Background
Rasmussen encephalitis (RE) is a rare pediatric neurological disease with an estimated incidence in children under the age 18 years of 2–3 per 10 million [
1‐
3]. The acute phase of the disease is characterized by intense uncontrolled partial or generalized seizures, and MRI FLAIR imaging often shows inflammation in one cerebral hemisphere [
3]. As the disease progresses, unilateral loss of cerebral tissue leaves the patient with severe hemiparesis and other neurological deficits. Corticosteroids may provide short term benefit but ultimately fail to halt the disease. Early treatment with tacrolimus or intravenous immunoglobulins may stabilize the neurological deterioration, but they do not reverse the intractable epilepsy [
2]. An inflammatory response involving T cells and activated microglia confined to the affected hemisphere appears to be the cause of the clinical symptoms. However, what precipitates the immune response is not known. Several types of Herpesviridae have been detected in surgical brain specimens from RE patients; however, to date, there is no consistent evidence for a pathogen that is common to all RE cases [
4‐
7]. Likewise, autoantibodies have been described in RE cases indicative of an autoimmune disease, but autoantibodies have not been found in all RE cases [
8‐
11].
The observation of polarized granzyme B-containing CD8
+ T cells in brain parenchyma in close proximity to neurons and astrocytes has pointed to a role for major histocompatibility complex (MHC) class I-restricted CD8
+ cytotoxic T cells in RE [
12]. The cytotoxic T cells are likely reacting to foreign or self-antigens displayed by neurons and astrocytes in the affected cerebral hemisphere. Confinement of the T cells to one cerebral hemisphere suggests that the initial inflammatory reaction may have been spatially restricted. Such a reaction would have triggered a localized innate immune response by brain resident macrophages (microglia) and could have led to the recruitment of nonresident non-MHC-restricted immune cells, such as natural killer cells and γδ T cells followed by primed MHC-restricted αβ T cells. In the present study, we document for the first time the presence of clonally restricted γδ T cells in brain tissue from RE patients, indicating a role for this T cell subtype in the inflammatory response in RE.
Methods
RE patient cohort and clinical variables
Under the University of California, Los Angeles, Institutional Review Board (UCLA IRB) approval (IRB #11-00030), brain tissue and blood were collected at surgery as part of UCLA’s Pediatric Epilepsy Surgery Program. For cases that were not treated at UCLA, tissue and blood were provided to UCLA under the auspices of the UCLA IRB approved Rare Brain Disease Tissue Bank (IRB# 13-001213). All of the patients or their parents or legal guardians provided informed consent for the use of the surgical remnant and blood for research purposes. All specimens were collected using the same standard operating procedures (SOPs); SOPs were provided by UCLA to the contributing institutions. De-identified patient information was collected with informed consent including age at seizure onset, age at surgery, gender, and affected cerebral hemisphere.
Isolation of peripheral blood lymphocytes and brain-infiltrating lymphocytes
Peripheral blood mononuclear cells (PBMCs) were isolated by density gradient centrifugation using Ficoll-Paque PLUS (GE Healthcare, Piscataway, NJ). Brain-infiltrating lymphocytes (BILs) were isolated from collagenase-treated brain tissue by fractionation on a step gradient. Briefly, brain tissue was diced manually on ice in dissociation solution (HBSS with 20 mM HEPES pH 7.0, 5 mM glucose, and 50 U/ml penicillin/streptomycin). Tissue fragments were incubated with agitation in dissociation solution containing 0.5 mg/ml type IV collagenase (Worthington Biochemical Corp., Lakewood, NJ) and 5 % filtered human serum (Mediatech Inc., Manassas, VA) at 37 °C for 3 h or at room temperature overnight. The dissociated tissue was fractionated on a 30:70 % Percoll® (SigmaAldrich, St. Louis, MO) step gradient in RPMI containing 20 mM HEPES. PBMCs and BILs were cryopreserved in 90 % human serum/10 % DMSO.
