Regulatory T cells in thymic epithelium-induced tolerance. I. Suppression of mature peripheral non-tolerant T cells

Transient depletion of dividing T lymphocytes in mice induces the emergence of regulatory T cells and dominant tolerance to islet allografts

We previously showed that transient depletion of dividing T cells at the time of an allogeneic transplantation induces long-term tolerance to the allograft. Here we investigated the role of homeostatic perturbation and regulatory T cells (Treg) in such tolerance. Transient depletion of dividing T cells was induced at the time of an allogeneic pancreatic islets graft, by administration of ganciclovir for 14 days, into diabetic transgenic mice expressing a thymidine kinase (TK) conditional suicide gene in T cells. Allograft tolerance was obtained in 63% of treated mice. It was not due to global immunosuppression, permanent deletion or anergy of donor-alloantigens specific T cells but to a dominant tolerance process since lymphocytes from tolerant mice could transfer tolerance to naïve allografted recipients. The transient depletion of dividing T cells induces a 2- to 3-fold increase in the proportion of CD4(+)CD25(+)Foxp3(+) Treg, within 3 weeks that persisted only in allograft-bearing mice but not in nongrafted mice. Tolerance with similar increased proportion of Treg cells was also obtained after a cytostatic hydroxyurea treatment in normal mice. Thus, the transient depletion of dividing T cells represents a novel means of immuno-intervention based on disturbance of T-cell homeostasis and subsequent increase in Treg proportion.

Plasticity of neuroendocrine-thymus interactions during ontogeny and ageing: role of zinc and arginine

Thymic re-growth and reactivation of thymic functions may be achieved in old animals by different endocrinological or nutritional manipulations such as, (a) treatment with melatonin, (b) implantation of a growth hormone (GH) secreting tumour cell line (GH3 cells) or treatment with exogenous GH, (c) castration or treatment with exogenous luteinizing hormone-releasing hormone (LHRH), (d) treatment with exogenous thyroxin or triiodothyronine, and (e) nutritional interventions such as arginine or zinc supplementation. These data strongly suggest that thymic involution is a phenomenon secondary to age-related alterations in neuroendocrine-thymus interactions and that it is the disruption of these interactions in old age that is responsible for age-associated immune-neuroendocrine dysfunctions. The targets involved in hormones-induced thymic reconstitution may directly or indirectly involve hormone receptors, cytokines, arginine, and a trace element such as zinc, which is pivotal for the efficiency of neuroendocrine-immune network during the whole life of an organism. The effect of GH, thyroid hormones, and LHRH may be due to specific hormone receptors on thymocytes and on thymic epithelial cells (TECs), which synthesize thymic peptides. Melatonin may also act through specific receptors on T-cells. In this context, the role of zinc, which turnover is reduced in old age, is pivotal because of its involvement through zinc fingers in the gene expression of hormone receptors. In addition, the effects of zinc are multifaceted: from the reactivation of zinc-dependent enzymes, to cell proliferation and apoptosis, to cytokines expression and to the reactivation of thymulin, which is a zinc-dependent thymic hormone required for intrathymic T-cell differentiation and maturation as well as for the homing of stem cells into the thymus. Zinc is also required for arginine action, via NO pathway. The role of zinc is therefore crucial in neuroendocrine-thymus interactions. According to data in animals and humans, the above reported endocrinological manipulations (GH, thyroid hormones, and melatonin) or arginine treatment may also act via zinc pool in restoring thymic activity in ageing allowing improvements on peripheral immune efficiency.

