Clonal anergy induced in mature V beta 6+ T lymphocytes on immunizing Mls-1b mice with Mls-1a expressing cells

Distinct mechanisms of neonatal tolerance induced by dendritic cells and thymic B cells

To assess the role of different types of antigen-presenting cells (APC) in the induction of tolerance, we isolated B cells, macrophages, and dendritic cells from thymus and spleen, and injected these into neonatal BALB/c mice across an Mls-1 antigenic barrier. One week after injection of APC from Mls-1-incompatible mice or from control syngeneic mice, we measured the number of thymic, Mls-1a-reactive, V beta 6+ T cells and the capacity of thymocytes to induce a graft-vs.-host (GVH) reaction in popliteal lymph nodes of Mls-1a mice. Injection of thymic but not spleen B cells deleted thymic, Mls-1a-reactive V beta 6+ T cells and induced tolerance in the GVH assay. The thymic B cells were primarily of the CD5+ type, and fluorescence-activated cell sorter-purified CD5+ thymic B cells were active. Injection of dendritic cells from spleen or thymus also induced tolerance, but the V beta 6 cells were anergized rather than deleted. Macrophages from thymus did not induce tolerance. Dendritic cells and thymic B cells were also effective in inducing tolerance even when injected into Mls-, major histocompatibility complex-incompatible, I-E- mice, but only thymic B cells depleted V beta 6-expressing T cells. Therefore, different types of bone marrow-derived APC have different capacities for inducing tolerance, and the active cell types (dendritic cells and CD5+ thymic B cells) can act by distinct mechanisms.

Ontogeny of thymic B cells in normal mice

Ontogeny of thymic B cells and their surface characteristics were analyzed using monoclonal antibodies (mAbs) against B220 molecules (CD45, CD45R). A small number of B cells were detected in fetal thymus on Gestation Day 14 (approximately 3.5% of the low-density fraction). Similarly, the percentage of B cells in the low-density fraction was 3.2% on Gestation Day 18, and 3.5% on Day 1 after birth. These were the same level as that of adult mice. CD5+ B cells, which form the major population of thymic B cells, were also found in the fetal life (0.5% on Day 14 and 2.2% on Day 16 in the low-density cells). The percentage of CD5+ B cells in B cell-enriched fraction was about 65% on Day 1 after birth, which is the same level as that in adult mice. These results indicate that a small number of B cells or cells in the B-cell lineage are present in the fetal thymus and also suggest the importance of these thymic B cells in the negative selection of T cells during early developmental stages.

Cytokine gene expression: analysis using northern blotting, polymerase chain reaction and in situ hybridization

We describe here the use of northern blotting, PCR and in situ hybridization for the analysis of cytokine gene expression. These techniques, each with their advantages and disadvantages, have been used to monitor cytokine gene expression in sites of immune reactivity and in the developing thymus. Whilst expression of a gene usually correlates well with protein production from that gene, this may not always be the case. The development of methods to analyze protein production in situ, for instance by immunohistochemistry, together with analysis of mRNA expression will allow us to begin to understand the role of cytokines within the immune system of the intact animal.

Effect of high ligand concentration on West Nile virus-specific T cell proliferation

In this paper the phenomenon of suppression of proliferation in vitro of 14 day primed, West Nile Virus (WNV)-specific, murine CD4+ T cells by large numbers of antigen-presenting macrophages and B cells has been investigated. Suppression was apparently not mediated by prostaglandins, as the use of indomethacin in cultures at four times the usual concentration did not reverse suppression. Experiments were designed to evaluate the contribution of major histocompatibility complex (MHC) Class II and nominal WNV antigens in causing suppression of T cell proliferation. Listeria- or thioglycollate-induced macrophages from CBA/H (H-2k) mice, when treated with heat-killed Listeria in vitro for 1 h to maintain or increase, respectively, MHC Class II levels before the addition of alloreactive Iak-specific T cells caused inverse dose-responses; the highest T cell proliferation occurred at a stimulator to responder (S : R) ratio of 0.25 and profound suppression at a S : R ratio of 1 or 2. In contrast, untreated thioglycollate-induced macrophages, which express low MHC Class II levels, gave a direct dose-response with increasing T cell proliferation as antigen-presenting cell (APC) numbers increased. Addition of anti-Ia antibodies (or their Fab fragments) to cultures caused a significant reversal of suppression of anti-WNV T cells imposed by high numbers of Listeria-induced macrophages or 14 day WNV-primed B cell APC. Suppression was also reversed by reducing the concentration of WNV antigen. These observations support the notion that the suppression of T cell proliferation observed at high S : R ratios was due to high concentrations of ligand (WNV-derived peptide complexed with Class II MHC) on APC.

Programmed cell death and extrathymic reduction of Vbeta8+ CD4+ T cells in mice tolerant to Staphylococcus aureus enterotoxin B

Clonal deletion and functional inactivation of self-reactive cells have been invoked as mechanisms underlying intrathymic development of T-cell tolerance. The relative importance of these mechanisms in the development of tolerance of more mature, peripheral T cells either to self or to exogenous antigens is unclear, although recent data relate the development of T-cell tolerance in the periphery to clonal anergy. We have now investigated the induction of extrathymic tolerance using BALB/c mice that were made tolerant to Staphylococcus aureus enterotoxin B, a superantigen which specifically interacts in such mice with T cells bearing V beta 8 antigen receptors. Both euthymic and athymic mice made tolerant to S. aureus enterotoxin B had a markedly reduced number of V beta 8.1,2+ CD4+ peripheral T cells. This reduction was accompanied by genomic DNA fragmentation that is associated with cell death. These results indicate that a deletional mechanism can contribute to the induction of T-cell tolerance in peripheral lymphoid cells.

