Negative selection of semimature CD4(+)8(-)HSA+ thymocytes requires the BH3-only protein Bim but is independent of death receptor signaling

T cell receptor/CD3 ligation induces apoptosis in semimature CD4(+)8(-)HSA+ thymocytes, and this helps establish immunological tolerance and constitutes one of the safeguards against autoimmune disease. We analyzed several knockout and transgenic mouse lines and found that T cell receptor/CD3-ligation-induced killing of semimature thymocytes occurred independently of Fas and "death receptor" signaling in general but required the proapoptotic BH3-only protein Bim and could be inhibited by Bcl-2. Loss of Apaf-1 or caspase-9, which act downstream of the Bcl-2 family protein family, provided only minor protection, indicating that the "apoptosome" functions as an amplifier rather than as an essential initiator of this death program. These results reveal the mechanisms of apoptosis in negative selection of semimature thymocytes and have implications for immunological tolerance and autoimmunity.

Commentary: TCR–MHC/peptide interactions: kissing-cousins or a shotgun wedding?

The purpose of this Commentary is to put into modern-day perspective Jerne's hypothesis that antigen receptors encoded in the genome have been evolutionarily selected for their ability to react with major histocompatibility proteins and that the process of eliminating self reactivity is the catalyst for the generation of diversity of antigen receptors. In writing his hypothesis Jerne was trying to deal with the obsession of the immune system with the MHC, an obsession that was manifest in his days by the strong reactions of the immune system with allogeneic MHC proteins. However, Jerne's hypothesis also took on other issues that were not understood at the time--issues that included lymphocyte selection and tolerance, the generation of somatic diversity and the ability of the MHC to control responses to other antigens. In so doing, Jerne generated a hypothesis that accounted remarkably satisfactorily for what was known in 1971. Whilst the details of much of the hypothesis have since turned out to be incorrect, in his ideas Jerne did anticipate many of the most interesting and surprising findings of the subsequent 33 years.

B cell-dependent T cell development

T and B cells are thought to develop independently. While it is widely recognized that T cells help B cells in the production of antibodies to protein antigens, less well understood is whether or how B cells contribute to T cell development and function. Defects in cell-mediated immunity in individuals with B cell deficiency and in B cell-deficient mice suggest that B cells contribute to T cell function. The question of whether T cell development is B cell dependent was revisited using two novel mouse strains: mice with monoclonal T cells (MT) and mice with monoclonal compartments of both B and T cells (MBT). It was found that T cell development and thymocyte selection is modified by the presence of B cells. These results suggest that B cells, or B cell products, contribute to thymocyte selection and T cell development.

The somatic generation of immune recognition

Antibody specificity is determined by structural v-genes that code for the amino acid sequences of the variable regions of antibody polypeptide chains. The present hypothesis proposes that the germ-cells of an animal carry a set of v-genes determining the combining sites of antibodies directed against a complete set of certain class of histocompatibility antigens of the species to which this animal belongs. The evolutionary development of this set of v-genes in phylogeny is traced back to the requirements for cell to cell recognition in all metazoa. The hypothesis leads to a distinction between two populations of antigen-sensitive cells. One population consists of cells forming antibodies against foreign antigens; these lymphocytes have arisen as mutants in clones descending from lymphocytic stem cells which expressed v-genes belonging to the subset (subset S) coding for antibody against histocompatibility antigens that the individual happens to possess. The other population consists of allograft rejecting lymphocytes that express v-genes of the remaining subset (subset A) coding for antibody against histocompatibility antigens of the species that the individual does not possess. The primary lymphoid organs are viewed as mutant-breeding organs. In these organs (e.g. in the thymus), the proliferation of lymphocytes expressing the v-genes of subset S and the subsequent suppression of the cells of these "forbidden" clones, leads to the selection of mutants cells expressing v-genes that have been modified by spontaneous random somatic mutation. This process generates self-tolerance as well as a diverse population of antigen-sensitive cells that reflects antibody diversity. The proliferation in the primary lymphoid organs of lymphocytes expressing v-genes of subset A generates the antigen-sensitive cell population that is responsible for allo-aggression. The theory explains how a functional immune system can develop through a selection pressure exerted by self-antigens, starting during a period in early ontogeny that precedes clonal selection by foreign antigens. The hypothesis provides explanations for the variability of the N-terminal regions of antibody polypeptide chains, for the dominant genetic control of specific immune responsiveness by histocompatibility alleles, for the relative preponderance of antigen-sensitive cells directed against allogeneic histocompatibility antigens, for antibody-idiotypes, for allelic exclusion, for the precommitment of any given antigen-sensitive lymphocyte to form antibodies of only one molecular species and for the cellular dynamics in the primary lymphoid tissues.

