The effect of thymus environment on T cell development and tolerance

Enhancement of swine progenitor chimerism in mixed swine/human bone marrow cultures with swine cytokines

OBJECTIVE

The induction of transplantation tolerance across xenogeneic barriers by bone marrow transplantation holds great promise, but engraftment of xenogeneic stem cells has been difficult to achieve. Part of this difficulty is due to species-specific differences in regulatory cytokines and elements of the stromal microenvironment, which we studied here.

MATERIALS AND METHODS

We developed a system where fresh bone marrow cells from swine and human are cultured on human bone marrow stroma in order to study these limiting factors in a clinically relevant species combination.

RESULTS

We report here the ability of recombinant swine interleukin (IL)-3 and c-kit ligand (KL) to specifically enhance swine hematopoietic chimerism in this system. In the absence of exogenous swine cytokines, there were about half as many swine progenitors as human progenitors at 1, 2, and 4 weeks of culture. When used alone, swine IL-3 led to a notable but transient increase in the relative ratio of swine progenitors, while addition of swine KL increased the ratio of swine progenitors only modestly and only at later time points. In contrast, when swine IL-3 and KL were added together, there was a two- to fourfold increase in the ratio of swine to human progenitors at all times tested.

CONCLUSION

These data demonstrate that both swine IL-3 and KL are needed for prolonged enhancement of swine progenitor chimerism under these conditions, and suggest that the species specificity of either one or both of these cytokines may represent an important barrier to prolonged engraftment of swine bone marrow in humans.

Self-Reactive T Cells Selected on Thymic Cortical Epithelium Are Polyclonal and Are Pathogenic In Vivo

Positive selection of CD4+ T cells requires that the TCR of a developing thymocyte interact with self MHC class II molecules on thymic cortical epithelium. In contrast, clonal deletion is mediated by dendritic cells and medullary epithelium. We previously generated K14 mice expressing MHC class II only on thymic cortical epithelium. K14 CD4+ T cells were positively, but not negatively, selected and had significant in vitro autoreactivity. Here, we examine the function of these autoreactive CD4+ T cells in more detail. Analysis of a series of K14-derived T hybrids demonstrated that the autoreactive population of CD4+ T cells is phenotypically and functionally diverse. Purified K14 CD4+ T cells transferred into lethally irradiated wild-type B6 mice cause acute graft vs host disease with bone marrow failure. Further, these autoreactive CD4+ T cells cause hypergammaglobulinemia and the production of autoantibodies when transferred into unirradiated wild-type hosts. Thus, positive selection by normal thymic cortical epithelial cells, unopposed by negative selection, produces polyclonal CD4+ T cells that are pathologic.

Antigen-pulsed CD8alpha+ dendritic cells generate an immune response after subcutaneous injection without homing to the draining lymph node

Two subsets of murine splenic dendritic cells, derived from distinct precursors, can be distinguished by surface expression of CD8alpha homodimers. The functions of the two subsets remain controversial, although it has been suggested that the lymphoid-derived (CD8alpha+) subset induces tolerance, whereas the myeloid-derived (CD8alpha-) subset has been shown to prime naive T cells and to generate memory responses. To study their capacity to prime or tolerize naive CD4(+) T cells in vivo, purified CD8alpha+ or CD8alpha- dendritic cells were injected subcutaneously into normal mice. In contrast to CD8alpha- dendritic cells, the CD8alpha+ fraction failed to traffic to the draining lymph node and did not generate responses to intravenous peptide. However, after in vitro pulsing with peptide, strong in vivo T cell responses to purified CD8alpha+ dendritic cells could be detected. Such responses may have been initiated via transfer of peptide-major histocompatibility complex complexes to migratory host CD8alpha- dendritic cells after injection. These data suggest that correlation of T helper cell type 1 (Th1) and Th2 priming with injection of CD8alpha+ and CD8alpha- dendritic cells, respectively, may not result from direct T cell activation by lymphoid versus myeloid dendritic cells, but rather from indirect modification of the response to immunogenic CD8alpha- dendritic cells by CD8alpha+ dendritic cells.

Alternative Splicing and Hypermutation of a Nonproductively Rearranged TCR α-Chain in a T Cell Hybridoma

Like Ig genes, TCR genes are formed by somatic rearrangements of noncontiguous genomic V, J, and C regions. Unlike Ig genes, somatic hypermutation of TCR V regions is an infrequent event. We describe the occurrence of spontaneous hypermutation in a nonproductively rearranged TCR α-chain gene in a clonal T cell hybridoma that had lost its productively rearranged α-chain. The mutating hybridoma was eventually supplanted in culture by a nonmutating variant that had restored an open reading frame in the nonproductively rearranged TCR α-chain through the use of cryptic splice sites in the Vα region. Evidence is presented for the presence of cDNA reverse transcripts of the TCR α-chain within the hybridoma, suggesting a role for reverse transcriptase in the generation of mutations.

Transfer of porcine MHC DRalpha into IEalpha-deficient murine bone marrow results in reduced IE-restricted Vbeta usage

BACKGROUND

Allogeneic bone marrow transplantation has proven effective for inducing specific tolerance to subsequent solid organ allografts, although the clinical applicability of this approach is limited by the morbidity and mortality associated with this procedure. As an alternative, we are investigating the transfer of allogeneic MHC class II genes into recipient bone marrow cells (BMC), using the miniature swine as a model.

METHODS

To understand the mechanism of tolerance induction achieved through class II gene transfer, BMC from C57BL/10 mice, which lack expression of the MHC class II DRalpha equivalent (H-2 IEalpha), were transduced with a retrovirus vector for swine DRalpha.

RESULTS

Expression of the DRA-vector in bone marrow-derived cells was demonstrated by Northern analysis of colonies grown in vitro from transduced myeloid progenitors. Taking advantage of the fact that the introduced DRalpha chain was able to form heterodimers with endogenous IEbeta, surface expression of the transgene was demonstrated on splenocytes harvested 1, 17, and 28 weeks after bone marrow transplantation. Transgene expression was confirmed by reverse transcriptase-polymerase chain reaction in the thymus of those animals killed at weeks 17 and 28. Finally, the effects of bone marrow transduction on central tolerance induction was demonstrated by the progressive decrease of IE-reactive T-cell clones bearing Vbeta5 and Vbeta11 T cell receptors in the peripheral blood cells of engineered recipients.

CONCLUSIONS

Our results support the notion that transplantation tolerance, induced by class II gene transfer into syngeneic BMC, results in part from durable deletional unresponsiveness of graft-specific alloreactive T cells.