A role for CD8+ T lymphocytes in the pathogenesis of AIDS

Experimental and clinical evidence is presented which supports the hypothesis that CD8+ T lymphocytes aimed at suppressing HIV replication in infected CD4+ T cells may have an important role in the pathogenesis of AIDS by directly causing a decrease in CD4+ T lymphocyte numbers. Possible models to test this hypothesis are discussed.

Immunological tolerance: The key feature in human filariasis?

Filariasis is a widespread tropical disease caused by a group of nematode parasites that can survive for many years in immunocompetent hosts. The paradox of filariasis has always been the inverse association between parasite density, in terms of circulating microfilariae in the blood, and severe pathology. In this review, Rick Maizels and Rachel Lawrence argue that microfilariae and adult parasites induce a form of immunological tolerance which prevents both parasite elimination and progression to disease. The breakdown of this unresponsiveness is seen as the critical step towards pathogenesis. However, not every exposed individual progresses through infection to disease. The authors discuss evidence for protective immunity acting on antigens from the mosquito-borne infective larva, and propose that this stage represents a vulnerable target outside the scope of tolerance and pathogenesis. Stage-specific larval antigens, to which asymptomatic hosts are known to respond, may therefore represent the most effective and safe choice for an anti filarial vaccine.

Peripheral clonal elimination of functional T cells

A major mechanism for generating tolerance in developing T cells is the intrathymic clonal deletion of T cells that have receptors for those self antigens that are presented on hematopoietic cells. The mechanisms of tolerance induction to antigens not expressed in the thymus remain unclear. Tolerance to self antigens can be generated extrathymically through the induction of clonal nonresponsiveness in T cells with self-reactive receptors. A second mechanism of extrathymic tolerance was identified: clonal elimination of mature T cells with self-reactive receptors that had previously displayed functional reactivity.

Extrathymic tolerance of mature T cells: clonal elimination as a consequence of immunity

The mechanism by which T lymphocytes are tolerized to self or foreign antigens is still controversial. Clonal deletion is the major mechanism of tolerance for immature thymocytes; for mature T cells, tolerance is considered to reflect anergy rather than deletion, and to be a consequence of defective presentation of antigen. This paper documents a novel form of tolerance resulting when mature T cells encounter antigen in immunogenic form. Evidence is presented that exposure of mature T cells to Mlsa antigens in vivo leads to specific tolerance and disappearance of Mlsa-reactive V beta 6+ T cells. Surprisingly, the clonal elimination of V beta 6+ cells is preceded by marked expansion of these cells. Thus, tolerance induction can be the end result of a powerful immune response. These data raise important questions concerning the relationship of tolerance and memory.

Transgenic models of T-cell self tolerance and autoimmunity

Self-tolerance is generally induced by intrathymic clonal deletion of T cells with reactivity directed to antigens synthesized within the thymus (Kappler et al. 1987, Kisielow et al. 1988). It may also be induced in peripheral T cells when these encounter antigens unique to extra-thymic tissues. Two transgenic models have been particularly useful in the study of peripheral self tolerance: in one model, a known antigen is expressed in a particular extra-thymic site; in the other, the T-cell repertoire is predominantly reactive to this antigen. We, and others, have shown that expression of class I or II MHC molecules in defined extra-thymic sites leads to a state of T-cell tolerance. To account for this, we have proposed two hypotheses which have different implications for autoimmune disease. According to one, tolerance is imposed by deletion or functional silencing of specific high-affinity cytolytic T cells; alternatively, the target cell for tolerance induction may be a regulatory IL-2-producing T-cell, rather than the effector cell itself. To distinguish between these hypotheses it is essential to examine the fate of T cells which have the potential to react to the transgene product. Since the frequency of such T cells is low and there is no dominant clonotype for H-2Kb, which is the class I molecule we used, it was necessary to create double transgenic mice by mating class I transgenic mice with transgenic mice whose T-cell pool was compared of cells reactive to H-2Kb and could be detected by an antibody directed to the TCR. Initial studies showed that such T cells did persist despite the presence of antigen to which they may be reactive. If these double transgenic mice can be shown to be tolerant, they will offer a rich source of tolerant T cells for detailed investigation of their phenotype and fate, and they will be most useful in enabling us to probe the mechanisms responsible for the induction of peripheral self tolerance. Transgenic mouse technology has also been used successfully to unravel the genetic influences which may lead to or prevent autoimmunity. In particular, we have prevented autoimmune diabetes in the nonobese diabetic mouse by introducing a non-NOD MHC class II gene and further work is implicating the failure of intrathymic positive selection of a protective cell as one step in the pathogenesis of diabetes in NOD mice.

Mechanisms of autoimmunity in the context of T-cell tolerance: insights from natural and transgenic animal model systems

There are a number of mechanisms which cooperate to produce and maintain T-cell tolerance. First, and perhaps most important, is the clonal deletion in the thymus of T cells with high affinity for self antigens. However, to ensure that a wide repertoire of T cells is available in the periphery to combat foreign antigens, the threshold of clonal deletion may be set low enough so that T cells whose TCR's have sub-threshold affinity for self antigens mature and migrate to the periphery. T cells which recognize self antigen-derived peptides not expressed or presented in the thymus will also fail to be deleted. For those self-reactive T cells which are not deleted in the thymus, other mechanisms may produce tolerance, including an undefined alteration of signalling pathways which produces clonal anergy, and lowering the avidity of the TCR for its ligand by downregulating coreceptor and accessory molecules. Active suppression of T-cell responses in another well-described phenomenon whose mechanism is undefined. From our observations with the model systems discussed here, we have observed three distinct mechanisms by which T-cell tolerance can be circumvented, allowing autoimmune phenomena to occur. These mechanisms may have relevance for different types of autoimmune diseases seen in humans. In gld mice, the autoimmune disease seems to be related to a global defect in T-cell differentiation and function, which allows for the expansion of autoimmune B cells. While we showed that clonal deletion of V beta-bearing T cells is appropriate in certain cases, aberrant lymphokine secretion by the abnormal T cells or disruption of immune system regulation are most probably responsible for allowing autoantibody production. While human lupus erythematosis shares much of the pathology of lpr and gld mice, there is no expansion of T cells with a similar phenotype in human lupus. There are environmental factors which must play a role in the development of human lupus, since the incidence of the disease does not follow an absolute genetic pattern. The escape from clonal deletion and subsequent reactivation of autoimmune T cells which we observed in V beta 8.1 TCR-transgenic mice can be a model for human autoimmune diseases such as multiple sclerosis and type I diabetes, in which T cells are directed against a specific autoantigen. According to this model, susceptibility loci for autoimmune disease such as the MHC would function by producing different repertoires of T cells which in some cases could gain autoreactivity following activation.(ABSTRACT TRUNCATED AT 400 WORDS)

