Virus and autoimmunity: induction of autoimmune disease in mice by mouse T lymphotropic virus (MTLV) destroying CD4+ T cells

Neonatal infection of the mouse T lymphotropic virus (MTLV), a member of herpes viridae, causes various organ-specific autoimmune diseases, such as autoimmune gastritis, in selected strains of normal mice. The infection selectively depletes CD4+ T cells in the thymus and periphery for 2-3 wk from 1 wk after infection. Thymectomy 3 wk after neonatal MTLV infection enhances the autoimmune responses and produces autoimmune diseases at higher incidences and in a wider spectrum of organs than MTLV infection alone. On the other hand, inoculation of peripheral CD4+ cells from syngeneic noninfected adult mice prevents the autoimmune development. These autoimmune diseases can be adoptively transferred to syngeneic athymic nude mice by CD4+ T cells. The virus is not detected by bioassay in the organs/tissues damaged by the autoimmune responses. Furthermore, similar autoimmune diseases can be induced in normal mice by manipulating the neonatal thymus/T cells (e.g., by neonatal thymectomy) without virus infection. These results taken together indicate that neonatal MTLV infection elicits autoimmune disease by primarily affecting thymocytes/T cells, not self Ags. It may provoke or enhance thymic production of CD4+ pathogenic self-reactive T cells by altering the thymic clonal deletion mechanism, or reduce the production of CD4+ regulatory T cells controlling self-reactive T cells, or both. The possibility is discussed that other T cell-tropic viruses may cause autoimmunity in humans and animals by affecting the T cell immune system, not the self Ags to be targeted by the autoimmunity.     

Altered expression of self-reactive T cell receptor V beta regions in autoimmune mice.

Intrathymic tolerance results in elimination of T cells bearing self-reactive TCR V beta regions in mice expressing certain combinations of I-E and minor lymphocyte stimulatory (Mls) phenotypes. To determine if autoimmune strains of mice have a defect in intrathymic deletion of self-reactive TCR V beta regions, expression of V beta 3, V beta 6, V beta 8.1, and V beta 11 were examined in lpr/lpr and +/+ strains of mice; MRL/MpJ(H-2K, I-E+, Mlsb,), C57BL/6J(H-2b, I-E-, Mlsb,), C3H/HeJ(H-2k, I-E+, Mlsc), AKR/J(H-2k, I-E+, Mlsa); and in autoimmune NZB/N(H-2d, I-E+, Mlsa) and BXSB(H-2b, I-E-, Mlsb) mice. The results suggest that, during intrathymic development, self-reactive T cells are deleted in autoimmune strains of mice as found in normal control strains of mice. However, the TCR V beta repertoire is skewed in autoimmune strains compared to normal strains of mice. For example, MRL-lpr/lpr mice, but not other lpr/lpr strains, had increased expression of V beta 6 relative to expression in control MRL(-)+/+ mice, which is associated with collagen-induced arthritis. These data are consistent with a model of normal affinity for negative selection of self-reactive T cells in the thymus of autoimmune strains of mice followed by expansion of autoreactive T cell clones in the peripheral lymphoid organs. The peripheral lymphoid organs of lpr/lpr mice contain an expanded population of abnormal CD4-, CD8-, 6B2+ T cells. Elimination of self-reactive peripheral T cells suggests that these abnormal cells are derived from a CD4+ subpopulation in the thymus. Flow cytometry analysis of peripheral lymph node T cells from MRL-lpr/lpr mice reveal three populations of CD4+ T cells expressing low, intermediate and high intensity of B220 (6B2). This supports the hypothesis that in lpr/lpr mice, self-reactive CD4+ T cells are eliminated in the thymus, and that these cells lose expression of CD4 and acquire expression of 6B2 in the periphery.     

