Negative regulation of lymphocyte activation and autoimmunity by the molecular adaptor Cbl-b

The signalling thresholds of antigen receptors and co-stimulatory receptors determine immunity or tolerance to self molecules. Changes in co-stimulatory pathways can lead to enhanced activation of lymphocytes and autoimmunity, or the induction of clonal anergy. The molecular mechanisms that maintain immunotolerance in vivo and integrate co-stimulatory signals with antigen receptor signals in T and B lymphocytes are poorly understood. Members of the Cbl/Sli family of molecular adaptors function downstream from growth factor and antigen receptors. Here we show that gene-targeted mice lacking the adaptor Cbl-b develop spontaneous autoimmunity characterized by auto-antibody production, infiltration of activated T and B lymphocytes into multiple organs, and parenchymal damage. Resting cbl-b(-/-) lymphocytes hyperproliferate upon antigen receptor stimulation, and cbl-b(-/-) T cells display specific hyperproduction of the T-cell growth factor interleukin-2, but not interferon-gamma or tumour necrosis factor-alpha. Mutation of Cbl-b uncouples T-cell proliferation, interleukin-2 production and phosphorylation of the GDP/GTP exchange factor Vav1 from the requirement for CD28 co-stimulation. Cbl-b is thus a key regulator of activation thresholds in mature lymphocytes and immunological tolerance and autoimmunity.

Chlamydia infections and heart disease linked through antigenic mimicry

Chlamydia infections are epidemiologically linked to human heart disease. A peptide from the murine heart muscle-specific alpha myosin heavy chain that has sequence homology to the 60-kilodalton cysteine-rich outer membrane proteins of Chlamydia pneumoniae, C. psittaci, and C. trachomatis was shown to induce autoimmune inflammatory heart disease in mice. Injection of the homologous Chlamydia peptides into mice also induced perivascular inflammation, fibrotic changes, and blood vessel occlusion in the heart, as well as triggering T and B cell reactivity to the homologous endogenous heart muscle-specific peptide. Chlamydia DNA functioned as an adjuvant in the triggering of peptide-induced inflammatory heart disease. Infection with C. trachomatis led to the production of autoantibodies to heart muscle-specific epitopes. Thus, Chlamydia-mediated heart disease is induced by antigenic mimicry of a heart muscle-specific protein.

Cbl-b is a negative regulator of receptor clustering and raft aggregation in T cells

Stimulation of T cells via the antigen and costimulatory receptors leads to the organization of a supramolecular activation cluster called the immune synapse. We report that loss of the molecular adaptor Cbl-b in T cells frees antigen receptor-triggered receptor clustering, lipid raft aggregation, and sustained tyrosine phosphorylation from the requirement for CD28 costimulation. Introduction of the cbl-b mutation into a vav1-/- background relieved the functional defects of vav1-/- T cells and caused spontaneous autoimmunity. Wiscott Aldrich Syndrome protein (WASP) was found to be essential for deregulated proliferation and membrane receptor reorganization of cbl-b mutant T cells. Antigen receptor-triggered Ca2+ mobilization, cytokine production, and receptor clustering can be genetically uncoupled in cbl-b mutant T cells. Thus, Cbl-b functions as a negative regulator of receptor clustering and raft aggregation in T cells.

Identification of cardiac myosin peptides capable of inducing autoimmune myocarditis in BALB/c mice

Immunization with cardiac myosin induces T cell-mediated myocarditis in genetically predisposed mice and serves as a model for autoimmune heart disease. This study was undertaken to identify pathogenic epitopes on the myosin molecule. Our approach was based on the comparison of the pathogenicity between cardiac (alpha-)myosin and soleus muscle (beta-)myosin. We show that alpha-myosin is the immunodominant isoform and induces myocarditis at high severity and prevalence whereas beta-myosin induces little disease. Therefore the immunodominant epitopes of alpha-myosin must reside in regions of different amino acid sequence between alpha- and beta-myosin isoforms. Cardiac myosin peptides corresponding to these regions of difference were synthesized and tested for their ability to induce inflammatory heart disease. Three pathogenic peptides were identified. One peptide that is located in the head portion of the molecule induced severe myocarditis, whereas two others that reside in the rod portion possessed only minor pathogenicity. The identification of pathogenic epitopes on the cardiac myosin molecule will allow detailed studies on the recognition of this antigen by the immune system and might be used to downmodulate ongoing heart disease.

