Efficient T cell repertoire selection in tetraparental chimeric mice independent of thymic epithelial MHC

MHC heterozygosity limits T cell receptor variability in CD4 T cells

αβ T cell receptor (TCR) V(D)J genes code for billions of TCR combinations. However, only some appear on peripheral T cells in any individual because, to mature, thymocytes must react with low affinity but not high affinity with thymus expressed major histocompatibility (MHC)/peptides. MHC proteins are very polymorphic. Different alleles bind different peptides. Therefore, any individual might express many different MHC alleles to ensure that some peptides from an invader are bound to MHC and activate T cells. However, most individuals express limited numbers of MHC alleles. To explore this, we compared the TCR repertoires of naïve CD4 T cells in mice expressing one or two MHC alleles. Unexpectedly, the TCRs in heterozygotes were less diverse that those in the sum of their MHC homozygous relatives. Our results suggest that thymus negative selection cancels out the advantages of increased thymic positive selection in the MHC heterozygotes.

Chimerism and tolerance without GVHD or engraftment syndrome in HLA-mismatched combined kidney and hematopoietic stem cell transplantation

The toxicity of chronic immunosuppressive agents required for organ transplant maintenance has prompted investigators to pursue approaches to induce immune tolerance. We developed an approach using a bioengineered mobilized cellular product enriched for hematopoietic stem cells (HSCs) and tolerogenic graft facilitating cells (FCs) combined with nonmyeloablative conditioning; this approach resulted in engraftment, durable chimerism, and tolerance induction in recipients with highly mismatched related and unrelated donors. Eight recipients of human leukocyte antigen (HLA)-mismatched kidney and FC/HSC transplants underwent conditioning with fludarabine, 200-centigray total body irradiation, and cyclophosphamide followed by posttransplant immunosuppression with tacrolimus and mycophenolate mofetil. Subjects ranged in age from 29 to 56 years. HLA match ranged from five of six loci with related donors to one of six loci with unrelated donors. The absolute neutrophil counts reached a nadir about 1 week after transplant, with recovery by 2 weeks. Multilineage chimerism at 1 month ranged from 6 to 100%. The conditioning was well tolerated, with outpatient management after postoperative day 2. Two subjects exhibited transient chimerism and were maintained on low-dose tacrolimus monotherapy. One subject developed viral sepsis 2 months after transplant and experienced renal artery thrombosis. Five subjects experienced durable chimerism, demonstrated immunocompetence and donor-specific tolerance by in vitro proliferative assays, and were successfully weaned off all immunosuppression 1 year after transplant. None of the recipients produced anti-donor antibody or exhibited engraftment syndrome or graft-versus-host disease. These results suggest that manipulation of a mobilized stem cell graft and nonmyeloablative conditioning represents a safe, practical, and reproducible means of inducing durable chimerism and donor-specific tolerance in solid organ transplant recipients.

Mouse chimeras as a system to investigate development, cell and tissue function, disease mechanisms and organ regeneration

Chimeras are organisms composed of at least two genetically distinct cell lineages originating from different zygotes. In the laboratory, mouse chimeras can be produced experimentally; various techniques allow combining different early stage mouse embryos with each other or with pluripotent stem cells. Identification of the progeny of the different lineages in chimeras permits to follow cell fate and function, enabling correlation of genotype with phenotype. Mouse chimeras have become a tool to investigate critical developmental processes, including cell specification, differentiation, patterning, and the function of specific genes. In addition, chimeras can also be generated to address biological processes in the adult, including mechanisms underlying diseases or tissue repair and regeneration. This review summarizes the different types of chimeras and how they have been generated and provides examples of how mouse chimeras offer a unique and powerful system to investigate questions pertaining to cell and tissue function in the developing and adult organism.