Provision of granulocyte-macrophage colony-stimulating factor converts an autoimmune response to a self-antigen into an antitumor response

Many tumor Ags recognized by T cells are self-Ags. Because high avidity, self-reactive T cells are deleted in the thymus, any residual self-reactive T cells existing in the periphery are likely to be low avidity and nonresponsive due to peripheral tolerance mechanisms. Activation of these residual T cells is critical for targeting tumors for immunotherapy. In this study, we studied immune responses against the murine B16 melanoma using a tyrosinase-related protein 2 (TRP-2) peptide as a model tumor/self-Ag. Our results showed that TRP-2 peptide vaccination alone elicited a weak T cell response and modestly decreased B16 lung tumor nodules. The combination of peptide vaccination and treatment with an Ab directed against the inhibitory receptor CTLA-4 enhanced the immune response against TRP-2 peptide, inducing autoimmune depigmentation and further decreasing lung tumor nodules. However, both vaccination methods failed to protect against orthotopic (s.c.) B16 tumor challenge. The addition of an irradiated GM-CSF-expressing, amelanotic tumor cell vaccine significantly delayed s.c. B16 tumor growth. Subsequent studies revealed that provision of GM-CSF increased dendritic cell numbers in lymph nodes and spleen. Furthermore, addition of CTLA-4 blockade increased the frequency of TRP-2-specific, IFN-secreting T cells in spleen and lymph nodes. Overall, our results indicate that combining enhancement of Ag presentation with removal of CTLA-4-mediated inhibition can convert a "weaker" autoimmune response into a more potent antitumor immune response.     

Thymic expression of a gastritogenic epitope results in positive selection of self-reactive pathogenic T cells

Intrathymic expression of tissue-specific self-Ags can mediate tolerance of self-reactive T cells. However, in this study we define circumstances by which thymic expression of a tissue-specific autoepitope enhances positive selection of disease-causing, self-reactive T cells. An immunodominant gastritogenic epitope, namely the gastric H/K ATPase beta subunit(253-277) (H/Kbeta(253-277)), was attached to the C terminus of the invariant chain (Ii) and the hybrid Ii (Ii-H/Kbeta(253-277)) expressed in mice under control of the Ii promoter. The Ii-H/Kbeta(253-277) fusion protein was localized to MHC class II-expressing cells in the thymus and periphery of Ii-H/Kbeta(253-277) transgenic mice. In one transgenic line the level of presentation in the periphery (spleen) was insufficient to activate naive, low affinity H/Kbeta(253-277)-specific transgenic T cells (1E4-TCR), whereas thymic presentation of H/Kbeta(253-277) enhanced positive selection of 1E4-TCR cells in Ii-H/Kbeta(253-277)/1E4-TCR double-transgenic mice. Furthermore, Ii-H/Kbeta(253-277)/1E4-TCR double-transgenic mice had an increased incidence of autoimmune gastritis compared with 1E4-TCR single-transgenic mice, demonstrating that the 1E4 T cells that seeded the periphery of Ii-H/Kbeta(253-277) mice were pathogenic. Therefore, low levels of tissue-specific Ags in the thymus can result in positive selection of low avidity, self-reactive T cells. These findings also suggest that the precise level of tissue-specific Ags in the thymus may be an important consideration in protection against autoimmune disease and that perturbation of the levels of self-Ags may be detrimental.     

Protection against autoimmunity in nonlymphopenic hosts by CD4+ CD25+ regulatory T cells is antigen-specific and requires IL-10 and TGF-beta

CD4+ CD25+ regulatory T cells (T(Reg)) play a critical role in the control of autoimmunity. However, little is known about how T(Reg) suppress self-reactive T cells in vivo, thus limiting the development of T(Reg)-based therapy for treating autoimmune diseases. This is in large part due to the dependency on a state of lymphopenia to demonstrate T(Reg)-mediated suppression in vivo and the unknown Ag specificity of T(Reg) in most experimental models. Using a nonlymphopenic model of autoimmune pneumonitis and T(Reg) with known Ag specificity, in this study we demonstrated that these T(Reg) can actively suppress activation of self-reactive T cells and protect mice from fatal autoimmune pneumonitis. The protection required T(Reg) with the same Ag specificity as the self-reactive T cells and depended on IL-10 and TGF-beta. These results suggest that suppression of autoimmunity by T(Reg) in vivo consists of multiple layers of regulation and advocate for a strategy involving Ag-specific T(Reg) for treating organ-specific autoimmunity, because they do not cause generalized immune suppression.     

