Transduction of inflammation from peripheral immune cells to the hippocampus induces neuronal hyperexcitability mediated by Caspase-1 activation

Abstract

Background Recent studies report infiltration of peripheral blood mononuclear cells (PBMCs) into the central nervous system (CNS) in epileptic disorders, suggestive of a potential contribution of PBMC extravasation to the generation of seizures. Nevertheless, the underlying mechanisms involved in PBMC infiltrates promoting neuronal predisposition to ictogenesis remain unclear. Therefore, we developed an in vitro model mimicking PBMC infiltration into the brain in order to investigate potential transduction of inflammatory signals from PBMCs to the CNS.MethodsTo establish our model, we first extracted PBMCs from rat spleen, then, we immunologically primed PBMCs with lipopolysaccharide (LPS), followed by immunological activation with Nigericin. Thereafter, we cultured PBMCs on top of organotypic cortico-hippocampal brain slice cultures (OCHSCs) derived from the same rat, and compared PBMC-OCHSC co-cultures to OCHSCs exposed to PBMCs in the culture media. Also, we targeted a potential molecular pathway underlying transduction of peripheral inflammation to OCHSCs by incubating OCHSCs with the Caspase-1 inhibitor VX-765 prior to co-culturing PBMCs with OCHSCs. After 24 hours, we immunohistochemically analyzed inflammation markers in the cortex and the hippocampus. In addition, we performed whole-cell patch-clamp recordings in cortical layer II/III and hippocampal CA1 pyramidal neurons.ResultsIn the cortex, co-culturing immunoreactive PBMCs treated with LPS + Nigericin on top of OCHSCs induced ectopic expression of inflammation markers and enhanced neuronal excitation. In contrast, no excitability changes were detected after adding primed PBMCs, i.e. treated with LPS only, to OCHSCs. Strikingly, in the hippocampus, both immunoreactive and primed PBMCs elicited similar pro-inflammatory and pro-excitatory effects. However, when immunoreactive and primed PBMCs were cultured in the media separately from OCHSCs, only immunoreactive PBMCs gave rise to neuroinflammation and hyperexcitability in the hippocampus, whereas primed PBMCs failed to produce any significant changes. Finally, VX-765 application to OCHSCs, co-cultured with either immunoreactive or primed PBMCs, protected them from neuroinflammation and hippocampal hyperexcitability.ConclusionsOur study shows a higher susceptibility of the hippocampus to peripheral inflammation as compared to the cortex, mediated via Caspase-1-dependent signaling pathways. Thus, our findings suggest that Caspase-1 inhibition may potentially provide therapeutic benefits during hippocampal neuroinflammation and hyperexcitability secondary to peripheral innate immunity.