Chronic encephalitis associated with epilepsy: immunohistochemical and ultrastructural studies

Progress in pathogenesis and therapy of Rasmussen's encephalitis

Rasmussen's encephalitis (RE) is a rare condition of unknown etiology that causes a severe chronically neurological disorder with mostly affecting children. The main clinical feature of RE includes frequent seizures with drug-resistant, unilateral hemispheric atrophy, and progressive neurological deficits. In this review, we summarized five pathogenesis on the basis of the current research including virus infection, antibody-mediated degeneration, cell-mediated immunity, microglia-induced degeneration, and genetic mutations. So far, no exact virus in RE brain tissue or definite antigen in humoral immune system was confirmed as the determined etiology. The importance of cytotoxic CD8⁺ T lymphocytes and activated microglial and the role of their immune mechanism in RE development are gradually emerging with the deep study. Genetic researches support the notion that the pathogenesis of RE is probably associated with single nucleotide polymorphisms on immune-related genes, which is driven by affecting inherent antiretroviral innate immunity. Recent advances in treatment suggest immunotherapy could partially slows down the progression of RE according to the histopathology and clinical presentation, which aimed at the initial damage to the brain by T cells and microglia in the early stage. However, the cerebral hemispherectomy is an effective means to controlling the intractable seizure, which is accompanied by neurological complications inevitably. So, the optimal timing for surgical intervention is still a challenge for RE patient. On the contrary, exploration on other aspects of pathogenesis such as dysfunction of adenosine system may offer a new therapeutic option for the treatment of RE in future.

Rasmussen encephalitis: Predisposing factors and their potential role in unilaterality

OBJECTIVE

Rasmussen encephalitis (RE) is a progressive and destructive inflammatory disease of one hemisphere. Its cause is unknown. We investigated comorbidity and laterality factors that might predispose to RE.

METHODS

We retrospectively compared the histories of 160 RE patients to those with genetic generalized epilepsy (n = 154) and those with focal cortical dysplasia Type II (FCD II; n = 148).

RESULTS

The median/mean age at symptom onset in RE was 7/10 years (range = 1-53 years), and 58.1% of the patients were female. The female sex predominated in RE patients, with age > 7 years at disease manifestation. The left hemisphere was affected in 65.6%. Perinatal complications (preterm birth, twin pregnancies, early acquired brain lesions) were more frequent in RE than in control patients. Ipsilateral facial autoimmune conditions (scleroderma en coup de sabre, uveitis, or chorioretinitis) were only observed in RE patients (6.9%). Onset of RE was more frequently associated with fever than that of FCD II. In 33.1% of RE patients, ≥1 potential risk factor was found. Interestingly, 11.9% of patients had one-sided early brain lesions or facial autoimmune lesions ipsilateral to subsequent RE; none had such a lesion contralaterally.

SIGNIFICANCE

Perinatal complications and facial autoimmune conditions may act as predisposing factors for RE. Fever might trigger RE manifestation. Further genetic or infectious contributors may be identified in the future. Single or combined hits may be required to elicit or facilitate the start of the disease. Ipsilateral early comorbid lesions or facial autoimmune processes might in part explain the enigmatic unilaterality of RE.

Genetic Factors in Rasmussen's Encephalitis Characterized by Whole-Exome Sequencing

Rasmussen’s encephalitis (RE) is a rare chronic neurological disorder characterized by unihemispheric brain atrophy and epileptic seizures. The mechanisms of RE are complex. Adaptive immunity, innate immunity and viral infection are all involved in the development of RE. However, there are few studies on the role of genetic factors in the mechanisms of RE. Thus, the objective of this study was to reveal the genetic factors in the mechanisms of RE. Whole-exome sequencing (WES) was performed in 15 RE patients. Ten patients with temporal lobe epilepsy (TLE), which is a common and frequently intractable seizure disorder, were used as the controls. Thirty-one non-silent single nucleotide variants (SNVs) affecting 16 genes were identified in the RE cases. The functions of the genes with SNVs were associated with antigen presentation, antiviral infection, epilepsy, schizophrenia and nerve cell regeneration. Genetic factors of RE were found first in this study. These results suggest that RE patients have congenital abnormalities in adaptive immunity and are susceptible to some harmful factors, which lead to polygenic abnormal disease.

Clonally Focused Public and Private T Cells in Resected Brain Tissue From Surgeries to Treat Children With Intractable Seizures

Using a targeted transcriptomics approach, we have analyzed resected brain tissue from a cohort of 53 pediatric epilepsy surgery cases, and have found that there is a spectrum of involvement of both the innate and adaptive immune systems as evidenced by the differential expression of immune-specific genes in the affected brain tissue. The specimens with the highest expression of immune-specific genes were from two Rasmussen encephalitis cases, which is known to be a neuro-immunological disease, but also from tuberous sclerosis complex (TSC), focal cortical dysplasia, and hemimegalencephaly surgery cases. We obtained T cell receptor (TCR) Vβ chain sequence data from brain tissue and blood from patients with the highest levels of T cell transcripts. The clonality indices and the frequency of the top 50 Vβ clonotypes indicated that T cells in the brain were clonally restricted. The top 50 Vβ clonotypes comprised both public and private (patient specific) clonotypes, and the TCR Vβ chain third complementarity region (CDR3) of the most abundant public Vβ clonotype in each brain sample was strikingly similar to a CDR3 that recognizes an immunodominant epitope in either human cytomegalovirus or Epstein Barr virus, or influenza virus A. We found that the frequency of 14 of the top 50 brain Vβ clonotypes from a TSC surgery case had significantly increased in brain tissue removed to control recurrent seizures 11 months after the first surgery. Conversely, we found that the frequency in the blood of 18 of the top 50 brain clonotypes from a second TSC patient, who was seizure free, had significantly decreased 5 months after surgery indicating that T cell clones found in the brain had contracted in the periphery after removal of the brain area associated with seizure activity and inflammation. However, the frequency of a public and a private clonotype significantly increased in the brain after seizures recurred and the patient underwent a second surgery. Combined single cell gene expression and TCR sequencing of brain-infiltrating leukocytes from the second surgery showed that the two clones were CD8 effector T cells, indicating that they are likely to be pathologically relevant.