B cells participate in thymic negative selection of murine auto-reactive CD4+ T cells

It is well documented that thymic epithelial cells participate in the process of negative selection in the thymus. In recent years it was reported that also dendritic cells enter the thymus and contribute to this process, thus allowing for the depletion of thymocytes that are specific to peripherally expressed self-antigens. Here we report that also B cells may take part in the elimination of auto-reactive thymocytes. Using a unique mouse model we show that B cells induce negative selection of self-reactive thymocytes in a process that leads to the deletion of these cells whereas regulatory T cells are spared. These findings have direct implication in autoimmunity, as expression of a myelin antigen by B cells in the thymus renders the mice resistant to autoimmune inflammation of the CNS.     

Selection and fine-tuning of the autoimmune T-cell repertoire

The immune system must avoid aggressive T-cell responses against self-antigens. But, paradoxically, exposure to self-peptides seems to have an important role in positive selection in the thymus and the maintenance of a broad T-cell repertoire in the periphery. Recent experiments have highlighted situations that allow high-avidity self-reactive T cells to avoid negative selection in the thymus. Accumulating evidence indicates that other, non-deleting mechanisms control the avidity with which T cells recognize self-antigens--a phenomenon that is known as 'tuning'. This might maximize the peripheral T-cell repertoire by allowing the survival of T cells that can respond to self, but only at concentrations that are not normally reached in vivo.     

Autoimmunity in Coxsackievirus B3 induced myocarditis: role of estrogen in suppressing autoimmunity

Picornaviruses are small, non-enveloped, single stranded, positive sense RNA viruses which cause multiple diseases including myocarditis/dilated cardiomyopathy, type 1 diabetes, encephalitis, myositis, orchitis and hepatitis. Although picornaviruses directly kill cells, tissue injury primarily results from autoimmunity to self antigens. Viruses induce autoimmunity by: aborting deletion of self-reactive T cells during T cell ontogeny; reversing anergy of peripheral autoimmune T cells; eliminating T regulatory cells; stimulating self-reactive T cells through antigenic mimicry or cryptic epitopes; and acting as an adjuvant for self molecules released during virus infection. Most autoimmune diseases (SLE, rheumatoid arthritis, Grave's disease) predominate in females, but diseases associated with picornavirus infections predominate in males. T regulatory cells are activated in infected females because of the combined effects of estrogen and innate immunity.     

Human autoantibodies modulate the T cell epitope repertoire but fail to unmask a pathogenic cryptic epitope

Abs can tune the responses of Ag-specific T cells by influencing the nature of the epitope repertoire displayed by APCs. We explored the interaction between human self-reactive T cells and human monoclonal autoantibodies from combinatorial Ig-gene libraries derived from autoimmune thyroiditis patients and specific for the main autoantigen thyroid peroxidase (TPO). All human mAbs extensively influenced the T cell epitope repertoire recognized by different TPO-specific T cell clones. The action of the human mAbs was complex, because sometimes the same Ab suppressed or enhanced the epitopes recognized by the 10 different TPO-specific T cell clones. The human mAbs could modulate the epitope repertoire when TPO was added exogenously and when expressed constitutively on the surface of APCs. However, they could not unmask an immunodominant cryptic TPO epitope. In this study, we show that human autoantibodies influence the activity of self-reactive T cells and prove their relevance in concealing or exposing epitopes recognized by self-reactive T cells. However, our results further stress the biological significance of the immunodominant cryptic epitope we have defined and its potential importance in the evolution of autoimmunity.     

