Positive selection of antigen-specific T cells in thymus by restricting MHC molecules

Highly restricted T cell repertoire shaped by a single major histocompatibility complex-peptide ligand in the presence of a single rearranged T cell receptor beta chain

The T cell repertoire is shaped by positive and negative selection of thymocytes through the interaction of alpha/beta-T cell receptors (TCR) with self-peptides bound to self-major histocompatibility complex (MHC) molecules. However, the involvement of specific TCR-peptide contacts in positive selection remains unclear. By fixing TCR-beta chains with a single rearranged TCR-beta irrelevant to the selecting ligand, we show here that T cells selected to mature on a single MHC-peptide complex express highly restricted TCR-alpha chains in terms of Valpha usage and amino acid residue of their CDR3 loops, whereas such restriction was not observed with those selected by the same MHC with diverse sets of self-peptides including this peptide. Thus, we visualized the TCR structure required to survive positive selection directed by this single ligand. Our findings provide definitive evidence that specific recognition of self-peptides by TCR could be involved in positive selection of thymocytes.

Distinct Stage-Specific cis-Active Transcriptional Mechanisms Control Expression of T Cell Coreceptor CD8α at Double- and Single-Positive Stages of Thymic Development

Developing thymocytes that give rise to CD8+ (cytotoxic) and CD4+ (helper) αβ-TCR T lymphocytes go through progressive stages of expression of coreceptors CD8 and CD4 from being negative for both (the double-negative stage), to coexpressing both (the double-positive (DP) stage), to a mutually exclusive sublineage-specific expression of one or the other (the single-positive (SP) stage). To delineate the mechanisms underlying regulation of CD8 during these developmental transitions, we have examined expression of a series of mouse CD8α gene constructs in developing T cells of conventional and CD8α “knock-out” transgenic mice. Our results indicate that cis-active transcriptional control sequences essential for stage- and sublineage-specific expression lie within a 5′ 40-kb segment of the CD8 locus, ∼12 kb upstream of the CD8α gene. Studies to characterize and sublocalize these cis sequences showed that a 17-kb 5′ subfragment is able to direct expression of the CD8α gene up to the CD3intermediate DP stage but not in more mature DP or SP cells. These results indicate that stage-specific expression of CD8α in developing T cells is mediated by the differential activity of multiple functionally distinct cis-active transcriptional control mechanisms. It will be important to determine the relationship of “switching” between these cis mechanisms and selection.

Thymic lymphomas are resistant to Nur77-mediated apoptosis

We reported previously that thymic lymphomas from mice expressing transgenic TCR autoreactive against male (HY) antigen were resistant to anti-CD3 antibody-mediated induction of apoptosis although they were responding to TCR triggering. To test whether thymic lymphomas were specifically resistant to TCR-dependent Ca(++)-mediated induction of apoptosis, we have measured apoptosis of cells treated with Ca(++)-dependent (ionomycin, A23187) and Ca(++)-independent (etoposide, dexamethasone) inducers of apoptosis. Here we show that, unlike thymocytes, all thymic lymphomas were resistant to Ca(++)-dependent but not to Ca(++)-independent induction of apoptosis. These results excluded a general defect of apoptosis in lymphoma cells and suggested a specific inhibition of the calcium-mediated (TCR-dependent) pathway of apoptosis in lymphomas. Interestingly however, nuclear expression of a specific mediator of TCR-dependent apoptosis Nur77 was induced in ionomycin-resistant lymphomas indicating that, unlike normal thymocytes, thymic lymphomas are resistant to Nur77-mediated apoptosis.

Positive selection through a motif in the alphabeta T cell receptor

The two lineages of T cells, alphabeta and gammadelta, differ in their developmental requirements: only alphabeta T cells require major histocompatibility complex recognition, a process known as positive selection. The alphabeta T cell receptor (TCR), but not its gammadelta counterpart, contains a motif within the alpha-chain connecting peptide domain (alpha-CPM) that has been conserved over the last 500 million years. In transgenic mice expressing an alphabeta TCR lacking the alpha-CPM, thymocytes were blocked in positive selection but could undergo negative selection. Thus, the alpha-CPM seems to participate in the generation of signals required for positive selection.

An opposite pattern of selection of a single T cell antigen receptor in the thymus and among intraepithelial lymphocytes

The differentiation of intestinal intraepithelial lymphocytes (IEL) remains controversial, which may be due in part to the phenotypic complexity of these T cells. We have investigated here the development of IEL in mice on the recombination activating gene (RAG)-2(-/-) background which express a T cell antigen receptor (TCR) transgene specific for an H-Y peptide presented by Db (H-Y/Db x RAG-2(-) mice). In contrast to the thymus, the small intestine in female H-Y/Db x RAG-2(-) mice is severely deficient in the number of IEL; TCR transgene+ CD8alphaalpha and CD8alphabeta are virtually absent. This is similar to the number and phenotype of IEL in transgenic mice that do not express the Db class I molecule, and which therefore fail positive selection. Paradoxically, in male mice, the small intestine contains large numbers of TCR+ IEL that express high levels of CD8alphaalpha homodimers. The IEL isolated from male mice are functional, as they respond upon TCR cross-linking, although they are not autoreactive to stimulator cells from male mice. We hypothesize that the H-Y/Db TCR fails to undergo selection in IEL of female mice due to the reduced avidity of the TCR for major histocompatibility complex peptide in conjunction with the CD8alphaalpha homodimers expressed by many cells in this lineage. By contrast, this reduced TCR/CD8alphaalpha avidity may permit positive rather than negative selection of this TCR in male mice. Therefore, the data presented provide conclusive evidence that a TCR which is positively selected in the thymus will not necessarily be selected in IEL, and furthermore, that the expression of a distinct CD8 isoform by IEL may be a critical determinant of the differential pattern of selection of these T cells.

Intrathymic deletion of alloreactive T cells in mixed bone marrow chimeras prepared with a nonmyeloablative conditioning regimen

BACKGROUND

Mixed hematopoietic chimerism induced with a nonmyeloablative conditioning regimen leads to donor-specific transplantation tolerance. Analyses of specific Vbeta-bearing T-cell families that recognize endogenous superantigens demonstrated that donor-specific tolerance is due mainly to an intrathymic deletional mechanism in these mixed chimeras. However, superantigens are not known to behave as classical transplantation antigens. We therefore used T-cell receptor (TCR) transgenic (Tg) recipients expressing a clonotypic TCR specific for an allogeneic major histocompatibility complex antigen to further assess deletional tolerance.

