Targeted depletion of PD-1-expressing cells induces immune tolerance through peripheral clonal deletion

Thymic negative selection of the T cell receptor (TCR) repertoire is essential for establishing self-tolerance and acquired allograft tolerance following organ transplantation. However, it is unclear whether and how peripheral clonal deletion of alloreactive T cells induces transplantation tolerance. Here, we establish that programmed cell death protein 1 (PD-1) is a hallmark of alloreactive T cells and is associated with clonal expansion after alloantigen encounter. Moreover, we found that diphtheria toxin receptor (DTR)-mediated ablation of PD-1+ cells reshaped the TCR repertoire through peripheral clonal deletion of alloreactive T cells and promoted tolerance in mouse transplantation models. In addition, by using PD-1-specific depleting antibodies, we found that antibody-mediated depletion of PD-1+ cells prevented heart transplant rejection and the development of experimental autoimmune encephalomyelitis (EAE) in humanized PD-1 mice. Thus, these data suggest that PD-1 is an attractive target for peripheral clonal deletion and induction of immune tolerance.

Proteasome-Generated cis-Spliced Peptides and Their Potential Role in CD8+ T Cell Tolerance

The human immune system relies on the capability of CD8+ T cells to patrol body cells, spot infected cells and eliminate them. This cytotoxic response is supposed to be limited to infected cells to avoid killing of healthy cells. To enable this, CD8+ T cells have T Cell Receptors (TCRs) which should discriminate between self and non-self through the recognition of antigenic peptides bound to Human Leukocyte Antigen class I (HLA-I) complexes-i.e., HLA-I immunopeptidomes-of patrolled cells. The majority of these antigenic peptides are produced by proteasomes through either peptide hydrolysis or peptide splicing. Proteasome-generated cis-spliced peptides derive from a given antigen, are immunogenic and frequently presented by HLA-I complexes. Theoretically, they also have a very large sequence variability, which might impinge upon our model of self/non-self discrimination and central and peripheral CD8+ T cell tolerance. Indeed, a large variety of cis-spliced epitopes might enlarge the pool of viral-human zwitter epitopes, i.e., peptides that may be generated with the exact same sequence from both self (human) and non-self (viral) antigens. Antigenic viral-human zwitter peptides may be recognized by CD8+ thymocytes and T cells, induce clonal deletion or other tolerance processes, thereby restraining CD8+ T cell response against viruses. To test this hypothesis, we computed in silico the theoretical frequency of zwitter non-spliced and cis-spliced epitope candidates derived from human proteome (self) and from the proteomes of a large pool of viruses (non-self). We considered their binding affinity to the representative HLA-A*02:01 complex, self-antigen expression in Medullary Thymic Epithelial cells (mTECs) and the relative frequency of non-spliced and cis-spliced peptides in HLA-I immunopeptidomes. Based on the present knowledge of proteasome-catalyzed peptide splicing and neglecting CD8+ TCR degeneracy, our study suggests that, despite their frequency, the portion of the cis-spliced peptides we investigated could only marginally impinge upon the variety of functional CD8+ cytotoxic T cells (CTLs) involved in anti-viral response.

Pulling RANK on Cancer: Blocking Aire-Mediated Central Tolerance to Enhance Immunotherapy

A major breakthrough in cancer treatment occurred with the development of strategies that overcome T-cell tolerance toward tumor cells. These approaches enhance antitumor immunity by overcoming mechanisms that are normally in place to prevent autoimmunity but simultaneously prevent rejection of tumor cells. Although tolerance mechanisms that restrict antitumor immunity take place both in the thymus and periphery, only immunotherapies that target peripheral tolerance mechanisms occurring outside of the thymus are currently available. We review here recent gains in our understanding of how thymic tolerance mediated by the autoimmune regulator (Aire) impedes antitumor immunity. It is now clear that transient depletion of Aire-expressing cells in the thymus can be achieved with RANKL blockade. Finally, we discuss key findings that support the repurposing of anti-RANKL as a cancer immunotherapy with a unique mechanism of action.

Clonal Deletion of Tumor-Specific T Cells by Interferon-γ Confers Therapeutic Resistance to Combination Immune Checkpoint Blockade

Resistance to checkpoint-blockade treatments is a challenge in the clinic. We found that although treatment with combined anti-CTLA-4 and anti-PD-1 improved control of established tumors, this combination compromised anti-tumor immunity in the low tumor burden (LTB) state in pre-clinical models as well as in melanoma patients. Activated tumor-specific T cells expressed higher amounts of interferon-γ (IFN-γ) receptor and were more susceptible to apoptosis than naive T cells. Combination treatment induced deletion of tumor-specific T cells and altered the T cell repertoire landscape, skewing the distribution of T cells toward lower-frequency clonotypes. Additionally, combination therapy induced higher IFN-γ production in the LTB state than in the high tumor burden (HTB) state on a per-cell basis, reflecting a less exhausted immune status in the LTB state. Thus, elevated IFN-γ secretion in the LTB state contributes to the development of an immune-intrinsic mechanism of resistance to combination checkpoint blockade, highlighting the importance of achieving the optimal magnitude of immune stimulation for successful combination immunotherapy strategies.

Immune monitoring of transplant patients in transient mixed chimerism tolerance trials

This review focuses on mechanistic studies performed in recipients of non-myeloablative bone marrow transplant regimens developed at Massachusetts General Hospital in HLA-identical and HLA-mismatched haploidentical combinations, initially as a platform for treatment of hematologic malignancies with immunotherapy in the form of donor leukocyte infusions, and later in combination with donor kidney transplantation for the induction of allograft tolerance. In patients with permanent mixed chimerism, central deletion may be a major mechanism of long-term tolerance. In patients in whom donor chimerism is only transient, the kidney itself plays a significant role in maintaining long-term tolerance. A high throughput sequencing approach to identifying and tracking a significant portion of the alloreactive T cell receptor repertoire has demonstrated biological significance in transplant patients and has been useful in pointing to clonal deletion as a long-term tolerance mechanism in recipients of HLA-mismatched combined kidney and bone marrow transplants with only transient chimerism.

Mode of Tolerance Induction and Requirement for Aire Are Governed by the Cell Types That Express Self-Antigen and Those That Present Antigen

Aire controls the fate of autoreactive thymocytes (i.e., clonal deletion or development into regulatory T cells [Tregs]) through transcriptional control of the expression of tissue-restricted self-antigens (TRAs) from medullary thymic epithelial cells (mTECs) and bone marrow (BM)-derived cells. Although TRAs expressed by mTECs and BM-derived cells are suggested to complement each other to generate a full spectrum of TRAs, little is known about the relative contribution of TRAs from each component for establishment of self-tolerance. Furthermore, the precise role of Aire in specific types of Aire-expressing APCs remains elusive. We have approached these issues by generating two different types of transgenic mouse (Tg) model, which express a prefixed model self-antigen driven by the insulin promoter or the Aire promoter. In the insulin-promoter Tg model, mTECs alone were insufficient for clonal deletion, and BM-derived APCs were required for this action by utilizing Ag transferred from mTECs. In contrast, mTECs alone were able to induce Tregs, although at a much lower efficiency in the absence of BM-derived APCs. Importantly, lack of Aire in mTECs, but not in BM-derived APCs, impaired both clonal deletion and production of Tregs. In the Aire-promoter Tg model, both mTECs and BM-derived APCs could independently induce clonal deletion without Aire, and production of Tregs was impaired by the lack of Aire in mTECs, but not in BM-derived APCs. These results suggest that the fate of autoreactive thymocytes together with the requirement for Aire depend on the cell types that express self-antigens and the types of APCs involved in tolerance induction.