Autoreactive T cells revealed in the normal repertoire: escape from negative selection and peripheral tolerance

Self-reactive T cells are known to be eliminated by negative selection in the thymus or by the induction of tolerance in the periphery. However, developmental pathways that allow self-reactive T cells to inhabit the normal repertoire are not well-characterized. In this investigation, we made use of anti-small nuclear ribonucleoprotein particle (snRNP) Ig transgenic (Tg) mice (2-12 Tg) to demonstrate that autoreactive T cells can be detected and activated in both normal naive mice and autoimmune-prone MRL lpr/lpr mice. In contrast, autoreactive T cells of nonautoimmune Tg mice are tolerized by Tg B cells in the periphery. In adoptive transfer studies, autoreactive T cells from MRL lpr/lpr mice can stimulate autoantibody synthesis in nonautoimmune anti-snRNP Tg mice. Transferred CD4 T cells migrate to regions of the spleen proximal to the B cell follicles, suggesting that cognate B cell-T cell interactions are critical to the autoimmune response. Taken together, our studies suggest that anti-snRNP B cells are important APCs for T cell activation in autoimmune-prone mice. Additionally, we have demonstrated that anti-snRNP B cell anergy in nonautoimmune mice may be reversed by appropriate T cell help.     

TGFbeta signalling in control of T-cell-mediated self-reactivity

In the immune system, transforming growth factor-beta (TGFbeta) affects multiple cell lineages by either promoting or opposing their differentiation, survival and proliferation. Understanding the cellular mechanisms of TGFbeta-mediated regulation is complicated due to a broad distribution of TGFbeta receptors on the surface of different immune-cell types. Recent studies using in vivo genetic approaches revealed a critical role for TGFbeta signalling in T cells in restraining fatal autoimmune lesions. Here, we review recent advances in our understanding of a role for TGFbeta signalling in the regulation of T-cell differentiation in the thymus and in the periphery, with a particular emphasis on TGFbeta-mediated control of self-reactive T cells.     

Thymus, innate immunity and autoimmune arthritis: interplay of gene and environment

A hypomorphic mutation of the gene encoding zeta-associated protein-70 (ZAP-70), a signaling molecule in T cells, produces autoimmune arthritis in mice under a microbially conventional condition but not in a clean environment. The genetic anomaly alters thymic selection of self-reactive T cells as well as natural regulatory T cells and their respective functions. Highly self-reactive polyclonal T cells, including arthritogenic ones, thus produced by the thymus strongly recognize self-antigens presented by antigen-presenting cells, stimulate them to up-regulate co-stimulatory molecules and secrete cytokines that drive na?ve self-reactive T cells to differentiate into autoimmune effector Th17 cells. Administration of microbial products and activation of complement can facilitate the differentiation, evoking clinically overt arthritis in a microbially clean environment. Furthermore, mutation-dependent graded attenuation of T cell receptor signaling alters disease phenotypes and the dependency of disease occurrence on the environment. These findings provide a model of how genetic and environmental factors, in association, cause autoimmune diseases such as rheumatoid arthritis.     

CD4+CD25+ regulatory T cells in HIV infection

The immune system faces the difficult task of discerning between foreign, potentially pathogen-derived antigens and self-antigens. Several mechanisms, including deletion of self-reactive T cells in the thymus, have been shown to contribute to the acceptance of self-antigens and the reciprocal reactivity to foreign antigens. Over the last decade it has become increasingly clear that CD4(+)CD25(+) T(Reg) cells are crucial for maintenance of T cell tolerance to self-antigens in the periphery, and to avoid development of autoimmune disorders. Recently, evidence has also emerged that demonstrates that CD4(+)CD25(+) T(Reg) cells can also suppress T cell responses to foreign pathogens, including viruses such as HIV. In this article we review the current knowledge and potential role of CD4(+)CD25(+) T(Reg) cells in HIV infection.     

Continuing education of the immune system--dendritic cells, immune regulation and tolerance

T cells, as they develop in the thymus come to express antigen receptors. The specificity of these receptors cannot be predicted and must include many with potential anti-self reactivity. Those that encounter self-antigens, in association with self-MHC (major histocompatibility complex), with high affinity are inactivated and do not leave the thymus. Not all self-antigens however are expressed in the thymus and thus many potentially self-reactive T cells enter the periphery. It poses therefore a fundamental immunological question: how peripheral self-tolerance is maintained in health? Dendritic cells (DC) play a central role in the activation of T cells, especially na?ve T cells. Their importance in initiating immune responses against pathogens has been well established. However, DC represent complex populations of cells. Recent advances in our knowledge including molecular understanding of DC/T cell interactions have begun to reveal another important dimension of DC functions in the periphery, being not only initiators but also regulators of the immune system. This review summarises recent findings on the roles of DC in the regulation of immune responses and the maintenance of peripheral tolerance, in an attempt to explain how break down of this may lead to immunopathologies and autoimmunity. The concept of a regulatory DC and its possible role in the generation of T regulatory cells in health and in diseases are also discussed. Based on these, the need for a "continuing education" of the immune system throughout one's life, in which DC are again the "tutors", is postulated.     

