Expression in transgenic mice of class I histocompatibility antigens controlled by the metallothionein promoter

Splenectomy induces biochemical remission and regeneration in experimental murine autoimmune hepatitis

Autoimmune hepatitis (AIH) is a chronic immune-mediated inflammatory liver disease. It is known that AIH originates not from the spleen but from the liver itself. Nonetheless, most details of the etiology and pathophysiology are unknown. We induced experimental murine AIH (emAIH) in NOD/Ltj mice by single administration of a replication-deficient adenovirus and performed splenectomy during late-stage disease. Biochemical disease remission occurred, which was characterized by improvement in transaminase levels. The causes of this remission included a shift in the transcriptomic signature of serum proteins toward regeneration. At the cellular level, there was a marked decrease in activated CD8⁺ T cells and an increase in intrahepatic regulatory T cells (Tregs). Here, intrahepatic Treg numbers correlated with biochemical remission. Notably, an imbalance in the T-cell/B-cell ratio was observed, with a disproportionate increase in total B cells. In summary, intrahepatic increases in Tregs, biochemical remission, and regeneration could be induced by splenectomy in the late stage of emAIH.

Effector T cell function rather than survival determines extent and duration of hepatitis in mice

BACKGROUND & AIMS

Acute hepatitis is often mediated by cytotoxic T lymphocytes (CTLs); however, the intrinsic parameters that limit CTL-mediated liver injury are not well understood.

METHODS

To investigate whether acute liver damage is limited by molecules that decrease the lifespan or effector function of CTLs, we used a well-characterized transgenic (Tg) mouse model in which acute liver damage develops upon transfer of T cell receptor (TCR) Tg CD8 T cells. Recipient Tg mice received donor TCR Tg T cells deficient for either the pro-apoptotic molecule Bim, which regulates CTL survival, or suppressor of cytokine signaling-1 (SOCS-1), which controls expression of common gamma chain cytokines; the effects of anti-PD-L1 neutralizing antibodies were also assessed.

RESULTS

Use of Bim-deficient donor T cells and/or PD-L1 blockade increased the number of intrahepatic T cells without affecting the degree and kinetic of acute hepatitis. In contrast, SOCS-1-deficient T cells induced a heightened, prolonged acute hepatitis caused by their enhanced cytotoxic function and increased expansion. Although they inflicted more severe acute liver damage, SOCS-1-deficient T cells never precipitated chronic hepatitis and became exhausted.

CONCLUSIONS

The degree of acute hepatitis is regulated by the function of CD8 T cells, but is not affected by changes in CTL lifespan. Although manipulation of the examined parameters affected acute hepatitis, persistent hepatitis did not ensue, indicating that, in the presence of high intrahepatic antigen load, changes in these factors in isolation were not sufficient to prevent T cell exhaustion and mediate progression to chronic hepatitis.

Medullary thymic epithelial cells and central tolerance in autoimmune hepatitis development: novel perspective from a new mouse model

Autoimmune hepatitis (AIH) is an immune-mediated disorder that affects the liver parenchyma. Diagnosis usually occurs at the later stages of the disease, complicating efforts towards understanding the causes of disease development. While animal models are useful for studying the etiology of autoimmune disorders, most of the existing animal models of AIH do not recapitulate the chronic course of the human condition. In addition, approaches to mimic AIH-associated liver inflammation have instead led to liver tolerance, consistent with the high tolerogenic capacity of the liver. Recently, we described a new mouse model that exhibited spontaneous and chronic liver inflammation that recapitulated the known histopathological and immunological parameters of AIH. The approach involved liver-extrinsic genetic engineering that interfered with the induction of T-cell tolerance in the thymus, the very process thought to inhibit AIH induction by liver-specific expression of exogenous antigens. The mutation led to depletion of specialized thymic epithelial cells that present self-antigens and eliminate autoreactive T-cells before they exit the thymus. Based on our findings, which are summarized below, we believe that this mouse model represents a relevant experimental tool towards elucidating the cellular and molecular aspects of AIH development and developing novel therapeutic strategies for treating this disease.

Hepatitis mouse models: from acute-to-chronic autoimmune hepatitis

Autoimmune hepatitis (AIH) is a chronic inflammatory liver disease associated with interface hepatitis, raised plasma liver enzymes, the presence of autoantibodies and regulatory T-cell (Tregs) dysfunction. The clinical course is heterogeneous, manifested by a fulminant or indolent course. Although genetic predisposition is well accepted, the combination with currently undefined environmental factors is crucial for the development of the disease. Progress in the development of reliable animal models provides added understanding of the pathophysiology of AIH, and these will be very useful in evaluating potential therapeutics. It appears that artificially breaking tolerance in the liver is easy. However, maintaining this state of tolerance breakdown, to get chronic hepatitis, is difficult because liver immune homeostasis is strongly regulated by several immune response inhibitory mechanisms. For example, Tregs are crucial regulators in acute and chronic hepatitis, and C57BL/6 mice are most prone to experimental AIH. Immunization of C57BL/6 mice with liver (AIH) autoantigens (CYP2D6/FTCD or IL-4R) and the disturbance of liver regulatory mechanism(s), leading to experimental AIH, are likely to be most representative of human AIH pathology.

