Central nervous system neuronal surface antibody-associated syndromes. Clinical and Laboratory Characterization
Year: 2012
Type: dissertation
Study design: Observational Study
Abstract: The concept of antibody–associated central nervous system (CNS) disorders is relatively recent. The classical CNS paraneoplastic neurological syndromes are thought to be T-cell mediated, and the onconeural antibodies merely biomarkers for the presence of the tumour. Accordingly it was thought that antibodies rarely, if ever, caused CNS disease. Recently, however, it has become increasingly clear that there are CNS syndromes associated with antibodies that bind to cell surface determinants on neuronal cells and are likely to be pathogenic. Over the last ten years identification of autoimmune forms of encephalitis with antibodies against neuronal surface (NS) antigens, particularly the VGKC-complex proteins or the glutamate NMDA receptor, have shown that CNS disorders, often without associated tumours, can be antibody-mediated and benefit from immunomodulatory therapies. The clinical spectrum of these diseases is not yet fully explored, there may be others yet to be discovered, and some types of more common disorders, as epilepsy or psychosis, may prove to have an autoimmune basis. Here, we call these antibodies neuronal surface antibodies (NSAbs), and the diseases associated with them NSAb syndromes or NSAS. The aim of the research performed in the three years of PhD study has been to characterize NSAS from both a laboratory and a clinical point of view. The first year of the PhD was spent at the Oxford laboratory under the supervision of Prof. Angela Vincent. During this period I contributed to the laboratory characterization of the two most important groups of NSAS: encephalitis associated with VGKC-complex-Abs and NMDAR-Abs. The results of these studies have been published (Irani et al., Brain 2010a; Irani et al., 2010b). My personal contribution in these studies included the development and optimization of immunofluorescence techniques for the detection of autoantibodies against neuronal surface antigens (NSAbs) using both primary cultures of rat hippocampal neurons and mammalian cells transfected with the antigen of interest. In addition, during my time at Oxford I investigated a possible novel proteomics approach to identify new NSAb by means of immunoprecipitation and mass spectrometry. These experiments have subsequently been applied to screen samples of patients with autoimmune cerebellar syndromes. The results of this study are described in the third part of the thesis. The doctoral thesis is divided into three parts. In the first part I have reviewed the state of the art of syndromes associated with neuronal surface antibodies, starting with a comparison of the conditions associated with antibodies to intracellular antigens and those associated with antibodies to cell surface antigens. I have then summarised the main clinical and paraclinical features of the syndromes that have already been identified. On the basis of these observations, I have designed guidelines for recognizing these and other immune-mediated conditions in the future. The guidelines work has been supervised by Prof. Angela Vincent (Oxford, UK) and critically reviewed by Prof. Francesc Graus (Barcelona, Spain), Prof. Christian Bien (Bielefeld, Germany) and Dr. Bruno Giometto (Treviso, Italy). The second part of the thesis describes the clinical and immunological characterization of a cohort of patients with suspected autoimmune encephalitis and VGKC-complex-Abs that I collected and reviewed in Padua during the 3-year period of the PhD. Recently the Oxford group showed that most antibodies previously attributed to VGKC (by radioimmunoprecipitation) target two proteins complexed with the channels Lgi1 and Caspr2, which also tend to associate with LE and Morvan’s syndrome phenotypes, respectively (see (Irani et al., Brain 2010b)). I therefore decided to focus my research on patients with autoimmune encephalitis associated with ‘VGKC’-complex-Abs in order to report their clinical and immunological profile, with a view to showing a possible antigen-phenotype correlation. During the 3-year period of my PhD, I screened 503 samples for NSAbs from 366 patients with suspected autoimmune encephalitis sent to our laboratory between January 2005 and December 2011: 279 serum (and cerebrospinal fluid) samples from 232 patients were tested for VGKC-complex-Abs; in addition 226 samples were tested for NMDAR-Abs, 59 for AMPAR1,2- and GABAbR-Abs, 91 for GAD-Abs; 21 samples were also tested on live hippocampal neurons. Screening methods included: radioimmunoprecipitation (RIA) assay for VGKC-Abs and GAD65-Abs; cell-based assay for antibodies binding human NMDAR, AMPAR1, AMPAR2, GABAbR, Lgi1 and Caspr2; indirect immunofluorescence on live hippocampal cell cultures. Thirty-two of the 232 patients tested for VGKC-complex-Abs were positive. Clinical data were available for 18 patients: 10 patients harboured Lgi1-Abs in their sera, 3 Caspr2-Abs, 5 patients were positive on RIA but not on IIF. Two patients had Morvan’s syndrome, one associated with Lgi1-ab and the other with Caspr2-Ab and a thymoma. Response to immune modulation was reported in all patients with the exception of a patient who died from lung cancer. A consistently high VGKC-complex-Abs titre despite clinical remission prompted the discovery and subsequent removal of an enlarged hyperplastic thymus in a patient with Lgi1-Abs associated LE; one Caspr2-Abs positive patient had mild encephalitis overlapping with an anti-phospholipid syndrome; another patient with an immunotherapy-responsive encephalomyelitis tested negative on IIF but had high-titre VGKC-complex-Abs on RIA. Striatal hypermetabolism was found in 4 patients with Lgi1-Abs in one case associated with faciobrachial dystonic seizures. In conclusion, these results have expanded the clinical spectrum of autoimmune encephalopathy associated with VGKC-complex antibody. It has been shown that VGKC-LE can associate not only with thymomas, but also with a hyperplastic thymus, similarly to the well-known finding in myasthenia gravis and therefore that the persistence of high VGKC-Abs titres should prompt the search for an enlarged thymus. The third part of the thesis describes a study whose aim was to identify new autoantibodies in cerebellar syndromes of possible autoimmune origin. Dr. Esther Becker from Oxford University contributed equally to me in this work. Relatively few studies have searched for potentially pathogenic antibodies in non-paraneoplastic patients with cerebellar ataxia. We first screened sera from more 52 idiopathic ataxia patients for the binding of serum IgG antibodies to cerebellar neurons. One strong-binding serum was selected for immunoprecipitation and mass spectrometry, which resulted in the identification of contactin-associated protein 2 (Caspr2) as a major antigen. Caspr2 antibodies were then found by a cell-based assay in 9/88 (10%) ataxia patients, compared to 3/144 (2%) multiple sclerosis or dementia controls. Caspr2 is strongly expressed in the cerebellum, only partly in association with voltage-gated potassium channels. Prospective studies are needed to see whether identification of Caspr2-antibodies has relevance for the diagnosis and treatment of idiopathic cerebellar ataxia.
Author: Zuliani, Luigi
Language: English
Subfield: Clinical Neurology
Field: Medicine
Domain: Health Sciences
Open Access status: green
OpenAlex ID: W79514353