First use of thymus transplantation therapy for FOXN1 deficiency (nude/SCID): a report of 2 cases

FOXN1 deficiency is a primary immunodeficiency characterized by athymia, alopecia totalis, and nail dystrophy. Two infants with FOXN1 deficiency were transplanted with cultured postnatal thymus tissue. Subject 1 presented with disseminated Bacillus Calmette-Guérin infection and oligoclonal T cells with no naive markers. Subject 2 had respiratory failure, human herpes virus 6 infection, cytopenias, and no circulating T cells. The subjects were given thymus transplants at 14 and 9 months of life, respectively. Subject 1 received immunosuppression before and for 10 months after transplantation. With follow up of 4.9 and 2.9 years, subjects 1 and 2 are well without infectious complications. The pretransplantation mycobacterial disease in subject 1 and cytopenias in subject 2 resolved. Subject 2 developed autoimmune thyroid disease 1.6 years after transplantation. Both subjects developed functional immunity. Subjects 1 and 2 have 1053/mm(3) and 1232/mm(3) CD3(+) cells, 647/mm(3) and 868/mm(3) CD4(+) T cells, 213/mm(3) and 425/mm(3) naive CD4(+) T cells, and 10 200 and 5700 T-cell receptor rearrangement excision circles per 100 000 CD3(+) cells, respectively. They have normal CD4 T-cell receptor β variable repertoires. Both subjects developed antigen-specific proliferative responses and have discontinued immunoglobulin replacement. In summary, thymus transplantation led to T-cell reconstitution and function in these FOXN1 deficient infants.

Thymoproteasome shapes immunocompetent repertoire of CD8+ T cells

How self-peptides displayed in the thymus contribute to the development of immunocompetent and self-protective T cells is largely unknown. In contrast, the role of thymic self-peptides in eliminating self-reactive T cells and thereby preventing autoimmunity is well established. A type of proteasome, termed thymoproteasome, is specifically expressed by thymic cortical epithelial cells (cTECs) and is required for the generation of optimal cellularity of CD8+ T cells. Here, we show that cTECs displayed thymoproteasome-specific peptide-MHC class I complexes essential for the positive selection of major and diverse repertoire of MHC class I-restricted T cells. CD8+ T cells generated in the absence of thymoproteasomes displayed a markedly altered T cell receptor repertoire that was defective in both allogeneic and antiviral responses. These results demonstrate that thymoproteasome-dependent self-peptide production is required for the development of an immunocompetent repertoire of CD8+ T cells.

Functionally mature CD4 and CD8 TCRalphabeta cells are generated in OP9-DL1 cultures from human CD34+ hematopoietic cells

Human CD34(+) hematopoietic precursor cells cultured on delta-like ligand 1 expressing OP9 (OP9-DL1) stromal cells differentiate to T lineage cells. The nature of the T cells generated in these cultures has not been studied in detail. Since these cultures do not contain thymic epithelial cells which are the main cell type mediating positive selection in vivo, generation of conventional helper CD4(+) and cytotoxic CD8(+) TCRalphabeta cells is not expected. Phenotypically mature CD27(+)CD1(-) TCRgammadelta as well as TCRalphabeta cells were generated in OP9-DL1 cultures. CD8 and few mature CD4 single-positive TCRalphabeta cells were observed. Mature CD8 single-positive cells consisted of two subpopulations: one expressing mainly CD8alphabeta and one expressing CD8alphaalpha dimers. TCRalphabeta CD8alphaalpha and TCRgammadelta cells both expressed the IL2Rbeta receptor constitutively and proliferated on IL-15, a characteristic of unconventional T cells. CD8alphabeta(+) and CD4(+) TCRalphabeta cells were unresponsive to IL-15, but could be expanded upon TCR stimulation as mature CD8alphabeta(+) and CD4(+) T cells. These T cells had the characteristics of conventional T cells: CD4(+) cells expressed ThPOK, CD40L, and high levels of IL-2 and IL-4; CD8(+) cells expressed Eomes, Runx3, and high levels of granzyme, perforin, and IFN-gamma. Induction of murine or human MHC class I expression on OP9-DL1 cells had no influence on the differentiation of mature CD8(+) cells. Similarly, the presence of dendritic cells was not required for the generation of mature CD4(+) or CD8(+) T cells. These data suggest that positive selection of these cells is induced by interaction between T precursor cells.