The induction of skin graft tolerance in major histocompatibility complex-mismatched or primed recipients: primed T cells can be tolerized in the periphery with anti-CD4 and anti-CD8 antibodies

Mice given short courses of anti-CD4 and anti-CD8 monoclonal antibodies became tolerant of allogeneic skin grafted at the same time. Tolerance could be obtained without T cell depletion across multiple minor antigen mismatches, both in naive and primed animals, demonstrating that peripheral T cells could be tolerized, even if they had been previously activated. Where donor and recipient were incompatible across the whole major histocompatibility complex, specific tolerance could be achieved by using a combination of depleting followed by non-depleting antibodies, where each alone was unsuccessful. Although mice clearly tolerated their original skin grafts, we observed in some strain combinations that a second fresh, but genotypically identical graft, was slowly rejected. Such mice also possessed T cells which could proliferate to donor-type stimulator cells in vitro. Whatever the mechanisms, we have demonstrated that operational transplantation tolerance can be achieved with simple, non-toxic antibody therapy. The introduction of comparable tolerance-inducing regimens in clinical organ transplantation could obviate the need for long-term immunosuppression and its unfortunate side effects.

Neonatal exposure to thymotropic gross murine leukemia virus induces virus-specific immunologic nonresponsiveness

Neonatal exposure to Gross murine leukemia virus results in a profound inhibition of the virus-specific T and B cell responses of adult animals. Animals exposed to virus as neonates exhibit a marked depression in virus-specific T cell function as measured by the virtual absence of in vivo delayed type hypersensitivity responses and in vitro proliferative responses to virally infected stimulator cells. Further, serum obtained from neonatally treated mice failed to either immunoprecipitate viral proteins or neutralize virus in an in vitro plaque assay, suggesting the concurrent induction of a state of B cell hyporesponsiveness. The specificity of this effect at the levels of both T and B cells was demonstrated by the ability of neonatally treated mice to respond normally after adult challenge with either irrelevant reovirus or influenza virus. The replication of Gross virus within both stromal and lymphocytic compartments of the neonatal thymus suggests that thymic education plays a key role in the induction of immunologic nonresponsiveness to viruses.

Induction of tolerance in peripheral T cells with monoclonal antibodies

Our goal has been to develop ways to tolerize the mature immune system to any defined antigen. In this report we show that peripheral (post-thymic) T cells of mice can become tolerant to a range of antigens (human and rat immunoglobulins, and bone marrow and skin grafts that differ at multiple minor transplantation antigens). In the case of human gamma globulin (HGG), this required that the antigen be given under the cover of a short course of non-depleting anti-CD4 antibody, while for tolerance to skin and marrow grafts anti-CD8 antibody was also required. Tolerance to HGG could be reinforced by repeated injections of HGG, but was lost in the absence of any further exposure to antigen. This reversal of tolerance with time was due to new T cells being exported from the thymus, as it was not observed in tolerized, adult thymectomized mice. In contrast, tolerance to marrow and skin grafts was permanent, presumably because the established grafts acted as a continuous source of antigen to reinforce the tolerant state. Tolerance could not be broken by the infusion of unprimed spleen cells and in one example (tolerance to Mls-1a) there was clear evidence that specific peripheral T cells were anergic. We propose that anergic cells may themselves participate in reinforcing the tolerant state by competing at sites of antigen presentation.

Induction of specific clonal anergy in human T lymphocytes by Staphylococcus aureus enterotoxins

The exotoxins produced by certain strains of Staphylococcus aureus are able to stimulate powerful polyclonal proliferative responses and to induce nonresponsiveness by clonal deletion of T lymphocytes expressing the appropriate T-cell antigen receptor V beta gene products. This paper examines the ability of S. aureus enterotoxins to modulate the responsiveness of human CD4+ T lymphocytes with defined antigen specificity. It was observed that certain S. aureus toxins were able to activate and induce anergy in hemagglutinin-reactive T cells expressing V beta 3+ elements. After exposure to S. aureus enterotoxins A, B, and D in the absence of antigen-presenting cells, the T cells failed to respond to their natural ligand presented in an immunogenic form, despite enhanced proliferation to exogenous interleukin 2. The S. aureus toxin-induced anergy was associated with modulation of T-cell membrane receptors; down-regulation of the T-cell antigen receptor was concomitant with enhanced expression of CD2 and CD25. Interestingly, CD28 was increased only on stimulation, suggesting this protein may be differentially expressed by activated and anergic T cells. These results indicate that bacterial toxins are able to induce antigen-specific nonresponsiveness in human T cells, the application of which may be relevant in the regulation of T cells expressing a particular family of V beta gene products.

