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

Immune Responses: costimulatory receptors have their say

Lymphocytes are more likely to make an immune response if costimulatory and antigen receptors coincidently signal; the way the signals are integrated illustrates how a lymphocyte learns to distinguish self from foreign antigens, and provides a model for coincident signaling through more than one receptor.

Cytokines and their roles in pancreatic islet beta-cell destruction and insulin-dependent diabetes mellitus

Insulin-dependent diabetes mellitus (IDDM) is a disease that results from autoimmune destruction of the insulin-producing beta-cells in the pancreatic islets of Langerhans. The autoimmune response against islet beta-cells is believed to result from a disorder of immunoregulation. According to this concept, a T helper 1 (Th1) subset of T cells and their cytokine products, i.e. Type 1 cytokines--interleukin 2 (IL-2), interferon gamma (IFNgamma), and tumor necrosis factor beta (TNFbeta), dominate over an immunoregulatory (suppressor) Th2 subset of T cells and their cytokine products, i.e. Type 2 cytokines--IL-4 and IL-10. This allows Type 1 cytokines to initiate a cascade of immune/inflammatory processes in the islet (insulitis), culminating in beta-cell destruction. Type 1 cytokines activate (1) cytotoxic T cells that interact specifically with beta-cells and destroy them, and (2) macrophages to produce proinflammatory cytokines (IL-1 and TNFalpha), and oxygen and nitrogen free radicals that are highly toxic to islet beta-cells. Furthermore, the cytokines IL-1, TNFalpha, and IFNgamma are cytotoxic to beta-cells, in large part by inducing the formation of oxygen free radicals, nitric oxide, and peroxynitrite in the beta-cells themselves. Therefore, it would appear that prevention of islet beta-cell destruction and IDDM should be aimed at stimulating the production and/or action of Type 2 cytokines, inhibiting the production and/or action of Type 1 cytokines, and inhibiting the production and/or action of oxygen and nitrogen free radicals in the pancreatic islets.

The threshold for autoimmune T cell killing is influenced by B7-1

The concept that naive CD4+ and CD8+ T cells require co-stimulatory signals for activation and proliferation is well documented. Less clear is the need for co-stimulation during the effector phase of the T cell response. Here we examined the influence of B7-1 (CD80) during the effector phase of an autoimmune response to pancreatic islets using transgenic mouse lines which expressed B7-1 in either all or only some of their beta cells ("confluent" or "patchy" RIP-B7-1 mice). Transgenic expression of B7-1 in normal mouse islets that co-expressed the pro-inflammatory cytokine, IL-2, resulted in early spontaneous autoimmunity. Islets with IL-2 and "confluent" B7-1 expression were destroyed whereas islets with IL-2 and "patchy" B7-1 expression showed selective killing of the B7-1+ beta cells. Islet-reactive T cells, circulating in the RIP-B7-1/IL-2 mice, rejected syngeneic islet grafts, but only if these expressed B7-1. Introduction of the B7-1 transgene into the nonobese diabetic (NOD) genetic background likewise resulted in early spontaneous autoimmunity, but splenocytes from the diabetic animals could only transfer diabetes to NOD scid recipients that expressed B7-1 on their beta cells. In both these transgenic models, therefore, islet destruction required continuous B7-1 expression by target beta cells. Thus, although the normal repertoire contains T cells with potential islet reactivity, these T cells remain harmless because parenchymal cells like the beta cell cannot normally express B7-1. Our results also have implications for tumor immunotherapy in that the ability of T cells to kill poorly immunogenic targets may be dependent upon B7-1 expression by the target cell itself.

Reduction of arthritis and pneumonitis in motheaten mice by soluble tumor necrosis factor receptor

OBJECTIVE

To determine the effects of anti-tumor necrosis factor (anti-TNF) therapy in the inflammatory and autoimmune disease in motheaten (me/me) mice, which exhibit a Fas apoptosis signaling defect.

