The effects of citrullination or variable amino-terminus acylation on the encephalitogenicity of human myelin basic protein in the PL/J mouse

Lyso-Lipid-Induced Oligodendrocyte Maturation Underlies Restoration of Optic Nerve Function

Protein hyperdeimination and deficiency of lyso-phospholipids (LPC 18:1) has been associated with the pathology of demyelinating disease in both humans and mice. We uncovered interesting biology of LPC 18:1, in which LPC 18:1 induced optic nerve function restoration through oligodendrocyte maturation and remyelination in mouse model systems. Our in vitro studies show LPC 18:1 protection against neuron-ectopic hyperdeimination and stimulation of oligodendrocyte maturation, while in vivo investigations recorded optic nerve function improvement following optic nerve injections of LPC 18:1, in contrast with LPC 18:0. Thus, just a change in a single bond renders a dramatic alternation in biological function. The incorporation of isobaric C13-histidine in newly synthesized myelin proteins and quantitative proteome shifts are consistent with remyelination underlying restoration in optic nerve function. These results suggest that exogenous LPC 18:1 may provide a therapeutic avenue for stemming vision loss in demyelinating diseases.

Myelin Basic Protein Phospholipid Complexation Likely Competes with Deimination in Experimental Autoimmune Encephalomyelitis Mouse Model

Multiple sclerosis has complex pathogenesis encompassing a variety of components (immunologic, genetic, and environmental). The autoimmunogenicity against the host's myelin basic protein is a major contributor. An increase in myelin basic protein deimination (a post-translational modification) and a change in phospholipid composition have been associated with multiple sclerosis. The interaction of myelin basic protein with phospholipids in the myelin membrane is an important contributor to the stability and maintenance of proper myelin sheath function. The study of this aspect of multiple sclerosis is an area that has yet to be fully explored and that the present study seeks to understand. Several biochemical methods, a capillary electrophoresis coupled system and mass spectrometry, were used in this study. These methods identified four specific phospholipids complexing with myelin basic protein. We show that lysophosphatidylcholine 18:1 provides a robust competitive effect against hyper-deimination. Our data suggest that lysophosphatidylcholine 18:1 has a different biochemical behavior when compared to other phospholipids and lysophosphatidylcholines 14:0, 16:0, and 18:0.

Lewis Rat Model of Experimental Autoimmune Encephalomyelitis

In this unit, we describe in detail the most common methods used to break immunological tolerance for central myelin antigens and induce experimental autoimmune encephalomyelitis (EAE) in Lewis rats as an animal model of multiple sclerosis. The resulting disease course ranges from an acute monophasic disease to a chronic relapsing or chronic progressive course, which strongly resembles the human disease. These models enable the study of cellular and humoral autoimmunity against major antigenic epitopes of the myelin basic protein, myelin oligodendrocyte glycoprotein, or proteolipid protein. We provide an overview of common immunization protocols for induction of active and passive EAE, assessment and analysis of clinical score, preparation and purification of myelin basic protein, and derivation of neuroantigen-specific rat T cell lines. Finally, we describe the major clinical characteristics of these models. © 2017 by John Wiley & Sons, Inc.

β cell ER stress and the implications for immunogenicity in type 1 diabetes

Type 1 diabetes (T1D) is a chronic autoimmune disease characterized by hyperglycemia due to progressive immune-mediated destruction of insulin-producing pancreatic islet β cells. Although many elegant studies have identified β cell autoantigens that are targeted by the autoimmune response, the mechanisms by which these autoantigens are generated remain poorly understood. Normal β cell physiology includes a high demand for insulin production and secretion in response to dynamic glucose sensing. This secretory function predisposes β cells to significantly higher levels of endoplasmic reticulum (ER) stress compared to nonsecretory cells. In addition, many environmental triggers associated with T1D onset further augment this inherent ER stress in β cells. ER stress may increase abnormal post-translational modification (PTM) of endogenous β cell proteins. Indeed, in other autoimmune disorders such as celiac disease, systemic lupus erythematosus, multiple sclerosis, and rheumatoid arthritis, abnormally modified neo-antigens are presented by antigen presenting cells (APCs) in draining lymph nodes. In the context of genetic susceptibility to autoimmunity, presentation of neo-antigens activates auto-reactive T cells and pathology ensues. Therefore, the ER stress induced by normal β cell secretory physiology and environmental triggers may be sufficient to generate neo-antigens for the autoimmune response in T1D. This review summarizes what is currently known about ER stress and protein PTM in target organs of other autoimmune disease models, as well as the data supporting a role for ER stress-induced neo-antigen formation in β cells in T1D.