Pig MHC Mediates Positive Selection of Mouse CD4+ T Cells with a Mouse MHC-Restricted TCR in Pig Thymus Grafts

Remarkably normal immune function and specific T cell tolerance to discordant xenogeneic donors can be achieved by grafting fetal pig thymus and liver (FP THY/LIV) tissue to T cell and NK cell-depleted, thymectomized (ATX) mice. To determine whether or not host class II MHC molecules participate in the positive selection of mouse CD4+ T cells in FP THY/LIV grafts, we compared their development in ATX “AND” TCR-transgenic mice with positive selecting or nonselecting host MHC genotypes. Mouse TCR-transgenic CD4 single positive T cells repopulated the periphery significantly and to a similar extent in both T/NK cell-depleted, ATX AND mice with positive-selecting or nonselecting MHC backgrounds after grafting with FP THY/LIV. Therefore, MHC molecules from a widely disparate xenogeneic species can positively select T cells bearing a host class II MHC-restricted TCR without a contribution from the host MHC. These results, in combination with previous studies performed in this model, suggest that the T cell repertoire that is generated by the combination of positive selection on xenogeneic MHC and negative selection on both recipient and xenogeneic porcine MHC is tolerant of both donor and recipient and has sufficient cross-reactivity with host MHC/foreign peptide complexes to confer a high level of immunocompetence. The results have implications for the potential clinical applicability of xenogeneic thymic transplantation and also suggest a predominant role for the TCR recognition of species-conserved MHC residues in positive selection.

An alternate pathway for T cell development supported by the bone marrow microenvironment: recapitulation of thymic maturation

In the principal pathway of alpha/beta T cell maturation, T cell precursors from the bone marrow migrate to the thymus and proceed through several well-characterized developmental stages into mature CD4+ and CD8+ T cells. This study demonstrates an alternative pathway in which the bone marrow microenvironment also supports the differentiation of T cell precursors into CD4+ and CD8+ T cells. The marrow pathway recapitulates developmental stages of thymic maturation including a CD4+CD8+ intermediary cell and positive and negative selection, and is strongly inhibited by the presence of mature T cells. The contribution of the marrow pathway in vivo requires further study in mice with normal and deficient thymic or immune function.

Role of intrathymic rat class II+ cells in maintaining deletional tolerance in xenogeneic rat-->mouse bone marrow chimeras

BACKGROUND

Mixed xenogeneic bone marrow chimerism and tolerance can be induced in mice conditioned with a nonmyeloablative regimen followed by injection of T cell-depleted rat bone marrow cells. We hypothesized that, despite a gradual decline in rat hematopoiesis observed in these chimeras, as long as rat class II+ antigen-presenting cells remain in their thymi, tolerance will persist as a result of deletion of donor-reactive thymocytes.

METHODS

The level of chimerism and of mouse Vbeta5 and Vbeta11 T-cell deletion was followed over time. These results were correlated with the presence of rat class II+ cells in the thymus by immunohistochemistry and the presence of tolerance in long-term chimeras by in vivo and in vitro assays.

RESULTS

(1) Proliferation and cytotoxicity assays, as well as skin graft survival, demonstrated the presence of specific tolerance to host and to donor rat, with normal reactivity to third-party rat and mouse stimulators, even as late as 85 weeks after bone marrow transplantation. (2) The absence of mature Vbeta5+ and Vbeta11+ host T cells in the thymus and periphery was always associated with the presence of rat class II+ cells in the thymus, and incomplete deletion of T cells expressing these Vbeta families was observed in thymi in which rat class II+ cells were not detectable.

CONCLUSIONS

Donor-specific T-cell tolerance is maintained during the period when donor-type reconstitution declines, and is most likely mediated by intrathymic clonal deletion of T cells that recognize antigens expressed on class II+ rat cells.

Cellular interactions in the thymus regulate the protein kinase C signaling pathway

We provide evidence that thymocytes receive signals from the thymic microenvironment which regulate the protein kinase C (PKC) signaling pathway. Thus, phorbol 12-myristate 13-acetate (PMA) causes a PKC-dependent down-regulation of CD4 expression and induces apoptosis in isolated thymocytes but has little effect on thymocytes maintained within intact thymic lobes or in reaggregate lobes containing purified thymocytes with either thymic or non-thymic stromal cells. Moreover, compact pellets of thymocytes alone are protected from the effects of PMA. This protection is maintained when the compacted thymocytes are rigorously depleted of MHC class II-expressing cells. We conclude that signals arising from thymocyte-thymocyte contact control the utilization of the PKC cascade. These observations have implications for thymocyte signaling in general as well as for the interpretation of studies carried out on thymocyte suspensions.

Interleukin-12 prevents severe acute graft-versus-host disease (GVHD) and GVHD-associated immune dysfunction in a fully major histocompatibility complex haplotype-mismatched murine bone marrow transplantation model

BACKGROUND

We have recently reported that interleukin (IL)-12 prevents acute graft-versus-host disease (GVHD)-induced mortality in a full major histocompatibility complex- plus multiple minor antigen-mismatched A/J-->B10 bone marrow transplantation (BMT) model. Because most patients have access to a haploidentical, one haplotype-mismatched donor, we have now investigated the protective effect of IL-12 against GVHD and GVHD-associated immune dysfunction in a haploidentical CBD2F1 (H2kxd) --> B6D2F1 (H2bxd) strain combination.

METHODS

GVHD was induced by injecting CBD2F1 marrow and spleen cells into lethally irradiated B6D2F1 mice.

RESULTS

In untreated control mice, GVHD resulted in 87% mortality by day 8 after BMT, with no survivors beyond day 17. Treatment with a single injection of IL-12 on the day of BMT led to 87% long-term survival, with no significant weight loss, diarrhea or GVHD skin changes. The majority of T cells recovering in these mice showed the CD62L+, CD44low, CD45RBhigh naive phenotype. These T cells showed specific tolerance to both host and donor histocompatibility antigens, but normal anti-third party (H2s) alloresponses in vitro. B-cell proliferative responses to lipopolysaccharide were also normal in IL-12-protected mice. Moreover, normal negative selection of thymocytes bearing T cell receptors with Vbeta that recognize endogenous superantigens was observed among CD4+CD8- thymocytes, indicating a lack of GVHD-associated thymic selection abnormalities in IL-12-protected allogeneic BMT recipients.

