Transgenic mice with I-A on islet cells are normoglycemic but immunologically intolerant

Islet Dysfunction in a Novel Transgenic Model of T Cell Insulitis

The newly established CD3FLAG-mIR transgenic mouse model on a C57Bl/6 background has a FLAG tag on the mouse Insulin Receptor (mIR), specifically on T cells, as the FLAG-tagged mIR gene was engineered behind CD3 promoter and enhancer. The IR is a chemotactic molecule for insulin and the Flag-tagged mIR T cells in the BL/6-CD3FLAGmIR transgenic mice can migrate into the pancreas, as shown by immunofluorescent staining. While the transgenic mice do not become diabetic, there are phenotypic and metabolic changes in the islets. The transgenic islets become enlarged and disorganized by 15 weeks and those phenotypes continue out to 35 weeks of age. We examined the islets by RT-PCR for cell markers, ER stress markers, beta cell proliferation markers, and cytokines, as well as measuring serum insulin and insulin content in the pancreas at 15, 25, and 35 weeks of age. In transgenic mice, insulin in serum was increased at 15 weeks of age and glucose intolerance developed by 25 weeks of age. Passage of transgenic spleen cells into C57Bl/6 RAG⁻/⁻ mice resulted in enlarged and disorganized islets with T infiltration by 4 to 5 weeks post-transfer, replicating the transgenic mouse studies. Therefore, migration of non-antigen-specific T cells into islets has ramifications for islet organization and function.

Insulin Receptor-Expressing T Cells Appear in Individuals at Risk for Type 1 Diabetes and Can Move into the Pancreas in C57BL/6 Transgenic Mice

Insulin receptor (IR) expression on the T cell surface can indicate an activated state; however, the IR is also chemotactic, enabling T cells with high IR expression to physically move toward insulin. In humans with type 1 diabetes (T1D) and the NOD mouse model, a T cell-mediated autoimmune destruction of insulin-producing pancreatic β cells occurs. In previous work, when purified IR⁺ and IR^- T cells were sorted from diabetic NOD mice and transferred into irradiated nondiabetic NOD mice, only those that received IR⁺ T cells developed insulitis and diabetes. In this study, peripheral blood samples from individuals with T1D (new onset to 14 y of duration), relatives at high-risk for T1D, defined by positivity for islet autoantibodies, and healthy controls were examined for frequency of IR⁺ T cells. High-risk individuals had significantly higher numbers of IR⁺ T cells as compared with those with T1D (p < 0.01) and controls (p < 0.001); however, the percentage of IR⁺ T cells in circulation did not differ significantly between T1D and control subjects. With the hypothesis that IR⁺ T cells traffic to the pancreas in T1D, we developed a (to our knowledge) novel mouse model exhibiting a FLAG-tagged mouse IR on T cells on the C57BL/6 background, which is not susceptible to developing T1D. Interestingly, these C57BL/6-CD3FLAGmIR/mfm mice showed evidence of increased IR⁺ T cell trafficking into the islets compared with C57BL/6 controls (p < 0.001). This transgenic animal model provides a (to our knowledge) novel platform for investigating the influence of IR expression on T cell trafficking and the development of insulitis.

Autoreactive T cells in type 1 diabetes

Type 1 diabetes (T1D) is a chronic autoimmune disease that causes severe loss of pancreatic β cells. Autoreactive T cells are key mediators of β cell destruction. Studies of organ donors with T1D that have examined T cells in pancreas, the diabetogenic insulitis lesion, and lymphoid tissues have revealed a broad repertoire of target antigens and T cell receptor (TCR) usage, with initial evidence of public TCR sequences that are shared by individuals with T1D. Neoepitopes derived from post-translational modifications of native antigens are emerging as novel targets that are more likely to evade self-tolerance. Further studies will determine whether T cell responses to neoepitopes are major disease drivers that could impact prediction, prevention, and therapy. This Review provides an overview of recent progress in our knowledge of autoreactive T cells that has emerged from experimental and clinical research as well as pathology investigations.

Autoimmune Diabetes: An Overview of Experimental Models and Novel Therapeutics

Type 1 diabetes (T1D) results from a chronic and selective destruction of insulin-secreting β-cells within the islets of Langerhans of the pancreas by autoreactive CD4(+) and CD8(+) T lymphocytes. The use of animal models of T1D was instrumental for deciphering the steps of the autoimmune process leading to T1D. The non-obese diabetic (NOD) mouse and the bio-breeding (BB) rat spontaneously develop the disease similar to the human pathology in terms of the immune responses triggering autoimmune diabetes and of the genetic and environmental factors influencing disease susceptibility. The generation of genetically modified models allowed refining our understanding of the etiology and the pathogenesis of the disease. In the present review, we provide an overview of the experimental models generated and used to gain knowledge on the molecular and cellular mechanisms underlying the breakdown of self-tolerance in T1D and the progression of the autoimmune response. Immunotherapeutic interventions designed in these animal models and translated into the clinical arena in T1D patients will also be discussed.