MHC restriction and allogeneic immune responses

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

Purified hematopoietic stem cell allografts reconstitute immunity superior to bone marrow

Antigen-specific immune responses are impaired after allogeneic hematopoietic cell transplantation (HCT). The events contributing to this impairment include host hematolymphoid ablation and donor cell regeneration, which is altered by pharmacologic immune suppression to prevent graft-versus-host disease (GVHD). A generally accepted concept is that graft T cell depletion performed to avoid GVHD yields poorer immune recovery because mature donor T cells are thought to be the major mediators of protective immunity early post-HCT. Our findings contradict the idea that removal of mature donor cells worsens immune recovery post-HCT. By transplantation of purified hematopoietic stem cells (HSC) compared with bone marrow (BM) across donor and recipient pairs of increasing genetic disparity, we show that grafts composed of the purified progenitor population give uniformly superior lymphoid reconstitution, both qualitatively and quantitatively. Subclinical GVHD by T cells in donor BM likely caused this lympho-depleting GVHD. We further determined in the major histocompatibility complex (MHC)-mismatched pairs, that T cell restricted proliferative responses were dictated by donor rather than host elements. We interpret these latter findings to show the importance of peripheral antigen presentation in the selection and maintenance of the T cell repertoire.

The immunopathology of thymic GVHD

The clinical success of allogeneic hematopoietic stem cell transplantation (HSCT) depends on the appropriate reconstitution of the host's immune system. While recovery of T-cell immunity may occur in transplant recipients via both thymus-dependent and thymus-independent pathways, the regeneration of a population of phenotypically naive T cells with a broad receptor repertoire relies entirely on the de novo generation of T-cells in the thymus. Preclinical models and clinical studies of allogeneic HSCT have identified the thymus as a target of graft-versus-host disease (GVHD), thus limiting T-cell regeneration. The present review focuses on recent insight into how GVHD affects thymic structure and function and how this knowledge may aid in the design of new strategies to improve T-cell reconstitution following allogeneic HSCT.

Factors affecting success of thymus transplantation for complete DiGeorge anomaly

Thymus transplantation shows promise for the treatment of athymia in complete DiGeorge anomaly. This report reviews the effects of dose of thymus tissue, ABO compatibility, HLA matching, culture conditions, age of donor and immunosuppression of recipient on immune outcomes at 1 year after transplantation. Forty-nine athymic subjects have been treated with cultured postnatal allogeneic thymus tissue; 36 (73%) survive with only one subject on immunosuppression at 1.5 years. Of 31 surviving subjects more than 1 year after transplantation, 30 (97%) developed naive T cells, T-cell proliferative responses to mitogens and a diverse T-cell receptor beta variable (TCRBV) repertoire. The dose of thymus tissue, HLA matching and use of immunosuppression had nonsignificant effects on these outcome variables. Removal of deoxyguanosine from culture medium and length of culture did not adversely affect outcomes. Use of thymus tissue from donors over 1 month of age, versus under 1 month, resulted in higher total T-cell numbers (p = 0.03). However, this finding must be confirmed in a prospective trial. Although subtle immune effects may yet be associated with some of the factors tested, it is remarkable that consistently good immune outcomes result despite variation in dose, HLA matching and use of immunosuppression.

Thymus organogenesis

The epithelial architecture of the thymus fosters growth, differentiation, and T cell receptor repertoire selection of large numbers of immature T cells that continuously feed the mature peripheral T cell pool. Failure to build or to maintain a proper thymus structure can lead to defects ranging from immunodeficiency to autoimmunity. There has been long-standing interest in unraveling the cellular and molecular basis of thymus organogenesis. Earlier studies gave important morphological clues on thymus development. More recent cell biological and genetic approaches yielded new and conclusive insights regarding the germ layer origin of the epithelium and the composition of the medulla as a mosaic of clonally derived islets. The existence of epithelial progenitors common for cortex and medulla with the capacity for forming functional thymus after birth has been uncovered. In addition to the thymus in the chest, mice can have a cervical thymus that is small, but functional, and produces T cells only after birth. It will be important to elucidate the pathways from putative thymus stem cells to mature thymus epithelial cells, and the properties and regulation of these pathways from ontogeny to thymus involution.