Viral Infection at the Blood-Brain Barrier in Multiple Sclerosis: – An Ultrastructural Study of Tissues from a Uk Regional Brain Bank

Although viral infections are often invoked as environmental factors in the aetiology and pathogenesis of multiple sclerosis (MS) it is only recently that a specific, indirect, cytokine-mediated mechanism for triggering of relapses during viral infections has been demonstrated. It is not yet clear however whether this indirect mechanism can account for all reported viral associations with the aetiopathogenesis of MS. A direct causal role of central nervous system (CNS) viral infection in MS has largely been discounted following repeated failures to demonstrate virus within the oligodendrocyte-myelin unit In the light of increasing evidence of blood-brain barrier (BBB) dysfunction in MS and to further explore the issue of possible viral involvement in MS, an ultrastructural search for viruses was undertaken in the CNS microvasculature, in autopsy and biopsy tissue from human CNS primary demyelinating diseases, including MS (20 cases), idiopathic monophasic CNS demyelinating disease (Mdemy, four cases) and metabolic or immunopathological demyelinating disease (two cases). For comparison, tissues from CNS viral disease in which demyelination is a major feature (nine cases) were examined in the same way. Control CNS tissues (nine cases) from a range of other neurological and non-neurological diseases were also examined. Outside the MS and Mdemy groups, morphological evidence of virus associations with the BBB were found only in the acute and subacute viral encephalitides (three cases subacute sclerosing panencephalitis, one case of Herpes encephalitis) and in one case of disseminated Cytomegalovirus infection. In a small proportion of MS and Mdemy cases, particles resembling either adenovirus (one case of MS) or paramyxovirus (one case of MS, one case of Mdemy) were found in the vicinity of microvessels. In each case a different cell type or extracellular compartment was involved and an exact correlation between the virus particles and the demyelinating lesions could not be demonstrated. Furthermore, corroborative clinical or laboratory evidence of current CNS infection in these primary demyelinating disease cases was available only from the single positive Mdemy case and not from the two cases of MS. This and other previously published evidence from MS (which implicated a Coronavirus) and other diseases highlights the potential vulnerability to viral infection of cells associated with the BBB. Furthermore it is concluded that the detection rate of such infections in pathological tissue could underestimate their true frequency. A possible role of transient virus-BBB interactions in triggering focal inflammation, BBB breakdown and demyelination in some cases of MS and parainfectious demyelinating disease cannot be discounted.

Autoimmune Epilepsies

Autoimmune epilepsies describe clinical syndromes wherein the immune system is suspected to be involved in the pathogenesis of seizures or as a mechanism for neuronal injury following seizures. These diseases typically affect otherwise healthy children and are characterized by explosive onset of focal seizures, encephalopathy, cognitive deterioration, or other focal neurological deficits, or all of these. Traditional neurological diagnostics lack sensitivity and specificity in the diagnosis of autoimmune epilepsies, and results must be considered in the clinical context. Consideration of an autoimmune etiology early in the clinical course is important to ensure timely initiation of immunotherapy, as appropriate, as conventional antiepileptic drugs alone are typically unable to control seizures and other neurological symptoms. This article discusses the autoimmune epilepsies of autoimmune encephalitis (including anti-N-methyl-D-aspartate receptor encephalitis), Rasmussen's encephalitis, and febrile infection-related epilepsy syndrome. Further research is needed to better understand pathogenic mechanisms, optimal immunotherapy, and the effect of treatment on prognosis.

Expanding Role of T Cells in Human Autoimmune Diseases of the Central Nervous System

It is being increasingly recognized that a dysregulation of the immune system plays a vital role in neurological disorders and shapes the treatment of the disease. Aberrant T cell responses, in particular, are key in driving autoimmunity and have been traditionally associated with multiple sclerosis. Yet, it is evident that there are other neurological diseases in which autoreactive T cells have an active role in pathogenesis. In this review, we report on the recent progress in profiling and assessing the functionality of autoreactive T cells in central nervous system (CNS) autoimmune disorders that are currently postulated to be primarily T cell driven. We also explore the autoreactive T cell response in a recently emerging group of syndromes characterized by autoantibodies against neuronal cell-surface proteins. Common methodology implemented in T cell biology is further considered as it is an important determinant in their detection and characterization. An improved understanding of the contribution of autoreactive T cells expands our knowledge of the autoimmune response in CNS disorders and can offer novel methods of therapeutic intervention.

Upregulation of HMGB1, toll-like receptor and RAGE in human Rasmussen's encephalitis

Rasmussen encephalitis (RE) is a rare neurological disorder of childhood characterized by uni-hemispheric inflammation, progressive neurological deficits and intractable focal epilepsy. The pathogenesis of RE is still enigmatic. Activation of endogenous high-mobility group box-1 (HMGB1) and Toll-like receptor (TLR) has been proved to be with pro-inflammatory as well as pro-convulsant effects. We hypothesized that the epileptogenic mechanisms underlying RE are related to activation of HMGB1/TLR signaling. Immunnohistochemistry approach was used to examine the expression of HMGB1, TLR2, TLR4, receptor for advanced glycation end products (RAGE) in surgically resected human epileptic cortical specimens from RE (n=12), and compared that with control cortical issue (n=6). HMGB1 was ubiquitously detected in nuclei of astrocytes while its receptors were not detected in control cortex specimens. Marked expression of the receptors were observed in the lesions of RE. In particular, HMGB1 was in stead detected in cytoplasm of reactive astrocytes in RE cortex, predictive its release from glial cells. Significant greater HMGB1 and its receptors expression in RE vs. control was demonstrated by western blot. These results provide the novel evidence of intrinsic activation of these pro-inflammation pathways in RE, which suggest the specific targets in the treatment of epilepsy associated with RE.

CD8(+) T-cell pathogenicity in Rasmussen encephalitis elucidated by large-scale T-cell receptor sequencing

Rasmussen encephalitis (RE) is a rare paediatric epilepsy with uni-hemispheric inflammation and progressive neurological deficits. To elucidate RE immunopathology, we applied T-cell receptor (TCR) sequencing to blood (n=23), cerebrospinal fluid (n=2) and brain biopsies (n=5) of RE patients, and paediatric controls. RE patients present with peripheral CD8⁺ T-cell expansion and its strength correlates with disease severity. In addition, RE is the only paediatric epilepsy with prominent T-cell expansions in the CNS. Consistently, common clones are shared between RE patients, who also share MHC-I alleles. Public RE clones share Vβ genes and length of the CDR3. Rituximab/natalizumab/basiliximab treatment does not change TCR diversity, stem cell transplantation replaces the TCR repertoire with minimal overlap between donor and recipient, as observed in individual cases. Our study supports the hypothesis of an antigen-specific attack of peripherally expanded CD8⁺ lymphocytes against CNS structures in RE, which might be ameliorated by restricting access to the CNS.