Role of major histocompatibility complex class II expression by non-hematopoietic cells in autoimmune and inflammatory disorders: facts and fiction

It is well established that interactions between CD4(+) T cells and major histocompatibility complex class II (MHCII) positive antigen-presenting cells (APCs) of hematopoietic origin play key roles in both the maintenance of tolerance and the initiation and development of autoimmune and inflammatory disorders. In sharp contrast, despite nearly three decades of intensive research, the functional relevance of MHCII expression by non-hematopoietic tissue-resident cells has remained obscure. The widespread assumption that MHCII expression by non-hematopoietic APCs has an impact on autoimmune and inflammatory diseases has in most instances neither been confirmed nor excluded by indisputable in vivo data. Here we review and put into perspective conflicting in vitro and in vivo results on the putative impact of MHCII expression by non-hematopoietic APCs--in both target organs and secondary lymphoid tissues--on the initiation and development of representative autoimmune and inflammatory disorders. Emphasis will be placed on the lacunar status of our knowledge in this field. We also discuss new mouse models--developed on the basis of our understanding of the molecular mechanisms that regulate MHCII expression--that constitute valuable tools for filling the severe gaps in our knowledge on the functions of non-hematopoietic APCs in inflammatory conditions.

Induction of GVHD-Like Skin Disease by Passively Transferred CD8+ T-Cell Receptor Transgenic T Cells into Keratin 14-Ovalbumin Transgenic Mice

To understand the mechanisms involved in immunological tolerance to skin-associated antigens, we have developed transgenic (Tg) mice that express a model self-antigen, membrane-bound chicken ovalbumin (OVA), under the control of a keratin 14 (K14) promoter. K14-mOVA Tg mice express OVA mRNA in the epidermis, and appear normal. K14-mOVA Tg mice failed to mount T cell and delayed type hypersensitivity reactions to OVA, suggesting that the Tg mice were tolerant to OVA. Skin dendritic cells, including Langerhans cells, may contribute to the tolerance induction because migratory skin DC derived from K14-mOVA efficiently activated CD8(+) T cells from OVA-specific T-cell receptor (Va2/Vb5) Tg (OT-I) mice. OT-I cells expanded and accumulated in skin-draining lymph nodes after intravenous injected into K14-mOVA mice and exhibited activation markers. Graft-versus-host disease-like skin lesions appeared in K14-mOVA mice by day 7 after injection of OT-I cells. These studies demonstrate that K14-mOVA Tg mice are susceptible to an autoimmunelike skin disease induced by passively transferred naïve CD8(+) OVA T-cell receptor Tg T cells, and serve as a good model for understanding self-tolerance and for the investigation of the pathogenesis, treatment and potential prevention of cell-mediated autoimmune reactions in skin.

Transgenic mice producing major histocompatibility complex class II molecules on thyroid cells do not develop apparent autoimmune thyroid diseases

The expression of major histocompatibility complex (MHC) class II molecules on thyrocytes has been demonstrated in autoimmune thyroid diseases. However, the role of this aberrant MHC class II in disease development is controversial. In particular, it remains unknown whether MHC class II expression on thyrocytes, which are nonprofessional antigenpresenting cells, plays a role in inducing autoimmune processes. To clarify this issue, we have produced transgenic mice harboring an MHC class II gene ligated to the promoter of the rat TSH receptor. We obtained three lines of transgenic mice, and the expression of MHC class II by the thyrocytes was demonstrated by immunofluorescence staining and flow cytometry. Our examination revealed no obvious abnormalities in thyroid histology or in thyroid autoantibody production in these transgenic mice. Although serum-free T(4) levels were slightly lower than those of their nontransgenic littermates, no transgenic mouse suffered from clinical hypothyroidism or hyperthyroidism. Furthermore, thyroid lymphocytic infiltration was absent, and MHC class II-expressing thyrocytes obtained from transgenic mice failed to stimulate the proliferation of autologous T cells in vitro. Taken together, these results show that transgenic mice with MHC class II molecules on their thyrocytes do not develop apparent autoimmune thyroid diseases, suggesting that aberrant MHC class II expression alone is not sufficient to induce thyroid autoimmunity.

A novel reporter strain to follow Cre-mediated recombination in T and NK cells

The Cre-loxP system permits the generation of mouse models in which the fate of a cell can be followed through time. Such approach is of great value in immunology because it may allow lineage studies and the dissection of the contribution of specific effector T cells to long-term memory responses or autoimmune responses. An essential component of such a strategy is the development of appropriate reporter strains of mice in which the inducible reporter molecule is not immunogenic and is well expressed at the cell surface of T cells. We describe here a novel reporter strain of mice that is designed to fulfill these criteria and show that this strain permits the monitoring of Cre-mediated recombination in both T cells and NK cells.

Vigorous allograft rejection in the absence of danger

Tolerance to self is a necessary attribute of the immune system. It is thought that most autoreactive T cells are deleted in the thymus during the process of negative selection. However, peripheral tolerance mechanisms also exist to prevent development of autoimmune diseases against peripheral self-Ags. It has been proposed that T cells develop tolerance to peripheral self-Ags encountered in the absence of inflammation or "danger" signals. We have used immunodeficient Rag 1-/- mice to study the response of T cells to neo-self peripheral Ags in the form of well-healed skin and vascularized cardiac allografts. In this paper we report that skin and cardiac allografts without evidence of inflammation are vigorously rejected by transferred T cells or when recipients are reconstituted with T cells at a physiologic rate by nude bone graft transplantation. These results provide new insights into the role of inflammation or "danger" in the initiation of T cell-dependent immune responses. These findings also have profound implications in organ transplantation and suggest that in the absence of central deletional tolerance, peripheral tolerance mechanisms are not sufficient to inhibit alloimmune responses even in the absence of inflammation or danger.

Antigen presentation by parenchymal cells: a route to peripheral tolerance?