The tyrosine kinase p56lck is essential in coxsackievirus B3-mediated heart disease

Infections are thought to be important in the pathogenesis of many heart diseases. Coxsackievirus B3 (CVB3) has been linked to chronic dilated cardiomyopathy, a common cause of progressive heart disease, heart failure and sudden death. We show here that the sarcoma (Src) family kinase Lck (p56lck) is required for efficient CVB3 replication in T-cell lines and for viral replication and persistence in vivo. Whereas infection of wild-type mice with human pathogenic CVB3 caused acute and very severe myocarditis, meningitis, hepatitis, pancreatitis and dilated cardiomyopathy, mice lacking the p56lck gene were completely protected from CVB3-induced acute pathogenicity and chronic heart disease. These data identify a previously unknown function of Src family kinases and indicate that p56lck is the essential host factor that controls the replication and pathogenicity of CVB3.

Colorectal carcinomas in mice lacking the catalytic subunit of PI(3)Kgamma

Phosphoinositide-3-OH kinases (PI(3)Ks) constitute a family of evolutionarily conserved lipid kinases that regulate a vast array of fundamental cellular responses, including proliferation, transformation, differentiation and protection from apoptosis. PI(3)K-mediated activation of the cell survival kinase PKB/Akt, and negative regulation of PI(3)K signalling by the tumour suppressor PTEN (refs 3, 4) are key regulatory events in tumorigenesis. Thus, a model has arisen that PI(3)Ks promote development of cancers. Here we report that genetic inactivation of the p110gamma catalytic subunit of PI(3)Kgamma (ref. 8) leads to development of invasive colorectal adenocarcinomas in mice. In humans, p110gamma protein expression is lost in primary colorectal adenocarcinomas from patients and in colon cancer cell lines. Overexpression of wild-type or kinase-dead p110gamma in human colon cancer cells with mutations of the tumour suppressors APC and p53, or the oncogenes beta-catenin and Ki-ras, suppressed tumorigenesis. Thus, loss of p110gamma in mice leads to spontaneous, malignant epithelial tumours in the colorectum and p110gamma can block the growth of human colon cancer cells.

Low-molecular-weight tumor necrosis factor receptor p55 controls induction of autoimmune heart disease

BACKGROUND

Tumor necrosis factor-alpha (TNF-alpha) is involved in the pathogenesis of myocarditis and can bind to either tumor necrosis factor receptor (TNF-R) p55 or TNF-Rp75. However, it is not known which TNF-R mediates the specific functions of TNF in disease. To determine the role of the TNF/TNF-R system in chronic heart disease, we used a murine model of cardiac myosin-induced myocarditis that closely resembles the chronic stages of virus-induced myocarditis in humans.

METHODS AND RESULTS

Mice lacking TNF-Rp55 expression after targeted disruption of the TNF-Rp55 gene were backcrossed into a genetic background susceptible to the induction of myocarditis with cardiac myosin. Here, we demonstrate that TNF-Rp55 gene-deficient mice did not develop any inflammatory infiltration into the heart after autoantigen injection, whereas control littermates showed autoimmune myocarditis at high prevalence and severity. Despite the absence of autoimmune heart disease, TNF-Rp55-/- mice produced cardiac myosin-specific IgG autoantibodies, indicating that activation of autoaggressive T and B lymphocytes had occurred. However, heart interstitial cells failed to express major histocompatibility complex (MHC) class II molecules in TNF-Rp55-/- animals, and adoptive transfer of autoreactive T cells resulted in heart disease only in TNF-Rp55-/- but not in TNF-Rp55-/- littermates.

CONCLUSIONS

Cardiac myosin-induced myocarditis is dependent on autoaggressive T cells and on autoantigen presentation in association with MHC class II molecules within the heart. Thus, lack of TNF-Rp55 expression could interfere with either lymphocyte activation or target organ susceptibility. The data presented here show that the TNF-Rp55 is a key regulator for the induction of autoimmune heart disease by its controlling target organ susceptibility.

