The effect of thymus environment on T cell development and tolerance

The yin/yang balance of the MHC-self-immunopeptidome

The MHC-self immunopeptidome of professional antigen presenting cells is a cognate ligand for the TCRs expressed on both conventional and thymic-derived natural regulatory T cells. In regulatory T cells, the TCR signaling associated with MHC-peptide recognition induces antigen specific as well as bystander immunosuppression. On the other hand, TCR activation of conventional T cells is associated with protective immunity. As such the peripheral T cell repertoire is populated by a number of T cells with different phenotypes and different TCRs, which can recognize the same MHC-self-peptide complex, resulting in opposite immunological outcomes. This article summarizes what is known about regulatory and conventional T cell recognition of the MHC-self-immunopeptidome at steady state and in inflammatory conditions associated with increased T and B cell self-reactivity, discussing how changes in the MHC-ligandome including epitope copy number and post-translational modifications can tilt the balance toward the expansion of pro-inflammatory or regulatory T cells.

Failures in thymus medulla regeneration during immune recovery cause tolerance loss and prime recipients for auto-GVHD

Bone marrow transplantation (BMT) is a widely used therapy for blood cancers and primary immunodeficiency. Following transplant, the thymus plays a key role in immune reconstitution by generating a naive αβT cell pool from transplant-derived progenitors. While donor-derived thymopoiesis during the early post-transplant period is well studied, the ability of the thymus to synchronize T cell development with essential tolerance mechanisms is poorly understood. Using a syngeneic mouse transplant model, we analyzed T cell recovery alongside the regeneration and function of intrathymic microenvironments. We report a specific and prolonged failure in the post-transplant recovery of medullary thymic epithelial cells (mTECs). This manifests as loss of medulla-dependent tolerance mechanisms, including failures in Foxp3⁺ regulatory T cell development and formation of the intrathymic dendritic cell pool. In addition, defective negative selection enables escape of self-reactive conventional αβT cells that promote autoimmunity. Collectively, we show that post-transplant T cell recovery involves an uncoupling of thymopoiesis from thymic tolerance, which results in autoimmune reconstitution caused by failures in thymic medulla regeneration.

Visualizing semipermeability of the cell membrane using a pH-responsive ratiometric AIEgen

In clinical chemotherapy, some basic drugs cannot enter the hydrophobic cell membrane because of ionization in the acidic tumor microenvironment, a phenomenon known as ion trapping. In this study, we developed a method to visualize this ion trapping phenomenon by utilizing a pH-responsive ratiometric AIEgen, dihydro berberine (dhBBR). By observing the intracellular fluorescence of dhBBR, we found that non-ionized dhBBR can enter cells more easily than ionized forms, which is in accordance with the concept of ion trapping. In addition, dhBBR shows superior anti-photobleaching ability to Curcumin thanks to its AIE properties. These results suggest that dhBBR can serve as a bioprobe for ion trapping.

The potential underlying mechanism of the leukemia caused by MLL-fusion and potential treatments

A majority of infant and pediatric leukemias are caused by the mixed-lineage leukemia gene (MLL) fused with a variety of candidates. Several underlying mechanisms have been proposed. One currently popular view is that truncated MLL1 fusion and its associated complex constitutively hijacks super elongation complex, including positive transcription elongation factor b, CDK9, and cyclin T1 complex and DOT1L, to enhance the expression of transcription factors that maintain or restore stemness of leukocytes, as well as prevent the differentiation of hematopoietic progenitor cells. An alternative emerging view proposes that MLL1-fusion promotes the recruitment of TATA binding protein and RNA polymerase II (Pol II) initiation complex, so as to increase the expression levels of target genes. The fundamental mechanism of both theories are gain of function for truncated MLL1 fusions, either through Pol II elongation or initiation. Our recent progress in transcription regulation of paused Pol II through JMJD5, JMJD6, and JMJD7, combined with the repressive role of H3K4me3 revealed by others, prompted us to introduce a contrarian hypothesis: the failure to shut down transcribing units by MLL-fusions triggers the transformation: loss of function of truncated MLL1 fusions coupled with the loss of conversion of H3K4me1 to H3K4me3, leading to the constitutive expression of transcription factors that are in charge of maintenance of hematopoietic progenitor cells, may trigger the transformation of normal cells into cancer cells. Following this track, a potential treatment to eliminate these fusion proteins, which may ultimately cure the disease, is proposed.

Isolation of human monoclonal antibodies that bind to two different antigens and are encoded by germline VH and VL genes

This paper reports isolation of two monoclonal antibodies (mAbs) that bind to both a membrane protein and a cytoplasmic protein. Most Abs established as markers for autoimmune disease bind to cytoplasmic or nuclear substances. However, it remains unknown how these Abs are produced. On the other hand, there were examples where clones originally isolated as Abs that bind to membrane proteins also showed binding activity to cytoplasmic or nuclear substances. Based on these results, the following hypothesis has been proposed. The Abs that had been originally produced against a membrane protein showed cross-reactivity against cytoplasmic or nuclear substances. In the present study we reported isolation of Abs that bound to both a membrane protein, CADM1, and a cytoplasmic protein, α-actinin-4. The method adopted in the present study could be generally applicable to isolation of Abs showing such dual specificity. Firstly, we constructed a huge human Ab library using various organs including naïve B-cell-rich organs such as bone marrow and umbilical cords. Then, we developed a comprehensive screening method for isolation of Abs that bound to cell surface antigens. Through extensive screenings with many kinds of cell we newly obtained a library composed of around 4000 independent clones that bind to membrane proteins. We screened this library with α-actinin-4 and succeeded in isolating two Abs. They bound to α-actinin-4 and a membrane protein CADM1. Furthermore, they are encoded by naïve heavy and light chain variable genes (V_H & V_L). These results suggested that cross-reactive Abs to both a membrane protein and a cytoplasmic protein could be present in germline repertoire of Ab in humans. This methodology adopted in the present study could be applied to isolation of cross-reactive Abs possibly involved in autoimmune diseases.