T-cell activation and the development of efficient immune responses requires the delivery, by the antigen-presenting cell, of two distinct signals. The first results from the engagement of the TCR:CD3:CD4 complex, and the second from the interaction of CD28 with the B7 family of co-stimulatory molecules. In this context, the physiological significance and the functional consequences of antigen presentation by B7-deficient parenchymal cells, which express MHC class II molecules as a result of inflammation, remains a matter of debate. In this paper we have attempted to critically review the often conflicting reports on the functional effects of antigen presentation by epithelial and endothelial cells to T cells, both in vitro and in vivo. Our own findings are summarised in a model which is consistent with the suggestion of an important role for antigen presentation by parenchymal cells in the induction and the maintenance of peripheral tolerance.

E expression is needed on both bone marrow derived cells and thymic epithelium to increase IL-4 production and achieve protection in NOD bone marrow chimeras

The NOD mouse is an animal model for insulin-dependent diabetes with many similarities to the human disease. NOD mice which are transgenic for the Ea gene, allowing expression of the E molecule, are protected from diabetes and rarely develop insulitis. We have constructed bone marrow chimeras between transgenic and non-transgenic NOD mice to study the correlation of E expression on bone marrow derived cells and thymic epithelium vs the production of IL-4 and IFN-gamma. We show that NOD-E-->NOD-E and NOD-E-->NOD chimeras have elevated levels of IL-4 compared to NOD-->NOD and NOD-->NOD-E chimeras in the thymus. However, in the periphery the protected NOD-E-->NOD-E show much higher IL-4 levels than any of the other chimeras. This drop in peripheral IL-4 production seen in NOD-E-->NOD, NOD-->NOD-E and NOD-->NOD chimeras correlates with the increased insulitis seen in these mice compared to NOD-E-->NOD-E. In contrast, there were no differences in IFN-gamma production between the chimeras. We suggest that the precommitted, regulatory T cells, selected in an E-expressing thymic environment, need continuous interaction with E-expressing primary antigen presenting cells in the periphery for optimal IL-4 production. Decrease in IL-4 production correlates with increased insulitis.

Identification of a cross-reactive self ligand in virus-mediated autoimmunity

Molecular mimicry has been considered to be one of the potential mechanisms underlying the induction of autoimmune diseases. Using a TCR-transgenic model specific for lymphocytic choriomeningitis virus (LCMV) we have examined the potential for cross-reactive recognition of tissue-restricted self peptides. Several peptides were identified that were able to cross-react with the TCR-transgenic virus-specific T cells in vitro. One peptide was derived from dopamine beta-mono-oxygenase, an enzyme expressed in the adrenal medulla. Interestingly, after activation of the transgenic T cells with LCMV glycoprotein peptides or viruses, infiltration of the adrenal medulla was detected in conjunction with alterations in dopamine metabolism. However, complete destruction of the adrenal medulla was not observed. This suggests that molecular mimicry may be sufficient for self recognition and infiltration, but other factors clearly contribute to chronic autoimmune disease.

Are there unique autoantigens triggering autoimmune diseases?

Using three reference disease models--insulin-dependent diabetes mellitus (IDDM) as a prototype of T-cell mediated organ-specific autoimmune disease, myasthenia gravis (MG) as a prototype of autoantibody-mediated organ-specific autoimmune disease and systemic lupus erythematosus (SLE) as a prototype of non-organ-specific autoimmune disease--we have reached several conclusions: 1) All three diseases are associated with the presence of multiple autoantibodies and/or autoreactive T cells that recognize a large number of antigenic molecules. The apparent predominant role of certain antibodies in some diseases could relate to their functional properties such as acetylcholine receptor (AChR) blockade for anti-AChR autoantibodies in MG or anti-dsDNA in SLE. 2) Major target antigens are clustered in the target cell affected by organ-specific autoimmune diseases: beta cells in IDDM, striated-muscle cells in MG, or apoptotic cells in the case of SLE. 3) Antibodies and T cells recognize multiple epitopes in these molecules. 4) The most evident explanation for the observed clustering and diversity is autoantigen spreading. Spreading probably involves T cells secreting proinflammatory cytokines but also possibly antibodies as in the case of nucleosome autoantibodies in SLE. 5) The counterpart of antigen spreading is bystander suppression in which regulatory cytokines deviate the immune response towards a protective response. 6) The mechanisms underlying the initiation of the autoimmune response and antigen spreading are still undetermined. They could imply a direct abnormality of the target cell in the case of organ-specific autoimmune diseases (e.g. infection with a virus showing a selective tropism for the target cell in organ-specific autoimmune diseases, or loss of physiological regulation of major histocompatibility complex molecule expression) or could be consequence of a ubiquitous cell abnormality such as increased apoptosis in SLE. The respective roles of genetic and environmental factors in these triggering events remain to be determined.

Endogenous myelin basic protein inactivates the high avidity T cell repertoire

To study the contribution of endogenous myelin basic protein (MBP) to the positive and/or negative selection of the MBP-specific T cell repertoire, we studied the T cell response to MBP in MBP-deficient shiverer and MBP-expressing congenic C3H mice. Immunization with MBP induced a vigorous T cell response in shiverer mice directed against a single I-Ak- restricted immunodominant determinant, the core of which is peptide MBP:79-87 (DENPVVHFF). Injection of this peptide induced a high avidity T cell repertoire in shiverer mice that primarily consisted of clones capable of recognizing the native MBP protein in addition to the peptide itself. These data show that endogenous MBP is not required for the positive selection of an MBP-specific T cell repertoire. C3H mice, in contrast, were selectively unresponsive to the MBP protein and injection of MBP:79-87 peptide induced a low avidity repertoire that could be stimulated only by the peptide, not by the protein. Therefore, endogenous MBP induced profound inactivation of high avidity clones specific for the immunodominant determinant making that determinant appear cryptic.

