Immunological function of a defined T-cell population tolerized to low-affinity self antigens

Designing Multivalent and Multispecific Biologics

In the era of precision medicine, multivalent and multispecific therapeutics present a promising approach for targeted disease intervention. These therapeutics are designed to interact with multiple targets simultaneously, promising enhanced efficacy, reduced side effects, and resilience against drug resistance. We dissect the principles guiding the design of multivalent biologics, highlighting challenges and strategies that must be considered to maximize therapeutic effect. Engineerable elements in multivalent and multispecific biologic design-domain affinities, valency, and spatial presentation-must be considered in the context of the molecular targets as well as the balance of important properties such as target avidity and specificity. We illuminate recent applications of these principles in designing protein and cell therapies and identify exciting future directions in this field, underscored by advances in biomolecular and cellular engineering and computational approaches. Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering , Volume 15 is June 2024. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Nanoparticle-enabled innate immune stimulation activates endogenous tumor-infiltrating T cells with broad antigen specificities

Tumors are often infiltrated by T lymphocytes recognizing either self- or mutated antigens but are generally inactive, although they often show signs of prior clonal expansion. Hypothesizing that this may be due to peripheral tolerance, we formulated nanoparticles containing innate immune stimulants that we found were sufficient to activate self-specific CD8+ T cells and injected them into two different mouse tumor models, B16F10 and MC38. These nanoparticles robustly activated and/or expanded antigen-specific CD8+ tumor-infiltrating T cells, along with a decrease in regulatory CD4+ T cells and an increase in Interleukin-17 producers, resulting in significant tumor growth retardation or elimination and the establishment of immune memory in surviving mice. Furthermore, nanoparticles with modification of stimulating human T cells enabled the robust activation of endogenous T cells in patient-derived tumor organoids. These results indicate that breaking peripheral tolerance without regard to the antigen specificity creates a promising pathway for cancer immunotherapy.

CD4+ T Cell Help Selectively Enhances High-Avidity Tumor Antigen-Specific CD8+ T Cells

Maintaining antitumor immunity remains a persistent impediment to cancer immunotherapy. We and others have previously reported that high-avidity CD8(+) T cells are more susceptible to tolerance induction in the tumor microenvironment. In the present study, we used a novel model where T cells derived from two independent TCR transgenic mouse lines recognize the same melanoma antigenic epitope but differ in their avidity. We tested whether providing CD4(+) T cell help would improve T cell responsiveness as a function of effector T cell avidity. Interestingly, delivery of CD4(+) T cell help during in vitro priming of CD8(+) T cells improved cytokine secretion and lytic capacity of high-avidity T cells, but not low-avidity T cells. Consistent with this observation, copriming with CD4(+) T cells improved antitumor immunity mediated by higher avidity, melanoma-specific CD8(+) T cells, but not T cells with similar specificity but lower avidity. Enhanced tumor immunity was associated with improved CD8(+) T cell expansion and reduced tolerization, and it was dependent on presentation of both CD4(+) and CD8(+) T cell epitopes by the same dendritic cell population. Our findings demonstrate that CD4(+) T cell help preferentially augments high-avidity CD8(+) T cells and provide important insight for understanding the requirements to elicit and maintain durable tumor immunity.

Dynamic tuning of lymphocytes: physiological basis, mechanisms, and function

Dynamic tuning of cellular responsiveness as a result of repeated stimuli improves the ability of cells to distinguish physiologically meaningful signals from each other and from noise. In particular, lymphocyte activation thresholds are subject to tuning, which contributes to maintaining tolerance to self-antigens and persisting foreign antigens, averting autoimmunity and immune pathogenesis, but allowing responses to strong, structured perturbations that are typically associated with acute infection. Such tuning is also implicated in conferring flexibility to positive selection in the thymus, in controlling the magnitude of the immune response, and in generating memory cells. Additional functional properties are dynamically and differentially tuned in parallel via subthreshold contact interactions between developing or mature lymphocytes and self-antigen-presenting cells. These interactions facilitate and regulate lymphocyte viability, maintain their functional integrity, and influence their responses to foreign antigens and accessory signals, qualitatively and quantitatively. Bidirectional tuning of T cells and antigen-presenting cells leads to the definition of homeostatic set points, thus maximizing clonal diversity.

Apoptosis-regulated low-avidity cancer-specific CD8(+) T cells can be rescued to eliminate HER2/neu-expressing tumors by costimulatory agonists in tolerized mice

A major barrier to vaccines in cancer treatment is their failure to activate and maintain a complete cancer-specific CD8(+) effector T-cell repertoire. Low-avidity T cells are more likely to escape clonal deletion in the thymus when compared with high-avidity T cells, and therefore comprise the major population of effector T cells available for activation in patients with cancer. However, low-avidity T cells fail to traffic into the tumor microenvironment and function in eradicating tumor under optimal vaccination conditions as opposed to high-avidity T cells that escape clonal deletion and function in tumor killing. We used high- and low-avidity T-cell receptor transgenic CD8(+) T cells specific for the immunodominant epitope HER2/neu (RNEU420-429) to identify signaling pathways responsible for the inferior activity of the low-avidity T cells. Adoptive transfer of these cells into tumor-bearing vaccinated mice identified the members of apoptosis pathways that are upregulated in low-avidity T cells. The increased expression of proapoptotic proteins by low-avidity T cells promoted their own cell death and also that of other tumor-specific CD8(+) T cells within their local environment. Importantly, we show that this proapoptotic effect can be overcome by using a strong costimulatory signal that prevents the activation-induced cell death and enables the low-avidity T cells to traffic into the tumor and assist in tumor clearance. These findings identify new therapeutic opportunities for activating the most potent anticancer T-cell responses.

High-avidity T cells are preferentially tolerized in the tumor microenvironment

One obstacle in eliciting potent antitumor immune responses is the induction of tolerance to tumor antigens. TCR(lo) mice bearing a TCR transgene specific for the melanoma antigen tyrosinase-related protein-2 (TRP-2, Dct) harbor T cells that maintain tumor antigen responsiveness but lack the ability to control melanoma outgrowth. We used this model to determine whether higher avidity T cells could control tumor growth without becoming tolerized. As a part of the current study, we developed a second TRP-2-specific TCR transgenic mouse line (TCR(hi)) that bears higher avidity T cells and spontaneously developed autoimmune depigmentation. In contrast to TCR(lo) T cells, which were ignorant of tumor-derived antigen, TCR(hi) T cells initially delayed subcutaneous B16 melanoma tumor growth. However, persistence in the tumor microenvironment resulted in reduced IFN-γ production and CD107a (Lamp1) mobilization, hallmarks of T-cell tolerization. IFN-γ expression by TCR(hi) T cells was critical for upregulation of MHC-I on tumor cells and control of tumor growth. Blockade of PD-1 signals prevented T-cell tolerization and restored tumor immunity. Depletion of tumor-associated dendritic cells (TADC) reduced tolerization of TCR(hi) T cells and enhanced their antitumor activity. In addition, TADCs tolerized TCR(hi) T cells but not TCR(lo) T cells in vitro. Our findings show that T-cell avidity is a critical determinant of not only tumor control but also susceptibility to tolerization in the tumor microenvironment. For this reason, care should be exercised when considering T-cell avidity in designing cancer immunotherapeutics.