CONCLUSIONS

IL-12 provides permanent protection against an otherwise severe, rapidly lethal GVHD, with no clinical manifestations of chronic GVHD, immunosuppression or autoimmune features, in a full major histocompatibilty complex haplotype-mismatched murine BMT model.

Mixed hematopoietic chimerism and transplantation tolerance

Durable transplantation tolerance can be reliably achieved by inducing engraftment of hematopoietic cells in recipients initially depleted of T-lymphocytes. Engraftment of donor pluripotent hematopoietic stem cells (PPHSC) produces mixed hematopoietic chimeras in which both host and donor cells coexist and are tolerant of each other. The major mechanism of tolerance in these chimeras is central, intrathymic clonal deletion, which is induced and maintained by immigration of both host and donor marrow-derived cells to the host thymus, ensuring the ongoing central deletion of donor- and host-reactive cells. In this article, approaches developed in our laboratory to induce stable mixed hematopoietic chimerism and specific central deletional allogeneic and xenogeneic tolerance without toxic or myeloablative host conditioning are reviewed.

Differential effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, bis(tri-n-butyltin) oxide and cyclosporine on thymus histophysiology

Recent advances in the histophysiology of the normal thymus have revealed its complex architecture, showing distinct microenvironments at the light and electron microscopic level. The epithelium comprising the major component of the thymic stroma is not only involved in the positive selection of thymocytes, but also in their negative selection. Dendritic cells, however, are more efficient than epithelial cells in mediating negative selection. Thymocytes are dependent on the epithelium for normal development. Conversely, epithelial cells need the presence of thymocytes to maintain their integrity. The thymus rapidly responds to immunotoxic injury. Both the thymocytes and the nonlymphoid compartment of the organ can be targets of exposure. Disturbance of positive and negative thymocyte selection may have a major impact on the immunological function of the thymus. Suppression of peripheral T-cell-dependent immunity as a consequence of thymus toxicity is primarily seen after perinatal exposure when the thymus is most active. Autoimmunity may be another manifestation of chemically mediated thymus toxicity. Although the regenerative capacity of thymus structure is remarkable, it remains to be clarified whether this also applies to thymus function. In-depth mechanistic studies on chemical-induced dysfunction of the thymus have been conducted with the environmental contaminants 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and bis(tri-n-butyltin)oxide (TBTO) as well as the pharmaceutical immunosuppressant cyclosporine (CsA). Each of these compounds exerts a differential effect on the morphology of the thymus, depending on the cellular targets for toxicity. TCDD and TBTO exposure results in cortical lymphodepletion, albeit by different mechanisms. An important feature of TCDD-mediated thymus toxicity is the disruption of epithelial cells in the cortex. TBTO primarily induces cortical thymocyte cell death. In contrast CsA administration results in major alterations in the medulla, the cortex remaining largely intact. Medullary epithelial cells and dendritic cells are particularly sensitive to CsA. The differential effects of these three immunotoxicants suggest unique susceptibilities of the various cell types and regions that make up the thymus.

From stem cells to lymphocytes: biology and transplantation

We review the development of the hematopoietic system, focusing on the transition from hematopoietic stem cells (HSCs) to T cells. This includes the isolation of HSCs, and recent progress in understanding their ontogeny, homing properties, and differentiation. HSC transplantation is reviewed, including the kinetics of reconstitution, engraftment across histocompatibility barriers, the facilitation of allogeneic engraftment, and the mechanisms of graft rejection. We describe progress in understanding T-cell development in the bone marrow and thymus as well as the establishment of lymph nodes. Finally, the role of bcl-2 in regulating homeostasis in the hematopoietic system is discussed.

Premature thymic involution, observed at the ultrastructural level, in two lineages of human-SOD-1 transgenic mice

The human Cu/Zn superoxide dismutase (hSOD-1) gene, catalyses the dismutation of O2 to H2O2 and O2. It is located on chromosome 21 in q22.1 and is overexpressed in Down's syndrome (DS) patients. These patients present various abnormalities including mental retardation, congenital heart disease, immunological deficits and premature aging. In order to explore the potential role of SOD-1 overexpression in DS, we have generated two lineages of transgenic mice for the hSOD-1 gene and studied, at the ultrastructural level, the effect of hSOD-1 overexpression on the thymic microenvironment. Modification of the cellular architecture and morphology associated with a lipidic invasion, signs of a premature involution of the thymus, were observed in both lineages. A rupture of the filamentous network in the extracellular and probably also in the intracellular matrix was first observed. These results correlate the thymic alterations visualized in light microscopy, on the thymus from DS patients, and raise the question of the relationship between the SOD-1 overexpression and the different morphological alterations associated with the premature thymic involution observed in SOD-1 transgenic mice. They suggest that thymic and immunological impairments present in DS patients may be related to the SOD-1 gene dosage effect.

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

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

Generation of NK1.1+ T cell antigen receptor alpha/beta+ thymocytes associated with intact thymic structure

The development of T cells within the thymus is largely dependent on intact cortical and medullary epithelial cells. However, it has been reported that positive selection of natural killer antigen 1.1+ (NK1.1+) T cell antigen receptor (TCR)-alpha/beta+ thymocytes recently identified among CD4+8- and CD4-8- subpopulations is attributable to major histocompatibility complex class Ib ligands expressed on bone marrow (BM)-derived components in the thymus. In the present study, we investigated generation of NK1.1+ TCR-alpha/beta+ cells in the thymus of the aly/aly mouse which lacks lymph nodes and Peyer's patches and shows abnormalities of thymic and splenic structure. We found that the proportion of the NK1.1+ TCR-alpha/beta+ thymocytes was extremely low in these mice as compared with aly/+ and normal C57BL/6 mice. Thymic reconstitution by BM cells from aly/+ mice that possess a normal population of NK1.1+ TCR-alpha/beta+ cell population did not restore the NK1.1+ TCR-alpha/beta+ cell population in the thymus of lethally irradiated aly/aly mouse. When deoxyguanosine-treated fetal thymi from (B6 x B10.G)F1 mice were transplanted to aly/aly mice that had been thymectomized and reconstituted with BM cells of aly/aly mice, normal proportions of the NK1.1+ TCR-alpha/beta+ thymocytes were present in the thymus grafts. These findings demonstrate that the development of NK1.1+ TCR-alpha/beta+ thymocytes is accomplished under the influence not only of BM-derived components, but also of irradiation-resistant or deoxyguanosine-resistant components and an intact microenvironment of the thymus.