METHODS

2C TCR Tg mice (H2b), whose Tg TCR recognizes major histocompatibility complex class I Ld, were used as recipients of Ld+ bone marrow cells after conditioning with depleting anti-CD4 and CD8 monoclonal antibodies, 3 Gy whole-body irradiation, and 7 Gy thymic irradiation. Chimerism and deletion of CD8+ 2C recipient T cells was evaluated by flow cytometry and by immunohistochemical staining. Tolerance was tested with in vitro cell-mediated lympholysis assays and in vivo by grafting with donor skin.

RESULTS

Intrathymic and peripheral deletion of 2C+ CD8-single-positive T cells was evident in mixed chimeras, and deletion correlated with the presence of donor-type cells with dendritic morphology in the thymus, and with chimerism in lymphohematopoietic tissues. Chimeras showed tolerance to the donor in cell-mediated lympholysis assays and specifically accepted donor skin grafts.

CONCLUSIONS

Tolerance to transplantation antigens is achieved through intrathymic deletion of donor-reactive T cells in mixed chimeras prepared with a nonmyeloablative conditioning regimen and allogeneic bone marrow transplantation.

Skewed TCRV beta repertoire in human thymus persists after thymic emigration: influence of genomic imposition, thymic maturation and environmental challenge on human TCRV beta usage in vivo

In order to investigate the mechanisms involved in originating a diverse TCR repertoire in human peripheral blood we analyzed TCRV beta surface expression in different T cell subsets of unrelated individuals. The relative frequencies of 11 distinct V beta chains were determined for immature double positive (DP) as well as for mature CD4 single positive (4SP) and CD8 single positive (8SP) thymocytes, respectively. By comparing these data with expression in peripheral blood T lymphocytes of the same donors we were able to show that usage of TCRV beta in peripheral T cells is significantly (p < 0.001) depending on the pattern in mature SP thymocytes whereas the frequency of TCRV beta families in immature DP thymocytes has no impact (p > 0.2). No association with distinct HLA-haplotypes was observed. Preferential usage of V beta-families in either CD4- or CD8-positive peripheral T cells also correlates with the status in mature thymic precursors (p < 0.001). Altogether, this first combined study of TCR frequencies within different stages of human T cell ontogeny indicates that TCRV beta repertoire is determined mainly through selectional processes within the thymus. Since neither genomically imposed expression nor modulating events in the periphery seem to have strong influence on the relative expression of TCRV beta chains these findings have to be considered in future studies of human diseases.

T Cells Expressing Receptors of Different Affinity for Antigen Ligands Reveal a Unique Role for p59fyn in T Cell Development and Optimal Stimulation of T Cells by Antigen

Signaling from the TCR involves the protein tyrosine kinase p59fyn (Fyn). Previous studies have shown that T cell development occurs normally in Fyn−/− mice. In this study, we investigated the requirement for Fyn in the development and function of T cells expressing either the transgenic 2C TCR, with high affinity for its Ag ligand, or the transgenic H-Y TCR, representative of a low affinity TCR. Although Fyn was not essential for positive selection of thymocytes expressing either the 2C or the H-Y TCR, it facilitated the down-regulation of the heat-stable Ag in positively selected CD4−CD8+ thymocytes in both 2C and H-Y mice. Negative selection of thymocytes expressing the H-Y TCR also occurs efficiently in Fyn−/− mice. However, in Fyn−/− mice, there was a preferential survival of thymocytes that expressed higher levels of the CD8 coreceptor and the transgenic TCR. Positively selected CD4−CD8+ thymocytes and peripheral T cells expressing either the 2C or the H-Y TCR differed in their requirement of Fyn for optimal proliferation responses to stimulation by antigenic ligands. Whereas 2C Fyn−/− or 2C Fyn+/+ thymocytes and peripheral T cells responded optimally to stimulation by the specific Ag, H-Y Fyn−/− thymocytes and peripheral T cells were hyporesponsive compared with Fyn+/+ cells. Significantly, in response to a defined low affinity ligand, both 2C Fyn−/− thymocytes and peripheral T cells required Fyn for optimal response to Ag stimulation. Thus, Fyn plays a role during thymocyte development and is required for optimal responses to low affinity/avidity ligands.

Quantitative analysis of the T cell repertoire selected by a single peptide-major histocompatibility complex

The positive selection of CD4+ T cells requires the expression of major histocompatibility complex (MHC) class II molecules in the thymus, but the role of self-peptides complexed to class II molecules is still a matter of debate. Recently, it was observed that transgenic mice expressing a single peptide-MHC class II complex positively select significant numbers of diverse CD4+ T cells in the thymus. However, the number of selected T cell specificities has not been evaluated so far. Here, we have sequenced 700 junctional complementarity determining regions 3 (CDR3) from T cell receptors (TCRs) carrying Vbeta11-Jbeta1.1 or Vbeta12-Jbeta1.1 rearrangements. We found that a single peptide-MHC class II complex positively selects at least 10(5) different Vbeta rearrangements. Our data yield a first evaluation of the size of the T cell repertoire. In addition, they provide evidence that the single Ealpha52-68-I-Ab complex skews the amino acid frequency in the TCR CDR3 loop of positively selected T cells. A detailed analysis of CDR3 sequences indicates that a fraction of the beta chain repertoire bears the imprint of the selecting self-peptide.

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.

Cellular Mechanisms Involved in Protection Against Influenza Virus Infection in Transgenic Mice Expressing a TCR Receptor Specific for Class II Hemagglutinin Peptide in CD4+ and CD8+ T Cells

Mice transgenic for a TCR that recognizes peptide110–120 of hemagglutinin of PR8 influenza virus in the context of MHC class II I-Ed molecules express the transgenes in both CD4+ and CD8+ T cells. We have found that these TCR-hemagglutinin (TCR-HA) transgenic mice display a significantly increased resistance to the primary infection with PR8 virus compared with the wild-type mice. The TCR-HA transgenic mice mounted significant MHC type II and enhanced MHC type I-restricted cytotoxicity as well as increased cytokine responses in both spleen and lungs after infection with PR8 virus. In contrast, the primary humoral response against PR8 virus was not significantly different from that of the wild-type mice. In vivo depletion and adoptive cell transfer experiments demonstrated that both CD4+ and CD8+ TCR-HA+ T cell subsets were required for the complete clearance of pulmonary virus following infection with a dose that is 100% lethal in wild-type mice. Whereas CD4+ TCR-HA+ T cells were necessary for effective activation and local recruitment of CD8+ T cells, CD8+ TCR-HA+ T cells showed a Th1-biased pattern and MHC type II-restricted cytotoxicity. However, in the absence of in vivo expression of MHC type I molecules on the infected cells, the protection conferred by the TCR-HA+ T cells was impaired, indicating that the enhanced MHC class I-restricted cytotoxicity due to TCR-HA+ CD4+ Th cells was a critical element for clearance of the pulmonary virus by the transgenic mice.