Rasmussen Encephalitis is a rare neurological disease accompanied by inflammation and T cell infiltration in the brain. Here the authors show that the severity of this disease correlates with clonal CD8 T cell expansion.

Semi-quantitative analyses of antibodies to N-methyl-d-aspartate type glutamate receptor subunits (GluN2B & GluN1) in the clinical course of Rasmussen syndrome

OBJECTIVE

In Rasmussen syndrome (RS), in addition to the predominant involvement of cytotoxic T cells, heterogeneous autoantibodies against neural molecules are also found, but their function has not been elucidated. We examined antibodies to N-methyl-d-aspartate (NMDA) type glutamate receptor (GluR) subunits (GluN2B & GluN1) semi-quantitatively in cerebrospinal fluid (CSF) samples from RS patients, and evaluated their changes over time and their roles in immunopathogenesis.

METHODS

Autoantibodies against N-terminal and C-terminal of GluN2B and GluN1 were examined in 40 CSF samples collected from 18 RS patients 5 to 180 months after the onset of RS. Epileptic patients without infectious etiology or progressive clinical course served as disease controls (n=23). Synthesized peptides encoding the extracellular and intracellular domains of human GluN2B and GluN1 subunits were used as antigens in ELISA. We defined the cut-off for these antibodies as mean +2 standard deviations (optimal density) of the disease controls. MRI were evaluated according to the MRI staging proposed by Bien et al. (2002b, Neurology 58, 250).

RESULTS

CSF levels of antibodies against N-terminal and C-terminal of GluN2B were higher in RS patients than in disease controls (p<0.01). Likewise, CSF levels of antibodies against N-terminal and C-terminal of GluN1 were also higher in RS patients than in disease controls (p<0.01). All four antibodies tested were below cut-off levels in almost all CSF samples collected within one year from epilepsy onset. The proportions of CSF samples with these antibodies above cut-off levels were highest from 12 to 23 months after epilepsy onset, and declined after 24 months. CSF levels of these antibodies were higher when seizure occurred daily than when seizure occurred less frequently (p<0.01), and were higher at MRI stage 3 than at MRI stages 0, 2 and 4 (p<0.05), except for anti-GluN1-CT antibody at stage 2.

CONCLUSIONS

Broad epitope recognition spectrum and delayed production of autoantibodies to NMDA type GluR in CSF of RS patients suggest that the autoantibodies are produced against NMDA type GluR antigens derived from cytotoxic T cell-mediated neuronal damages. These antibodies may impact the pathophysiology of RS in the most active stage, and could be a marker for active inflammation in the clinical course of RS. Further studies including passive transfer of the antibodies to mice may reveal the pivotal roles of the antibodies in RS.

In Rasmussen encephalitis, hemichannels associated with microglial activation are linked to cortical pyramidal neuron coupling: a possible mechanism for cellular hyperexcitability

AIMS

Rasmussen encephalitis (RE) is a rare but devastating condition, mainly in children, characterized by sustained brain inflammation, atrophy of one cerebral hemisphere, epilepsy, and progressive cognitive deterioration. The etiology of RE-induced seizures associated with the inflammatory process remains unknown.

METHODS

Cortical tissue samples from children undergoing surgical resections for the treatment of RE (n = 16) and non-RE (n = 12) were compared using electrophysiological, morphological, and immunohistochemical techniques to examine neuronal properties and the relationship with microglial activation using the specific microglia/macrophage calcium-binding protein, IBA1 in conjunction with connexins and pannexin expression.

RESULTS

Compared with non-RE cases, pyramidal neurons from RE cases displayed increased cell capacitance and reduced input resistance. However, neuronal somatic areas were not increased in size. Instead, intracellular injection of biocytin led to increased dye coupling between neurons from RE cases. By Western blot, expression of IBA1 and pannexin was increased while connexin 32 was decreased in RE cases compared with non-RE cases. IBA1 immunostaining overlapped with pannexin and connexin 36 in RE cases.

CONCLUSIONS

In RE, these results support the notion that a possible mechanism for cellular hyperexcitability may be related to increased intercellular coupling from pannexin linked to increased microglial activation. Such findings suggest that a possible antiseizure treatment for RE may involve the use of gap junction blockers.

Rasmussen encephalitis and comorbid autoimmune diseases: A window into disease mechanism?

OBJECTIVE

To describe a potential association between comorbid autoimmune disease and Rasmussen encephalitis (RE) and discuss potential insights into underlying RE pathogenesis.

METHODS

We report a case series of 4 patients with RE in whom a comorbid autoimmune disease was subsequently diagnosed and review the literature on possible common susceptibility factors.

RESULTS

In 4 patients who presented with typical clinical features of RE, a comorbid autoimmune disease was subsequently diagnosed: Hashimoto thyroiditis, ulcerative colitis, Crohn disease, and systemic lupus erythematosus. We discuss the possible common predisposing factors.

CONCLUSIONS

The association of RE, a rare entity, with a comorbid autoimmune disease raises the possibility of shared mechanisms of susceptibility, including common immunogenetic and/or environmental risk factors.

Mechanisms of epileptogenesis in pediatric epileptic syndromes: Rasmussen encephalitis, infantile spasms, and febrile infection-related epilepsy syndrome (FIRES)

The mechanisms of epileptogenesis in pediatric epileptic syndromes are diverse, and may involve disturbances of neurodevelopmental trajectories, synaptic homeostasis, and cortical connectivity, which may occur during brain development, early infancy, or childhood. Although genetic or structural/metabolic factors are frequently associated with age-specific epileptic syndromes, such as infantile spasms and West syndrome, other syndromes may be determined by the effect of immunopathogenic mechanisms or energy-dependent processes in response to environmental challenges, such as infections or fever in normally-developed children during early or late childhood. Immune-mediated mechanisms have been suggested in selected pediatric epileptic syndromes in which acute and rapidly progressive encephalopathies preceded by fever and/or infections, such as febrile infection-related epilepsy syndrome, or in chronic progressive encephalopathies, such as Rasmussen encephalitis. A definite involvement of adaptive and innate immune mechanisms driven by cytotoxic CD8(+) T lymphocytes and neuroglial responses has been demonstrated in Rasmussen encephalitis, although the triggering factor of these responses remains unknown. Although the beneficial response to steroids and adrenocorticotropic hormone of infantile spasms, or preceding fever or infection in FIRES, may support a potential role of neuroinflammation as pathogenic factor, no definite demonstration of such involvement has been achieved, and genetic or metabolic factors are suspected. A major challenge for the future is discovering pathogenic mechanisms and etiological factors that facilitate the introduction of novel targets for drug intervention aimed at interfering with the disease mechanisms, therefore providing putative disease-modifying treatments in these pediatric epileptic syndromes.

