Preferential differentiation of T cell receptor specificities based on the MHC glycoproteins encountered during development. Evidence for positive selection

Kidins220 and Aiolos promote thymic iNKT cell development by reducing TCR signals

Development of T cells is controlled by the signal strength of the TCR. The scaffold protein kinase D-interacting substrate of 220 kilodalton (Kidins220) binds to the TCR; however, its role in T cell development was unknown. Here, we show that T cell-specific Kidins220 knockout (T-KO) mice have strongly reduced invariant natural killer T (iNKT) cell numbers and modest decreases in conventional T cells. Enhanced apoptosis due to increased TCR signaling in T-KO iNKT thymocytes of developmental stages 2 and 3 shows that Kidins220 down-regulates TCR signaling at these stages. scRNA-seq  indicated that the transcription factor Aiolos is down-regulated in Kidins220-deficient iNKT cells. Analysis of an Aiolos KO demonstrated that Aiolos is a downstream effector of Kidins220 during iNKT cell development. In the periphery, T-KO iNKT cells show reduced TCR signaling upon stimulation with α-galactosylceramide, suggesting that Kidins220 promotes TCR signaling in peripheral iNKT cells. Thus, Kidins220 reduces or promotes signaling dependent on the iNKT cell developmental stage.

MHC restriction and allogeneic immune responses

Discovery of major histocompatability complex (MHC) restriction helped in the understanding of how T-lymphocytes recognize antigens on bacteria, viruses, and tumor cells. It was initially accepted that MHC restriction was a consequence of "adaptive differentiation" in the thymus; during differentiation, the forming repertoire of T-lymphocytes "learned" a low affinity for self MHC molecules via positive selection. This view was later countered by discovery of artifacts in underlying studies and the fact that adaptive differentiation could not explain direct allogeneic and allorestricted recognition phenomena. Data from experiments with TCR transgenic animals, individual MHC/peptide complex expression, and recipients of xenogenic thymus glands yielded evidence of an ability to adapt to microenvironment and a low specificity of positive selection. These facts led to an alternative interpretation of MHC restriction explained, in part, by specificity of a pool of effector cells activated by primary immunization. Details of this phenomenon were defined in studies that noted differential primary structures of peptides that bound various allelic forms of MHC molecules. Here, the T-lymphocyte repertoire formed in the thymus was a result, in part, of random rearrangement of germinal sequences of TCR gene fragments. Such pre-selected repertoires were inherently capable of reacting with different allelic forms of MHC molecules. In contrast, MHC molecules were characterized by significant intraspecies polymorphisms; negative and positive selections were aimed at adaptation of a pre-selected repertoire to a specific microenvironment in an individual. Via elimination of autoreactive clones and sparing of a broad spectrum of specificity to potential pathogens, selection in the thymus could be considered a life-long allogeneic reaction of a pre-selected repertoire to self MHC molecules resulting in tolerance to "self," increased responsiveness to foreign MHC molecules, and cross-reactivity of the mature T-lymphocyte repertoire to individual foreign peptides plus self MHC.

T-cell development and the CD4-CD8 lineage decision

Cell-fate decisions are controlled typically by conserved receptors that interact with co-evolved ligands. Therefore, the lineage-specific differentiation of immature CD4+ CD8+ T cells into CD4+ or CD8+ mature T cells is unusual in that it is regulated by clonally expressed, somatically generated T-cell receptors (TCRs) of unpredictable fine specificity. Yet, each mature T cell generally retains expression of the co-receptor molecule (CD4 or CD8) that has an MHC-binding property that matches that of its TCR. Two models were proposed initially to explain this remarkable outcome--'instruction' of lineage choice by initial signalling events or 'selection' after a stochastic fate decision that limits further development to cells with coordinated TCR and co-receptor specificities. Aspects of both models now appear to be correct; mistake-prone instruction of lineage choice precedes a subsequent selection step that filters out most incorrect decisions.