Dexamethasone therapy for preventing delayed encephalopathy after carbon monoxide poisoning

We investigated dexamethasone therapy for preventing delayed encephalopathy after carbon monoxide (CO) poisoning. Eighty healthy male rats were exposed to CO and randomly divided into four groups: hyperbaric oxygen treatment (H), treatment (D), combined hyperbaric and dexamethasone treatment (C), and a control (M) group in which the rats inhaled CO to coma in the hyperbaric oxygen chamber, then were removed without further treatment. Twelve rats were put into the hyperbaric oxygen chamber and treated with air for 60 min (N) group. An eight arm maze was used to evaluate cognitive and memory abilities of these mice. Serum myelin basic protein (MBP) levels were evaluated using ELISA, and magnetic resonance imaging was used to observe brain demyelination and morbidity associated with delayed encephalopathy. A sample of the hippocampus from each group was examined by light microscopy. Cognitive and memory functions decreased in the control group M. Three days after CO poisoning, the serum MBP level of each group increased significantly. On Day 10 after CO poisoning, the MBP levels in groups C and D decreased significantly, but returned to normal on Day 18. MBP levels in the M and H groups were elevated at all time points. Brain MRIs showed significant differences among C, D, H and control M groups. Hematoxylin & eosin staining of the hippocampus showed greater damage in the control M and H groups. Early dexamethasone treatment may be useful for preventing delayed encephalopathy after CO poisoning and may reduce serum MBP levels.

Contribution of myelin autoantigen citrullination to T cell autoaggression in the central nervous system

Breakdown in immunological self tolerance, leading to autoimmune diseases such as multiple sclerosis, might arise from immune recognition of self proteins that have undergone heightened posttranslational modification under pathophysiological conditions. A posttranslational modification of particular interest is the deimination of Arg to citrulline, catalyzed by peptidylarginyl deiminase (PAD) enzymes. As a CD4(+) T cell-driven model of multiple sclerosis, we used experimental autoimmune encephalomyelitis (EAE) induced with the immunodominant 35-55 peptide of myelin oligodendrocyte glycoprotein (pMOG) in C57BL/6 mice to test whether citrullination of a T cell epitope can contribute to disease etiopathology. Immunization with an altered peptide ligand (APL) of pMOG with an Arg-->citrulline conversion at a TCR contact (residue 41) led to the activation of two populations of APL-responsive T cells that either did, or did not cross-react with the native pMOG peptide. This APL could induce EAE. However, this reflected the activation of T cells that cross-reacted with the native pMOG epitope, because prior tolerization of these T cells using pMOG prevented APL-induced EAE. Using a passive transfer model, we found that T cells that responded specifically to the citrullinated form of pMOG were neither necessary, nor sufficient to initiate the EAE lesion. Nevertheless, these cells could provoke exacerbation of pathology if transferred into mice with ongoing EAE. The PAD2 and PAD4 enzymes were markedly upregulated in the inflamed CNS. Therefore, once inflammation is established, citrullination of target autoantigens can allow an expanded repertoire of T cells to contribute to CNS pathology.