Mammalian target of rapamycin integrates diverse inputs to guide the outcome of antigen recognition in T cells

T cells must integrate a diverse array of intrinsic and extrinsic signals upon Ag recognition. Although these signals have canonically been categorized into three distinct events--Signal 1 (TCR engagement), Signal 2 (costimulation or inhibition), and Signal 3 (cytokine exposure)--it is now appreciated that many other environmental cues also dictate the outcome of T cell activation. These include nutrient availability, the presence of growth factors and stress signals, as well as chemokine exposure. Although all of these distinct inputs initiate unique signaling cascades, they also modulate the activity of the evolutionarily conserved serine/threonine kinase mammalian target of rapamycin (mTOR). Indeed, mTOR serves to integrate these diverse environmental inputs, ultimately transmitting a signaling program that determines the fate of newly activated T cells. In this review, we highlight how diverse signals from the immune microenvironment can guide the outcome of TCR activation through the activation of the mTOR pathway.

Junctional diversity prevents negative selection of an antigen-specific T cell repertoire

Endogenous mouse mammary tumor proviruses (MMTV; Mtv loci) deletes Vbeta6 expressing T cells in the thymus of Mtv-7(+) DBA/2 (H2(d)) mice through negative selection. We found that in Mtv-7(-) BALB/c (H2(d)) mice, Vbeta6 is a dominant V gene used in T cell responses to an 18 amino acid long peptide antigen: EYKEYAEYAEYAEYAEYA [abbreviated as K5 or EYK(EYA)(5)]. It was therefore surprising to find that despite the deletion of Vbeta6+ T cells, vigorous K5 specific T cell responses that use Vbeta6 can be raised in DBA/2 mice. Sequence analysis of Vbeta6 junctional diversity in K5 specific T cell lines revealed that the DBA/2 K5 repertoire compensates for the loss of most Vbeta6 T cells by overusing and amplifying Vbeta6+ T cells escaping central deletion and peripheral tolerization. In order to address the inability of some Vbeta6 T cells to recognize Mtv-7(+) we analyzed a panel of BALB/c Vbeta6 expressing T cell hybridomas. This data supported the argument that certain Vbeta6 junctional sequences preclude Mtv recognition and allows their escape from central deletion in DBA/2 mice. These cells are not anergic and can be activated with cognate peptide antigen in periphery. We suggest that junctional diversity at the V region of some of the T cell receptors does not allow these cells to recognize self-superantigens with high enough affinity and thus they escape negative selection in the thymus. These results for the first time provide a molecular explanation of how the immune system compensates for "hole in the repertoire" caused by deletion of the majority of T cells carrying certain V region segments.

Melanoma progression despite infiltration by in vivo-primed TRP-2-specific T cells

Many antigens recognized by tumor-reactive cytotoxic CD8+ T cells are self-antigens. Tyrosinase-related protein 2 (TRP-2) is a melanogenic enzyme expressed by both melanocytes and melanomas that is reported to be a candidate melanoma rejection antigen. To study the role of self-reactive CD8+ T cells in tumor immunity and autoimmunity, we generated mice that bear a T-cell receptor transgene (TCR Tg) specific for the TRP-2(180-188) epitope. TRP-2 TCR Tg mice did not spontaneously develop depigmentation despite systemic expression of TRP-2 in the skin. Peripheral T cells from these TCR Tg mice exhibited a naive phenotype and proliferated in response to TRP-2 in vitro. In addition, transfer of in vitro-activated Tg T cells reduced B16 pulmonary tumor burden, but not subcutaneous tumors. We next sought to determine the in vivo responses of the Tg T cells to endogenous and tumor-derived TRP-2. Adoptive transfer of naive TCR Tg T cells into wild-type C57BL/6 mice, in combination with a TRP-2-pulsed dendritic cell vaccine, induced proliferation of the Tg T cells and resulted in migration of the Tg T cells into a subcutaneous B16 melanoma tumor. Although these tumor-infiltrating Tg T cells remained reactive against TRP-2, they did not reduce growth of the primary subcutaneous tumor; similarly, these in vivo-primed effector cells had no significant effect on the growth of pulmonary nodules. These data demonstrate that despite in vivo priming, tumor-infiltrating T cells may fail to reduce tumor burden. Determining the basis for the inability of the tumor microenvironment to sustain effective antitumor responses will be critical for designing newer, more potent antitumor immunotherapies.

How the immune system achieves self-nonself discrimination during adaptive immunity

We propose an "Avidity Model of Self-Nonself Discrimination" in which self-nonself discrimination is achieved by both central thymic selection and peripheral immune regulation. The conceptual framework that links these two events is the understanding that both in the thymus and in the periphery the survival or the fate of T cells is determined by the avidity of the interactions between T cell receptors (TCRs) on T cells, specific to any antigens and MHC/antigen peptides presented by antigen-presenting cells (APCs). We envision that the immune system achieves self-nonself discrimination, during adaptive immunity, not by recognizing the structural differences between self versus foreign antigens, but rather by perceiving the avidity of T cell activation. Intrathymic deletion of high avidity T cell clones responding to the majority of self-antigens generates a truncated peripheral self-reactive repertoire composed of mainly intermediate and low but devoid of high avidity T cells compared with the foreign-reactive repertoire. The existence of intermediate avidity self-reactive T cells in the periphery represents a potential danger of pathogenic autoimmunity inherited in each individual because potentially pathogenic self-reactive T cells are included in the pool of intermediate avidity T cells and can often be functionally activated to elicit autoimmune diseases. The distinct composition of peripheral T cell repertoires to self versus to foreign antigens provides a unique opportunity for the immune system to discriminate self from nonself, in the periphery, by selectively downregulating intermediate avidity T cells to both self and foreign antigens. Selective downregulation of the intermediate avidity T cell populations containing the potentially pathogenic self-reactive T cells enables the immune system to specifically control autoimmune diseases without damaging the effective anti-infection immunity, which is, largely, mediated by high avidity T cells specific to the infectious pathogens. In this regard, it has been recently shown that Qa-1-restricted CD8(+) T cells selectively downregulate intermediate avidity T cells, to both self and foreign antigens, and as a consequence, specifically dampen autoimmunity yet optimize the immune response to foreign antigens. Selective downregulation of intermediate avidity T cells is accomplished via specific recognition, by the Qa-1-restricted CD8(+) T cells, of particular Qa-1/self-peptide complexes, such as Qa-1/Hsp60sp, which function as a common surrogate target structure and preferentially expressed on the activated intermediate avidity T cells. This regulatory pathway thus represents one example of the peripheral mechanisms that the immune system evolved to complete self-nonself discrimination that is achieved, imperfectly, by thymic negative selection, in order to maintain self-tolerance. The conceptual framework of the "Avidity Model" differs from, but contains intellectual wisdom of certain conceptual elements of, the "Tunable Activation Thresholds Hypothesis," the "Danger Model," and the "Ergotypic Regulation Phenomenon." It provides a unified and simple paradigm to explain various seemingly unrelated biomedical problems inherent in immunological disorders that cannot be uniformly interpreted by any currently existing paradigms. The potential impact of the conceptual framework of the "Avidity Model" on our understanding of the development and control of commonly seen autoimmune diseases is also discussed.