Upregulation of adenosine kinase in Rasmussen encephalitis

Rasmussen encephalitis (RE) is a rare neurologic disorder of childhood characterized by unihemispheric inflammation, progressive neurologic deficits, and intractable focal epilepsy. The pathogenesis of RE is still enigmatic. Adenosine is a key endogenous signaling molecule with anticonvulsive and anti-inflammatory effects, and our previous work demonstrated that dysfunction of the adenosine kinase (ADK)–adenosine system and astrogliosis are the hallmarks of epilepsy. We hypothesized that the epileptogenic mechanisms underlying RE are related to changes in ADK expression and that those changes might be associated with the development of epilepsy in RE patients. Immunohistochemistry was used to examine the expression of ADK and glial fibrillary acidic protein in surgically resected human epileptic cortical specimens from RE patients (n = 12) and compared with control cortical tissues (n = 6). Adenosine kinase expression using Western blot and enzymatic activity for ADK were assessed in RE versus control samples. Focal astrogliosis and marked expression of ADK were observed in the lesions of RE. Significantly greater ADK expression in RE versus controls was demonstrated by Western blot, and greater enzymatic activity for ADK was demonstrated using an enzyme-coupled bioluminescent assay. These results suggest that upregulation of ADK is a common pathologic hallmark of RE and that ADK might be a target in the treatment of epilepsy associated with RE.

Extrarolandic electroclinical findings in the evolution of adult-onset Rasmussen's encephalitis

Rasmussen's encephalitis (RE) is a rare immunomediated disorder characterized by unilateral hemispheric atrophy, drug-resistant focal epilepsy, and progressive neurological deficits. Its onset typically occurs in childhood, though it has also been reported in adult age (A-RE) with atypical clinical features. The aim of this study was to describe the electroclinical features in a group of seven patients with A-RE. We retrospectively studied seven women aged 23-43years (mean: 32.1years) with a diagnosis of RE according to commonly accepted diagnostic criteria. All the patients were clinically evaluated and underwent prolonged video-EEG monitoring, laboratory investigations, and high-resolution MRI follow-up. All the patients displayed an ictal electroclinical pattern whose evolution varied. We identified an early phase characterized by polymorphic ictal electroclinical manifestations (temporal semiology in five cases, frontal in one, and parietal in the remaining case) and a late phase clinically characterized by viscerosensitive phenomena followed by somatosensitive signs, experiential symptoms, and motor signs in all the cases. In the late phase, the ictal EEG pattern was characterized by monomorphic, pseudorhythmic, repetitive slow-wave theta activity over the frontal and central regions, with ipsilateral propagation and/or secondary spreading to contralateral perisylvian structures. Patients were treated with a combination of AEDs and immunotherapy (steroids and IVIg); epilepsy surgery was performed in 3 cases. Our results show that A-RE is characterized by early and late clinical- and EEG-different features which may reflect a progressive involvement of a specific "extrarolandic" network in the advanced phase of the disease and may suggest that the electroclinical expression of RE varies according to the different stages of the pathological process.

Differential expression of interferon-γ and chemokine genes distinguishes Rasmussen encephalitis from cortical dysplasia and provides evidence for an early Th1 immune response

Background

Rasmussen encephalitis (RE) is a rare complex inflammatory disease, primarily seen in young children, that is characterized by severe partial seizures and brain atrophy. Surgery is currently the only effective treatment option. To identify genes specifically associated with the immunopathology in RE, RNA transcripts of genes involved in inflammation and autoimmunity were measured in brain tissue from RE surgeries and compared with those in surgical specimens of cortical dysplasia (CD), a major cause of intractable pediatric epilepsy.

Methods

Quantitative polymerase chain reactions measured the relative expression of 84 genes related to inflammation and autoimmunity in 12 RE specimens and in the reference group of 12 CD surgical specimens. Data were analyzed by consensus clustering using the entire dataset, and by pairwise comparison of gene expression levels between the RE and CD cohorts using the Harrell-Davis distribution-free quantile estimator method.

Results

Consensus clustering identified six RE cases that were clearly distinguished from the CD cases and from other RE cases. Pairwise comparison showed that seven mRNAs encoding interferon-γ, CCL5, CCL22, CCL23, CXCL9, CXCL10, and Fas ligand were higher in the RE specimens compared with the CD specimens, whereas the mRNA encoding hypoxanthine-guanine phosphoribosyltransferase was reduced. Interferon-γ, CXCL5, CXCL9 and CXCL10 mRNA levels negatively correlated with time from seizure onset to surgery (P <0.05), whereas CCL23 and Fas ligand transcript levels positively correlated with the degree of tissue destruction and inflammation, respectively (P <0.05), as determined from magnetic resonance imaging (MRI) T2 and FLAIR images. Accumulation of CD4⁺ lymphocytes in leptomeninges and perivascular spaces was a prominent feature in RE specimens resected within a year of seizure onset.

Conclusions

Active disease is characterized by a Th1 immune response that appears to involve both CD8⁺ and CD4⁺ T cells. Our findings suggest therapeutic intervention targeting specific chemokine/chemokine receptors may be useful in early stage RE.

Neuropathologic Studies of 'Epileptogenic' Tissue

This mini-review will summarize some of the major contributions of Neuropathology to understanding the structural basis of several forms of intractable epilepsy. Because of space limitations, only selected morphologically defined brain abnormalities will be considered and illustrated. By definition, the types of specimen encountered by Neuropathologists are those resected to treat intractable seizure disorders. One can therefore infer that the 'lesions' encountered in some way caused or at least contributed to the epilepsy, though the leap from observed neuropathologic abnormalities to 'seizuregenesis' is not always straightforward—indeed often is very tortuous and obscure. Nevertheless, a Neuropathologist working with a team of neurologists, neurosurgeons, electrophysiologists, neuropsychologists, and others, can contribute to a richer understanding of why seizures are triggered within human brain tissue, and therefore how they may be more effectively treated.