Immune tolerance induced by platelet-targeted factor VIII gene therapy in hemophilia A mice is CD4 T cell mediated

Essentials The immune response is a significant concern in gene therapy. Platelet-targeted gene therapy can restore hemostasis and induce immune tolerance. CD4 T cell compartment is tolerized after platelet gene therapy. Preconditioning regimen affects immune tolerance induction in platelet gene therapy.

SUMMARY

Background Immune responses are a major concern in gene therapy. Our previous studies demonstrated that platelet-targeted factor VIII (FVIII) (2bF8) gene therapy together with in vivo drug selection of transduced cells can rescue the bleeding diathesis and induce immune tolerance in FVIIInull mice. Objective To investigate whether non-selectable 2bF8 lentiviral vector (LV) for the induction of platelet-FVIII expression is sufficient to induce immune tolerance and how immune tolerance is induced after 2bF8LV gene therapy. Methods Platelet-FVIII expression was introduced by 2bF8LV transduction and transplantation. FVIII assays and tail bleeding tests were used to confirm the success of platelet gene therapy. Animals were challenged with rhF8 to explore if immune tolerance was induced after gene therapy. Treg cell analysis, T-cell proliferation assay and memory B-cell-mediated ELISPOT assay were used to investigate the potential mechanisms of immune tolerance. Results We showed that platelet-FVIII expression was sustained and the bleeding diathesis was restored in FVIIInull mice after 2bF8LV gene therapy. None of the transduced recipients developed anti-FVIII inhibitory antibodies in the groups preconditioned with 660 cGy irradiation or busulfan plus ATG treatment even after rhF8 challenge. Treg cells significantly increased in 2bF8LV-transduced recipients and the immune tolerance developed was transferable. CD4+ T cells from treated animals failed to proliferate in response to rhF8 re-stimulation, but memory B cells could differentiate into antibody secreting cells in 2bF8LV-transduced recipients. Conclusion 2bF8LV gene transfer without in vivo selection of manipulated cells can introduce immune tolerance in hemophilia A mice and this immune tolerance is CD4+ T cell mediated.

Human T-Cell Development and Thymic Egress: An Infectious Disease Perspective

Emigration of mature naïve CD4 SP T cells from the human thymus to the periphery is not fully understood, although elucidation of the mechanisms that govern egress of T cells is crucial to understanding both basic immunology and the immune response in diseases such as HIV infection. Recent work has brought to light the requirement for sphingosine-1-phosphate (S1P) and its receptors in a variety of fields including mature naïve T-cell egress from the thymus of mice. We are examining the expression and function of this novel requisite T-cell egress receptor within the human thymus, characterizing changes observed in the expression and function of this receptor in infectious diseases. To perform this work, we use a variety of humanized murine models reviewed in this article. Future work in the field of T-cell egress, especially as it pertains to S1P receptors, should advance the fields of basic T-cell immunology and immunopathology and open new avenues for exploration into novel therapeutics.

The path to reactivation of antitumor immunity and checkpoint immunotherapy

Cancer immunology has recently made major therapeutic inroads that represent clinical application of basic insights into mechanisms that govern immunity against tumors. Research into fundamental elements of T-cell and natural killer-cell biology, including the basis of antigen recognition, activation, proliferation, and survival, has informed the design of new therapeutic approaches to augment the body's natural anticancer immune response. Here, we describe some of the key steps that have provided the foundation for current strategies of immunotherapy.

Perspectives on antigen presenting cells in zebrafish

Antigen presentation is a critical step in the activation of naïve T lymphocytes. In mammals, dendritic cells (DCs), macrophages, and B lymphocytes can all function as antigen presenting cells (APCs). Although APCs have been identified in zebrafish, it is unclear if they fulfill similar roles in the initiation of adaptive immunity. Here we review the characterization of zebrafish macrophages, DCs, and B cells and evidence of their function as true APCs. Finally, we discuss the conservation of APC activity in vertebrates and the use of zebrafish to provide a new perspective on the evolution of these functions.

CTLA-4 controls the thymic development of both conventional and regulatory T cells through modulation of the TCR repertoire

Cytotoxic T lymphocyte-associated antigen-4 (CTLA-4; CD152) is of pivotal importance for self-tolerance, with deficiency or unfavorable polymorphisms leading to autoimmune disease. Tolerance to self-antigens is achieved through thymic deletion of highly autoreactive conventional T (Tconv) cells and generation of FoxP3(+) regulatory T (Treg) cells. The main costimulatory molecule, CD28, augments the negative selection of Tconv cells and promotes the generation of FoxP3(+) Treg cells. The role of its antagonistic homolog CTLA-4, however, remains a topic of debate. To address this topic, we investigated the thymic development of T cells in the presence and absence of CTLA-4 in a T-cell receptor (TCR) transgenic mouse model specific for the myelin basic protein peptide Ac1-9. We reveal that CTLA-4 is expressed in the corticomedullary region of the thymus. Its absence alters the response of CD4(+)CD8(-) thymocytes to self-antigen recognition, which affects the quantity of the Treg cells generated and broadens the repertoire of peripheral Tconv cells. T-cell repertoire alteration after deletion of CTLA-4 results from changes in TCR Vα and Jα segment selection as well as CDR3α composition in Tconv and Treg cells. CTLA-4, therefore, regulates the early development of self-reactive T cells in the thymus and plays a key role in central tolerance.

The regulatory role of dendritic cells in the induction and maintenance of T-cell tolerance

The induction and maintenance of T-cell tolerance to tissue antigens is essential to prevent autoimmunity. Combinations of central and peripheral mechanisms act in parallel to inactivated, eliminated or control autoreactive T cells. Both centrally and peripherally, a key requirement for self-tolerance is the presentation of self-antigens in a correct context. There is now evidence to suggest that dendritic cells (DCs) play a fundamental role in the development of central and peripheral tolerance. In this review, we summarize recent progress toward the definition of the multiple roles of DCs in these processes. We will also discuss the association between defects in the DC compartment and the development of autoimmune responses, with particular reference to DC deregulation in the context of type I diabetes.

