Branched chain α-ketoacid dehydrogenase kinase 111-130, a T cell epitope that induces both autoimmune myocarditis and hepatitis in A/J mice

The role of amino acid metabolism in autoimmune hepatitis

Autoimmune hepatitis (AIH) is an inflammatory chronic liver disease with persistent and recurrent immune-mediated liver injury. The exact cause of AIH is still not fully understood, but it is believed to be primarily due to an abnormal activation of the immune system, leading to autoimmune injury caused by the breakdown of autoimmune tolerance. Although the pathogenesis of AIH remains unclear, recent studies have shown that abnormalities in amino acid metabolism play significant roles in its development. These abnormalities in amino acid metabolism can lead to remodeling of metabolic processes, activation of signaling pathways, and immune responses, which may present new opportunities for clinical intervention in AIH. In this paper, we first briefly outline the recent progress of clinically relevant research on AIH, focusing on the role of specific amino acid metabolism (including glutamine, cysteine, tryptophan, branched-chain amino acids, etc.) and their associated metabolites, as well as related pathways, in the development of AIH. Furthermore, we discuss the scientific issues that remain to be resolved regarding amino acid metabolism, AIH development and related clinical interventions, with the aim of contributing to the future development of amino acid metabolism-based as a new target for the clinical diagnosis and treatment of AIH.

Transgenic Mice Expressing Functional TCRs Specific to Cardiac Myhc-α 334-352 on Both CD4 and CD8 T Cells Are Resistant to the Development of Myocarditis on C57BL/6 Genetic Background

Myocarditis is a predominant cause of congestive heart failure and sudden death in children and young adolescents that can lead to dilated cardiomyopathy. Lymphocytic myocarditis mediated by T cells can result from the recognition of cardiac antigens that may involve CD4 or CD8 T cells or both. In this report, we describe the generation of T cell receptor (TCR) transgenic mice on a C57BL/6 genetic background specific to cardiac myosin heavy chain (Myhc)-α 334-352 and make the following observations: First, we verified that Myhc-α 334-352 was immunogenic in wild-type C57BL/6 mice and induced antigen-specific CD4 T cell responses despite being a poor binder of IAb; however, the immunized animals developed only mild myocarditis. Second, TCRs specific to Myhc-α 334-352 in transgenic mice were expressed in both CD4 and CD8 T cells, suggesting that the expression of epitope-specific TCR is common to both cell types. Third, although T cells from naïve transgenic mice did not respond to Myhc-α 334-352, both CD4 and CD8 T cells from animals immunized with Myhc-α 334-352 responded to the peptide, indicating that antigen priming is necessary to break tolerance. Fourth, although the transgenic T cells could produce significant amounts of interferon-γ and interleukin-17, the immunized animals developed only mild disease, indicating that other soluble factors might be necessary for developing severe myocarditis. Alternatively, the C57BL/6 genetic background might be a major contributing factor for resistance to the development of myocarditis. Taken together, our model permits the determination of the roles of both CD4 and CD8 T cells to understand the disease-resistance mechanisms of myocarditis in a single transgenic system antigen-specifically.

Attenuated strain of CVB3 with a mutation in the CAR-interacting region protects against both myocarditis and pancreatitis

Coxsackievirus B3 (CVB3), is commonly implicated in myocarditis, which can lead to dilated cardiomyopathy, in addition to causing acute pancreatitis and meningitis. Yet, no vaccines are currently available to prevent this infection. Here, we describe the derivation of a live attenuated vaccine virus, termed mutant (Mt) 10, encoding a single amino acid substitution H790A within the viral protein 1, that prevents CVB3 infection in mice and protects from both myocarditis and pancreatitis in challenge studies. We noted that animals vaccinated with Mt 10 developed virus-neutralizing antibodies, predominantly containing IgG2a and IgG2b, and to a lesser extent IgG3 and IgG1. Furthermore, by using major histocompatibility complex class II dextramers and tetramers, we demonstrated that Mt 10 induces antigen-specific T cell responses that preferentially produce interferon-γ. Finally, neither vaccine recipients nor those challenged with the wild-type virus revealed evidence of autoimmunity or cardiac injury as determined by T cell response to cardiac myosin and measurement of circulating cardiac troponin I levels, respectively. Together, our data suggest that Mt 10 is a vaccine candidate that prevents CVB3 infection through the induction of neutralizing antibodies and antigen-specific T cell responses, the two critical components needed for complete protection against virus infections in vaccine studies.