Dual Pathology in Rasmussen's Encephalitis: A Report of Coexistent Focal Cortical Dysplasia and Review of the Literature

Rasmussen's encephalitis is a well-established, albeit rare cause of medically intractable epilepsy. In a small number of Rasmussen's cases, a second pathology is identified, which independently can cause medically intractable seizures (dual pathology). This paper documents a case of a 13-year-old male who presented with medically intractable epilepsy. The patient underwent a series of surgical resections, early on resulting in a diagnosis of focal cortical dysplasia and later yielding a diagnosis of coexistent Rasmussen's encephalitis, marked by chronic inflammation, microglial nodules, and focal cortical atrophy, combined with focal cortical dysplasia (Palmini et al. type IIA, ILAE type IIA). The literature on dual pathology in the setting of Rasmussen's encephalitis is reviewed.

Epileptic encephalitis: the role of the innate and adaptive immune system

Seizures are a prominent clinical feature of encephalitis. Recent data suggest the adaptive as well as innate immune system to be involved directly in the pathomechanism of epileptogenesis. Cytotoxic T-cells and antibody-mediated complement activation are major components of the adaptive immune system, which can induce neurodegeneration, thereby probably contributing to epileptic encephalitis. The innate immune system operates via interleukin-1 and toll-like receptor-associated mechanisms and was shown to play a direct role in epileptogenesis. Here, we review neuropathology hallmarks of various encephalitis conditions such as Rasmussen encephalitis (RE) but also introduce the more recently discovered antibody-associated voltage-gated potassium channel complex (VGKC), N-methyl-D-aspartate receptor (NMDAR) or glutamic acid decarboxylase (GAD) 65 encephalitides. Neuropathological investigations are used to determine specific cellular components and molecular mechanisms used by the immune system to provoke neurodegeneration and to promote epileptogenesis. Based on recent findings, we propose concepts for the stratification of epileptic encephalitis. Knowledge of the role of the innate immunity has already translated into clinical treatment strategies and may help to discover novel drug targets for these epileptic disorders.

Neuropathology of temporal lobe epilepsy

Pathologic findings in surgical resections from patients with temporal lobe epilepsy include a wide range of diagnostic possibilities that can be categorized into different groups on the basis of etiology. This paper outlines the various pathologic entities described in temporal lobe epilepsy, including some newly recognized epilepsy-associated tumors, and briefly touch on the recent classification of focal cortical dysplasia. This classification takes into account coexistent pathologic lesions in focal cortical dysplasia.

Intimal thickening of meningeal arteries after serial corticectomies for Rasmussen encephalitis

Rasmussen encephalitis is a rare cause of intractable epilepsy in children. Between 2008 and 2010, 4 patients had second cortical resections performed after a primary corticectomy for Rasmussen encephalitis. In each case, we observed some degree of vessel wall change in leptomeningeal arteries, consisting of moderate to moderately severe intimal hyperplasia. The intervals between original resection and second operation ranged from 8 months to 10 years. Ages of the patients ranged from 9 to 12 years at their first resection and from 10 to 19 years at the time of revision. Four other Rasmussen encephalitis cases operated upon in the years 2006 to 2010 and 2 surgical revisions for severe cortical dysplasia, 1 for mild cortical dysplasia and 1 for recurrent dysembryoplastic neuroepithelial tumor, did not show significant vascular abnormalities (with surgical intervals of 10 months to 16 years). Leptomeningeal intimal hyperplasia appears to develop in the interval between repeated cortical resections for Rasmussen encephalitis, an inflammatory disorder. The pathogenesis of this vascular change may be related to meningeal inflammation in Rasmussen encephalitis. This finding in children undergoing surgical resection for Rasmussen encephalitis may itself lead to "secondary" ischemic change that contributes to worsening of epilepsy.

Astrocyte immune responses in epilepsy

Astrocytes, the major glial cell type of the central nervous system (CNS), are known to play a major role in the regulation of the immune/inflammatory response in several human CNS diseases. In epilepsy-associated pathologies, the presence of astrogliosis has stimulated extensive research focused on the role of reactive astrocytes in the pathophysiological processes that underlie the development of epilepsy. In brain tissue from patients with epilepsy, astrocytes undergo significant changes in their physiological properties, including the activation of inflammatory pathways. Accumulating experimental evidence suggests that proinflammatory molecules can alter glio-neuronal communications contributing to the generation of seizures and seizure-related neuronal damage. In particular, both in vitro and in vivo data point to the role of astrocytes as both major source and target of epileptogenic inflammatory signaling. In this context, understanding the astroglial inflammatory response occurring in epileptic brain tissue may provide new strategies for targeting astrocyte-mediated epileptogenesis. This article reviews current evidence regarding the role of astrocytes in the regulation of the innate immune responses in epilepsy. Both clinical observations in drug-resistant human epilepsies and experimental findings in clinically relevant models will be discussed and elaborated, highlighting specific inflammatory pathways (such as interleukin-1β/toll-like receptor 4) that could be potential targets for antiepileptic, disease-modifying therapeutic strategies.

Rasmussen's encephalitis: update on pathogenesis and treatment

Rasmussen's encephalitis is a devastating disease characterized by refractory epilepsy and progressive atrophy of one cerebral hemisphere. The only treatment option able to prevent seizures is the surgical disconnection of the affected hemisphere. The etiology of the disease remains unknown, but several laboratory findings have recently indicated the involvement of the immune system. Whether these findings represent an initiating event or the consequence of a not yet identified trigger factor must be elucidated. Nevertheless, these observations suggest the use of different immunomodulatory approaches (including corticosteroids, plasmapheresis, intravenous immunoglobulins and immunosuppressive drugs) that proved to be of variable efficacy in some patients, confirming the potential role of the immune system, at least in the perpetuation of the disease.

Diagnostic challenges in the evaluation of chronic epilepsy-related surgical neuropathology

For patients with pharmacoresistent epilepsy, surgical excision is often an effective means of controlling and sometimes curing the seizures. Excised tissue frequently provides information regarding the precise etiology of the epilepsy. This review examines some of the more commonly encountered pathologies encountered in the neuropathology arena in this clinical venue. The most common identifiable causes of chronic epilepsy including low-grade tumors, hippocampal sclerosis, malformation of cortical development (cortical dysplasia), and remote infarcts will be discussed. Differential diagnostic difficulties will be examined.