BIM Deficiency Protects NOD Mice From Diabetes by Diverting Thymocytes to Regulatory T Cells

Because regulatory T-cell (Treg) development can be induced by the same agonist self-antigens that induce negative selection, perturbation of apoptosis will affect both negative selection and Treg development. But how the processes of thymocyte deletion versus Treg differentiation bifurcate and their relative importance for tolerance have not been studied in spontaneous organ-specific autoimmune disease. We addressed these questions by removing a critical mediator of thymocyte deletion, BIM, in the NOD mouse model of autoimmune diabetes. Despite substantial defects in the deletion of autoreactive thymocytes, BIM-deficient NOD (NODBim(-/-)) mice developed less insulitis and were protected from diabetes. BIM deficiency did not impair effector T-cell function; however, NODBim(-/-) mice had increased numbers of Tregs, including those specific for proinsulin, in the thymus and peripheral lymphoid tissues. Increased levels of Nur77, CD5, GITR, and phosphorylated IκB-α in thymocytes from NODBim(-/-) mice suggest that autoreactive cells receiving strong T-cell receptor signals that would normally delete them escape apoptosis and are diverted into the Treg pathway. Paradoxically, in the NOD model, reduced thymic deletion ameliorates autoimmune diabetes by increasing Tregs. Thus, modulating apoptosis may be one of the ways to increase antigen-specific Tregs and prevent autoimmune disease.

Negative selection assay based on stimulation of T cell receptor transgenic thymocytes with peptide-MHC tetramers

Thymocyte negative selection is a requirement for the development of self tolerance. Although it is possible to assay the induction of cell death in thymocytes in vitro using antibody cross-linking, this stimulus is much stronger than the normal range of T cell receptor ligands that could be encountered during normal development. Signaling in thymocytes is finely balanced between positive and negative selection stimuli, where a negative selecting ligand can be only marginally higher affinity than a positive selecting ligand. We have therefore developed an assay for the induction of negative selection that can distinguish such cases, and that is amenable to high-throughput analysis. The assay is based on the induction of activated caspase 3 in thymocytes expressing a defined T cell receptor, after stimulation with MHC-peptide tetramers in vitro for 24 hours or less.

Presentation of neuroendocrine self in the thymus: a necessity for integrated evolution of the immune and neuroendocrine systems

During evolution, from ancestor thymoids scattered in gill baskets of the lamprey, the first unique thymus appeared in jawed cartilaginous fishes around 450-500 millions years ago, concomitantly or shortly after the emergence of recombinase-dependent adaptive immunity. The major biological function of the thymus is to generate a diverse repertoire of T cell receptors that are self tolerant. The thymus achieves this role by using two complementary and intimately associated mechanisms: apoptotic deletion of T cell clones bearing a TCR with high affinity for self-antigens presented by MHC proteins on thymic epithelial cells (TECs) and dendritic cells (DCs); and generation of self-antigen-specific natural regulatory T (nT(reg)) cells. Moreover, the escape from thymic central self-tolerance plays a primary role in the development of autoimmune diseases that are a significant burden for the quality of life and health-care cost. Our new knowledge in thymus physiology and physiopathology is currently translated into innovative therapeutic strategies against these devastating chronic diseases.

Detection of an autoreactive T-cell population within the polyclonal repertoire that undergoes distinct autoimmune regulator (Aire)-mediated selection

The autoimmune regulator (Aire) plays a critical role in central tolerance by promoting the display of tissue-specific antigens in the thymus. To study the influence of Aire on thymic selection in a physiological setting, we used tetramer reagents to detect autoreactive T cells specific for the Aire-dependent tissue-specific antigen interphotoreceptor retinoid-binding protein (IRBP), in the polyclonal repertoire. Two class II tetramer reagents were designed to identify T cells specific for two different peptide epitopes of IRBP. Analyses of the polyclonal T-cell repertoire showed a high frequency of activated T cells specific for both IRBP tetramers in Aire(-/-) mice, but not in Aire(+/+) mice. Surprisingly, although one tetramer-binding T-cell population was efficiently deleted in the thymus in an Aire-dependent manner, the second tetramer-binding population was not deleted and could be detected in both the Aire(-/-) and Aire(+/+) T-cell repertoires. We found that Aire-dependent thymic deletion of IRBP-specific T cells relies on intercellular transfer of IRBP between thymic stroma and bone marrow-derived antigen-presenting cells. Furthermore, our data suggest that Aire-mediated deletion relies not only on thymic expression of IRBP, but also on proper antigen processing and presentation of IRBP by thymic antigen-presenting cells.

CD24 on thymic APCs regulates negative selection of myelin antigen-specific T lymphocytes

Negative selection plays a key role in the clonal deletion of autoreactive T cells in the thymus. However, negative selection is incomplete; as high numbers of autoreactive T cells can be detected in normal individuals, mechanisms that regulate negative selection must exist. In this regard, we previously reported that CD24, a GPI-anchored glycoprotein, is required for thymic generation of autoreactive T lymphocytes. The CD24-deficient 2D2 TCR transgenic mice (2D2(+) CD24(-/-) ), whose TCR recognizes myelin oligodendrocyte glycoprotein (MOG), fail to generate functional 2D2 T cells. However, it was unclear if CD24 regulated negative selection, and if so, what cellular mechanisms were involved. Here, we show that elimination of MOG or Aire gene expression in 2D2(+) CD24(-/-) mice - through the creation of 2D2(+) CD24(-/-) MOG(-/-) or 2D2(+) CD24(/) ∼Aire(-/-) mice - completely restores thymic cellularity and function of 2D2 T cells. Restoration of CD24 expression on DCs, but not on thymocytes also partially restores 2D2 T-cell generation in 2D2(+) CD24(-/-) mice. Taken together, we propose that CD24 expression on thymic antigen-presenting cells (mTECs, DCs) down-regulates autoantigen-mediated clonal deletion of autoreactive thymocytes.

Aire regulates the transfer of antigen from mTECs to dendritic cells for induction of thymic tolerance

To investigate the role of Aire in thymic selection, we examined the cellular requirements for generation of ovalbumin (OVA)-specific CD4 and CD8 T cells in mice expressing OVA under the control of the rat insulin promoter. Aire deficiency reduced the number of mature single-positive OVA-specific CD4(+) or CD8(+) T cells in the thymus, independent of OVA expression. Importantly, it also contributed in 2 ways to OVA-dependent negative selection depending on the T-cell type. Aire-dependent negative selection of OVA-specific CD8 T cells correlated with Aire-regulated expression of OVA. By contrast, for OVA-specific CD4 T cells, Aire affected tolerance induction by a mechanism that operated independent of the level of OVA expression, controlling access of antigen presenting cells to medullary thymic epithelial cell (mTEC)-expressed OVA. This study supports the view that one mechanism by which Aire controls thymic negative selection is by regulating the indirect presentation of mTEC-derived antigens by thymic dendritic cells. It also indicates that mTECs can mediate tolerance by direct presentation of Aire-regulated antigens to both CD4 and CD8 T cells.