Systemic targeting of CpG-ODN to the tumor microenvironment with anti-neu-CpG hybrid molecule and T regulatory cell depletion induces memory responses in BALB-neuT tolerant mice

We have shown that neu transgenic mice are immunotolerant and that immunizations with dendritic cells (DC) pulsed with neu-derived antigens were not able to control tumor growth in these animals. We tested whether, by modulating the tumor microenvironment with Toll-like receptor ligands, it could be possible to induce the activation of antitumor responses in neu mice. Our results indicate that only intratumoral (i.t.) injections of CpG-ODN induce an antitumor response in neu mice. To target the CpG-ODN to the tumor site anywhere within the body, we chemically conjugated an anti-Her-2/neu monoclonal antibody (mAb) with CpG-ODN. The anti-neu-CpG hybrid molecule retained its ability to bind to Her-2/neu(+) tumors, activate DCs, and induce antitumor responses. Our results indicated that injections of anti-neu-CpG induced the rejection of primary tumors in 100% of BALB/c mice and only in approximately 30% of BALB-neuT mice. After challenging the BALB/c and BALB-neuT mice, we observed that BALB/c mice developed a protective memory response; in contrast, BALB-neuT mice succumbed to the challenge. After injections of anti-neu-CpG, T regulatory cells (T-reg) were drastically reduced at the tumor site, but a large number were still present in the lymphoid organs. When BALB-neuT mice were treated with anti-neu-CpG plus anti-GITR mAb, but not with anti-CD25 mAb, 100% of the BALB-neuT mice rejected the primary tumor and developed a protective memory response indicating the critical role of T-regs in regulating the repertoire against self antigens. Taken together, these results indicate that CpG-ODN-targeted therapy and depletion of T-regs optimally activate a primary response and generate a protective memory response against self-tumor antigens.

Vaccination with Human HER-2/neu (435-443) CTL Peptide Induces Effective Antitumor Immunity against HER-2/neu-Expressing Tumor Cells In vivo

HER-2/neu is a self-antigen expressed by tumors and nonmalignant epithelial tissues. The possibility of self-tolerance to HER-2/neu-derived epitopes has raised questions concerning their utility in antitumor immunotherapy. Altered HER-2/neu peptide ligands capable of eliciting enhanced immunity to tumor-associated HER-2/neu epitopes may circumvent this problem. The human CTL peptide HER-2/neu (435-443) [hHER-2(9(435))] represents a xenogeneic altered peptide ligand of its mouse homologue, differing by one amino acid residue at position 4. In contrast to mHER-2(9(435)), vaccination of HLA-A*0201 transgenic (HHD) mice with hHER-2(9(435)) significantly increased the frequency of mHER-2(9(435))-specific CTL and also induced strong protective and therapeutic immunity against the transplantable ALC tumor cell line transfected to coexpress HLA-A*0201 and hHER-2/neu or rHER-2/neu. Similar results were also obtained with wild-type C57BL/6 mice inoculated with HER-2/neu transfectants of ALC. Adoptive transfer of CD8(+) CTL from mice immunized with hHER-2(9(435)) efficiently protected naive syngeneic mice inoculated with ALC tumors. In conclusion, our results show that HER-2(9(435)) serves as a tumor rejection molecule. They also propose a novel approach for generating enhanced immunity against a self-HER-2/neu CTL epitope by vaccinating with xenogeneic altered peptide ligands and provide useful insights for the design of improved peptide-based vaccines for the treatment of patients with HER-2/neu-overexpressing tumors.

Identification of Cross-Reactive Peptides Using Combinatorial Libraries Circumvents Tolerance against Her-2/neu-Immunodominant Epitope

The majority of the currently defined tumor-associated Ags are often overexpressed products of normal cellular genes. Therefore, tolerance deletes high-affinity T cells directed against the TAAs, leaving only a low-affinity repertoire. We have demonstrated previously that the T cell repertoire against the immunodominant p773-782 A2.1-Her-2/neu-restricted peptide has low affinity in A2xneu mice (Her-2/neu mice crossed with A2.1/Kb mice), compared with A2xFVB mice (A2.1/Kb crossed with FVB-wild-type mice). Immunizations with this peptide have a minor impact in preventing tumor growth in A2xneu mice. Therefore, attempts to expand these responses may be of little clinical value. We hypothesized that if not all possible cross-reactive peptides (CPs) are naturally processed and presented, the possibility exists that T cells against these CPs persist in the repertoire and can be used to induce antitumor responses with higher avidity against native epitopes present on the tumor cells. We have used the positional scanning synthetic peptide combinatorial library methodology to screen the p773-782 T cell clone. The screening data identified potential amino acids that can be substituted in the primary sequences of the p773-782 peptide. The designed CPs induce CTL responses of higher affinity in A2xneu mice compared with the native p773-783 peptide. These CTLs recognize A2+-Her-2/neu(+) tumors with high efficiency. Moreover, multiple immunizations with CPs significantly prolonged the survival of tumor-bearing A2xneu mice. These results have demonstrated that it was possible to circumvent tolerance with the identification of CPs and that these peptides could be of significant clinical value.

Immunogenic HER-2/neu peptides as tumor vaccines

During the last decade, a large number of tumor-associated antigens (TAA) have been identified, which can be recognized by T cells. This has led to renewed interest in the use of active immunization as a modality for the treatment of cancer. HER-2/neu is a 185-KDa receptor-like glycoprotein that is overexpressed by a variety of tumors including breast, ovarian, lung, prostate and colorectal carcinomata. Several immunogenic HER-2/neu peptides recognized by cytotoxic T lymphocytes (CTL) or helper T lymphocytes (TH) have been identified thus far. Patients with HER-2/neu over-expressing cancers exhibit increased frequencies of peripheral blood T cells recognizing immunogenic HER-2/neu peptides. Various protocols for generating T cell-mediated immune responses specific for HER-2/neu peptides have been examined in pre-clinical models or in clinical trials. Vaccination studies in animals utilizing HER-2/neu peptides have been successful in eliminating tumor growth. In humans, however, although immunological responses have been detected against the peptides used for vaccination, no clinical responses have been described. Because HER-2/neu is a self-antigen, functional immune responses against it may be limited through tolerance mechanisms. Therefore, it would be interesting to determine whether abrogation of tolerance to HER-2/neu using appropriate adjuvants and/or peptide analogs may lead to the development of immune responses to HER-2/neu epitopes that can be of relevance to cancer immunotherapy. Vaccine preparations containing mixtures of HER-2/neu peptides and peptide from other tumor-related antigens might also enhance efficacy of therapeutic vaccination.