METHODS

Arthritis, pneumonitis, and mortality were analyzed in me/me mice treated with a novel, soluble, dimeric TNF receptor I (sTNFRI) molecule capable of high-affinity binding and neutralization of TNFalpha.

RESULTS

Soluble TNFRI reduced serum levels of TNFalpha and led to a 2-fold increase in the lifespan of me/me mice, compared with the control treatment group. The treatment also reduced the development of the "motheaten" skin patches and alleviated pneumonitis and inflammatory lesions in the extremities of me/me mice compared with controls. However, the serum levels of IgM and IgM anti-double-stranded DNA autoantibody were comparable to those of untreated control mice.

CONCLUSION

TNFalpha is an important cytokine involved in the pathogenesis of inflammatory disease in me/me mice, resulting in tissue damage and early mortality. Therapies directed at blocking TNF/TNFR interactions, such as the sTNFRI used in these experiments, may be effective in diseases associated with apoptosis defects leading to overutilization of the TNF/TNFR pathway.

Role of interleukin 12 and costimulators in T cell anergy in vivo

The induction of T cell anergy in vivo is thought to result from antigen recognition in the absence of co-stimulation and inflammation, and is associated with a block in T cell proliferation and Th1 differentiation. Here we have examined the role of interleukin (IL)-12, a potent inducer of Th1 responses, in regulating this process. T cell tolerance was induced by the administration of protein antigen without adjuvant in normal mice, and in recipients of adoptively transferred T cells from T cell receptor transgenic mice. The administration of IL-12 at the time of tolerance induction stimulates Th1 differentiation, but does not promote antigen-specific T cell proliferation. Conversely, inhibiting CTLA-4 engagement during anergy induction reverses the block in T cell proliferation, but does not promote full Th1 differentiation. T cells exposed to tolerogenic antigen in the presence of both IL-12 and anti-CTLA-4 antibody are not anergized, and behave identically to T cells which have encountered immunogenic antigen. These results suggest that two processes contribute to the induction of anergy in vivo; CTLA-4 engagement, which leads to a block in the ability of T cells to proliferate to antigen, and the absence of a prototypic inflammatory cytokine, IL-12, which prevents the differentiation of T cells into Th1 effector cells. The combination of IL-12 and anti-CTLA-4 antibody is sufficient to convert a normally tolerogenic stimulus to an immunogenic one.

Immunological tolerance to a pancreatic antigen as a result of local expression of TNFalpha by islet beta cells

Recent experiments have suggested that tumor necrosis factor alpha (TNFalpha) can down-regulate islet-specific T cells and prevent the development of autoimmune diabetes. Here we demonstrate that transgenic mice expressing both TNFalpha and the Leishmania major LACK antigen in the pancreas (RIP-TNFalpha/RIP-LACK) exhibit an impaired ability to mount a CD4+ T cell response against LACK. In addition, peripheral CD4+ T cells from TCR transgenic mice (TCR-LACK/RIP-TNFalpha/RIP-LACK) produced reduced interleukin-2 but elevated levels of T helper 2 cytokines in response to LACK peptide in vitro. Taken together, our data suggest that TNFalpha may act in vivo to modulate a potentially damaging self-reactive T cell response by inducing tolerance to pancreatic antigens.

Ex vivo adenovirus-mediated gene delivery leads to long-term expression in pancreatic islet transplants

BACKGROUND

Replication-deficient adenovirus, one of the most efficient vectors in gene therapy, has been limited by transient transgene expression due to its episomal location and loss during cell division, as well as a host immune response against viral proteins.

METHODS

Murine pancreatic islets were infected ex vivo with ad5-cytomegalovirus (CMV)-beta-galactosidase and transplanted into diabetic recipients with normalization of glucose metabolism.