Correlation between magnetic resonance imaging and histopathologic grades in Rasmussen syndrome

The aim of this study was to investigate the correlation between magnetic resonance imaging (MRI) and histopathologic findings in Rasmussen syndrome. Serial MRIs were obtained for five patients who had histologically proven Rasmussen syndrome. The histopathologic grades of the lesions were subdivided into phases: active 1-3, resolving 1-3, and chronic inflammatory. The images were then correlated with histopathologic findings. Neuropathologic findings in the central areas on MRI demonstrated only the chronic and resolving grades, but active inflammatory abnormalities were present not only at the margins of the lesions, but also in areas of subtle signal abnormality on MRI. Atrophic areas on MRI exhibited all grades of histopathologic abnormalities, but chronic and resolving grades were predominant. Seizure duration of less than 6 months was associated with very active grades, duration of 1-2 years with variable grades, and duration greater than 6 years with chronic and resolving grades only. The MRI images correlated highly with histopathologic analysis. These findings suggest that the lesions initially arise from one site in the brain, and so support the centrifugal spreading theory of this disease. Findings also suggest that the margin rather than the center of the MRI abnormality may be the most ideal site for biopsy.

Epileptic encephalopathy in children possibly related to immune-mediated pathogenesis

Severe epilepsy in the paediatric population negatively influences neurological and cognitive development. Different etiological factors could be responsible of these severe epilepsies, and an early diagnosis could change, in some cases, the neurological and cognitive development. Immune mechanisms have been reported in epilepsy. Epilepsy has been associated with systemic lupus erythematosus, with the presence of anti-phospholipid antibodies (aPL), anti-cardiolipin antibodies, anti-nuclear antibodies, Beta2-glycoprotein antibodies, and anti-glutamic acid decarboxylase (anti-GAD) antibodies. CNS inflammation and markers of adaptive immunity have been, also, associated with some epileptic syndromes, such as West syndrome, temporal lobe epilepsy, febrile seizures, tonic-clonic seizures, and tuberous sclerosis. Inflammation and blood-brain barrier (BBB) disruption could be one of the mechanisms responsible for seizure recurrence. Recently clinical entities, characterized by severe epilepsy with a febrile, acute or sub-acute onset, sometimes associated with status epilepticus, followed by drug-resistant, partial epilepsy have been described. Some of these publications also suggested acronyms for the condition described: Acute Encephalitis with Refractory, Repetitive Partial Seizures (AERRPS) reported by Japanese authors, Devastating Epileptic Encephalopathy in School-aged Children (DESC) reported by French authors. Among children with acquired symptomatic severe epilepsy, we identified a group of previously normal children who had developed severe partial epilepsy after an acute/sub-acute illness resembling encephalitis. The etiological factors for those patients seems to remain unknown, and a possible immune-mediating or inflammatory process as pathogenesis of the disease could be hypothesized. More studies need to be addressed to finally define this peculiar epileptic entity.

Treatment of Rasmussen encephalitis half a century after its initial description: promising prospects and a dilemma

Rasmussen encephalitis (RE), initially described half a century ago, is an inflammatory unihemispheric brain disorder. Its two clinical key facets are the progressive tissue and function loss and the epilepsy, often in form of epilepsia partialis continua. For both, treatment options are available. Anti-seizure effect of anti-epilepsy drugs is usually limited to secondarily generalized seizures and complex partial seizures whereas epilepsia partialis continua usually is totally refractory. Hemispherectomy in one of its modern variants offers a very high chance of seizure freedom, however at the price of irreversible loss of functions located in the affected hemisphere. In a proportion of patients, long-term immunotherapy is able to prevent or slow down hemispheric tissue loss and the associated functional decline. It does, however, mostly not improve the epilepsy. Whereas for many patients unequivocal treatment proposals can be readily made, a dilemma may emerge in those with severe epilepsy but still preserved hemispheric function.

Increased activation of Iba1+ microglia in pediatric epilepsy patients with Rasmussen's encephalitis compared with cortical dysplasia and tuberous sclerosis complex

Microgliosis is prominent in Rasmussen's encephalitis (RE), a disease with severe seizure activity. However, it is unclear if microglial activation is similar with different histopathologic substrates. Iba1-immunolabelled microglial activation was assessed in neocortex from pediatric epilepsy surgery patients with RE (n=8), cortical dysplasia (CD; n=6) and tuberous sclerosis complex (TSC; n=6). Microglial reactivity was increased, in severely affected RE areas (29% labeling) compared with minimally affected areas of RE cases (15%) and cases of TSC (14%) and CD (12%). There was no qualitative association of Iba1 immunolabelling with the presence of CD8(+) cytotoxic T-cells and no statistical association with clinical epilepsy variables, such as seizure duration or frequency. Iba1 appears to be an excellent marker for detecting extensive microglial activation in patients with RE. In addition, this study supports the notion that Iba1-labeled microglial activation is increased in patients with active RE, compared with cases of CD and TSC.

Astrocytes are a specific immunological target in Rasmussen's encephalitis

OBJECTIVE

The current histopathological criteria of Rasmussen's encephalitis (RE) include the presence of T-cell-dominated inflammation, microglial activation, neuronal loss, and astrocytic activation. An in vitro study, however, suggested glutamate receptor 3 (GluR3) antibody-mediated astrocytic loss. Therefore, we investigated astrocytic apoptosis and loss in situ.

METHODS

Histochemical, immunohistochemical, terminal deoxynucleotidyltransferase-mediated biotin-dUTP nick end labeling and in situ hybridization techniques were applied to paraffin sections of 20 RE cases, 6 healthy control subjects, and 6 paraneoplastic encephalomyelitis, 10 Ammon's horn sclerosis, and 5 focal cortical dysplasia cases.

RESULTS

Astrocytic apoptosis and subsequent loss of these cells is a specific feature of RE. Such lesions are not found in the control groups. In RE, astrocytic apoptosis and loss was present both in cortical and in white matter areas. Astrocytes in these tissues showed major histocompatibility complex class I expression. Furthermore, granzyme-B(+) lymphocytes were found in close apposition to astrocytes bordering astrocyte-deficient lesions. Granzyme-B(+) granules in these lymphocytes were polarized and faced the astrocytic membranes. No evidence was found for an antibody-mediated destruction.

INTERPRETATION

We suggest a specific attack by cytotoxic T lymphocytes as a possible mechanism responsible for astrocytic degeneration in RE. The loss of astrocytes might play a role in neuronal dysfunction, seizure induction, and enhancement of neuronal cell death.