LIGHT/TNFSR14 can regulate hepatic lipase expression by hepatocytes independent of T cells and Kupffer cells

LIGHT/TNFSF14 is a costimulatory molecule expressed on activated T cells for activation and maintenance of T cell homeostasis. LIGHT over expressed in T cells also down regulates hepatic lipase levels in mice through lymphotoxin beta receptor (LTβR) signaling. It is unclear whether LIGHT regulates hepatic lipase directly by interacting with LTβR expressing cells in the liver or indirectly by activation of T cells, and whether Kupffer cells, a major cell populations in the liver that expresses the LTβR, are required. Here we report that LIGHT expression via an adenoviral vector (Ad-LIGHT) is sufficient to down regulate hepatic lipase expression in mice. Depletion of Kupffer cells using clodronate liposomes had no effect on LIGHT-mediated down regulation of hepatic lipase. LIGHT-mediated regulation of hepatic lipase is also independent of LIGHT expression by T cells or activation of T cells. This is demonstrated by the decreased hepatic lipase expression in the liver of Ad-LIGHT infected recombination activating gene deficient mice that lack mature T cells and by the Ad-LIGHT infection of primary hepatocytes. Hepatic lipase expression was not responsive to LIGHT when mice lacking LTβR globally or only on hepatocytes were infected with Ad-LIGHT. Therefore, our data argues that interaction of LIGHT with LTβR on hepatocytes, but not Kupffer cells, is sufficient to down regulate hepatic lipase expression and that this effect can be independent of LIGHT’s costimulatory function.

Animal models of cutaneous and hepatic fibrosis

Fibrosis occurs as a part of normal wound healing. However, excessive or dysregulated fibrosis can lead to severe organ dysfunction and is a feature of a variety of diseases. Due to its insidious onset, fibrosis tends to go undetected in its early stages. This is in part why these diseases remain so poorly understood. Animal models have provided a means to examine these early stages and to isolate and understand the effect of perturbations in signaling pathways, chemokines, and cytokines. Here, we summarize recent progress in the understanding of the molecular pathogenesis of fibrosis, both its initiation and its maintenance phases, from animal models of fibrosis in the skin and liver. Due to these organs' properties, modeling fibrosis in them poses unique challenges. Elegant solutions have therefore been developed for modeling fibrosis in each, and now, great potential for animal models to contribute to our understanding appears scientifically imminent.

The benefit of animal models for autoimmune hepatitis

Autoimmune hepatitis (AIH) is a chronic liver disease which is normally recognized during late stage of the disease. Due to limited knowledge about the onset and course of disease and need for chronic immunosuppression with significant side-effects there is a requirement for a good preclinical animal model, mirroring main characteristics of AIH. In addition to the exclusion of other liver diseases, AIH is characterized by elevated serum transaminases, specific autoantibodies and elevated gammaglobulins as well as a specific liver histopathology. A good preclinical model should mirror most of these criteria. In the last decades several models have been published using different approaches to break hepatic tolerance and induce liver damage. The induction of a chronic hepatitis similar to the human disease remained a difficult challenge. Nevertheless, these models helped to get more information about the aspects of AIH induction and liver immunology.

Receptor editing in self-reactive bone marrow B cells. The Journal of Experimental Medicine. 1993. 177: 1009-1020

A central paradigm of immunology is clonal selection: lymphocytes displaying clonally distributed antigen receptors are generated and subsequently selected by antigen for growth or elimination. Here we show that in mice transgenic for anti-H-2Kk,b antibody genes, in which a homogeneous clone of developing B cells can be analyzed for the outcome of autoantigen encounter, surface immunoglobulin M+/idiotype+ immature B cells binding to self-antigens in the bone marrow are induced to alter the specificity of their antigen receptors. Transgenic bone marrow B cells encountering membrane-bound Kb or Kk proteins modify their receptors by expressing the V(D)J recombinase activator genes and assembling endogenously encoded immunoglobulin light chain variable genes. This (auto)antigen-directed change in the specificity of newly generated lymphocytes is termed receptor editing.

Microanatomy of the liver immune system

The critical metabolic functions of the liver often eclipse any perception of its role as an immune organ. However, the liver as a mediator of systemic and local innate immunity and an important site of immune regulation is now an accepted concept. Complex repertoires of lymphoid and non-lymphoid cells are key to hepatic defense and immunoregulation. Hepatic cells of myeloid lineage include Kupffer cells and dendritic cells. Intrahepatic lymphocytes are distinct both in phenotype and function from their counterparts in any other organ and include both conventional (CD4+ and CD8+ alphabeta T cell receptor (TCR)+ T cells, B cells, natural killer (NK) cells) and nonconventional lymphoid cells (natural killer T (NKT) cells, gamma delta TCR+ T cells, CD4- CD8- T cells). Many hepatic T cells express the TCR at an intermediate level and the great majority of them either coexpress NK cell markers (NKT cells) or they are apoptosing peripheral T cells. The percentage of activated (CD69+) and memory (CD45RB low+) lymphocytes is much higher while naive (CD62L high) and resting T cells as well as B lymphocytes are underrepresented in the liver. The discovery of major populations of lymphoid cells in the liver that differ phenotypically, functionally and even perhaps developmentally from populations in other regions has been key to the evolving perception of the liver as a regulatory lymphoid organ. This chapter will focus on these populations and how they contribute to immune surveillance against malignant, infectious and autoimmune disease of the liver.

Intrahepatic murine CD8 T-cell activation associates with a distinct phenotype leading to Bim-dependent death

BACKGROUND & AIMS

Chronic infections by hepatotropic viruses such as hepatitis B and C are generally associated with an impaired CD8 T-cell immune response that is unable to clear the virus. The liver is increasingly recognized as an alternative site in which primary activation of CD8 T cells takes place, a property that might explain its role in inducing tolerance. However, the molecular mechanism by which intrahepatically activated T cells become tolerant is unknown. Here, we investigated the phenotype and fate of naïve CD8 T cells activated by hepatocytes in vivo.

METHODS

Transgenic mouse models in which the antigen is expressed in lymph nodes and/or in the liver were adoptively transferred with naïve CD8 T cells specific for the hepatic antigen.