Targeted expression of major histocompatibility complex (MHC) class II molecules demonstrates that dendritic cells can induce negative but not positive selection of thymocytes in vivo

It is well established that lymphoid dendritic cells (DC) play an important role in the immune system. Beside their role as potent inducers of primary T cell responses, DC seem to play a crucial part as major histocompatibility complex (MHC) class II+ "interdigitating cells" in the thymus during thymocyte development. Thymic DC have been implicated in tolerance induction and also by some authors in inducing major histocompatibility complex restriction of thymocytes. Most of our knowledge about thymic DC was obtained using highly invasive and manipulatory experimental protocols such as thymus reaggregation cultures, suspension cultures, thymus grafting, and bone marrow reconstitution experiments. The DC used in those studies had to go through extensive isolation procedures or were cultured with recombinant growth factors. Since the functions of DC after these in vitro manipulations have been reported to be not identical to those of DC in vivo, we intended to establish a system that would allow us to investigate DC function avoiding artificial interferences due to handling. Here we present a transgenic mouse model in which we targeted gene expression specifically to DC. Using the CD 11c promoter we expressed MHC class II I-E molecules specifically on DC of all tissues, but not on other cell types. We report that I-E expression on thymic DC is sufficient to negatively select I-E reactive CD4+ T cells, and to a less complete extent, CD8+ T cells. In contrast, it only DC expressed I-E in a class II-deficient background, positive selection of CD4+ T cells could not be observed. Thus negative, but not positive, selection events can be induced by DC in vivo.

The discovery of immunologic tolerance

The phenomenon of tolerance can be said to have begun with the seminal observations in 1945 by R. D. Owen that cattle dizygotic twins display red cell (chimerism--mosaicism as he called it--in adult life. Owen interpreted this extraordinary finding in terms of the much earlier discovery by F. R. Lillie that the placentae of cattle dizygotic twins undergo anastomosis early in fetal life, and he speculated that this would have permitted blood cells and their precursors to move from one twin to the other. Owen's discovery came out of the blue and it was ignored by immunologists until F. M. Burnet and F. Fenner highlighted it four years later in their influential monograph The Production of Antibodies, in which they predicted the existence of tolerance as a general phenomenon and developed their notion of "self-markers" to explain why the body does not react against self. Though it was Medawar's group that showed conclusively in 1953 that tolerance can be experimentally induced in fetal mice and in chick embryos, their entry into this field came from a totally different direction, an attempt to distinguish between mono- and dizygotic cattle twins by the exchange of skin grafts. This led to the seemingly paradoxic result that grafts exchanged between dizygotic twins were accepted (1951) and it was not until their cattle experiments had been virtually completed that they became aware of Owen's earlier discovery. Following the work of Billingham, Brent, and Medawar, and of Hasek, tolerance became incorporated into general immunologic theory and it helped to explain the fact that mammals do not normally suffer from injurious autoimmune manifestations. Ray Owen's discovery therefore has a secure place in the history of immunology.

Thymic stromal cell specialization and the T-cell receptor repertoire

Ten years ago, we proposed a model for thymus function in which thymic epithelial cells are primarily responsible for imprinting major histocompatibility complex (MHC)-restricted specificity, and bone marrow-derived macrophages or dendritic cells are responsible for the induction of self-tolerance. Since then, transgenic and knockout models have allowed for a dissection of thymic stromal components in vivo, leading to a new understanding of their specialized functions. We have determined that with regard to class II-restricted CD4 T-cell development, two distinct subsets of thymic epithelium help shape the repertoire: Cortical epithelium appears solely responsible for positive selection, whereas a fucose-bearing subset of medullary epithelium is specialized for negative selection. This absolute separation of positive and negative selection into two distinct spatial and temporal compartments leads to a much simpler view of the process of repertoire selection. Finally, a novel view of the function of the thymic medulla is discussed.

Chimerism and tolerance: from freemartin cattle and neonatal mice to humans

Bone marrow transplantation (BMT) results in hematopoietic chimeras that demonstrate donor specific tolerance to tissue and cellular grafts. The clinical application of chimerism to induce tolerance is limited by the morbidity associated with human BMT: failure of engraftment, graft-versus-host disease (GVHD), and toxic host conditioning. BMT in an immunologically mature host has until recently been believed to require complete ablation of the host's immune system to allow donor engraftment. Lethal conditioning is associated with significant morbidity and mortality. Stable multilineage mixed allogeneic chimerism has more recently been achieved in mice using partial myeloablation prior to BMT. Chimeras prepared in this fashion exhibit donor specific tolerance in vitro and in vivo similar to lethally-conditioned recipients. A second factor that has limited the widespread application of BMT to nonmalignant disease, including attempts to induce tolerance, is GVHD. Although T-cell depletion of donor marrow reduces the incidence of GVHD, engraftment is often jeopardized. Although highly purified stem cells (SC) engraft at relatively low doses in syngeneic recipients, they do not durably engraft in MHC-disparate recipients. It has recently become clear that a second cell (facilitating cell) that enhances bone marrow engraftment and minimizes the occurrence of GVHD is required for SC to engraft in MHC-disparate recipients. Methods to optimize engraftment yet minimize GVHD may provide an approach to apply BMT clinically. With decreased morbidity through incomplete recipient conditioning and the ability to engineer a bone marrow graft to contain only the desired cells to optimize engraftment, BMT may provide a reasonable strategy to treat nonmalignant diseases including enzyme deficiencies, hemoglobinopathies, autoimmune diseases, and species-specific viral infections such as HIV. BMT-induced donor specific tolerance may benefit recipients of solid organ transplants by eliminating the need for nonspecific immunosuppression and by preventing chronic rejection. This review will focus on approaches to enable BMT yet minimize recipient morbidity and mortality.

Interleukin 2 production, not the pattern of early T-cell antigen receptor-dependent tyrosine phosphorylation, controls anergy induction by both agonists and partial agonists

Full activation of T cells requires signaling through the T-cell antigen receptor (TCR) and additional surface molecules interacting with ligands on the antigen-presenting cell. TCR recognition of agonist ligands in the absence of accessory signals frequently results in the induction of a state of unresponsiveness termed anergy. However, even in the presence of costimulation, anergy can be induced by TCR partial agonists. The unique pattern of early receptor-induced tyrosine phosphorylation events induced by partial agonists has led to the hypothesis that altered TCR signaling is directly responsible for the development of anergy. Here we show that anergy induction is neither correlated with nor irreversibly determined by the pattern of early TCR-induced phosphorylation. Rather, it appears to result from the absence of downstream events related to interleukin 2 receptor occupancy and/or cell division. This implies that the anergic state can be manipulated independently of the precise pattern of early biochemical changes following TCR occupancy, a finding with implications for understanding the induction of self-tolerance and the use of partial agonist ligands in the treatment of autoimmune diseases.