The role of Aire in clonal selection

In his clonal selection theory, Frank Macfarlane Burnet predicted that autoreactive lymphocytes are deleted to prevent autoimmunity. This and other principles of lymphocyte behavior outlined by Burnet guided many studies that lead to our current understanding of thymic selection. Thus, when the genetic mutation responsible for autoimmune polyglandular syndrome type 1 was mapped to the autoimmune regulator (AIRE) gene, and Aire was found to be highly expressed in thymic epithelium, studying the role of Aire in negative selection made sense in the context of modern models of thymic selection. We now know Aire is a transcription factor required for the expression of many tissue-specific antigens (TSAs) in the thymus. In the absence of functional Aire, human patients and mice develop multi-organ autoimmune disease because of a defect in thymic negative selection. In addition to its role in the thymus, recent work in our lab suggests that extrathymic Aire-expressing cells have an important role in the clonal deletion of autoreactive CD8+ T cells. In this review, we summarize the latest studies on thymic and peripheral Aire-expressing cells, as well as other TSA-expressing stromal cell populations in peripheral lymphoid organs. We also discuss theoretical differences in thymic and peripheral Aire function that warrant further studies.

Removal of physiologic corticosteroid doses results in HLA antibody appearance and allograft dysfunction in patients transplanted under a clonal deletion protocol

Many transplant protocols now use high-intensity induction immunosuppression immediately post-transplant, followed by weaning from immunosuppressives. This chapter reports on the risks of post-transplant immunosuppression reduction/removal revealed by a close monitoring of HLA antibodies in 14 patients who recently received living-donor kidney grafts under clonal deletion at the IKDRC-ITS in Ahmedabad, India. While all 14 patients still have good function at a median follow-up of 13.5 months (7.3-20.1), only two had no dysfunction or antibody appearance. Antibodies appeared in 10 of the other 12, and serum creatinine rose in six of the 12. In light of the high rate of immune activation following immunosuppression reduction/withdrawal, it is clear that close monitoring is required to ensure safety in these patients. The tests that are most effective as early warning of immune activation in these kidney recipients were elevation of serum creatinine and antibodies monitoring, the latter being, perhaps, the more important for reasons discussed in this chapter.

Clonal deletion using total lymphoid irradiation with no maintenance immunosuppression in renal allograft recipients

A total of 69 individuals received a kidney from a living donor after a TLI-based clonal deletion protocol with no post-transplant maintenance immunosuppression planned. If needed, immunosuppression was started on a patient-specific basis, adding one drug at a time, a strategy we AWN". call "Drugs Added When Needed," or "DAWN. Following this strategy, at last follow-up 40 of the 69 patients (58%) had to be rescued by conventional immunosuppression, 23 (33%) had to be started on daily prednisone and six (9%) remained with no maintenance immunosuppression. The overall rate of de novo donor-specific antibody produced was 36% (in 25 of the 69 patients), and mean time to detection was about four months. The incidence of acute rejection episodes that displayed humoral components was 27% (19 cases), of which 14 were pure antibody-mediated rejection, five combined antibody- and T-cell-mediated rejection, and six were episodes (9%) of pure T-cell-mediated rejection. Finally, this study shows that although complete clonal deletion was not achieved, an important proportion of patients--42%, or 29 of the original 69--could be maintained with prednisone alone or even with no immunosuppression for a total mean follow-up of 13.3 months. Moreover, 16 patients with recent follow-up are surviving with no maintenance immunosuppression or just on prednisone. The mean serum creatinine at last follow-up for these 16 patients is 1.33 +/- 0.2 mg/dL with a mean follow-up of 19.3 months. Clonal deletion can be used to transplant patients without maintenance immunosuppression, adding drugs only as needed.

[Cloning deletion of mouse medullary CD4SP thymocyte subgroups induced by superantigen staphylococcal enterotoxin B]

OBJECTIVE

To investigate the clonal deletion of medullary CD4SP thymocyte subgroups induced by superantigen SEB, and to understand the stage specificity of thymic negative selection based on the developmental pathway of medullary CD4SP thymocytes we established previously.

METHODS

SEB was intravenously injected into C57BL/6 mice, and then the expression of surface molecules and apoptosis of thymocytes were detected by FACS. The reduction of each subgroup of medullary CD4SP thymocytes was analyzed by FACS assay and cell counting.

RESULTS

The percentage of apoptotic CD4SP thymocytes significantly increased after the intravenous injection of SEB. The cell number of CD4SP decreased about 43.8%. Among TCR Vbeta8(+)CD4SP thymocytes, the percentage of 6C10(+)CD69(+)Qa-2(-)thymocytes(SP1) was reduced by two thirds and the percentage of Qa-2(+) thymocytes(SP4) was comparatively increased, while the cell numbers of SP1 to SP3 decreased over 80% and that of SP4 decreased about 50%.

CONCLUSION

Thymic negative selection occurs during the stage of SP and goes through the whole developmental progress from SP1 to SP4.The thymocytes in the stage of SP1 to SP3 are more sensitive to apoptosis than those in the stage of SP4.

Breakdown of T cell tolerance and autoimmunity in primary immunodeficiency--lessons learned from monogenic disorders in mice and men

A key feature of the immune system is the capacity to monitor and control infections from non-self pathogens while maintaining tolerance to self-antigens. Primary immunodeficiencies (PID) are characterized by an increased susceptibility to infections, often associated with aberrant inflammatory responses and a concomitant high prevalence of autoimmunity. Autoimmunity in PID raises a conundrum: How can an immune system fail to respond to non-self pathogens while reacting vigorously to self-antigens? Recent advances from studies of PID patients and related animal models have revealed the crucial role of Aire-induced expression of self-antigens for deletion of autoreactive T cells in the thymus (central tolerance). Moreover, lessons from PID have provided unequivocal evidence for the essential role of regulatory T cells in suppressing autoreactive T cells in the periphery. Finally, findings from PID have broadened our understanding of how homeostatic proliferation and increased load or decreased clearance of apoptotic cells and non-self pathogens can lead to breakdown of peripheral tolerance.

H2E-derived Ealpha52-68 peptide presented by H2Ab interferes with clonal deletion of autoreactive T cells in autoimmune thyroiditis

Susceptibility and resistance to experimental autoimmune thyroiditis is encoded by MHC H2A genes. We reported that traditionally resistant B10 (H2(b)) mice permit thyroiditis induction with mouse thyroglobulin (mTg) after depleting regulatory T cells (Tregs), supporting A(b) presentation to thyroiditogenic T cells. Yet, Ea(k) transgenic mice, expressing A(b) and normally absent E(b) molecules (E(+)B10 mice), are susceptible to thyroiditis induction without Treg depletion. To explore the effect of E(b) expression on mTg presentation by A(b), seven putative A(b)-binding, 15-16-mer peptides were synthesized. Five were immunogenic for both B10 and E(+)B10 mice. The effect of E(b) expression was tested by competition with an Ealpha52-68 peptide, because Ealpha52-68 occupies approximately 15% of A(b) molecules in E(+)B10 mice, binding with high affinity. Ealpha52-68 competitively reduced the proliferative response to mTg, mTg1677, and mTg2342 of lymph node cells primed to each Ag. Moreover, mTg1677 induced mild thyroiditis in Treg-depleted B10 mice, and in E(+)B10 mice without the need for Treg depletion. Ealpha52-68 competition with mTg-derived peptides may impede clonal deletion of pathogenic, mTg-specific T cells in the thymus.