Viral myocarditis involves the generation of autoreactive T cells with multiple antigen specificities that localize in lymphoid and non-lymphoid organs in the mouse model of CVB3 infection

Autoreactive T cells may contribute to post-viral myocarditis induced with Coxsackievirus B3 (CVB3), but the underlying mechanisms of their generation are unclear. Here, we have comprehensively analyzed the generation of antigen-specific, autoreactive T cells in the mouse model of CVB3 infection for antigens implicated in patients with myocarditis/dilated cardiomyopathy. First, comparative analysis of CVB3 proteome with five autoantigens led us to identify three mimicry epitopes, one each from adenine nucleotide translocator 1 (ANT), sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase 2a (SERCA2a) and cardiac troponin I. None of these induced cross-reactive T cell responses. Next, we generated major histocompatibility complex (MHC) class II dextramers to enumerate the frequencies of antigen-specific T cells to determine whether T cells with multiple antigen specificities are generated by CVB3 infection. These analyses revealed appearance of CD4 T cells positive for SERCA2a 971-990, and cardiac myosin heavy chain-α (Myhc) 334-352 dextramers, both in the periphery and also in the hearts of CVB3-infected animals. While ANT 21-40 dextramer⁺ T cells were inconsistently detected, the β1-adrenergic receptor 181-200/211-230 or branched chain α-ketoacid dehydrogenase kinase 111-130 dextramer⁺ cells were absent. Interestingly, SERCA2a 971-990, Myhc 334-352 and ANT 21-40 dextramer⁺ cells were also detected in the liver indicating that they may have a pathogenic role. Finally, we demonstrate that the SERCA2a 971-990-reactive T cells generated in CVB3 infection could transfer disease to naïve mice. The data suggest that CVB3 infection can lead to the generation of autoreactive T cells for multiple antigens indicating a possibility that the autoreactive T cells localized in the liver can potentially circulate and contribute to the development of viral myocarditis.

Identification of Immunogenic Epitopes That Permit the Detection of Antigen-Specific T Cell Responses in Multiple Serotypes of Group B Coxsackievirus Infections

Coxsackievirus group B (CVB) contains six serotypes that can affect various organs. Some of these organ-specific diseases such as myocarditis and pancreatitis can be caused by more than one serotype. Thus, development of immunological tools common to multiple serotypes is desired. This is especially critical for analyzing antigen-specific T cell responses at a single cell level. To this end, we made efforts to identify the immunogenic epitopes of CVB3 leading us to localize three T cell epitopes within the viral protein 1 (VP1) namely, VP1 681-700, VP1 721-740 and VP1 771-790. First, we confirmed their immunogenicity in the immunization settings. Second, we sought to verify the ability of VP1 epitopes to bind major histocompatibility complex (MHC) class II (IA^k) molecules. Third, we created MHC class II (IA^k) dextramers and tetramers and ascertained the T cell responses to be antigen-specific. Fourth, we analyzed the T cell responses in animals infected with CVB3 and noted the magnitude of antigen-specific T cell responses occurring in the order of VP1 721-740 and VP1 681-700 followed by VP1 771-790 as verified by proliferation assay and IA^k tetramer staining. All epitopes induced interferon (IFN)-γ as a major cytokine. Finally, we investigated whether the VP1 tools generated for CVB3 can also be used to verify T cell responses in infections caused by other serotypes. To this end, we established the CVB4 infection model in A/J mice and found that the CVB4 infection led to the induction of IFN-γ-producing T cell responses primarily for VP1 721-740 and VP1 681-700. Thus, the VP1-specific tools, particularly IA^k tetramers can be used to monitor anti-viral T cell responses in multiple CVB serotypes.

An evidence for surface expression of an immunogenic epitope of sarcoplasmic/endoplasmic reticulum calcium-ATPase2a on antigen-presenting cells from naive mice in the mediation of autoimmune myocarditis