Mouse models of autoimmune disease suggest that self-tolerance is maintained by unresponsive autoreactive T cells

Multiple organ-localized autoimmune diseases, such as thyroiditis and gastritis, spontaneously develop in BALB/c nu/nu (nude) mice receiving embryonic rat thymus grafts (TG) under their renal capsules (TG nude mice). When thyroid was grafted into the rat thymus of TG nude mice, development of autoimmune thyroiditis, but not other diseases, was completely prevented. However, when such mice received thyroid antigen plus complete Freund's adjuvant (CFA), severe autoimmune thyroiditis developed, suggesting that some thyroid-specific autoreactive T cells migrate into the periphery, but remain unresponsive. Development of autoimmune diseases, including thyroiditis, in TG nude mice was prevented by a single intraperitoneal injection of splenic CD4+ cells from normal BALB/c mice and also from mice with intrathymic thyroid grafts, indicating that thyroid-specific suppressor T cells are present in normal mice and that such T cells are neither deleted nor inactivated by the intrathymic thyroid grafts, in contrast to autoreactive T cells. Thus clonal deletion in the thymus, and clonal anergy and/or ignorance in the periphery, of autoreactive cells is important to maintain immune tolerance to organ-specific antigen, but CD4 suppressor T cells may play a more important role in tolerance, and the failure of education of this population may cause autoimmune diseases in the TG nude mouse model.

Establishment of tissue-specific tolerance is driven by regulatory T cells selected by thymic epithelium

Grafts of thymic epithelium (TE) rudiments restore T cell development and function in allogeneic athymic mice. These TE chimeras are specifically tolerant to grafts of peripheral tissues (e.g. skin and heart) from the TE donor strain, although they harbor peripheral immunocompetent T cells capable of rejecting those grafts. Initial analysis has shown that TE chimeras also harbor TE-selected CD4 T lymphocytes that inhibit graft rejection by tissue-reactive T cells in immunocompetent recipients. Peripheral tolerance in TE chimeras is thus maintained by dominant mechanisms dependent on regulatory CD4 T lymphocytes. Here we show that TE-selected regulatory T cells recruit nontolerant tissue-reactive CD4 and CD8 T cells to express similar regulatory functions. Only recent thymic emigrants, but not peripheral resident mature T cells are susceptible to this process of functional education, which also requires exposure to specific antigens and occurs entirely in the periphery. We propose that these mechanisms play a major role in establishing and maintaining natural self tolerance to tissue-specific antigens.

The c-kit+ maturation pathway in mouse thymic T cell development: lineages and selection

Positive selection of T cells begins with TCR alpha beta lo thymic progenitors. Here, we show that the most efficient TCRlo progenitors are c-kit+ with intermediate levels of CD4 and CD8 (DPint). Positive selection of DPint TCRlo c-kit+ cells results in TCRmed CD69+ c-kit+ transitional intermediates that show increased TCRV beta frequencies to selecting superantigen (SAg) that are committed to the CD4 or CD8 pathway. The cells on the c-kit+ maturation pathway maintain Bcl-2 expression. Most DPint c-kit+ progenitors fail positive selection, and become DPhi c-kit- cells that lose Bcl-2 expression. Some DPhi c-kit blast cells can be salvaged to produce mature single-positive (SP) cells. DPint c-kit+ maturation to SP cells can occur in <12 hr in vitro on thymic stromal monolayers.

Antigen compartmentation and T helper cell tolerance induction

The process of antigen recognition depends in part on the amount of peptide antigen available and the affinity of the T cell receptor for a particular peptide-major histocompatibility complex (MHC) molecule complex. The availability of self antigen is limited by antigen processing, which is compartmentalized such that peptide antigens presented by MHC class I molecules originate in the cytoplasm, whereas peptide antigens presented by MHC class II molecules are acquired from the endocytic pathway. This segregation of the antigen-processing pathways may limit the diversity of antigens that influence the development and selection of, e.g., CD4-positive, MHC class II-specific T cells. Selection in this case might involve only a subset of self-encoded proteins, specifically those that are plasma membrane bound or secreted. To study these aspects of immune development, we engineered pigeon cytochrome for expression in transgenic mice in two forms: one in which it was expressed as a type II plasma membrane protein, and a second in which it was targeted to the mitochondria after cytoplasmic synthesis. Experiments with these mice clearly show that tolerance is induced in the thymus, irrespective of antigen compartmentation. Using radiation bone marrow chimeras, we further show that cytoplasmic/mitochondrial antigen gains access to the MHC class II pathway by direct presentation. As a result of studying the anatomy of the thymus, we show that the amount of antigen and the affinity of the TCR affect the location and time point of thymocytes under-going apoptosis.

Additional monoclonal antibody (mAB) injections can replace thymic irradiation to allow induction of mixed chimerism and tolerance in mice receiving bone marrow transplantation after conditioning with anti-T cell mABs and 3-Gy whole body irradiation

While allogeneic bone marrow transplantation (BMT) has long been known to be capable of inducing donor-specific tolerance and hence permitting allograft acceptance without immunosuppressive pharmacotherapy, the toxicity of conditioning regimens required to achieve marrow engraftment has precluded the application of this approach to clinical organ transplantation. A relatively nontoxic method of conditioning mice that allows allogeneic bone marrow engraftment and induction of donor-specific skin allograft tolerance has recently been described. This regimen included anti-CD4 and anti-CD8 mAbs administered on day -5, followed by 3-Gy whole body irradiation (WBI) and 7-Gy thymic irradiation (TI) on day 0. To further reduce the potential toxicity of this regimen, we have now attempted to overcome the requirement for TI by administering additional mAb injections before and after BMT. Mixed chimerism and prolonged donor-specific skin graft acceptance were induced in 90% of B10 mice conditioned with anti-CD4 and -CD8 mAbs on days -6 and -1 and 3-Gy WBI on day 0 without TI. Despite long-term acceptance of donor-specific skin grafts, however, some of these animals showed a gradual decline in donor-type hematopoietic repopulation, and 2 of 10 mice regrafted with a second donor-type skin graft 5-9 months after BMT rejected the second and/or the original graft. This rejection after repeat donor-specific skin grafting correlated with a decline in the percentage of donor-type T cells between 6 and 12 weeks after BMT. In contrast, all animals receiving additional mAb injections 7 and 14 days following BMT after conditioning with mAbs on days -6 and -1 and 3-Gy WBI showed stable chimerism and accepted both primary and secondary donor-specific skin grafts. Animals receiving TI in addition to mAb and 3-Gy WBI also showed stable chimerism and long-term acceptance of initial (at 7 weeks) and later repeat donor-specific grafts. In contrast, the majority of mice receiving mAbs only on day -5 or on day -1 only, followed by 3-Gy WBI on day 0 without TI, did not accept initial donor-specific skin grafts, and showed only transient chimerism. Thus, the requirement for thymic irradiation to allow permanent mixed chimerism and donor-specific tolerance induction can be overcome by the administration of additional T cell-depleting mAb injections. These results establish a less toxic method of inducing donor-specific tolerance, thus increasing the potential clinical applicability of this approach to inducing organ allograft acceptance without chronic immunosuppressive therapy.