The lymphotoxin pathway regulates Aire-independent expression of ectopic genes and chemokines in thymic stromal cells

Medullary thymic epithelial cells (mTEC) play an important and unique role in central tolerance, expressing tissue-restricted Ags (TRA) which delete thymocytes autoreactive to peripheral organs. Since deficiencies in this cell type or activity can lead to devastating autoimmune diseases, it is important to understand the factors which regulate mTEC differentiation and function. Lymphotoxin (LT) ligands and the LTbetaR have been recently shown to be important regulators of mTEC biology; however, the precise role of this pathway in the thymus is not clear. In this study, we have investigated the impact of this signaling pathway in greater detail, focusing not only on mTEC but also on other thymic stromal cell subsets. LTbetaR expression was found in all TEC subsets, but the highest levels were detected in MTS-15(+) thymic fibroblasts. Rather than directing the expression of the autoimmune regulator Aire in mTEC, we found LTbetaR signals were important for TRA expression in a distinct population of mTEC characterized by low levels of MHC class II (mTEC(low)), as well as maintenance of MTS-15(+) fibroblasts. In addition, thymic stromal cell subsets from LT-deficient mice exhibit defects in chemokine production similar to that found in peripheral lymphoid organs of Lta(-/-) and Ltbr(-/-) mice. Thus, we propose a broader role for LTalpha1beta2-LTbetaR signaling in the maintenance of the thymic microenvironments, specifically by regulating TRA and chemokine expression in mTEC(low) for efficient induction of central tolerance.

[Prediction of T-cell immune reconstitution by investigating T cell receptor excision circle and T-cell receptor beta-chain variable region clonal repertoire in leukemia patients after allogeneic hematopoietic stem cell transplantation]

OBJECTIVE

To predict T-cell immune reconstitution by investigating T cell receptor excision circles (TREC) and T-cell receptor beta-chain variable region (TCRBV) clonal repertoire in leukemia patients after allogeneic hematopoietic stem cell transplantation (allo-HSCT).

METHODS

Real-time quantitative PCR was used to detect the TREC in 43 leukemia patients undergoing matched sibling donor bone marrow transplantation (MSD BMT), matched unrelated donor (MUD) BMT, or haploidentical-stem cell transplantation (HID-SCT), and in 70 normal individuals. RT-PCR was used to amplify 24 subfamily genes of T-cell receptor beta-chain variable region (TCRBV) in 24 of the 43 patients and 5 normal donors as control. The PCR products were further analyzed by genescane to evaluate the clonality of BV subfamily, characteristics of complementarity determining region 3 (CDR3), and the usage rate in BV subfamily.

RESULTS

There were (335.1 +/- 782.5) copies/10(5) cells in the 43 patients before transplantation, far lower than the normal value. The TREC values of the patients of the 3 groups all decreased obviously 3 months after transplantation. The TREC value of the MSD-BMT group recovered faster than the other two groups and reached the value before transplantation in 24 months. The recovery of TREC value in the HID-BMT group was delayed. 3 - 19 months after transplantation, the usage of TCRBV subfamilies was still restricted. There were 6 - 16 BV subfamilies expressed and 33% - 48% of them were polyclonals, the others were monoclones and oligoclones and existed in 24 BV subfamilies, no common monoclone BV subfamilies was expressed.

CONCLUSION

Investigation of the TREC and TCRBV clonal repertoire showed that the number of naive T cell is lower and the usage of TCRBV subfamilies skewed 3 - 24 months after allo-HSCT. The immune deficiency of the patients undergoing HID-BMT is more prominent and consistent with the clinical process.

CD45RO: A Marker for BCR-mediated Selection

We previously showed that IgH sequence alone minimally influenced germinal centre (GC) B-cell survival fate. As end-stage effector B cells are typically more mutated than founder GC B cells, we worked to develop an assay that would enrich for populations of GC B cells with progressively increasing numbers of somatic mutations, which could potentially be used as an indicator of positive selection. We targeted CD45 as it has been shown to influence activation-induced cytidine deaminase (AID) expression. In this study, anti-CD77 and anti-CD45RO (RO) were used to subdivide CD19(+)IgD(-)CD38(+)CD77(+) centroblasts (CB) and CD19(+)IgD(-)CD38(+)CD77(-) centrocytes (CC) into three contiguous RO fractions (RO(-), RO(+/-) and RO(+)) and assessed whether mutation frequency and characteristics associated with selection varied with respect to increasing RO expression. Here, we show that the average number of mutations per IgV(H)4 transcript increased concordantly with RO for CC, but not for CB. CC also exhibited an RO-associated increase in replacement mutations. Comparative analysis of clonally related sequences revealed that increased mutations were not due to the exclusive persistence of surface RO on highly mutated cells. RO-expressing CC and CB pools showed increased signs of activation (CD69(+)) and were enriched for surface Ig(+) cells. BCR-crosslinking induced a significant increase in surface RO on total tonsillar and GC B cells, which collectively suggests that the RO-associated increase in mutations is attributable, at least in part, to the cycling of cells that may have recently undergone BCR-mediated selection, or are potentially in developmental transition between CC and CB stages.

Peripheral deletion of mature alloreactive B cells induced by costimulation blockade

Alloreactive B cells can contribute to graft rejection. Anti-CD154 treatment together with donor-specific transfusion (DST) results in the long-term survival of MHC-mismatched mouse heart grafts and inhibition of alloantibody production. To characterize the mechanism of B cell tolerance induced by the anti-CD154 and DST, we used 3-83Igi mice, on BALB/c (H-2K(d)) background, that express a B cell receptor that reacts with MHC class I antigens H-2K(b). Transplanting C57BL/6 (H-2K(b)) hearts into 3-83Igi mice, followed by tolerance induction, resulted in the peripheral deletion of mature but not immature 3-83 B cells. The sustained deletion of mature alloreactive B cells required the presence of the allograft and can be explained by the absence of T cell help.

Cutting Edge: Bim is required for superantigen-mediated B cell death

To impair B cell clonal regulation, the microbial virulence factor, protein A of Staphylococcus aureus, can interact with evolutionarily conserved BCR-binding sites to induce a form of Fas-independent activation-associated B cell death that results in selective immune tolerance. We now show that this in vivo death pathway is associated with induction of increased transcript and protein levels of Bim, a BH3-only proapoptotic Bcl-2 family protein, which is inhibited by excess B cell-activating factor. An absolute requirement for Bim was documented, since Bim-deficient B cells were protected from in vivo superantigen-induced death and instead underwent persistent massive supraclonal expansion without functional impairment. These studies characterize a BCR-dependent negative clonal selection pathway that has been co-opted by a common bacterial pathogen to induce selective defects in host immune defenses.