Distinct fates of self-specific T cells developing in irradiation bone marrow chimeras: clonal deletion, clonal anergy, or in vitro responsiveness to self-Mls-1a controlled by hemopoietic cells in the thymus

Elimination of potentially self-reactive T lymphocytes during their maturation in the thymus has been shown to be a major mechanism in accomplishing self-tolerance. Previous reports demonstrated that clonal deletion of self-Mls-1a-specific V beta 6+ T lymphocyte is controlled by a radiosensitive I-E+ thymic component. Irradiation chimeras reconstituted with I-E- bone marrow showed substantial numbers of mature V beta 6+ T cells despite host Mls-1a expression. Analysis of the functional properties of such chimeric T cells revealed a surprising variability in their in vitro reactivity to host Mls-1a, depending on the H-2 haplotype of stem cells used for reconstitution. In chimeras reconstituted with B10.S (H-2s) stem cells, mature V beta 6+ lymphocytes were present but functionally anergic to host-type Mls-1a in vitro. In contrast, in chimeras reconstituted with B10.G (H-2q) bone marrow, nondeleted V beta 6+ cells were highly responsive to Mls-1a in vitro. These findings suggest that clonal anergy of V beta 6+ cells to self-Mls-1a may be controlled by the affinity/avidity of T cell receptor interactions with bone marrow-derived cells in the thymus depending on the major histocompatibility complex class II molecules involved. Furthermore, chimeras bearing host (Mls-1a)-reactive V beta 6+ cells did not differ clinically from those with anergic or deleted V beta 6+ cells and survived more than one year without signs of autoimmune disease. Interestingly, their spleen cells had no Mls-1a stimulatory capacity in vitro. Therefore, regulation at the level of antigen presentation may be an alternative mechanism for maintenance of tolerance to certain self-antigens such as Mls-1a.

IL-4 acts synergistically on the IL-2 response of an autoreactive T-cell clone; synergism correlates with increased intracellular IL-2, but not with a modified IL-2 receptor expression

TE44, an H-2b-restricted, self-reactive T-cell line, did not produce autocrine-acting growth factors, neither after antigenic nor after mitogenic activation; they remained for their proliferation completely dependent on exogenously added IL-2. Administration of IL-4, which poorly promotes growth by itself, resulted in a 5- to 10-fold enhancement of the specific biological activity of IL-2 on antigen-activated TE44-cells. This synergism was exerted nonreciprocally and required the presence of both lymphokines. IL-4 did not affect the number, nor the affinity, nor the rate of internalization of the high-affinity receptors for IL-2. However, increased levels of intracellular IL-2 were observed, suggesting an effect of IL-4 on the turnover of IL-2. This might allow a prolonged activity of IL-2 or IL-2-associated molecules inside the cell. Furthermore, the lack of autocrine growth factor production by antigen-stimulated TE44 is discussed in terms of its relationship to the autoimmune specificity of these T-cells.

Selective anergy of V beta 8+,CD4+ T cells in Staphylococcus enterotoxin B-primed mice

The cellular basis of the in vitro and in vivo T cell responses to Staphylococcus enterotoxin B (SEB) has been investigated. The proliferation and cytotoxicity of V beta 8.1,2+,CD4+ and CD8+ T cells were observed in in vitro response to SEB. In primary cytotoxicity assays, CD4+ T cells from control spleens were more active than their CD8+ counterparts, however, in cells derived from SEB-primed mice, CD8+ T cells were dominant in SEB-specific cytotoxicity. In vivo priming with SEB abrogated the response of V beta 8.1,2+,CD4+ T cells despite the fact that these cells exist in significant number. This SEB-specific anergy occurred only in V beta 8.1,2+,CD4+ T cells but not in CD8+ T cells. These findings indicate that the requirement for the induction of antigen-specific anergy is different between CD4+ and CD8+ T cells in post-thymic tolerance, and the existence of coanergic signals for the induction of T cell anergy is suggested.

The role of thymic epithelium in the acquisition of tolerance

There are two separate mechanisms of induction of T-cell tolerance in the thymus. First, MHC molecules expressed on bone-marrow-derived cells can cause clonal deletion of autoreactive cells. Second, as discussed here by Elisabeth Houssaint and Martin Flajnik, thymic epithelial cells can generate a form of tolerance that does not eliminate self-reactive clones. This nondeletional mechanism, which is also a feature of the other MHC class-II-bearing epithelia, may contribute to the establishment of tolerance-maintaining regulatory networks.

In vivo induction of anergy in peripheral V beta 8+ T cells by staphylococcal enterotoxin B

We have developed a model of peripheral in vivo T cell tolerance that is induced by administration of the protein superantigen staphylococcal enterotoxin B (SEB). Rather than activating V beta 8+ T cells, in vivo administration of SEB induced in them a profound state of anergy. This was shown by their failure to proliferate to subsequent in vitro restimulation with SEB or to anti-V beta 8 antibodies. This unresponsiveness was V beta 8 specific since T cells from SEB-immunized mice responded normally to other antigens. 8 d after SEB administration, there was no reduction in the number of V beta 8+ T cells or in the intensity of V beta 8 T cell receptor (TCR) expression. Although a portion of the V beta 8+ T cells from SEB-primed mice were able to express interleukin 2 receptors (IL-2Rs), they failed to proliferate in response to exogenous IL-2, indicating they were defective in their IL-2 responsiveness. 2-4 wk after SEB administration, there was a reduction of approximately 50% in the number of V beta 8+ cells in immunized compared with control animals. There was, however, no reduction in the level of TCR expression on the remaining V beta 8+ cells. These data demonstrate that proteins that activate T cells in vitro in a V beta-specific manner can induce a state of anergy in peripheral T cells in vivo and may possibly further mediate clonal deletion in a portion of the tolerized cells.