T cell clonal conditioning: a phase occurring early after antigen presentation but before clonal expansion is impacted by Toll-like receptor stimulation

After in vivo immunization, Ag-specific T cells disappear from circulation and become sequestered in lymphoid tissue where they encounter Ag presented by dendritic cells. In the same site and just after Ag presentation, they "disappear" a second time and we investigated this process. Using a mouse model of T cell deletion (without Toll-like receptor (TLR) stimulation) vs survival (with TLR stimulation), Ag-specific T cells indeed became undetectable by flow cytometry, however were readily detected by immunohistochemistry. Thus, whether or not the activated T cells were destined to delete or survive, they were difficult to extract from lymphoid tissue and did not disappear but in fact were abundantly present. Nevertheless, profound differences were observed during this time period when tolerizing conditions were compared with immunizing conditions. TLR stimulation induced an increase in CD25 expression, acquisition of surface MHC class II, and abnormally high increases in forward and side scatter of the peptide-specific T cells. Using a modified adoptive transfer approach, we demonstrated by flow cytometry that in the presence of TLR stimulation the Ag-specific T cells were tightly coupled to dendritic cells, explaining the unusual increases in size and granularity. Ultimately, these events induced the specific T cells to differentiate into memory cells. We postulate that this is a stage where T cells are either conditioned to survive or to delete depending upon the activation status of the innate immune system.

In Vivo VL-Targeted Activation-Induced Apoptotic Supraclonal Deletion by a Microbial B Cell Toxin

To interfere with host immune responses, some microbial pathogens produce proteins with the properties of superantigens, which can interact via conserved V region framework subdomains of the Ag receptors of lymphocytes rather than the complementarity-determining region involved in the binding of conventional Ags. In recent studies, we have elucidated how a model B cell superantigen affects the host immune system by targeting a conserved V(H) site on the Ag receptors of B lymphocytes. To determine whether these findings represent a general paradigm, we investigated the in vivo immunobiologic properties of protein L of Peptostreptococcus magnus (PpL), a microbial Ig-binding protein specific for a V region site on Ig L chains. Our studies confirmed that PpL binding is restricted to a subset of murine Vkappa-expressing B cells, and found that B cells with stronger PpL-binding activity are associated with certain B cell subsets: splenic marginal zone (CD21(high) CD23(low)), splenic CD1(+), peritoneal B-1a (IgD(low) CD5(+)), and CD21(high) CD24(high) B cells in peripheral lymph nodes, mesenteric lymph nodes, and Peyer's patches. Infusion of PpL triggered a sequence of events in B cell receptor (BCR)-targeted B cells, with rapid down-regulation of BCR, the induction of an activation phenotype, and limited rounds of proliferation. Apoptosis followed through a process heralded by the dissipation of mitochondrial membrane potential, the induction of the caspase pathway, DNA fragmentation, and the deposition of B cell apoptotic bodies. These studies define a common pathway by which microbial toxins that target V region-associated BCR sites induce programmed cell death.

Tolerance-inducing strategies in transplantation surgery?current status and perspectives

BACKGROUND

Life-long immunosuppressive medication has to be administered to the majority of solid-organ recipients after transplantation of genetically mismatched organs in order to circumvent acute graft loss due to alloreactive rejection responses triggered by the host's immune system. However, life-long suppression of the immune system implicitly limits the host's ability to respond appropriately to infectious, fungal and carcinogenic threats. Simultaneously non-targeted inhibition of immunological defense mechanisms coincides with substantial morbidity and mortality for the host. Thus, for the past five decades research in the field of transplantation medicine has focused on innovative strategies to induce graft tolerance to donor alloantigens, a state in which the recipient's lymphocytes have learned to accept the foreign organ or tissue as "self" or "non-dangerous" without the need of chronic immunosuppression. Achieving that specific goal of donor-specific tolerance would not only minimize the risk of the recipient to suffer from serious side effects resulting from continuous immunosuppressive therapy, but would also prevent loss of long-term graft function caused by chronic rejection processes. Recently, numerous insights into the dynamic interrelationships of host immune responses elicited by donor antigen-presentation, either on the graft itself or on specialized antigen-presenting cells, have substantially broadened our understanding of the cascade of events that result in the acquisition of tolerance.