The PI3K p110delta is required for down-regulation of RAG expression in immature B cells

At the immature B cell stage the BCR signals the down-regulation of the RAG genes and Ig L chain (LC) allelic and isotype exclusion. The signaling pathway that regulates these events is poorly characterized. We demonstrate that immature B cells from mice deficient in the PI3K catalytic subunit p110delta fail to suppress RAG expression and inappropriately recombine kappa and lambda LC loci. In addition, in the presence of the autoantigen, clonal deletion and receptor editing still takes place, demonstrating that these processes are independent of p110delta. These results demonstrate a role for p110delta in the regulation of RAG gene expression and thereby LC allelic/isotype exclusion.

IFN-gamma mediates CD4+ T-cell loss and impairs secondary antitumor responses after successful initial immunotherapy

Protective cell-mediated immune responses in cancer are critically dependent on T-helper type 1 (T(H)1) cytokines such as interferon-gamma (IFN-gamma). We have previously shown that the combination of CD40 stimulation and interleukin-2 (IL-2) leads to synergistic antitumor responses in several models of advanced metastatic disease. We now report that after this treatment and other immunotherapy regimens, the CD4+ T-cell population, in contrast to CD8+ T cells, did not significantly increase but rather exhibited a substantial level of apoptosis that was dependent on IFN-gamma. Mice immunized with tumor cells and treated with an immunotherapy regimen that was initially protective were later unable to mount effective memory responses compared with immunized mice not receiving immunotherapy. Immunotherapy given to tumor-bearing Ifngr-/- mice resulted in restoration of secondary responses. Thus, although immunotherapeutic regimens inducing strong IFN-gamma responses can lead to successful early antitumor efficacy, they may also impair the development of durable antitumor responses.

Cognate CD4+ Help Elicited by Resting Dendritic Cells Does Not Impair the Induction of Peripheral Tolerance in CD8+ T Cells

Peripheral tolerance is required to prevent autoimmune tissue destruction by self-reactive T cells that escape negative selection in the thymus. One mechanism of peripheral tolerance in CD8(+) T cells is their activation by resting dendritic cells (DC). In contrast, DC can be "licensed" by CD4(+) T cells to induce cytotoxic function in CD8(+) T cells. The question that then arises, whether CD4(+) T cell help could impair peripheral tolerance induction in self-reactive CD8(+) T cells, has not been addressed. In this study we show that CD4(+) T cell activation by resting DC results in helper function that transiently promotes the expansion and differentiation of cognate CD8(+) T cells. However, both the CD4(+) and CD8(+) T cell populations ultimately undergo partial deletion and acquire Ag unresponsiveness, disabling their ability to destroy OVA-expressing pancreatic beta cells and cause diabetes. Thus, effective peripheral tolerance can be induced by resting DC in the presence of CD4(+) and CD8(+) T cells with specificity for the same Ag.

Direct impact of inactivated HIV-1 virions on B lymphocyte subsets

Although there is no convincing evidence that HIV infects primary B cells, marked changes in B cell responses have been described in HIV-1-infected subjects, including B cell repertoire perturbations, depression of B cell memory and paucity of CD5(+) B cells. As it is hard to assess the consequences of these in vitro and ex vivo observations in patients, the pathogenic mechanisms responsible for the B cell deficit are unclear, and direct and indirect effects of HIV-1 remain possible. To gain further insight into the impact of HIV-1 on the B cell compartment in vivo, we used XenoMouse mice, mice genetically engineered to express human antibodies with an absence of mouse antibody expression. In these transgenic animals, B cells expressing a virtually full human Ig repertoire develop, which allows investigation of the in vivo consequences of confronting B cells expressing human immunoglobulins with HIV-1. We found that soluble gp120 induced an inversion in the B-1a/B-1b cell ratios, without impacting B-2 cells or affecting substantially the T cell compartment. Virion treatment specifically and dramatically depressed B-1a cells, which represent the majority of B-1 cells in normal mice. The observed B cell changes were associated with a functional alteration of the humoral response to tetanus toxoid. Thus, the results reveal a capacity of HIV-1 to specifically impact a highly specialized B cell subpopulation. Because there is evidence that human IgM memory B cells are functionally equivalent to murine B-1a cells, our findings suggest that gp120 may have a direct deleting activity on B cell memory.

Cell death and thymic export during fetal life

In the fetus the peripheral T cell pool expands as the fetus grows, but the mechanisms that regulate T cell homeostasis during fetal life are unknown. Here, we show that the peripheral T cell pool in the sheep fetus is established by the export from the fetal thymus of twice as many CD8+ as CD4+ thymic emigrants every day. Clonal deletion of CD4+ thymocytes in the fetal thymus appeared to be more stringent than was the case for CD8+ thymocytes because only 1 in 35 single-positive CD4 (SPCD4) thymocytes was exported from the thymus whereas the majority (2/3) of the single-positive CD8 (SPCD8) thymocytes were exported from the fetal thymus each day. Furthermore, within the thymus, the number of apoptotic SPCD4 thymocytes was 40 times greater than the number of apoptotic SPCD8 thymocytes. A tissue-specific migration of CD8+ emigrants localizing in the spleen was also established in the fetus in contrast to CD4+ emigrants, which migrated randomly to spleen and LN.

Clonal deletion of thymocytes by circulating dendritic cells homing to the thymus

Dendritic cell (DC) presentation of self antigen to thymocytes is essential to the establishment of central tolerance. We show here that circulating DCs were recruited to the thymic medulla through a three-step adhesion cascade involving P-selectin, interactions of the integrin VLA-4 with its ligand VCAM-1, and pertussis toxin-sensitive chemoattractant signaling. Ovalbumin-specific OT-II thymocytes were selectively deleted after intravenous injection of antigen-loaded exogenous DCs. We documented migration of endogenous DCs to the thymus in parabiotic mice and after painting mouse skin with fluorescein isothiocyanate. Antibody to VLA-4 blocked the accumulation of peripheral tissue-derived DCs in the thymus and also inhibited the deletion of OT-II thymocytes in mice expressing membrane-bound ovalbumin in cardiac myocytes. These findings identify a migratory route by which peripheral DCs may contribute to central tolerance.

The effects of thymic selection on the range of T cell cross-reactivity

Based on the results of a computational model of thymic selection, we propose a mechanism that produces the observed wide range of T cell cross-reactivity. The model suggests that the cross-reactivity of a T cell that survives thymic selection is correlated with its affinity for self peptides. In order to survive thymic selection, a T cell with low affinity for all self peptides expressed in the thymus must have high affinity for major histocompatibility complex (MHC), which makes it highly cross-reactive. A T cell with high affinity for any self peptide must have low MHC affinity to survive selection, which makes it highly specific for its cognate peptide. Our model predicts that (1) positive selection reduces by only 17% the number of T cells that can detect any given foreign peptide, even though it eliminates over 95% of pre-selection cells; (2) negative selection decreases the average cross-reactivity of the pre-selection repertoire by fivefold; and (3) T cells responding to foreign peptides similar to self peptides will have a lower average cross-reactivity than cells responding to epitopes dissimilar to self.