We recently reported identification of sarcoplasmic/endoplasmic reticulum calcium-ATPase2a (SERCA2a) 971-990, which induces atrial myocarditis by generating autoreactive T cells in A/J mice. However, it was unknown how antigen-sensitized T cells could recognize SERCA2a 971-990, since SERCA2a-expression is confined to an intracellular compartment. In this report, we present evidence that antigen-presenting cells (APCs) from lymphoid and non-lymphoid organs in naïve animals present SERCA2a 971-990 and stimulate antigen-specific T cells. Using major histocompatibility complex (MHC) class II dextramers for SERCA2a 971-990, we created a panel of T cell hybridomas and demonstrated that splenocytes from naïve A/J mice stimulated the hybridoma cells without exogenous supplementation of SERCA2a 971-990. We then recapitulated this phenomenon by using SERCA2a 971-990 -specific primary T cells, verifying that the T cell responses were MHC-restricted. Furthermore, SERCA2a 971-990 -sensitzed T cells exposed to APCs from naïve mice were found to produce the inflammatory cytokines interferon-γ, granulocyte macrophage colony stimulating factor, and interleukin-17A, which are implicated in the induction of myocarditis. Finally, while T cells exposed to mononuclear cells (MNCs) obtained from heart and liver also responded similarly to splenocytes, endothelial cells (ECs) generated from the corresponding organs displayed opposing effects, in that the proliferative responses were suppressed with the heart ECs, but not with the liver ECs. Taken together, our data suggest that the surface expression of SERCA2a 971-990 by naïve APCs can potentially trigger pathogenic autoreactive T cell responses under conditions of autoimmunity, which may have implications in endothelial dysfunction.

Epitope Mapping of SERCA2a Identifies an Antigenic Determinant That Induces Mainly Atrial Myocarditis in A/J Mice

Sarcoplasmic/endoplasmic reticulum Ca2+ adenosine triphosphatase (SERCA)2a, a critical regulator of calcium homeostasis, is known to be decreased in heart failure. Patients with myocarditis or dilated cardiomyopathy develop autoantibodies to SERCA2a suggesting that they may have pathogenetic significance. In this report, we describe epitope mapping analysis of SERCA2a in A/J mice that leads us to make five observations: 1) SERCA2a contains multiple T cell epitopes that induce varying degrees of myocarditis. One epitope, SERCA2a 971-990, induces widespread atrial inflammation without affecting noncardiac tissues; the cardiac abnormalities could be noninvasively captured by echocardiography, electrocardiography, and magnetic resonance microscopy imaging. 2) SERCA2a 971-990-induced disease was associated with the induction of CD4 T cell responses and the epitope preferentially binds MHC class II/IAk rather than IEk By creating IAk/and IEk/SERCA2a 971-990 dextramers, the T cell responses were determined by flow cytometry to be Ag specific. 3) SERCA2a 971-990-sensitized T cells produce both Th1 and Th17 cytokines. 4) Animals immunized with SERCA2a 971-990 showed Ag-specific Abs with enhanced production of IgG2a and IgG2b isotypes, suggesting that SERCA2a 971-990 can potentially act as a common epitope for both T cells and B cells. 5) Finally, SERCA2a 971-990-sensitized T cells were able to transfer disease to naive recipients. Together, these data indicate that SERCA2a is a critical autoantigen in the mediation of atrial inflammation in mice and that our model may be helpful to study the inflammatory events that underlie the development of conditions such as atrial fibrillation in humans.

β1-Adrenergic Receptor Contains Multiple IAk and IEk Binding Epitopes That Induce T Cell Responses with Varying Degrees of Autoimmune Myocarditis in A/J Mice

Myocarditis/dilated cardiomyopathy (DCM) patients can develop autoantibodies to various cardiac antigens and one major antigen is β1-adrenergic receptor (β₁AR). Previous reports indicate that animals immunized with a β₁AR fragment encompassing, 197–222 amino acids for a prolonged period can develop DCM by producing autoantibodies, but existence of T cell epitopes, if any, were unknown. Using A/J mice that are highly susceptible to lymphocytic myocarditis, we have identified β₁AR 171–190, β₁AR 181–200, and β₁AR 211–230 as the major T cell epitopes that bind major histocompatibility complex class II/IA^k or IE^k alleles, and by creating IA^k and IE^k dextramers, we demonstrate that the CD4 T cell responses to be antigen-specific. Of note, all the three epitopes were found also to stimulate CD8 T cells suggesting that they can act as common epitopes for both CD4 and CD8 T cells. While, all epitopes induced only mild myocarditis, the disease-incidence was enhanced in animals immunized with all the three peptides together as a cocktail. Although, antigen-sensitized T cells produced mainly interleukin-17A, their transfer into naive animals yielded no disease. But, steering for T helper 1 response led the T cells reacting to one epitope, β₁AR 181–200 to induce severe myocarditis in naive mice. Finally, we demonstrate that all three β₁AR epitopes to be unique for T cells as none of them induced antibody responses. Conversely, animals immunized with a non-T cell activator, β₁AR 201–220, an equivalent of β₁AR 197–222, had antibodies comprising of all IgG isotypes and IgM except, IgA and IgE. Thus, identification of T cell and B cell epitopes of β₁AR may be helpful to determine β₁AR-reactive autoimmune responses in various experimental settings in A/J mice.