Epigenetic mechanisms facilitating oligodendrocyte development, maturation, and aging

The process of oligodendrocyte differentiation is regulated by a dynamic interaction between a genetic and an epigenetic program. Recent studies, addressing nucleosomal histone modifications have considerably increased our knowledge regarding epigenetic regulation of gene expression during oligodendrocyte development and aging. These results have generated new hypotheses regarding the mechanisms underlying the decreased efficiency of endogenous remyelination in response to demyelinating injuries with increasing age. In this review, we present an overview of the epigenetic mechanisms regulating gene expression at specific stages of oligodendrocyte differentiation and maturation as well as the changes that occur with aging.

Evaluating epigenetic landmarks in the brain of multiple sclerosis patients: a contribution to the current debate on disease pathogenesis

The evidence suggesting a role of epigenetics in the definition of complex trait diseases is rapidly increasing. The gender prevalence of multiple sclerosis, the low level concordance in homozygous twins and the linkage to several genetic loci, suggest an epigenetic component to the definition of this demyelinating disorder. While the immune etio-pathogenetic mechanism of disease progression has been well characterized, still relatively little is known about the initial events contributing to onset and progression of the demyelinating lesion. This article addresses the challenging question of whether loss of the mechanisms of epigenetic regulation of gene expression in the myelinating cells may contribute to the pathogenesis of multiple sclerosis, by affecting the repair process and by modulating the levels of enzymes involved in neo-epitope formation. The role of altered post-translational modifications of nucleosomal histones and DNA methylation in white matter oligodendroglial cells are presented in terms of pathogenetic concepts and the relevance to therapeutic intervention is then discussed.

Passive induction of experimental allergic encephalomyelitis

Experimental allergic encephalomyelitis (EAE) is a widely used animal model of the human demyelinating disease multiple sclerosis. EAE is initiated by immunization with myelin antigens in adjuvant or by adoptive transfer of myelin-specific T cells, resulting in inflammatory infiltrates and demyelination in the central nervous system. Induction of EAE in rodents typically results in ascending flaccid paralysis with inflammation primarily targeting the spinal cord. This protocol describes passive induction of EAE by adoptive transfer of T cells isolated from mice primed with myelin antigens into naïve mice. The advantages of using this method versus active induction of EAE are discussed.

Active induction of experimental allergic encephalomyelitis

This protocol details a method to actively induce experimental allergic encephalomyelitis (EAE), a widely used animal model for studies of multiple sclerosis. EAE is induced by stimulating T-cell-mediated immunity to myelin antigens. Active induction of EAE is accomplished by immunization with myelin antigens emulsified in adjuvant. This protocol focuses on induction of EAE in mice; however, the same principles apply to EAE induction in other species. EAE in rodents is manifested typically as ascending flaccid paralysis with inflammation targeting the spinal cord. However, more diverse clinical signs can occur in certain strain/antigen combinations in rodents and in other species, reflecting increased inflammation in the brain.

Distinct T cell recognition of naturally processed and cryptic epitopes within the immunodominant 35-55 region of myelin oligodendrocyte glycoprotein

We have assessed the complexity in T cell recognition of the immunodominant 35-55 region of myelin oligodendrocyte glycoprotein (MOG) in C57BL/6 mice. Immunization with the p35-55 peptide generated two types of T cell, recognizing either a cryptic, or a naturally-processed epitope. Clear differences in the recognition of residues within a core sequence of 40-48 were observed. The majority of the p35-55-reactive repertoire in vivo appeared responsive to the intact autoantigen, supporting the notion of a failure of central tolerance to this region of MOG. Our data also provide a basis for exploring the requirements for antigen processing of MOG.

A Tale of Two Citrullines—Structural and Functional Aspects of Myelin Basic Protein Deimination in Health and Disease

Myelin basic protein (MBP) binds to negatively charged lipids on the cytosolic surface of oligodendrocyte membranes and is responsible for adhesion of these surfaces in the multilayered myelin sheath. The pattern of extensive post-translational modifications of MBP is dynamic during normal central nervous system (CNS) development and during myelin degeneration in multiple sclerosis (MS), affecting its interactions with the myelin membranes and with other molecules. In particular, the degree of deimination (or citrullination) of MBP is correlated with the severity of MS, and may represent a primary defect that precedes neurodegeneration due to autoimmune attack. That the degree of MBP deimination is also high in early CNS development indicates that this modification plays major physiological roles in myelin assembly. In this review, we describe the structural and functional consequences of MBP deimination in healthy and diseased myelin.