NKT cells play critical roles in the induction of oral tolerance by inducing regulatory T cells producing IL-10 and transforming growth factor beta, and by clonally deleting antigen-specific T cells

Oral tolerance is the systemic unresponsiveness induced by orally administered proteins. To explore the roles of natural killer T (NKT) cells in oral tolerance, we induced oral tolerance to ovalbumin (OVA) in NKT cell-deficient mice. In CD1d-/- mice, the induction of tolerance to orally administered high- or low-dose OVA was impaired. Dendritic cells (DCs) in the Peyer's patches (PPs) of CD1d-/- mice fed OVA showed high expression of major histocompatibility complex (MHC) class II and B7 molecules, whereas DCs of control mice fed OVA expressed low levels of these molecules. The adoptive transfer of NKT cells restored oral tolerance and induction of tolerogenic DCs in the PPs and spleens of CD1d-/- mice. Moreover, interleukin (IL)-10 and transforming growth factor (TGF)-beta1 production in vitro were reduced in cells from the spleen and PPs of CD1d-/- mice compared with those of control mice fed OVA. The numbers of OVA-specific CD4+ KJ1-26+ T cells were significantly reduced in the PPs and spleens of DO11.10 mice fed OVA. In contrast, OVA-specific CD4+ KJ1-26+ T cells were not deleted in the PPs or spleens of DO11.10 CD1d-/- mice. In conclusion, NKT cells were found to play an indispensable role in oral tolerance by inducing regulatory T cells, and clonally deleting antigen-specific CD4+ T cells.

Deletion is neither sufficient nor necessary for the induction of peripheral tolerance in mature CD8+ T cells

Previous reports have demonstrated clonal deletion of CD8(+) T cells during peripheral tolerance induction to tissue antigens. However, direct evidence demonstrating a causal connection between deletion and tolerance has not been reported because of model limitations in which the tissue antigens were expressed in vital organs. Thus, studies were initiated in a mouse model where expression of a membrane-bound ovalbumin fusion protein (mOVA) was driven by a prostate specific androgen regulated probasin promotor, providing restricted expression in a non-vital organ where antigen levels can be abrogated through androgen deprivation. Adoptive transfer of mOVA specific CD8(+) T cells (OT-I) was used to assess the development of peripheral tolerance. Proliferation of OT-I cells was observed, as was partial deletion of transferred OT-I cells. Although deletion occurred, the long-term persistence of a stable level of OT-I cells was observed. Importantly, the persistent OT-I cells lost antigen responsiveness within 3 weeks of transfer. Castration resulted in loss of high-level prostate mOVA expression, with a resultant abrogation of tolerance induction, but surprisingly did not affect the deletion rate of OT-I cells. In contrast, abrogation of deletion through the adoptive transfer of OT-I cells from third generation CD95-deficient mice had no effect on tolerance induction. These data demonstrate the necessity for continued expression of tissue antigen throughout the establishment of peripheral tolerance. Furthermore, these findings demonstrate that deletion is neither sufficient nor required for CD8(+) T-cell tolerance to tissue antigens, suggesting that regulatory events independent of deletion are necessary for peripheral tolerance induction to prostate antigens.

The presence of donor-reactive CD4 T cells in early-phase liver-induced tolerance in rats: analysis using donor passenger leukocytes from the recipient

In previous studies, we showed that the recipient spleen 12 h after liver transplantation included highly immunogenic donor passenger leukocytes (PLs) in the non-rejector rat strain combination (Lewis: LEW-->Wistar Furth: WF). Furthermore, we noted that giving anti-CD4 mAb to WF recipients at the time of liver grafting prolonged the liver damage due to primary rejection reaction, suggesting that CD4 T-cell regulation but not the deletional mechanism played a dominant role during the induction phase of tolerance in this combination. We proposed that in vitro CD4 T-cell response to the donor PLs from the recipient lymphoid organ may be observed in the early phase of tolerance induction. We initially observed donor-specific acceptance by 10 days after liver transplantation from the result of secondary heart transplantation, and showed that donor (LEW) PLs in recipient WF spleen 12 h after grafting were able to prime cytotoxic CD8 T cells in vivo. We then isolated functional donor PLs from the recipient WF spleen and established in vitro proliferation assay. When the enriched donor PLs were used as stimulator cells in the proliferation assay, a powerful alloimmunostimulatory ability was observed. Moreover, a potent response of recipient splenocytes to the PLs was observed on postoperative day 14. From these observations, we confirmed that clonal deletion of donor-reactive CD4 T cells did not participate in early phase of tolerance induction in this setting.

Programmed death-1 (PD-1):PD-ligand 1 interactions inhibit TCR-mediated positive selection of thymocytes

Positive selection during thymocyte development is driven by the affinity and avidity of the TCR for MHC-peptide complexes expressed in the thymus. In this study, we show that programmed death-1 (PD-1), a member of the B7/CD28 family of costimulatory receptors, inhibits TCR-mediated positive selection through PD-1 ligand 1 (PD-L1):PD-1 interactions. Transgenic mice that constitutively overexpress PD-1 on CD4+CD8+ thymocytes display defects in positive selection in vivo. Using an in vitro model system, we find that PD-1 is up-regulated following TCR engagement on CD4+CD8+ murine thymocytes. Coligation of TCR and PD-1 on CD4+CD8+ thymocytes with a novel PD-1 agonistic mAb inhibits the activation of ERK and up-regulation of bcl-2, both of which are downstream mediators essential for positive selection. Inhibitory signals through PD-1 can overcome the ability of positive costimulators, such as CD2 and CD28, to facilitate positive selection. Finally, defects in positive selection that result from PD-1 overexpression in thymocytes resolve upon elimination of PD-L1, but not PD-1 ligand 2, expression. PD-L1-deficient mice have increased numbers of CD4+CD8+ and CD4+ thymocytes, indicating that PD-L1 is involved in normal thymic selection. These data demonstrate that PD-1:PD-L1 interactions are critical to positive selection and play a role in shaping the T cell repertoire.

Analyzing expression of perforin, Runx3, and Thpok genes during positive selection reveals activation of CD8-differentiation programs by MHC II-signaled thymocytes

Intrathymic positive selection matches CD4-CD8 lineage differentiation to MHC specificity. However, it is unclear whether MHC signals induce lineage choice or simply select thymocytes of the appropriate lineage. To investigate this issue, we assessed thymocytes undergoing positive selection for expression of the CD8 lineage markers perforin and Runx3. Using both population-based and single-cell RT-PCR analyses, we found large subsets of MHC class II (MHC-II)-signaled thymocytes expressing these genes within the CD4+ 8+ and CD4+ 8(int), but not the CD4+ 8- populations of signaling competent mice. This indicates that MHC-II signals normally fail to impose CD4 differentiation and further implies that the number of mature CD8 single-positive (SP) thymocytes greatly underestimates CD8 lineage choice. We next examined whether MHC-II-restricted CD4+ 8- thymocytes remain competent to initiate CD8 lineage gene expression. In mice in which expression of the tyrosine kinase Zap70 and thereby TCR signaling were impaired selectively in SP thymocytes, MHC-II-signaled CD4+ 8- thymocytes expressed perforin and Runx3 and failed to up-regulate the CD4 marker Thpok. This indicated that impairing TCR signals at the CD4 SP stage switched gene expression patterns from CD4- to CD8-lineage specific. We conclude from these findings that MHC-II-signaled thymocytes remain competent to initiate CD8-specific gene expression even after CD8 down-regulation and that CD4 lineage differentiation is not fixed before the CD4 SP stage.

