Innate and adaptive immunity during epileptogenesis and spontaneous seizures: Evidence from experimental models and human temporal lobe epilepsy
Introduction
Recent experimental and clinical evidence highlights the activation of inflammatory pathways in epilepsy (Vezzani and Granata, 2005); in particular, the demonstration that brain inflammatory reactions occur also in epilepsy disorders that do not feature an inflammatory pathophysiology (Sheng et al., 1994, Crespel et al., 2002, Maldonado et al., 2003, Aronica et al., 2006, Ravizza et al., 2006a) raises the possibility that this phenomenon may be a common factor contributing to the etiopathogenesis of seizures in various forms of epilepsy of different etiologies. Experimental findings in experimental models report a rapid induction of cytokines in glia and neurons after acute seizures or in kindling (Eriksson et al., 1999, Vezzani et al., 1999, De Simoni et al., 2000, Plata-Salaman et al., 2000, Shinoda et al., 2003, Turrin and Rivest, 2004, Gorter et al., 2006, Ravizza and Vezzani, 2006) in forebrain areas involved in epileptic activity. The increase in interleukin (IL)-1β, IL-6 and TNF-α in microglia and astrocytes is followed by a cascade of downstream inflammatory events which may recruit cells of the adaptive immune system (Nguyen et al., 2002, Vezzani and Granata, 2005). These findings are corroborated by clinical evidence showing that both IL-1β and IL-1 receptor type I (RI) are overexpressed in lesional brain tissue of patients with epilepsy-associated malformations of cortical development and glio-neuronal tumors (Ravizza et al., 2006a); moreover, IL-1α (Sheng et al., 1994) and NFkB complex (Crespel et al., 2002) are activated in reactive astrocytes and/or in neurons in mesial TLE hippocampal specimens and in cellular components of tubers in tuberous sclerosis (Maldonado et al., 2003). Functional studies in rodents demonstrate that cytokines (for review see Jankowsky and Patterson, 2001, Vezzani and Granata, 2005) such as IL-1β and TNF-α, alter neuronal excitability and the generation of seizures and may contribute to neuronal cell loss, astrogliosis and BBB damage (Vezzani et al., 1999, De Simoni et al., 2000, Vezzani et al., 2000, Vezzani et al., 2002, Allan et al., 2005, Balosso et al., 2005, Dubé et al., 2005, Heida and Pittman, 2005, Oby and Janigro, 2006, Ravizza et al., 2006b, Vezzani and Baram, 2007) supporting the hypothesis that inflammation may play an active role in the pathophysiology of seizures and the associated neuropathology in human epilepsy. No evidence is yet available if brain inflammation occurs during epileptogenesis, a transition phase devoid of EEG and behavioral seizures, that evolves from the initial brain insult and it is instrumental for the onset of chronic epilepsy (Pitkanen and Sutula, 2002). This information is required to elucidate the consequences of inflammation on brain function, and to determine the time-window during which the effects of anti-inflammatory treatments should be probed in experimental models of epilepsy. In this study, we investigated the expression of inflammatory markers of both innate and adaptive immunity during epileptogenesis in rat models of TLE provoked by electrically- or chemically-induced SE. The persistence of inflammation in chronic epileptic tissue was also investigated in spontaneous epileptic rats and in patients with TLE and hippocampal sclerosis (HS).
Section snippets
Animals
Adult male Sprague–Dawley rats (225–250 g) were purchased from Charles River (Calco, CO, Italy). Rats were housed at constant temperature (23 °C) and relative humidity (60%), with free access to food and water and a fixed 12-h light/dark cycle. Procedures involving animals and their care were conducted in conformity with the institutional guidelines that are in compliance with national (D.L.n.116; G.U., suppl. 40, Feb. 18, 1992; UK legislation under the 1986 Animals Scientific Procedures Act)
Experimental models
Pilocarpine-injected rats were killed at different time points after SE to investigate the pattern of neuronal damage, inflammation and BBB breakdown during the acute phase of SE, epileptogenesis and the chronic phase of spontaneous seizures. Immunohistochemical analysis was done in the hippocampus, frontoparietal and entorhinal cortices, amygdala and lateral thalamic nuclei. Results are depicted for frontoparietal cortex and hippocampus since these areas are representative of changes occurring
Discussion
We report novel experimental findings showing that specific inflammatory processes initiated in the brain by a triggering event, as exemplified by SE, persist during epileptogenesis. This evidence indicates that brain inflammation is a chronic process developing after the initial precipitating insult and it can persist in epileptogenic tissue in the absence of ongoing seizure activity. Inflammatory processes are also significantly present in rat chronic epileptic tissue as well as in human
Acknowledgments
The authors are grateful to Drs Annunciata Vecchi, Nicola Marchi, Silvia Balosso and Marina Sironi for their helpful and constructive discussions at the beginning of this study and to Miss J. Marcon for her contribution to part of these experiments. This study was supported by EPICURE (LSH-CT-2006-037315, A. V.), Fondazione Mariani Onlus (R-05-46) (A. V.), Negri Weizmann Programme (A. V.), Fondazione Monzino (A. V.), National Epilepsy Fund-“Power of the Small”/Hersenstichting Nederland (NEF
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