Spatial (Tbata) expression in mature medullary thymic epithelial cells

The Spatial gene is expressed in highly polarized cell types such as testis germ cells, brain neurons and thymic epithelial cells (TEC). Its expression was documented in testis and brain but poorly characterized in thymus. Here, we characterize for the first time Spatial-expressing TEC throughout ontogeny and adult mouse thymus. Spatial is expressed in thymic-fated domain by embryonic day E10.5 and persists in subcapsular, cortical, medullary epithelial cells and in MTS24(+) progenitor TEC. Using mouse strains in which thymocyte development is blocked at various stages, we show that Spatial expression is independent of thymocyte-derived signals during thymus organogenesis. Analyses on purified thymic cell subsets show that Spatial short isoforms are expressed in cortical TEC (cTEC) and mature medullary TEC (mTEC). Spatial long isoforms were detected in the same TEC population. Spatial presents a nuclear distribution specific to mature mTEC expressing UEA1 and Aire. Aire- and RANKL-deficient mice revealed that Spatial expression is drastically reduced in the thymus of these mutants. These findings reveal a critical function of Aire in regulating Spatial expression, which is compatible with promiscuous Spatial gene expression.

Cross-recognition of HLA DR4 alloantigen by virus-specific CD8+ T cells: a new paradigm for self-/nonself-recognition

The ability of CD8+ T cells to engage a diverse range of peptide–major histocompatibility complex (MHC) complexes can also lead to cross-recognition of self and nonself peptide-MHC complexes and thus directly contribute toward allograft rejection or autoimmunity. Here we present a novel form of cross-recognition by herpes virus–specific CD8+ cytotoxic T cells that challenges the current paradigm of self/non-self recognition. Functional characterization of a human leukocyte antigen (HLA) Cw*0602-restricted cytomegalovirus-specific CD8+ T-cell response revealed an unusual dual specificity toward a pp65 epitope and the alloantigen HLA DR4. This cross-recognition of HLA DR4 alloantigen was critically dependent on the coexpression of HLA DM and was preferentially directed toward the B-cell lineage. Furthermore, allostimulation of peripheral blood lymphocytes with HLA DRB*0401-expressing cells rapidly expanded CD8+ T cells, which recognized the pp65 epitope in the context of HLA Cw*0602. T-cell repertoire analysis revealed 2 dominant populations expressing T-cell receptor beta variable (TRBV)4-3 or TRBV13, with cross-reactivity exclusively mediated by the TRBV13+ clonotypes. More importantly, cross-reactive TRBV13+ clonotypes displayed markedly lower T-cell receptor binding affinity and a distinct pattern of peptide recognition, presumably mimicking a structure presented on the HLA DR4 allotype. These results illustrate a novel mechanism whereby virus-specific CD8+ T cells can cross-recognize HLA class II molecules and may contribute toward allograft rejection and/or autoimmunity.

Crucial contribution of thymic Sirp alpha+ conventional dendritic cells to central tolerance against blood-borne antigens in a CCR2-dependent manner

Thymic dendritic cells (DCs) as well as thymic epithelial cells are presumed to be major sentinels in central tolerance by inducing the apoptosis of autoreactive T progenitor cells. The thymic DC population is composed of heterogeneous subsets including CD11c(+)B220(+) plasmacytoid DCs, CD11c(+)B220(-)CD8alpha(+) signal regulatory protein alpha (Sirpalpha)(-) and CD11c(+)B220(-)CD8alpha(-)Sirpalpha(+) conventional DCs (cDCs). However, the distinctive role of each DC subset remains undefined. We show herein that Sirpalpha(+) cDCs, a minor subpopulation, was disseminated in the thymic cortical area with some of them uniquely localized inside perivascular regions and nearby small vessels in the thymus. The Sirpalpha(+) but not Sirpalpha(-) cDC subset can selectively capture blood-circulating Ags. Moreover, in CCR2-deficient mice, the thymic Sirpalpha(+) cDC subset, but not other thymic cell components, was moderately decreased especially in the perivascular regions. Concomitantly, these mice exhibited a modest impairment in intrathymic negative selection against blood-borne Ags, with the reduced capacity to uptake blood-borne Ags. Given their intrathymic cortical localization, CD11c(+)B220(-)CD8alpha(-)Sirpalpha(+) cDCs can have a unique role in the development of central tolerance against circulating peripheral Ags, at least partially in a CCR2-dependent manner.

TSLP in epithelial cell and dendritic cell cross talk

Dendritic cells (DCs) are professional antigen-presenting cells that have the ability to sense infection and tissue stress, sample and present antigen to T lymphocytes, and instruct the initiation of different forms of immunity and tolerance. The functional versatility of DCs depends on their remarkable ability to translate collectively the information from the invading microbes, as well as their resident tissue microenvironments. Recent progress in understanding Toll-like receptor (TLR) biology has illuminated the mechanisms by which DCs link innate and adaptive antimicrobial immune responses. However, how tissue microenvironments shape the function of DCs has remained elusive. Recent studies of TSLP (thymic stromal lymphopoietin), an epithelial cell-derived cytokine that strongly activates DCs, provide strong evidence at a molecular level that epithelial cells/tissue microenvironments directly communicate with DCs, the professional antigen-presenting cells of the immune system. We review recent progress on how TSLP expressed within thymus and peripheral lymphoid and nonlymphoid tissues regulates DC-mediated central tolerance, peripheral T cell homeostasis, and inflammatory Th2 responses.

TSLP: an epithelial cell cytokine that regulates T cell differentiation by conditioning dendritic cell maturation

Dendritic cells (DCs) are professional antigen-presenting cells that have the ability to sense infection and tissue stress, sample and present antigen to T lymphocytes, and induce different forms of immunity and tolerance. The functional versatility of DCs depends on their remarkable ability to translate collectively the information from both the invading microbes and their resident tissue microenvironments and then make an appropriate immune response. Recent progress in understanding TLR biology has illuminated the mechanisms by which DCs link innate and adaptive antimicrobial immune responses. However, how tissue microenvironments shape the function of DCs has remained elusive. Recent studies of TSLP (thymic stromal lymphopoietin), an epithelial cell-derived cytokine that strongly activates DCs, provide evidence at a molecular level that epithelial cells/tissue microenvironments directly communicate with DCs. We review recent progress on how TSLP expressed within thymus and peripheral lymphoid and nonlymphoid tissues regulates DC-mediated central tolerance, peripheral T cell homeostasis, and inflammatory Th2 responses.