Vaccination with dendritic cells pulsed with apoptotic tumors in combination with anti-OX40 and anti-4-1BB monoclonal antibodies induces T cell-mediated protective immunity in Her-2/neu transgenic mice

Tumor cells express tumor-associated antigens (TAAs), which can serve as targets for the immune system. However, the majority of TAAs are overexpressed products of normal cellular genes; as such, self-tolerance mechanisms have hindered their use for the induction of effective antitumor responses. One such normal self-protein is the growth factor receptor Her-2/neu, which is overexpressed in 25-35% of all mammary carcinomas in humans. In previous studies, we have demonstrated that Her-2/neu mice are functionally tolerant to neu antigens and contain only a low avidity T-cell repertoire to neu antigens. However, this residual low-avidity T-cell repertoire has antitumor activity. In this study, we compared the immune responses of Her-2/neu mice immunized with dendritic cells (DCs) pulsed with soluble neu protein or with apoptotic tumor cells. Analysis of the antitumor response shows that Her-2/neu mice vaccinated with DCs pulsed with Her-2/neu antigens retard tumor growth; however, vaccination with DCs pulsed with apoptotic tumor cells induces a stronger antitumor effect. Administration of multiple immunizations in combination with the costimulatory agonist anti-OX40 or anti-4-1BB MAb significantly enhanced the immune responses in these mice, resulting in complete tumor rejection if the tumor burden was small and substantial tumor reduction with a larger tumor burden. These results have important implications for the design of tumor vaccination strategies, suggesting that the use of vaccines that stimulate a broad immune response in combination with costimulatory molecules as immunomodulators could significantly improve the antitumor immune response in tolerant hosts.

Plasticity in the positive selection of T cells: affinity of the selecting antigen and IL-7 affect T cell responsiveness

The current study examines how responsiveness of T cells is affected by the avidity of the peptide/MHC engaged during positive selection of their thymocyte precursors. We used a thymus reaggregate culture system in which CD4(+)CD8(+) thymocytes from AND TCR transgenic mice were induced to undergo positive selection by pigeon cytochrome c (PCC) peptide or its analogs presented by I-E(k) class II MHC on a thymic epithelial cell line. When low-affinity peptide analogs drove positive selection, up to 100 microM was needed to produce >50% CD4(+) T cells, and these cells were highly responsive to PCC. In contrast, <0.2 microM high-affinity peptides was required to achieve similar selection efficiency, but the resultant cells failed to respond to PCC. However, these cells were not dead based on dye exclusion and capacity to respond to phorbal ester and to agonist if IL-2 was also present, supporting the view that non-responsiveness of cells selected on high-affinity peptides is a form of central T cell tolerance distinct from deletion. Cells selected on intermediate-affinity peptides showed variable responsiveness which was suppressed 5- to 10-fold by addition during reaggregate culture of antibody to the IL-7R. Similarly, supplementary IL-7 in the reaggregate culture produced CD4(+) T cells that were promiscuously responsive. Overall, this study demonstrates that the responsiveness of T cells is not rigidly controlled and that the presence of IL-7 during T cell development has the potential to negate central T cell tolerance and produce autoreactive T cells.

Distinct requirements for deletion versus anergy during CD8 T cell peripheral tolerance in vivo

Activation of naive T cells by quiescent APCs results in tolerance through deletion and anergy. The underlying basis for these distinct fates is unclear. Using clone 4 TCR transgenic animals as a source of naive CD8 T cells, we examined the requirements for peripheral deletion in vivo. Our results demonstrate that independent of the amount of Ag used for stimulation, a single dose was insufficient to achieve complete clonal deletion. Instead, further antigenic exposure was required to completely eliminate all of the activated T cells. Additionally, consecutive stimulations with low doses of Ag were highly effective in promoting deletion. In contrast, although stimulation with high doses of Ag initially led to the apoptosis of many of the activated T cells, it induced hyporesponsiveness in a portion of the responding cells, thereby sparing them from further activation and deletion. These data explain why some conditions promote tolerance through clonal deletion whereas others promote anergy. Furthermore, these data provide a framework to devise protocols for effective deletion of potentially autoreactive T cells.

An affinity/avidity model of peripheral T cell regulation

We show in these studies that Qa-1-dependent CD8+ T cells are involved in the establishment and maintenance of peripheral self tolerance as well as facilitating affinity maturation of CD4+ T cells responding to foreign antigen. We provide experimental evidence that the strategy used by the Qa-1-dependent CD8+ T cells to accomplish both these tasks in vivo is to selectively downregulate T cell clones that respond to both self and foreign antigens with intermediate, not high or low, affinity/avidity. Thus, the immune system evolved to regulate peripheral immunity using a unified mechanism that efficiently and effectively permits the system to safeguard peripheral self tolerance yet promote the capacity to deal with foreign invaders.

Delineation of Signals Required for Thymocyte Positive Selection

Peptide/MHC complexes capable of inducing positive selection in mouse fetal thymic organ cultures fail to do so in suspension culture. Furthermore, this type of culture does not promote initial stages of differentiation, such as coreceptor down-modulation, unless peptides used for stimulation have (at least) weak agonist activity. We show in this study that signals provided in suspension culture by nonagonist peptide/MHC complexes on the surface of macrophages, even though apparently silent, are sufficient to promote complete phenotypic differentiation when CD4+CD8+ thymocytes are subsequently placed in a proper anatomical setting. Furthermore, the synergistic actions of suboptimal concentrations of phorbol esters and nonagonist peptide/MHC complexes can make the initial stages of positive selection visible, without converting maturation into negative selection. Thus, the correlation between efficiency of positive selection and the degree of coreceptor down-modulation on CD4+CD8+ thymocytes is not linear. Furthermore, these results suggest that the unique role of thymic stromal cells in positive selection is related not to presentation of self-peptide/MHC complexes, but most likely to another ligand.

Epitope-dependent inhibition of T cell activation by the Ea transgene: an explanation for transgene-mediated protection from murine lupus

A high level expression of the Ea(d) transgene encoding the I-E alpha-chain is highly effective in the suppression of lupus autoantibody production in mice. To explore the possible modulation of the Ag-presenting capacity of B cells as a result of the transgene expression, we assessed the ability of the transgenic B cells to activate Ag-specific T cells in vitro. By using four different model Ag-MHC class II combinations, this analysis revealed that a high transgene expression in B cells markedly inhibits the activation of T cells in an epitope-dependent manner, without modulation of the I-E expression. The transgene-mediated suppression of T cell responses is likely to be related to the relative affinity of peptides derived from transgenic I-E alpha-chains (Ealpha peptides) vs antigenic peptides to individual class II molecules. Our results support a model of autoimmunity prevention based on competition for Ag presentation, in which the generation of large amounts of Ealpha peptides with high affinity to I-A molecules decreases the use of I-A for presentation of pathogenic self-peptides by B cells, thereby preventing excessive activation of autoreactive T and B cells.

Self-reactivity in thymic double-positive cells commits cells to a CD8αα lineage with characteristics of innate immune cells

Thymocytes displaying self-reactive T cell receptors usually undergo negative selection in the thymus. Here we demonstrate that agonist peptides can promote positive selection of immature double-positive thymocytes into distinct lineages, varying with the agonist concentration and the animal's age. Microarray gene expression analyses showed broad transcriptional alterations in a set of transcripts associated with the innate immune system, as well as silencing of CD8 beta expression. The resulting CD8 alpha alpha T cells showed a rapid effector cytokine response. Hence, T cells displaying self-reactive receptors can have the gene expression profile and phenotypic characteristics of innate immune cells.