Induction of regulatory T cells from mature T cells by allogeneic thymic epithelial cells in vitro

The ability of thymic epithelial cells (TEC) to re-educate mature T cells to be regulatory T cells has not been addressed. In the present study, this issue was directly investigated by co-culturing of mature T cells and allo-TECs. B6 macrophage cell line 1C21-cultured BALB/c splenocytes responded to B6 antigens in vitro. However, BALB/c splenocytes precultured with B6-derived TECs 1-4C18 or 1C6 did not proliferate to B6 antigens, but responded to rat antigens. Exogenous interleukin-2 (IL-2) failed to revise the unresponsiveness of these T cells. Allo-TEC-cultured T cells predominantly expressed Th2 cytokines (IL-4 and IL-10). B6 TEC-cultured BALB/c splenocytes markedly inhibited the immune responses of naïve BALB/c splenocytes to B6 antigens, but not to rat or the third-party mouse antigens. BALB/c nude mice that received naïve syngeneic splenocytes rejected B6 or rat skin grafts by 17 days postskin grafting; however, co-injection of B6 TEC-cultured BALB/c splenocytes significantly delayed B6 skin graft rejection (P < 0.01), with the unchanged rejection of rat skin grafts. These studies demonstrate that allo-TECs are able to 'educate' mature T cells to be regulatory cells, and suggest that regulatory cells derived from mature T cells by TECs may play an important role in T cell tolerance to allo- and auto-antigens.

Regulatory T cells in microbial infection

Natural T regulatory cells (NatTReg) limit immunopathology and protective immune responses induced upon microbial infection. In addition, infection increases the number and activity of NatTReg. These findings need to be conciliated with the process of 'self-nonself' discrimination based on the function of NatTReg committed intrathymically and positively selected (and activated) on thymic epithelial cells. A review of the available evidence comforts the assumptions that, in physiological conditions, NatTReg engaged in the immune responses to microbial infections are drawn from the autoreactive repertoire even if some may appear to be microbe specific. This contention also provides a suitable explanation for the 'hygiene hypothesis': infections re-enforce the physiological mechanisms of natural dominant tolerance, through the expansion of naturally occurring regulatory T cells. Accumulating evidence demonstrates that pro-inflammatory ligands of Toll-like receptors expressed by NatTReg, both of microbial (e.g., lipopolysaccharide, flagellin, peptidoglycans) and endogenous (e.g., stress proteins and degradation products of the extracellular matrix) origin, may play a critical role in their activation and expansion. As NatTReg vigorously respond to IL-2/IL-15 locally produced by ongoing effector responses, this whole set of mechanisms provides for a robust feedback process that limits tissue damage and accounts for an 'organism-centered' quality control of immune responses. Detailed knowledge on these molecular and cellular bases should open novel opportunities for intervention in a variety of critical conditions, such as autoimmunity, allergy, chronic infections, and cancer, for which we currently lack effective therapies.

Long-Term Acceptance of Fully Allogeneic Cardiac Grafts by Cotransplantation of Vascularized Thymus in Miniature Swine

BACKGROUND

We have previously reported the ability of both thymokidney and vascularized thymic lobe (VTL) allografts to induce transplantation tolerance to renal allografts across a full major histocompatibility complex (MHC) mismatch in thymectomized miniature swine. However, whether vascularized thymus is capable of inducing tolerance to less tolerogeneic organs when it is transplanted simultaneously is not yet known. The present study investigates cardiac allograft survival and the mechanism of long-term acceptance in recipient swine following cotransplantation of VTL and cardiac grafts from fully MHC-mismatched donors.

METHODS

Animals received a heart graft, a heart graft and a VTL, or a heart graft and a donor thymocyte infusion. Immunosuppressive regimens consisted of 12 or 28 days of tacrolimus.

RESULTS

All animals that received a VTL maintained their grafts significantly longer than their counterparts that received only a heart graft, and those receiving 28 days of tacrolimus maintained their heart grafts long-term. Recipients of a donor thymocyte infusion demonstrated slightly prolonged cardiac graft survival but all rejected their grafts, highlighting the importance of thymic stroma. Cytotoxic T-lymphocyte responses against third-party antigens by cells from tolerant animals showed restriction by both self and donor MHC, whereas responses of controls were restricted to self MHC only. The presence of donor dendritic cells in the VTL grafts and results of co-culture assays suggest that both central and regulatory mechanisms were involved in achieving long-term acceptance.