RESULTS

Liver-activated CD8 T cells displayed poor effector functions and a unique CD25(low) CD54(low) phenotype. This phenotype was associated with increased expression of the proapoptotic protein Bim and caspase-3, demonstrating that these cells are programmed to die following intrahepatic activation. Importantly, we show that T cells deficient for Bim survived following intrahepatic activation.

CONCLUSIONS

This study identifies Bim for the first time as a critical initiator of T-cell death in the liver. Thus, strategies inhibiting the up-regulation of this molecule could potentially be used to rescue CD8 T cells, clear the virus, and reverse the outcome of viral chronic infections affecting the liver.

Differential priming of CD8 and CD4 T-cells in animal models of autoimmune hepatitis and cholangitis

The pathogenesis of autoimmune liver diseases is poorly understood. Animal models are necessary to investigate antigen presentation and priming of T-cells in the context of autoimmunity in the liver. Transgenic mouse models were generated in which the model antigen ovalbumin is expressed in hepatocytes (TF-OVA) or cholangiocytes (ASBT-OVA). Transgenic OT-I (CD8) or OT-II (CD4) T-cells specific for ovalbumin were adoptively transferred into TF-OVA and ASBT-OVA mice to induce in vivo priming of antigen-specific T-cells. T-cell migration and activation, as well as induction of liver inflammation, were studied. OT-I T-cells preferentially located to the liver of both mouse strains whereas no migration of OT-II T-cells to the liver was observed. OT-I T-cells proliferated in the liver of TF-OVA mice and the liver and liver draining lymph nodes of ASBT-OVA mice. OT-II CD4 T-cells were activated in spleen and liver draining lymph node of TF-OVA mice but not in ASBT-OVA mice. Transfer of OT-I T-cells led to histologically distinct inflammatory conditions in the liver of ASBT-OVA and TF-OVA mice and caused liver injury as determined by the elevation of serum alanine aminotransferase.

CONCLUSION

An antigen expressed in hepatocytes is presented to CD8 and CD4 T-cells, whereas the same antigen expressed in cholangiocytes is presented to CD8 but not CD4 T-cells. In both models, activation of CD8 T-cells occurs within the liver and causes liver inflammation. The models presented here are valuable to investigate the priming of T-cells in the liver and their role in the development of autoimmune disease of the liver.

T lymphocytes interact with hepatocytes through fenestrations in murine liver sinusoidal endothelial cells

The liver has an established ability to induce tolerance. Recent evidence indicates that this unique property might be related to its distinctive architecture allowing T cells to be activated in situ independently of lymphoid tissues. Unlike lymph node-activated T cells, liver-activated T cells are short-lived, a mechanism that might contribute to the "liver tolerance effect." Although the potential role of hepatocytes as tolerogenic antigen-presenting cells has been demonstrated, the question as to whether these cells are able to interact with CD8(+) T cells in physiological settings remains controversial. Contradicting the immunological dogma stating that naïve T lymphocytes are prevented from interacting with parenchymal cells within non-lymphoid organs by an impenetrable endothelial barrier, we show here that the unique morphology of the liver sinusoidal endothelial cell (LSEC) permits interactions between lymphocytes and hepatocytes. Using electron microscopy, we demonstrate that liver resident lymphocytes as well as circulating naïve CD8(+) T cells make direct contact with hepatocytes through cytoplasmic extensions penetrating the endothelial fenestrations that perforate the LSECs. Furthermore, the expression of molecules required for primary T cell activation, MHC class I and ICAM-1, is polarized on hepatocytes to the perisinusoidal cell membrane, thus maximizing the opportunity for interactions with circulating lymphocytes. In conclusion, this study has identified, at the ultrastructural level, a unique type of interaction between naïve T lymphocytes and liver parenchymal cells in vivo. These results hold implications for the pathogenesis of viral hepatitis in which hepatocytes may represent the main antigen-presenting cell, and for the development of immune tolerance as lymphocytes pass through the liver.

A novel model of drug hapten-induced hepatitis with increased mast cells in the BALB/c mouse

Clinical evidence suggests that idiosyncratic hepatitis following administration of halogenated volatile anesthetics is mediated by autoimmune responses. No murine model to study mechanisms of anesthetic-induced or any other form of drug-induced idiosyncratic hepatitis exists. Anesthetics are believed to trigger hepatitis by covalently linking a trifluoroacetyl (TFA) chloride hapten to hepatic proteins, forming haptenated self-proteins. To test this hypothesis, we developed a hapten-induced model of hepatitis by immunization with syngeneic S100 liver proteins covalently coupled to TFA (TFA-S100). We found that TFA-S100 induced hepatitis was more severe than disease induced by S100 plus adjuvants or by the adjuvant alone and was characterized by neutrophil, mast cell, and eosinophil infiltration. TFA-specific IgG1 antibodies directly correlated with hepatitis, whereas S100 autoantibodies did not. TNF-alpha, IL-1beta, and IL-6 released from splenocytes collected 2 weeks after TFA-S100 inoculation were increased resembling the elevated serum cytokines reported in patients with autoimmune hepatitis (AIH). Three weeks after inoculation, the peak of hepatitis, we noted decreased numbers of Kupffer cells and lower levels of IL-6 and IL-10 in the liver, cytokines produced by Kupffer cells. This is the first report, to our knowledge, of a hapten-induced model of hepatitis with immune and autoimmune features.