RESULTS

High levels of beta-galactosidase activity were detectable histologically for at least 20 weeks after transplant, and beta-galactosidase and viral mRNA were also present that long. Sera from transplanted animals did not significantly inhibit ad5-CMV-interleukin-2-Ig infection of HeLa cells in vitro, whereas sera from intravenously delivered ad5-CMV-beta-galactosidase drastically diminished HeLa cell infection, suggesting the presence of reduced levels of antibodies in transplanted animals as compared with intravenously infected animals. Immunofluorescent staining of islet isografts infected with ad5-CMV-beta-galactosidase revealed the presence of CD8+ and CD4+ T lymphocytes at all time points, however, no islet destruction was seen.

CONCLUSIONS

Treatment of islet isografts ex vivo with ad5-CMV-beta-galactosidase results in prolonged transgene expression, possibly due to an attenuated immune response against adenovirus.

Increased expression of B7-1 costimulatory molecule on cerebrospinal fluid cells of patients with multiple sclerosis and infectious central nervous system disease

The expression of the costimulatory molecule B7-1 (BB-1; CD80) and its ligand CD28 was investigated on peripheral blood (PB) and cerebrospinal fluid (CSF) T and B lymphocytes and monocytes in 11 patients with relapsing-remitting multiple sclerosis (MS) 21 age-matched healthy controls and 10 patients with central nervous system (CNS) infectious disease (CID). Three channel flow cytometry was used with a novel gating technique in order to unambiguously identify the low numbers of B lymphocytes present in normal CSF. There was a significantly higher fraction of B7-1+ B lymphocytes in the CSF of patients with MS (72%) and CID (69%) when compared with healthy individuals (53%; p < 0.0001 and p < 0.002, respectively). Furthermore, two patients with a clinical picture of encephalitis showed a profoundly increased B7-1 expression on CSF monocytes. Comparison of absolute numbers of B7-1+ B lymphocytes/mL CSF between MS patients and healthy controls revealed a highly increased frequency of these cells among MS patients (235 cells/mL in MS patients versus 3.9 cells/mL in controls; p < 0.0001) with no overlap between the groups, which was otherwise seen for all other analyzed cell populations. We therefore hypothesize that activated B lymphocytes expressing high levels of B7-1 may be of pathogenetic importance in the development and maintenance of the MS disease.

The role of Fas in autoimmune diabetes

Immunologically privileged sites express Fas ligand (FasL), which protects them from attack by activated T cells that express Fas and die upon contact with FasL. In an attempt to protect nonobese diabetic mice (NOD) from autoimmune diabetes, we made FasL transgenic NOD mice using the beta cell-specific rat insulin-1 promoter. Surprisingly, these transgenic mice showed heightened sensitivity to diabetogenic T cells, which was due to self-destruction of beta cells upon T cell-mediated induction of Fas. Fas-negative NOD(lpr/lpr) animals were resistant to diabetogenic T cells and to spontaneous diabetes. Thus, induction of Fas expression on beta cells and their subsequent destruction constitutes the main pathogenic mechanism in autoimmune diabetes.

T cell-mediated elimination of B7.2 transgenic B cells

Transgenic mice were generated to explore the effects on lymphoid development and immune function of constitutive expression of murine B7.2 on B and T cells. The number of B lymphocytes in primary and secondary lymphoid tissues is normal in B7.2 transgenic lines expressing low levels of B7.2 on B cells, but markedly reduced in transgenic lines expressing moderate to high levels of the transgene on B cells. This reduction is not due to an intrinsic abnormality of the transgenic B cells, but is rather the consequence of an elimination by an immune mechanism requiring the engagement of CD28 on T cells. Interestingly, during cognate antigen-specific interaction with T cells in vivo, B7.2 transgenic B cells are not eliminated, but proliferate and differentiate normally. Our findings suggest that, in the absence of high affinity ligand for the TCR, the CD28-B7.2 system participates in the regulation of B cell homeostasis.