Neural crest origin of perivascular mesenchyme in the adult thymus

The endodermal epithelial thymus anlage develops in tight association with neural crest (NC)-derived mesenchyme. This epithelial-NC interaction is crucial for thymus development, but it is not known how NC supports thymus development or whether NC cells or their progeny make any significant contribution to the adult thymus. By nude mouse blastocyst complementation and by cell surface phenotype, we could previously separate thymus stroma into Foxn1-dependent epithelial cells and a Foxn1-independent mesenchymal cell population. These mesenchymal cells expressed vascular endothelial growth factor-A, and contributed to thymus vascularization. These data suggested a physical or functional association with thymic blood vessels, but the origin, location in the thymus, and function of these stromal cells remained unknown. Using a transgenic mouse expressing Cre recombinase in premigratory NC (Sox10-Cre), we have now fate-mapped the majority of these adult mesenchymal cells to a NC origin. NC-derived cells represent tightly vessel-associated pericytes that are sandwiched between endothelium and epithelium along the entire thymus vasculature. The ontogenetic, phenotypic, and positional definition of this distinct perivascular mesenchymal compartment provides a cellular basis for the role of NC in thymus development and possibly maintenance, and might be useful to address properties of the endothelial-epithelial barrier in the adult thymus.

Generating intrathymic microenvironments to establish T-cell tolerance

Alphabeta T cells pass through a series of lymphoid tissue stromal microenvironments to acquire self tolerance and functional competence. In the thymus, positive selection of the developing T-cell receptor repertoire occurs in the cortex, whereas events in the medulla purge the system of self reactivity. T cells that survive are exported to secondary lymphoid organs where they direct first primary and then memory immune responses. This Review focuses on the microenvironments that nurture T-cell development rather than on T cells themselves. We summarize current knowledge on the formation of thymic epithelial-cell microenvironments, and highlight similarities between the environments that produce T cells and those that select and maintain them during immune responses.

Fully MHC-Disparate Mixed Hemopoietic Chimeras Show Specific Defects in the Control of Chronic Viral Infections

The establishment of mixed allogeneic chimerism can induce donor-specific transplantation tolerance across full MHC barriers. However, a theoretical disadvantage of this approach is the possibility that the state of mixed chimerism might negatively affect the recipient's immune competence to control pathogens. Previous studies using murine models have not supported this hypothesis, because they indicate that acute viral infections are cleared by chimeric animals with similar kinetics to that of unmanipulated controls. However, chronic or persistent viral infections often require a more complex and sustained response with cooperation between CD4 Th cells, CTL, and B cells for effective control. The current study indicates that profound defects become manifest in the control of chronic pathogenic infections in MHC-disparate mixed allogeneic chimeric mice. Furthermore, we show that ineffective priming of the donor-restricted CTL response leads to virus persistence, as well as severe T cell exhaustion. Our results further suggest that either T cell adoptive immunotherapy or selected MHC haplotype matching partially restore immune competence. These approaches may facilitate the translation of mixed chimerism therapeutic regimens.

Target-organ specificity of autoimmunity is modified by thymic stroma and bone marrow-derived cells

Physical contact between thymocytes and the thymic stroma is essential for the establishment of self-tolerance, and Aire in thymic epithelial cells plays an important role in this action. As expected, the autoimmune phenotypes of Aire-deficient mice are thymic stroma-dependent. Interestingly, the spectrum of the organs involved differs depending on the genetic background of non-autoimmune-prone mouse strains. Furthermore, deficiency of Aire in an autoimmune-prone strain of NOD also modifies target-cell specificity in the pancreas. In order to clarify the factors that regulate target-organ specificity in Aire-dependent autoimmunity, I have generated both thymic and bone-marrow chimeras, making it possible to evaluate the contribution of thymic stroma and bone-marrow-derived cells to this pathogenic process. The findings suggested that the genetic background of bone-marrow-derived cells contributes to the strain-dependent target-organ specificity of non-autoimmune-prone strains. Furthermore, in a study using NOD mice with a fixed genetic background, thymic stromal cells but not bone-marrow-derived cells were found to be relevant to the Aire-dependent alteration of target-cell specificity in the pancreas. These results clearly underscore the significance of immunological and/or genetic complexity that underlies Aire-deficiency monogenic disease together with critical dialogue between thymic stroma and bone-marrow-derived cells in the organized thymic microenvironment.

