HHV-6 viral DNA has often been detected at higher levels in resected temporal lobe tissues than in non-epileptogenic control tissues, with the frequency ranging between approximately 30% and 70%. In Leibovitch and colleague’s study, using real-time PCR, 60-70% HHV-6B positivity was observed in the mesial temporal lobe epilepsy (MTLE) resections (Leibovitch 2015).
Kawamura and colleagues recently performed a study involving 75 intractable MTLE patients, including 52 MTS patients and 23 non-MTS patients. Resected hippocampus, amygdala, and mixed samples of amygdala and uncut samples were examined by real-time polymerase chain reaction (PCR) and reverse-transcriptase PCR to detect viral DNA and mRNA, respectively. Host gene expressions, including neural markers, were measured using the TaqMan Gene Expression Assay. Kawamura found HHV-6 DNA in approximately 22.7-28.6% of the studied tissues (hippocampus, amygdala, and mixed amygdala/uncus). Interestingly, significantly greater levels of viral DNA were detected in MTS than in non-MTS patient material, and the incidence of febrile seizures was also significantly higher in the MTS group (Kawamura 2015, Leibovitch 2015).
Furthermore, a German group found HHV-6 DNA in 56% of MTLE patients with a history of encephalitis, but no in the patients with a history of complex febrile seizures (Niehusmann 2010). However, for their nested PCR study, this group used formalin-fixed, paraffin embedded (FFPE) tissues, considered by many to be a less sensitive method for the detection of HHV-6. Recent studies have shown that fresh or flash frozen tissues are far more sensitive because the beta herpesvirus DNA is degraded when the FFPE tissues are used and that the DNA also degrades significantly with the passage of time (Wang 2013).
In addition, a study from the Royal Children’s Hospital in Melbourne, Australia found that at least 50% of patients who developed HHV-6 Post-Transplant Acute Limbic Encephalitis (HHV-6 PALE) developed symptomatic generalized epilepsy (SGE) after a silent period of 11-18 months (Howell 2012). Following additional clinical confirmation by a separate group, this condition is characterized by generalized seizures and cognitive regression following HHV-6 PALE (Raspall-Chaure 2013).
Neuroinflammation is increasingly recognized as a key component underlying epileptic disease pathogenesis (Vezzani 2014). Several studies have demonstrated increased inflammatory pathway expression in HHV-6B-positive patients with MTLE:
- Kawamura observed significantly increased expression of monocyte chemoattractant protein 1 (MCP-1) and glial fibrillary acidic protein (GFP) in the HHV-6-positive versus HHV-6-negative amygdala tissues. Moreover, these expression levels positively correlated with HHV-6 viral load and markers that directly (MCP-1) or indirectly (GFAP) reflect inflammatory or otherwise injurious processes (Kawamura 2015, Leibovitch 2015).
- Using nested PCR and immunohistochemistry, neurologists from the West China Hospital examined surgical resections from patients with Mesial Temporal Lobe Epilepsy (MTLE) and found HHV-6B in the brain tissue of 28% of 32 resected brain tissues compared to only 8% of controls. The Chinese group also found that an inflammatory marker, NF-κB, was up-regulated in the glial cells of patients positive for HHV-6B. The virus was found in the subset of patients with a history of febrile seizures (Li 2011). This finding confirmed an earlier report by investigators at the NINDS that showed HHV-6B replication in the hippocampal astrocytes in two-thirds of 24 patients with mesial temporal sclerosis, but not in patients with other causes of epilepsy. At the time, this group speculated that HHV-6B might cause seizures by interfering with the astrocytes’ ability to transport glutamate (Fotheringham 2007).
- Mao and colleagues reported significantly increased interleukin 17A levels in serum samples from epileptic patients during intricate periods, compared with healthy controls. In this cohort, serum interleukin 17A levels correlated with seizure severity and frequency. Observations from these translational studies corroborate findings from animal models of seizure induction, specifically that pro-inflammatory cytokines, such as interleukin 1β, contribute to seizure duration and recurrence and blood-brain barrier damage, which may then perpetuate brain inflammation (Mao 2013, Librizzi 2012, Leibovitch 2015).
How might these observations in MTS/MTLE of increased HHV-6 viral detection and increased markers of neuroinflammation and astrocyte activation be mechanistically associated with epilepsy?
- Inflammation and HHV-6 infection have each been demonstrated to induce dysregulation of glutamate homeostasis in astrocytes, which is hypothesized to play a central role in the pathogenesis of epilepsy. Excess glutamate may be excitotoxic, and contribute to neuronal depolarization. Furthermore, glutamate receptor antagonists have demonstrated anticonvulsant properties (Tian 2005, Leibovitch 2015)
- In vitro, HHV-6 infection of primary astrocytes has been shown to down-regulate levels of glutamate transporter expression, which supports the concomitant observation of decreased glutamate uptake in infected versus uninfected astrocytes (Fotheringham 2008, Leibovitch 2015).
- Inflammatory cytokines, such as interleukin 1β, can also inhibit astrocyte reuptake of glutamate (Vezzani 2014, Leibovitch 2015).
- Because HHV-6-infected astrocytes have been demonstrated in CNS disorders, including MTLE, and because the virus can induce a metabolic dysregulation that is considered to contribute to epileptogenesis, this mechanism is biologically plausible. Importantly, the role that HHV-6 (or other viruses) may play in the pathogenesis of epilepsy suggests new clinical interventional approaches that target virus infection in the CNS (Leibovitch 2015).
- Kawamura and colleagues observed a positive correlation between MCP-1 and GFAP expression and HHV-6 viral DNA loads, suggestive of a latent infection (Kawamura 2015). If the virus persistently infects resident glial cells, disrupting astrocyte homeostasis and triggering CNS immune responses toward latent antigens, this may be one mechanism by which the seizure threshold is lowered. Resultant inflammation from seizures may then contribute to the reactivation of HHV-6 from latency in glial or other CNS resident cells (Leibovitch 2015).