Post-translational modifications of self antigens: implications for autoimmunity

Alterations in amino acid sequence can generate neo-epitopes from self proteins, causing autoaggressive immune attack. There is a range of possible post-translational modifications (PTMs) of mammalian proteins that can allow immune recognition of neo-self epitopes. These effects can vary from overt increase in affinity of MHC or T-cell receptor binding, to more subtle effects on the activity of proteolytic enzymes involved in antigen processing. Furthermore, intriguing insights into how the complex interactions between inflammation, enzyme activity and protein modification can direct self recognition are beginning to be unearthed.

Delayed neuropathology after carbon monoxide poisoning is immune-mediated

The neuropathological sequelae of carbon monoxide (CO) poisoning cannot be explained by hypoxic stress alone. CO poisoning also causes adduct formation between myelin basic protein (MBP) and malonylaldehyde, a reactive product of lipid peroxidation, resulting in an immunological cascade. MBP loses its normal cationic characteristics, and antibody recognition of MBP is altered. Immunohistochemical evidence of degraded MBP occurs in brain over days, along with influx of macrophages and CD-4 lymphocytes. Lymphocytes from CO-poisoned rats subsequently exhibit an auto-reactive proliferative response to MBP, and there is a significant increase in the number of activated microglia in brain. Rats rendered immunologically tolerant to MBP before CO poisoning exhibit acute biochemical changes in MBP but no lymphocyte proliferative response or brain microglial activation. CO poisoning causes a decrement in learning that is not observed in immunologically tolerant rats. These results demonstrate that delayed CO-mediated neuropathology is linked to an adaptive immunological response to chemically modified MBP.

Thiopalmitoylation of myelin proteolipid protein epitopes enhances immunogenicity and encephalitogenicity

Proteolipid protein (PLP) is the most abundant protein of CNS myelin, and is posttranslationally acylated by covalent attachment of long chain fatty acids to cysteine residues via a thioester linkage. Two of the acylation sites are within epitopes of PLP that are encephalitogenic in SJL/J mice (PLP(104-117) and PLP(139-151)) and against which increased immune responses have been detected in some multiple sclerosis patients. It is known that attachment of certain types of lipid side chains to peptides can result in their enhanced immunogenicity. The aim of this study was to determine whether thioacylated PLP peptides, as occur in the native protein, are more immunogenic than their nonacylated counterparts, and whether thioacylation influences the development of autoreactivity and experimental autoimmune encephalomyelitis. The results show that in comparison with nonacylated peptides, thioacylated PLP lipopeptides can induce greater T cell and Ab responses to both the acylated and nonacylated peptides. They also enhanced the development and chronicity of experimental autoimmune encephalomyelitis. Synthetic peptides in which the fatty acid was attached via an amide linkage at the N terminus were not encephalitogenic, and they induced greater proportions of CD8+ cells in initial in vitro stimulation. Therefore, the lability and the site of the linkage between the peptide and fatty acid may be important for induction of encephalitogenic CD4+ T cells. These results suggest that immune responses induced by endogenous thioacylated lipopeptides may contribute to the immunopathogenesis of chronic experimental demyelinating diseases and multiple sclerosis.

SR proteins are autoantigens in patients with systemic lupus erythematosus. Importance of phosphoepitopes

OBJECTIVE

To determine whether members of the highly phosphorylated SR protein family are autoantigens and, if so, to determine the frequency and molecular basis of antigen recognition.

METHODS

Native human SR proteins were purified to homogeneity from HeLa cells, and an enzyme-linked immunosorbent assay (ELISA) was developed. Further studies employed immunoblotting of both phosphorylated and dephosphorylated SR proteins.