Induction of peripheral tolerance to class I major histocompatibility complex (MHC) alloantigens in adult mice: transfused class I MHC-incompatible splenocytes veto clonal responses of antigen-reactive Lyt-2+ T cells

The efficacy and the mode of action of pretransplant transfusion with class I major histocompatibility complex (MHC)-disparate splenocytes in establishing a state of peripheral tolerance in adult mice is analyzed. Adult mice injected intravenously with a critical number of approximately 5 x 10(7) allogenic splenocytes accept skin grafts and develop chimerism in the peripheral lymphatic tissues, but not in thymus and bone marrow. In parallel, a split tolerance evolves: the frequency of class I MHC-reactive Lyt-2+ cytotoxic T lymphocyte precursor (CTL-p)- and interleukin 2 (IL-2)-producing T cells falls off in the peripheral lymphoid tissue, but remains unaltered intrathymically. In particular, high affinity CTL-p become clonally undetectable. In vivo generation of tolerant cells is cyclosporin A resistant, but dependent on recipient L3T4+ T cells. Loss of Lyt-2+ CTL-p- and IL-2-producing T cell precursors is not due to active suppression, but is caused by clonal anergy. Donor-derived chimeric cells positively selected 7 d after intravenous transfusion exhibit in vitro the hallmarks of veto cells, i.e., paralyze CTL-p reactive to donor-type class I MHC alloantigens. We conclude that the peripheral (split) tolerance induced in vivo by pretransplant transfusion operates because donor-type cells develop in vivo efficiently into "veto cells," which in turn induce a state of clonal anergy within antigen-reactive Lyt-2+ T lymphocytes.

Self-reactive T cells can escape clonal deletion in T-cell receptor V beta 8.1 transgenic mice

To study the mechanisms of tolerance in detail, we have constructed transgenic mice expressing a V beta 8.1-D beta 2-J beta 2.3-C beta 2 T-cell receptor (TCR) gene. Since expression of V beta 8.1 is known to correlate with reactivity of CD4+CD8- T cells to minor lymphocyte-stimulating locus 1a (Mls-1a), we expected to induce tolerance in most CD4+CD8- T cells in V beta 8.1 transgenic mice of the Mls-1a allele. In one line of Mls-1b V beta 8.1 transgenic mice, the V beta 8.1 TCR was expressed on greater than 98% of mature T cells and their response to Mls-1a was highly enriched. In Mls-1a V beta 8.1 transgenic mice, CD4+CD8- T cells in these mice were severely reduced among both peripheral T cells and thymocytes. However, the deletion of these cells was not complete, and most of the residual CD4+CD8- mature T cells still expressed normal densities of V beta 8.1 TCR. The residual CD4+CD8- T cells did not respond to Mls-1a but were still able to proliferate in response to other stimuli via the TCR. Interestingly, CD4+CD8- V beta 8.1+ T-cell clones isolated from Mls-1a V beta 8.1 transgenic mice could respond to Mls-1a. We suggest that these types of T cells escape clonal deletion in the thymus.

Does the presence of self-reactive T cells indicate the breakdown of tolerance?

There are many experimental systems in which autoreactive T cells can easily be demonstrated but where the host does not normally develop autoimmune disease. How do these animals avoid autoimmunity? Does the presence of these self-reactive cells indicate the failure of self-tolerance? To answer these questions it is necessary to consider how some T cells might escape tolerance induction and why they are not activated in the host. There are several different explanations which can be broadly placed into one of two categories. First, although autoreactive cells may be easily stimulated under experimental conditions, the requirements for activation and likewise deletion may not be met under physiological conditions. The self-antigen may be poorly presented by APC or sequestered in a particular body compartment; alternatively, these T cells may have low affinity receptors needing high levels of antigen. The second category is characterized by the need for immunoregulation. A random selection of T cells may escape clonal inactivation in the thymus but may be kept under constant suppression, which provides a fail-safe mechanism for deletional tolerance. In this review we will discuss these mechanisms and their possible importance in the prevention of autoimmunity.

Elucidating the pathogenesis of autoimmune disease: recent advances at the molecular level and relevance to oral mucosal disease

Humoral and/or cellular immune responses directed against self antigen are currently thought to underlie a wide spectrum of systemic and organ-specific human and animal autoimmune diseases. Although the immunopathology of these diseases has been well characterized in most cases, the etiology still remains obscure. In order to gain a more fundamental insight of the abnormal processes leading to autoimmunity, efforts are currently being directed toward defining these diseases at a molecular level. Because of the clinical diversity and immunological complexity of these diseases, several directions are being pursued. Relevant to autoimmune oral disease, this review will focus on antigen processing and recognition (immunoglobulin and T cell receptor genes), the key role of the major histocompatibility complex, and the involvement of the cytokine network. Advances made in these fields have clear relevance for future diagnostic and therapeutic strategies related to autoimmune diseases affecting oral tissues.

Peripheral tolerance in mice expressing a liver-specific class I molecule: inactivation/deletion of a T-cell subpopulation

We previously demonstrated that C3H/HeJ transgenic (TG) mice that express a laboratory-engineered class I molecule, Q10/L, exclusively on liver parenchymal cells show no evidence of hepatic disease even after deliberate immunization. Nevertheless, these animals demonstrate cytotoxic T-lymphocyte (CTL) activity specific for Q10/L, although it is less than that obtained from non-TG littermates. We now show that this decrease in CTL activity is not a reflection of a decrease in precursors, since both TG and normal animals have similar numbers. When non-TG C3H mice are primed with H-2Ld and H-2Kbm1 antigens, which extensively crossreact with Q10/L, their specific in vitro CTL activity directed against H-2Ld, H-2Kbm1, and Q10/L is increased 10- to 20-fold, as expected. Although primed TG mice show similar increases in in vitro CTL activity directed against H-2Ld and H-2Kbm1, they display no increase in anti-Q10/L activity. Whereas anti-H-2Ld spleen cells from non-TG mice readily generate CTL lines and clones specific for H-2Ld and Q10/L, TG cells give rise to anti-H-2Ld lines or clones only. These data indicate that the tolerance in TG mice is accounted for by the inactivation or deletion of an important CTL subpopulation having the capability of recognizing the peripheral antigen in situ. To determine whether tolerance would persist in the absence of Q10/L, TG cells were transferred into non-TG recipients. Three weeks later Q10/L-specific lytic activity generated in in vitro bulk cultures remained reduced compared to non-TG cells, indicating that the tolerant phenotype was stable during this interval.