A range of CD4 T cell tolerance: partial inactivation to organ-specific antigen allows nondestructive thyroiditis or insulitis

T cell receptor (TCR) transgenic mice specific for hen egg lysozyme (HEL) were crossed with mice expressing HEL on the thyroid epithelium, on pancreatic islet beta cells, or systemically. Depending on the pattern of HEL expression, deletion of double-positive thymocytes ranged from minimal to complete, and peripheral CD4 cells exhibited graded reduction in TCR expression, in vitro responsiveness, and in vivo helper ability. CD4 cells were least tolerant in TCR/thyroid-HEL and TCR/islet-HEL mice, which developed an extensive lymphocytic thyroiditis or insulitis that nevertheless did not eliminate HEL-expressing endocrine cells. Autoreactive CD4 clones thus escape the thymus under a range of circumstances, retain sufficient function to initiate subclinical autoimmune inflammation when self-antigens are concentrated in the thyroid or pancreas, and may regulate progression of subclinical inflammation to destructive autoimmune disease.

Duration of TCR stimulation determines costimulatory requirement of T cells

Current models suggest that T cells that receive only signal-1 through antigenic stimulation of the T cell receptor (TCR) become anergic, but will mount an immune response when a costimulatory signal-2 is provided. Using mice deficient for an important costimulatory molecule, CD28, we show that a transient signal-1 alone, either through infection with an abortively replicating virus, or through injection of viral peptide, anergizes CD8+ T cells, demonstrating the biological relevance of T cell anergy in vivo. However, in the absence of CD28, continued presence of signal-1 alone, either through prolonged viral replication or repeated injection of peptide, prevents the induction of anergy and generates a functional T cell response in vivo.

Balancing immunity and tolerance: deleting and tuning lymphocyte repertoires

Immunological self-tolerance is ensured by eliminating or inhibiting self-reactive lymphocyte clones, creating physical or functional holes in the B- and T-lymphocyte antigen receptor repertoires. The nature and size of these gaps in our immune defenses must be balanced against the necessity of mounting rapid immune responses to an everchanging array of foreign pathogens. To achieve this balance, only a fraction of particularly hazardous self-reactive clones appears to be physically eliminated from the repertoire in a manner that fully prevents their recruitment into an antimicrobial immune response. Many self-reactive cells are retained with a variety of conditional and potentially flexible restraints: (i) their ability to be triggered by antigen is diminished by mechanisms that tune down signaling by their antigen receptors, (ii) their ability to carry out inflammatory effector functions can be inhibited, and (iii) their capacity to migrate and persist is constrained. This balance between tolerance and immunity can be shifted, altering susceptibility to autoimmune disease and to infection by genetic or environmental differences either in the way antigens are presented, in the tuning molecules that adjust triggering set points for lymphocyte responses to antigen, or in the effector molecules that eliminate, retain, or expand particular clones.

Suppression of anti-thyrocyte autoreactivity by the lymphocytes of normal fetal lambs

We have devised an experimental strategy to determine whether the developing immune system of normal fetal animals can spontaneously acquire the capacity to inhibit autoimmune responses by its cells as it matures. Whilst the existence of cells with the capacity to exert negative regulation and to curtail autoimmune responses has been demonstrated previously in response to the experimental induction of these responses, the relevance of such regulatory processes to the prevention of overt autoimmunity in normal animals has not been established. We have produced pairs of identical twin fetal lambs by splitting blastocysts and have subsequently deprived one of each pair of exposure to thyroid-specific antigens by surgical thyroidectomy before development of immunological self recognition. Thyroidectomized fetuses developed T lymphocytes autoreactive against self thyrocytes. However, their normal, identical co-twins were found to acquire a class of T lymphocytes with the capacity to block anti-thyrocyte autoreactive cells from the thyroidectomized fetal co-twin. Blocking of anti-thyroid autoreactivity required preliminary contact between these normal T lymphocytes and the target thyrocytes. Substitution of an allograft of fetal thyroid tissue for a fetal lamb's own thyroid gland failed to prevent the development of autoreactivity against autologous thyrocytes by the recipient's lymphocytes. However, the reactivity of those lymphocytes against thyrocytes from the specific allogeneic thyroid donor was markedly curtailed.

On the various manifestations of spontaneous autoimmune diabetes in rodent models

Autoimmune (type 1) diabetes mellitus in mouse, rat, and humans shares several features, including T lymphocyte infiltration into pancreatic islets and a dependence on permissive class II major histocompatibility complex (MHC) alleles. We report here on an experimental model involving mice that express influenza hemagglutinin (HA) under the control of the insulin promoter and, at the same time, a transgenic class II MHC-restricted T cell receptor (TcR) specific for an HA peptide. These mice spontaneously develop islet infiltrates resembling those found in NOD mice and most animals become diabetic within 8 weeks of age. Because of the availability of a clonotypic TcR antibody, we can be confident that the Ins-HA transgene does not induce any measurable alterations in the vast majority of T cells with the transgenic TcR in primary and secondary lymphoid organs. Continuous export of large numbers of HA-specific lymphocytes from the thymus was not required for the manifestation of the disease since mice thymectomized at 3 days after birth still developed the disease albeit with smaller infiltrates.

