The phenotypes of microglia and macrophages during epileptogenesis

Question: A growing body of evidence is now supporting a relationship between inflammation and epilepsy. Indeed, activated microglia, reactive astrocytes, local expression of pro-inflammatory cytokines, blood brain barrier leakage and peripheral immune cell infiltration have all been observed in temporal lobe epilepsy (TLE) animal models as well as in humans. Accordingly, inflammatory mechanisms are thought to play a central role in the initiation and maintenance of seizures, starting in the acute phase represented by status epilepticus (SE) induction. Microglia activation has been correlated with the expression of several pro-inflammatory cytokines which are thought to contribute to the neuronal cell death occurring after SE. Data point towards a pro-inflammatory phenotype of microglia that precedes neuronal injury and cell death. Because of this, microglia are generally considered to play a pro-epileptogenic role. However, infiltration of peripheral immune cells during epileptogenesis such as leukocytes, granulocytes and monocytes/macrophages might also contribute to the development of chronic epilepsy and recurrent seizures. Uncertainty on the role of these POSTER ABSTRACTS E355 GLIA different inflammatory cells depended on technical limitations in the discrimination of microglia from macrophages. For this reason, it is possible that the detrimental function that is currently attributed to microglia might be incorrect and should be ascribed to infiltrating macrophages. Methods: Both microglia and macrophages were acutely isolated from the hippocampi of control and pilocarpine-treated CD1 mice (24h and 96h after SE) and FACS sorted. Microglia were defined as CD11b+ CD45int Ly-6Cneg and infiltrated macrophages as CD11b+ CD45hi Ly-6Cpos. After sorting, qPCR and flow cytometry analysis were performed. Results: During epileptogenesis, microglia displayed a weakly immune-activated phenotype, based on the expression of MHCII, co-stimulatory molecule CD40 and pro-inflammatory gene IL-1. In contrast, infiltrated macrophages were strongly immune activated. Both cell types expressed high levels of the phagocytosis marker AXL. Conclusions: These data suggest that macrophages might be more detrimental than microglia during epileptogenesis.