Analysis of lymphocytes by flow cytometry
Phenotypic data were acquired on an analytical LSRII flow cytometer (Becton Dickinson, San Jose, CA). The following antibodies were used: APC-efluor® 780-conjugated CD3 (clone UCHT1; eBioscience Inc., San Diego, CA), PE/Cy7-conjugated CD4 (clone SK3; eBioscience Inc.), PerCP/Cy5.5-conjugated CD8 (clone RPA-T8; eBioscience Inc.), APC-conjugated T cell receptor (TCR) αβ (clone IP26; eBioscience Inc.), FITC-conjugated TCR γδ (clone B1.1; eBioscience Inc.), PE-conjugated CD69 (clone FN50; eBioscience Inc.), PE-conjugated TCR Vδ2 (clone B6; Biolegend), and FITC-conjugated TCR Vδ1 (Clone TS8.2; GeneTex Inc., Irvine, CA). Data were analyzed with FlowJo software (TreeStar Inc., Ashland, OR); plots were exported to CorelDRAW X6 (Corel Corporation, Ottawa, Canada). Statistical analyses and graphing utilized R-project programs (
www.r-project.org).
Spectratyping T cell receptor Vδ chains
Total RNA was purified from flash frozen blocks of involved tissue consisting of mostly cortical gray matter (~50 mg) using Trizol™ (Life Technologies, Carlsbad, CA) followed by column purification (Qiagen, Valencia, CA). RNA was reverse transcribed (Qiagen), and PCR reactions were carried out in an Applied Biosystems (ABI) GeneAmp® PCR system 9700 using AccuPrime™ Taq polymerase (Life Technologies). The same primer sets described by Dechanet et al. [
13] were used; forward primers were unique to Vδ1, Vδ2, and Vδ3, respectively, and two nested reverse primers were located in the constant region. The sequences of each primer are as follows: Vδ1 5′ CTGTCAACTTCAAGAAAGCAGCGAAATC 3′; Vδ2 5′ TACCGAGAAAAGGACATCTATGGC 3′; Vδ3 5′GGGGATAACAGCAGATCAGAAGGT 3′; Cδ1.1 5′ TGGGAGAGATGCAATAGCAGGATC 3′; Cδ1.2 5′ ACGGATGGTTTGGTAGAGGCTGA 3′. The Cδ1.2 primer was end-labeled with 6-carboxyfluorescein (FAM). The cycling conditions were as follows: 94 °C 2 min followed by 40 cycles of 94 °C 45 s, 60 °C 45 s, 68 °C 45 s, and a run off step at 68 °C for 4 min. For the second round of PCR, the number of cycles was reduced to 35 cycles. FAM-labeled products were separated on an ABI 3730 Capillary DNA Analyzer (Life Technologies), and peak areas were calculated using ABI Peak Scanner Software 2 (Life Technologies). In order to compare the relative amounts of each Vδ chain-specific fragment between samples, areas of individual peaks in each sample were normalized to the total peak area in each sample. Data were clustered and displayed as a heat map using the GENE-E bioinformatics package (
www.broadinstitute.org).
DNA sequencing T cell receptor Vδ CDR3s
First round Vδ2 and Vδ3 PCR fragments that yielded a single FAM-labeled fragment were re-amplified with the appropriate chain-specific forward primer and an unlabeled Cδ1.2 reverse primer. PCR products were treated with ExoSAP-IT (Affymetrix, Inc. Santa Clara, CA) and sequenced by the Sanger sequencing method using BigDye® cycle sequencing chemistry (Life Technologies). Sequences were analyzed using International Immunogenetics Information System (IMGT)/V-QUEST, the IMGT web portal for analysis of T cell receptor and immunoglobulin sequences [
14]. To sequence Vδ1 fragments, adaptors were added to the first round PCR products in the second PCR step, and the resulting DNA fragments were separated by agarose gel electrophoresis and purified (Qiagen). The following primers were used (TCR sequences are underlined): Vδ1 adaptor (forward primer) 5′ GAGACAGTCGTCGGCAGCGTCAGATGTATAACT
GTCAACTTCAAGAAAGCAGGAAATC 3′; Cδ1.2 adaptor (reverse primer) 5′ GTCTCGTGGGCTCGG AGATGTGTATAAGAGACAG
