perfusion and brain activity.

ABSTRACT

Recombinant human erythropoietin (rEPO) is a promising treatment for hypoxic-ischaemic brain injury. Disappointingly, a large randomized controlled trial in preterm infants found that prophylactic, repeated high-dose rEPO boluses started within 24 h of birth did not improve neurodevelopmental outcomes. We examined whether initiation of a continuous infusion of rEPO at the end of the latent phase after hypoxic-ischaemia (HI) might improve outcomes compared with intermittent bolus injections. Chronically instrumented preterm (0.7 gestation) fetal sheep received sham asphyxia or asphyxia induced by complete umbilical cord occlusion for 25 min. Six hours after asphyxia, fetuses received either a continuous infusion of rEPO (loading dose 2000 IU, infusion at 520 IU/h) from 6 to 72 h post-asphyxia or intravenous saline or 5000 IU rEPO, with repeated doses every 48 h for 5 days. Continuous infusion of rEPO did not improve EEG recovery, oligodendrocyte and neuronal survival at 1 week post-asphyxia. By contrast, intermittent rEPO boluses were associated with impaired EEG recovery and bilateral cystic injury of temporal lobe intragyral white matter in 6/8 fetuses. These studies demonstrate for the first time that initiation of intermittent rEPO boluses 6 h after HI, at a dose comparable with recent clinical trials, exacerbated neural injury. These data reinforce the importance of early initiation of many potential neuroprotective therapies.

Cooling and immunomodulation for treating hypoxic-ischemic brain injury

Therapeutic hypothermia is now well established to partially reduce disability in term and near-term infants with moderate-severe hypoxic-ischemic encephalopathy. Preclinical and clinical studies have confirmed that current protocols for therapeutic hypothermia are near optimal. The challenge is now to identify complementary therapies that can further improve outcomes, in combination with therapeutic hypothermia. Overall, anti-excitatory and anti-apoptotic agents have shown variable or even no benefit in combination with hypothermia, suggesting overlapping mechanisms of neuroprotection. Inflammation appears to play a critical role in the pathogenesis of injury in the neonatal brain, and thus, there is potential for drugs with immunomodulatory properties that target inflammation to be used as a therapy in neonates. In this review, we examine the evidence for neuroprotection with immunomodulation after hypoxia-ischemia. For example, stem cell therapy can reduce inflammation, increase cell survival, and promote cell maturation and repair. There are also encouraging preclinical data from small animals suggesting that stem cell therapy can augment hypothermic neuroprotection. However, there is conflicting evidence, and rigorous testing in translational animal models is now needed.