METHOD

We highlight areas of research that are currently particularly helpful not only to set up new strategies to induce donor-specific tolerance or long-term graft acceptance, but also to identify and describe parameters which serve to characterize those patients who have acquired a state of tolerance and are safe to be weaned off from their immunosuppressive regimen.

A fail-safe mechanism for negative selection of isotype-switched B cell precursors is regulated by the Fas/FasL pathway

In B lymphocytes, immunoglobulin (Ig)M receptors drive development and construction of naive repertoire, whereas IgG receptors promote formation of the memory B cell compartment. This isotype switching process requires appropriate B cell activation and T cell help. In the absence of T cell help, activated B cells undergo Fas-mediated apoptosis, a peripheral mechanism contributing to the establishment of self-tolerance. Using Igμ-deficient μMT mouse model, where B cell development is blocked at pro-B stage, here we show an alternative developmental pathway used by isotype-switched B cell precursors. We find that isotype switching occurs normally in B cell precursors and is T independent. Ongoing isotype switching was found in both normal and μMT B cell development as reflected by detection of IgG1 germline and postswitch transcripts as well as activation-induced cytidine deaminase expression, resulting in the generation of IgG-expressing cells. These isotype-switched B cells are negatively selected by Fas pathway, as blocking the Fas/FasL interaction rescues the development of isotype-switched B cells in vivo and in vitro. Similar to memory B cells, isotype-switched B cells have a marginal zone phenotype. We suggest a novel developmental pathway used by isotype-switched B cell precursors that effectively circumvents peripheral tolerance requirements. This developmental pathway, however, is strictly controlled by Fas/FasL interaction to prevent B cell autoimmunity.

Cell-cycle dysregulation in the immunopathogenesis of AIDS

For a number of years the pathogenesis of AIDS was thought to be essentially related to direct human immunodeficiency virus (HIV)-mediated killing of CD4+ T cells. More recently, attention has shifted to pathogenic models that emphasize the role of generalized immune system activation and the excess apoptosis of uninfected T cells in inducing HIV-associated CD4+ T-cell depletion. The main focus of our research is to better define the determinants and the consequences of these "indirect" mechanisms of immunodeficiency by studying both HIV-infected patients and nonhuman primates infected with simian immunodeficiency virus (SIV). We have discovered that pathogenic models of retroviral infections of primates (i.e., HIV infection in humans and SIV infection in rhesus macaques) are associated with the presence of a set of perturbations of normal cell-cycle control in T lymphocytes. These perturbations, to which we collectively refer to as cell-cycle dysregulation, or CCD, may represent an important biological link between chronic immune activation and excess apoptosis and therefore may play a significant role in the pathogenesis of AIDS. A better understanding of the determinants and consequences of CCD may pave the way for the introduction of new therapeutic strategies to be used in addition to standard antiretroviral therapy in HIV-infected patients.

Negative selection and autoimmunity

The impaired elimination of self-reactive T cells is one factor that contributes to autoimmunity. Although the mechanism of thymic negative selection has been studied for decades, recent data demonstrate that the mechanisms underlying this fundamental process remain extremely controversial. Nonetheless, new models such as the Aire-deficient mice have demonstrated the importance of thymic negative selection in autoimmune disease progression in mice and humans.

Immunity and the Animation of the Genome

Expression of genes in the right place at the right time is fundamental to all of life. The ontogeny of effector and memory T cells is a robust example of this important principle. Although lymphocytes represent a late addition in animal evolution, they are serving as an exceptional model system to unlock the mysteries surrounding the way in which the genome is animated. It will be speculated here that further insight into fundamental principles of genome function may arise from study of immunity. Likewise, such information may be key to understanding and altering immunobiology.