Long term response to steroid therapy in Rasmussen encephalitis

Rasmussen encephalitis (RE) is a severe and progressive focal epilepsy of unknown origin that leads to deterioration of motor and cognitive function. In a previous study, we described positive effect of high doses of steroids during the first year after the onset of RE. The objective of this study was to evaluate this therapy at long term. We reviewed 11 patients (7 girls and 4 boys) with RE of the right hemisphere (7) and the left (4) at a follow-up of 9+/-2 years. Age at onset of RE ranged from 2 to 14 years. Six patients had no benefit from steroid therapy and underwent hemispherotomy. Five had significant reduction of seizure frequency with disappearance of epilepsia partialis continua, and improved motor function. Of these, two died of unexpected sudden death 5 and 7 years after seizure control. Two others with initial response experienced progressive recurrence of seizures 1 to 4 years after the end of steroid therapy and required hemispherotomy. Finally, only one patient exhibited total cessation of seizures with steroids for 3 years, but seizures progressively recurred although the frequency was moderate. Our data confirm that although steroid treatment can be useful when given early in the course of RE, long term relapse can occur among the good responders requiring delayed hemispheric disconnection.

Rasmussen's encephalitis: interleukin-10-dependent Tc2 cell polarization may explain its pathophysiology and clinical course

Little is known about the cellular immune dynamics and pathophysiology of Rasmussen's encephalitis (RE). We investigated transcriptional expression of pro- and anti-inflammatory cytokines and characterized the T-cell subset types present in temporal and frontal lobe specimens obtained from a child with RE. Interleukin (IL)-10 and macrophage scavenger receptor type I mRNA assessed by semiquantitative reverse transcription polymerase chain reaction was found in temporal but not in affected frontal lobe tissue. Messenger RNA specific to tumor necrosis factor alpha, IL-l, IL-4, IL-6, IL-12, IL-15, IL-18, transforming growth factor beta, CD-14, and inducible nitric oxide synthase was not detected in either temporal or frontal tissue with histopathologically manifest evidence of disease. Virtually all lymphocytic infiltrate consisted of CD3+ CD8+ T cells. We speculate that RE is a disease mediated by Tc2 polarization of the immune response and that its immunohistopathology, natural history, and clinical evolution (chronic, staircase progression) reflect the dual/pleiotropic actions of IL-10, which, depending on the state of activation of the immune system, may be either cytolytic or immunosuppressant.

Infections as causative factors of epilepsy

Epilepsies bearing some relationship to infections or vaccinations are often encountered clinically. While the onset of epilepsy or aggravation of seizures may follow common infections or vaccinations, complete seizure control has also been observed after infections. However, the true mechanisms underlying the relationship between infections and epilepsies have not been fully elucidated. Recently, advances in immunology have contributed to the study of autoimmune mechanisms in Rasmussen’s syndrome, a prototype of autoimmune epilepsy related to infections. The roles of autoimmunity, including cytotoxic T cells and autoantibodies against neural molecules, have been demonstrated in Rasmussen’s syndrome. This review postulates the probable molecular mimicry of microbial and neural components in Rasmussen’s syndrome and proposes possible autoimmune mechanisms related to the development of symptomatic epilepsies.

Neurodevelopmental disorders as a cause of seizures: neuropathologic, genetic, and mechanistic considerations

This review will consider patterns of developmental neuropathologic abnormalities-malformations of cortical development (MCD)--encountered in infants (often with infantile spasms), children, and adults with intractable epilepsy. Treatment of epilepsy associated with some MCD, such as focal cortical dysplasia and tubers of tuberous sclerosis, may include cortical resection performed to remove the "dysplastic" region of cortex. In extreme situations (eg, hemimegalencephaly), hemispherectomy may be carried out on selected patients. Neuropathologic (including immunohistochemical) findings within these lesions will be considered. Other conditions that cause intractable epilepsy and often mental retardation, yet are not necessarily amenable to surgical treatment (eg, lissencephaly, periventricular nodular heterotopia, double cortex syndrome) will be discussed. Over the past 10 years there has been an explosion of information on the genetics of MCD. The genes responsible for many MCD (eg, TSC1, TSC2, LIS-1, DCX, FLN1) have been cloned and permit important mechanistic studies to be carried out with the purpose of understanding how mutations within these genes result in abnormal cortical cytoarchitecture and anomalous neuroglial differentiation. Finally, novel techniques allowing for analysis of patterns of gene expression within single cells, including neurons, is likely to provide answers to the most vexing and important question about these lesions: Why are they epileptogenic?

T-cells in human encephalitis

Encephalitis literally means inflammation of the brain. In general, this inflammation can result from a viral or bacterial infection in the brain itself or alternatively from a secondary autoimmune reaction against an infection or a tumor in the rest of the body. Besides this, encephalitis is present in (believed autoimmune) diseases with unknown etiology, such as multiple sclerosis or Rasmussen encephalitis (RE). This article summarizes the existing data on the role of T-cells in the pathogenesis of three types of human encephalitis: RE, paraneoplastic encephalomyelitis, and virus encephalitis. In all of them, T-cells play a major role in disease pathogenesis, mainly mediated by major histocompatiblity complex class I-restricted CD8⁺ T-lymphocytes.

Neue Erkenntnis zur Rasmussen-Enzephalitis

Rasmussen encephalitis (RE) is a severe and at the same time pathophysiologically fascinating condition. The chronic inflammation affects one of the two cerebral hemispheres and destroys it during the disease process that lasts from months to years. The patients -- mostly children -- suffer from frequent pharmacoresistant seizures, often in the form of epilepsia partialis continua. In parallel to the atrophy of the affected hemisphere, the neurological functions associated with it decline continuously. This results in a final stage with a usually high-grade sensorimotor hemisyndrome, hemianopia, cognitive impairment and -- if the language-dominant hemisphere is affected -- aphasia. Research results in the last 5 years have contributed to a deeper understanding of the pathogenesis of this condition. Formal diagnostic criteria have been proposed, and new therapeutic options have emerged by which the disease progression can be slowed or stopped. This article summarizes the current research results on the background of older data and gives recommendations regarding diagnostic and therapeutic procedures in RE patients.