ACGGATGGTTTGGTAGAGGCTGA 3′. Nextera XT indices (Illumina, San Diego, CA) were added by tagging PCR using a KAPA Hifi PCR kit (KAPA Biosystems, Boston, MA), and the PCR products were purified using the Agencourt AMPure XP system (Beckman Coulter, Brea, CA). Fragments were sized on a Bioanalyzer instrument (Agilent Technologies, Inc. Santa Clara, CA), quantified by qPCR (KAPA library quant kit), and pooled. Libraries were sequenced on a MiSeq desktop sequencer using V2 2 × 250 bp chemistry (Illumina). IMGT/HighV-QUEST, the IMGT web portal for high throughput analysis of T cell receptor and immunoglobulin sequences was used to curate all of the sequence data [
14]. Scripts were written in R and Python to collate and enumerate the sequences. The δ1-specific sequences from cortical dysplasia (CD) brain samples were generated as described above. In the second PCR step, CDR3 clone-specific forward primers were used together with an extended Cδ1.1 primer, 5′ CTGGGAGAGATGACAATAGCAGGATCAAACTCTG 3′. The following forward primers were used: CDR3 ALGDSIPRRIAYTDKLI, 5′ GCTCTTGGGGATTCCATTCCTAGGAGGATAGCGTACACCGATAAACTCATC 3′; CDR3 ALGGLGTGGYAYTDKLI, 5′ GCTCTTGGGGGGCTAGGTACTGGGGGATACGCCTACACCGATAAACTCATC 3′; CDR3 ALGVPPRPSLYWGIGSLGSYTDKLI, 5′ GCTCTTGGGGTCCCGCCTCGACCTTCCCTCTACTGGGGGATAGGAAGCTTGGGCTCGTACACCGATAAACTCATC 3′. The cycling conditions were as follows: 94 °C 2 min followed by 35 cycles of 94 °C 45 s, 68 °C 90 s, and a run off step at 68 °C for 4 min. PCR products were gel purified and sequenced by the Sanger sequencing method using BigDye® cycle sequencing chemistry (Life Technologies). Sequences were aligned in Seaview [
15].
Immunocytochemistry
Surgical tissue blocks were fixed in freshly prepared 4 % paraformaldehyde in phosphate-buffered saline (PBS) for 24–48 h, then cryoprotected by immersion overnight at 4 °C in 20 % sucrose-PBS followed by 30 % sucrose-PBS. Blocks were then frozen with powdered dry ice and stored at −80 °C. Immunostaining was performed on free-floating 30-μm cryostat-cut sections by first blocking in PBS with 5 % normal goat serum (Vector Laboratories, Burlingame, CA) and 0.3 % Triton X-100 for 1 h. Sections were then incubated in mouse anti-human TCR pan γδ (clone 5A6.E9, 1:100, Thermo Scientific, Waltham, MA) and an anti-human CD3 rabbit polyclonal antibody (1: 100, Dako North America, Inc. Carpinteria, CA) overnight at 4 °C followed by incubation in Alexa Fluor® 488 goat anti-rabbit and Alexa Fluor® 568 goat anti-mouse secondary antibodies (1:1000, Life Technologies) for 1 h at room temperature. Sections were mounted on slides and cover-slipped using ProLong® Gold anti-fade reagent containing DAPI (Life Technologies). Fluorescent images were acquired with an Olympus spinning disk confocal microscope (Olympus America, Inc., Center Valley, PA) controlled by SlideBook™ image acquisition and analysis software (Intelligent Imaging Innovations, Inc. Denver CO). Images were transferred to CorelDRAWX6 (Corel Corporation). Sections (5 μm) of paraffin-embedded involved tissue were deparaffinized and microwaved for 20 min in buffered citrate (10 mM, pH 6.0) to retrieve antigens. After blocking for 1 h (Impress Kit, Vector Laboratories, Burlingame, CA), sections were incubated overnight at 4 °C with rabbit anti-human CD3 (1:800, Dako) or mouse anti-human CD69 (clone CH11, 1:50, Abcam, Cambridge, MA). Peroxidase-conjugated anti-rabbit or anti-mouse secondary antibodies (1: 300; Impress Kit, Vector Laboratories) were added for 1 h at room temperature, followed by incubation with 3,3′-diaminobenzidine (DAB) substrate (MP Biomedicals, Santa Ana, CA) and subsequent counterstaining with hematoxylin. Omission of primary antibodies served as negative controls. Images of the entire sections were acquired with an Aperio ScanScope XT scanner (Aperio, Vista CA) and transferred to CorelDRAWX6 (Corel Corporation).