Human adaptive immune system Rag2-/-gamma(c)-/- mice

Although many biologic principles are conserved in mice and humans, species-specific differences exist, for example, in susceptibility and response to pathogens, that often do not allow direct implementation of findings in experimental mice to humans. Research in humans, however, for ethical and practical reasons, is largely restricted to in vitro assays that lack components and the complexity of a living organism. To nevertheless study the human hematopoietic and immune system in vivo, xenotransplantation assays have been developed that substitute human components to small animals. Here, we summarize our recent findings that transplantation of human cord blood CD34(+) cells to newborn Rag2(-/-)gamma(c)(-/-) mice leads to de novo development of major functional components of the human adaptive immune system. These human adaptive immune system Rag2(-/-)gamma(c)(-/-) (huAIS-RG) mice can now be used as a technically straightforward preclinical model to evaluate in vivo human adaptive immune system development as well as immune responses, for example, to vaccines or live infectious pathogens.

Functional CD8+ but not CD4+ T cell responses develop independent of thymic epithelial MHC

The role of nonthymic epithelial (non-TE) MHC in T cell repertoire selection remains controversial. To analyze the relative roles of thymic epithelial (TE) and non-TE MHC in T cell repertoire selection, we have generated tetraparental aggregation chimeras (B6-nude<=>BALB/c and B6<=>BALB/c-nude) harboring T and B cells from both parents, whereas TE cells originated exclusively from the non-nude donor. These chimeras mounted protective virus-specific TE and non-TE MHC-restricted T cell responses. To further evaluate whether non-TE MHC alone was sufficient to generate a functional T cell repertoire, we generated tetraparental aggregation chimeras lacking MHC class II (B6-nude<=>MHCII(-/-)) or both MHC molecules (B6-nude<=>MHCI(-/-)II(-/-)) on TE cells, but not on cells of B6-nude origin. Chimeras with MHC-deficient TE cells mounted functional virus-specific CD8+ but not CD4+ T cell responses. Thus, maturation of functional CD4+ T cell responses required MHC class II on thymic epithelium, whereas CD8+ T cells matured in the absence of TE MHC.

Control of graft-versus-host disease by regulatory T cells: Which level of antigen specificity?

Regulatory T cells (Tregs) are essential to suppress harmful self-reactive T cell responses. A paper in this issue of the European Journal of Immunology analyzes the role of Tregs in preventing syngeneic graft-versus-host disease (sGVHD). In this Commentary, we address the role of antigen specificity of Tregs in this context as well as future protocols for successful allogeneic grafts transplantation. Such ideal interventions will hopefully retain a diverse repertoire of pathogen-specific T cells, while maintenance of self-, foreign- and alloantigen-specific Tregs ensures life-long graft tolerance.

Proteasomes shape the repertoire of T cells participating in antigen-specific immune responses

Differences in the cleavage specificities of constitutive proteasomes and immunoproteasomes significantly affect the generation of MHC class I ligands and therefore the activation of CD8-positive T cells. Based on these findings, we investigated whether proteasomal specificity also influences CD8-positive T cells during thymic selection by peptides derived from self proteins. We find that one of the self peptides responsible for positive selection of ovalbumin-specific OT-1 T cells, which is derived from the f-actin capping protein (Cpalpha1), is efficiently generated only by immunoproteasomes. Furthermore, OT-1 mice backcrossed onto low molecular mass protein 7 (LMP7)-deficient mice show a 50% reduction of OT-1 cells. This deficiency is also observed after transfer of BM from OT-1 mice in LMP7-deficient mice and can be corrected by the injection of the Cpalpha1 peptide. Interestingly, WT and LMP7-deficient mice mount comparable immune responses to the ovalbumin-derived epitope SIINFEKL. However, their cytotoxic T lymphocytes (CTL) differ in the use of T cell receptor Vbeta genes. CTL derived from WT mice use Vbeta8 or Vbeta5 (the latter is also used by OT-1 cells), whereas SIINFEKL-specific CTL from LMP7-deficient mice are exclusively Vbeta8-positive. Taken together, our experiments provide strong evidence that proteasomal specificity shapes the repertoire of T cells participating in antigen-specific immune responses.