Central tolerance: good but imperfect

T-cell development is a highly coordinated process that depends on interactions between thymocytes, thymic epithelium, and bone marrow (BM)-derived dendritic cells (DCs). Before entering the peripheral T-cell pool, thymocytes are subject to negative selection, a process that eliminates (or deletes) T cells with high affinity toward self-antigens and therefore promotes self-tolerance. These self-antigens include those that are broadly expressed ubiquitous antigens and those whose expression is restricted to a few tissues, tissue-specific antigens (TSAs). Expression of TSAs in the thymus is mostly a property of medullary thymic epithelial cells (mTECs), and because these cells may be less capable than BM-derived DCs at mediating negative selection to ubiquitous antigens, we investigated the roles of both of these cell types in tolerance to TSAs. Here, we review our studies in which we found that mTECs were competent mediators of negative selection to a subset of TSA-reactive T cells, while thymic DCs extend the range of TSA-reactive T cells that undergo negative selection by capturing TSAs from mTECs. In addition, we recently investigated the efficiency of central tolerance to TSA during ontogeny, and we report that this process was less efficient in neonates than adult animals.

Morphological and immunohistochemical study of Down syndrome thymus

The incidence and severity of various histologic changes were studied in the thymuses from 35 Down syndrome (DS) patients. Variable histologic alterations up to fibrotic involution of the gland were found. In addition, immunohistochemical investigation showed a reduction or absence of interdigitating reticulum cells roughly correlated with the severity of morphologic changes. The complex architectural and cellular thymic alterations, concerning both cortical and medullary microenvironment, could be correlated with the imbalances of immunity in DS patients.

Operationally tolerant and minimally immunosuppressed kidney recipients display strongly altered blood T-cell clonal regulation

Most kidney transplant recipients who discontinue immunosuppression reject their graft. Nevertheless, a small number do not, suggesting that allogeneic tolerance state (referred to operational tolerance) is achievable in humans. So far, however, the rarity of such patients has limited their study. Because operational tolerance could be linked to anergy, ignorance or to an active regulatory mechanism, we analyzed the blood T-cell repertoire usage of these patients. We report on comparison of T-cell selection in drug-free operationally tolerant kidney recipients (or with minimal immunosuppression), recipients with stable graft function, chronic rejection and healthy individuals. The blood T cells of operationally tolerant patients display two major characteristics: an unexpected strongly altered T-cell receptor (TCR) Vbeta usage and high TCR transcript accumulation in selected T cells. The cytokine transcriptional patterns of sorted T cells with altered TCR usage show no accumulation of cytokine transcripts (IL10, IL2, IL13, IFN-gamma), suggesting a state of hyporesponsiveness in these patients. Identification of such a potential surrogate pattern of operational tolerance in transplant recipients under life-long immunosuppression may provide a new basis and rationale for exploration of tolerance state. However, these data obtained in a limited number of patients require further confirmation on larger series.

CD28 costimulation of developing thymocytes induces Foxp3 expression and regulatory T cell differentiation independently of interleukin 2

Efficient generation of regulatory T cells (T(reg) cells) in the thymus requires CD28 costimulation, but it is not known why. Here, molecular mapping of CD28 costimulation showed that T(reg) cell generation requires a motif that binds the tyrosine kinase Lck, precisely the same motif that is required for CD28 costimulation of interleukin 2 production. Nevertheless, CD28 costimulation provides more than interleukin 2 to developing T(reg) cells, as CD28 costimulation of T cell receptor-signaled double-positive thymocytes induced expression of Foxp3, considered to be the T(reg) 'master gene', as well as GITR and CTLA-4, two proteins expressed on T(reg) cells. Thus, CD28 costimulation directly signals developing thymocytes to express Foxp3 and to initiate the T(reg) cell differentiation program.

Dendritic Cell Biology

Dendritic cells (DCs) are a special type of leukocytes able to alert the immune system to the presence of infections. They play a central role in the initiation of both innate and adaptive immune responses. This particular DC feature is regulated by the activation of specific receptors at the cell surface called Toll-like receptors (TLRs) that bind a number of microbial products collectively referred to as microbial-associated molecular patterns (MAMP). TLRs initiate a cascade of events, which together define the process of DC maturation. This phenomenon allows DCs to progressively acquire varying specific functions. DC maturation depends on the nature of the perturbation and permits unique and efficient immune responses for each pathogen. In this review the discussion is focused on DCs in the context of interactions with pathogens and DC-specific functions are highlighted.

Is tolerance a prospective for clinical research?

Tolerance is an emotive issue in transplantation. It is the promised land for which we all strive and which we all hope we shall live to see. In such circumstances, tolerance must always be a prospective for clinical research! The question is, therefore, better posed in a more optimistic fashion and with a small act of faith: do we, in 1991, have that crucial combination of basic scientific knowledge and creative imagination to make it possible?

Impaired thymic negative selection causes autoimmune graft-versus-host disease

Animal models with impaired thymic negative selection do not always cause autoimmune diseases despite the development of an autoreactive T-cell repertoire. We investigated the requirements for the development of systemic autoimmune disease by using bone marrow chimeras that lacked expression of major histocompatibility complex (MHC) class II on thymic antigen-presenting cells (APCs), leading to impaired negative selection. We found that impaired negative selection mediated by absence of MHC class II, but not MHC class I, permitted the development of systemic autoimmune disease that is indistinguishable from acute graft-versus-host disease (GVHD). Thymectomy prevented disease, confirming the causal association of the thymus with its development. Adoptive transfer of CD4+ T cells caused GVHD in secondary hosts only when they were irradiated, and cotransfer of peripheral CD4+ and CD8+ T cells from naive mice prevented the disease. These results demonstrate that impaired thymic negative selection can cause lethal autoimmune disease indistinguishable from acute GVHD in the context of a proinflammatory milieu when peripheral regulatory mechanisms are absent.