TNF-alpha transgenic and knockout models of CNS inflammation and degeneration

Tumour necrosis factor-alpha (TNF-alpha) plays a central role in inflammatory events including those taking place in the central nervous system (CNS), and has been implicated as a key pathogenic mediator in several human inflammatory, infectious and autoimmune CNS disorders. Using transgenic and gene knockout mice we have investigated the role of deregulated TNF-alpha production in the CNS. We show that the overexpression of wild-type murine or human TNF-alpha transgenes by resident CNS astrocytes or neurons in sufficient to trigger a neurological disorder characterised by ataxia, seizures and paresis, with histopathological features of chronic CNS inflammation and white matter degeneration. Furthermore, we show that transmembrane human TNF-alpha is sufficient to trigger CNS inflammation and degeneration when overexpressed by astrocytes but not by neurons, indicating that target cells mediating the neuroinflammatory activities of TNF-alpha localise in the vicinity of astrocytes rather than neurons. Our results establish that both soluble and transmembrane molecular forms of TNF-alpha can play critical roles in vivo in the pathogenesis of CNS inflammation and demyelination, and validate TNF-alpha transgenic and mutant mice as important models for the further study of related human CNS diseases.

Prevention of autoimmune diabetes mellitus in NOD mice by transgenic expression of soluble tumor necrosis factor receptor p55

The non-obese diabetic (NOD) mouse represents a relevant animal model of autoimmunity for insulin-dependent diabetes mellitus. The pathogenic role of tumor necrosis factor (TNF) in insulitis and beta cell destruction observed in these mice remains controversial, since injections of TNF or of anti-TNF antibodies have been reported to exert protection or acceleration of diabetes, depending on the timing of administration. In this study, we demonstrate that, in contrast to the non-transgenic littermates, NOD mice with permanent neutralization of TNF by high blood levels of soluble TNF receptor p55-human FcIgG3-fusion molecules resulting from the expression of a transgene are protected from spontaneous diabetes. They are also protected from accelerated forms of disease caused by transfer of NOD spleen cells or cyclophosphamide injections. This protection is associated with a marked decrease in the severity and incidence of insulitis and in the expression of the adhesion molecules MAdCAM-1 and ICAM-1 on the venules of pancreatic islets. These data suggest a central role for TNF-alpha in the mediation of insulitis and of the subsequent destruction of insulin-secreting beta-cells observed in NOD mice. They may be relevant to cell-mediated autoimmune diseases in general, in which treatment with soluble TNF receptors might be beneficial.

Anomalous expression of costimulatory molecules B7-1, B7-2 and CD28 in primary biliary cirrhosis

BACKGROUND

T lymphocytes require two important signals for efficient activation: 1) recognition of antigens bound to self major histocompatibility complex antigens, and 2) simultaneous stimulation via so-called costimulatory molecules. Interaction of the costimulatory B7 molecules on antigen presenting cells with CD28 on T lymphocytes appears to be particularly important, as it modifies secretion of cytokines, especially interleukin 2. In primary biliary cirrhosis biliary epithelial cells aberrantly express major histocompatibility complex class II antigens and may function as antigen presenting cells.

METHODS

We studied expression of HLA-DR, B7-1, B7-2 and CD28 on cryostat liver sections in 16 patients with primary biliary cirrhosis, three patients each with autoimmune hepatitis and primary sclerosing cholangitis and nine patients with chronic viral hepatitis (five hepatitis B, four hepatitis C) using mouse monoclonal antibodies in an indirect immunoperoxidase technique.

RESULTS

In advanced primary biliary cirrhosis, HLA-DR was found on 57% of bile ducts, B7-2 on 5% of bile ducts, and B7-1 could not be detected on any bile duct. Neither B7-1 nor B7-2 was seen on bile ducts in the four patients with early primary biliary cirrhosis. HLA-DR+ bile ducts also lacked expression of B7 molecules in autoimmune hepatitis. In contrast, HLA-DR, B7-1 and B7-2 were expressed simultaneously on professional antigen presenting cells such as macrophages in epitheloid granulomas.