T-cell development: an extrathymic perspective

The lymph nodes (LNs) harbor a cryptic T-lymphopoietic pathway that is dramatically amplified by oncostatin M (OM). OM-transgenic mice generate massive amounts of T lymphocytes in the absence of Lin(-)c-Kit(hi)IL-7Ralpha- lymphoid progenitors and of reticular epithelial cells. Extrathymic T cells that develop along the OM-dependent LN pathway originate from Lin(-)c-Kit(lo)IL-7Ralpha+ lymphoid progenitors and are different from classic T cells in terms of turnover kinetics and function. Positive selection does not obey the same rules in the thymus and the LNs, where positive selection of developing T cells is supported primarily by epithelial and hematopoietic cells, respectively. Extrathymic T cells undergo enhanced homeostatic proliferation and thereby acquire some properties of memory T cells. Following antigen encounter, extrathymic T-cells initiate proliferation and cytokine secretion more readily than classic T cells, but their accumulation is limited by an exquisite susceptibility to apoptosis. Studies on in vitro and in vivo extrathymic T-cell development have yielded novel insights into the essence of a primary T-lymphoid organ. Furthermore, comparison of the thymic and OM-dependent extrathymic pathways shows how the division of labor between primary and secondary lymphoid organs influences the repertoire and homeostasis of T lymphocytes.

Establishment and functioning of intrathymic microenvironments

The thymus supports the production of self-tolerant T cells from immature precursors. Studying the mechanisms regulating the establishment and maintenance of stromal microenvironments within the thymus therefore is essential to our understanding of T-cell production and ultimately immune system functioning. Despite our ability to phenotypically define stromal cell compartments of the thymus, the mechanisms regulating their development and the ways by which they influence T-cell precursors are still unclear. Here, we review recent findings and highlight unresolved issues relating to the development and functioning of thymic stromal cells.

Regulation of MHC class II gene expression by the class II transactivator

MHC class II molecules are pivotal for the adaptive immune system, because they guide the development and activation of CD4+ T helper cells. Fulfilling these functions requires that the genes encoding MHC class II molecules are transcribed according to a strict cell-type-specific and quantitatively modulated pattern. This complex gene-expression profile is controlled almost exclusively by a single master regulatory factor, which is known as the class II transactivator. As we discuss here, differential activation of the three independent promoters that drive expression of the gene encoding the class II transactivator ultimately determines the exquisitely regulated pattern of MHC class II gene expression.

Thymocyte-Thymocyte Interaction for Efficient Positive Selection and Maturation of CD4 T Cells

Despite numerous reports on MHC class II expression by T cells from a wide spectrum of mammalian species including humans, the biological relevance of this phenomenon has never been tested with appropriately designed animal models. To address this issue, we developed mouse models in which immature thymocytes are the only positively selecting antigen-presenting cells in the thymus. In these mice, CD4+ T cells were generated with the appropriate maturation phenotype and showed a diverse repertoire of TCR Vbetas. The CD4+ T cells were functionally competent, mediating effective allogeneic responses that involved polyclonal TCR Vbetas. These results suggest that the thymocyte-thymocyte (T-T) interaction operates as an independent pathway for CD4+ T cell selection in the thymi of species with MHC II-positive thymocytes. This T-T interaction appears to be the basis for the generation of donor MHC-restricted CD4+ T cells in xenogeneic hosts.