Increased number of cytotoxic T cells within CD4+8- T cells in beta 2-microglobulin, major histocompatibility complex class I-deficient mice

Targeted disruption of beta 2-microglobulin gene results in deficient major histocompatibility complex class I expression and failure to develop CD4-8+ T cells. Despite this, beta 2 M-/- mice reject skin grafts and cope with most viral infections tested. We asked whether CD4+8- cytotoxic T cells would play a role in compensating for the defect in CD4-8+ cytotoxic T cell function. We found that the cytotoxic activity against class II+ targets is significantly higher among CD4+8- T cells of beta 2M-/- than among those of beta 2M+/+ mice. In the limiting dilution experiment, we showed that the precursor frequency for the cytotoxic, CD4+8-, class II-specific T cells is at least fivefold higher in beta 2M-/- than in beta 2M+/+ mice. These results suggest that CD4+8- cytotoxic T cells could play a major role in carrying out cytotoxic function in beta 2M-/- mice.

Ontogeny of T cell maturation in LEC mutant rats which bear a congenital arrest of maturation from CD4+CD8+ to CD4+CD8- thymocytes

LEC rats bear a congenital deficiency in CD4+CD8- thymocytes and peripheral CD4+ T cells, and consequently a deficiency in Th cell functions. Ontogeny of T cell maturation in normal and LEC mutant rats was, therefore, investigated. Prenatal development of thymocytes in normal rat strains, with respect to the expression of CD4/CD8 and TcR antigens, was similar to that of mice except that its kinetics was delayed by approximately 24 h. The kinetics of T cell maturation in LEC rats was comparable with that of normal rats up to day 19 of gestation, at which stage double-negative thymocytes (CD4-CD8-) developed into double positives (CD4+CD8+) through immature CD4-CD8+ subset. At day 19 of gestation in LEC as well as normal rats, double positives occupied approximately 80% of the total thymocytes, half of which were TcR-dull positive, indicating that TcR was normally rearranged and then expressed in LEC rat thymocytes. These data indicate that double negatives normally mature into at least double positives in LEC rats. Both single positives appeared after day 19 of gestation in normal rats, while in LEC rats CD4+CD8- cells did not appear, suggesting that the deficiency in CD4+CD8- cells is due to a congenital arrest of maturation from CD4+CD8+ to CD4+CD8- cells, but not due to a postnatal deletion.

Bone marrow-derived progenitor T cells convey the origin of maturational arrest from CD4+CD8+ to CD4-CD8+ thymocytes in LEC mutant rats

A mutant strain of rats, LEC, shows a novel arrest of T cell maturation from CD4+CD8+ to CD4+CD8- but not to CD4-CD8+ cells in the thymus. Transplantation of LEC rat fetal thymuses into the subcapsule of the kidney of athymic nude rats resulted in a normal maturation of thymocytes in the thymus graft. Furthermore, both single-positive thymocytes and peripheral lymph node T cells expressed T cell receptor alpha/beta antigen, and lymph node T cells acquired the ability to produce interleukin 2 upon mitogen stimulation. Transplantation of fetal thymuses from LEA rats, which express the same major histocompatibility complex haplotype as LEC rats, into LEC rat kidney subcapsule resulted in the maturational arrest from CD4+CD8+ to CD4+CD8- cells in the thymus graft. These data strongly suggest that bone marrow-derived progenitor T cells carry the cause of maturational arrest and that the thymic stroma of LEC rats has a normal potential to nurse thymocytes.

A unified model for T cell antigen recognition and thymic selection of the T cell repertoire