Con A activates an Akt/PKB dependent survival mechanism to modulate TCR induced cell death in double positive thymocytes

While low avidity ligation of the T cell receptor (TCR) leads to positive selection and further maturation of developing thymocytes providing the immune system with mature CD4(+) and CD8(+) (single positive) T cells, high avidity ligation triggers negative selection by apoptotic cell death and therefore the TCR repertoire is purged of autoreactive T cells. On peripheral T cells, however, high avidity ligation of the TCR triggers activation and survival not death. In the present study we used concanavalin A (Con A) and alpha-CD3 epsilon antibody to investigate a possible survival mechanism in connection with TCR ligation. Con A and alpha-CD3 epsilon were used in the study for the following reasons: (1) they both mimic the effects of high avidity TCR ligation by activating peripheral T cells, and (2) they trigger distinctively different physiological changes in developing thymocytes. While Con A supports events associated with cellular survival, alpha-CD3 epsilon induces apoptotic cell death. In our experimental system the TCR was cross-linked by Con A and alpha-CD3 epsilon in thymocytes of major histocompatibility complex (MHC) deficient thymus organ cultures, where signals from the TCR can be triggered on zero background signal level. We have found that TCR cross-linking by Con A and not by alpha-CD3 epsilon decreases the gene and protein expression of the pro-apoptotic molecule, Bad; and that Con A is capable of the activation of the survival signalling pathway including protein kinase B (Akt/PKB) independently of phosphatidyl inositol kinase (PI3K).

Experimental models linking dendritic cell lineage, phenotype and function

One of the important issues in dendritic cell (DC) biology today is how DC control the fate of T cells. Our data suggest that an important branch point in determining T cell fate is the decision between deletion and memory. We have previously hypothesized that this binary decision is determined by contact with DC derived from lymphoid- versus myeloid-restricted progenitors. However, the false attribution of CD8alpha expression as a reliable marker of lymphoid origin has underpinned a number of studies in which DC expressing CD8alpha did not induce deletion, thereby clouding the issue of whether deletion is indeed a function of lymphoid DC. By returning to basics, that is, functional testing of the progeny of lymphoid- and myeloid-restricted progenitors in vivo, we hope to provide clear evidence of the in vivo roles of lymphoid and myeloid DC subsets, independent of assumptions about the surface phenotypes they can assume.

Generation of a complete thymic microenvironment by MTS24(+) thymic epithelial cells

The epithelial component of the thymic microenvironment is indispensable for the generation of T lymphocytes. Although the heterogeneity of this epithelium is well documented, little is known about precursor-progeny relationships between distinct thymic epithelial lineages. Here we characterized a thymic epithelial cell subpopulation identified by the cell surface glycoprotein MTS24. These cells contained epithelial progenitor cells that were competent and sufficient to fully reconstitute the complex thymic epithelial microenvironment that supported normal T cell development.

Mixed chimerism induces donor-specific T-cell tolerance across a highly disparate xenogeneic barrier

Induction of tolerance is likely to be essential for successful xenotransplantation because immune responses across xenogeneic barriers are vigorous. Although mixed hematopoietic chimerism leads to stable donor-specific tolerance in allogeneic and closely related xenogeneic (eg, rat-to-mouse) combinations, the ability of this approach to induce tolerance across a highly disparate xenogeneic barrier has not yet been demonstrated. In this study, we investigated the immune responses of murine T cells that developed in mice with pre-established porcine hematopoietic chimerism. Our results show for the first time that induction of porcine hematopoietic chimerism can eliminate the development of antiporcine donor responses in a highly disparate xenogeneic species. Porcine hematopoietic chimeras showed donor-specific nonresponsiveness in the mixed lymphocyte reaction, lack of antidonor IgG antibody production, and acceptance of donor skin grafts. Thus, mixed chimerism is capable of inducing tolerance in a highly disparate xenogeneic combination and may have clinical potential to prevent xenograft rejection. (Blood. 2002;99:3823-3829)

A large quantity of CD3-/CD19-/CD16- lymphocytes in malignant pleural effusion from a patient with recurrent cholangio cell carcinoma

Tumor infiltrating lymphocytes (TILs) are candidates for adoptive cellular immunotherapy. Here we report on a patient whose TILs presented unusual lymphocyte antigens. Pleural effusions were collected from a 47-year-old man with recurrent cholangio cell carcinoma and malignant effusion. Effusion-associated lymphocytes (EALs) were separated by Ficoll-Hypaque gradient, and the EAL phenotype was determined by flow cytometry. The percentage of positive cells was determined for each lymphocyte-related differentiation antigen. The percentages of CD3+, CD19+, and CD16+ lymphocyte subpopulations among EALs were 20%, 7%, and 3%, respectively. Nearly 70% of EALs were CD3-/CD19-/CD56-/CD16- cells. The phenotypes of peripheral blood lymphocytes (PBLs) collected simultaneously from the patient's peripheral blood were CD3+ (52%), CD19+ (20%), and CD16+ (20%). When EALs were cultured in medium without pleural effusion, T cell-related antigens, but not B cell- or natural killer (NK) cell-related antigens, were newly expressed on EALs, and this expression reached a plateau after 48 h in culture. The proportions of CD3+, CD19+, and CD16+ cells were 69%, 7%, and 3%, respectively. However, when EALs were cultured in medium with pleural effusion, increased expression of T cell-related antigens was not observed; the proportions of CD3+, CD19+, and CD16+ cells were 16%, 6%, and 1%, respectively. Neither total cell numbers nor cellular viability of EALs changed significantly after in-vitro culture, suggesting that significant proliferation or death of EALs did not occur during the culture period. Co-culture of the patient's PBLs with autologous pleural effusion for 96 h did not alter the expression of lymphocyte-related antigens on the PBLs. These results indicate that expression of T cell-related antigens, but not B cell- or NK cell-related antigens, on EALs was blocked temporarily by the malignant pleural effusion. This is the first report concerning the existence of a large quantity of unclassified lymphocytes in which the T cell-related antigens were reversibly masked in the malignant pleural effusion.