Regulatory T cells in thymic epithelium-induced tolerance. I. Suppression of mature peripheral non-tolerant T cells

Athymic mice grafted at birth with allogeneic thymic epithelium (TE) display life-long tolerance to tissue grafts of the TE donor strain, in spite of harboring peripheral T cells capable of rejecting those grafts. Tolerance is maintained in these chimeras by TE-specific regulatory CD4 T cells. We presently address the quantification and the mechanisms of this dominant tolerance process. C57BL/6 mice containing variable but defined numbers of peripheral, resident T cells received cell transfers of graded numbers of peripheral T cells from B6(BALB E10) chimeras (C57BL/6 nude mice grafted with TE from 10-day-old BALB/c embryos), resulting in a series of animals containing a wide range of donor (tolerant) versus host (non-tolerant) T cell chimerism. Increasing the relative representation of donor T cells results in a progressive delay in the rejection of BALB/c skin grafts, life-long tolerance being achieved at a ratio of tolerant and non-tolerant T cell populations of 1. In recipients displaying full tolerance, graft-reactive non-tolerant T cells were not deleted, anergized or committed to noninflammatory functions. Thus, sorted host T cells from tolerant recipients readily rejected BALB/c skin grafts upon transfer to immunodeficient animals. Finally, measurements of "helper" and inflammatory activities, as well as interleukin-4 and interferon-gamma production, failed to discriminate between T cell populations from tolerant and non-tolerant animals after specific in vitro stimulation. We conclude that: (a) TE-selected regulatory T cells can suppress, in a quantitative manner, in vivo T cell responses against major and minor histocompatibility antigens expressed by the TE and, (b) this suppressive activity neither inactivates mature non-tolerant T cells, nor does it seem to drive their differentiation along noninflammatory pathways.

[Immunologic tolerance after experimental liver transplantation]

Allografts in the rat liver are rejected less vigorously than other primarily vascularized allografts; they show a better survival rate and induce donor-specific unresponsiveness or tolerance in some donor-recipient combinations. This overview focuses on the immunologic mechanisms of this privileged status of liver allografts. A variety of possible mechanisms, such as generation of suppressor T-cells, humoral factors and microchimerism, has been related to the observed hyporeactivity. A further analysis of these phenomena may enhance the development of clinical organ transplantation protocols that allow for establishment of donor-specific unresponsiveness without the need for life-long immunosuppression.

Lymphocytes selected in allogeneic thymic epithelium mediate dominant tolerance toward tissue grafts of the thymic epithelium haplotype

Athymic mice grafted at birth with allogeneic thymic epithelium (TE) from day 10 embryos before hematopoietic cell colonization reconstitute normal numbers of T cells and exhibit full life-long tolerance to skin grafts of the TE haplotype. Intravenous transfers of splenic cells, from these animals to adult syngeneic athymic recipients, reconstitute T-cell compartments and the ability to reject third-party skin grafts. The transfer of specific tolerance to skin grafts of the TE donor strain, however, is not observed in all reconstituted recipients, and the fraction of nontolerant recipients increases with decreasing numbers of cells transferred. Furthermore, transfers of high numbers of total or CD4+ T cells from TE chimeras to T-cell receptor-anti-H-Y antigen transgenic immunocompetent syngeneic hosts specifically hinder the rejection of skin grafts of the TE haplotype that normally occurs in such recipients. These observations demonstrate (i) that mice tolerized by allogeneic TE and bearing healthy skin grafts harbor peripheral immunocompetent T cells capable of rejecting this very same graft; and (ii) that TE selects for regulatory T cells that can inhibit effector activities of graft-reactive cells.

Chicken thymic nurse cells: an overview

Thymic nurse cells are multicellular complexes located in the subcortical area of the thymus of all avian, mammalian and amphibian species investigated so far. Since their first description in 1980 many studies have been carried out to characterize their morphological and functional properties. The purpose of this review is to summarize recent morphological as well a functional analyses of chicken thymic nurse cells which suggest a role of these cell complexes in T cell selection.

Thymic reticulum of mice. III. The connective compartment (innervation, vascularisation, fibrous tissues and myoid cells)

T lymphocytes interact at various levels of differentiation, with cells of the thymic reticulum, forming a peculiar and complex microenvironment. Following earlier descriptions by electron microscopy of three types of epithelial cells and two types of non-epithelial cells (macrophages and interdigitated cells) forming the thymic microenvironment, we report a study on a third compartment, the connective tissue, whose elements occur throughout the organ. The components of the capsule and trabeculae, the vascularisation and the innervation of the thymus and the presence of a few myoid cells are described. This is very rarely studied in ultrastructure. All these cells are completely imbricated and form a network trapping the lymphocytes, playing an essential role in the differentiation, maturation and selection of T cells.

