Tolerance induction in double specific T-cell receptor transgenic mice varies with antigen

Differential kinetics of various subsets of thymic bone marrow-derived stromal cells in rat chimeras

Identification of those cells within the thymic stroma which are responsible for tolerance induction remains controversial. Evidence derived from studies of bone marrow chimeras or thymus transplants attributed this function to cells of haematopoietic origin, usually class II positive medullary dendritic cells (DC). Recent data suggest, however, that a stromal element located in the thymus cortex might be involved in negative selection. To further explore this issue we used immunohistology and immunocytology with a combination of allotype and cell type specific monoclonal antibodies (MoAb) to study the turnover of thymic stromal cells of haematopoietic origin in different rat models of allogeneic and congenic bone marrow (BM) radiation chimeras. Use of CFU-GM cultured BM inoculum for congenic recipients allowed us to distinguish between direct homing of donor myeloid cells and the delayed migration of the donor stem cell progeny after the post-irradiation recovery of the recipient. Our data indicate a heterogeneity in the turnover rate of thymic mobile stromal cells. While DC and a subset of macrophages located in the cortex as well as in the medulla (ED1+), within 4 weeks were virtually all of donor type, cortical macrophages detected by ED2 MoAbs were still incompletely replaced after a period as long as 20 weeks. Slow turnover, location and variable class II expression may imply a role for thymic cortical macrophages in (self-) tolerance induction.

Positive and negative selection of the alpha beta T-cell repertoire in vivo

The selection of developing and mature alpha beta T cells, by intrathymic and extra-thymic ligands expressed on cells of hemopoietic and other origins, has been studied in a variety of systems. These experiments have increased our knowledge of the selection of alpha beta T cells in the thymus, in the presence or absence of specific peptides bound to MHC, and also made clear that additional selective mechanisms exist outside the thymus that can be exploited to silence mature T cells.

Detection of apoptosis of immature CD4+8+ thymocytes by flow cytometry

Apoptosis, i.e., programmed cell death, may be the mechanism by which both autoreactive and unselected immature CD4+8+ thymocytes are eliminated in the thymus. In the present paper we describe a simple and rapid flow cytometric method which permits one to study the induction and kinetics of apoptosis of CD4+8+ thymocytes using in vivo and in vitro suspension cultures. Analysing the level of surface expression of CD4 and CD8 molecules, forward light scatter and side (90 degrees) scatter as well as staining with ethidium bromide, three distinct stages of apoptosis of CD4+8+ thymocytes were defined. By counting cells passing through different stages of apoptosis one can attempt to quantitate this process. This method should be useful for in vitro studies on the mechanisms of negative and positive selection of CD4+8+ thymocytes, i.e., induction and inhibition of apoptosis respectively.

Phenotypic and functional stages in the intrathymic development of alpha beta T cells

The thymus is the main site of T-cell maturation. On arrival in the thymus, CD8-CD4- double-negative (DN) T-cell precursors undergo extensive gene rearrangement, phenotypic alteration and biochemical modification to yield the population of thymocytes that undergoes intrathymic selection. The selected mature thymocytes then go on to seed the periphery. In this article Janko Nikolić-Zugić discusses the labyrinthine development that constitutes alpha beta T-cell maturation and selection.

Role of MHC and antigens in T-cell development

The role of self-peptides in influencing the development of the T-cell repertoire has been the focus of recent studies. The findings suggest that the recognition of self-peptides bound to MHC proteins in the thymus is part of the thymic self-recognition process that results in selective maturation, or positive selection of T cells.

Towards an integrated view of thymopoiesis

One prediction from the complex series of steps in intrathymic T-cell differentiation is that to regulate it the stroma controlling the process must be equally complex: the attraction of precursors, commitment to the T-cell lineage, induction of T-cell receptor (TCR) gene rearrangement, accessory molecule expression, repertoire expansion, major histocompatibility complex (MHC) molecule-based selection (positive and negative), acquisition of functional maturity and migratory capacity must all be controlled. In this review, Richard Boyd and Patrice Hugo combine knowledge of T-cell differentiation with thymic stromal cell heterogeneity to offer an integrated view of thymopoiesis within the thymic microenvironment.