CONCLUSION

This is the first demonstration of the long-term acceptance of fully MHC-mismatched cardiac allografts in large animals.

Characterisation of CD8 monoclonal antibody-induced protection from diabetes in NOD mice

NOD mice can be protected from transferred diabetes for long periods by short-term treatment with CD8 mabs. This protection has previously been shown to be thymus-dependent as thymectomised mice do not show the long-term protection observed in intact mice. In this study we show that the thymus is required only during antibody treatment as its removal thereafter does not affect protection. Recent thymic emigrants (RTEs) are not necessary for long-term tolerance induction and irradiation plays no part as anti-CD8 treatment cannot protect NOD.scid recipients from diabetes development. IL-10 is also shown to play an important role in the anti-CD8 induced protection in intact mice as it is reversed by IL-10R blockade.

Control of autoimmunity by naturally arising regulatory CD4+ T cells

Naturally acquired immunological self-tolerance is not entirely accounted for by clonal deletion, anergy, and ignorance. It is now well established that the T cell-repertoire of healthy individuals harbors self-reactive lymphocytes with a potential to cause autoimmune disease and these lymphocytes are under dominant control by a unique subpopulation of CD4+ T cells now called regulatory T cells. Efforts to delineate these Treg cells naturally present in normal individuals have revealed that they are enriched in the CD25+ CD4+ population. The identification of the CD25 molecule as a useful marker for naturally arising CD4+ regulatory T cells has made it possible to investigate many key aspects of their immunobiology, including their antigen specificities and the cellular/molecular pathways involved in their development and their mechanisms of action. Furthermore, reduction or dysfunction of the CD25+ CD4+ regulatory T cell population can be responsible for certain autoimmune diseases in humans.

Whither T-suppressors: if they didn’t exist would we have to invent them?

Arriving at an understanding of the role of suppressor T-cells (regulatory T-cells, CD4(+)CD25+) depends on whether their functional repertoire is somatically selected to be anti-Self or anti-Nonself. Immunologists are ambivalent; often publications espousing opposite views share an author. Here the arguments are detailed that the suppressor repertoire is not somatically selected to be anti-Self, but rather it is anti-Nonself. Therefore, suppression cannot regulate the Self-Nonself discrimination; its function is to regulate the magnitude and class of the anti-Nonself effector response.

Influence of first-wave derived T lymphocytes in the long term functional reconstitution of allogeneic T cell deficient hosts

The functional immunological reconstitution and the patterns of cytokine secretion were comparatively studied in BALB/c nu/nu mice grafted with allogeneic B6.Thy-1.1+ E14 or E18 embryonic thymus. In spite of equivalent proliferative responses to both mitogen or MLR stimuli, the two groups presented different cytokine patterns. B6 E18-thymus grafted BALB/c nu/nu mice showed a predominant IL-2/IFN-gamma secretion in response to mitogen or to CBA haplotype, with insignificant secretion of either cytokine to the tolerated BALB/c or donor B6 haplotype. In contrast, E14 grafted mice showed a significant IL-10 secretion, both in response to mitogens or to the tolerated haplotypes, even in the absence of a detectable proliferative response. A significant IFN-gamma secretion appeared only accompanying high responses to CBA. The preferential Th2 profile associated to the E14 chimeras was coincident with a longer lifespan of the nude host kept in a conventional environment, higher CD3+ cells frequency in the blood and functional restoration of allogeneic skin graft rejection, not seen on the E18 chimeras. The meaning of these results is discussed in relation to the previously described longer persistence of the first-wave donor derived lymphocytes in the allogeneic BALB/c periphery, also exclusive of the E14 grafted group.