RESULTS

Anti-SR protein reactivity was frequently detected in the sera of patients with systemic lupus erythematosus (SLE). Sera from 52% of the SLE patients in a group of patients with a variety of autoimmune and other disorders (n = 137) and from 50% of the SLE patients in a separate group (n = 102) were positive in an ELISA. In contrast, sera from patients with other disorders, such as rheumatoid arthritis and primary antiphospholipid syndrome, reacted infrequently. Reactivity with double-stranded DNA (dsDNA), used in the diagnosis of SLE, did not correlate with SR protein reactivity. Anti-SR autoantisera did not bind highly charged unphosphorylated peptides related to the SR domain, which is rich in arginine and phosphoserine residues. Surprisingly, many of the epitopes were influenced by the presence or absence of SR protein phosphorylation. In immunoblots, some patient sera lost reactivity upon SR protein dephosphorylation, while others significantly gained reactivity.

CONCLUSION

We have identified a novel set of autoantigens in SLE, the SR protein family of non-small nuclear RNP pre-messenger RNA splicing factors. Anti-SR autoantibodies are distinct from those which bind dsDNA. The identification of this new set of autoantigens and the observation that the auto-epitope(s) involves posttranslational modification offer new possibilities for understanding autoimmunity and its development.

Analogous structural motifs in myelin basic protein and in MARCKS

Myelin basic protein (MBP) and myristoylated alanine-rich C-kinase substrate (MARCKS) are similar in terms of having extended conformations regulated by their environment (i.e., solubilised or lipid-associated), N-terminal modifications, a dual nature of interactions with lipids, binding to actin and Ca2+-calmodulin, and being substrates for different kinds of protein kinases. The further sequence similarities of segments of MBP with lipid effector regions of MARCKS, and numerous reports in the literature, support the thesis that some developmental isoform of MBP functions in signal transduction.

Tolerance induction by acylated peptides: effect on encephalitogenic T cell lines

We reported previously that acylation of an encephalitogenic peptide of myelin basic protein (MBP68-86) by attachment of palmitoyl chloride (PAL68-86) converted this peptide into a powerful tolerogen for EAE in the Lewis rat. In this study we show that T cell lines derived from a PAL68-86-protected rat proliferated poorly to MBP68-86 in vitro, even after repeated passages in this peptide and IL-2. Conversely, T cell lines derived from untreated rats that were challenged with MBP68-86 or PAL68-86 in CFA responded vigorously to MBP68-86 when propagated for many passages in this peptide but became gradually unresponsive after being propagated in the presence of PAL68-86. The modulation of the T cell lines by PAL68-86 in vitro was reflected by a significant reduction in their ability to transfer EAE to recipients. A high percentage of cells stained with an anti-Vbeta8.2 antibody, regardless of whether they were propagated in the presence of unmodified or acylated peptide. The results are consistent with the notion that tolerance induced by PAL68-86 operates by functional inactivation and provide the basis for the use of acylated peptides in the antigen-specific treatment of autoimmune diseases.

Citrullinated myelin basic protein induces experimental autoimmune encephalomyelitis in Lewis rats through a diverse T cell repertoire

An increased proportion of citrullinated MBP (MBP-C8) occurs in the brains of multiple sclerosis (MS) patients. In this study, MBP-C8 from guinea pig (GP) brains was isolated and found encephalitogenic in Lewis rats upon immunization. An encephalitogenic T cell line selected with MBP-C8 preferentially reacted with MBP-C8 over unmodified MBP. This T cell line responded weakly to the dominant encephalitogenic epitope, GP-MBP peptide 70-88, and did not display restricted TCR beta-chain usage (such as Vbeta88.2). The distinctive features of MBP-C8 were also demonstrated by its ability to reinduce active EAE in 70% of rats which had recovered from unmodified MBP induced EAE. These findings raise the possibility that citrullinated MBP may elicit a different pathogenic T cell repertoire for the recurrent phases of inflammatory demyelination.