Costimulator B7-1 confers antigen-presenting-cell function to parenchymal tissue and in conjunction with tumor necrosis factor alpha leads to autoimmunity in transgenic mice

Tolerance to peripheral antigens is thought to result from the inability of parenchymal tissue to stimulate T cells--an inability that is believed to relate to the lack of expression of the costimulatory signal(s) required for T-cell activation. To test this model, we generated transgenic mice expressing costimulatory molecule B7-1 on the B cells of the pancreas. We find that islets from these transgenic mice are immunogenic for naive T cells in vitro and in vivo. Nonetheless, mice expressing the costimulator B7-1 specifically on beta cells do not develop diabetes, suggesting that expression of the B7-1 costimulator is not sufficient to abrogate the tolerance to peripheral antigens. We have reported that tumor necrosis factor alpha subunit (TNF-alpha) expressed by beta cells leads to a local inflammation but no islet destruction. Strikingly, however, the combination of a local inflammation due to the expression of the cytokine TNF-alpha and the expression of B7-1 results in tissue destruction and diabetes.

Timely immunization subverts the development of peripheral nonresponsiveness and suppresses tumor development in simian virus 40 tumor antigen-transgenic mice

Tolerance to tumor cell-expressed molecules and selection of cells that evade immune surveillance during tumor progression create effective barriers to immunotherapy. We investigated the cytotoxic T-lymphocyte response to simian virus 40 (SV40) tumor (T/t) antigen in two lineages of transgenic mice bearing the same rat insulin promoter-SV40 T/t antigen (RIP Tag) hybrid gene. RIP1-Tag2 mice, which express Tag as embryos, are tolerant to Tag, whereas RIP1-Tag4 mice, which express the transgene in pancreatic islet beta cells several weeks after birth and develop insulinomas, can be immunized to generate active Tag-specific cytotoxic T lymphocytes as determined by in vitro assays. Indeed, RIP1-Tag4 mice immunized with Tag by SV40 infection prior to the time of endogenous transgene expression also mount an effective in vivo cellular immune response to the Tag-expressing pancreatic beta cells, and Tag-induced tumor growth is significantly delayed (up to 1 year). However, after the transgene is expressed, RIP1-Tag4 mice are unable to mount a tumor-inhibiting response upon immunization, although Tag-specific cytotoxic T cells can still be demonstrated in vitro. Our data suggest that Tag-specific T cells are rendered unresponsive in vivo in RIP1-Tag4 mice and that the establishment of this unresponsiveness to Tag can be prevented by SV40 immunization only before the onset of the transgene expression. In the older, successfully immunized mouse, decreased immune surveillance and selection of cells with down-regulation of major histocompatibility complex class I expression most likely set the stage for insulinoma development.

A vector driving the expression of foreign cDNAs in the MHC class II-positive cells of transgenic mice

We describe a plasmid vector that drives the expression of foreign cDNAs in transgenic mice, according to the dictates of an MHC class II gene promoter. Using this vector, we have often obtained mRNA and protein synthesis with a tissue and cell-type specificity indistinguishable from that of the endogenous MHC class II genes.

Autoimmune diabetes can be induced in transgenic major histocompatibility complex class II-deficient mice

Insulin-dependent diabetes mellitus (IDDM) is an autoimmune disease marked by hyperglycemia and mononuclear cell infiltration of insulin-producing beta islet cells. Predisposition to IDDM in humans has been linked to the class II major histocompatibility complex (MHC), and islet cells often become aberrantly class II positive during the course of the disease. We have used two recently described transgenic lines to investigate the role of class II molecules and CD4+ T cells in the onset of autoimmune insulitis. Mice that are class II deficient secondary to a targeted disruption of the A beta b gene were bred to mice carrying a transgene for the lymphocytic choriomenigitis virus (LCMV) glycoprotein (GP) targeted to the endocrine pancreas. Our results indicate that class II-deficient animals with and without the GP transgene produce a normal cytotoxic T lymphocyte response to whole LCMV. After infection with LCMV, GP-transgenic class II-deficient animals develop hyperglycemia as rapidly as their class II-positive littermates. Histologic examination of tissue sections from GP-transgenic class II-deficient animals reveals lymphocytic infiltrates of the pancreatic islets that are distinguishable from those of their class II-positive littermates only by the absence of infiltrating CD4+ T cells. These results suggest that in this model of autoimmune diabetes, CD4+ T cells and MHC class II molecules are not required for the development of disease.

The chondroitin sulfate form of invariant chain can enhance stimulation of T cell responses through interaction with CD44

Invariant chain (Ii) is a nonpolymorphic glycoprotein that associates with major histocompatibility complex class II molecules and has been shown to mediate several functions in class II-restricted antigen presentation. A small proportion of Ii is modified by the addition of chondroitin sulfate (Ii-CS), and this form of Ii is associated with class II on the surface of antigen-presenting cells. In this report we show that expression of Ii-CS dramatically enhanced the ability of class II-positive EL4 transfectants to stimulate class II-dependent allogeneic and mitogenic T cell responses. Antibody blocking studies and the ability of CD44 to bind directly to Ii-CS suggest that Ii-CS can function as an accessory molecule during T cell responses through interactions with CD44.