Deletion, but not anergy, is involved in TGF-beta-treated antigen-presenting cell-induced tolerance

Intravenous injection of transforming growth factor (TGF-)-beta-treated antigen-presenting cells (APC) pulsed with antigen induces antigen-specific tolerance in both naive and previously primed mice. Although TGF-beta-treated APC-induced tolerance is associated with induction of regulatory T cells and impaired delayed-type hypersensitivity (DTH) responses, the specific mechanisms that mediate this tolerance are not currently known. The goal of the present report was to study the mechanisms involved in TGF-beta-treated APC-induced tolerance by determining the fate of the antigen-specific effector T cells that are regulated. Using a well-characterized system that allows tracking of small numbers of TCR transgenic T cells, we have found that antigen-specific T cell expansion, either in vivo or in vitro, is inhibited in mice that have been injected with TGF-beta-treated APC. The failure of antigen-specific effector T cells to expand did not appear to be due to the induction of anergy, since carboxyfluorescein diacetate succinimidyl ester (CFSE)-labeled cells divided normally in response to antigen and adjuvant in vivo, and addition of exogenous IL-2 was unable to restore T cell expansion in in vitro assays. Interestingly, the percentage of CFSE-labeled cells was decreased after >7-8 divisions following culture in vitro, which correlated with a significant increase in cell death. Cell death was prevented and the ability to expand in vitro was restored by treatment with anti-Fas ligand (FasL) antibody. In conclusion, tolerance induced by TGF-beta-treated APC appears to be associated with deletion of antigen-specific T cells involving the Fas-FasL pathway.

A role for the B7-1/B7-2:CD28/CTLA-4 pathway during negative selection

Although costimulation plays an important role in activating naive T cells, its role in negative selection is controversial. By following thymocyte deletion induced by endogenous superantigens in mice lacking B7-1 and/or B7-2, we have identified a role for both B7-1 and B7-2 in negative selection. Studies using CD28-deficient and CD28/CTLA-4-double-deficient mice have revealed that either CD28 or another as yet undefined coreceptor can mediate these B7-dependent signals that promote negative selection. Finally, CTLA-4 delivers signals that inhibit selection, suggesting that CTLA-4 and CD28 have opposing functions in thymic development. Combined, the data demonstrate that B7-1/B7-2-dependent signals help shape the T cell repertoire.

Activation-induced cell death in T cells

A properly functioning immune system is dependent on programmed cell death at virtually every stage of lymphocyte development and activity. This review addresses the phenomenon of activation-induced cell death (AICD) in T lymphocytes, in which activation through the T-cell receptor results in apoptosis. AICD can occur in a cell-autonomous manner and is influenced by the nature of the initial T-cell activation events. It plays essential roles in both central and peripheral deletion events involved in tolerance and homeostasis, although it is likely that different forms of AICD proceed via different mechanisms. For example, while AICD in peripheral T cells is often caused by the induction of expression of the death ligand, Fas ligand (CD95 ligand, FasL), it does not appear to be involved in AICD in thymocytes. This and other mechanisms of AICD are discussed. One emerging model that may complement other forms of AICD involves the inducible expression of FasL by nonlymphoid tissues in response to activated T lymphocytes. Induction of nonlymphoid FasL in this manner may serve as a sensing mechanism for immune cell infiltration, which contributes to peripheral deletion.

Superantigen-induced regulatory T cells display different suppressive functions in the presence or absence of natural CD4+CD25+ regulatory T cells in vivo

Repeated exposures to both microbial and innocuous Ags in vivo have been reported to both eliminate and tolerize T cells after their initial activation and expansion. The remaining tolerant T cells have been shown to suppress the response of naive T cells in vitro. This feature is reminiscent of natural CD4(+)CD25(+) regulatory T cells. However, it is not known whether the regulatory function of in vivo-tolerized T cells is similar to the function of natural CD4(+)CD25(+) regulatory T cells. In this study, we demonstrate that CD4(+)CD25(+) as well as CD4(+)CD25(-) T cells isolated from mice treated with superantigen three consecutive times to induce tolerance were functionally comparable to natural CD4(+)CD25(+) regulatory T cells, albeit more potent. The different subpopulations of in vivo-tolerized CD4(+) T cells efficiently down-modulated costimulatory molecules on dendritic cells, and their suppressive functions were strictly cell contact dependent. Importantly, we demonstrate that conventional CD4(+)CD25(-) T cells could also be induced to acquire regulatory functions by the same regimen in the absence of natural regulatory T cells in vivo, but that such regulatory cells were functionally different.