Cytotoxic effects of irradiation and deoxyguanosine on fetal thymus

Effects of irradiation and deoxyguanosine on the fetal thymus were examined both in vitro and in vivo. Fetal thymi (gestation day 15) of C57BL/6 mice that had been irradiated (0-25 Gy) or treated with various doses of deoxyguanosine (dGuo) were engrafted under the renal capsules of BALB/c nu/nu mice, and the differentiation of T cells was investigated in the engrafted thymi or spleens of these mice. After in vitro treatment of fetal thymi with 1.35 mM dGuo (which was previously reported to be an optimal dose), T cell precursors still remained in some cultures, whereas 1.80 mM dGuo was highly cytotoxic not only to T cell precursors but also to thymic epithelial cells. In contrast, 25 Gy irradiation totally eliminated the T cell precursors from the fetal thymi, though the capacity of epithelial cells to induce T cell differentiation was retained. Although irradiated thymi had the capacity to induce T cell differentiation when assayed in an in vitro organ culture system, long-term observation of thymi engrafted into BALB/c nu/nu mice revealed that, if they had been irradiated (9.5 Gy or 25 Gy), the thymi became scarred by 12 wks after their transplantation. Furthermore, the expression of cell interaction molecules such as ICAM-1 and MHC class II on the thymus stromal cells decreased after irradiation. The interaction molecules decreased 3 wks after 25 Gy irradiation and 7 wks after 9.5 Gy irradiation. The alteration in T cell subsets in the thymus (decreases in both double- and single- positive cells and an increase in double-negative cells) correlated with the decreases in the interaction molecules. This indicates that irradiation (even 9.5 Gy) impairs the T cell-induction capacity of the thymus stromal cells, resulting in an alteration of the T cell subsets followed by a change in the T cell counts in the thymus. Therefore, the long-term effects of irradiation of the thymus should be considered in cases of fetal thymus grafts or total body irradiation before bone marrow transplantation, particularly in the newborn.

Retention and multilineage expression of human hematopoietic stem cells in human-sheep chimeras

We have taken advantage of the permissive environment of the early gestational age fetus to engraft human hematopoietic stem cells (HSC) into preimmune fetal lambs. The resulting chimeras exhibit long-term multilineage engraftment of human cells in the bone marrow (BM) and peripheral blood (PB). Long-term multilineage reconstitution of second generation recipients by human cells isolated from chimeric sheep BM indicates that the engraftment in this model involved long-term repopulating human HSC. The model appears to be sensitive enough to detect relatively small numbers of transplanted HSC from pre- and postnatal human sources. Finally, transplantation of mature T lymphocyte-containing cells from a variety of sources results in lethal graft-versus-host disease (GVHD), analogous to clinical BM transplantation, suggesting that, at least in some respects, the model is biologically relevant. The human-sheep model is promising and may have important advantages over murine models for the in vivo study of normal and abnormal hematopoiesis and as a potential assay system for human HSC.

Superantigenic characteristics of mouse mammary tumor viruses play a critical role in susceptibility to infection in mice

Mouse mammary tumor viruses (MMTV) are retroviruses that induce mammary carcinomas. An interesting feature of these viruses is the superantigen (SAg) encoded in an open reading frame within the 3' long terminal repeat. The mechanism by which ingestion of milk-borne virus results in infection of the host mammary tissue remains incompletely understood. However, a working model has been proposed in which the interaction between viral SAg, T-cell receptor and MHC class II I-E facilitates viral replication and hence infectivity. In this review we summarize current studies demonstrating the role of SAg stimulation in susceptibility to MMTV infection.

Thymic epithelium induces full tolerance to skin and heart but not to B lymphocyte grafts

Athymic nude mice reconstituted at birth with allogeneic thymic epithelia (TE) from day 10 embryos (E10), show life-long specific tolerance to skin and heart grafts, but eliminate B lymphocytes of the TE donor haplotype, nearly as well as those from a third strain. Previous immunizations with B cells do not alter the state of tolerance to skin grafts, but specifically accelerate elimination of lymphocytes. In contrast, transplantation of E15 allogeneic thymuses already seeded by hematopoietic cells resulted in chimeras tolerant to both skin and B lymphocytes. In vitro reactivities towards stimulator spleen cells of the haplotype of the thymus were observed in both E10 TE and E15 thymus chimeras. We conclude that induction of full in vivo tolerance to B cells requires hematopoietic cells, while this is not the case for induction of tolerance to skin and heart tissues; furthermore, in vitro reactivity to stimulator spleen cells of the tolerized haplotype is independent of in vivo tolerance.

Murine nursing thymic epithelial cell lines capable of inducing thymocyte apoptosis express the self-superantigen Mls-1a

Two cloned thymic epithelial cell (TEC) lines, D2.TEC-A3 and AKR TEC-K1, were established from minor lymphocyte-stimulating (Mls)-1a-positive normal, 4-week-old DBA/2 (H-2d, Mls-1a2a) mice and AKR (H-2k, Mls-1a2b) mice, respectively. Both cell lines were MHC class I and class II (both I-A and I-E) positive without stimulation by interferon-gamma. They were capable of infolding immature thymocytes to form thymic nurse cells (TNC; we call this type of TEC "nursing TEC") and induced apoptosis with DNA fragmentation in immature thymocytes. Using a primary Mls mixed lymphocyte reaction (MLR) we demonstrated that self-superantigen Mls-1a was expressed on these cloned nursing TEC lines. D2.TEC-A3 cells stimulated nylon-wool-purified splenic T cells obtained from H-2d-compatible BALB/c (Mls-1b2a) and B10.D2 (Mls-1b2b) mice with a maximal response at a stimulator:responder ratio of 1:40 after 4 days of the coculture. AKR TEC-K1 cells also stimulated purified T cells from H-2k-compatible C3H/He mice (Mls-1b2a) in a similar manner. The Mls MLR induced by the nursing TEC lines was completely inhibited in the presence of anti-mouse I-A and anti-mouse I-E monoclonal antibodies. These results suggest that nursing TEC/TNC could be involved in negative selection due to apoptosis.

The ER-TR4 monoclonal antibody recognizes murine thymic epithelial cells (type 1) and inhibits their capacity to interact with immature thymocytes: immuno-electron microscopic and functional studies

The thymic stroma is heterogeneous with regard to cellular morphology and cellular function. In this study, we employed the monoclonal antibody ER-TR4 to characterize stromal cells at the ultrastructural level. To identify the labelled cell type, we used two techniques: immunogold labelling on ultrathin frozen sections and immunoperoxidase staining on thick "vibratome" sections. ER-TR4 reacted with thymic Type 1 epithelial cells (according to our classification). A dense labelling appears in the cytoplasm of cortical cells using the two techniques. Immunogold labelling identified small cytoplasmic vesicles whereas the cytoplasm and the cell membrane seem to be labelled with the immunoperoxidase technique. ER-TR4 also identified isolated thymic nurse cells (TNC), and was observed in vitro to inhibit the capacity of some type 1 epithelial cells to establish interactions with immature thymocytes. This finding supports the hypothesis that the factor is involved in the formation of lymphoepithelial interactions within thymic nurse cells, and thus in the relations that immature thymocytes establish with the thymic microenvironment.

