Role of Astrocytic and Microglial Phenotype in the Biology of Hippocampal Sclerosis.

Aims: Astrocytic and microglial phenotypes (A1/A2 and M1/M2, respectively) with neuro-inflammatory and neuroprotective roles are key players in the pathogenesis of several neurological diseases. However, their role in epilepsy is underexplored. We investigated astrocytic and microglial phenotypes in the pathogenesis of epileptogenesis in hippocampal sclerosis (HS), causing drug-resistant temporal lobe epilepsy.

Methods: Astrocytic and microglial phenotypes were assessed in 10 histologically confirmed cases of surgically resected HS and four age-matched post-mortem controls by immunohistochemistry, using C3 and GBP2 (A1-astrocyte), pSTAT3 (A2-astrocyte), CD14 (M1-microglia) and CD163 and CD206 (M2-microglia), as well as Caspase 3 (apoptosis) and phosphorylated-tau and phosphorylated-neurofilament (pNF) (neuronal degeneration). These were semiquantitatively assessed for their expression in sclerotic hippocampus. Validation was carried out by immunoblotting and quantitative real-time polymerase chain reaction (RT-PCR). Results of the glial phenotypes were correlated with clinical parameters.

Results: Compared to the control group, a significant increase in pSTAT3+ A2-astrocytes was detected (range: 26.7-230.5 cells/mm²) compared to a mild increase in GBP2+ A1-astrocytes (0.4-11.4 cells/mm²). pSTAT3 expression was localised to subpial astrocytes and vascular endothelial cells, aligning with subpial gliosis and vascular sclerosis observed in HS. The findings were confirmed by RT-PCR, reflecting a predominantly reparative and neuroprotective A2 response to chronic seizure activity in HS. Among microglial phenotypes, perivascular staining with CD14+ M1 (6-23.7 vessels/mm²) and CD163+ M2-microglia (7.4-16 vessels/mm²) was significantly higher in hippocampal subfields showing neuronal loss and gliosis. Predominant inflammatory M1-microglia, along with an increase in M2-microglia, indicate an ongoing reparative response to chronic epilepsy. Surviving neurons in sclerotic Ammon's horn (AH) were labelled by pNF, indicating dysmorphic change. Several astrocytes and oligodendroglia in hippocampal AH and temporal white matter were labelled by caspase 3, probably promoting gliosis rather than apoptosis.

Conclusions: This is the first study providing a comprehensive assessment of astrocytic and microglial phenotypes in HS. These identified phenotypes can be targeted in multiple stages, offering potential modulation of epileptogenesis and contributing to the treatment of epilepsy, especially in cases marked by drug resistance.