Discussion
With the participation of eleven other institutions, we accrued fresh brain specimens from 20 RE surgeries over a 2.5-year period. We have carried out an initial characterization of T cells isolated from these specimens in comparison with T cells in peripheral blood from the same patients. Several new and confirmatory observations were made. The profile of brain-infiltrating CD3
+ T cells was relatively consistent, comprising mostly CD8
+ αβ and CD4
− CD8
− γδ T cells with fewer CD4
+ αβ T cells, whereas the profile of peripheral blood, collected at the time of surgery, was more variable. The predominance of CD8
+ T cells in the BIL populations agrees with published immunohistochemistry results [
12,
16‐
18], but the presence of γδ T cells is a new finding and has implications for the immune response in RE. This T cell subtype acts like an innate immune cell and plays an important role in both tumor and pathogen immune surveillance [
24,
25]. γδ T cells are mainly found in the skin, gut, lungs, and genitourinary tract and normally comprise only a small fraction of circulating T lymphocytes [
26,
27]. Unlike αβ T cells, γδ T cells recognize intact antigens and are not dependent on antigen presentation by classical MHC molecules [
19,
28]. They recognize transformed and infected cells via NKG2D- and TCR-dependent binding [
29]. Stress-induced self-proteins related to MHC molecules, lipid moieties bound to CD1 proteins, heat shock proteins, and phosphorylated metabolites made by certain pathogens and tumor cells are recognized by γδ T cells [
19,
28].
Vδ spectratyping showed that γδ T cells in RE brain tissue were oligoclonal and revealed the presence of dominant clonotypes. Vβ spectratyping has also provided evidence for oligoclonality of αβ T cells in RE [
30,
31]. Strikingly, the same CDR3 sequences in four abundant δ1 clonotypes were highly represented in every sample. This finding strongly indicates that in all of the RE cases analyzed, and possibly in all RE cases, there are clones of γδ T cells that recognize the same epitope(s). In a study of unrelated newborns who had been infected in utero with cytomegalovirus, T cells with identical TCR Vδ1 CDR3 sequences were found in the blood of every infected individual [
32]. It is thought that convergent recombination primarily accounts for the development of public T cell clones [
33].
The antigen(s) recognized by γδ T cells in RE are currently unknown. As mentioned above, CD1 proteins are ligands for γδ TCRs. The structures of two different δ1 TCRs complexed with CD1d-bound sulfatide and CD1d-bound α-galactocerebroside, respectively, were recently determined [
34,
35]. Two CDR3 motifs, YWG and TDK, that were found to directly interact with residues in CD1d are present in several of the clones that we sequenced (Fig.
6), suggesting the possibility that some γδ T cells in RE brain may bind CD1d. In addition, one of the prevalent CDR3 sequences that we identified contains the heptapeptide motif (AYTDKLI) that is also found in a δ1-containing TCR that binds a stress regulated, MHC class I related molecule, MHC class I polypeptide related sequence A (MICA) [
36]. It should be possible to derive clonal lines of γδ T cells from the RE BIL fractions and use binding assays to determine if they interact with these non-classical MHC class I molecules.
Using PCR primers designed to amplify the δ1 CDR3 region from three of the dominant clones, we detected the same sequences in dysplastic brain tissue. The PCR detection method used was not quantitative; thus, we do not yet know how prevalent these δ1 clones are in FCD brain. Further analysis of the γδ T cell repertoire in FCD is clearly warranted and will be the subject of future studies. The finding of γδ TCR sequences is not entirely unexpected as the presence of T cells in FCD brain has been reported [
23]. Finding the same δ1 clones in RE and FCD is also consistent with reports of dual pathology [
37‐
42]. Whether the γδ T cell clones that we have identified are only associated with inflammatory events in RE and FCD remains to be determined. If they can be detected in peripheral blood, then they might be useful as a diagnostic indicator of RE, FCD, or brain inflammation in general.
Both RE and FCD are associated with seizures, which have been shown to promote an inflammatory reaction in the brain [
43]. γδ T cells are likely to be among the first immune cells to cross the blood brain barrier in response to pro-inflammatory cytokines such as IL-1β and IL-18 released by inflammasomes [
43,
44]. The presence of inflammasomes associated with microglia in RE brain tissue has recently been described [
45]. In response to inflammatory cytokines, γδ T cells can act as an innate immune cell and release inflammatory cytokines in particular IL-17 without TCR engagement potentially perpetuating an inflammatory reaction [
46].