A positive role for thymus-derived steroids in formation of the T-cell repertoire

T cells undergo rigorous selection processes in the thymus that are necessary to prevent T cells with either autoreactive or nonfunctional T-cell receptors (TCRs) from entering the periphery. Although both positive and negative selection depend on TCR-mediated signals, the means by which a thymocyte interprets these signals to result in survival or death is not understood. Glucocorticoids are known to induce thymocyte apoptosis at high concentrations, but at lower concentrations glucocorticoids can antagonize TCR-mediated deletional signals and allow survival of thymocytes and T cell hybridomas. Interestingly, transgenic mice in which the expression of the glucocorticoid receptor has been downmodulated specifically in thymocytes have abnormal thymocyte differentiation, indicating that glucocorticoids play a significant role in T-cell development. Furthermore, we have demonstrated the presence of steroidogenic enzymes in the thymic epithelium and can show that, in vitro, these cells readily synthesize pregnenolone, the first product in the steroidogenic pathway, and deoxycorticosterone. Inhibition of local glucocorticoid biosynthesis in thymi from TCR transgenic mice during fetal thymic organ culture (FTOC) revealed significant alterations in the process of thymocyte selection. These data suggest that glucocorticoids do not simply suppress the immune system but rather are necessary for thymocyte survival and differentiation.

Defective TCR expression in transgenic mice constructed using cDNA-based alpha- and beta-chain genes under the control of heterologous regulatory elements

We describe the generation of ovalbumin (OVA)-specific, MHC class II-restricted alpha beta T cell receptor (TCR) transgenic mice. Initial attempts at generating these transgenic mice utilized heterologous regulatory elements to drive the expression of cDNA genes encoding the separate alpha- and beta-chains of the TCR. Unexpectedly, T cells bearing the transgenic alpha beta TCR failed to emerge from the thymus in these mice, although the transgenes did modify endogenous TCR expression. However, subsequent modification of the approach which enabled expression of the TCR beta-chain under the control of its natural regulatory elements generated mice whose peripheral T cells expressed the transgenic TCR and were capable of antigen-dependent proliferation. These results show that successful generation of MHC class II-restricted, OVA-specific alpha beta TCR transgenic mice was dependent upon combining cDNA- and genomic DNA-based constructs for expression of the respective alpha- and beta-chains of the TCR.

Opposite CD4/CD8 Lineage Decisions of CD4+8+ Mouse and Rat Thymocytes to Equivalent Triggering Signals: Correlation with Thymic Expression of a Truncated CD8α Chain in Mice But Not Rats

Unselected CD4+8+ rat thymocytes, generated in vitro from their direct precursors, are readily converted to functional TCRhigh T cells by stimulation with immobilized TCR-specific mAb plus IL-2. Lineage decision invariably occurs toward CD4−8+, regardless of the timing of TCR stimulation after entry into the CD4+8+ compartment or the concentration of TCR-specific mAb used for stimulation. CD4-specific mAb synergizes with suboptimal TCR-specific mAb in inducing T cell maturation, but lineage decision remains exclusively CD4−8+. These results contrast with those obtained in mice, in which Abs to the TCR complex were shown to promote CD4+8− T cell maturation from CD4+8+ thymocytes. Surprisingly, when rat and mouse CD4+8+ thymocytes were stimulated with PMA/ionomycin under identical conditions, the opposite lineage commitment was observed, i.e., mouse thymocytes responded with the generation of CD4+8− and rat thymocytes with the generation of CD4−8+ cells. It thus seems that CD4+8+ thymocytes of the two species respond with opposite lineage decisions to strong activating signals such as given by TCR-specific mAb or PMA/ionomycin. A possible key to this difference lies in the availability of p56lck for coreceptor-supported signaling. We show that in contrast to mouse CD4+8+ thymocytes, which express both a complete and a truncated CD8α-chain (CD8α′) unable to bind p56lck, rat thymocytes only express full-length CD8α molecules. Mice, but not rats, therefore may use CD8α′ as a “dominant negative” coreceptor chain to attenuate the CD8 signal, thereby facilitating MHC class II recognition through the higher amount of p56lck delivered, and rats may use a different mechanism for MHC class distinction during positive selection.

The role of the bcl-2/ced-9 gene family in cancer and general implications of defects in cell death control for tumourigenesis and resistance to chemotherapy

Cell production within an organ is determined by the rate of immigration, proliferation, differentiation, emigration and death of cells. Abnormalities in any one of these processes will disturb normal control of cell production, thereby eliciting hyperplasia can be an early event in neoplasia. Cell death, apoptosis, is a physiological process responsible for removing unwanted cells. It is used in multi-cellular organisms for tissue remodelling during embryogenesis, regulation of cell turnover and as a defence strategy against invading pathogens. In this review article we describe the role of the bcl-2/ced-9 gene family in cancer and discuss the general implications of defects in the apoptosis program for tumourigenesis and resistance of cancer cells to chemotherapy in light of current knowledge of the molecular mechanisms of cell death.

Peripheral T cell survival requires continual ligation of the T cell receptor to major histocompatibility complex-encoded molecules

In the thymus, T cells are selected according to their T cell receptor (TCR) specificity. After positive selection, mature cells are exported from primary lymphoid organs to seed the secondary lymphoid tissue. An important question is whether survival of mature T cells is an intrinsic property or requires continuous survival signals, i.e., engagement of the TCR by major histocompatibility complex (MHC) molecules in the periphery, perhaps in a similar way as occurring during thymic positive selection. To address this issue we used recombination-activating gene (Rag)-deficient H-2b mice expressing a transgenic TCR restricted by I-Ed class II MHC molecules. After engraftment with Rag-/- H-2d fetal thymi, CD4+8- peripheral T cells emerged. These cells were isolated and transferred into immunodeficient hosts of H-2b or H-2d haplotype, some of the latter being common cytokine receptor gamma chain deficient to exclude rejection of H-2b donor cells by host natural killer cells. Our results show that in the absence, but not in the presence, of selecting MHC molecules, peripheral mature T cells are short lived and disappear within 7 wk, indicating that continuous contact of the TCR with selecting MHC molecules is required for survival of T cells.