Gene targeting of VEGF-A in thymus epithelium disrupts thymus blood vessel architecture

The thymus harbors an organ-typical dense network of branching and anastomosing blood vessels. To address the molecular basis for morphogenesis of this thymus-specific vascular pattern, we have inactivated a key vascular growth factor, VEGF-A, in thymus epithelial cells (TECs). Both Vegf-A alleles were deleted in TECs by a complementation strategy termed nude mouse [mutated in the transcription factor Foxn1 (forkhead box N1)] blastocyst complementation. Injection of Foxn1(+/+) ES cells into Foxn1(nu/nu) blastocysts reconstituted a functional thymus. By dissecting thymus stromal cell subsets, we have defined, in addition to medullary TECs (mTECs) and cortical TECs (cTECs), another prominent stromal cell subset designated cortical mesenchymal cells (cMes). In chimeric thymi, mTECs and cTECs but not cMes were exclusively ES cell-derived. According to this distinct origin, the Vegf-A gene was deleted in mTECs and cTECs, whereas cMes still expressed Vegf-A. This genetic mosaic was associated with hypovascularization and disruption of the organ-typical network of vascular arcades. Thus, vascular growth factor production by TECs is required for normal thymus vascular architecture. These experiments provide insights into Foxn1-dependent and Foxn1-independent stromal cell development and demonstrate the value of this chimeric approach to analyzing gene function in thymus epithelium.

Severe combined immunodeficiency. A model disease for molecular immunology and therapy

Severe combined immunodeficiencies (SCIDs) consist of genetically determined arrest of T-cell differentiation. Ten different molecular defects have now been identified, which all lead to early death in the absence of therapy. Transplantation of allogeneic hematopoietic stem cells (HSCT) can restore T-cell development, thus saving the lives of SCID patients. In this review, the different characteristics of HSCT are discussed along with the available data regarding the long-term outcome. Transient thymopoiesis caused by an exhaustion of donor progenitor cells and possibly a progressive loss of thymus function can lead to a progressive decline in T-cell functions. The preliminary results of gene therapy show the correction of two SCID conditions. Based on the assumption that long-lasting pluripotent progenitor cells are transduced, these data suggest that gene therapy could overcome the long-term recurrence of the T-cell immunodeficiency. SCID is thus a disease model for experimental therapy in the hematopoietic system.

On 'reactivity' versus 'tolerance'

In Burnet's review on 'The impact of ideas on immunology' he considers himself an observer of nature using biochemical and molecular analysis for more detailed understanding, a description that applies also to me. I use three examples--repertoire selection of T cells, rules of immune reactivity versus non-reactivity and immunological memory--to illustrate the difficulties we all have in probing nature's immunological secrets and in critically testing immunologists' ideas. At one end of the spectrum of biological research one may argue everything is possible and therefore all results are correct, if correctly measured. But perhaps it is more important to always ask again and again what is frequent and enhances survival versus what is rare and an exception. At the same time one must keep in mind that special situations and special tricks may well be applied for medical benefits, although they may have little impact on physiology and species survival. I will attempt to use disease in virus-infected mice to obtain some answers to what I consider to be important immunological questions with the hope of improving the ratio of answers that are right for the right experimental reasons versus those that are right for the wrong reasons. Some of these experiments falsify hypotheses, previous experiments and interpretations and therefore are particularly important in correcting misleading concepts. They should help to find out which half of immunological ideas and truths in immunological text books written today are likely to be wrong. Ideas are important in immunology, but are often rather demagogically handled and therefore may cost us very dearly indeed. Evaluating immunity to infections and tumours in vivo should help prevent us from getting lost in immunology.

The Role of Dendritic Cells in Selection of Classical and Nonclassical CD8+T Cells In Vivo

T cell development is determined by positive and negative selection events. An intriguing question is how signals through the TCR can induce thymocyte survival and maturation in some and programmed cell death in other thymocytes. This paradox can be explained by the hypothesis that different thymic cell types expressing self-MHC/peptide ligands mediate either positive or negative selection events. Using transgenic mice that express MHC class I (MHC-I) selectively on DC, we demonstrate a compartmentalization of thymic functions and reveal that DC induce CTL tolerance to MHC-I-positive hemopoietic targets in vivo. However, in normal and bone marrow chimeric mice, MHC-I+ DC are sufficient to positively select neither MHC-Ib (H2-M3)- nor MHC-Ia (H2-K)-restricted CD8+ T cells. Thus, thymic DC are specialized in tolerance induction, but cannot positively select the vast majority of MHC-I-restricted CD8+ T cells.