Cellular basis of T-cell autoreactivity in autoimmune diseases

There is no doubt that T cells play a key role in the pathogenesis of autoimmune diseases (AD) both as effector and regulatory cells. Despite spectacular progress in the understanding of natural tolerance to self, owing particularly to transgenic technology, important questions remain open regarding the pathogenesis of AD, the conditions favoring the transition from benign or 'physiological' autoimmunity to deleterious autoimmunity, and the precise effector mechanisms. This review on the cellular basis of T-cell-mediated AD begins with an enumeration of the main arguments in favor of direct T-cell involvement, special emphasis being given to two animal models which have been most extensively investigated: experimental allergic encephalomyelitis, and the nonobese diabetic mouse. The question as to whether pathogenic T cells use a restricted repertoire of V beta genes is examined in the context of these two models. From here we proceed to an evaluation of the mechanisms of onset of AD, discussing both extrinsic and intrinsic factors responsible for the breakdown in T-cell tolerance and reviewing the arguments in favor of suppressor T cells being actively involved in the prevention of autoimmunity. The last two sections are devoted to the effector mechanisms responsible for tissue injury in organ-specific AD and to T-cell-directed therapeutic interventions, respectively. We discuss the two main pathogenic hypotheses based on direct intervention of cytotoxic T cells or indirect involvement of inflammatory cytokines and macrophages, and evaluate the importance of ecotaxis in leading autoreactive T cells to the site of injury. We conclude on a brief and nonexhaustive list of strategies aimed at selectively neutralizing potentially harmful T cells.

The role of thymic epithelium in the establishment of transplantation tolerance

From experimental observations on induction of transplantation tolerance, we discuss a model that accounts for tissue-specific tolerance to antigens not expressed inside the thymus. It is postulated that antigens presented to differentiating T cells by thymic epithelium (or at large within the thymic environment) positively select and activate self-reactive T cells. A developmental program and/or prevalent conditions in the thymic environment restrict the proliferative potential and the class of effector functions that can be exerted by differentiating T cells activated in the thymus. These do not mediate inflammatory or cytolytic activities, but instead will produce the appropriate mediators to inhibit aggressive effector activities by other T cells activated in their proximity. Such "regulatory" functions will be locally expressed at the periphery upon recognition of tissue antigens shared with the thymus, towards newly formed thymic emigrants directed at tissue-specific antigens expressed by the same "target" cells. This mechanism imposes "dominant tolerance", based on specific self-recognition and predominantly established in the embryonic and neonatal period. Throughout life, the process of thymic positive selection results in all newly-formed T cells being susceptible to such suppressive mechanisms, but becoming increasingly refractory with time in the resting, post-differentiative stage. Absence of antigen (nonself) in the embryonic and neonatal life therefore allows for the accumulation of such "suppression-resistant" antigen-reactive T cells that will mount aggressive responses upon antigenic exposure. Tolerance or immunity thus represent two classes of specific immune responses, the relative predominance of which is determined by the frequency of each type of effector T cell, representing the antigenic overlap between thymic and peripheral tissues, as well as the frequency of tissue-specific T-cell generation, and the kinetics of peripheral antigenic exposure. Tolerance induced by hemopoietic cells to all other tissues is also "dominant" and based on thymic colonization and persistence of antigenic cells, with the consequent positive selection of regulatory T cells and peripheral conditions for the establishment of suppression. Upon this simple model, that ensures "interclonal class regulation" by "bridging" regulatory and effector T cells through the recognition of different antigens on the same target cell, other mechanisms which are based on V-region interactions among T cells (Ben-Nun et al. 1981, Pereira et al. 1989, Webb & Sprent 1990, Gaur et al. 1993) might well operate to ensure "dominant tolerance" by self-reactivity and class regulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Thymus epithelium induces tissue-specific tolerance

Most current models of T cell development include a positive selection step in the thymus that occurs when T cells interact with thymic epithelium and a negative selection step after encounters with bone marrow-derived cells. We show here that developing T cells are tolerized when they recognize antigens expressed by thymic epithelium, that the tolerance is tissue specific, and that it can occur by deletion of the reactive T cells.

Exploring pathogenetic mechanisms using transgenic animals

Molecular technologies for the permanent germ-line transformation of animals are now well established and routine. These new strains of animals, called transgenic, offer an unprecedented opportunity to gain a basic understanding of human genetic disorders. In this brief review we discuss the role of transgenic animals in the creation of new models of human disease and their experimental use in biomedical research. Models are now available for the study of the genetic processes involved in the pathogenesis of neoplasia, diabetes, atherosclerosis, and developmental abnormalities. Many others are available and new ones are being produced at a great rate. Principles of gene replacement therapy are amenable to analysis with transgenic animals and the information gained will be important for the development of rational therapy.

Transgenic mice that express a myelin basic protein-specific T cell receptor develop spontaneous autoimmunity

We constructed a transgenic mouse model that mimics the human autoimmune disease multiple sclerosis in its spontaneous induction and pathology. Transgenic mice were constructed expressing genes encoding a rearranged T cell receptor specific for myelin basic protein (MBP). T cell tolerance was not induced in the periphery, and functional, autoreactive T cells were found in the spleen and lymph nodes of these mice. Transgenic mice developed experimental allergic encephalomyelitis (EAE) following immunization with MBP and adjuvant plus pertussis toxin as well as with administration of pertussis toxin alone. Spontaneous EAE can develop in transgenic mice housed in a non-sterile facility but not in those maintained in a sterile, specific pathogen-free facility. This model system affords a unique opportunity to dissect the genetic and environmental variables that may contribute to the development of spontaneous autoimmune disease.