Thymic re-entry of mature activated T cells and increased negative selection in vascularized allograft recipients

Transplantation tolerance is a dynamic state that involves several homeostatic mechanisms intrinsic to the host. One of these mechanisms is activation-induced T cell death (AICD). However, it is unclear where AICD takes place during alloreactive responses. Since activated T cells can re-enter the thymus, we hypothesized that mature T cells activated by an allograft could be deleted upon re-entry into the thymus. To test this hypothesis, we used wild-type or 2C TCR transgenic mice receiving syngeneic or allogeneic heterotopic, vascularized heart grafts. First, we demonstrated that ex vivo CFSE-labelled T cells re-entered the thymus when transferred into allograft recipients but not when transferred into isograft recipients. Next, we compared the changes in cell subset numbers and incidence of apoptosis in the thymi and spleens of allograft or isograft recipients. Seven days after transplantation, at a time in which all the allografts were undergoing rejection, cells expressing donor-MHC class II molecules had migrated to the thymus and to the spleen. In the thymus of allograft recipients, overall cellularity was significantly reduced by 40% and associated with an increase in the number of double negative (CD4-CD8-) thymocytes and a decrease in double positive (CD4+CD8+) thymocytes, consistent with increased negative selection of thymocytes. Additionally, thymi of allograft recipients showed an increase in the number of recently activated, mature T cells (TCRhi, CD25+, CD44+) and a significant increase in the number of apoptotic cells, especially in the thymic medulla, that involved mature T cells as indicated by the TCRhi, CD44+, CD4 or CD8 single positive phenotype. Spleens of allograft recipients were increased in size and cellularity but did not show any of the changes in cell subsets seen in the thymi. Our data show that after allografting there is an increase in apoptotic cell death that is associated with negative selection of developing thymocytes as well as of alloreactive mature T cells that have re-entered the thymus upon activation in the periphery. This may occur upon migration of graft-derived antigen-presenting cells to the thymus.

Identification and characterization of thymus LIM protein: targeted disruption reduces thymus cellularity

We have identified a novel LIM gene encoding the thymus LIM protein (TLP), expressed specifically in the thymus in a subset of cortical epithelial cells. TLP was identified as a gene product which is upregulated in a thymus in which selection of T cells is occurring (Rag(-/-) OT-1) compared to its expression in a thymus in which selection is blocked at the CD4+ CD8+ stage of T-cell development (Rag(-/-) Tap(-/-) OT-1). TLP has an apparent molecular mass of 23 kDa and exists as two isomers (TLP-A and TLP-B), which are generated by alternative splicing of the message. The sequences of TLP-A and TLP-B are identical except for the C-terminal 19 or 20 amino acids. Based on protein sequence alignment, TLP is most closely related to the cysteine-rich proteins, a subclass of the family of LIM-only proteins. In both medullary and cortical thymic epithelial cell lines transduced with TLP, the protein localizes to the cytoplasm but does not appear to be strongly associated with actin. In immunohistochemical studies, TLP seems to be localized in a subset of epithelial cells in the cortex and is most abundant near the corticomedullary junction. We generated mice with a targeted disruption of the Tlp locus. In the absence of TLP, thymocyte development and thymus architecture appear to be normal but thymocyte cellularity is reduced by approximately 30%, with a proportional reduction in each subpopulation.

Organ-specific autoimmunity in mice whose T cell repertoire is shaped by a single antigenic peptide

Organ-specific autoimmune diseases have been postulated to be the result of T cell response against organ-specific self-peptides bound to MHC molecules. Contrary to this paradigm, we report here that transgenic mice lacking MHC class I expression and expressing an MHC class II I-A(b) molecule that presents only a single peptide (E alpha 52-68) spontaneously develops peripheral nervous system-specific autoimmune disease with many of the histopathological features found in experimental allergic neuritis. Reciprocal bone marrow chimeras produced using susceptible and resistant lines revealed that bone marrow-derived cells determined disease susceptibility. While the expression of the I-A(b)-E alpha 52-68 complex in the periphery was readily detectable in both lines, its expression on thymic dendritic cells responsible for tolerance induction was markedly lower in the susceptible line than in the resistant line. Consistent with this, CD4(+) T cells that can be activated by the I-A(b)-E alpha 52-68 complex were found in the susceptible line, but not in the resistant line. Such CD4(+) T cells conferred the disease to the resistant line by adoptive transfer, and administration of Ab specific for the I-A(b)-E alpha 52-68 complex inhibited disease manifestation in the susceptible line. These results indicate that disease development involves systemic T cell reactivity to I-A(b)-E alpha 52-68 complex, probably caused by incomplete negative thymocyte selection.

Mixed chimerism

Induction of mixed chimerism has the potential to overcome the current limitations of transplantation, namely chronic rejection, complications of immunosuppressive therapy and the need for xenografts to overcome the current shortage of allogeneic organs. Successful achievement of mixed chimerism had been shown to tolerize T cells, B cells and possibly natural killer cells, the lymphocyte subsets that pose major barriers to allogeneic and xenogeneic transplants. Current understanding of the mechanisms involved in tolerization of each cell type is reviewed. Considerable advances have been made in reducing the potential toxicity of conditioning regimens required for the induction of mixed chimerism in rodent models, and translation of these strategies to large animal models and in a patient are important advances toward more widespread clinical application of the mixed chimerism approach for tolerance induction.

The Src-like adaptor protein downregulates the T cell receptor on CD4+CD8+ thymocytes and regulates positive selection

In this report, we show that the Src-like adaptor protein (SLAP) plays an important role in thymocyte development. SLAP expression is developmentally regulated; it is low in CD4-CD8- thymocytes, it peaks in the CD4+CD8+ subset, and it decreases to low levels in more mature cells. Disruption of the SLAP gene leads to a marked upregulation of TCR and CD5 expression at the CD4+CD8+ stage. The absence of SLAP was also developmentally significant because it enhanced positive selection in mice expressing the DO11.10 transgenic T cell receptor. Moreover, SLAP deletion at least partially rescued the development of ZAP-70-deficient thymocytes. These results demonstrate that SLAP participates in a novel mechanism of TCR downregulation at the CD4+CD8+ stage and regulates positive selection.