CD8 T Cell Sensory Adaptation Dependent on TCR Avidity for Self-Antigens

Adaptation of the T cell activation threshold may be one mechanism to control autoreactivity. To investigate its occurrence in vivo, we engineered a transgenic mouse model with increased TCR-dependent excitability by expressing a Zap70 gain-of-function mutant (ZAP-YEEI) in postselection CD8 thymocytes and T cells. Increased basal phosphorylation of the Zap70 substrate linker for activation of T cells was detected in ZAP-YEEI-bearing CD8 T cells. However, these cells were not activated, but had reduced levels of TCR and CD5. Moreover, they produced lower cytokine amounts and showed faster dephosphorylation of linker for activation of T cells and ERK upon activation. Normal TCR levels and cytokine production were restored by culturing cells in the absence of TCR/spMHC interaction, demonstrating dynamic tuning of peripheral T cell responses. The effect of avidity for self-ligand(s) on this sensory adaptation was studied by expressing ZAP-YEEI in P14 or HY TCR transgenic backgrounds. Unexpectedly, double-transgenic animals expressed ZAP-YEEI prematurely in double-positive thymocytes, but no overt alteration of selection processes was observed. Instead, modifications of TCR and CD5 expression due to ZAP-YEEI suggested that signal tuning occurred during thymic maturation. Importantly, although P14 x ZAP-YEEI peripheral CD8 T cells were reduced in number and showed lower Ag-induced cytokine production and limited lymphopenia-driven proliferation, the peripheral survival/expansion and Ag responsiveness of HY x ZAP-YEEI cells were enhanced. Our data provide support for central and peripheral sensory T cell adaptation induced as a function of TCR avidity for self-ligands and signaling level. This may contribute to buffer excessive autoreactivity while optimizing TCR repertoire usage.

Evidence of marginal-zone B cell-positive selection in spleen

Antigen receptor-mediated signaling is critical for the development and survival of B cells. However, it has not been established whether B cell development requires a signal from self-ligand engagement at the immature stage, a process known as "positive selection." Here, using a monoclonal B cell receptor (BCR) mouse line, specific for the self-Thy-1/CD90 glycoprotein, we demonstrate that BCR crosslinking by low-dose self-antigen promotes survival of immature B cells in culture. In spleen, an increase in BCR signaling strength, induced by low-dose self-antigen, directed naive immature B cells to mature, not into the default follicular B cell fate, but instead into the marginal-zone B cell subset. These data indicate that positive selection can occur in developing B cells and that BCR signal strength is a key factor in deciding between two functionally distinct mature B cell compartments in the microenvironment of the spleen.

Central tolerance: learning self-control in the thymus

In the past few years, there has been a flurry of discoveries and advancements in our understanding of how the thymus prepares T cells to exist at peace in normal healthy tissue: that is, to be self-tolerant. In the thymus, one of the main mechanisms of T-cell central tolerance is clonal deletion, although the selection of regulatory T cells is also important and is gaining enormous interest. In this Review, we discuss the emerging consensus about which models of clonal deletion are most physiological, and we review recent data that define the molecular mechanisms of central tolerance.

Induction of central T cell tolerance: Recombinant antibodies deliver peptides for deletion of antigen-specific CD4+8+ thymocytes

In order to prevent or ameliorate autoimmune disease, it would be desirable to induce central tolerance to peripheral self-antigens. We have investigated whether recombinant antibodies (Ab) that deliver T cell epitopes to antigen-presenting cells (APC) in the thymus can be used to induce thymocyte deletion. Troybodies are recombinant Ab with V regions specific for APC surface molecules that have T cell epitopes genetically introduced in their C domains. When MHC class II-specific Troybodies with the lambda2(315)T cell epitope were injected into lambda2(315)-specific TCR transgenic mice, a profound deletion of (CD4+)8+ thymocytes was observed. MHC class II-specific Troybodies were 10-100-fold more efficient than non-targeting peptide Ab, and 500-fold more efficient than synthetic peptide at inducing deletion. Similar findings were observed when MHC class II-specific Troybodies with the OVA(323-339) T cell epitope were injected into OVA-specific TCR transgenic mice. Although deletion was transient after a single injection, newborn mice repeatedly injected with MHC class II-specific Troybodies for 4 weeks, had reduced antigen-specific T cells in peripheral lymphoid tissues and reduced T cell responses. These experiments suggest that Troybodies constructed to target specifically thymic APC could be useful tools for induction and maintenance of central T cell tolerance in autoimmune diseases.

Low dose busulfan facilitates chimerism and tolerance in a murine model

T cell depletion, sirolimus and "mega" dose donor specific bone marrow (DSBM) infusion promotes stable multilineage chimerism and indefinite survival of skin allografts in completely mismatched mice. The purpose of this study is to determine whether the addition of low dose busulfan can reduce the amount of DSBM required while preserving efficacy. C57BL/6 recipients of BALB/c skin allografts were treated with alphaCD4 and alphaCD8 monoclonal antibodies, DSBM, sirolimus and various doses of busulfan. The kinetics and phenotype of chimerism and the presence of clonal deletion of alloreactive T-cells were defined using flow cytometry. In vitro reactivity was determined using mixed lymphocyte culture. Second skin grafts confirmed the presence of tolerance. All doses of busulfan resulted in engraftment when combined with this regimen using a reduced dose of donor marrow. The level, kinetics and character of chimerism observed were dose related. Chimerism was associated with indefinite allograft acceptance (>200 days). Tolerance was documented both in vitro/in vivo and was associated with clonal deletion. Addition of a single low dose of busulfan to an established tolerance protocol reduced the required DSBM dose by over 80% while still promoting comparable levels of donor chimerism and donor-specific tolerance.

Germinal center exclusion of autoreactive B cells is defective in human systemic lupus erythematosus

Breach of B cell tolerance is central to the pathogenesis of systemic lupus erythematosus (SLE). However, how B cell tolerance is subverted in human SLE is poorly understood due to difficulties in identifying relevant autoreactive B cells and in obtaining lymphoid tissue. We have circumvented these limitations by using tonsil biopsies to study autoreactive B cells (9G4 B cells), whose regulation is abnormal in SLE. Here we show that 9G4 B cells are physiologically excluded during the early stages of the GC reaction before acquiring a centroblast phenotype. Furthermore, we provide evidence to indicate that an anergic response to B cell receptor stimulation may be responsible for such behavior. In contrast, in SLE, 9G4 B cells progressed unimpeded through this checkpoint, successfully participated in GC reactions, and expanded within the post-GC IgG memory and plasma cell compartments. The faulty regulation of 9G4 B cells was not shared by RA patients. To our knowledge, this work represents the first comparative analysis of the fate of a specific autoreactive human B cell population. The results identify a defective tolerance checkpoint that appears to be specific for human SLE.