Ectopic synthesis of epidermal cytokeratins in pancreatic islet cells of transgenic mice interferes with cytoskeletal order and insulin production

The members of the multigene family of intermediate filament (IF) proteins are expressed in various combinations and amounts that are specific for a given pathway or state of differentiation. Previous experiments in which the cell type-specific IF cytoskeleton was altered by introducing foreign IF proteins into cultured cells or certain tissues of transgenic animals have shown a remarkable tolerance, without detectable interference with cell functions. To examine the importance of the cell type-specific cytokeratin (CK) IF pattern, we have studied the ectopic expression of CK genes in different epithelia of transgenic mice. Here we report changes observed in the beta cells of pancreatic islets expressing the genes for human epidermal CKs 1 and/or 10 brought under control of the rat insulin promoter. Both genes were efficiently expressed, resulting in the appearance of numerous and massive bundles of aggregated IFs, resembling those of epidermal keratinocytes. While the synthesis of epidermal CK 10 was readily accommodated and compatible with cell function, mice expressing CK 1 in their beta cells, alone or in combination with CK 10, developed a special form of diabetes characterized by a drastic reduction of insulin-secretory vesicles and of insulin-and CK 1-producing cells. In many CK 1-producing cells, accumulations of fibrous or granular material containing CK 1 were also seen in the nucleus. This demonstration of functional importance of the specific CK-complement in an epithelial cell indicates a contribution of cell type-specific factors to cytoplasmic IF compartmentalization and that the specific CK complement can be crucial for functions and longevity of a given kind of epithelium.

Vaccination or tolerance to prevent diabetes

Experiments with transgenic mice expressing the glycoprotein (GP) of lymphocytic choriomeningitis virus (LCMV) under the control of the rat insulin promoter (RIP) have demonstrated that potentially self-reactive T cells that normally ignore self peptides may nevertheless be induced by self peptides or "cross-reactive" foreign (e.g. viral) peptides that arise in the host in an immunogenic form; once activated these potentially self-reactive T cells may cause autoaggressive diseases (e.g. diabetes). The possibility of vaccinating against such T cell-mediated immunopathological diseases was evaluated in the RIP-GP transgenic mouse line Bln. Any attempt to vaccinate with the self antigen itself (e.g. recombinant vaccinia virus expressing LCMV-GP) failed to protect mice from disease. However, immunization with a recombinant vaccinia virus expressing LCMV-nucleoprotein (vacc-NP) as a non-GP LCMV vaccine was able to modulate the immune response and prevented autoaggressive disease in a MHC-dependent fashion. In contrast, tolerance induction neonatally or, more generally applicable, by lethal irradiation and reconstitution with neo-self antigen-expressing bone marrow cells always resulted in prevention of virally induced diabetes in this model situation.

Prevention of autoimmune insulitis in nonobese diabetic mice by expression of major histocompatibility complex class I Ld molecules

Nonobese diabetic (NOD) mice spontaneously develop a T-cell-mediated autoimmune disease that is similar in many respects to insulin-dependent diabetes mellitus in humans. NOD mice were shown to express major histocompatibility complex class I Kd and Db antigens. To examine the possible involvement of major histocompatibility complex class I molecules in the development of autoimmune insulitis, we attempted to express a different type of class I molecule in NOD mice by crossing C57BL/6 mice transgenic for the class I Ld gene with NOD mice. The backcross progeny expressed the Ld antigen on the peripheral blood lymphocytes at a level comparable with that of the BALB/c mice. The cell surface expression of endogenous class I and class II antigens on the peripheral blood lymphocytes was not affected. Analysis of these mice revealed that the expression of the class I Ld antigen significantly reduced the incidence of insulitis at 20 weeks of age. In situ hybridization of a biotinylated probe on mouse chromosomes showed that the Ld transgene was located in the E area of chromosome 6 with which no genetic linkage to insulin-dependent diabetes mellitus was demonstrated. These results suggest that the NOD-type class I molecules are involved in the development of insulitis in NOD mice.

A fail-safe mechanism for maintaining self-tolerance

Using cytotoxic T lymphocyte (CTL) responses to the class I histocompatibility antigen Qa1 and to the minor histocompatibility antigen H-Y, we show that the immune system maintains a peripheral screening process that is able to tolerize a wide variety of potentially autoimmune CTL. The critical factor is the presence or absence of specific T helper cells. If T help is available, CTL precursors that recognize antigen are activated. In the absence of help, they are tolerized. Thus, T helper cells are guardians of peripheral tolerance in CTL.

Organ injury associated with extrathymic induction of immune tolerance in doubly transgenic mice

The developmental fate of autoreactive T cells encountering extrathymically expressed self-antigen was studied in a doubly transgenic mouse model system where pancreatic acinar cells expressed H-2Ld and T cells expressed an antigen receptor (2C TCR) specific for H-2Ld. Thymocytes bearing 2C TCR differentiated normally. They were positively selected without evidence of intrathymic clonal deletion. Survival of H-2Ld-bearing skin allografts was significantly prolonged in pancreatic H-2Ld singly and doubly transgenic mice, consistent with an in vivo state of T-cell tolerance. The mechanism of tolerance induction was determined and found to have two components. First, up to 80% of peripheral CD8+2C TCR+ T cells were eliminated. Second, those T cells which escaped elimination had a significantly reduced proliferative response to H-2Ld. Thus, autoreactive T cells can be made self-tolerant through interaction with self-antigen located extrathymically. This is accomplished by a reduction in the percentage of autoreactive T cells as well as by a reduction in the functional capacity of residual T cells. Surprisingly, although pancreatic lymphocytic infiltration and organ injury were absent in exocrine tissue of singly transgenic mice, it was present in doubly transgenic mice. This suggests that when the percentage of autoreactive T cells is high, tolerance induction can be associated with an inflammatory infiltrate in extrathymic tissue where self-antigen is presented.