Induction of tumor cell apoptosis in vivo increases tumor antigen cross-presentation, cross-priming rather than cross-tolerizing host tumor-specific CD8 T cells

Cross-presentation of cell-bound Ags from established, solid tumors to CD8 cells is efficient and likely to have a role in determining host response to tumor. A number of investigators have predicted that when tumor Ags are derived from apoptotic cells either no response, due to Ag "sequestration," or CD8 cross-tolerance would ensue. Because the crucial issue of whether this happens in vivo has never been addressed, we induced apoptosis of established hemagglutinin (HA)-transfected AB1 tumors in BALB/c mice using the apoptosis-inducing reagent gemcitabine. This shrank the tumor by approximately 80%. This induction of apoptosis increased cross-presentation of HA to CD8 cells yet neither gross deletion nor functional tolerance of HA-specific CD8 cells were observed, based on tetramer analysis, proliferation of specific CD8 T cells, and in vivo CTL activity. Interestingly, apoptosis primed the host for a strong antitumor response to a second, virus-generated HA-specific signal in that administration of an HA-expressing virus after gemcitabine administration markedly decreased tumor growth compared with viral administration without gemcitabine. Thus tumor cell apoptosis in vivo neither sequesters tumor Ags nor cross-tolerizes tumor-specific CD8 cells. This observation has fundamental consequences for the development of tumor immunotherapy protocols and for understanding T cell reactivity to tumors and the in vivo immune responses to apoptotic cells.

The possibility of B-cell-dependent T-cell development

The development of T cells is thought to be independent of B cells. However, defects in cell-mediated immunity in individuals with B-cell deficiency suggest the contrary. To test whether B cells affect T-lymphocyte development, we constructed mice with a monoclonal T-cell compartment (MT) and monoclonal B- and T-cell compartments (MBTs). In these mice, the T cells expressed a DO 11.10 transgenic (DO-T) cell receptor restricted to major histocompatibility complex (MHC) class IId. While CD4+ DO-T lymphocytes are rare in transgenic H-2b MT mice, we found that in H-2b MBT mice under the influence of B cells, DO-T lymphocytes mature into large numbers of CD4+ peripheral T cells. H-2b MBT mice have more CD4+ thymocytes than H-2b MT mice. These data are consistent with the view that B cells play some role in thymocyte development.

LIGHT (a cellular ligand for herpes virus entry mediator and lymphotoxin receptor)-mediated thymocyte deletion is dependent on the interaction between TCR and MHC/self-peptide

Negative selection serves as a major mechanism to maintain self-tolerance. We previously reported that LIGHT (a cellular ligand for herpes virus entry mediator and lymphotoxin receptor), a TNF family member, plays an important role in thymocyte development via promoting apoptosis of double-positive thymocytes. Here, we demonstrated that LIGHT-mediated deletion of thymocyte requires the strong interaction of TCR with MHC/self-peptide. Transgenic mice overexpressing LIGHT in thymocytes were bred with a transgenic mouse line expressing a TCR recognizing the H-Y male Ag in the context of H-2b class I MHC molecules. In male H-Y/LIGHT double-transgenic mice, more efficient negative selection of H-Y T cells occurred, and total thymocyte number was further reduced compared with H-Y/negative littermates. In contrast, the presence of LIGHT transgene had no evident impact on the thymocyte development of female H-Y/LIGHT double-transgenic mice. Taken together, LIGHT plays a role in negative selection of thymocytes via inducing the apoptosis of thymocytes bearing high affinity TCR during negative selection.

Aire regulates negative selection of organ-specific T cells

Autoimmune polyendocrinopathy syndrome type 1 is a recessive Mendelian disorder resulting from mutations in a novel gene, AIRE, and is characterized by a spectrum of organ-specific autoimmune diseases. It is not known what tolerance mechanisms are defective as a result of AIRE mutation. By tracing the fate of autoreactive CD4+ T cells with high affinity for a pancreatic antigen in transgenic mice with an Aire mutation, we show here that Aire deficiency causes almost complete failure to delete the organ-specific cells in the thymus. These results indicate that autoimmune polyendocrinopathy syndrome 1 is caused by failure of a specialized mechanism for deleting forbidden T cell clones, establishing a central role for this tolerance mechanism.

