AbstractObjectiveThe influx of immune cells and serum proteins from the periphery into the brain due to a dysfunctional blood‐brain barrier (BBB) has been proposed to contribute to the pathogenesis of seizures in various forms of epilepsy and encephalitis. We evaluated the pathophysiological impact of activated peripheral blood mononuclear cells (PBMCs) and serum albumin on neuronal excitability in an in vitro brain preparation.MethodsA condition of mild endothelial activation induced by arterial perfusion of lipopolysaccharide (LPS) was induced in the whole brain preparation of guinea pigs maintained in vitro by arterial perfusion. We analyzed the effects of co‐perfusion of human recombinant serum albumin with human PBMCs activated with concanavalin A on neuronal excitability, BBB permeability (measured by FITC‐albumin extravasation), and microglial activation.ResultsBioplex analysis in supernatants of concanavalin A–stimulated PBMCs revealed increased levels of several inflammatory mediators, in particular interleukin (IL)‐1β, tumor necrosis factor (TNF)‐α, interferon (INF)‐γ, IL‐6, IL‐10, IL‐17A, and MIP3α. LPS and human albumin arterially co‐perfused with either concanavalin A–activated PBMCs or the cytokine‐enriched supernatant of activated PBMCs (1) modulated calcium‐calmodulin–dependent protein kinase II at excitatory synapses, (2) enhanced BBB permeability, (3) induced microglial activation, and (4) promoted seizure‐like events. Separate perfusions of either nonactivated PBMCs or concanavalin A–activated PBMCs without LPS/human albumin (hALB) failed to induce inflammatory and excitability changes.SignificanceActivated peripheral immune cells, such as PBMCs, and the extravasation of serum proteins in a condition of BBB impairment contribute to seizure generation.