On Immunity Against Infections and Vaccines: Credo 2004

Resistance of vertebrate hosts against infections comprises important natural or innate resistance combined with adaptive immune responses of T and B cells. Viruses, bacteria or classical parasites all probe the limit of immune responses and of immunity. They, therefore, offer an excellent opportunity to assess the biology, physiology and molecular aspects of immune responses and help in characterizing the three basic parameters of immunology-- specificity, tolerance and memory. Various experiments are summarized that indicate that the rules of antiviral, antitumour, antiorgan graft and of autoimmune responses are basically the same. The practical specificity repertoire of T and B cells is probably in the order of 10(4)-10(5) specificities expressed by T cells or by neutralizing antibodies. Tolerance is best defined by rules of reactivity to eliminate infections while avoiding destruction of normal cells by complete elimination of T cells that are specific for antigens persisting within the blood and lymphatic (lymphohaemopoietic) system. Induction of a T-cell response is the result of antigens newly entering lymph nodes or spleen, initially in a local fashion and exhibiting an optimal distribution kinetics within the lymphohaemopoietic system. Antigen staying outside lymphatic tissues are immunologically ignored (e.g. are non-events). Thus immune reactivity is regulated by antigen dose, time and relative distribution kinetics. Memory is the fact that a host is resistant against disease caused by reinfection with the same agent. Memory correlates best with antigen-dependent maintenance of elevated antibody titres in serum and mucosal secretions, or with an antigen-driven activation of T cells, such that they are protective immediately against peripheral reinfections in solid tissues. While antibodies transferred from mother to offspring are a prerequisite for the survival of otherwise unprotected immuno-incompetent offsprings, activated memory T cells cannot be transmitted. Thus, attenuation of infections in newborns and babies by maternal antibodies is the physiological correlate of man-made vaccines. T cells not only play an essential role in maintaining T-help-dependent memory antibody titres, but also in controlling the many infections that persist in a host at rather low levels (such as tuberculosis, measles and HIV).

B cell-dependent TCR diversification

T cell diversity was once thought to depend on the interaction of T cell precursors with thymic epithelial cells. Recent evidence suggests, however, that diversity might arise through the interaction of developing T cells with other cells, the identity of which is not known. In this study we show that T cell diversity is driven by B cells and Ig. The TCR V beta diversity of thymocytes in mice that lack B cells and Ig is reduced to 6 x 10(2) from wild-type values of 1.1 x 10(8); in mice with oligoclonal B cells, the TCR V beta diversity of thymocytes is 0.01% that in wild-type mice. Adoptive transfer of diverse B cells or administration of polyclonal Ig increases thymocyte diversity in mice that lack B cells 8- and 7-fold, respectively, whereas adoptive transfer of monoclonal B cells or monoclonal Ig does not. These findings reveal a heretofore unrecognized and vital function of B cells and Ig for generation of T cell diversity and suggest a potential approach to immune reconstitution.

Do thymically and strictly extrathymically developing T cells generate similar immune responses?

If present in sufficient numbers, could extrathymic T cells substitute for thymus-derived T cells? To address this issue, we studied extrathymic T cells that develop in athymic mice under the influence of oncostatin M (OM). In this model, extensive T-cell development is probably due to amplification of a minor pathway of T-cell differentiation taking place only in the lymph nodes. Extrathymic CD4 T cells expanded poorly and were deficient in providing B-cell help after infection with vesicular stomatitis virus (VSV) and lymphocytic choriomeningitis virus (LCMV). Compared with classic T cells, stimulated extrathymic CD8 T cells produced copious amounts of interferon gamma (IFN-gamma), and their expansion was precocious but of limited amplitude because of a high apoptosis rate. Consequently, although extrathymic cytotoxic T lymphocytes (CTLs) responded to LCMV infection, as evidenced by the expansion of GP33-41 tetramer-positive CD8 T cells, they were unable to eradicate the virus. Our data indicate that the site of development impinges on T-cell quality and function and that extrathymic T cells functionally cannot substitute for classical thymic T cells.