The site of primary T cell activation is a determinant of the balance between intrahepatic tolerance and immunity

Hepatic immunobiology is paradoxical: although the liver possesses unusual tolerogenic properties, it is also the site of effective immune responses against multiple pathogens and subject to immune-mediated pathology. The mechanisms underlying this dichotomy remain unclear. Following previous work demonstrating that the liver may act as a site of primary T cell activation, we demonstrate here that the balance between immunity and tolerance in this organ is established by competition for primary activation of CD8+ T cells between the liver and secondary lymphoid tissues, with the immune outcome determined by the initial site of activation. Using a transgenic mouse model in which antigen is expressed within both liver and lymph nodes, we show that while naive CD8+ T cells activated within the lymph nodes were capable of mediating hepatitis, cells undergoing primary activation within the liver exhibited defective cytotoxic function and shortened half-life and did not mediate hepatocellular injury. The implications of these novel findings may pertain not only to the normal maintenance of peripheral tolerance, but also to hepatic allograft tolerance and the immunopathogenesis of chronic viral hepatitis.

Antigen Nonspecific Suppression of T Cell Responses by Activated Stimulation-Refractory CD4+ T Cells

Several classes of anergic T cells are capable of suppressing naive T cell proliferation and thereby limiting immune responses. Activated T cells, although not anergic, are transiently refractory to restimulation with Ag. We examine in this study whether activated refractory murine T cells can also suppress naive T cell responses. We find that they can, and that they exhibit many of the suppressive properties of anergic T cells. The activated cells strongly diminish Ag-mediated T cell proliferation, an activity that correlates with their refractory period. Suppression is independent of APC numbers and requires cell contact or proximity. Naive T cells stimulated in the presence of activated refractory cells up-regulate CD25 and CD69, but fail to produce IL-2. The addition of IL-2 to culture medium, however, does not prevent the suppression, which is therefore not solely due to the absence of this growth factor. Persistence of the suppressor cells is also not essential. T cells stimulated in their presence and then isolated from them and cultured do not divide. The suppressive cells, however, do not confer a refractory or anergic state on the target T lymphocytes, which can fully respond to antigenic stimulation if removed from the suppressors. Our results therefore provide evidence that activated T cells act as transient suppressor cells, severely constraining bystander T cell stimulation and thereby restricting their response. These results have potentially broad implications for the development and regulation of immune responses.

Antigen-specific primary activation of CD8+ T cells within the liver

It is generally accepted that naive T cells recirculate via the blood and lymph, but do not enter nonlymphoid tissues without prior activation and differentiation. In this study, we demonstrate that the liver is an exception to this rule. Naive Des-TCR transgenic CD8(+) T cells specific for H-2K(b) were selectively retained in the liver within a few minutes of adoptive transfer into transgenic Met-K(b) mice expressing H-2K(b) in the liver. Activated CD8(+) cells were found in the liver, but not the blood, as soon as 2 h after transfer and underwent cell division and started to recirculate within 24 h of transfer. In contrast, CD8(+) cells activated in the lymph nodes remained sequestered at that site for 2 days before entering the blood. Our results therefore suggest that, in addition to its previously described role as a non Ag-specific activated T cell graveyard, the liver is involved in Ag-specific activation of naive recirculating CD8(+) T cells. This particular property of the liver, combined with the previously demonstrated ability of hepatocytes to induce tolerance by means of premature CD8(+) T cell death, may be a major mechanism contributing to the acceptance of liver allografts and the chronicity of viral hepatitis.

T cell-independent rescue of B lymphocytes from peripheral immune tolerance

Autoimmunity arises when immune tolerance to specific self-antigens is broken. The mechanisms leading to such a failure remain poorly understood. One hypothesis proposes that infectious agents or antigens can break B or T lymphocyte self-tolerance by expressing epitopes that mimic self. Using a transgenic immunoglobulin model, we show that challenge with self-mimicking foreign antigen rescues B cells from peripheral tolerance independent of T cell help, resulting in the accumulation of self-reactive cells in the lymph nodes and secretion of immunoglobulins that bind to a liver-expressed self-antigen. Therefore, our studies reveal a potentially important mechanism by which B lymphocytes can escape self-tolerance.

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.

Periodontal manifestations of collagen vascular disorders

Chronic degeneration of connective tissue components can be produced by a variety of autoimmune mechanisms. The designations connective tissue disease and collagen-vascular disease are commonly used to describe such conditions when a patient exhibits chronic, immune-mediated deterioration of connective tissue structures in a systemic distribution. Recognized conditions that fit this definition include rheumatoid arthritis, lupus erythematosus, progressive systemic sclerosis, CREST syndrome, and mixed connective tissue disease. Several characteristic oral manifestations of these conditions are recognized. Xerostomia associated with any of these conditions in addition to dryness of the eyes is the definition of secondary Sjögren's syndrome. Fibrosis of facial skin and the resulting limited jaw opening are diagnostic features of progressive systemic sclerosis. Several periodontal manifestations are associated with these connective tissue disorders. Dramatic periodontal ligament space widening that is associated with some cases of progressive systemic sclerosis has been appreciated for more than five decades. However, it has been more recently reported that the majority of progressive systemic sclerosis patients exhibit at least subtle generalized periodontal ligament widening when intraoral radiographs are carefully evaluated. This finding is, however, of limited periodontal significance because the teeth are typically not mobile. Comparisons of periodontitis indices such as pocket depth between healthy subjects and patients with progressive systemic sclerosis do not reveal significant differences (21). In addition, recent evidence suggests a tendency for more severe or progressive manifestations of periodontitis as a consequence of xerostomia that may result from these diseases.

Distinct signal thresholds for the unique antigen receptor-linked gene expression programs in mature and immature B cells

Although it is well established that immature B lymphocytes are exquisitely sensitive to tolerance induction compared with their mature counterparts, the molecular basis for this difference is unknown. We demonstrate that signaling by B cell antigen receptors leads to distinct and mutually exclusive biologic responses in mature and immature B cells: upregulation of CD86, CD69, and MHC class II in mature cells and receptor editing in immature cells. These responses can be induced simply by elevation of intracellular free calcium levels, as occurs after receptor aggregation. Importantly, induction of immature B cell responses requires much smaller increases in intracellular free calcium than does induction of mature B cell responses. These differences in biologic response and sensitivity to intracellular free calcium likely contributes to selective elimination at the immature stage of even those B cells that express low affinity for self-antigens.