CONCLUSION

HLA-DR+ biliary epithelial cells in primary biliary cirrhosis insufficiently co-express B7-1 or B7-2 molecules. Therefore, they must either use different costimulatory molecules, or otherwise are deficient in lymphocyte activation. Since recognition of antigen in the absence of B7-CD28 interaction may lead to anergy of lymphocytes, this might contribute to the impaired cytokine secretion found in primary biliary cirrhosis.

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.

Costimulation and autoimmunity

The past year has seen significant advances in our understanding of the role of the B7-CD28/CTLA-4 pathway in T cell activation and self-tolerance. Recent studies have demonstrated that CTLA-4 is a critical negative regulator of T cell activation and autoreactivity, revealing a previously unsuspected means by which costimulation is involved in the maintenance and breakdown of self-tolerance. Manipulation of this costimulatory pathway in animal models of autoimmunity has shown an important role for this pathway in both the initiation and progression of autoimmune diseases.

Virus-induced autoimmune disease

The breaking of tolerance or unresponsiveness to self-antigens, involving the activation of autoreactive lymphocytes, is a critical event leading to autoimmune diseases. The precise mechanisms by which this can occur are mostly unknown. Viruses have been implicated in this process, among other etiological factors, such as genetic predisposition and cytokine activity. Several ways have been proposed by which a viral infection might break tolerance to self and trigger an autoreactive cascade that ultimately leads to the destruction of a specific cell type or an entire organ. The process termed "molecular mimicry' and the use of transgenic models in which viral and host genes can be manipulated to analyze their effects in causing autoimmunity have been particular focuses for research. For example, there is a transgenic murine model of virus-induced autoimmune disease, in which a known viral gene is selectively expressed as a self-antigen in beta cells of the pancreas. In these mice, insulin-dependent diabetes develops after either a viral infection, the release of a cytokine such as IFN-gamma, or the expression of the costimulatory molecule B7.1 in the islets of Langerhans. Recent studies using this model have contributed to the understanding of the pathogenesis of virus-induced autoimmune disease and have furthered the design and testing of novel immunotherapeutic approaches.

Using transgenic mouse models to dissect the pathogenesis of virus-induced autoimmune disorders of the islets of Langerhans and the central nervous system

Viruses have often been associated with autoimmune diseases. One mechanism by which self-destruction can be triggered is molecular mimicry. Many examples of cross-reactive immune responses between pathogens and self-antigens have been described. This review presents two transgenic models of autoimmune disease induced by a virus through activation of anti-self lymphocytes. Viral antigens are expressed as transgenes either in beta-cells of the pancreas or in the oligodendrocytes of the CNS. Infection by a virus encoding the same gene activated autoreactive T cells that cleared the viral infection, and as a consequence of transgene expression resulted in organ-specific autoimmune disease. In both transgenic mouse models, autoreactive lymphocytes that escaped thymic negative selection were present in the periphery. Several factors are described that play a role in the regulation of the self-reactive process precipitated by a viral infection. These include the quantity of activated autoreactive T cells, the affinity of these T cells, the number of memory T cells generated following primary infection, costimulation by accessory molecules, and the types and locations of cytokines produced. In addition, unique barriers exist in target tissues that prevent or suppress autoreactive responses and define to a large extent the outcome of disease. Restimulation of autoreactive memory lymphocytes may be required to bypass these barriers and enhance autoimmune disease. Therapy directed at modifying these factors can reduce and even prevent autoimmune disease after it has been initiated.