The role of regulatory T cells in controlling immunologic self-tolerance

Accumulating evidence indicates that T cell-mediated dominant control of self-reactive T cells contributes to the maintenance of immunologic self-tolerance and its alternation may lead to development of autoimmune disease. Efforts to delineate such a regulatory T cell population have revealed that CD25+ cells within the CD4+ population in normal naive animals including humans possess the regulatory activity. The CD25+CD4+ regulatory T cells are produced by the normal thymus as a functionally distinct subpopulation of T cells. They play critical roles not only in preventing autoimmunity but also in controlling various immune reactions.

Neuroprotective autoimmunity: naturally occurring CD4+CD25+ regulatory T cells suppress the ability to withstand injury to the central nervous system

The ability of rats or mice to withstand the consequences of injury to myelinated axons in the CNS was previously shown to depend on the ability to manifest a T cell-mediated protective immune response, which is amenable to boosting by myelin-specific T cells. Here we show that this ability, assessed by retinal ganglion cell survival after optic nerve injury or locomotor activity after spinal cord contusion, is decreased if the animals were immunized as neonates with myelin proteins (resulting in their nonresponsiveness as adults to myelin proteins) or injected with naturally occurring regulatory CD4(+)CD25(+) T cells immediately after the injury, and is improved by elimination of these regulatory T cells. In nude BALBc mice replenished with a splenocyte population lacking CD4(+)CD25(+) regulatory T cells, significantly more neurons survived after optic nerve injury than in nude mice replenished with a complete splenocyte population or in matched wild-type controls. In contrast, neuronal survival in wild-type BALBc mice injected with CD4(+)CD25(+) regulatory T cells immediately after injury was significantly worse than in noninjected controls. These findings suggest that the ability to cope with the sequelae of a CNS insult is affected unfavorably by nonresponsiveness to myelin self-antigens and favorably by conditions allowing rapid expression of an autoimmune response. The regulatory T cells might represent an evolutionary compromise between the need to avoid autoimmune diseases and the need for autoimmunity on alert for the purpose of tissue maintenance.

Grafts of supplementary thymuses injected with allogeneic pancreatic islets protect nonobese diabetic mice against diabetes

In nonobese diabetic (NOD) mice, the autoimmune attack of the beta-cells in pancreatic islets is now believed to result from abnormal thymic selection. Accordingly, grafts of thymic epithelium from NOD donors to athymic recipients promote autoimmune islet inflammation in normal strains, and intrathymic islet grafts decrease the incidence of disease in NOD animals. Two competing hypotheses of abnormal thymic selection in diabetic mice have been proposed: deficient negative selection with poor elimination of aggressive organ-specific T cells vs. deficient positive selection of protective T regulatory cells. We have now addressed these alternatives by grafting, into young NOD mice whose own thymus was left intact, newborn NOD thymuses containing allogeneic pancreatic islets. If the NOD defect represented poor negative selection, these animals would develop disease at control rates, as the generation of autoreactive T cells proceeds undisturbed in the autologous thymus. In contrast, if NOD thymuses are defective in the production of T regulatory cells, lower disease incidence is expected in the chimeras, as more protective cells can be produced in the grafted thymus. The results show a reduced incidence of diabetes in the chimeras (24%) as compared with control (72%) NOD mice, throughout adult life. We conclude that amelioration of NOD mice by intrathymic islet grafts is not caused by enhanced negative selection and suggest that autoimmune diabetes in this system is the result of inefficient generation of T regulatory cells in the thymus.