Ultrastructural analysis of mouse thymocyte subpopulations

To understand the lineage relationship and to define morphological characteristics of each thymocyte subset, we have performed ultrastructural analysis of highly purified thymocyte subpopulations. By flow cytometry, five subpopulations were sorted based on the expression of CD4 and CD8 and on cell size (forward scatter): large and small CD4+8+, CD4-8-, CD4+8-, and CD4-8+ thymocytes. Small CD4+8+ thymocytes were the smallest among lymphoid cells, and had a round and smooth cell outline with condensed nuclei, the cytoplasm was scanty and the cell organelles were not developed, suggesting the majority of this subset might be inactive by morphological criteria. CD4+8- thymocytes appeared to be similar to peripheral CD4+ T cells. The CD4-8- thymocyte subset contained morphologically immature cells in terms of cell size, presence of cell surface villi, and euchromatic appearance of the nucleus. CD4-8+ thymocytes heterogeneous in cell size, nuclear chromatin contents and amount of cytoplasm, could be divided into two distinct types. Type 1 CD4-8+ thymocytes were intermediate in size, and therefore similar to peripheral mature CD8+ T cells. Type 2 CD4-8+ thymocytes were large and irregular in shape (large CD4-8+) with irregular-shaped and euchromatic nuclei. Large CD4-8+ cells were, thus, considered to be at the transitional stage from CD4-8- to CD4+8+. At least two groups of large CD4+8+ cells were ultrastructurally classified by the nuclear chromatin content. Large CD4+8+ cells with heterochromatic nuclei were round with a smooth cell membrane, whereas large CD4+8+ cells with euchromatic nuclei were spherical with projections. Cytological features of heterochromatic large CD4+8+ cells are similar to those of small CD4+8+ thymocytes except for cell size. Euchromatic large CD4+8+ cells could be regarded as active blasts potentially leading to mature cells. Taken together, this is the first report that describes the ultrastructural characteristics of each thymocyte subset highly purified by flow cytometry.

Autoantibody production and cytokine profiles of MHC class I (beta2-microglobulin) gene deleted New Zealand black (NZB) mice

We established a colony of MHC class I deleted (knockout) NZB mice, which lack the beta2 microglobulin gene (NZB.beta2m-/-), to characterize the contribution of MHC class I to the thymic microenvironment abnormalities, autoantibody production and lupus-like disease of NZB mice. Using an extensive panel of well characterized monoclonal antibodies defining thymic epithelial and other stromal elements, we demonstrated that deletion of MHC class I molecules does not change the thymic abnormalities, including the presence of a cortical epithelial cell free region, ectopic expression of medullary epithelial antigens, and the irregular shape of the medullary epithelial network of NZB mice. Moreover, the decreased staining of MTS 33(+) cells, a marker of premature thymocyte maturation, was also seen in NZB.beta2m-/-. However, although NZB.beta2m-/- mice had approximately the same levels of IgM and IgG anti-ss and dsDNA antibodies when compared to control NZB mice, there were significant alterations in the incidence and onset of anti-erythrocyte antibody levels. NZB.beta2m-/- had a lower incidence and a delayed onset of anti-erythrocyte autoantibody production compared to that seen in NZB mice. We also compared constitutive and PHA-P-driven levels of IFN-gamma, IL-4, IL-6, and IL-12 in cells from NZB, NZB.beta-/-2, and control C57BL/6 mice. Mitogen stimulated cells showed a decreased IFN-gamma, and a marked increase in IL-6 and IL-12 in NZB and NZB.beta2m-/- mice.

Discrimination between maintenance- and differentiation-inducing signals during initial and intermediate stages of positive selection

As well as signaling through the alphabeta T cell receptor complex, positive selection of immature CD4+ 8+ thymocytes involves additional ill-defined accessory interactions provided by thymic epithelial cells. Here, we have isolated CD4+ 8+ thymocytes at a pre-positive selection stage of development (TCR- CD69- 4+ 8+ cells), or after initiation of positive selection (CD69+ 4+ 8+ cells), from mice where the normal lifespan of thymocytes is extended by the presence of a bcl-2 transgene, to allow us to discriminate between requirements for maintenance and differentiation signals during positive selection. We find that MHC class II+ thymic epithelial cells drive positive selection of TCR- CD69- 4+ 8+ bcl-2 tg thymocytes to the CD4+ and CD8+ stage, while no such mature subsets are observed when thymocytes are cultured alone or with major histocompatibility complex (MHC) class II+ salivary epithelial cells. However, CD4+ 8+ cells remain in such cultures in considerable numbers, and retain the potential for positive selection if re-cultured with thymic epithelium, suggesting that thymic epithelial cells provide specific differentiation-inducing signals for positive selection. In contrast, intermediate CD69+ 4+ 8+ thymocytes show some capacity for phenotypic conversion in the absence of thymic stromal cells although strikingly the single-positive CD4+ and CD8+ cells generated are not functionally competent. Finally, we show that prior culture of thymic epithelial cells under monolayer conditions abrogates their ability to support the initiation of positive selection, suggesting that the epithelial cell molecules necessary for the provision of differentiation signals during positive selection are down-regulated under such conditions.

Peptide-induced positive selection of TCR transgenic thymocytes in a coreceptor-independent manner

T cell receptor (TCR) transgenic thymocytes specific for the LCMV gp peptide are normally positively selected to the CD8 lineage. Transgenic thymocyte development was substantially reduced in the absence of these CD8 coreceptors. However, efficient positive selection was restored when TCR transgenic CD8-/- fetal thymic lobes were cultured with a peptide variant of the wild-type ligand. These mature thymocytes were functional, as shown by their ability to respond against strong peptide agonists. Additional experiments demonstrated that transgenic positive selection was peptide-specific. These results prove that CD8 does not possess essential signaling properties that are necessary for T cell development. In addition, the unilateral commitment of transgenic thymocytes to mature CD4-TCR(hi) T cells expressing intracellular perforin suggests that there must be some instructive component to CD4 down-regulation and lineage commitment during thymocyte selection.

Positive and negative CD4+ thymocyte selection by a single MHC class II/peptide ligand affected by its expression level in the thymus

The central event in thymic selection of T cells bearing alpha beta TCRs is their interaction with self-peptides bound to self-MHC molecules. With the use of transgenic mouse lines expressing a single peptide/MHC class II complex, we show that CD4+ T cells with the preferential usage of particular TCR V(alpha)s and V(beta)s were selected to mature on this complex in lines with the lower expression, whereas such CD4+ T cells were eliminated in the thymus in a line with the relatively high expression. When a low expressing line was crossed with a high expressing line, the frequency of CD4+ T cells selected by this complex markedly decreased. Thus, these results suggest that a single peptide/MHC class II complex, being affected by its cell surface density in the thymus, can serve as both positively and negatively selecting ligand in vivo.