Serum cardiac troponin T and creatine kinase-MB elevations in murine autoimmune myocarditis

BACKGROUND

We used a murine model of autoimmune myocarditis to investigate systematically whether serum markers of myocardial cell injury, that is, cardiac troponin T (TnT) and the MB isoenzyme of creatine kinase (CK-MB) are useful for the diagnosis of inflammatory heart disease.

METHODS AND RESULTS

Fifty-two A.SW mice were immunized with cardiac myosin to induce myocarditis. The disease was evident on day 12 after the initial immunization in 14 of 22 immunized mice, on day 16 in 7 of 10 mice, on day 19 in 6 of 10 mice, and on day 23 in 5 of 10. The severity of myocarditis increased between days 12 and 16 and remained constant thereafter. TnT was elevated in a considerable number of mice with myocarditis, resulting in a diagnostic sensitivity (number of marker elevations per number of mice with myocarditis) of 0.43 on day 12, 0.71 on day 16, and 0.50 on day 19. CK-MB elevations were not seen on day 12 but resulted in a diagnostic sensitivity of 0.71 on day 16 and of 0.33 on day 19. No elevations of CK-MB or TnT were observed on day 23. All elevations were specific for the disease, as none of the mice lacking myocarditis showed increased markers.

CONCLUSIONS

In murine autoimmune myocarditis, TnT is a more sensitive marker for the disease than CK-MB. Elevations clearly indicate myocarditis, but negative test results do not exclude the presence of the disease. These data suggest that the determination of CK-MB and, in particular, of TnT, can be useful for the diagnostic evaluation of patients with suspected myocarditis.

Review of microbial infections and the immune response to cardiac antigens

Heart disease is the most prevalent cause of morbidity and mortality in rich countries. Multiple pathogens are epidemiologically linked to human heart disease, and autoinflammatory responses to heart-specific epitopes revealed to the host's immune system (e.g., due to the cytopathic effects of cardiotropic viruses) or attacked by autoaggresive lymphocytes activated by mimicking peptides present in bacteria may be causative in the pathogenesis of chronic inflammatory cardiomyopathy. The experimental system of murine chronic autoimmune myocarditis has been used to analyze aspects of the host immune response. This review presents insights gained by use of this murine model system into molecular mechanisms governing activation of autoaggressive lymphocytes, target organ susceptibility, and cardiopathogenic epitope mapping and discusses mimicking endogenous epitopes found in pathogens.

The oncogene product Vav is a crucial regulator of primary cytotoxic T cell responses but has no apparent role in CD28-mediated co-stimulation

The guanine nucleotide-exchange factor Vav is a regulator of antigen-mediated cytoskeletal reorganization required for receptor clustering, proliferation and thymic selection. Moreover, Vav has been identified as a major substrate in the CD28 signal transduction pathway and overexpression of Vav enhances TCR-mediated IL-2 secretion in T cells. Here we show that CD3- plus CD28-mediated proliferation and IL-2 production were reduced in vav gene-deficient T cells. However, Vav had no apparent role in phorbol 12-myristate 13-acetate-plus CD28-mediated proliferation and IL-2 production, suggesting that Vav acts downstream of the TCR/CD3 complex. In vivo, Vav expression was crucial to generate primary vesicular stomatitis virus (VSV)-specific cytotoxic T cell responses. In contrast, vav-/- mice exhibited a reduced but significant footpad swelling after lymphocytic choriomeningitis virus (LCMV) infections and mounted a measurable primary cytotoxic T cell response to LCMV. Upon in vitro restimulation, cytotoxic T cell responses of both VSV- and LCMV-infected mice reached near normal levels. Our data provide the first genetic evidence that Vav is an important effector molecule that relays antigen receptor signaling to IL-2 production and activation of cytotoxic T cells.