In RE, γδ T cells may provide a link between inflammation in the brain and an adaptive immune response involving antigen-specific CD8
+ T cells. In multiple sclerosis plaques and psoriatic lesions, γδ T cells are predominantly IL-17
+, linking this functional subtype to autoimmunity [
46‐
48]. On the other hand, they can act as an adaptive immune cell and bind a cognate antigen and produce IFN-γ [
49]. Neurons and astrocytes that express specific stress-induced autoantigens might be directly engaged by Th1-polarized γδ T cells. In mice, the costimulatory receptor CD27 is expressed by IFN-γ
+ but not IL-17
+ γδ T cells [
50]. Immunostaining BILs with CD27 antibodies may allow us to differentiate between these two functional types of γδ T cells in RE brain tissue. Th1 cytokines such as IFN-γ released by γδ T cells would also be expected to promote a cytotoxic CD8
+ αβ T cell response [
51]. IFN-γ increases MHC class I on neurons, which would render them selectively vulnerable to CD8
+ cytotoxic αβ T cells [
52]. γδ T cells may also play an immunosuppressive role depending upon the stage of the disease, as appears be the case in multiple sclerosis [
47]. Multiple roles for γδ T cells in RE might explain why the percentage of γδ T cells in CD3
+ BILs isolated at the time of surgery appeared to be unrelated to the length of time between seizure onset and surgery.
The cause of RE is unknown. A role for predisposing genetic factors that would influence the outcome of an immune response resulting from seizure-induced inflammation has been proposed [
53]. An infectious agent may also be involved in the etiology of RE [
4‐
7,
53,
54], which would fit with the known role of γδ T cells in pathogen surveillance. In a separate RNAseq study, we have looked for evidence of a persistent viral infection in resected RE brain tissue but have not found any gene transcripts encoded by known viruses (unpublished results). In the absence of a persistent infection, RE could involve T cells that recognize an epitope common to both a foreign and a self-antigen, T cells that express dual receptors, or epitope spreading [
55‐
58].
Acknowledgements
We are indebted to the following investigators for providing specimens from RE surgeries: Drs. Carlos Pardo and Adam Hartman (Johns Hopkins School of Medicine, Baltimore, MD); Drs. William Gaillard and Judy Liu (Children’s National Medical Center, Washington, DC); Drs. Sookyong Koh and Douglas Nordli (Northwestern University Feinberg School of Medicine, Chicago IL); Dr. Brent O’Neill (University of Colorado School of Medicine); Dr. Gerald Grant (Stanford University School of Medicine); Drs. Tiziana Granata and Elena Freri (Instituto Neurologico Carlo Besta, Milan, Italy); Dr. Alexandre Rainha Campos (CUF Descobertas Hospital, Lisbon, Portugal); Dr. Stephen Malone and Alex Micati (Children’s Hospital at Westmead, Australia); Drs. Tonicarlo Velasco and Helio Marcado (Hospital das Clínicas da Faculdade de Medicina de Ribeirão Preto da Universidade de São Paulo); Dr. Matt Wheatley (University of Alberta School of Medicine, Edmonton, Canada); and Dr. Amanda Yaun, (University of Oklahoma College of Medicine, Oklahoma City, OK). The Rasmussen Encephalitis Consortium contributes to the tissue transfer program, which is supported by the RE Children’s Project (
www.REchildrens.org). We would also like to thank Sugandha Dandekar and Hemani Wijesuriya, UCLA GenoSeq Core Laboratory for spectratyping and DNA sequencing. Financial support was provided by the RE Children’s Project (CAK/GWM/GCO), NIH R01 NS083823 (GWM), NIH R01CA125244 (CAK), R01CA154256 (CAK), and the University of California Pediatric Neuropathology Consortium (HVV).
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
GCO designed the study, performed PCRs, analyzed the data, and drafted the manuscript; KLE prepared PBMCs and carried out flow cytometry; CCM prepared BILs and carried out flow cytometry; CP performed the bioinformatics analysis; MH prepared BILs and carried out immunocytochemistry; JWC prepared BILs and PBMCs; TC coordinated acquisition of RE specimens; HVV provided tissue sections and helped draft the manuscript; GWM provided surgical specimens and helped draft the manuscript; CAK helped design the study and draft the manuscript. All authors read and approved the final manuscript.