IL-10 is required for regulatory T cells to mediate tolerance to alloantigens in vivo

We present evidence that donor-reactive CD4(+) T cells present in mice tolerant to donor alloantigens are phenotypically and functionally heterogeneous. CD4(+) T cells contained within the CD45RB(high) fraction remained capable of mediating graft rejection when transferred to donor alloantigen-grafted T cell-depleted mice. In contrast, the CD45RB(low) CD4(+) and CD25(+)CD4(+) populations failed to induce rejection, but rather, were able to inhibit rejection initiated by naive CD45RB(high) CD4(+) T cells. Analysis of the mechanism of immunoregulation transferred by CD45RB(low) CD4(+) T cells in vivo revealed that it was donor Ag specific and could be inhibited by neutralizing Abs reactive with IL-10, but not IL-4. CD45RB(low) CD4(+) T cells from tolerant mice were also immune suppressive in vitro, as coculture of these cells with naive CD45RB(high) CD4(+) T cells inhibited proliferation and Th1 cytokine production in response to donor alloantigens presented via the indirect pathway. These results demonstrate that alloantigen-specific regulatory T cells contained within the CD45RB(low) CD4(+) T cell population are responsible for the maintenance of tolerance to donor alloantigens in vivo and require IL-10 for functional activity.

Germ-line selection ensures embryonic autoreactivity and a positive discrimination of self mediated by supraclonal mechanisms

It is necessary to clarify principles and mechanisms of natural tolerance to body tissues, in order to derive appropriate diagnostics, therapeutics and prognostics of autoimmune diseases (AID). I will argue that AIDs result from deficits in autoreactive regulatory T cell generation and/or function, and propose a model that explains why relatively few prototypes of AID exist, as well as their organ-specificity or systemic nature. The model suggests that natural tolerance is achieved through evolutionarily selected developmental genetic programs: (i) for patterns of V-region expression early in life that ensure auto(multi)reactivity at the outset of the system; (ii) for a cellular composition of thymic stroma that 'breeds' and activates regulatory (autoreactive) T cells in early development; (iii) for lymphocyte differentiation and population dynamics, that results in peripheral 'education' of regulatory tissue-specific cells, while allowing for 'unregulated' clonal responses to nonself. In the present model, S/NS discrimination is 'supraclonal' and 'dominant', related to other 'systemic' properties such as the regulation of total lymphocyte numbers, the 'open-endedness' of repertoires, and their differences in health and disease. Dominant tolerance models in general, also solve the paradox that pathogenic autoreactivity is rare, in spite of the extensive V-region degeneracy of lymphocyte recognition and the high frequency of cross-reactivity between S/NS; in short, it is astonishing that we are not autoimmune every time we get infected. As in other areas of biomedical science, time is perhaps ripe to move from component (clonal) analysis to system's biology, as some have proned for years.

The third function of the thymus

Nonresponsiveness of mammalian T cells to self-antigens is not totally accounted for in terms of clonal deletion, T-cell anergy and T-cell ignorance: studies have shown that the T-cell repertoire of healthy individuals contains cells with the potential to cause autoimmune disease. This article describes a T-cell-mediated mechanism that prevents the realization of this potential and indicates how its failure can lead to the development of autoimmunity.

Role of Immunoregulatory Donor T Cells in Suppression of Graft-Versus-Host Disease Following Donor Leukocyte Infusion Therapy

In murine models of allogeneic bone marrow transplantation (BMT), MHC-mismatched recipients given a delayed infusion of donor leukocytes (DLI) at 21 days posttransplant develop significant GVHD whereas MHC-matched recipients do not. The current study was initially designed to test the hypothesis that small numbers of T cells in the MHC-mismatched donor bone marrow (BM) graft exacerbated graft-vs-host disease (GVHD) when DLI was administered at 21 days after BMT. Ex vivo depletion of Thy1+ cells from the donor BM had no impact on the severity of GVHD after DLI. However, depletion of donor T cells in vivo with a Thy1 allele-specific mAb given after BMT resulted in significantly more severe GVHD after DLI. Similar results were obtained in a MHC-matched model of allogeneic BMT, indicating that this was a general phenomenon and not model dependent. These results indicated that a population of donor-derived Thy1+ cells suppressed graft-vs-host reactivity after DLI. Results of experiments with thymectomized recipients demonstrated that an intact thymus was required for generation of the immunoregulatory donor cells. Experiments using TCR β-chain knockout mice as BM donors indicated that the immunosuppressive Thy1+ cells coexpressed αβTCR heterodimers. Similar experiments with CD4 and CD8 knockout donor BM suggested that the immunoregulatory Thy1+αβTCR+ cells consisted of two subpopulations: a CD4+CD8− subpopulation and a CD4−CD8− subpopulation. Together, these results show that thymus-derived, Thy1+αβTCR+ donor cells generated early after allogeneic BMT suppress the graft-vs-host reactivity of T cells given as DLI. These cells may mediate dominant peripheral tolerance after allogeneic BMT.