B cells drive early T cell autoimmunity in vivo prior to dendritic cell-mediated autoantigen presentation

Both B cells and dendritic cells (DCs) have been implicated as autoantigen-presenting cells in the activation of self-reactive T cells. However, most self-proteins are ubiquitously and/or developmentally expressed, making it difficult to determine the source and the exposure of autoantigens to APCs in a controlled manner. In this study, we have used an Ig transgenic mouse model to examine the mechanisms by which B cells and other APCs acquire and present lupus autoantigens in vivo. Targeting a lupus autoantigen, the small nuclear ribonucleoprotein particle D protein, to the BCR activates autoreactive T cells in the periphery. Our in vivo studies demonstrate that autoantigen-specific B cells, when present in the repertoire, are the first subset of APCs to capture and present self-proteins for activating T cells. Thereafter, DCs acquire self-Ag and become effective APCs for stimulating the same subsets of autoreactive T cells. This mechanism provides one explanation of how early steps in autoimmunity can focus responses, via BCR, at a small group of self-proteins among the total milieu of intracellular self-proteins. Subsequently, DCs and other professional APCs may then amplify and perpetuate the autoimmune T cell response.     

T-Cell Tolerance: Central and Peripheral

Somatic recombination of TCR genes in immature thymocytes results in some cells with useful TCR specificities, but also many with useless or potentially self-reactive specificities. Thus thymic selection mechanisms operate to shape the T-cell repertoire. Thymocytes that have a TCR with low affinity for self-peptide–MHC complexes are positively selected to further differentiate and function in adaptive immunity, whereas useless ones die by neglect. Clonal deletion and clonal diversion (Treg differentiation) are the major processes in the thymus that eliminate or control self-reactive T cells. Although these processes are thought to be efficient, they fail to control self-reactivity in all circumstances. Thus, peripheral tolerance processes exist wherein self-reactive T cells become functionally unresponsive (anergy) or are deleted after encountering self-antigens outside of the thymus. Recent advances in mechanistic studies of central and peripheral T-cell tolerance are promoting the development of therapeutic strategies to treat autoimmune disease and cancer and improve transplantation outcome.T cells recognize pathogen fragments in the context of surface MHC molecules on host cells. As such, they have the potential to do enormous damage to healthy tissue when they are not appropriately directed, that is, when they respond to self-antigens as opposed to foreign antigens. T lymphocyte tolerance is particularly important, because it impacts B-cell tolerance as well, through the requirement of T cell help in antibody responses. Thus, failure of T-cell tolerance can lead to many different autoimmune diseases. The tolerance of T cells begins as soon as a T-cell receptor is formed and expressed on the cell surface of a T-cell progenitor in the thymus. Tolerance mechanisms that operate in the thymus before the maturation and circulation of T cells are referred to as “central tolerance.” However, not all antigens that T cells need to be tolerant of are expressed in the thymus, and thus central tolerance mechanisms alone are insufficient. Fortunately, additional tolerance mechanisms exist that restrain the numbers and or function of T cells that are reactive to developmental or food antigens, which are not thymically expressed. These mechanisms act on mature circulating T cells and are referred to as “peripheral tolerance.”     

Tolerance mechanism in experimental ovarian and gastric autoimmune diseases.