Generation of humanized mice susceptible to peptide-induced inflammatory heart disease

BACKGROUND

Dilated cardiomyopathy (DCM) is a major cause of sudden cardiac death. In certain mouse major histocompatibility complex (MHC) backgrounds, myocarditis and inflammatory cardiomyopathy can be triggered by immunization with heart muscle-specific proteins. Similarly, chronic heart disease in humans has been linked to certain HLA alleles, such as HLA-DQ6. However, there is no experimental evidence showing that human MHC class II molecules and peptides derived from human proteins are involved in the pathogenesis of myocarditis and DCM.

METHODS AND RESULTS

We generated double CD4- and CD8-deficient mice transgenic for human CD4 (hCD4) and human HLA-DQ6 to specifically reconstitute the human CD4/DQ6 arm of the immune system in mice. Transgenic hCD4 and HLA-DQ6 expression rendered genetically resistant C57BL/6 mice susceptible to the induction of autoimmune myocarditis induced by immunization with cardiac myosin. Moreover, we identified heart-specific peptides derived from both mouse and human alpha-myosin heavy chains capable of inducing inflammatory heart disease in hCD4 and HLA-DQ6 double transgenic mice but not in hCD4 single transgenic littermates. The autoimmune inflammatory heart disease induced by the human heart muscle-specific peptide in hCD4 and HLA-DQ6 double transgenic mice shared functional and phenotypic features with the disease occurring in disease-susceptible nontransgenic mice.

CONCLUSIONS

Our data provide the first genetic and functional evidence that human MHC class II molecules and a human alpha-myosin heavy chain-derived peptide can cause inflammatory heart disease and suggest that human inflammatory cardiomyopathy can be caused by organ-specific autoimmunity. The humanized mice generated in this study will be an ideal animal model to further elucidate the pathogenesis of inflammatory heart disease and facilitate the development of rational treatment strategies.

Cellular and molecular mechanisms of murine autoimmune myocarditis

Dilated cardiomyopathy is a prevalent cause of progressive heart disease and sudden death, and most patients with cardiomyopathy have a history of viral myocarditis. Coxsackie B3 (CB3) picornaviruses can be detected in as many as 50% of these patients and CB3 infections have been epidemiologically linked to chronic heart disease. Several clinical and experimental studies suggest that chronic stages of disease are mediated by an autoimmune response against heart muscle myosin. Human heart disease can be mimicked in mice using cardiac myosin as autoantigen. Murine cardiac myosin-induced myocarditis is an organ-specific autoimmune disease and mediated by CD4+ T cells that recognize a myosin-specific peptide in association with MHC class II molecules. Here, the recent discovery of autoimmune epitopes derived from the alpha isoform of cardiac myosin, the functional roles of surface receptor and signal transduction molecules, and the molecular mechanisms of target organ susceptibility will be discussed.

Chlamydia and antigenic mimicry

Chlamydial infections are among the most common human infections. Every year, in millions of humans, they cause infections of the eyes, the respiratory tract, the genital tract, joints, and the vasculature. Chlamydiae are obligate intracellular prokaryotic pathogens. Chlamydiae promote, in susceptible host cells that include mucosal epithelial cells, vascular endothelial cells, smooth muscle cells, and monocytes and macrophages, their survival while causing disease of varying clinical importance and consequence in their hosts. Chlamydia infections often precede the initiation of autoimmune diseases, and Chlamydiae are often found within autoimmune lesions. Thus, they have been suspected in the etiology and pathogenesis of autoimmune diseases. Autoimmune diseases have many causes. Genes, notably genes encoding cell-surface proteins that display peptides for immune recognition, the major histocompatibility complex (MHC), the environment, and the microbial diversity within the human body determine the susceptibility to autoimmune diseases. One mechanism by which infection is linked to the initiation of autoimmunity is termed molecular mimicry. Molecular mimicry describes the phenomenon of protein products from dissimilar genes sharing similar structures that elicit an immune response to both self and microbial proteins. Molecular mimicry might thus be a mechanism by which infections trigger autoimmune diseases. For the purpose of this chapter, we will focus on chlamydial proteins that mimic host self-proteins and thus contribute to initiation and maintenance of autoimmune diseases. Thus far, the strongest cases for molecular mimicry seem to have been made for chlamydial heat shock proteins 60, the DNA primase of Chlamydia trachomatis, and chlamydial OmcB proteins.