Thymic selection in CD8 transgenic mice supports an instructive model for commitment to a CD4 or CD8 lineage

Immature thymocytes, which coexpress CD4 and CD8, give rise to mature CD4+CD8- and CD4-CD8+ T cells. Only those T cells that recognize self-MHC are selected to mature, a process known as positive selection. The specificity of the T cell antigen receptor (TCR) for class I or class II MHC influences the commitment to a CD4 or CD8 lineage. This may occur by a directed mechanism or by stochastic commitment followed by a selection step that allows only CD8+, class I-specific and CD4+, class II-specific cells to survive. We have generated a mouse line expressing a CD8 transgene under the control of the T cell-specific CD2 regulatory sequences. Although constitutive CD8 expression does not affect thymic selection of CD4+ cells, selection of a class I-specific TCR in the CD8 subset is substantially improved. This outcome is consistent with a model for positive selection in which selection occurs at a developmental stage in which both CD4 and CD8 are expressed, and positive selection by class I MHC generates an instructive signal that directs differentiation to a CD8 lineage.

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.

Induction by antigen of intrathymic apoptosis of CD4+CD8+TCRlo thymocytes in vivo

In order to examine the mechanisms by which clonal deletion of autoreactive T cells occurs, a peptide antigen was used to induce deletion of antigen-reactive thymocytes in vivo. Mice transgenic for a T cell receptor (TCR) that reacts to this peptide contain thymocytes that progress from the immature to the mature phenotype. Intraperitoneal administration of the peptide antigen to transgenic mice results in a rapid deletion of the immature CD4+ CD8+ TCRlo thymocytes. Apoptosis of cortical thymocytes can be seen within 20 hours of treatment. These results provide direct evidence for the in vivo role of apoptosis in the development of antigen-induced tolerance.

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.

Failure of clonal deletion in neonatally thymectomized mice: tolerance is preserved through clonal anergy

Self-tolerance is achieved in part through intrathymic deletion of self-reactive T cells. The necessity of the thymus for this process is suggested by the development of autoimmune diseases in neonatally thymectomized (neoTx) mice and by the failure of clonal deletion in nude mice. Indeed, the present study demonstrates that neonatal thymectomy on day 3 after birth results in the failure of clonal deletion of V beta 11+ T cells in BALB/c mice and V beta 5+ and V beta 6+ T cells in DBA/2 mice. However, these potentially autoreactive cells are nonfunctional as measured by proliferation and lymphokine production after stimulation with appropriate anti-V beta mAbs or stimulator cells. It appears that this induction of nonresponsiveness may have occurred extrathymically: the early neonatal thymus (presumably the source of the peripheral T cells observed in neoTx mice) also contains T cells with self-reactive receptors, but these cells are fully functional. Therefore, neonatal thymectomy aborts deletion of self-reactive T cells, but self-tolerance is maintained through functional inactivation of potentially self-reactive clones.

Newly generated thymocytes are not refractory to deletion when the alpha/beta component of the T cell receptor is engaged by the superantigen staphylococcal enterotoxin B

It has been reported that, following the initial expression of the T cell receptor (TcR) alpha/beta, newly generated thymocytes pass through a developmental window characterized by ineffective coupling between the alpha/beta and CD3 components resulting in resistance to deletion (negative selection). However, we now provide evidence that the TcR alpha/beta on developing thymocytes is capable of delivering deletional signals in response to the superantigen staphylococcal enterotoxin B (SEB) as soon as the receptor is expressed. We also show that if TcR+ thymocytes are allowed to mature in organ cultures of embryonic thymus before SEB is added, they respond by proliferation giving rise to blast cells of CD4-CD8-, CD4+CD8- or CD4-CD8+ phenotypes.

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.

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.

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.

