Plasma cytokines associated with febrile status epilepticus in children: A potential biomarker for acute hippocampal injury

Abstract

Objective: Our aim was to explore the association between plasma cytokines and febrile status epilepticus (FSE) in children, as well as their potential as biomarkers of acute hippocampal injury.

Methods: Analysis was performed on residual samples of children with FSE (n = 33) as part of the Consequences of Prolonged Febrile Seizures in Childhood study (FEBSTAT) and compared to children with fever (n = 17). Magnetic resonance imaging (MRI) was obtained as part of FEBSTAT within 72 h of FSE. Cytokine levels and ratios of antiinflammatory versus proinflammatory cytokines in children with and without hippocampal T2 hyperintensity were assessed as biomarkers of acute hippocampal injury after FSE.

Results: Levels of interleukin (IL)-8 and epidermal growth factor (EGF) were significantly elevated after FSE in comparison to controls. IL-1β levels trended higher and IL-1RA trended lower following FSE, but did not reach statistical significance. Children with FSE were found to have significantly lower ratios of IL-1RA/IL-1β and IL-1RA/IL-8. Specific levels of any one individual cytokine were not associated with FSE. However, lower ratios of IL-1RA/IL-1β, IL-1RA/1L-6, and IL-1RA/ IL-8 were all associated with FSE. IL-6 and IL-8 levels were significantly higher and ratios of IL-1RA/IL-6 and IL-1RA/IL-8 were significantly lower in children with T2 hippocampal hyperintensity on MRI after FSE in comparison to those without hippocampal signal abnormalities. Neither individual cytokine levels nor ratios of IL-1RA/IL-1β or IL-1RA/IL-8 were predictive of MRI changes. However, a lower ratio of IL-1RA/IL-6 was strongly predictive (odds ratio [OR] 21.5, 95% confidence interval [CI] 1.17-393) of hippocampal T2 hyperintensity after FSE.

Significance: Our data support involvement of the IL-1 cytokine system, IL-6, and IL-8 in FSE in children. The identification of the IL-1RA/IL-6 ratio as a potential biomarker of acute hippocampal injury following FSE is the most significant finding. If replicated in another study, the IL-1RA/IL-6 ratio could represent a serologic biomarker that offers rapid identification of patients at risk for ultimately developing mesial temporal lobe epilepsy (MTLE).

Keywords: Cytokine; Febrile status epilepticus; Hippocampal injury; Interleukin; Mesial temporal lobe epilepsy.

Figure 1

Plasma cytokine profiles of children within 72 hour of febrile status epilepticus (FSE) and children with fever alone (control). Plasma levels of IL-8, EGF, and IL-2R were all statistically higher in children with FSE in comparison to controls. The ratios of IL-1RA/IL-1β and IL-1RA/IL-8 were significantly lower in the FSE group. (* = statistically significant, p ≤ 0.0024).

Figure 2

Relationship between IL-1β and IL-1RA in the development of febrile status epilepticus (FSE). Patients with FSE (blue diamonds) favored and imbalance between IL-1β (higher) and IL-1RA (lower) as opposed to control patients with fever only (red boxes). As the balance shifts toward higher IL-1β and lower IL-1RA (orange) seizure threshold goes down and the likelihood of FSE increases (orange). As the balance shifts towards lower IL-1β and higher IL-1RA (purple) seizure threshold is higher and the likelihood of FSE decreases.

Figure 3

Potential role of the IL-1 system in the development of FSE and epilepsy. Precipitating events result in activation of cytokine secreting cells (orange boxes). Neurons are represented by the light blue boxes, and all other cytokine secreting cells (glia in the CNS, peripheral leukocytes, and endothelial cells of the blood brain barrier) by the light green boxes. Activated cells release IL-1β which induces a robust reciprocal production and release of IL-1RA at significantly higher concentrations of IL-1RA to IL-1β (A). Normally, IL-1RA competitively inhibits the effects of IL-1β on the on IL-1 receptor 1. As such, signal transduction pathways are minimally activated, ion channels continue to function normally, there is no excessive release of other inflammatory cytokines, and the blood brain barrier remains intact without brain hyperexcitabilty, and no seizure activity occurs (B and C). In children with abnormal IL-1 systems, activation results in a lower ratio of IL-1RA to IL-1β (D). This may occur through excessive IL-1β release, inadequate release of IL-1RA, or inadequate IL-1RA binding to the IL-1R1, all of which may be driven by underlying genetic predisposition. Increased IL-1β results in upregulation of IL-1β receptors. The process further perpetuates itself by IL-1β competing with EGF for binding to the EGF receptor, with resultant increase in free EGF. EGF increases the posttranslational production of IL-1β. The end result is excessive IL-1 β binding to the IL-1R. In neurons, excessive IL-1β activation results in neuronal hyperexcitability through 1) activation of Src-mediated phosphorylation of the NR2B subunit of the NMDA receptor resulting in rapid influx of Ca2+, 2) augmentation of Ca2+ influx through voltage-gated dependent Ca2+ channels (VGC C) 3) inhibition of K+ efflux via Ca2+ dependent K+ channels (KCaC) 4) and inhibition of Cl− influx through GABA channels (E). In other cytokine producing cells (including neurons), signal transduction pathways activated by IL-1β induce gene transcription (F) resulting in production and release of more inflammatory cytokines (IL-6 and IL-8) from cells within the CNS (may occur independent of the presence of systemic inflammation) or if in the periphery cross over the BBB at the circumventricular organs, as well as contribute to blood brain barrier breakdown themselves. These cytokines also induce intracellular signal transduction pathways regulating gene transcription. This perpetuates the inflammatory cascade within the CNS and along with their impact on ion channels, contributes to the cycle of increased excitability, lower seizure threshold, and ultimately development of status epilepticus. Long term, gene transcription produces cellular changes, which along with chronic inflammation, leads to the eventual development epilepsy.