The CD8+ T cell repertoire against Her-2/neu antigens in neu transgenic mice is of low avidity with antitumor activity

The majority of tumor-associated antigens are aberrantly expressed or overexpressed normal gene products. Therefore, mechanisms responsible for self tolerance dampen immune responses against these antigens. To evaluate the effect that tolerance has on the immune responses against tumor antigens, we characterized the CD8+ T cell responses in neu mice. T cell responses against the A2.1/neu p369-377 and p773-782 peptides were evaluated in neu mice that were crossed with A2.1/Kb transgenic mice (A2 x neu). Tetramer binding and cytotoxic activity demonstrate that, compared to CTL from A2.1/Kb x FVB wild-type mice (A2 x FVB), CD8+ T cells from A2 x neu mice were of lower avidity for the peptides. Despite the fact that A2 x neu mice are tolerant, multiple immunizations with DC pulsed with the p369-377 or p773-782 peptides in the presence of IL-2 retarded tumor growth in A2 x neu mice, and immunizations in combination with the anti-OX40 mAb further enhanced the antitumor response. Taken together, these data indicate that low-avidity T cells for neu antigens persisting in A2 x neu mice have the capacity to develop antitumor responses as long as they are provided with efficient costimulation. These results underscore the potential role of low-avidity T cells in antitumor immunity and may offer an important component for vaccination immunotherapies.

Donor-specific antigen transfusion-mediated skin-graft tolerance results from the peripheral deletion of donor-reactive CD8+ T cells

BACKGROUND

The mechanism of donor-specific transfusion (DST)-induced long-term skin-graft survival is examined in 2CF1 (2C x dm2) transgenic and B6F1 (C57BL/6 x dm2) nontransgenic mice in which CB6F1 (Balb/c x B6) DST and donor skin grafts differ from 2CF1 or B6F1 recipients only at major histocompatibility complex class I Ld.

METHODS

Saline (control) or allogeneic CB6F1 spleen cells were injected intravenously into 2CF1 and B6F1 mice. One week later, CB6F1 tail skin was transplanted onto the dorsum of these mice. Fluorescence-activated cell sorter analysis (flow cytometric analysis) of peripheral blood was performed 2 days before DST, 5 days after DST, and 7, 14, 21, 28, and 75 days after skin grafting. Splenocyte responsiveness was measured by in vitro mixed lymphocyte culture and cytotoxic T lymphocyte. Cytokine protein production (interleukin [IL]-2 and interferon-gamma) was measured by enzyme-linked immunosorbent assay.

RESULTS

Whereas all CB6F1 skin grafts in control saline-treated 2CF1 and B6F1 mice were rejected, 100% of 2CF1 and B6F1 pretreated with CB6F1 DST accepted the class I Ld disparate donor skin indefinitely. DST followed by a CB6F1 skin graft led to a significant deletion of donor-reactive CD8+ T cells by fluorescence-activated cell sorter analysis and decreased production of the inflammatory cytokines IL-2 and interferon-gamma. The hyporesponsiveness of residual CD8+ T cells in mixed lymphocyte culture and cytotoxic T lymphocyte to Ld after DST was restored to normal by IL-2.

CONCLUSION

These findings demonstrate that administration of DST uniformly results in long-term Ld+ skin-allograft acceptance. This tolerance induction is related to both a significant decrease in donor-reactive CD8+ transgenic T cells and anergy of the residual CD8+ T cells.

Zinc-binding sites in the N terminus of Mycoplasma arthritidis-derived mitogen permit the dimer formation required for high affinity binding to HLA-DR and for T cell activation

Zinc-dependent superantigens can be divided into two subfamilies based on how they use zinc ions for interactions with major histocompatibility complex (MHC) class II molecules. Members of the first subfamily use zinc ions for interactions with histidine 81 on the beta-chain of MHC class II molecules, whereas members of the second subfamily use zinc ions for dimer formation. The zinc-binding motif is located in the C terminus of the molecule in both subfamilies. While our recent studies with Mycoplasma arthritidis-derived mitogen (MAM) have provided the first direct evidence demonstrating the binding to MHC class II molecules in a zinc-dependent manner, it still not known how zinc coordinates the interaction. Data presented here show that the zinc ion is mainly required to induce MAM/MAM dimer formation. Residues in the N terminus of MAM are involved in dimer formation and MHC class II binding, while histidine 14 and aspartic acid 31 of the MAM sequence are the major residues mediating MAM/MAM dimerization. Zinc-induced dimer formation is necessary for MAM binding, MHC class II-induced cell-cell adhesion, and efficient T cell activation. Together these results depict the unique mode of interaction of MAM in comparison with other superantigens.

Mechanism of portal venous tolerant long-term MHC Class I L(d)-specific skin graft survival in transgenic 2CF1 mice

BACKGROUND

Administration of alloantigen via the portal vein (PV) in non-transgenic animals has been shown to promote immunologic tolerance and enhance transplant allograft survival. The underlying mechanisms remain unclear. In 2C x dm2 F1 (2CF1) transgenic mice, the monoclonal antibody, 1B2, identifies specific 2C TCR transgenic CD8+ T cells that are cytotoxic against Class I MHC L(d). In these mice, the specific response by these cells to L(d+) skin grafts after PV administration of L(d+) antigen was determined.

MATERIALS AND METHODS

Saline (control) or allogeneic C57BL/6 x BALB/c F1 (CB6F1) spleen cells (25 x 10(6)), which differ from 2CF1 only at L(d), were injected PV into 2CF1 mice. One week later, CB6F1 tail skin was transplanted onto the dorsum of these 2CF1 mice. Skin graft rejection was defined as >50% loss of the graft. Parallel experiments were performed in non-transgenic littermates [B6F1 (C57BL/6 x dm2)]. FACS analysis of 2CF1 peripheral blood for 1B2+, CD4+, and CD8+ T cells was performed 2 days before PV injection (9 days prior to skin grafting), 5 days after PV injection (2 days prior to skin grafting), and 7, 14, 21, 28, and 60 days after skin grafting. FACS analysis of nai;ve, saline control, and CB6F1 PV-treated 2CF1 thymocytes was also performed. Responsiveness of saline (control)-treated and PV-treated 2CF1 splenocytes was measured by in vitro cytotoxic T lymphocyte (CTL).

RESULTS

All CB6F1 skin grafts were rejected in <14 days by PV saline controls. However, a single PV injection of donor L(d+) CB6F1 cells was sufficient to induce indefinite CB6F1 (L(d+)) skin allograft survival in 100% of non-transgenic B6F1 and transgenic 2CF1 (anti-L(d)) TCR transgenic recipients. FACS analysis of 1B2+ T cells demonstrated that PV injection of donor antigen followed by a CB6F1 skin graft led to a 70% decrease in peripheral donor-reactive 1B2+ CD8+ T cells by day 7, while central thymocytes were unchanged. CTL of 2CF1 splenocytes following PV CB6F1 demonstrated that they were hyporesponsive to L(d) compared to saline-treated 2CF1 splenocytes. Despite recovery of peripheral CD8+ T cells to near normal levels by 60 days post-transplantation, skin graft survival persisted indefinitely.

CONCLUSIONS

Administration of specific PV antigen results in exquisite long-term L(d+) skin allograft acceptance. This tolerance induction is related to a significant peripheral deletion of donor-reactive 1B2+ CD8+ transgenic T cells and anergy of the residual T cells.