Control of immunodeficiency and lymphoproliferation in mouse AIDS: studies of mice deficient in CD8+ T cells or perforin

CD8+ T cells were previously shown to be important in preventing lymphoproliferation and immunodeficiency following infection of murine AIDS (MAIDS)-resistant mice with the LP-BM5 mixture of murine leukemia viruses. To further evaluate the mechanisms contributing to MAIDS resistance, we studied mice lacking CD8+ T cells or deficient in perforin due to knockout of the beta2-microglobulin (beta2M) or perforin gene, respectively. In contrast to wild-type, MAIDS-resistant controls, B10.A mice homozygous for the beta2M mutation and B10.D2 mice homozygous for the perforin mutation were diagnosed as having MAIDS by 5 to 8 weeks after infection by the criteria of lymphoproliferation, impaired proliferative responses to mitogens, and changes in cell populations as judged by histopathology and flow cytometry. Unexpectedly, there was no progression of lymphoproliferation through 24 weeks, even though immune functions were severely compromised. Expression of the defective virus responsible for MAIDS was enhanced in spleens of the knockouts in comparison with wild-type mice. These results demonstrate that perforin-dependent functions of CD8+ T cells contribute to MAIDS resistance but that other, non-CD8-dependent mechanisms are of equal or greater importance.

Thymosin alpha 1 antagonizes dexamethasone and CD3-induced apoptosis of CD4+ CD8+ thymocytes through the activation of cAMP and protein kinase C dependent second messenger pathways

It is well established that glucocorticoid hormones and anti-CD3 monoclonal antibodies induce apoptosis in immature developing thymocytes. This process can be modulated by soluble factors, anti-oxidants and adhesion receptors. Previously we have demonstrated that thymosin alpha 1 (T alpha 1), a 28-amino acid thymic peptide hormone, is a dose and time dependent antagonist of dexamethasone (DEX) and CD# induced DNA fragmentation of murine thymocytes in vitro. To further investigate the mechanism of T alpha 1 action we determined a T alpha 1 sensitive thymocyte population and examined some of the molecular events associated with T alpha 1 anti-apoptotic activity. Phenotypic analysis of the sub-populations of thymocytes, based on CD4 and CD8 expression, revealed that T alpha 1 exerts its effect on CD4+ CD8+ immature thymocytes. T alpha 1 treatment of thymocytes delays the production of free radicals and the subsequent consumption of glutathione, that is observed during both DEX and CD3 induced apoptosis. We further demonstrate that T alpha 1 stimulates the production of cAMP and activates PKC in thymocytes. These data suggest that T alpha 1 exerts an influence on the development of a population of immature T-cells in the thymus by effecting the sensitivity of thymocytes to apoptosis during the pre-selection stages of thymic development. Our studies also suggest that the mechanism of T alpha 1 action involves the induction of both cAMP and PKC dependent second messenger pathways.

The agonist-antagonist balance in positive selection

Peptide antigens expressed in the thymus, in combination with self major histocompatibility complex molecules play a crucial role in thymocyte selection and shaping of the mature T-cell repertoire. Here, it is proposed that a single thymocyte may be exposed to numerous different peptide ligands as it matures, such that its fate is determined by the sum of signals produced by these interactions.

CD2 Regulates the Positive Selection and Function of Antigen-Specific CD4−CD8+ T Cells

The CD2 glycoprotein has been implicated in both positive and negative regulation of T-cell mitogenesis. To study the involvement of CD2 in T-lymphocyte development and immune responses, we have analyzed two lines of CD2-null mice, each expressing a distinct class I major histocompatibility complex (MHC)-restricted T-cell receptor (TCR). In both situations, the absence of CD2 appeared to promote the positive selection of cells in a manner that is similar to that which occurs in the absence of CD5. Consistent with this, compound homozygotes that lacked both CD2 and CD5 showed evidence of enhanced positive selection even in the absence of a transgenic TCR. Despite the observed enhancement of positive selection, the lack of CD2 was associated with defects in proliferative responses and interferon-γ production when transgenic thymocytes and mature T lymphocytes were stimulated with the appropriate antigens. These findings raise the possibility that impaired sensitivity to selecting ligands in the thymus may provide a selective advantage that improves the efficiency of positive selection for certain TCRs. Furthermore, the results highlight the potential for a differential role for CD2 in thymocyte selection and T-cell immune responses.

A transgenic T cell receptor restores thymocyte differentiation in interleukin-7 receptor alpha chain-deficient mice

Interleukin-7 (IL-7) receptor alpha chain-deficient (IL-7R alpha-/-) mice have severely depleted lymphocyte populations and thymocyte development is arrested at the double-negative (DN) stage. We show that thymocyte development in these mice can be reconstituted by the introduction of a transgenic T cell receptor (TCR), implying that one function of the IL-7R alpha chain is to initiate TCR gene rearrangement. Expression of the recombinase-activating genes RAG1 and RAG2 was greatly reduced in the IL-7R alpha-/- thymuses, and in DN thymocytes from the TCR transgenic IL-7R alpha-/- mice, but was restored in double-positive thymocytes from the TCR transgenic IL-7R alpha-/- mice. These data suggest that the IL-7R alpha chain controls RAG expression and initiation of TCR beta chain VDJ rearrangement in DN cells. In contrast, once cells have progressed beyond the DN stage of development the IL-7R alpha chain becomes no longer essential for RAG expression.

Requirement of the familial Alzheimer's disease gene PS2 for apoptosis. Opposing effect of ALG-3

ALG-3, a truncated mouse homologue of the chromosome 1 familial Alzheimer's disease gene PS2, rescues T hybridoma 3DO cells from T-cell receptor-induced apoptosis by inhibiting Fas ligand induction and Fas signaling. Here we show that ALG-3 transfected 3DO cells express a COOH-terminal PS2 polypeptide. Overexpression of PS2 in ALG-3 transfected 3DO cells reconstitutes sensitivity to receptor-induced cell death, suggesting that the artificial PS2 polypeptide functions as a dominant negative mutant of PS2. ALG-3 and antisense PS2 protect PC12 cells from glutamate-induced apoptosis but not from death induced by hydrogen peroxide or the free radical MPP+. Thus, the PS2 gene is required for some forms of cell death in diverse cell types, and its function is opposed by ALG-3.