Long-term repopulating ability of xenogeneic transplanted human fetal liver hematopoietic stem cells in sheep

We previously reported on the successful engraftment and long-term multilineage expression (erythroid, myeloid, lymphoid) of human fetal liver hematopoietic stem cells in sheep after transplantation in utero. That the engraftment of long-term repopulating pluripotent stem cells occurred in these animals was shown here by the fact that transplantation of human CD45+ cells isolated from bone marrow of these chimeric animals into preimmune fetal sheep resulted in engraftment and expression of human cells. Marrow cells were obtained from three chimeric sheep at 3.2-3.6 yr after transplant. The relative percentage of human CD45+ cells present in these marrows was 3.3 +/- 0.32%. A total of 29 x 10(6) CD45+ cells were isolated by panning, pooled, and transplanted into six preimmune sheep fetuses (4.8 x 10(6) cells/fetus). All six recipients were born alive. Hematopoietic progenitors exhibiting human karyotype were detected in marrows of two lambs soon after birth. Cells expressing human CD45 antigen were also detected in blood and marrow of both lambs. Human cell expression has been multilineage and has persisted for > 1 yr. These results demonstrate that the expression of human cells in this large animal model resulted from engraftment of long-term repopulating pluripotent human stem cells.

FK506 inhibits the differentiation of developing thymocytes but not negative selection of T cell receptor V beta 5+ and V beta 11+ T lymphocytes in vivo

To examine the influence of FK506 on lymphocyte development, we employed a syngeneic bone marrow transplantation model using MHC-disparate B10 (H-2b, I-Ab) and B10.BR (H-2k, I-Ak, I-Ek) mice. B10 mice, which do not express class II I-E, do not delete any known T cell receptor (TCR)-V beta, while B10.BR mice (MHC class II I-Ek, I-Ak) delete V beta 5+ and V beta 11+ TCR. Continuous daily treatment of syngeneically reconstituted B10 mice with FK506 delayed the development of thymocytes from the CD4+CD8+ to CD4+CD8- stage, while no effect was observed at the earlier CD4-CD8- to CD4+CD8+ stage. At the same time, there was a significant reduction in TCRhigh thymocytes compared with untreated, syngeneically reconstituted controls. These results suggest that FK506 treatment interfered with thymic positive selection. We also examined whether FK506 treatment would influence negative selection. Levels of expression of V beta 5+ and V beta 11+ T cells in FK506-treated B10.BR-->B10.BR recipients were similar to those observed in unmanipulated, syngeneically reconstituted B10.BR-->B10.BR controls. This was not due to the inhibition of clonal proliferation by FK506, since 35 days after drug withdrawal complete recovery of the peripheral Thy1.2+ population was observed, while the percentages of V beta 5+ and V beta 11+Thy1.2+ T cells were maintained at values similar to controls. Surprisingly, clonal proliferation stimulated by monoclonal antibody against V beta 5 and V beta 11 TCRs was observed in CsA-treated, syngeneically reconstituted B10.BR mice but not in FK506-treated mice, suggesting that CsA may be more likely to induce autoreactivity. Differences in thymic architecture between FK506- and CsA-treated animals further suggested that the drugs may differ in their effects on T cell development in vivo.

T cell development in a major histocompatibility complex class II-deficient patient

In this report we show that the major histocompatibility complex (MHC) class II-negative thymus of a bare lymphocyte syndrome (BLS) patient contains a reduced CD4+ CD8- T cell population when compared to thymocytes derived from a MHC class II-expressing thymus. Of these CD4+ CD8- BLS thymocytes, approximately only one third co-expressed the CD3 antigen, moreover at a lower expression level when compared to control thymocytes. This suggests a partial maturation of the CD4+ CD8- T cells in the absence of MHC class II expression. Among the BLS thymocytes, CD4+ CD8+ thymocytes could easily be detected. Noteworthy, the number of CD4- CD8+ thymocytes was significantly increased. CD4+ CD8- T cells could also be found among the BLS peripheral blood mononuclear cells, albeit at reduced numbers. Despite the absence of peripheral MHC class II expression, the majority of these CD4+ CD8- T cells co-expressed the CD45RO marker. In the BLS patient, thymocytes as well as peripheral CD4+ CD8- T cells were not restricted in the use of the available T cell receptor (TcR) V gene family pool. However, the lack of detectable levels of thymic and peripheral MHC class II antigen expression in the BLS patient had altered the CD4-skewing patterns of TcR V gene families which were present in normal individuals. In conclusion, the lack of MHC class II expression in the BLS patient does not completely inhibit the CD4+ CD8- T cell development.

Expression of a MHC class II transgene determines both superantigenicity and susceptibility to mammary tumor virus infection

Milk-borne mouse mammary tumor virus (MMTV) is a type B retrovirus that induces mammary carcinoma. Infectious MMTV, as well as genomically integrated mouse mammary proviruses, encode superantigens that are recognized by T cells that express appropriate T cell receptor V beta products. To determine the relationship between the superantigenic property of milk-borne MMTV and its in vivo infectivity, mice which were either positive or negative for expression of a transgene-encoded E alpha E beta class II major histocompatibility complex (MHC) product were exposed to milk borne C3H MMTV. Superantigen-mediated deletion of V beta 14-expressing T cells occurred only in E alpha transgene-positive mice, indicating that the deletion was E alpha E beta dependent. When mice were analyzed for viral infection by assaying viral p28 in the milk of recipient females, significant p28 levels were found only in E alpha E beta transgene-positive mice. Similarly, the presence of C3H MMTV LTR mRNA in mammary glands, as detected by PCR, paralleled p28 levels. These findings indicate that E alpha expression or the E alpha-dependent T cell response to viral superantigen is causally related to susceptibility to MMTV infection, and that lack of a permissive class II product can protect mice from virus infection.

Positive selection of V beta 8+ CD4-8- thymocytes by class I molecules expressed by hematopoietic cells

A small subset of T cells of mature phenotype express the alpha/beta T cell receptor, but not CD4 and CD8 coreceptors (alpha/beta double-negative [DN] cells). The repertoire of V beta usage of alpha/beta DN cells is strongly biased towards V beta 8 expression, suggesting that the formation of the population is subject to selection. We now report that deficiency of class I expression leads to a strongly depressed frequency of V beta 8+ DN cells, but has little effect on V beta 8- DN cells. Studies of hematopoietic chimeras between class I+ and class I- mice demonstrated that expression of class I molecules by hematopoietic cells is necessary and sufficient for selection of most V beta 8 DN cells. The lack of a role for class I expression by thymic epithelial cells suggests that the mechanism of selection of these cells by class I differs significantly from the mechanism of selection of conventional T cells. Models to explain the selection of these cells as well as their possible function in vivo are discussed.