Future aspects of epilepsy research

This contribution in honour of Prof. Gerhard Pendl first reviews some recent studies on resected tissue, migrational disorders, and Rasmussen's Syndrome. These areas of basic research profit from recent advances of molecular biology and genetics. On the clinical side, some studies dealing with proton magnetic resonance spectroscopy are reviewed. In order to highlight the progress in clinical epilepsy research using modern methods of structural and functional imaging, functional outcome prediction is also reviewed. This kind of advanced clinical research is dealt with by discussing risk factor assessment associated with postsurgical decrements in memory. With regard to motor functions, we compare the yield of functional MR and intraoperative cortical stimulation in patients with lesions in or close to the Rolandic cortex. Progress in the field of advanced EEG analysis is reviewed in the context of "seizure prediction" and cognitive event-related potentials. Finally some of the new epilepsy treatment options, such as Gamma Knife treatment, where Prof. Pendl's group made pioneering contributions, are dealt with.

Histopathology of brain tissue from patients with infantile spasms

This chapter reviews the patterns of neuropathologic abnormality encountered in cortical resections performed for the treatment of infantile spasms (ISS) in the broader context of the larger "universe" of neuropathology seen in the central nervous system of infants with this syndrome as well as older children with intractable epilepsy. Although destructive lesions (encephalomalacia), Sturge-Weber-Dimitri syndrome, and even neoplasms can cause ISS, its most common neuropathologic substrate is cortical dysplasia (CD), representing the result of aberrant neuronal migration to the neocortex from the germinal matrix. Ways to recognize CD morphologically and assess its severity are reviewed, as are special stains and immunohistochemical methods that may be useful in confirming the diagnosis. The similarity and relationship between (sporadic) CD and tubers of tuberous sclerosis are discussed. Future potentially fruitful research directions aimed at refining the clinicopathologic analysis of the role of CD in ISS and epilepsies of older children are briefly considered.

The natural history of Rasmussen's encephalitis

Rasmussen's encephalitis (RE) is a chronic inflammatory disease of unknown origin, usually affecting one brain hemisphere. In the present study, a comprehensive assessment of the natural history of the disorder is presented. Seizure frequency, degree of hemiparesis and degree of cerebral hemiatrophy in 13 patients with histopathologically proven RE are analysed over the time course prior to resective epilepsy surgery or introduction of long-term immunosuppressive pharmacotherapy. For the assessment of the degree of cerebral hemiatrophy, on defined slices comprising the Sylvian fissure of hard copies of serial MRI investigations, the hemispheric ratio (HR) was determined. The data show an initial prodromal phase with an intermediate frequency of focal onset seizures and mostly no hemiparesis. The occurrence of this stage was mainly observed in the adolescent and adult patients. All patients went through an acute phase with a median duration of 8 months. During this stage, there were frequent simple partial motor seizures, development of hemiparesis and volume loss of the affected hemisphere. After this, the patients passed into a residual stage with a marked decrease in seizure frequency. Twelve months after the onset of the acute stage, the average HR was 0.72. These data allow an estimation of the prognosis of newly affected patients, and demonstrate that most of the brain damage in RE occurs during the first 8-12 months. These findings should be taken into consideration when future therapeutic approaches to RE are evaluated.

Advances in pathogenic concepts and therapeutic agents in Rasmussen's encephalitis

Rasmussen's encephalitis is a rare inflammatory brain disease which occurs mainly in children and is characterised by affection of only one hemisphere. Pathogenetic concepts have considered three different, not mutually exclusive, key factors contributing to the initiating or perpetuating events in the central nervous system. These include viruses, autoimmune antibodies and autoimmune cytotoxic T lymphocytes. Based on these concepts, different therapeutic strategies have been pursued, such as antiviral agents, plasmapheresis, immuno-adsorption, immunosuppression or immunomodulation with intravenous immunoglobulins. However, due to the lack of larger studies, to date there is no established therapeutic strategy of this devastating disease. An overview of the current state of immunepathogenic concepts for Rasmussen's encephalitis is given and past and present treatment attempts are discussed, including an outline of future perspectives. An opinion on symptomatic treatment with anticonvulsive drugs is included.

Rasmussen encephalitis: a clinicopathologic and immunohistochemical study of seven patients

We retrospectively reviewed the clinicopathologic features and immunohistochemical profiles of 7 patients with Rasmussen encephalitis (age range, 3.5-15 years at surgery). All had medically intractable seizures (6 months' to 7 years' duration); all but 1 developed unilateral hemiparesis. Histologically, all cases were characterized by leptomeningeal and parenchymal perivascular chronic inflammation consisting primarily of T lymphocytes (CD3+, CD5+, CD7+). In all but 1 case, a predominance of CD8+ T-cytotoxic/suppressor lymphoid cells over CD4+ cells was observed. All cases had rare B lymphocytes (CD79a+, CD20+). Rare CD10+ and no CD56+ cells were noted. All cases were marked by diffuse proliferation of microglial cells, highlighted on CD68 immunostaining. Focal microglial nodule formations were observed in 4 cases and focal cortical atrophy in 5 cases. Viral inclusions were not noted. There was no evidence of Epstein-Barr virus by LMP-1 antibody immunostaining. The histologic findings of Rasmussen encephalitis resemble those of viral meningoencephalitis. The pathologicfindings may be only focally present, and missed, if diagnosis is made or confirmed with biopsy alone. Most lymphoid cells have a T-cell immunophenotype, with a predominance of CD8+ cells in most cases.

Rasmussen's encephalitis: a role for autoimmune cytotoxic T lymphocytes

The present review describes advances in Rasmussen's encephalitis (also known as Rasmussen's syndrome), an unihemispheric intractable epileptic disease with persistent inflammation. Specific attention is given to the recent recognition of cytotoxicity by CD8+/granzyme-B-positive T lymphocytes as a new pathogenic mechanism of neuronal damage.

Destruction of neurons by cytotoxic T cells: a new pathogenic mechanism in Rasmussen's encephalitis

Rasmussen's encephalitis is a progressive epileptic disorder characterized by unihemispheric lymphocytic infiltrates, microglial nodules, and neuronal loss leading to the destruction of the affected hemisphere. In this study, immunohistochemical evaluation of specimens from 11 patients revealed lymphocytic infiltrates that consisted mainly of CD3(+)CD8(+) T cells. Of these cells, 7.0% lay in direct apposition to MHC class I(+) neurons. Confocal laser microscopy revealed that these lymphocytes contained granzyme B in a polar orientation toward these perikarya. Single neurons underwent apoptosis. These findings indicate that a T-cell-mediated cytotoxic reaction induces neuronal death in Rasmussen's encephalitis. This study directly shows, for what we believe is the first time, that a cytotoxic T-cell mechanism contributes to loss of neurons in human brain disease.