Reduced self-reactivity of an autoreactive T cell after activation with cross-reactive non-self-ligand

Autoreactive CD4(+) T lymphocytes are critical to the induction of autoimmune disease, but because of the degenerate nature of T cell receptor (TCR) activation such receptors also respond to other ligands. Interaction of autoreactive T cells with other non-self-ligands has been shown to activate and expand self-reactive cells and induce autoimmunity. To understand the effect on the autoreactivity of naive cross-reactive T cells of activation with a potent nonself ligand, we have generated a TCR transgenic mouse which expresses a TCR with a broad cross-reactivity to a number of ligands including self-antigen. The activation of naive transgenic recombination activating gene (Rag)2(-)(/)(-) T cells with a potent non-self-ligand did not result in a enhancement of reactivity to self, but made these T cells nonresponsive to the self-ligand and anti-CD3, although they retained a degree of responsiveness to the non-self-ligand. These desensitized cells had many characteristics of anergic T cells. Interleukin (IL)-2 production was selectively reduced compared with interferon (IFN)-gamma. p21(ras) activity was reduced and p38 mitogen-activated protein kinase (MAPK) was relatively spared, consistent with known biochemical characteristics of anergy. Surprisingly, calcium fluxes were also affected and the anergic phenotype could not be reversed by exogenous IL-2. Therefore, activation with a hyperstimulating non-self-ligand changes functional specificity of an autoreactive T cell without altering the TCR. This mechanism may preserve the useful reactivity of peripheral T cells to foreign antigen while eliminating responses to self.

Aberrant regulation of superantigen responses during T-cell reconstitution and graft-versus-host disease in immunodeficient mice

Acute graft-versus-host disease (GVHD) after allogeneic stem cell transplantation is associated with impaired deletion and anergy of host-reactive T cells. To elucidate the immunoregulatory events that may contribute to such dysregulated T-cell responses in GVHD, we studied superantigen (SAg) responses after adoptive T-cell transfer into severe combined immunodeficient (SCID) mice. SAg responses are normally regulated by mechanisms involving deletion and anergy, with SAg-reactive T cells typically being deleted rapidly in vivo. In a SCID mouse model of GVHD, however, allogeneic host SAg-reactive T cells were not deleted rapidly, but rather persisted in increased numbers for several months. Moreover, depending on the timing of SAg stimulation and the numbers of T cells transferred, dysregulation (impaired deletion and anergy) of SAg responses could be demonstrated following the adoptive transfer of syngeneic T cells into SCID mice as well. Transgenic T-cell receptor-bearing KJ1-26.1+ T cells were then used to determine the fate of weakly reactive T cells after adoptive transfer and SAg stimulation. When transferred alone, KJ1-26.1+ T cells demonstrated impaired deletion and anergy. In the presence of more strongly staphylococcal enterotoxin B (SEB)–reactive T cells, however, KJ1-26.1+ T cells were regulated normally, in a manner that could be prevented by inhibiting the effects of more strongly SEB-reactive cells or by increasing the level of activation of the KJ1-26.1+ T cells themselves. We suggest that the control mechanisms that normally regulate strongly activated T cells in immunocompetent animals are lost following adoptive transfer into immunodeficient hosts, and that this impairment contributes to the development of GVHD.

Aberrant regulation of superantigen responses during T-cell reconstitution and graft-versus-host disease in immunodeficient mice

Acute graft-versus-host disease (GVHD) after allogeneic stem cell transplantation is associated with impaired deletion and anergy of host-reactive T cells. To elucidate the immunoregulatory events that may contribute to such dysregulated T-cell responses in GVHD, we studied superantigen (SAg) responses after adoptive T-cell transfer into severe combined immunodeficient (SCID) mice. SAg responses are normally regulated by mechanisms involving deletion and anergy, with SAg-reactive T cells typically being deleted rapidly in vivo. In a SCID mouse model of GVHD, however, allogeneic host SAg-reactive T cells were not deleted rapidly, but rather persisted in increased numbers for several months. Moreover, depending on the timing of SAg stimulation and the numbers of T cells transferred, dysregulation (impaired deletion and anergy) of SAg responses could be demonstrated following the adoptive transfer of syngeneic T cells into SCID mice as well. Transgenic T-cell receptor-bearing KJ1-26.1+ T cells were then used to determine the fate of weakly reactive T cells after adoptive transfer and SAg stimulation. When transferred alone, KJ1-26.1+ T cells demonstrated impaired deletion and anergy. In the presence of more strongly staphylococcal enterotoxin B (SEB)–reactive T cells, however, KJ1-26.1+ T cells were regulated normally, in a manner that could be prevented by inhibiting the effects of more strongly SEB-reactive cells or by increasing the level of activation of the KJ1-26.1+ T cells themselves. We suggest that the control mechanisms that normally regulate strongly activated T cells in immunocompetent animals are lost following adoptive transfer into immunodeficient hosts, and that this impairment contributes to the development of GVHD.

Autoreactivity, dynamic tuning and selectivity

The immune system adjusts its response to the context in which antigens, including self-antigens, are recognized. Recent observations support a conceptual framework for understanding how this may be achieved at the cellular and cell-population levels. At both levels, 'perturbations' elicit competition between excitation and de-excitation, resulting either in adaptation or in various responses. The responsiveness of individual cells is dynamically tuned, reflecting their recent experience. The tuning of T-cell activation thresholds by self-ligands facilitates positive selection and continuously regulates the level of autoreactivity in the periphery. Autoreactivity appears to be involved in regulation of the immune response, homeostasis, maintaining of the functional integrity of naïve and memory cells, and in other physiological functions.

Alloantigen-induced specific immunological unresponsiveness

When the immune system encounters alloantigen it can respond in any one of a number of different ways. The choice that is made will take into account factors such as where, when and how the contact with the alloantigen takes place, as well as the environmental conditions that prevail at the time the alloantigen is encountered. Alloantigen administration before transplantation either alone or in combination with therapeutic agents that modulate the functional activity of the responding leucocytes can be a powerful way of inducing specific unresponsiveness to alloantigens in vivo. The molecular mechanisms that influence the way the outcome of the immune response to alloantigen develops, either activation or unresponsiveness to the triggering antigen, hold the key to our ability to manipulate the immune system effectively by exposing it to donor antigen for therapeutic purposes. This review will focus on alloantigen-induced immunological unresponsiveness and how insights into the mechanisms of unresponsiveness have driven the development of novel tolerance-induction strategies that show promise for translation into the clinic in the future.

Modulation of CD8+ T cell response to antigen by the levels of self MHC class I

The response of H-Y-specific TCR-transgenic CD8(+) T cells to Ag is characterized by poor proliferation, cytolytic activity, and IFN-gamma secretion. IFN-gamma secretion, but not cytotoxic function, can be rescued by the B7.1 molecule, suggesting that costimulation can selectively enhance some, but not all, effector CD8(+) T cell responses. Although the H-Y epitope binds H-2D(b) relatively less well than some other epitopes, it can induce potent CTL responses in nontransgenic mice, suggesting that the observed poor responsiveness of transgenic CD8(+) T cells cannot be ascribed to the epitope itself. Previously reported reactivity of this TCR to H-2A(b) is also not the cause of the poor responsiveness of the H-Y-specific CD8(+) T cells, as H-Y-specific CD8(+) T cells obtained from genetic backgrounds lacking H-2A(b) also responded poorly. Rather, reducing the levels of H-2(b) class I molecules by breeding the mice to (C57BL/6 x B10.D2)F(1) or TAP1(+/-) backgrounds partially restored cytotoxic activity and enhanced proliferative responses. These findings demonstrate that the self MHC class I gene dosage may regulate the extent of CD8(+) T cell responsiveness to Ag.