Molecular basis for the recognition of two structurally different major histocompatibility complex/peptide complexes by a single T-cell receptor

2C is a typical alloreactive cytotoxic T lymphocyte clone that recognizes two different ligands. These ligands are adducts of the allo-major histocompatibility complex (MHC) molecule H-2Ld and an endogenous octapeptide, and of the self-MHC molecule H-2Kb and another peptide. MHC-binding and T-cell assays with synthetic peptides in combination with molecular modeling studies were employed to analyze the structural basis for this crossreactivity. The molecular surfaces of the two complexes differ greatly in densities and distributions of positive and negative charges. However, modifications of the peptides that increase similarity decrease the capacities of the resulting MHC peptide complexes to induce T-cell responses. Moreover, the roles of the peptides in ligand recognition are different for self- and allo-MHC-restricted T-cell responses. The self-MHC-restricted T-cell responses were finely tuned to recognition of the peptide. The allo-MHC-restricted responses, on the other hand, largely ignore modifications of the peptide. The results strongly suggest that adaptation of the T-cell receptor to the different ligand structures, rather than molecular mimicry by the ligands, is the basis for the crossreactivity of 2C. This conclusion has important implications for T-cell immunology and for the understanding of immunological disorders.

Completely allogeneic spleen cells induced cytolytic neonatal tolerance to alloantigens, but failed to establish allo-helper interactions with host T cells

The injection of spleen cells from F1 mice into-newborns from a parental strain results in the establishment of cytolytic tolerance to donor alloantigens and the development of a lupus-like disease. This syndrome is the consequence of the recognition by alloreactive host CD4+ T cells of discordant major histocompatibility complex (MHC) class II antigens on semi-allogeneic donor B cells. We have analysed whether completely allogeneic spleen cells are as able as semi-allogeneic spleen cells to induce cytolytic tolerance to donor alloantigens and to co-operate with alloreactive T cells for autoantibody production. BALB/c mice were injected at birth with Thy-1-depleted spleen cells from (C57BL/6 x BALB/c)F1 or C57BL/6 mice, either alone or in combination. Cytolytic tolerance was always induced, as manifested by persistence of chimerism and acceptance of skin allografts. However, only F1 semi-allogeneic B cells were activated by alloreactive host T cells to produce anti-DNA IgG antibody. The deficient co-operation between BALB/c CD4+ T cells and completely allogeneic C57BL/6 B cells was confirmed after neonatal injection of (C57BL/ 6 x BALB/c)F1(Igha) spleen cells together with C57BL/6(Ighb) spleen cells. These mice developed anti-DNA antibodies bearing only the Igha allotype. Similar results were observed in experiments of allogeneic interaction in vitro, in which BALB/c CD4+ T cells were cocultured with either (C57BL/6 x BALB/c)F1 or C57BL/6 B cells. The present results demonstrate that completely allogeneic spleen cells efficiently induced cytolytic unresponsiveness to donor alloantigens, but B cells contained in this spleen cell population were unable to establish allo-helper interactions with alloreactive CD4+ T cells, suggesting that cytolytic and helper T-cell interactions involved in alloreactivity may be different.

Raf regulates positive selection

T cell development is regulated by extracellular signals that mediate cellular proliferation and differentiation via specific signal transduction pathways. To determine the importance of the mitogen-activated protein kinase (MAP kinase) pathway in thymocyte development, we analyzed transgenic mice expressing dominant negative Raf (DN Raf) and a constitutively active v-Raf under the control of the p56lck proximal promoter. DN Raf had a profound effect on T cell receptor (TCR)-mediated signaling events as assessed by the inhibition of mitogen-induced proliferation of thymocytes in vitro. Overall thymocyte numbers were decreased by at most twofold from nontransgenic littermates. Positive selection was inhibited in DN Raf transgenic mice, as evidenced by both reduced numbers of mature thymocytes and a decrease in CD8+ thymocytes in female mice doubly transgenic for DN-Raf and a class I-restricted H-Y TCR. In contrast, the differentiation of double-positive thymocytes to single-positive thymocytes was enhanced in H-YTCR transgenic mice expressing constitutively active Raf (v-Raf). Thus, Raf regulates positive selection in the thymus.

Development of functional rat-derived T cells in SCID mice engrafted with the fetal thymus of LEC rats which are defective in CD4+ T cells

We reported that LEC rats are genetically deficient in the development of thymic CD4+8- cells and that this defect is caused by bone marrow (BM)-derived stem cells. To determine which BM-derived cells are responsible for the arrest of T-cell development in LEC rats, fetal thymuses of LEC rats, or LEA rats which bear the same major histocompatibility complex (MHC) as LEC rats but are immunologically normal, were engrafted under the kidney capsule of severe combined immunodeficiency (SCID) mice (LEC-TG and LEA-TG mice, respectively). We than examined the differentiation of T cells and their immunological functions in the SCID mice. A large number of rat-derived CD4+ T cells appeared in the peripheral blood, lymph nodes (LN) and spleens in LEC-TG mice. Furthermore, the peripheral LN cells in LEC-TG mice appeared to be functional. These cells produced IL-2 upon Con A stimulation, whereas LN cells from LEC rats produced no IL-2 in the same conditions. Thymopoiesis was observed at 3 weeks in LEC-TG as well as LEA-TG mice. The distribution of thymocyte subsets with respect to CD4 and CD8 expression in LEC-TG mice closely resembled that of LEA rat thymus and that in LEA-TG mice, suggesting that normal T-cell differentiation occurred in LEC-TG mice. The results indicated that BM-derived progenitor T cells of LEC rats could differentiate to functional CD4+ T cells.

Administration of anti-interleukin-2 receptor alpha antibody in vivo induces localized autoimmune disease

Neonatal thymectomy (Tx) of mice at day 3 after birth (Tx-3), but not day 7 (Tx-7), induces organ-localized autoimmune diseases such as oophoritis and gastritis. Lesions in Tx-3 mice can be prevented by injection of splenic CD4+ cells from syngeneic normal mice, and this CD4+ population with suppressor activity is activated extrathymically by self antigens. Since it is speculated that these CD4+ T suppressor cells (Ts) express the interleukin-2 receptor (IL-2R) as an activated T cell population, an attempt was made to eliminate these Ts from the developing immune system of Tx-7 mice and normal mice by i.p. injection of anti-IL-2R alpha monoclonal antibodies. Interestingly, organ-localized autoimmune disease with quite similar characteristics to those observed after neonatal Tx developed in not only Tx-7 mice, but also normal mice. The results thus indicate that CD4+ cells expressing IL-2R alpha play an important role, as Ts in the periphery, in maintaining immune tolerance.