What is the role of regulatory T cells in transplantation tolerance?

There has been considerable recent progress in the characterization of the regulatory T cells that mediate tolerance in a number of transplantation models. The conditions that facilitate the generation of regulatory T cells point to the thymus, the nature of immune suppression and the dose of immunosuppressive agent(s) being important. Putative mechanisms of immune regulation by regulatory T cells, particularly in the 'infectious' tolerance pathway, include Th2-type cytokines (IL-4, IL-10 and transforming growth factor beta) that may play a direct role as an indispensable requirement or may contribute indirectly as a favorable milieu for acquisition of tolerance. Anergic T cells may suppress immune responses via either cytokine competition or antigen-presenting cells. Models of autoimmune disease, in which regulatory T cells were shown to represent a distinct thymus-derived T cell subset, also suggest the role of antigen-presenting cells in mediating immune suppression. Progress has also been made in generating and characterizing regulatory T cells in vitro.

Cutting Edge: Linked Suppression of Skin Graft Rejection Can Operate Through Indirect Recognition

Adult mice can be rendered immunologically tolerant of allogeneic tissues if transplanted under cover of mAbs to CD4 and CD8. Tolerance generated in this manner is characterized by the presence of regulatory CD4+ T cells that can recruit naive T cells to become tolerant also through “infectious tolerance.” Regulatory CD4+ T cells can also suppress rejection of third party transplant Ags provided they are expressed on the same graft as the tolerated Ags. This process of linked suppression can act across whole MHC barriers and represents a powerful mechanism with therapeutic potential. Tolerance can also be induced to reprocessed minor transplantation Ags presented through host APCs (indirect recognition). We here demonstrate that linked suppression can also be induced through the indirect pathway. This finding may be important in the development of transplantation tolerance in the clinic.

Intrathymic expression of genes involved in organ specific autoimmune disease

Insulin, thyroglobulin and myelin basic protein (MBP) are implicated as autoantigens in the autoimmune diseases, insulin-dependent diabetes mellitus (IDDM), autoimmune thyroid-disease and multiple sclerosis. Self tolerance to these antigens, until recently only thought to be present extrathymically, is generally considered to be maintained by 'peripheral' mechanisms, such as clonal anergy or clonal ignorance. The techniques of reverse transcription and polymerase chain reaction (RT-PCR) were used to investigate the intrathymic expression of these genes. Expression was examined in mRNA isolated from complete adult rat thymus, various mouse thymic cell-types isolated from fetal thymic-organ cultures and from neonatal-mouse thymocyte subsets. mRNA for insulin, thyroglobulin and MBP were detected in unfractionated adult rat and embryonic mouse thymus. Rat thymus expressed both insulin I and II, while mouse thymus only expressed insulin II. Thyroglobulin and MBP, but not insulin mRNA were detected in mouse MHC class II+ thymic epthelial cells and class II+ dendritic cells and in certain thymocyte subsets. The presence of insulin, thyroglobubin and MBP mRNA in the thymus has important implications for the development of the T-cell repertoire, particularly for the mechanisms of tolerance that prevent autoreactivity to these antigens in healthy individuals.