Induction of autoimmunity in the absence of CD28 costimulation

Ag-specific activation of T lymphocytes requires two signals, one by the TCR and a second by costimulatory molecules. In a CD4+ T helper cell-dependent experimental autoimmune myocarditis model, we provide genetic evidence that cardiac myosin-induced autoimmune myocarditis and the production of IgG auto-Abs is dependent on functional T cells and did not occur in mice lacking the tyrosine kinase p56lck or the tyrosine phosphatase CD45. By contrast, animals lacking the T cell-costimulatory molecule CD28 (CD28 -/-) developed autoimmune heart disease, although at significantly lower severity than in heterozygous littermates, and produced IgG auto-Abs depending on the concentration of the autoantigen administered. In addition, the isotypes of IgG auto-Abs specific for cardiac myosin differed between CD28 +/- and CD28 -/- mice. Whereas CD28 +/- mice predominantly produced Th2-mediated IgG1 auto-Abs, CD28 -/- mice produced predominantly IgG2a. These data suggest that CD28 costimulation plays a crucial role in induction and maintenance of autoimmune heart disease and that CD28 expression is required for predominant Th2-IgG1 responses in an autoimmune setting.

iNOS expression and nitrotyrosine formation in the myocardium in response to inflammation is controlled by the interferon regulatory transcription factor 1

BACKGROUND

Production of NO by inducible NO synthase (iNOS) has been implicated in the pathology of spontaneous and antigen-induced autoimmune diseases, and iNOS is expressed in the myocardium of patients with heart failure. It is not clear whether inflammatory murine autoimmune heart disease, an experimental model for human postviral heart disease, is characterized by increased iNOS expression within the heart and whether iNOS and NO are essential in the pathogenesis of autoimmune myocarditis.

METHODS AND RESULTS

In the murine model of cardiac myosin-induced myocarditis, we demonstrate that iNOS expression was elicited in inflammatory macrophages and in distinct cardiomyocytes. Autoimmune heart disease was accompanied by formation of the NO reaction product nitrotyrosine in inflammatory macrophages as well as in cardiomyocytes. iNOS expression and nitrotyrosine formation were strictly dependent on myocardial inflammation. Focal myocarditis was sufficient to induce nitrotyrosine formation throughout the whole heart muscle. Mice defective for the interferon regulatory transcription factor-1 (IRF-1(-/-)) after gene targeting failed to induce iNOS expression and nitrotyrosine formation in the heart but developed cardiac myosin-induced myocarditis at prevalence and severity similar to those of heterozygous littermates (IRF-1(+/-)).

CONCLUSIONS

These data provide the first in vivo evidence that iNOS expression and NO synthesis in macrophages and distinct cardiomyocytes are elicited in experimental murine inflammatory heart disease. The transcription factor IRF-1 controls iNOS expression and NO synthesis in disease. Because autoimmune myocarditis can develop in animals lacking IRF-1, these mice will be useful to elucidate the link between iNOS expression in inflammatory heart disease and the development of dilated cardiomyopathy and heart failure.

Induction of autoimmunity in the absence of CD28 costimulation

Ag-specific activation of T lymphocytes requires two signals, one by the TCR and a second by costimulatory molecules. In a CD4+ T helper cell-dependent experimental autoimmune myocarditis model, we provide genetic evidence that cardiac myosin-induced autoimmune myocarditis and the production of IgG auto-Abs is dependent on functional T cells and did not occur in mice lacking the tyrosine kinase p56lck or the tyrosine phosphatase CD45. By contrast, animals lacking the T cell-costimulatory molecule CD28 (CD28 -/-) developed autoimmune heart disease, although at significantly lower severity than in heterozygous littermates, and produced IgG auto-Abs depending on the concentration of the autoantigen administered. In addition, the isotypes of IgG auto-Abs specific for cardiac myosin differed between CD28 +/- and CD28 -/- mice. Whereas CD28 +/- mice predominantly produced Th2-mediated IgG1 auto-Abs, CD28 -/- mice produced predominantly IgG2a. These data suggest that CD28 costimulation plays a crucial role in induction and maintenance of autoimmune heart disease and that CD28 expression is required for predominant Th2-IgG1 responses in an autoimmune setting.