The requirement of intrathymic mixed chimerism and clonal deletion for a long-lasting skin allograft tolerance in cyclophosphamide-induced tolerance

Mechanisms of cyclophosphamide (CY)-induced tolerance were studied. When C3H/He Slc (C3H; H-2k, Mls-1b) mice were primed i.v. with 1 x 10(8) viable spleen cells from H-2-identical AKR/J Sea (AKR; H-2k, Mls-1a) mice and treated with 200 mg/kg of CY 2 days later, a long-lasting skin allograft tolerance to AKR was established. When [C57BL/6 Sea (B6; H-2b, Mls-1b) x AKR]F1 (B6AKF1) cells were used as the tolerogen, however, only a moderate, but not long-lasting, skin tolerance to AKR was observed. In the C3H mice treated with AKR cells and CY, the intrathymic clonal deletion of V beta 6+ T cells, which are strongly correlated with reactivity to Mls-1a antigens, was observed in the chimeric thymus on day 35, although neither the clonal deletion of V beta 6-bearing T cells nor the mixed chimerism was observed in the thymus on day 14. In the C3H mice treated with B6AFKF1 cells followed by CY, however, neither the clonal deletion of V beta 6+ T cells nor the mixed chimerism was observed in the thymus throughout the test period. In the lymph nodes of the C3H mice treated with AKR cells and CY, only CD4+ V beta 6+ T cells, bur not CD8+V beta 6+ T cells, had selectively decreased by day 14, and they were hardly detectable on day 35. The selective decrease of CD4+V beta 6+ T cells in the lymph nodes was also observed by day 14 when B6AKF1 cells were used as the tolerogen, although CD4+V beta 6+ T cells gradually increased on day 35, at which time almost all skin grafts from AKR had already been rejected. These results strongly support the necessity of the intrathymic mixed chimerism and clonal deletion of donor-reactive T cells for a long-lasting skin allograft tolerance in CY-induced tolerance.

Loss of pancreatic islet tolerance induced by beta-cell expression of interferon-gamma

Interferon-gamma (IFN-gamma) is produced during the response to infection and participates in immunostimulatory events. We have previously reported the induction of diabetes in transgenic mice (ins-IFN-gamma) in which the expression of the lymphokine IFN-gamma is directed by the insulin promoter. This diabetes is a result of the progressive destruction of pancreatic islets that occurs with the influx of inflammatory cells. Here we demonstrate that this islet cell loss is mediated by lymphocytes, that engrafted histocompatible islets are destroyed, and that lymphocytes from the transgenic mice are cytotoxic to normal islets in vitro. These results indicate that the pancreatic expression of IFN-gamma can result in a loss of tolerance to normal islets, consistent with its role as an inducer of costimulatory activity, which is essential for lymphocyte activation during an immune response.

Intrinsic B-cell hyporesponsiveness accounts for self-tolerance in lysozyme/anti-lysozyme double-transgenic mice

In double-transgenic mice expressing a gene construct encoding hen egg lysozyme as well as rearranged anti-lysozyme antibody genes, large numbers of anti-lysozyme B cells are present in peripheral lymphoid tissues but are profoundly tolerant. The cellular basis for this form of non-deletional self-tolerance was explored. The tolerant anti-lysozyme B cells from double-transgenic mice were found to produce much less antibody than nontransgenic controls in T-cell-dependent antigen-specific responses, in adoptive transfer in vivo, and in hanging-drop cultures in vitro, as well as in response to stimulation with the nonspecific mitogen lipopolysaccharide. The diminished responsiveness of the tolerant B cells was not due to a reduction in the number of responding B-cell precursors per se nor were suppressor cells detected in titration, depletion, or mixing experiments. Nondeletional tolerance in this model, therefore, appears to result from an intrinsic functional change in the self-reactive B cells themselves.

T-cell clones that react against autologous human tumors

T cells (derived from peripheral blood lymphocytes [PBL], lymph nodes or tumor tissues and restimulated with autologous tumor cells and expanded in interleukin-2 [IL-2]), when cloned, produce three functional classes of clone. Class I T-cell clones exhibit the phenotype of alpha/beta cytotoxic T lymphocytes (CD3+, CD8+, CD4-, WT31+), use their CD3-alpha/beta complexes for cognate function, and lyse the autologous tumor cells specifically in a major histocompatibility complex (MHC) Class I-restricted manner. The second class of T cell clone expresses identical phenotype but exhibits a rather broad cytotoxic profile against the autologous and allogeneic tumor cells derived from tumors with similar and/or dissimilar histologies. Although these CTL clones can, at times, show MHC Class I-restricted killing and use their T-cell receptors (TCR) complexes for function, activation via certain accessory molecules, particularly lymphocyte-function associated (LFA-1) antigens, might induce their broad cytotoxic behavior. The nature of the tumor antigen recognized by the Class I antigen-specific CTL clones remains unknown. It is evident, however, that more than one antigen can be associated with a given tumor and they are recognized by different CTL clones from individual patients. The third class of T-cell clone is usually of CD4+ alpha/beta T cells (CD3+, CD4+, CD8-, WT31) and these T-cell clones exhibit no cytotoxicity toward the autologous or allogeneic target cells. When tested for potential regulatory property, one type of CD4+ T-cell clone exhibits the characteristics of helper T cells. This type induces or amplifies cytotoxic response in fresh PBL by elaborating interleukin-2 (IL-2) and interferon-gamma). These helper T-cell clones can proliferate against the autologous tumor cells and demonstrate functional specificity for the autologous tumor cells. The other type of CD4+ T-cell clone exhibits the phenotype of the helper T-cell clone (CD3+, CD4+, CD8-, WT31+) but suppresses the cytotoxic response of the autologous PBL in co-culture in the presence of the autologous tumor cells and exogenous IL-2. In some situations, these CD4+ suppressor T-cell clones exhibit considerable specificity for the autologous tumor cells. They do not suppress the cytotoxic response against allogeneic targets or against EBV-infected autologous lymphoblastoid cells. Furthermore, they specifically up-regulate their IL-2 receptors (IL-2R) when stimulated by the autologous tumor cells or with autologous tumor cell-pulsed antigen-presenting cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Thymus: a direct target tissue in graft-versus-host reaction after allogeneic bone marrow transplantation that results in abrogation of induction of self-tolerance