Efficient peripheral clonal elimination of B lymphocytes in MRL/lpr mice bearing autoantibody transgenes

Peripheral B cell tolerance was studied in mice of the autoimmune-prone, Fas-deficient MRL/ lpr.H-2(d) genetic background by introducing a transgene that directs expression of membrane-bound H-2Kb antigen to liver and kidney (MT-Kb) and a second transgene encoding antibody reactive with this antigen (3-83mu delta, anti-Kk,b). Control immunoglobulin transgenic (Ig-Tg) MRL/lpr.H-2(d) mice lacking the Kb antigen had large numbers of splenic and lymph node B cells bearing the transgene-encoded specificity, whereas B cells of the double transgenic (Dbl-Tg) MRL/lpr.H-2(d) mice were deleted as efficiently as in Dbl-Tg mice of a nonautoimmune B10.D2 genetic background. In spite of the severely restricted peripheral B cell repertoire of the Ig-Tg MRL/lpr.H-2(d) mice, and notwithstanding deletion of the autospecific B cell population in the Dbl-Tg MRL/lpr.H-2(d) mice, both types of mice developed lymphoproliferation and exhibited elevated levels of IgG anti-chromatin autoantibodies. Interestingly, Dbl-Tg MRL/lpr.H-2(d) mice had a shorter lifespan than Ig-Tg MRL/lpr.H-2(d) mice, apparently as an indirect result of their relative B cell lymphopenia. These data suggest that in MRL/lpr mice peripheral B cell tolerance is not globally defective, but that certain B cells with receptors specific for nuclear antigens are regulated differently than are cells reactive to membrane autoantigens.

CD4 T cell tolerance to human C-reactive protein, an inducible serum protein, is mediated by medullary thymic epithelium

Inducible serum proteins whose concentrations oscillate between nontolerogenic and tolerogenic levels pose a particular challenge to the maintenance of self-tolerance. Temporal restrictions of intrathymic antigen supply should prevent continuous central tolerization of T cells, in analogy to the spatial limitation imposed by tissue-restricted antigen expression. Major acute-phase proteins such as human C-reactive protein (hCRP) are typical examples for such inducible self-antigens. The circulating concentration of hCRP, which is secreted by hepatocytes, is induced up to 1,000-fold during an acute-phase reaction. We have analyzed tolerance to hCRP expressed in transgenic mice under its autologous regulatory regions. Physiological regulation of basal levels (<10(-9) M) and inducibility (>500-fold) are preserved in female transgenics, whereas male transgenics constitutively display induced levels. Surprisingly, crossing of hCRP transgenic mice to two lines of T cell receptor transgenic mice (specific for either a dominant or a subdominant epitope) showed that tolerance is mediated by intrathymic deletion of immature thymocytes, irrespective of widely differing serum levels. In the absence of induction, hCRP expressed by thymic medullary epithelial cells rather than liver-derived hCRP is necessary and sufficient to induce tolerance. Importantly, medullary epithelial cells also express two homologous mouse acute-phase proteins. These results support a physiological role of "ectopic" thymic expression in tolerance induction to acute-phase proteins and possibly other inducible self-antigens and have implications for delineating the relative contributions of central versus peripheral tolerance.

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.

All or none peripheral tolerance induction in H-Y antigen-specific TCR transgenic mice

In in vivo tolerance induction, the dose of tolerogen injected is generally linearly correlated to the length of tolerance induced. Small, medium and large doses are related to no, partial and long-term tolerance, respectively. However, even with injection of substantially large doses of tolerogen, the length of tolerance induced varies over a wide range. Most of the recipients can still reject donor grafts. In this study, it is shown that the linear dose-response can be altered into an all or nothing response in a H-Y antigen-specific TCR transgenic (Tg) mouse model. In thymectomized female Tg mice, injection of 3, 30 and 100 x 10(6) male spleen cells was correlated to no, partial and massive deletion of Tg (alpha T beta T) CD8 cells, respectively. When the thymectomized Tg mice were injected with 9 x 10(6) T cell-enriched (T+) male cells, one half of the recipients showed no deletion of alpha T beta T cells, and in the other half massive deletion occurred. In complete correlation with deletion, all male skin grafts were rejected in the undeleted group as PBS-injected controls, whereas with massive deletion they were indefinitely tolerized. Thus, partial deletion and partial tolerance can be avoided. Injection of 18 x 10(6) male T+ cells induced long-term tolerance in all the recipients. The all or none T cell deletion and long-term tolerance induction has not only significant implications in understanding the mechanism of peripheral tolerance induction, but also in tolerance induction in transplantation, gene therapy and the prevention and treatment of autoimmune diseases.