Chronic inflammation caused by lymphotoxin is lymphoid neogenesis

In presenting a unifying concept for chronic inflammation and lymphoid organogenesis, we suggest that lymphotoxin's (LT, LT-alpha, TNF-beta) crucial role in these processes is pivotal and similar. Chronic inflammatory lesions that developed in the kidney and pancreas at the sites of transgene expression in rat insulin promoter-LT (RIP-LT) mice resembled lymph nodes with regard to cellular composition (T cells, B cells, plasma cells, and antigen-presenting cells), delineated T and B cell areas, primary and secondary follicles, characteristic morphologic and antigenic (ICAM-1, VCAM-1, MAdCAM-1, and PNAd) features of high endothelial venules, and ability to respond to antigen and undergo Ig class switching when obtained from mice immunized with SRBC. The vascular changes, with the exception of PNAd, appear to be the direct consequence of transgene derived LT expression, as they were also observed in RIP-LT mice lacking mature T and B cells. These data show that LT-induced chronic inflammation has the characteristics of organized lymphoid tissue.

CD28/B7 system of T cell costimulation

T cells play a central role in the initiation and regulation of the immune response to antigen. Both the engagement of the TCR with MHC/Ag and a second signal are needed for the complete activation of the T cell. The CD28/B7 receptor/ligand system is one of the dominant costimulatory pathways. Interruption of this signaling pathway with CD28 antagonists not only results in the suppression of the immune response, but in some cases induces antigen-specific tolerance. However, the CD28/B7 system is increasingly complex due to the identification of multiple receptors and ligands with positive and negative signaling activities. This review summarizes the state of CD28/B7 immunobiology both in vitro and in vivo; summarizes the many experiments that have led to our current understanding of the participants in this complex receptor/ligand system; and illustrates the current models for CD28/B7-mediated T cell and B cell regulation. It is our hope and expectation that this review will provoke additional research that will unravel this important, yet complex, signaling pathway.

Checkpoints in the progression of autoimmune disease: lessons from diabetes models

In the last few years, data from experiments employing transgenic models of autoimmune disease have strengthened a particular concept of autoimmunity: disease results not so much from cracks in tolerance induction systems, leading to the generation of anti-self repertoire, as from the breakdown of secondary systems that keep these cells in check. T cells with anti-self specificities are readily found in disease-free individuals but ignore target tissues. This is also the case in some transgenic models, in spite of overwhelming numbers of autoreactive cells. In other instances, local infiltration and inflammation result, but they are well tolerated for long periods of time and do not terminally destroy target tissue. We review the possible molecular and cellular mechanisms that underlie these situations, with a particular emphasis on the destruction of pancreatic beta cells in transgenic models of insulin-dependent disease.

The anatomy of T-cell activation and tolerance

The mammalian immune system must specifically recognize and eliminate foreign invaders but refrain from damaging the host. This task is accomplished in part by the production of a large number of T lymphocytes, each bearing a different antigen receptor to match the enormous variety of antigens present in the microbial world. However, because antigen receptor diversity is generated by a random mechanism, the immune system must tolerate the function of T lymphocytes that by chance express a self-reactive antigen receptor. Therefore, during early development, T cells that are specific for antigens expressed in the thymus are physically deleted. The population of T cells that leaves the thymus and seeds the secondary lymphoid organs contains helpful cells that are specific for antigens from microbes but also potentially dangerous T cells that are specific for innocuous extrathymic self antigens. The outcome of an encounter by a peripheral T cell with these two types of antigens is to a great extent determined by the inability of naive T cells to enter nonlymphoid tissues or to be productively activated in the absence of inflammation.

The presence of costimulatory molecules CD86 and CD28 in rheumatoid arthritis synovium

OBJECTIVE

To investigate the expressions of co-stimulatory molecules CD86 (B7-2, B70) and CD28 by cells obtained from the synovial tissues (ST) and synovial fluids (SF) of patients with rheumatoid arthritis (RA).

METHODS

Monoclonal antibodies (MAb) against CD86 and CD28 were used for immunochemical study of synovia from 18 RA patients, 4 osteoarthritis (OA) patients, and 4 normal subjects. These MAb were also used for flow cytometry of isolated ST cells from 8 RA and 5 OA patients and of SF mononuclear cells from 5 RA and 5 OA patients.