Positive selection of T cells during thymic differentiation predisposes mature T cells to recognize glycoproteins encoded by "self" alleles of the major histocompatibility complex (MHC) as "restricting elements" for antigen presentation. Yet, negative selection also occurs during thymic differentiation resulting in the clonal deletion of T cells reactive with "self" MHC glycoproteins. Thymic processes of positive and negative selection represent a paradox because the "altered self" view of T cell receptor (TcR) recognition indicates that the same type of TcR-MHC glycoprotein binding interaction mediates both positive and negative selection of the T cell repertoire. Most contemporary models of thymic selection attempt to explain these paradoxical observations by quantitative differences of affinity. That is, TcR interactions with MHC ligands that are of medium affinity lead to positive selection whereas those that are of high affinity lead to negative selection. The purpose of this manuscript is to provide an alternative model of thymic selection based on the efficacy (the ability of a ligand to catalyze receptor mediated biological activity) of TcR-MHC ligand interactions. The "efficacy" model predicts that among those thymocytes exhibiting affinity for self MHC ligands, some clones bind self MHC ligands without efficacy whereas others bind these ligands with efficacy. Immature T cells that bind MHC ligands without efficacy do not undergo TcR mediated activation and thereby escape clonal deletion. Instead, these T cells compete for growth--promoting sites on thymic antigen presenting cells (APC) based upon their clonotypic TcR affinity for self MHC ligands. These T cells experience positive thymic selection and eventually dominate a repertoire of mature T cells predisposed to exhibit non-efficacious binding to "self" MHC ligands. In contrast, immature T cells that exhibit efficacious binding to self MHC ligands are deleted from the T cell repertoire during thymic maturation. By this mechanism, the mature T cell repertoire is selected so that clonotypic T cells are predisposed to bind the very sites on MHC glycoproteins responsible for antigen presentation without risk of autoimmunity. Given the clonotypic diversity of the mature repertoire, complexes of foreign peptides and self MHC glycoproteins would be recognized by clones of the appropriate specificity as highly efficacious ligands. In summary, the "efficacy" model is entirely consistent with the "altered self" concept of T cell antigen recognition and readily accounts for both positive and negative selection of the T cell repertoire.

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

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

Towards an integrated view of thymopoiesis

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

Kinetics of negative and positive selection in the thymus

Recent experiments show that CD4+8+ thymocytes represent the critical stage in T cell development at which the specificity of randomly generated alpha beta T cell receptors is screened. These cells are deleted when the receptor binds to the MHC molecule plus specific peptide presented by bone marrow derived cells but are rescued from cell death and induced to mature if the receptor binds to the MHC molecule on thymic epithelium in the absence of the specific peptide. Different tolerogens delete CD4+8+ thymocytes earlier or later during their lifespan and negative selection can occur prior to positive selection. The specificity of the alpha beta T cell receptor for either class I or class II thymic MHC molecules determines the CD4-8+ and CD4+8- phenotype of mature T cells.

Maturational arrest from CD4+8+ to CD4+8- thymocytes in a mutant strain (LEC) of rat

A mutant strain (LEC) of rats was found to have a novel defect in T cell maturation, that is, arrest of differentiation from CD4+8+ to CD4+8- but not to CD4-8+ thymocytes. FACS analyses demonstrated a deficiency in the CD4+8- T cell subset in the thymus and a marked decrease in CD4+ T cells in peripheral lymphoid organs. Expression of the T cell receptor (TCR)/CD3 complex in CD4+8+ and CD4-8+ thymocytes of LEC rats was normal. Expression of class II major histocompatibility complex (MHC) in the thymus of LEC rats was also the same as that of normal rats. These results indicate that maturational arrest occurs only in the transition pathway from CD4+8+ to CD4+8- thymocytes, and that this mutation can not be attributed to the default of expression of either TCR/CD3, CD4, or class II MHC antigen. Consequently, dysfunction of helper T cells was observed in LEC rats, while killer T cells and B cells functioned normally. Although the complete identification of the origin of this mutation requires further studies, it is hoped that such investigations will throw light on the mechanism of positive selection.

Dynamics of positive and negative selection in the thymus: review and hypothesis

T cells recognize with a single receptor both a product of antigens processed by antigen presenting cells (APC1) and a self-marker molecule, encoded by the major histocompatibility complex (MHC, a property termed MHC-restricted recognition of antigen). During their differentiation in the thymus, T cells "learn" what to regard as self-MHC molecules, and only the cells once able to recognize antigen in the context of self-MHC will be "positively selected" to exit the thymus. The cells, once capable of reacting to self molecules, do not exit the thymus. They are "negatively selected" (deleted). Both "positive" and "negative" selection depends on the T-cell-receptor (TCR) specificity. Furthermore, the TCR specificity determines the final phenotype of the mature T cells; namely, the cells with receptors specific for the MHC-class I molecule will acquire the CD4-CD8+ phenotype, while the cells with receptors specific for the MHC-class II molecule will acquire the CD4+CD8- phenotype. However, a few mature T cells in the periphery do not follow the rule: CD4 expression class II restriction and CD8 expression class I restriction. We believe that these T lymphocytes have a receptor with very high affinity for one class of MHC molecules and cross-react with another class of MHC molecules (with somewhat lower affinity). The majority of T lymphocytes with such receptors bind the thymic MHC molecule, for which they have the highest affinity. Since this affinity is too high for further differentiation, such clones are deleted in the thymus. However, a small fraction of these cells bind the alternative class of MHC molecules, due to cross-reactivity of their receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Abnormal anti-viral immune response in mice is corrected in HLA-B27.2-transgenic mice