Selective loss of mouse embryos due to the expression of transgenic major histocompatibility class I molecules early in embryogenesis

Among the numerous hypotheses proposed to explain the absence of fetal rejection by the mother in mammals, it has been suggested that regulation of expression of the polymorphic major histocompatibility complex (MHC) at the fetal-maternal interface plays a major role. In addition to a lack of MHC gene expression in the placenta throughout gestation, the absence of polymorphic MHC molecules on the early embryo, as well as their low level of expression after midgestation, could contribute to this important biologic phenomenon. In order to test this hypothesis, we have produced transgenic mice able to express polymorphic MHC class I molecules early in embryogenesis. We have placed the MHC class la gene H-2Kb under the control of a housekeeping gene promoter, the hydroxy-methyl-glutaryl coenzyme A reductase (HMG) gene minimal promoter. This construct has been tested for functionality after transfection into mouse fibroblast L cells. The analysis of three founder transgenic mice and their progeny suggested that fetoplacental units that could express the H-2Kb heavy chains are unable to survive in utero beyond midgestation. We have shown further that a much higher resorption rate, on days 11 to 13 of embryonic development, is observed among transgenic embryos developing from eggs microinjected at the one-cell stage with the pHMG-Kb construct than in control embryos. This lethality is not due to immune phenomena, since it is observed in histocompatible combinations between mother and fetus. These results are discussed in the context of what is currently known about the regulation of MHC expression at the fetal-maternal interface and in various transgenic mouse models.

Enforced Bcl-2 expression inhibits antigen-mediated clonal elimination of peripheral B cells in an antigen dose-dependent manner and promotes receptor editing in autoreactive, immature B cells

The mechanisms that establish immune tolerance in immature and mature B cells appear to be distinct. Membrane-bound autoantigen is thought to induce developmental arrest and receptor editing in immature B cells, whereas mature B cells have shortened lifespans when exposed to the same stimulus. In this study, we used Emu-bcl-2-22 transgenic (Tg) mice to test the prediction that enforced expression of the Bcl-2 apoptotic inhibitor in B cells would rescue mature, but not immature, B cells from tolerance induction. To monitor tolerance to the natural membrane autoantigen H-2Kb, we bred 3-83mudelta (anti-Kk,b) Ig Tg mice to H-2(b) mice or to mice expressing transgene-driven Kb in the periphery. In 3-83mudelta/bcl-2 Tg mice, deletion of autoreactive B cells induced by peripheral Kb antigen expression in the liver (MT-Kb Tg) or epithelia (KerIV-Kb Tg), was partly or completely inhibited, respectively. Furthermore, Bcl-2 protected peritoneal B-2 B cells from deletion mediated by acute antigen exposure, but this protection could be overcome by higher antigen dose. In contrast to its ability to block peripheral self-tolerance, Bcl-2 overexpression failed to inhibit central tolerance induced by bone marrow antigen expression, but instead, enhanced the receptor editing process. These studies indicate that apoptosis plays distinct roles in central and peripheral B cell tolerance.

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.

Prolonged survival of mouse skin allografts in recipients treated with donor splenocytes and antibody to CD40 ligand

Combined treatment with antibody against CD40 ligand and one transfusion of donor splenocytes prolonged survival of fully mismatched BALB/c skin allografts on C57BL/6 recipients, with approximately 20% of grafts surviving > 100 days. In vitro alloresponsiveness in treated animals was reduced in the immediate post-transplantation period, but by day 100 was increased despite the presence of a successful allograft. The presence of alloreactivity on day 100 was confirmed in vivo by adoptive transfer, which suggests that our protocol had induced either a state of "split tolerance" or "graft accommodation." Mice with skin grafts that had survived for > or = 100 days revealed no evidence of lymphoid chimerism. Treatment with donor splenocytes and antibody against CD40 ligand permits long-term survival of highly antigenic donor skin allografts despite the presence of functionally intact alloreactive lymphocytes.

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.

Dysmyelination in class I MHC transgenic mice

Why is it that oligodendrocytes do not normally express major histocompatibility complex (MHC) molecules? To examine the effect of aberrant MHC expression in oligodendrocytes, transgenic mice have been produced which expressed the class I MHC gene, H-2Kb, under direction of the MBP promoter [Turnley et al. (1991b) Nature, 353:566-569; Yoshioka et al. (1991) Mol. Cell. Biol., 11:5479-5486]. A proportion of these mice exhibited a shivering phenotype, with tonic seizures and early death. Oligodendrocyte function and viability was shown to be affected, resulting in severe dysmyelination of the CNS. Is this phenomenon of cell damage due to aberrant expression of MHC molecules restricted to oligodendrocytes, and could other, non-MHC molecules, when aberrantly expressed, result in similar cell damage? This paper discusses these questions and examines possible mechanisms for the oligodendrocyte damage and hypomyelination observed in these transgenic mice. Finally, the implications of aberrant MHC expression in oligodendrocytes for demyelinating diseases such as multiple sclerosis are discussed.

Peripheral deletion of autoreactive CD8+ T cells in transgenic mice expressing H-2Kb in the liver

The response of T cells specific for liver antigens was examined in transgenic mice expressing the allogeneic major histocompatibility complex class I molecule H-2Kb (Kb) under the control of the sheep metallothionein promoter (Met-Kb mice). To follow the fate of Kb-specific T cells, and to prevent any aberrant thymic expression of the Kb transgene, the mice were thymectomized, lethally irradiated, protected with bone marrow cells from transgenic mice expressing in their T cells a Kb-specific T cell receptor identifiable by a clonotypic antibody, and given syngeneic non-transgenic thymus grafts. Although Kb-specific CD8+ T cells were produced in the thymus grafts of these manipulated Met-Kb mice, only small numbers of such cells could be detected in the spleen and lymph nodes. The livers, however, showed signs of damage and were heavily infiltrated by actively dividing CD8+ T cells. We provide strong evidence that the hepatocytes, not generally regarded as antigen-presenting cells, activated the Kb-specific CD8+ T cells and that these disappeared after a vigorous autoimmune response that resulted in deletion.