Ligation of retinoic acid receptor alpha regulates negative selection of thymocytes by inhibiting both DNA binding of nur77 and synthesis of bim

Negative selection refers to the selective deletion of autoreactive thymocytes. Its molecular mechanisms have not been well defined. Previous studies in our laboratory have demonstrated that retinoic acids, physiological ligands for the nuclear retinoid receptors, selectively inhibit TCR-mediated death under in vitro conditions, and the inhibition is mediated via the retinoic acid receptor (RAR) alpha. The present studies were undertaken to investigate whether ligation of RARalpha leads to inhibition of TCR-mediated death in vivo and to identify the molecular mechanisms involved. Three models of TCR-mediated death were studied: anti-CD3-mediated death of thymocytes in wild-type mice, and Ag- and bacterial superantigen-driven thymocyte death in TCR-transgenic mice expressing a receptor specific for a fragment of pigeon cytochrome c in the context of the E(k) (class II MHC) molecule. Our data demonstrate that the molecular program of both anti-CD3- and Ag-driven, but not that of superantigen-mediated apoptosis involves up-regulation of nur77, an orphan nuclear receptor, and bim, a BH3-only member of the proapoptotic bcl-2 protein family, proteins previously implicated to participate in the negative selection. Ligation of RARalpha by the synthetic agonist CD336 inhibited apoptosis, DNA binding of nur77, and synthesis of bim induced by anti-CD3 or the specific Ag, but had no effect on the superantigen-driven cell death. Our data imply that retinoids are able to inhibit negative selection in vivo as well, and they interfere with multiple steps of the T cell selection signal pathway.

Defective thymocyte apoptosis and accelerated autoimmune diseases in TRAIL-/- mice

TRAIL, the tumor necrosis factor-related apoptosis-inducing ligand, selectively induces apoptosis of tumor cells, but not most normal cells. Its role in normal, nontransformed tissues is not clear. We report here that mice deficient in TRAIL have a severe defect in thymocyte apoptosis-thus, thymic deletion induced by T cell receptor ligation is severely impaired. TRAIL-deficient mice are also hypersensitive to collagen-induced arthritis and streptozotocin-induced diabetes and develop heightened autoimmune responses. Thus, TRAIL mediates thymocyte apoptosis and is important in the induction of autoimmune diseases.

Preferential escape of subdominant CD8+ T cells during negative selection results in an altered antiviral T cell hierarchy

Negative selection is designed to purge the immune system of high-avidity, self-reactive T cells and thereby protect the host from overt autoimmunity. In this in vivo viral infection model, we show that there is a previously unappreciated dichotomy involved in negative selection in which high-avidity CD8(+) T cells specific for a dominant epitope are eliminated, whereas T cells specific for a subdominant epitope on the same protein preferentially escape deletion. Although this resulted in significant skewing of immunodominance and a substantial depletion of the most promiscuous T cells, thymic and/or peripheral deletion of high-avidity CD8(+) T cells was not accompanied by any major change in the TCR V beta gene family usage or an absolute deletion of a single preferred complementarity-determining region 3 length polymorphism. This suggests that negative selection allows high-avidity CD8(+) T cells specific for subdominant or cryptic epitopes to persist while effectively deleting high-avidity T cells specific for dominant epitopes. By allowing the escape of subdominant T cells, this process still preserves a relatively broad peripheral TCR repertoire that can actively participate in antiviral and/or autoreactive immune responses.

Cutting edge: identification of the targets of clonal deletion in an unmanipulated thymus

Autoreactive thymocytes can be eliminated by clonal deletion during their development in the thymus. The precise developmental stage(s) at which clonal deletion occurs in a normal thymus has been difficult to assess, in large part because of the absence of a specific marker for TCR-mediated apoptosis. In this report, we reveal that Nur77 expression can be used as a specific marker of clonal deletion in an unmanipulated thymus and directly identify TCRintCD4+CD8+ and semimature CD4+CD8- thymocytes as the principal targets of deletion. These data indicate that clonal deletion normally occurs at a relatively late stage of development, as cells mature from CD4+CD8+ thymocytes to single-positive T cells.