In contrast to the strong Sendai virus-specific cytotoxic T-lymphocyte (CTL) responses in C57BL/6 mice. H-2Kb mutant bm1 mice are nonresponders to Sendai virus. By appropriate crossings between HLA-B27 double-transgenic mice and Kb mutant bm1 mice, and after subsequent selection, H-2bm1 homozygous mice were produced expressing the human HLA-B27.2 and beta 2-microglobulin genes. Here we show that the introduction of a human HLA class I gene into the genome of the H-2bm1 Sendai virus-nonresponder mutant mice resulted in good responsiveness to Sendai virus, and in normal levels of Sendai virus-specific CTL precursors. The CTL response in the HLA-B27.2 double-transgenic H-2bm1 mice against Sendai virus was restricted by the HLA-B27.2 molecule. These results show the direct involvement of HLA class I molecules in regulation of the anti-viral CTL repertoire and represent for the first time a correction of abnormal anti-viral immunity in mice by incorporation of a human MHC class I (HLA-B27.2) gene.

CD8 is required during positive selection of CD4-/CD8+ T cells

Interactions between self-MHC molecules and T cells are necessary for the proper development of mature T cells, in part due to an absolute requirement for self-MHC-TCR interactions. Recently, we showed that CD4-mediated interactions also participate in shaping the T cell repertoire during thymic maturation. We now examine the possible role of the CD8 molecule during in vivo T cell development. Our results demonstrate that perinatal thymi treated with intact anti-CD8 mAb fail to generate CD8 single-positive T cells, while the generation of the other main phenotypes remains unchanged. Most importantly, the use of F(ab')2 anti-CD8 mAb fragments gave identical results, i.e., lack of generation of CD4-/CD8+ cells, with no effect on the generation of CD4+/CD8+. Furthermore, selective blocking of one CD8 allele with F(ab')2 mAbs in F1 mice expressing both CD8 alleles did not interfere with the development of CD4-/CD8+ cells, demonstrating that the absence of CD8+ T cells in homozygous mice is not due to depletion, but rather is caused by a lack of positive selection. This is most likely attributable to a deficient CD8-MHC class I interaction. Our findings strongly advocate that CD8 molecules are vital to the selection process that leads to the development of mature single-positive CD8 T cells.

Role of CD4 in thymocyte selection and maturation

We examined the possible role of CD4 molecules during in vivo and in vitro fetal thymic development. Our results show that fetal thymi treated with intact anti-CD4 mAbs fail to generate CD4 single-positive T cells, while the generation of the other phenotypes remains unchanged. Most importantly, the use of F(ab')2 and Fab anti-CD4 mAb gave identical results, i.e., failure to generate CD4+/CD8- T cells, with no effect on the generation of CD4+/CD8+ T cells. Since F(ab')2 and Fab anti-CD4 fail to deplete CD4+/CD8- in adult mice, these results strongly argue that the absence of CD4+/CD8- T cells is not due to depletion, but rather, is caused by a lack of positive selection, attributable to an obstructed CD4-MHC class II interaction. Furthermore, we also observed an increase in TCR/CD3 expression after anti-CD4 (divalent or monovalent) mAb treatment. The TCR/CD3 upregulation occurs in the double-positive population, and may result from CD4 signaling after mAb engagement, or may be a consequence of the blocked CD4-class II interactions. One proposed model argues that the CD3 upregulation occurs in an effort to compensate for the reduction in avidity or signaling that is normally provided by the interaction of the CD4 accessory molecule and its ligand. As a whole, our findings advocate that CD4 molecules play a decisive role in the differentiation of thymocytes.