Zinc inhibits turnover of labile mRNAs in intact cells

For immediate early genes such as the c-fos proto-oncogene, mRNA breakdown is very rapid and is largely responsible for the transient nature of mRNA accumulation after transcription is stimulated. We found that in several types of cultured cells and in mice, Zn++ caused marked accumulation of c-fos mRNA and that of another labile mRNA, that encoding the tristetraprolin (TTP) protein. Exposure of TK-L cells to 100 microM ZnSO4 caused an increase of c-fos and TTP mRNA levels within 1 h that reached peak levels in 4-8 h and remained constant to 12 h. Increases in fos protein accumulation were also noted. When the cells were exposed to Zn++ for 4 h and then exposed to actinomycin D, both c-fos and TTP mRNA levels remained constant for up to 10 h, indicating that Zn++ was preventing the breakdown of both c-fos and TTP mRNA. Also, 100 microM ZnSO4 inhibited protein synthesis in TK-L cells, suggesting that the effect on mRNA accumulation could have been an indirect effect resulting from inhibited protein synthesis. Zn++ was unable to inhibit the breakdown of TTP and c-fos mRNA in vitro; however, extracts from cells exposed to Zn++ were less able to cause the breakdown of TTP and c-fos mRNAs than were extracts from control cells, again suggesting that Zn++ indirectly affects mRNA stability through inhibition of protein synthesis. These studies suggest that in addition to their effects on gene transcription, Zn++ and other divalent cations may regulate gene expression by affecting mRNA stability.

Characterization of human thymic epithelial cell surface antigens: phenotypic similarity of thymic epithelial cells to epidermal keratinocytes

Cellular interactions between developing thymocytes and cells of the thymic microenvironment are necessary for maturation of thymocytes into mature T cells. While much is known about the molecules on developing T cells that mediate these interactions, little is known about the surface molecules of human thymic epithelial (TE) cells. In this study, using a panel of 276 MAb including 255 MAb from the 5th International Workshop on Human Leukocyte Differentiation Antigens (HLDA-V), we have determined the expression of CD1 through CDw130 and other surface molecules on resting and IFN-gamma-activated cultured human TE cells and on resting epidermal keratinocytes (EK). We demonstrate the surface expression of 50 of the 161 molecules assayed for on TE cells, including a number of adhesion molecules, cytokine receptors, Apo-1, and MHC-encoded molecules. While activation of TE cells with IFN-gamma for 48 hr induced a greater than fivefold increase in the expression of four surface molecules (CD38, CD54, MHC class I, and MHC class II), it also induced a greater than 50% increase in the expression of 14 other surface molecules (CD12, CD29, CD40, CD44, CD47, CD49b, CD49c, CD49e, CD55, CD66, CD87, CD104, TE4, and STE3) and a decrease in the expression of three molecules (CDw65, CDw109, and STE2). In comparing the phenotype of TE cells to 83 other cell lines studied in HLDA-V, we found that TE cells were strikingly more similar to EK than to any of the other cell types tested.

Autoantigen-induced deletion of peripheral self-reactive T cells

There are now numerous reports documenting the deletion of mature peripheral specific T cells following massive antigenic stimulation. Such a phenomenon can be regarded as a homeostatic mechanism to prevent unrestricted growth of antigen-activated clones and to safeguard against autoimmunity. Some of the reports examining this issue are summarized in this review and the role of the liver in tolerance and autoimmunity is discussed based on recent work performed with transgenic mice.

B lymphocyte ontogeny and immunoglobulin production

In this paper we review different aspects of B cell development on the path from the proB cell to the memory B cell and the plasmocyte. Emphasis is given to the positive and negative selection effects mediated by the changing forms of the surface immunoglobulin (Ig) receptor under successive microenvironments. Positive selection is linked to lambda chain expression at the pro- and preB cell stage in fetal liver and bone marrow. Negative selection takes place when surface (s)IgM is being cross-linked by autoantigens before the immature B cell can leave, or after it has left, the bone marrow. After somatic mutation, major expansion becomes possible for B cells with high-affinity sIg receptors. This takes place in the germinal centres of the secondary lymphoid organs in the context of major histocompatibility complex (MHC) restriction and provided the necessary T cell help is given. Kinetic data on B cell replenishment in the rodent models are used to draw a schematic view of an established B cell repertoire.

Testis-specific expression of a metallothionein I-driven transgene correlates with undermethylation of the locus in testicular DNA

Mice carrying a chimeric transgene of the human testis-specific lactate dehydrogenase cDNA driven by mouse metallothionein I promoter have been reported to express the transgene in a testis-specific manner in six founder lines. To study the mechanism by which this testis-specific expression is mediated, we have examined genomic placement, expression pattern, and methylation status of the transgene. Our results indicate that transgene expression is repressed in all somatic tissues examined even when heavy metals are administered. Nuclear run-on assays indicate that failure of expression in the liver (in which the metallothionein I promoter is highly active) occurs at the transcriptional level. In contrast, the transgene mRNA is transcribed in male germ cells and is developmentally regulated during spermatogenesis. Examination of the transgene methylation status reveals that expression is inversely correlated with hypermethylation of the locus; all CpG dinucleotides examined in the promoter region were found to be fully methylated in kidney and liver but were undermethylated in testis. Since methylation of the murine metallothionein I promoter is sufficient to inhibit its activity, it is likely that suppression of the transgene in somatic tissues is mediated by methylation.