Role of Maternal Ig in the Induction of Cκ-Specific CD8+ T Cell Tolerance

Although the influence of maternal Ig on the B cell repertoire and subsequent Ab response has been extensively studied, much less attention has been devoted to their effects on T cell responses of the offspring. To address this question, we have studied the influence of maternal κ-positive Ig (Igκ) on the Cκ-specific CD8+ T cell response of κ knock-out (κ−/−) pups resulting from various crosses and foster nursings. These systems allowed control of physiologic transmission of Igκ at defined periods of ontogeny. Our data show that conventional transfer of maternal Ig via the placenta plus colostrum/milk or adoptive transfer via only the colostrum/milk were the most efficient at tolerizing Cκ-specific CD8+ responses. Surprisingly, tolerance was not detected in κ−/− pups born to κ+/− females obtained by cesarean delivery and suckled by κ−/− mothers (transplacental supply only). Tolerance, which was strong until 5 wk of age, was reversible and waned with the decrease of Igκ serum concentration. Depletion of CD4+ T cells at the time of Cκ peptide immunization abolished the tolerance of Cκ-specific CD8+ T cells. These data suggest that an oral supply of Ig is very efficient at inducing and maintaining tolerance of Cκ-specific CD8+ T cells, at least for several weeks after birth, and that suppression rather than deletion is responsible for this tolerance. In addition, they strengthen the view that tolerance of CD8+ T cells to a soluble Ag is never permanently acquired even if it is present in large quantities during ontogeny.

Abnormal T cell selection on nod thymic epithelium is sufficient to induce autoimmune manifestations in C57BL/6 athymic nude mice

To investigate the role of primary T cell repertoire selection in the immunopathogenesis of autoimmune diseases, pure thymic epithelium (TE) from nonobese diabetic (NOD) embryos was grafted into non autoimmune prone newborn C57BL/6 athymic mice. The results show that NOD TE selects host T cell repertoires that establish autoimmunity in otherwise nondiabetic animals. Thus, such chimeras regularly show CD4 and CD8 T cell-mediated insulitis and sialitis, in contrast with syngeneic or allogeneic chimeras produced with TE from nonautoimmune strains. This is the first demonstration that autoimmunity to pancreatic beta cells and salivary glands can be established by the sole alteration of the thymic environment involved in T cell selection, regardless of the nature and presentation of both major histocompatibility complex and tissue-specific antigens on the target organ. These data indicate that T cell repertoire selection by the NOD thymic epithelium is sufficient to induce specific autoimmune characteristics in the context of an otherwise normal host.

Establishment of tissue-specific tolerance is driven by regulatory T cells selected by thymic epithelium

Grafts of thymic epithelium (TE) rudiments restore T cell development and function in allogeneic athymic mice. These TE chimeras are specifically tolerant to grafts of peripheral tissues (e.g. skin and heart) from the TE donor strain, although they harbor peripheral immunocompetent T cells capable of rejecting those grafts. Initial analysis has shown that TE chimeras also harbor TE-selected CD4 T lymphocytes that inhibit graft rejection by tissue-reactive T cells in immunocompetent recipients. Peripheral tolerance in TE chimeras is thus maintained by dominant mechanisms dependent on regulatory CD4 T lymphocytes. Here we show that TE-selected regulatory T cells recruit nontolerant tissue-reactive CD4 and CD8 T cells to express similar regulatory functions. Only recent thymic emigrants, but not peripheral resident mature T cells are susceptible to this process of functional education, which also requires exposure to specific antigens and occurs entirely in the periphery. We propose that these mechanisms play a major role in establishing and maintaining natural self tolerance to tissue-specific antigens.