Neonatal splenocytes, neonatal thymocytes, or phenotypically mature adult thymocytes, transferred from normal BALB/c mice to syngeneic athymic nu/nu (or SCID) mice, led to autoimmune oophoritis and autoimmune gastritis, with corresponding serum autoantibodies, in the recipients. The overall disease incidence was 73%; the pathology ranged from mild to severe, with complete loss of ovarian follicles and gastric parietal cells. CD4+ neonatal spleen cells and CD4+ CD8- adult thymocytes were required for autoimmune disease induction. Adult spleen cells did not elicit disease, but they prevented disease when co-transferred with neonatal spleen cells. However, in confirmation of an earlier report by Sakaguchi et al., (J. Exp. Med. 161:72, 1985), a subset of adult splenic T cells expressing a low level of CD5 molecules elicited similar autoimmune diseases. Thus, self-reactive T cells responsible for autoimmune disease of the stomach and ovary are not effectively deleted in the thymus, and they exist in the peripheral lymphoid organs of normal mice. We conclude that the functional expression of the self-reactive T cells is ontogenetically regulated; whereas T cells in the neonatal mice readily elicited autoimmune diseases in nu/nu recipients, regulatory cells may render self-reactive T cells in the normal adults unresponsive.     

NF-kappa B-inducing kinase establishes self-tolerance in a thymic stroma-dependent manner

Physical contact between thymocytes and the thymic stroma is essential for T cell maturation and shapes the T cell repertoire in the periphery. Stromal elements that control these processes still remain elusive. We used a mouse strain with mutant NF-kappaB-inducing kinase (NIK) to examine the mechanisms underlying the breakdown of self-tolerance. This NIK-mutant strain manifests autoimmunity and disorganized thymic structure with abnormal expression of Rel proteins in the stroma. Production of immunoregulatory T cells that control autoreactive T cells was impaired in NIK-mutant mice. The autoimmune disease seen in NIK-mutant mice was reproduced in athymic nude mice by grafting embryonic thymus from NIK-mutant mice, and this was rescued by supply of exogenous immunoregulatory T cells. Impaired production of immunoregulatory T cells by thymic stroma without normal NIK was associated with altered expression of peripheral tissue-restricted Ags, suggesting an essential role of NIK in the thymic microenvironment in the establishment of central tolerance.     

Too dangerous to ignore: self-tolerance and the control of ignorant autoreactive T cells

Self-reactive T cells often escape thymic negative selection and are released into the periphery. While many of these T cells are tolerized by peripheral deletion or anergy, a proportion persists in a na?ve (or ignorant) state. Self-ignorant T cells are probably one of the greatest threats to the maintenance of self-tolerance, as inadvertent activation of these cells may provoke autoimmune pathology. Nevertheless, despite the presence of self-ignorant T cells within most individuals, the majority of people fail to develop autoimmunity. The means by which self-ignorant T cells are silenced by the immune system remains a major issue within the tolerance field and it has received surprisingly little attention within the literature. In this review, we first summarize the factors that allow such cells to persist in a self-ignorant state, with a particular focus on the role of self-antigen dose. We next consider the conditions under which such self-reactive cells may become activated and speculate on how the immune system is able to prevent such cells from precipitating autoimmune disease.     

Experimental autoimmune encephalomyelitis induction in naive mice by dendritic cells presenting a self-peptide

Self-reactive T cells escape deletion in the thymus and are found in the peripheral repertoire. Because bone-marrow-derived dendritic cells (BM-DC) are potent activators of antigen-specific T cells, these cells could theoretically activate self-reactive T cells leading to autoimmunity. We investigated whether BM-DC could induce the autoimmune disease experimental autoimmune encephalomyelitis (EAE). Our results show that transfer of BM-DC presenting a self-peptide from the myelin oligodendrocyte glycoprotein (MOG35-55) into naive mice induced EAE 7-14 days later. MOG35-55-specific T cells of the Th1 phenotype were present in the lymph nodes and spleens of mice that received live peptide-pulsed BM-DC. Heat-killed or formaldehyde-fixed BM-DC presenting MOG35-55 could induce neither clinical signs of EAE nor a measurable T-cell response in vitro. These data show that live BM-DC presenting a self-antigen can induce the organ-specific autoimmune disorder EAE in a non-transgenic system. Therefore, this new EAE model could be used as a more clinically relevant model for the human disease multiple sclerosis. These findings could also have implications for the use of DC immunotherapy in a clinical setting.