Lessons from BXSB and related mouse models

The BXSB murine strain spontaneously develops an autoimmune syndrome with features of systemic lupus erythematosus (SLE) that affects males much earlier than females, due to the presence of an as yet unidentified mutant gene located on its Y chromosome, designated Yaa (Y-linked autoimmune acceleration). The Yaa gene by itself is unable to induce significant autoimmune responses in mice without an apparent SLE background, while it can induce and accelerate the development of an SLE in combination with autosomal susceptibility alleles present in lupus-prone mice. Although the genes encoded within or closely linked to the MHC locus play an important role in the development or protection of SLE, the MHC effect can be completely masked by the presence of the Yaa gene in mice highly predisposed to SLE. The role of the Yaa gene for the acceleration of SLE is apparently two-fold; it enhances overall autoimmune responses against autoantigens to which mice respond relatively weakly, and promotes Th 1 responses against autoantigens to which mice respond relatively well, leading to the production of more pathogenic autoantibodies, i.e., FcgammaR-fixing IgG2a and cryoglobulin IgG3 autoantibodies. Yaa+ - Yaa- double bone marrow chimera experiments revealed that the Yaa defect is expressed in B cells, but not in T cells, and that T cells from non-autoimmune mice are capable of providing help for autoimmune responses by collaborating Yaa+ B cells. We speculate that the Yaa defect may decrease the threshold for antigen receptor-dependent stimulation, leading to the triggering and excessive stimulation of autoreactive T and B cells.

T cell tolerance based on avidity thresholds rather than complete deletion allows maintenance of maximal repertoire diversity

Given the flexible nature of TCR specificity, deletion or permanent disabling of all T cells with the capacity to recognize self peptides would severely limit the diversity of the repertoire and the capacity to recognize foreign Ags. To address this, we have investigated the patterns of CD8+ CTL reactivity to a naturally H-2Kb-presented self peptide derived from the elongation factor 1alpha (EF1alpha). EF1alpha occurs as two differentially expressed isoforms differing at one position of the relevant peptide. Low avidity CTLs could be raised against both variants of the EF1alpha peptide. These CTLs required 100-fold more peptide-H-2Kb complexes on the target cell compared with CTLs against a viral peptide, and did not recognize the naturally expressed levels of EF1alpha peptides. Thus, low avidity T cells specific for these self peptides escape tolerance by deletion, despite expression of both EF1alpha isoforms in dendritic cells known to mediate negative selection in the thymus. The low avidity in CTL recognition of these peptides correlated with low TCR affinity. However, self peptide-specific CTLs expressed elevated levels of CD8. Furthermore, CTLs generated against altered self peptide variants displayed intermediate avidity, indicating cross-reactivity in induction of tolerance. We interpret these data, together with results previously published by others, in an avidity pit model based on avidity thresholds for maintenance of both maximal diversity and optimal self tolerance in the CD8+ T cell repertoire.

The use of HLA A2.1/p53 peptide tetramers to visualize the impact of self tolerance on the TCR repertoire

p53 is an attractive target for cancer immunotherapy since it is overexpressed in half of all tumors. However, it is also expressed in normal lymphoid tissue, and self tolerance leaves a p53-specific repertoire purged of high avidity CTL. To better understand the mechanism of tolerance and the basis for such low avidity interaction, p53-specific CTL from p53 deficient (p53-) and sufficient (p53+) A2.1/Kb transgenic mice were compared with respect to their ability to bind HLA-A2.1 tetramers containing cognate murine p53 peptide Ag, p53 261-269. Since the murine CD8 molecule cannot interact with human HLA-A2.1, this tests the ability of the TCR to bind the A2.1/peptide complex tetramer. CTL from p53- mice demonstrated strong binding of such A2.1/p53 261-269 tetramers; however, the CTL from tolerant p53+ mice were devoid of tetramer-binding CD8+ T cells. Examination of TCR expression at the clonal level revealed that CTL from p53+ and p53- mice each expressed comparable levels of the p53-specific TCR. These results indicate that normal expression of p53 promotes elimination of T cells expressing TCRs with sufficient affinity to achieve stable binding of the A2.1/p53 261-269 tetramers.

Characterization of CD8+ T lymphocytes that persist after peripheral tolerance to a self antigen expressed in the pancreas

As a result of expression of the influenza hemagglutinin (HA) in the pancreatic islets, the repertoire of HA-specific CD8+ T lymphocytes in InsHA transgenic mice (D2 mice expressing the HA transgene under control of the rat insulin promoter) is comprised of cells that are less responsive to cognate Ag than are HA-specific CD8+ T lymphocytes from conventional mice. Previous studies of tolerance induction involving TCR transgenic T lymphocytes suggested that a variety of different mechanisms can reduce avidity for Ag, including altered cell surface expression of molecules involved in Ag recognition and a deficiency in signaling through the TCR complex. To determine which, if any, of these mechanisms pertain to CD8+ T lymphocytes within a conventional repertoire, HA-specific CD8+ T lymphocytes from B10.D2 mice and B10.D2 InsHA transgenic mice were compared with respect to expression of cell surface molecules, TCR gene utilization, binding of tetrameric KdHA complexes, lytic mechanisms, and diabetogenic potential. No evidence was found for reduced expression of TCR or CD8 by InsHA-derived CTL, nor was there evidence for a defect in triggering lytic activity. However, avidity differences between CD8+ clones correlated with their ability to bind KdHA tetramers. These results argue that most of the KdHA-specific T lymphocytes in InsHA mice are not intrinsically different from KdHA-specific T lymphocytes isolated from conventional animals. They simply express TCRs that are less avid in their binding to KdHA.

Anergic CD8+ T Cells Can Persist and Function In Vivo

Using a mouse model system, we demonstrate that anergic CD8+ T cells can persist and retain some functional capabilities in vivo, even after the induction of tolerance. In TCR Vβ5 transgenic mice, mature CD8+Vβ5+ T cells transit through a CD8lowVβ5low deletional intermediate during tolerance induction. CD8low cells are characterized by an activated phenotype, are functionally compromised in vitro, and are slated for deletion in vivo. We now demonstrate that CD8low cells derive from a proliferative compartment, but do not divide in vivo. CD8low cells persist in vivo with a t1/2 of 3–5 days, in contrast to their in vitro t1/2 of 0.5–1 day. During this unexpectedly long in vivo life span, CD8low cells are capable of producing IFN-γ in vivo despite their inability to proliferate or to kill target cells in vitro. CD8low cells also accumulate at sites of inflammation, where they produce IFN-γ. Therefore, rather than withdrawing from the pool of functional CD8+ T cells, anergic CD8low cells retain a potential regulatory role despite losing their capacity to proliferate. The ability of anergic cells to persist and function in vivo adds another level of complexity to the process of tolerance induction in the lymphoid periphery.

Selection of the T cell repertoire

Advances in gene technology have allowed the manipulation of molecular interactions that shape the T cell repertoire. Although recognized as fundamental aspects of T lymphocyte development, only recently have the mechanisms governing positive and negative selection been examined at a molecular level. Positive selection refers to the active process of rescuing MHC-restricted thymocytes from programmed cell death. Negative selection refers to the deletion or inactivation of potentially autoreactive thymocytes. This review focuses on interactions during thymocyte maturation that define the T cell repertoire, with an emphasis placed on current literature within this field.