Graft-versus-host reaction (GVHR) following allogeneic bone marrow (BM) transplantation was investigated by analyzing expression of antigen receptors on T cells specific for recipient antigens. GVHR chimeras were prepared by transplanting mixtures of splenic T cells and T-cell-depleted BM cells from B10 (I-E-, Mls-1b) or B10.AQR (I-E+, Mls-1b) mice into lethally irradiated AKR/J (I-E+, Mls-1a) recipients. Increased proportions of V beta 6+ T cells reactive to recipient antigens (I-E and Mls-1a) were observed in thymuses from such chimeras 1 or 5 wk after BM transplantation. V beta 6+ T cells observed 1 wk after BM transplantation were derived from mature T cells that had been inoculated into recipients. These cells responded to recipient antigens expressed in the thymus. After 5 wk, thymocytes brightly positive for V beta 6+ were shown not to descend from mature T cells but to differentiate from precursor cells present in the BM inocula. Since V beta 6+ T cells were eliminated in thymuses from non-GVHR chimeras 5 wk after BM transplantation using T-cell-depleted BM cells alone, it appears that GVHR occurring in the thymus at an early stage abrogates thymic stromal functions essential to induction of self-tolerance in the T-cell repertoire. These findings propose a mechanism (autoimmunity) to explain in part the pathogenesis of chronic GVHR.

Clonal anergy in self-reactive alpha/beta T cells is abrogated by heat-shock protein-reactive gamma/delta T cells in aged athymic nude mice

Although T cells proliferate and differentiate primarily in the thymus, athymic nude mice contain an appreciable level of T cell receptor alpha/beta and gamma/delta T cells, suggesting the existence of the extrathymic pathway in the development of both T cells. Recent studies with nude mice indicate that clonal deletion of self-reactive T cells does not occur extrathymically. In the present study, we have investigated the responsiveness of self-reactive T cells differentiating along an extrathymic pathway in aged BALB/c (H-2d, Mls-1b2a, I-E+, 7-8 month old) nude mice. Consistent with recent reports, T cells bearing V beta 3 or V beta 11, which are important for recognizing proteins encoded by the Mls-2a or the I-E allele, respectively, are readily detected in age nude mice. The V beta 3- or V beta 11-bearing T cells, however, do not proliferate in response to staphylococcal enterotoxin A which specifically stimulates V beta 3- or V beta 11-bearing T cells. When exogenous recombinant interleukin 2 was added to the culture, the V beta 3-bearing T cells in aged nude mice significantly proliferated in response to staphylococcal enterotoxin A. Aged nude mice also contained a substantial level of gamma/delta T cells which account for 15.6% of all Thy-1.2+ cells. The gamma/delta T cells proliferated and produced a significant level of interleukin 2 in response to the 65-kDa mycobacterial heat-shock protein, which is highly homologous to its eukaryotic counterpart. These results suggest that unresponsiveness of self-reactive T cells may be reversed by T cells responding to stress proteins expressed by the invading microbes and/or the stressed autologous cells.

Clonal deletion versus clonal anergy: the role of the thymus in inducing self tolerance

During development in the thymus, T cells are rendered tolerant to self antigens. It is now apparent that thymocytes bearing self-reactive T cell receptors can be tolerized by processes that result in physical elimination (clonal deletion) or functional inactivation (clonal anergy). As these mechanisms have important clinical implications for transplantation and autoimmunity, current investigations are focused on understanding the cellular and molecular interactions that generate these forms of tolerance.

The influence of allogeneic cells on the human T and B cell repertoire

Clinical transplantation is often complicated by rejection episodes, in which the immune system of the recipient reacts to the foreign transplantation (HLA) antigens on the graft. This immune response includes humoral and cellular components. In the first, B lymphocytes form antibodies to the HLA alloantigens. In the second, CD8+ T lymphocytes recognize and react to HLA class I antigens, and CD4+ T cells react to HLA class II antigens. The frequency and severity of these rejection episodes can be diminished by immunosuppressive drugs, HLA matching between donor and recipient, and immune modulation by blood transfusion. Effective HLA matching between donor and recipient is not always possible and often not necessary. Insight into the factors that influence the T and B cell repertoire after blood transfusion might lead to new approaches to improve graft survival.

A cell culture model for T lymphocyte clonal anergy

T lymphocytes respond to foreign antigens both by producing protein effector molecules known as lymphokines and by multiplying. Complete activation requires two signaling events, one through the antigen-specific receptor and one through the receptor for a costimulatory molecule. In the absence of the latter signal, the T cell makes only a partial response and, more importantly, enters an unresponsive state known as clonal anergy in which the T cell is incapable of producing its own growth hormone, interleukin-2, on restimulation. Our current understanding at the molecular level of this modulatory process and its relevance to T cell tolerance are reviewed.