A model for developmentally acquired thymus-dependent tolerance to central and peripheral antigens

Current models of tolerance to peripheral, tissue-specific antigens contain some major caveats. First, they consider peripheral tolerance independently from intrathymic T cell selection, a dichotomy that is challenged by observations on TE-induced tolerance. Second, they do not account for the fact that vertebrates are more readily tolerised in development than in adult life. Third, they do not explain the fact that embryonic/neonatal tolerance to foreign tissues can only be induced by HC or TE. A model of thymic selection and peripheral tolerance is developed here that resolves those problems, by assuming two classes of T cell effector functions, one being regulatory and the other aggressive. Three postulates are required: (1) both epithelial and hemopoietic cellular compartments of the thymic stroma can support both positive and negative selection of T cells, but with vastly different avidity requirements and efficiency; (2) positively selected T cells with the highest avidity that escape deletion are activated intrathymically and irreversibly committed for regulatory effector functions; (3) the functional phenotype of all other thymic emigrants is determined in the periphery upon encounter with antigen. Functional commitment in the periphery depends on the maturity stage (RTE or PMR) of the immunocompetent cell, on the nature of the antigen-presenting cells, and on the effector classes of other T lymphocytes interacting on the same presenting cell. This model explains a number of observations on experimental autoimmune disease and transplantation tolerance, and it contains several readily testable predictions.

Evidence for a thymus-dependent form of tolerance that is not based on elimination or anergy of reactive T cells

The avian embryo has provided an appropriate model to study the ontogeny of the primary lymphoid organs, thymus and bursa of Fabricius. By using the quail-chick marker system the embryonic origin of the highly intricate cell components which form these organs could be traced back to the initial endodermal, mesodermal and ectodermal germ layers. The timing and dynamics of the incoming and outcoming flows of hemopoietic cells which characterize their lymphopoietic activity could be revealed in both quail and chick embryos. This knowledge served as a basis for an investigation on the role of the epithelial component of the thymus (derived from the pharyngeal endoderm) on tolerance to tissue graft and, by extension, tolerance to self. When this work was undertaken, the prevailing view was that exposure of the developing immune system to foreign antigens in the embryo allows them to be assimilated to self components in the mature animal. In fact, this was found to be true for allogeneic grafts between MHC-distinct chickens, of certain tissues, such as for instance wing tissues. However, in heterospecific transplantations, i.e. when a limb bud was grafted from quail to chick embryos, the chick host acutely rejected the foreign limb soon after birth. In contrast, grafts of the quail thymic epithelial (TE) rudiment resulted in the development of a chimeric thymus in which the foreign epithelial component was not only tolerated but able to induce full tolerance of the grafted wing from the same donor. By monitoring the amount of quail TE implanted we showed in addition that only part of the peripheral T-cell population had to differentiate in the context of the quail epithelial cells to induce tolerance to quail tissues. This pointed to the generation in the thymus of regulatory T cells, coexisting with specific anti-quail reactive T cells, but able to inhibit them from reacting against the quail wing antigenic determinants. A mammalian model was then devised to further study this mechanism of tolerance that we have qualified as "dominant" by opposition to the current model based on either clonal elimination or anergy which can be considered as recessive or passive. Nude mice of MHC type A were grafted with TE of E10 type B embryos. They became reconstituted for T-cell function but tolerant for B skin allografts. Spleen cells from such tolerant animals injected to naive A nude mice reconstituted T cell function in the recipient and transferred the tolerance to B skin grafts. Reducing the number of donor cells resulted in the segregation of the two phenomena. For low numbers the recipients were restored but not tolerant, thus showing the coexistence in the tolerant donor of anti-B reactive T cells together with regulatory cells able to abolish their reactivity against B determinants. Other experiments demonstrated that TE-induced tolerance does not rely on clonal deletion or anergy. This was shown on systems where elimination of cells directed toward superantigens was screened. It turned out that tolerance to skin grafts and superantigen T-cell deletion are unrelated phenomena. These observations strongly suggest that tolerance to self results at least in part from the interplay between cells potentially harmful for self component and others which exert a strong control on their reactivity. The latter cell type depends upon interactions of thymocytes with the endodermal component of the thymus.