Human leukocyte antigen system and the immune response to it

Immunologic rejection of tissue grafts follows recognition of donor alloantigens; either those resulting from ABO incompatibility of those encoded by the human major histocompatibility complex, HLA. Alloantigens encoded by HLA are present on membrane proteins that are expressed constitutively by tissues or whose expression can be induced by cytokines released during inflammation. Genes of the HLA complex are highly polymorphic resulting in variations in amino acid sequence that shape the peptide binding pocket of HLA molecules and define the complementary structure that interacts with the T lymphocyte receptor for antigen. Variants of HLA proteins expressed by the allografts that are not expressed by the recipient can stimulate the immune response to the allograft resulting in rejection both by humoral antibody and through attack by T lymphocytes. Class II HLA antigens on donor cells can stimulate these responses directly by contact with recipient T cells. However, rejection also may result when HLA antigens are released from the graft, processed to peptides, and presented to recipient T cells by cells expressing recipient HLA Class II molecules. Rejection can be avoided by preventing activation of T lymphocytes, by minimizing differences in HLA proteins between recipient and donor or by avoiding preexisting responses to donor HLA antigens.

Permissive recognition during positive selection

In the periphery alpha beta T lymphocytes recognize antigens in conjunction with major histocompatibility complex (MHC) molecules. In the thymus immature T cells are positively selected on MHC molecules in the apparent absence of cognate peptides. Thus, at different developmental stages a T cell responds to different epitopes, yet uses the identical alpha beta T cell antigen receptor (TcR). To explain this paradox it has been hypothesized that during positive selection immature T cells see peptides/ligands unique to the thymus, are selected by specific antagonists related to their cognate peptides, or are driven by lowered affinity thresholds of their TcR. Though different in detail, these theories rely on defined peptides uniquely matched to select certain TcR. However, we find that in a TcR-transgenic (TcR(trans +)) mouse severely limiting the diversity of peptides does not impair positive selection. We show that many unrelated peptides, including some naturally occurring on the cell surface, induce maturation of CD4-CD8+TcR(high) thymocytes. The same peptides when presented in conjunction with the selecting MHC molecule, are not recognized by peripheral T cells expressing the same TcR(trans). Therefore, these findings point to a promiscuous rather than discriminate recognition mode used by immature T cells.

Inhibition of Nur77/Nurr1 leads to inefficient clonal deletion of self-reactive T cells

The Nur77/Nurr1 family of DNA binding proteins has been reported to be required for the signal transduction of CD3/T cell receptor (TCR)-mediated apoptosis in T cell hybridomas. To determine the role of this family of DNA-binding proteins in thymic clonal deletion, transgenic (Tg) mice bearing a dominant negative mutation were produced. The transgene consisted of a truncated Nur77 (deltaNur77) gene encoding the DNA-binding domain of Nur77 ligated to a TCR-beta enhancer resulting in early expression in thymocytes. Apoptosis of CD4+CD8+ thymocytes mediated by CD3/TCR signaling was greatly inhibited in the deltaNur77 Tg mice, compared with non-Tg littermates, after treatment with anti-CD3 or anti-TCR antibody in vivo and in vitro. Clonal deletion of self-reactive T cells was investigated in deltaNur77-Db/HY TCR-alpha/beta double Tg mice. There was a five-fold increase in the total number of thymocytes expressing self-reactive Db/HY TCR-alpha/beta in the deltaNur77-TCR-alpha/beta double Tg male mice. Deficient clonal deletion of self-reactive thymocytes was demonstrated by a 10-fold increase in the CD4+CD8+ thymocytes that expressed Tg TCR-alpha/beta. There was an eightfold increase in the CD8+, Db/HY TCR-alpha/beta T cells in the lymph nodes (LN) of delta Nur77-Db/HY TCR-alpha/beta double Tg compared with Db/HY TCR-alpha/beta Tg male mice. In spite of defective clonal deletion, the T cells expressing the Tg TCR were functionally anergic. In vivo analysis revealed increased activation and apoptosis of T cells associated with increased expression of Fas and Fas ligand in LN of deltaNur77-Db/HY TCR-alpha/beta double male mice. These results indicate that inhibition of Nur77/Nurr1 DNA binding in T cells leads to inefficient thymic clonal deletion, but T cell tolerance is maintained by Fas-dependent clonal deletion in LN and spleen.

MHC class II-specific T cells can develop in the CD8 lineage when CD4 is absent

The generation of mature CD4 T cells from CD4+CD8+ precursor thymocytes usually requires corecognition of class II MHC by a TCR and CD4, while the production of mature CD8 T cells requires corecognition of class I MHC by a TCR and CD8. To assess the role of the CD4 coreceptor in development and lineage commitment, we generated CD4-deficient mice expressing a transgenic class II-specific TCR. Surprisingly, in the absence of CD4 a large number of T cells mature, but these cells appear in the CD8 lineage. Thus, when CD4 is present, the majority of immature T cells with this class II-specific TCR choose the CD4 lineage but develop in the CD8 pathway when CD4 is absent. The results indicate that even for TCRs that are not dependent on coreceptor for MHC recognition, the coreceptor can influence the lineage choice. These findings are considered in terms of a quantitative signaling model for CD4/CD8 lineage commitment.

Asymmetric redundancy in CD4 silencer function

We and others have defined a transcriptional silencer critical for the proper expression of the CD4 gene at all stages of T cell development. In this report, we use biochemical techniques to identify three different factor-binding sites within the CD4 silencer, denoted sites I, II, and III. Using transgenic analyses, we determine that although all three factor-binding sites are important for silencer activity, there is significant redundancy in that the presence of either site II alone, or the combination of sites I and III permits silencer function. Thus, our data indicate that the mechanism of function of the CD4 silencer is extremely complex. Further biochemical analyses indicate that the factor binding to site II has the same sequence specificity as a factor binding to an E box site in the CD4 enhancer; thus, a member of the bHLH factor family may be important in mediating silencer function.

The cytoplasmic domain of CD4 promotes the development of CD4 lineage T cells

Thymocytes must bind major histocompatibility complex (MHC) proteins on thymic epithelial cells in order to mature into either CD8+ cytotoxic T cells or CD4+ helper T cells. Thymic precursors express both CD8 and CD4, and it has been suggested that the intracellular signals generated by CD8 or CD4 binding to class I or II MHC, respectively, might influence the fate of uncommitted cells. Here we test the notion that intracellular signaling by CD4 directs the development of thymocytes to a CD4 lineage. A hybrid protein consisting of the CD8 extracellular and transmembrane domains and the cytoplasmic domain of CD4 (CD884) should bind class I MHC but deliver a CD4 intracellular signal. We find that expression of a hybrid CD884 protein in thymocytes of transgenic mice leads to the development of large numbers of class I MHC-specific, CD4 lineage T cells. We discuss these results in terms of current models for CD4 and CD8 lineage commitment.