The need for central and peripheral tolerance in the B cell repertoire

The immune system normally avoids producing antibodies that react with autologous ("self") antigens by censoring self-reactive T and B cells. Unlike the T cell repertoire, antibody diversity is generated within the B cell repertoire in two phases; the first occurs by gene rearrangement in primary lymphoid organs, and the second phase involves antigen-driven hypermutation in peripheral lymphoid organs. The possibility that distinct cellular mechanisms may impose self tolerance at these two different phases of B cell diversification may explain recent findings in transgenic mouse models, in which self-reactive B cells appear to be silenced both by functional inactivation and by physical elimination.

Tolerance in transgenic mice expressing major histocompatibility molecules extrathymically on pancreatic cells

Transgenic mice with defined expression of major histocompatibility complex (MHC) proteins provide novel systems for understanding the fundamental question of T cell tolerance to nonlymphoid self components. The MHC class II I-E and I-A and class I H-2K molecules expressed specifically on pancreatic islet or acinar cells serve as model self antigens. In these systems, transgenic proteins are not detected in the thymus or other lymphoid tissues. Yet mice are tolerant to the pancreatic MHC products in vivo; this tolerance is not induced by clonal deletion. These studies have been aided by monoclonal antibodies specific for I-E-reactive T cells and indicate that clonal anergy may be an important mechanism of tolerance to peripheral proteins.

The role of the T cell receptor in positive and negative selection of developing T cells

Although many combinations of alpha beta T cell receptors are available to the T cells in any given organism, far fewer are actually used by mature T cells. The combinations used are limited by two selective processes, positive selection of T cells bearing receptors that will be useful to the host, and clonal elimination or inactivation of T cells bearing receptors that will be damaging to the host. The ways in which these two apparently contradictory processes occur, and the hypotheses that have been suggested to reconcile them, are discussed.

A role for clonal inactivation in T cell tolerance to Mls-1a

Clonal deletion plays a major part in the maintenance of natural self-tolerance in both normal and transgenic mice. Self antigens that are expressed in the thymus result in the physical elimination of autoreactive thymocytes at a particular stage in their development. For example, the majority V beta 6- and V beta 8.1-bearing T cells that recognize the minor lymphocyte-stimulating antigen, Mls-1a (ref. 10) , are clonally deleted in the thymuses of normal mice and transgenic mice expressing Mls-1a (refs 2, 3, 9). In contrast, a very different mechanism of tolerance involving the functional inactivation, but not elimination, of autoreactive cells, termed clonal inactivation or clonal anergy, has been implicated in some experimentally manipulated systems of tolerance. To test further the mechanisms involved in self-tolerance, we have generated transgenic mice expressing a V beta 8.1 beta chain on greater than 95% of peripheral T cells and have tested tolerance to Mls-1a in these mice. Surprisingly, a significant fraction of the CD4+ peripheral cells that survived deletion were non-responsive in vitro to any stimulus tested. Naturally occurring tolerance to a self antigen expressed in the thymus can thus be mediated by clonal anergy, as well as by clonal deletion.

Clonal deletion: a mechanism of tolerance in mixed bone marrow chimeras

The mechanism of antigen-specific immunologic unresponsiveness which results from lethal irradiation and mixed (syngeneic-allogeneic) bone marrow cell (BMC) reconstitution is unknown. To determine whether clonal deletion is the mechanism of tolerance in this model, monoclonal antibody (Mab) RR-4-4, specific for a T-cell receptor (V beta 6) reactive against the minor alloantigen MLsa, was employed. Six-week-old B10 mice (H-2b, Mlsb, Thyl.2) were tolerized to AKR antigens (H-2k, Mlsa, Thyl.1) by whole body irradiation (950 R) and iv infusion of T-cell-depleted (TCD) B10 BMC + non-TCD AKR BMC. Chimerism and antigen-specific tolerance were documented by flow microfluorometry (FMF), skin grafting, mixed lymphocyte reaction, and cell-mediated lympholysis. When tolerant B10 mice (n = 15) had accepted AKR skin grafts for greater than 100 days, these animals were studied for the presence of host V beta 6+ T cells using Mab RR-4-4. FMF revealed that 0-5% of host (B10) lymph node and spleen cells from chimeras were V beta 6+ while 15-20% of lymph node and spleen cells from control B10 mice expressed V beta 6. These data demonstrate that clonal deletion occurs in the lethal irradiation-mixed reconstitution model as evidenced by the near total elimination of Mlsa-reactive V beta 6+ T cells and suggest that it maybe a mechanism responsible for tolerance in adult mice.

Cell components required for deletion of an autoreactive T cell repertoire

T cells become tolerant to self antigens during their development in the thymus. Clonal deletion of thymocytes bearing T cell receptor (TcR) which recognize self antigens is a major mechanism for generating tolerance. In the present study we have used allogeneic bone marrow (BM) chimeras, prepared with various combinations of mouse strains and focusing especially on expressions of I-E molecules and Mls-1a antigens on the cell surface, to investigate both immunohistochemically and by flow cytometry the cell components that contribute to the clonal deletion of T cells positive for V beta 6 TcR. The V beta 6 TcR expression is strongly associated with T cell recognition of both I-E and Mls-1a antigens. We found that I-E+ cells derived from donor BM (and thus not of recipient lineage) represented a primary requirement for deletion of Mls-1a-reactive thymocytes which bear V beta 6 TcR. Immunohistochemical analysis revealed that the donor-derived I-E+ cells were distributed mainly to the thymic medulla and that the V beta 6+ cells were eliminated from the thymic medulla between 2 and 3 weeks following BM transplantation. In contrast, Mls-1a+ cells of either donor or recipient origin might be responsible for the deletion, even though cortical epithelial cells appeared not to express Mls-1a antigens.