RESULTS

Immunohistochemical examination revealed that CD86+ cells occurred in 11 of the 18 RA synovia, but in none of the 4 OA or 4 normal synovia. Most of the positive cells had macrophage-like morphology, and surrounded lymphoid aggregates. Most cells within lymphoid aggregates were stained positively for CD28. Flow cytometry showed that CD86+ cells comprised 2.9-33.4% (average 14.3%) of the total ST cells and 2.1-14.9% (average 6.1%) of the total SF mononuclear cells from RA patients. Approximately 40% of the CD86+ cells expressed CD14. A majority (mean 72%, range 57-89%) of the T cells in ST and SF expressed CD28. RA synovia expressed more CD86 molecules than did OA synovia (mean frequency of positive cells 14.3% versus 2.8%; mean fluorescence intensity 104.6 versus 40.9).

The contribution of insulitis to diabetes development in tumor necrosis factor transgenic mice

The inflammatory response mediated by cytokines such as TNF can promote recruitment of lymphocytes to a tissue. Moreover, if other conditions are met, this can provide a predisposing role to autoimmune disease. TNFs induce the appearance of adhesion molecules (and presumably, therefore, extravasation of lymphocytes into tissue from the vasculature) and increase the levels of MHC class I on tissue. However, it is not clear which of these effects plays the key role in induction of disease. This should be the subject of further study. The data substantiate the hypothesis that chronic inflammation might play a precipitating role in autoimmunity and could be one of the environmental factors of importance in the development of so many autoimmune diseases.

Tolerance and immunity to the inducible self antigen C-reactive protein in transgenic mice

The understanding of immunological tolerance has been greatly aided by the development of transgenic animal models in which expression of a specific T cell receptor (or B cell receptor) and its cognate self antigen is experimentally controlled. In most cases, expression of the self antigen was constitutive and did not allow for variation of its time- and dose-dependent expression pattern, parameters which are known to influence the balance of tolerance versus immunity. We describe a transgenic model in which expression of human C-reactive protein (hCRP), an acute-phase protein, is tightly controlled at basal levels (female mice express around 10(-9) M and male mice 5 x 10(-7) M circulating hCRP) and is highly inducible (induction factor of 25-500). T cells from C57BL/6 mice recognize two epitopes of hCRP termed A (residues 79-95) and B (residues 87-102). Different efficacies of presentation in vitro and in vivo identify epitope A as sub-dominant and epitope B as dominant. T cells of non-induced hCRP transgenic mice are tolerant to the dominant epitope, but reactive to the subdominant epitope. A hCRP-specific IgG antibody response is detectable in transgenic mice, but is weaker than in littermates. Upon induction of hCRP, both T cell epitopes are presented by thymic and splenic antigen-presenting cells (APC) in vivo. Kinetics of presentation by splenic APC closely match serum kinetics of hCRP, whereas presentation in the thymus is considerably prolonged. This model enables epitope-specific T cell tolerance to be studied as a function of time- and dose-dependent expression of the self antigen.

Transgenic/knockout mice--tools to study autoimmunity

Transgenic and knockout mice have been valuable tools for clarifying the roles of individual cell types and effector molecules in the pathogenesis of autoimmunity. During the past year, new strains have been added to the large array of transgenic mice, with broad or tissue-specific expression of transgene products. These laboratory models, as well as knockout mice lacking genes for a particular molecule, have greatly enhanced our understanding of autoimmune disease.

Sensitization to self (virus) antigen by in situ expression of murine interferon-gamma