Major histocompatibility complex class II-positive cortical epithelium mediates the selection of CD4(+)25(+) immunoregulatory T cells

CD4(+)25(+) T cells are a unique population of immunoregulatory T cells which are critical for the prevention of autoimmunity. To address the thymic selection of these cells we have used two models of attenuated thymic deletion. In K14-A(beta)(b) mice, major histocompatibility complex (MHC) class II I-A(b) expression is limited to thymic cortical epithelium and deletion by hematopoietic antigen-presenting cells does not occur. In H2-DMalpha-deficient mice, MHC class II molecules contain a limited array of self-peptides resulting in inefficient clonal deletion. We find that CD4(+)25(+) T cells are present in the thymus and periphery of K14-A(beta)(b) and H2-DMalpha-deficient mice and, like their wild-type counterparts, suppress the proliferation of cocultured CD4(+)25(-) effector T cells. In contrast, CD4(+)25(+) T cells from MHC class II-deficient mice do not suppress responder CD4(+) T cells in vitro or in vivo. Thus, development of regulatory CD4(+)25(+) T cells is dependent on MHC class II-positive thymic cortical epithelium. Furthermore, analysis of the specificities of CD4(+)25(+) T cells in K14-A(beta)(b) and H2-DMalpha-deficient mice suggests that a subset of CD4(+)25(+) T cells is subject to negative selection on hematopoietic antigen-presenting cells.

Induction of stable prenatal tolerance to beta-galactosidase by in utero gene transfer into preimmune sheep fetuses

The successful transduction of hematopoietic stem cells and long-term (28 months) transgene expression within the hematopoietic system following the direct injection of high-titer retroviral vectors into preimmune fetal sheep was previously demonstrated. The present studies extended these analyses for 40 months postinjection and evaluated whether the longevity of transgene expression in this model system was the result of induction of prenatal tolerance to the transgene product. The intraperitoneal injection of retroviral vectors into preimmune sheep fetuses transduces thymic epithelial cells thought to present antigen and thus define self during immune system development. To directly demonstrate induction of tolerance, postnatal sheep were boosted with purified beta-galactosidase and showed that the peripheral blood lymphocytes from in utero-transduced sheep exhibited significantly lower stimulation indices to transduced autologous cells than did control animals and that the in utero-transduced sheep had a reduced ability to mount an antibody response to the vector-encoded beta-galactosidase protein compared with control sheep. Collectively, our results provide evidence that the direct injection of retroviral vectors into preimmune sheep fetuses induces cellular and humoral tolerance to the vector/transgene products and provide an explanation for the duration and stability of transgene expression seen in this model. These results also suggest that even relatively low levels of gene transfer in utero may render the recipient tolerant to the exogenous gene and thus potentially permit the successful postnatal treatment of the recipient. (Blood. 2001;97:3417-3423)

Ultrastructural study of the FVB/N mouse thymus: presence of an immature epithelial cell in the medulla and premature involution

FVB/N inbred mice have been widely used to generate a variety of transgenic lines, but their physiology and especially their immunological characteristics are poorly documented. We therefore studied the ultrastructure of the thymus and the distribution of thymocyte subpopulations in FVB/N mice at several ages. In young FVB/N mice the stromal microenvironment exhibits the three types of epithelial cells and the two types of bone-marrow derived cells (macrophages and interdigitated cells) previously described in other strains of mice. Moreover, in the thymic medulla of young FVB/N mice, a fourth cell type with the morphological characteristics of an immature epithelial cell was present in relatively high number. Furthermore, thymocyte subpopulations distribution shows an earlier thymocyte maturation than in other strains. Finally, changes associated with thymic involution were observed about 5 months earlier than in many other mouse strains. Our results demonstrated that the FVB/N strain has a specific immunological status.

Bone marrow cell graft engineering: from bench to bedside

Bone marrow transplantation (BMT) has the potential to treat hemoglobinopathies (sickle cell and thalassemia) autoimmunity (diabetes, lupus, multiple sclerosis, rheumatoid arthritis, Crohn's colitis) and enzyme deficiency states. Graft versus host disease (GVHD) is a major complication and limitation to the therapeutic application of BMT. There have been many clinical trials and experimental animal models that have attempted to control GVHD through the engineering of the donor bone marrow cells (BMC). Historically, several methods have demonstrated effectiveness in controlling GVHD; however they were also associated with a marked increase in the rate of graft failure. Highly purified hematopoietic stem cells (HSC) engraft quite readily in genetically-matched recipients while they do not engraft as easily in MHC-disparate recipients. The numbers of HSC must be increased 100-200 fold in order to overcome the allogeneic barrier. We were the first to phenotypically and to functionally characterize a novel cell in the bone marrow that enables engraftment of highly purified HSC in allogeneic recipients. The discovery of graft facilitating cell populations has resulted in the restoration of the engraftment-potential of purified HSC between genetically-disparate individuals. The addition of facilitating cells (FC) to T cell-depleted BMC grafts results in allogeneic engraftment without GVHD or graft failure. New strategies of BMC engineering that retain FC and HSC but avoid GVHD have allowed successful engraftment in mismatched and older recipients. These techniques have expanded the therapeutic potential of BMT to virtually every candidate as well as to non-malignant diseases in which the morbidity associated with conventional BMT could not be accepted. This article reviews the transition of the FC technology from bench to bedside and discuss the potentially broad-reaching applications of BMT and mixed chimerism.