Chronic Modulation of the TCR Repertoire in the Lymphoid Periphery

Using TCR Vβ5 transgenic mice as a model system, we demonstrate that the induction of peripheral tolerance can mold the TCR repertoire throughout adult life. In these mice, three distinct populations of peripheral T cells are affected by chronic selective events in the lymphoid periphery. First, CD4+Vβ5+ T cells are deleted in the lymphoid periphery by superantigens encoded by mouse mammary tumor viruses-8 and -9 in an MHC class II-dependent manner. Second, mature CD8+Vβ5+ T cells transit through a CD8lowVβ5low deletional intermediate during tolerance induction by a process that depends upon neither mouse mammary tumor virus-encoded superantigens nor MHC class II expression. Third, a population of CD4−CD8−Vβ5+ T cells arises in the lymphoid periphery in an age-dependent manner. We analyzed the TCR Vα repertoire of each of these cellular compartments in both Vβ5 transgenic and nontransgenic C57BL/6 mice as a function of age. This analysis revealed age-related changes in the expression of Vα families among different cellular compartments, highlighting the dynamic state of the peripheral immune repertoire. Our work indicates that the chronic processes maintaining peripheral T cell tolerance can dramatically shape the available TCR repertoire.

Dual MHC Class I and Class II Restriction of a Single T Cell Receptor: Distinct Modes of Tolerance Induction by Two Classes of Autoantigens

In the final stages of thymic development, immature T cells undergo three distinct processes (positive selection, negative selection, and lineage commitment) that all depend on interactions of thymocyte TCRs with MHC molecules. It is currently thought that TCRs are preferentially restricted by either MHC class I or class II molecules. In this report, we present direct evidence that the TCR previously described as H-Y/H-2Db specific cross-reacts with H-2IAb if expressed in CD4+ cells. We also demonstrate an increase in thymocyte numbers in H-Y TCR-trangenic mice deficient in MHC class II, suggesting a relatively discrete form of negative selection by MHC class II compared with that induced by H-Y/H-2Db. We propose that inability to generate CD4+ T cells expressing H-Y TCR in different experimental settings may be due to tolerance to self-MHC class II. These results, therefore, support an intriguing possibility that tolerance to self may influence and/or interfere with the outcome of the lineage commitment.

Degree of TCR Internalization and Ca2+ Flux Correlates with Thymocyte Selection

Recent evidence suggests that TCR down-regulation directly reflects the number of TCRs that have engaged MHC/peptide ligand complexes. Here, we examined the influence of defined peptides on thymic selection based on their ability to induce differential TCR internalization. Our results demonstrate that there is a direct correlation: peptides that induce strong TCR down-regulation are most efficient at mediating negative selection, whereas peptides that induce suboptimal TCR internalization are more efficient at triggering positive selection. As a consequence of suboptimal TCR internalization, a proportion of TCR complexes that remain on the cell surface may be able to relay continual signals required for survival and differentiation. In addition, we show that the magnitude of Ca2+ influx set by these peptides reflects the hierarchy of TCR down-regulation and correlates with positive vs negative selection of transgenic thymocytes. Together, our data suggest that T cell selection is mediated by differing intensities of the same TCR-mediated signal, rather than by distinct signals.

Infectious tolerance

Infectious tolerance can be induced in many ways, does not require a thymus or clonal deletion and can spread to third-party antigens linked on the same antigen-presenting cell-the process being variously described as linked-, bystanderor epitope-suppression. We here review the evidence concerning the mechanisms involved and attempt to make a consistent hypothesis, that during tolerance induction in the Th1-mediated autoimmune diseases and transplantation systems there would seem to be a phase of immune deviation towards Th2 cytokines, like IL-4 and IL-10; however, this may lead to an IL-10-induced form of anergy or nonresponsiveness and generation of the recently characterized Th3/T-regulatory-1 CD4+ T cell subset which is thought to downregulate the antigen-presenting cell, possibly via transforming growth factor beta.

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.

Interactions with multiple peptide ligands determine the fate of developing thymocytes

Thymocytes are positively or negatively selected depending on interactions between their T cell receptors (TCR) and peptides presented by major histocompatibility complex molecules. We have previously shown that apoptosis of thymocytes from an alpha beta TCR-transgenic mouse (F5), induced by antigenic peptide, can be inhibited specifically by an antagonist peptide variant in an in vitro culture model. We have now extended these experiments by demonstrating that the antagonist peptide can inhibit natural negative selection of maturing thymocytes, induced by endogenously expressed antigen, in fetal thymic organ cultures (FTOC). This inhibition resulted in the rescue and maturation of thymocytes that would otherwise have been deleted. Mature T cells generated in these cultures were able to respond to antigen by producing limited quantities of interferon-gamma, but unlike T cells from control FTOC, they required exogenous interleukin-2 to generate cytolytic effector cells. Interestingly, the antagonist peptide also accelerated the development of F5 thymocytes in the absence of the negatively selecting ligand. These data suggest that the developmental fate of a thymocyte may be determined by the recognition of multiple distinct peptide ligands during thymic selection. Alterations in the profiles of selecting peptides presented in the thymus would thus have profound effects on the size and autoreactive potential of the T cell repertoire generated.

Mice expressing the E7 oncogene of HPV16 in epithelium show central tolerance, and evidence of peripheral anergising tolerance, to E7-encoded cytotoxic T-lymphocyte epitopes

In order to derive mice which expressed both the E7 open reading frame transgene of human papillomavirus type 16 in skin and MHC class 1 restriction elements for several E7-encoded cytotoxic T-lymphocyte (CTL) epitopes, K14.HPV16E7 mice which express E7 in basal keratinocytes were crossed to the F1 generation with A2.1 Kb transgenic mice which express the MHC binding cleft domains of human HLA A*0201, and murine H-2b. F1 mice (denoted K14E7 x A2.1) expressed E7 in the thymus at least as early as 2-5 days before birth. Immunisation of FVB x A2.1 control mice (transgenic for HLA A*0201 and H-2b but not for E7), with two HLA A*0201-restricted epitopes of E7 and one H-2b-restricted CTL epitope of E7, gave strong primary CTL responses recognising epitope-pulsed or constitutively E7-expressing syngeneic target cells. In contrast, in immunised K14E7 x A2.1 mice, the CTL responses to the H-2b epitope and one of the HLA A*0201 CTL epitopes were strongly down-regulated, and to the other HLA A*0201 epitope, completely abolished, as demonstrated by percentage specific killing by bulk splenocyte cultures in cytotoxicity assays, and by CTL precursor frequency analysis. In thymus-transplanted bone marrow radiation chimeras in which the immune system of K14E7 x A2.1 mice was replaced by a FVB x A2.1 immune system, specific immunisation did not result in reemergence of strong E7-directed CTL responses. In agreement with these in vitro findings, specific immunisation failed to significantly alter the course of E7-associated tumour development in K14E7 x A2.1 mice. These data are consistent with a model of central deletional CTL tolerance to E7-encoded epitopes recognised in the context of two distinct MHC class 1 restriction elements, and with the possibility of peripheral T-cell anergy maintained by expression of E7 in the skin.