Autoimmune disease results from inflammatory destruction of tissues by aberrant self-reactive lymphocytes. We studied the autoimmune potential of T lymphocytes immunologically ignorant of viral antigens acting as self antigens and whether the host defense molecule IFN-gamma could stimulate these cells to cytotoxic competency. For this purpose, we produced double transgenic mice expressing pancreatic IFN-gamma as well as lymphocytic choriomeningitis virus (LCMV) nucleoprotein (NP) or glycoprotein (GP) antigen. 100% of the NP+/IFN-gamma+ mice became diabetic before 2 mo of age, while none of the NP single transgenic littermates and only 10% of IFN-gamma single transgenic littermates did. Strikingly, NP+/IFN-gamma+ mice spontaneously developed cytotoxic T lymphocyte activity on LCMV-infected targets and vaccinia virus-NP-infected ones without prior LCMV infection but NP+/IFN-gamma- mice did not, which indicates specific sensitization to the viral antigen by IFN-gamma. These results suggest that lymphocytes ignorant of self antigens can be activated by IFN-gamma released after immunologic stimulation such as viral infection. This mechanism may account for the loss of apparent tolerance to self antigens in autoimmune diseases such as insulin-dependent diabetes mellitus.

Costimulation in tolerance and autoimmunity

The two signal model of T cell activation predicts that a second costimulatory signal provided by antigen presenting cells (APC) is required in conjunction with the antigenic signal to trigger T cell activation. Considerable evidence indicates that indeed, T cell activation requires such a costimulatory signal which results, at least in part, from the interaction of CD28 with its ligands B7 expressed on all antigen-presenting cells (APC). The second prediction of the two signal model is that T cell receptor engagement in the absence of such a costimulatory signal would lead to specific inactivation of antigen reactive cells. Thus, tissue cells that do not express costimulatory signals would not trigger T cell activation but rather lead to specific inactivation of auto-reactive T cells. By such a model, tolerance to peripheral antigens would be permanently re-established. We review here the evidence suggesting that the CD28-B7 costimulatory pathway might play an important role in T cell tolerance and in the development of autoimmune responses.

Keratinocyte expression of B7-1 in transgenic mice amplifies the primary immune response to cutaneous antigens

Resting epidermal keratinocytes do not express B7-1 and other known CD28 counterligands with costimulatory activity. The absence of these costimulators on keratinocytes correlates with their ability to preferentially induce T-cell anergy instead of T-cell activation. To test the hypothesis that keratinocytes expressing a CD28 counterligand would be more effective inducers of T-cell-mediated immune responses in skin, we prepared transgenic mice in which expression of the B7-1 costimulator was targeted to basal keratinocytes by using the human K14 promoter. Keratinocytes from the K14/B7-1 transgenic line expressed high levels of surface B7-1. No spontaneous inflammatory changes were seen in transgenic skin, but epicutaneous application of contact sensitizers to these mice elicited a stronger primary ear swelling response than in controls. Sites of initial hapten application in transgenic mice also responded much more strongly to reapplication of hapten to a remote cutaneous site. Epidermal cell suspensions from transgenic mice contained normal numbers of Langerhans cells and dendritic epidermal T cells when analyzed by flow cytometry. Systemic treatment of the transgenic mice with interferon gamma induced high levels of class II major histocompatibility complex expression on keratinocytes but was not sufficient to initiate an inflammatory response. We conclude that the constitutive expression of the B7-1 molecule in vivo on a nonprofessional antigen-presenting cell is not by itself sufficient to trigger inflammatory changes, but B7-1 expression amplifies the host immune responses after exposure to nonself antigens presented by B7-1-expressing cells.

A CD8+ T-lymphocyte-mediated and CD4+ T-lymphocyte-independent autoimmune diabetes of early onset in transgenic mice

While transgenic mice expressing tumour necrosis factor-alpha under the control of the beta-cell-specific insulin promoter display a marked lymphocytic infiltration of the islets, they never develop insulin-dependent diabetes mellitus (IDDM). In striking contrast, "double" transgenic mice whose beta cells express both tumour necrosis factor-alpha as well as the co-stimulatory B7-1 molecule all develop IDDM at an early age. Furthermore, administration of anti-CD8 but not anti-CD4 immunoglobulins prevents diabetes onset. These results indicate that while tumour necrosis factor-alpha induced lymphocytic infiltration is not sufficient to effect beta-cell destruction, locally co-stimulated islet-infiltrating CD8+ T lymphocytes could play a critical role in the development of IDDM.