T cell receptor-mediated negative selection of autoreactive T lymphocyte precursors occurs after commitment to the CD4 or CD8 lineages

To identify the maturational stage(s) during which T cell receptor (TCR)-mediated positive and negative selection occurs, we followed the development of CD4+8- and CD4-8+ T cells from TCRlo CD4+8+ thymic blasts in the presence of different positive and negative selecting (major histocompatibility complex or Mls) elements. We describe novel lineage-committed transitional intermediates that are TCRmed CD4+8lo or TCRmed CD4lo8+, and that show evidence of having been positively selected. Furthermore, negative selection is not evident until after cells have attained one of the TCRmed transitional phenotypes. Accordingly, we propose that negative selection in normal mice occurs only after TCRlo CD4+8+ precursors have been positively selected into either the CD4 or CD8 lineage.

Distinct sequence of negative or positive selection implied by thymocyte T-cell receptor densities

Recent evidence suggests that positive and negative selection of thymocytes bearing alpha beta T-cell receptors occurs during the predominant double-positive (CD4+CD8+) stage. But the sequence or stage at which positive or negative selection occurs during thymocyte maturation has not been well defined. Here we use transgenic mice to show that the CD4+CD8+ stage might be further subdivided into CD3lo (low) and CD3in (intermediate) stages. The CD3in stage could represent T cells that have been positively selected, as this stage is dependent on the presence of the appropriate major histocompatibility complex restriction element. In addition, we use two different tolerizing antigens to show that negative selection may occur either before or after this CD3in stage.

Viral escape by selection of cytotoxic T cell-resistant virus variants in vivo

Viruses persist in an immune population, as in the case of influenza, or in an individual, as postulated for human immunodeficiency virus, when they are able to escape existent neutralizing antibody responses by changing their antigens. It is now shown that viruses can in principle escape the immunosurveillance of virus-specific cytotoxic T cells by mutations that alter the relevant T-cell epitope.

Intrathymic nurse cell lymphocytes can induce a specific graft-versus-host reaction

Single chicken thymic nurse cells (TNC) placed onto the chorionallantoic membrane (CAM), showed that intra-TNC lymphocytes (TNC-L) possess a strong graft-versus-host reactivity (GVHR) in allogeneic MHC combinations. This reaction shows the morphological, phenotypic, and functional characteristics of a classical GVH reaction (GVHR). The induction of a GVHR was significantly higher for TNC-L as compared with thymocytes or peripheral blood lymphocytes (PBL). The specificity of the GVHR was shown by serial transfer experiments onto appropriate allogeneic and syngeneic secondary embryonic hosts. In immunofluorescence analyses with monoclonal antibodies (mAb) to the chicken alpha/beta and gamma/delta T cell receptors (TCR) and the CD3, CD4, and CD8 equivalents, an enrichment of CD3+/CD4+/CD8- and CD3+/CD-4-/CD8+, TCR-alpha/beta + and TCR- gamma/delta + cells was observed inside TNC as compared with extra-TNC thymocytes. A large proportion of CD4+ and/or CD8+ TCR- gamma/delta + cells were demonstrated inside TNC. A minor population among TCR- gamma/delta extra-TNC thymocytes also expressed CD4 and/or CD8 molecules. Based on functional tests and double staining experiments, we propose that CD4+/CD8+ thymocytes enter the TNC where they may undergo positive selection for MHC restriction and further differentiation to CD4 or CD8 single-positive cells. Taken together these data support the concept that TNC contribute a specialized thymic microenvironment for T cell differentiation and maturation.

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.

Self-nonself discrimination by T cells

The alpha beta T cell receptor (TCR) recognizes antigens that are presented by major histocompatibility complex (MHC)-encoded cell surface molecules by binding to both the antigen and the MHC molecules. Discrimination of self from nonself antigens and MHC molecules is achieved by negative and positive selection of T cells in the thymus: potentially harmful T cells with receptors that bind to self antigens plus self MHC molecules are deleted before they can mount immune responses. In contrast, the maturation of useful T cells with receptors that bind foreign antigens plus self MHC molecules requires the binding of their receptor to MHC molecules on thymic epithelium in the absence of foreign antigen. The binding of the TCR to either class I or class II MHC molecules directs differentiation of the selected cells into either CD4-8+ (killer) or CD4+8- (helper) T cells, respectively.

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.