Developmental regulation of thymocyte susceptibility to deletion by "self"-peptide

The specificity of the T cell receptor (TCR) repertoire for foreign peptide bound to self-major histocompatibility complex (MHC) molecules is determined in large part by positive and negative selection processes in the thymus, yet the mechanisms of these selection events remain unknown. Using in vitro organ culture of thymi isolated from mice transgenic for a TCR-alpha/beta specific for cytochrome c peptide bound to I-Ek, we analyzed the developmental timing of negative selection (deletion). On the basis of the experiments described below, we conclude that all CD4+8+ thymocytes, and only CD4+8+ thymocytes, are susceptible to negative selection mediated by the cytochrome c peptide antigen. First, we found that deletion of thymocytes resulting from addition of the cytochrome c peptide to the thymic organ cultures can occur at the earliest stage of TCR, CD4, and CD8 coexpression. Second, we found that CD4+8+ thymocytes isolated from positively selecting or nonselecting MHC haplotypes were equally efficiently deleted in vitro, suggesting that positive selection is not a prerequisite for deletion. Third, we examined the effects of TCR/ligand avidity on the developmental timing of deletion by varying the concentration of cytochrome c peptide added to the organ cultures. We detected deletion only at the CD4+8+ stage: intermediate concentrations of peptide that resulted in partial deletion of CD4+8+ cells did not eliminate the appearance of mature CD4+8- cells. Finally, we found that CD4+8- thymocytes were resistant to deletion as well as activation by peptide antigen added to the intact organ cultures. Nevertheless, the CD4+8- thymocytes isolated from the peptide-treated organ cultures responded vigorously to peptide presented by spleen cells in vitro. Thus, the T cells were tolerant of (but not anergized by) self-antigen encountered in thymic organ culture. Together, these results indicate that thymocytes susceptible to negative selection are not developmentally distinct from those susceptible to positive selection, and further, that the thymic microenvironment plays a role in regulating the outcome of TCR/ligand interactions.

Immune regulation in self tolerance: functional elimination of a self-reactive, counterregulatory CD8+ T lymphocyte circuit by neonatal transfer of encephalitogenic CD4+ T cells lines

Transfer of encephalitogenic, CD4+ T lymphocyte lines into syngeneic adult Lewis rats not only leads to the development of experimental autoimmune encephalomyelitis (EAE), but, in addition, to the expansion of counterregulatory, CD8+ T lymphocyte clones which are able to lyse specifically the encephalitogenic T cells in vitro and to neutralize their encephalitogenic capacity in vivo. In striking contrast, in neonatal rats, which still lack myelin (autoantigens), injection of the same encephalitogenic lines neither mediates EAE, nor confers protection in later life against the myelin-specific T cells. In fact, this treatment results in the life-long functional elimination of counterregulatory, clonotypic CD8+ T lymphocytes, which cannot even be reinduced by repeated injections of the relevant CD4+ T line. These data seem to point to a self-protective T cell control mechanism which is developed within the immune system prior to, and thus independent of the appearance of the appropriate self antigen.

T-cell responsiveness to an oncogenic peripheral protein and spontaneous autoimmunity in transgenic mice

Why T cells develop autoimmune reactivity to some antigens and tolerance to others is unknown. Various mechanisms can provide for T-cell tolerance. These include deletion in the thymus, exhaustive differentiation in the periphery, T-cell receptor and coreceptor downregulation, and anergy. Which mechanisms normally provide for tolerance to antigens expressed on specific tissues and why they sometimes fail is unclear. To understand this, we analyzed how a tissue-specific protein with defined timing and location of expression is recognized by T cells so as to induce tolerance or autoimmunity. We crossed mice expressing the simian virus 40 large tumor antigen on pancreatic acini beginning 4-25 days after birth with mice transgenic for a rearranged T-cell receptor that recognizes this antigen presented by the class I major histocompatibility complex molecule H-2Kk. No T-cell tolerance was found; rather, T-cell reactivity accompanied lymphocytic infiltration and pancreatic acinar destruction. This result argues that T cells may become spontaneously autoreactive to certain postnatally expressed peripheral proteins and that this reactivity may lead to autoimmune disease.

Peripheral tolerance to an islet cell-specific hemagglutinin transgene affects both CD4+ and CD8+ T cells

To study the basis for immunological tolerance of peripheral tissue-specific antigens, a transgenic mouse line was established that expresses the influenza hemagglutinin (HA) on pancreatic islet beta cells (Ins-HA transgenic mice). When followed up to 14 months of age, Ins-HA transgenic mice did not develop spontaneous autoimmune disease. Upon immunization with HA-expressing viruses, high titers of HA-specific circulating antibody were detected; however, T cell responses by both the T helper and T cytolytic compartment were markedly reduced as compared with transgene-negative littermates, and no evidence could be found for islet infiltrates. Adoptive transfer of histocompatible lymphocytes from transgene-negative mice plus virus into irradiated Ins-HA hosts resulted in islet inflammation dominated by CD4+ T cells, indicating that the HA antigen was accessible to activated T cells. These results suggest that T cells can be rendered tolerant of antigens expressed outside the thymus.