Regulation of AP-1 and NFAT transcription factors during thymic selection of T cells

The ability of thymocytes to express cytokine genes changes during the different stages of thymic development. Although CD4- CD8- thymocytes are able to produce a wide spectrum of cytokines in response to a T-cell receptor (TcR)-independent stimulus, as they approach the double-positive (DP) CD4+ CD8+ stage, they lose the ability to produce cytokine. After the DP stage, thymocytes become single-positive CD4+ or CD8+ thymocytes which reacquire the ability to secrete cytokines. In an attempt to understand the molecular basis of this specific regulatin, we use AP-1-luciferase and newly generated NFAT-luciferase transgenic mice to analyze the transcriptional and DNA-binding activities of these two transcription factors that are involved in the regulation of cytokine gene expression. Here, we show that both AP-1 and NFAT transcriptional activities are not inducible in the majority of DP cells but that during the differentiation of DP cells to the mature single-positive stage, thymocytes regain this inducibility. Subpopulation analysis demonstrates that this inducibility is reacquired at the DP stage before the down-modulation of one of the coreceptors. Indeed AP-1 inducibility, just like the ability to express the interleukin-2 gene, is reacquired during the differentiation of DP TcRlow CD69low heat-stable antigen (HSA)high thymocytes to DP TcRhigh CD69high HSAhigh cells, which is considered to be the consequence of the first signal that initiates positive selection. We therefore propose that the inability of DP thymocytes to induce AP-1 and NFAT activities is one of the causes for the lack of cytokine gene expression at this stage and that this inducibility is reacquired at the latest stage of DP differentiation as a consequence of positive selection. This could be a mechanism to prevent the activation of DP thymocytes before selection has taken place.

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

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

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

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

Resistance to tolerance induction in the diabetes-prone biobreeding rat as one manifestation of abnormal responses to superantigens

T cells taken from normal rats treated with an exogenous source of bacterial superantigen in vivo specifically failed to proliferate following re-stimulation with the same superantigen in vitro. Responsiveness was restored following the addition of an exogenous source of interleukin-2 indicating that the T cells had been made functionally tolerant and not deleted. While staphylococcal enterotoxin treatment of normal rats virtually abolished T-cell proliferation to the same enterotoxin in vitro, T cells from similarly treated diabetes-prone Biobreeding (BB-DP) rats were markedly resistant to this in vivo effect. Responses in BB-DP rats were never reduced by more than 50% even when a 4 times more effective dose of enterotoxin was employed. The resistance of BB-DP peripheral T cells to staphylococcal enterotoxin-induced tolerance could not be attributed to differences in T-cell receptor V beta chain family usage of BB-DP vs normal T cells but was associated with qualitative differences in the way in which BB-DP T cells responded to staphylococcal enterotoxins in vitro. While under optimal stimulatory conditions BB-DP T-cell proliferative responses to staphylococcal enterotoxins appeared comparable to those from non-diabetes-prone animals, under superoptimal conditions BB-DP, but not diabetes-resistant, donor T-cell proliferative responses to staphylococcal enterotoxins could be blocked in vitro with antibodies to CD4 antigens. In addition, BB-DP T-cell proliferative responses were more sensitive to suboptimal staphylococcal enterotoxin doses in vitro. We discuss ways in which abnormal BB-DP T-cell responses to superantigens in general and resistance to staphylococcal enterotoxin-mediated tolerance induction in particular may play a role in the generation of a peripheral T-cell repertoire prone to autoimmunity.

Both MHC and background gene heterozygosity alter T cell receptor repertoire selection in an antigen-specific response

Many autoimmune diseases are associated with specific class II MHC alleles; however, this association is not complete. One explanation for the variable expression of disease in susceptible individuals is that variability in the TCR repertoire may alter the potential to generate pathogenic autoreactive T cells. The current study was undertaken to examine the possibility that MHC and background heterozygosity, which is the norm in the outbred human population, alters the expressed TCR repertoire and, if so, whether this has an impact on peptide recognition and antigenic specificity. We, therefore, systematically analysed the beef insulin-specific TCR repertoire in inbred BALB/c mice before and after introduction of MHC heterozygosity (BALB/c x BALB.K)F1 mice, or MHC and background gene heterozygosity (BALB/c x A/J)F1 mice. We show that T cells from all three repertoires are predominantly Ad-restricted and recognize the same immunodominant peptide. Despite this, the beef insulin-specific TCR repertoires in F1 mice differ from those seen in BALB/c mice with the most dramatic changes seen in (BALB/c x A/J)F1 mice. These changes are accompanied by subtle differences in the antigenic specificity of the T cells. The results demonstrate that both MHC and background gene heterozygosity affect TCR repertoire selection, suggesting that the variable expression of autoimmune disease in individuals with a susceptible MHC allele may result, in part, from variability in the TCR repertoire introduced by this heterozygosity.

HLA-DRB1 molecules and antigenic experience shape the repertoire of CD4 T cells

Forces influencing the composition of the mature TCR repertoire have been well studied in the mouse. In particular, the contribution of MHC molecules in negative and positive selection events of T lymphocytes has been established. To understand whether the allelic polymorphism of HLA-DRB1 molecules can shape the human TCR repertoire, we compared the usage of TCR V beta segments in two cohorts of unrelated individuals who were selected for the expression of HLA-DRB1 alleles. To investigate the potential role of antigenic experience in shaping the TCR repertoire, we compared the usage of V beta gene elements in CD45RO- CD4+ (naive) T cells versus CD45RO+ CD4+ (memory) T cells. A correlation between V beta gene segment usage and HLA-DRB1 alleles could be demonstrated for the repertoire of the naive CD4+ T cells, suggesting a shaping force of the HLA-DRB1 allele on the peripheral TCR repertoire. While the HLA-DRB1 imposed profile in V beta distribution was maintained in CD45RO+ CD4+ T cells, it was less pronounced, indicating that antigenic experience modulates the functional TCR repertoire.

A mutation in zap-70 protein tyrosine kinase results in a selective immunodeficiency

We have previously described a new type of selective T-cell deficiency characterized by persistent infections reminiscent of severe combined immunodeficiency. We show here that selective T-cell deficiency patients carry a mutation of zap-70 protein tyrosine kinase, resulting in a loss of the activity of this kinase. The thymus of zap-70(-1-1) patients shows the presence of CD4CD8 double-positive cells in the cortex, however, only CD4, and not CD8, single-positive cells are present in the medulla. Peripheral CD4+ T cells from the zap-70(-1-1) patients exhibit markedly reduced tyrosine phosphorylation, fail to produce interleukin-2, and do not proliferate in response to T-cell receptor stimulation by mitogens or antigens. Thus zap-70 kinase appears to be indispensable for the development of CD8 single-positive T cells as well as for the signal transduction and function of single-positive CD4 T cells.