Both gamma delta T cells and NK cells inhibit the engraftment of xenogeneic rat bone marrow cells and the induction of xenograft tolerance in mice

In murine allogeneic bone marrow transplantation recipients, treatment of the hosts with a nonmyeloablative regimen, including depleting anti-CD4 and anti-CD8 mAbs, allows establishment of long-term mixed chimerism and donor-specific tolerance. However, in the xenogeneic rat-to-mouse combination, additional anti-Thy1.2 and anti-NK1.1 mAbs are required. We have now attempted to identify the xenoresistant mouse cell populations that are targeted by anti-NK1.1 and anti-Thy1.2 mAbs. C57BL/6 (B6) wild-type, B6 TCRbeta(-/-), and B6 TCRdelta(-/-) mice received anti-CD4 and anti-CD8 mAbs, followed by 3 Gy of whole body irradiation, 7 Gy of thymic irradiation, and transplantation of T cell-depleted rat bone marrow cells. Anti-NK1.1 and anti-Thy1.2 mAbs were additionally administered to some groups. Increased rat chimerism was observed in TCRdelta(-/-) mice treated with anti-CD4, anti-CD8, and anti-NK1.1 mAbs compared with similarly treated TCRbeta(-/-) mice. In TCRbeta(-/-) mice, but not in TCR delta(-/-) mice, donor chimerism was increased by treatment with anti-Thy1.2 mAb, indicating that CD4(-)CD8(-)TCRgammadelta(+)Thy1. 2(+)NK1.1(-) cells (gammadelta T cells) are involved in the rejection of rat marrow. In addition, chimerism was enhanced in both TCRbeta(-/-) and TCRdelta(-/-) mice treated with anti-CD4, anti-CD8, and anti-Thy1.2 mAbs by the addition of anti-NK1.1 mAb to the conditioning regimen. Donor-specific skin graft prolongation was enhanced by anti-Thy1.2 and anti-NK1.1 mAbs in TCRdelta(-/-) mice. Therefore, in addition to CD4 and CD8 T cells, gammadelta T cells and NK cells play a role in resisting engraftment of rat marrow and the induction of xenograft tolerance in mice.

DNA vaccination and the immune responsiveness of neonates

Neonates often respond poorly to conventional vaccines or microbial infections. Immaturity of the immune system has been considered to play a role in this regard. However, accumulating evidence shows that in certain conditions, neonatal inoculation of antigens leads to protective immunity. In the particular case of DNA vaccines administered to neonates, the rule is immunity rather than tolerance. Exceptions to the rule give opportunities to further understand the neonatal responsiveness and the mechanism of DNA vaccination. Due to the very nature of the vaccine vector, inhibition of neonatal DNA vaccination by maternal antibodies may be limited to the humoral immunity.

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

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

Functional comparison of thymic B cells and dendritic cells in vivo

In this report we present a transgenic mouse model in which we targeted gene expression specifically to B-lymphocytes. Using the human CD19 promoter, we expressed major histocompatibility complex class II I-E molecules specifically on B cells of all tissues, but not on other cell types. If only B cells expressed I-E in a class II-deficient background, positive selection of CD4+ T cells could not be observed. A comparison of the frequencies of I-E reactive Vβ5+ and Vβ11+ T cells shows that I-E expression on thymic B cells is sufficient to negatively select I-E reactive CD4+ T cells partially, but not CD8+ T cells. Thus partial negative but no positive selection events can be induced by B-lymphocytes in vivo.

Functional comparison of thymic B cells and dendritic cells in vivo

In this report we present a transgenic mouse model in which we targeted gene expression specifically to B-lymphocytes. Using the human CD19 promoter, we expressed major histocompatibility complex class II I-E molecules specifically on B cells of all tissues, but not on other cell types. If only B cells expressed I-E in a class II-deficient background, positive selection of CD4+ T cells could not be observed. A comparison of the frequencies of I-E reactive Vβ5+ and Vβ11+ T cells shows that I-E expression on thymic B cells is sufficient to negatively select I-E reactive CD4+ T cells partially, but not CD8+ T cells. Thus partial negative but no positive selection events can be induced by B-lymphocytes in vivo.

Induction of rapid T cell activation, division, and recirculation by intratracheal injection of dendritic cells in a TCR transgenic model

Dendritic cells (DCs) are thought to be responsible for sensitization to inhaled Ag and induction of adaptive immunity in the lung. The characteristics of T cell activation in the lung were studied after transfer of Ag-pulsed bone marrow-derived DCs into the airways of naive mice. Cell division of Ag-specific T cells in vivo was followed in a carboxyfluorescein diacetate succinimidyl ester-labeled cohort of naive moth cytochrome c-reactive TCR transgenic T cells. Our adoptive transfer system was such that transferred DCs were the only cells expressing the MHC molecule required for presentation of cytochrome c to transgenic T cells. Ag-specific T cell activation and proliferation occurred rapidly in the draining lymph nodes of the lung, but not in nondraining lymph nodes or spleen. No bystander activation of non-Ag-specific T cells was induced. Division of Ag-specific T cells was accompanied by transient expression of CD69, while up-regulation of CD44 increased with each cell division. Divided cells had recirculated to nondraining lymph nodes and spleen by day 4 of the response. In vitro restimulation with specific Ag revealed that T cells were primed to proliferate more strongly and to produce higher amounts of cytokines per cell. These data are consistent with the notion that DCs in the lung are extremely efficient in selecting Ag-reactive T cells from a diverse repertoire. The response is initially localized in the mediastinal lymph nodes, but subsequently spreads systemically. This system should allow us to study the early events leading to sensitization to inhaled Ag.

T-cell recovery following marrow transplant: experience with delayed lymphocyte infusions to accelerate immune recovery or treat infectious problems

All forms of hematopoietic stem-cell transplantation are complicated by delayed immune reconstitution, which results in an increased risk of infectious complications and relapse of disease. Donor lymphocyte infusions have been used in an attempt to enhance immune recovery and for the prevention and treatment of specific infections following transplantation. While there is little data to support the use of donor lymphocytes for the enhancement of general immune function post-transplant, unselected and virus-specific donor T cells may have efficacy for the prophylaxis and treatment of infections and disease caused by Epstein-Barr virus (EBV) and cytomegalovirus (CMV). While donor lymphocyte infusions may cause significant morbidity and mortality, they are a novel and potentially powerful approach for the treatment of frequently fatal post-transplant infectious complications.

Prospects for in utero human gene therapy

Gene therapy for the treatment of disease in children and adults is being actively pursued at many medical centers. However, a number of genetic disorders result in irreversible damage to the fetus before birth. In these cases, as well as for those with genetic diseases who may benefit from therapy before symptoms are manifested, in utero gene therapy (IUGT) could be beneficial. Although some successes with in utero gene transfer have been reported in animals, significant questions remain to be answered before IUGT clinical trials would be acceptable. This review analyzes the state of the art and delineates the studies that still need to be performed before it would be appropriate to consider human IUGT.