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)

An evaluation of the potential to use tumor-associated antigens as targets for antitumor T cell therapy using transgenic mice expressing a retroviral tumor antigen in normal lymphoid tissues

A major obstacle to the development of T cell therapy for the treatment of human tumors has been the difficulty generating T cells specifically reactive with the tumor. Most of the characterized human tumor antigens have been classified as tumor associated, because of demonstrable expression at low levels in some normal cells, and thus have not been extensively studied as potential targets of a therapeutic immune response. However, the quantitative difference in expression of such antigens between the tumor and normal cells might permit the generation of antigen-specific T cells capable of selective antitumor and not autoimmune activity. To address this issue, transgenic (TG) mice were generated that expressed low levels of Friend murine leukemia virus (FMuLV) envelope protein in lymphoid cells under the control of an immunoglobulin promoter. This protein is expressed at high levels by a Friend virus-induced erythroleukemia of C57BL/6 (B6) origin, FBL, and has been shown to serve as an efficient tumor-specific rejection antigen in B6 mice. The env-TG mice were tolerant to envelope, as reflected by the failure to detect an envelope-specific response after in vivo priming and in vitro stimulation with preparations of FMuLV envelope. However, adoptively transferred envelope-specific T cells from immunized non-TG B6 mice mediated complete eradication of FBL tumor cells in TG mice, and did not induce detectable autoimmune damage to TG lymphoid tissues. The transferred immune cells were not permanently inactivated in the TG mice, since donor T cells responded to envelope after removal from the TG mice. The lack of autoimmune injury did not reflect inadequate expression of envelope by TG lymphocytes for recognition by T cells, since TG lymphocytes functioned effectively in vitro as stimulators for envelope-specific T cells. The results suggest that this and analogous strains of TG mice may prove useful for elucidating principles for the generation and therapeutic use of tumor-reactive T cells specific for tumor-associated antigens.

Self-nonself discrimination and tolerance in T and B lymphocytes

The immune system must not only fight off infections, but also ensure that it does not react against its own body tissues. Since clones of lymphocytes have predetermined reactivities, some will be self-reactive and have the potential to cause damage. They should therefore be neutralized in some way. In a system as complex and important as that governing self-tolerance, many mechanisms must exist to neutralize autoaggressive lymphocytes. They may be classified under two main groups. In one the tolerant state arises from the physical or functional silencing of potentially autoaggressive lymphocytes after antigen encounter. This may involve clonal deletion, clonal abortion or clonal anergy. In the second, regulatory mechanisms of the immune system itself may hold autoreactive lymphocytes in check, for example through the operation of idiotypic network interactions and the action of specialized suppressor cells. Much evidence has accumulated for the physical deletion of autoreactive T cells as they mature in the thymus. The fate of any that escape thymus censorship has been the subject of recent research and is discussed here. Under certain conditions, self-tolerance must also be imposed at the B-cell level to prevent the production of potentially damaging autoantibodies. Although the mechanisms which silence self-reactive lymphocytes are very efficient, self-tolerance can break down, and autoimmunity will thus ensue. The main factors responsible for this are briefly described here.

Receptor editing in self-reactive bone marrow B cells

A central paradigm of immunology is clonal selection: lymphocytes displaying clonally distributed antigen receptors are generated and subsequently selected by antigen for growth or elimination. Here we show that in mice transgenic for anti-H-2Kk,b antibody genes, in which a homogeneous clone of developing B cells can be analyzed for the outcome of autoantigen encounter, surface immunoglobulin M+/idiotype+ immature B cells binding to self-antigens in the bone marrow are induced to alter the specificity of their antigen receptors. Transgenic bone marrow B cells encountering membrane-bound Kb or Kk proteins modify their receptors by expressing the V(D)J recombinase activator genes and assembling endogenously encoded immunoglobulin light chain variable genes. This (auto)antigen-directed change in the specificity of newly generated lymphocytes is termed receptor editing.

Mechanisms of transplantation immunity

In summary, this chapter describes the biology and genetics of the major and minor histocompatibility antigens and the nature of in vitro and in vivo immune responses to them and to tissue-specific antigens. It reviews the nature and action of immune response genes. It gives an account of how tolerance to histocompatibility antigens was originally defined and the prospects of intervention aimed at establishing tolerance to these and tissue-specific antigens in adult animals, including man.

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.

Cellular and molecular mechanisms of B lymphocyte tolerance

A paradox of immunology is that the immune system is distributed so widely in the body, as a large number of cells that discharge most of their effector functions as single cells; but, at the same time, the elements of the system are so very interdependent, not only via specialized cell clusters and microenvironments, but also by mobile feedback loops, cellular and molecular. The end result is that one cannot really understand one element of the system without understanding every other, at least to a degree. Certainly, tolerance cannot be isolated from immune activation, nor B cell from T cell tolerance, rendering the task of the reviewer somewhat thankless. This being said, the last few years have seen wonderful progress in our grasp of B cell tolerance, to which the transgenic revolution has contributed a great deal. The fact that B cell tolerance exists as an important component of self-tolerance has been firmly established, as have the limits of the process in terms of both the survival of low-affinity antiself clonotypes and the question of location and concentration of antigen required for tolerance induction. Two processes have been identified as key alternatives: clonal abortion/maturation arrest/deletion and induction of clonal anergy. The latter requires a less strong Ig receptor crosslinking signal, may be partial, and is reversible. Recognition of these facts has prompted both experimentation and speculation on possible functions of the anergic cell. One unsatisfactory area, which we have not addressed because nothing like a consensus has been reached, is T cell-mediated suppression and its possible effects on tolerant states, including anergy induction in B cells. The phenomenology of suppression is too striking to sweep under the carpet, and suppressor T cell memory in particular (Adelstein et al., 1990) requires much more investigation; however, suppression has not been shown to play a major role in any of the best-studied transgenic models. These can readily be explained on the basis of direct interactions between the B cell target for abortion or anergy and the self antigen in question. The biochemical basis of discrimination between immunity and tolerance has also progressed, but not as fast. This is understandable, as so many signaling pathways have to come together for full immune induction, and as immaturity of the signal transduction pathway plays a profound role that must be studied in normal cells, with all the attendant difficulties of cell separation.(ABSTRACT TRUNCATED AT 400 WORDS)