Dwindling competition with constant demand: Can homeostatic adjustments explain age-associated changes in peripheral B cell selection?

The close relationship between specificity-based selection and homeostatic processes in maintaining peripheral B cell pools has become increasingly evident. Thus, age-associated changes observed within these pools may reflect homeostatic responses to proximal primary lesions. Marked shifts in the size and dynamics of most B lymphocyte subsets and their progenitors occur with age: perturbations in B lineage precursors result in reduced production of immature B lymphocytes in the bone marrow and transitional pools in the periphery, but these effects appear to be offset by compensatory homeostatic processes at the marrow-periphery interface. We propose a model whereby these "distal" homeostatic adjustments relax the stringency of specificity based selection, affording a potential explanation for the increased frequency of autoreactive specificities with age.

In vivo assessment of the relative contributions of deletion, anergy, and editing to B cell self-tolerance

In normal B cell development, a large percentage of newly formed cells bear receptors with high levels of self-reactivity that must be tolerized before entry into the mature B cell pool. We followed the fate of self-reactive B cells expressing high affinity anti-hen egg lysozyme (HEL) Ag receptors exposed in vivo to membrane HEL in a setting in which the anti-HEL L chain was "knocked-in" at the endogenous L chain locus. These mice demonstrated extensive and efficient L chain receptor editing responses and had B cell numbers comparable to those found in animals lacking membrane Ag. BrdU labeling indicated that the time required for editing in response to membrane HEL was approximately 6 h. In mice transgenic for soluble HEL, anti-HEL B cells capable of editing showed evidence for both editing and anergy. These data identify receptor editing as a major physiologic mechanism by which highly self-reactive B cells are tolerized to membrane and soluble self-Ags.

Two Lymphoid Roads Diverge— but Does Antigen Bade B Cells to Take the Road Less Traveled?

The contribution of a self-antigen to marginal-zone B lymphocytes is described in this issue (). Other interpretations of these important findings are considered here.

Efficacy of clonal deletion vs. anergy of self-reactive CD4 T-cells for the prevention and reversal of autoimmune diabetes

The self-reactive CD4 T-cells play an essential role in triggering and sustaining organ-specific autoimmune diseases. Silencing or elimination of these cells can prevent and reverse an autoimmune process. We have previously showed that a single dose-administration of a soluble dimeric MHC II-peptide chimera (DEF) in double-transgenic mice delayed the onset autoimmune diabetes, and restored the euglycemia in already diabetic mice for a period of 1 week. DEF dimer protection relied on induction of anergy of diabetogenic CD4 T-cells in spleen, and stimulation of IL-10-secreting T regulatory type 1 cells in pancreas. Herein, we show that an octameric form of DEF has doubled the period of protection and reversal of disease by clonal deletion of diabetogenic CD4 T-cells in both the thymic and peripheral compartments. Deletion occurred by activation-induced cell death subsequent to repartitioning and signaling of FAS-FADD apoptotic module in the plasma membrane lipid rafts. Our previous and present data indicated first, that DEF valence translates into various effects on the antigen-specific CD4 T-cells, i.e., Th2 immune deviation, anergy, and apoptosis. Second, the present findings argue for a better efficacy of clonal deletion than anergy of diabetogenic CD4 T-cells for the protection and reversal of autoimmune diabetes.

The cellular mechanism of Aire control of T cell tolerance

Aire promotes the tolerization of thymocytes by inducing the expression of a battery of peripheral-tissue antigens in thymic medullary epithelial cells. We demonstrate that the cellular mechanism by which Aire exerts its tolerance-promoting function is not primarily positive selection of regulatory T cells, but rather negative selection of T effector cells. Surprisingly, supplementing its influence on the transcription of genes encoding peripheral-tissue antigens, Aire somehow enhances the antigen-presentation capability of medullary epithelial cells. Thus, this transcriptional control element promotes central tolerance both by furnishing a specific thymic stromal cell type with a repertoire of self antigens and by better arming such cells to present these antigens to differentiating thymocytes. In Aire's absence, autoimmunity and ultimately overt autoimmune disease develops.

Gammadelta T cell development--having the strength to get there

Gammadelta T cells play critical roles in immune regulation, tumour surveillance and specific primary immune responses. Mature gammadelta cells derive from thymic precursors that also generate alphabeta T cells. Recent reports have highlighted the impact of the strength of signal received via the T cell receptor on T cell lineage commitment, and the importance of cross-talk between committed alphabeta thymocytes and bipotential progenitors for normal gammadelta T cell differentiation. Studies on T cell receptor-mediated selection of gammadelta cells have supported the view that these unconventional T cells are positively rather than negatively selected on cognate self antigen.

Antigen Presentation by an Immature Myeloid Dendritic Cell Line Does Not Cause CTL Deletion In Vivo, but Generates CD8+ Central Memory-Like T Cells That Can Be Rescued for Full Effector Function

Immature dendritic cells (DC), in contrast to their mature counterparts, are incapable of mobilizing a CD8+ CTL response, and, instead, have been reported to induce CTL tolerance. We directly addressed the impact of immature vs mature DC on CTL responses by infusing adenovirus peptide-loaded DC (of the D1 cell line) into mice that had received adenovirus-specific naive TCR-transgenic CD8+ T cells. Whereas i.v. injection of mature DC triggered vigorous CTL expansion, immature DC elicited little proliferation involving only a minority of the TCR-transgenic CTL. Even though the latter CTL developed effector functions, including cytolytic activity and proinflammatory cytokine secretion, these cells differed significantly from CTL primed by mature DC in that they did not exhibit down-regulation of CD62L and CCR7, receptors involved in trapping of T cells in the lymphoid organs. Interestingly, adoptive transfer of CTL effector cells harvested after priming by either mature or immature DC into naive recipient mice, followed by exposure to adenovirus, yielded quantitatively and qualitatively indistinguishable CTL memory responses. Therefore, in vivo priming of naive CD8+ T cells by immature DC, although failing to induce a full-blown, systemic CTL response, resulted in the formation of central memory-like T cells that were able to expand and produce IFN-gamma upon secondary antigenic stimulation.

Selective expression of the 21-kilodalton tyrosine-phosphorylated form of TCR zeta promotes the emergence of T cells with autoreactive potential

T cells undergo negative selection in the thymus to eliminate potentially autoreactive cells. The signals generated through the alphabeta TCR following receptor interactions with peptide/MHC complexes in the thymus control these selection processes. Following receptor ligation, a fraction of the TCR zeta subunit appears as two distinct tyrosine-phosphorylated forms of 21 and 23 kDa (p21 and p23). Previous data have reported elevated levels of p21 in some murine models of autoimmunity. We have examined the contributions of both the p21 and p23 to T cell negative selection in the HY TCR-transgenic system using ITAM-substituted TCR zeta and CD3 epsilon transgenic mice. Expression of just p21, in the absence of p23, partially impairs negative selection of self-reactive HY-specific T cells. This results in the emergence of potentially autoreactive peripheral T cells and an elevated population of CD11b(+)B220(+) B cells in the spleen. These data clearly identify a specific and unique role for p21 during negative selection.