Induction of neonatal tolerance to Mlsa antigens by CD8+ T cells

Direct presentation of inflammation-associated self-antigens by thymic innate-like T cells induces elimination of autoreactive CD8+ thymocytes

Upregulation of diverse self-antigens that constitute components of the inflammatory response overlaps spatially and temporally with the emergence of pathogen-derived foreign antigens. Therefore, discrimination between these inflammation-associated self-antigens and pathogen-derived molecules represents a unique challenge for the adaptive immune system. Here, we demonstrate that CD8+ T cell tolerance to T cell-derived inflammation-associated self-antigens is efficiently induced in the thymus and supported by redundancy in cell types expressing these molecules. In addition to thymic epithelial cells, this included thymic eosinophils and innate-like T cells, a population that expressed molecules characteristic for all major activated T cell subsets. We show that direct T cell-to-T cell antigen presentation by minute numbers of innate-like T cells was sufficient to eliminate autoreactive CD8+ thymocytes. Tolerance to such effector molecules was of critical importance, as its breach caused by decreased thymic abundance of a single model inflammation-associated self-antigen resulted in autoimmune elimination of an entire class of effector T cells.

Recirculating Th2 cells induce severe thymic dysfunction via IL-4/STAT6 signaling pathway

Thymic involution happened early in life, but a certain ratio of activated CD4⁺ T cells will persistently recirculate into the thymus from the periphery and it have been suggested to be able to inhibit the development of embryonic thymocytes. Our present study was aimed to elucidate the specific mechanism how activated CD4⁺ T cells could influence upon developing thymocytes by using fetal thymic organ culture (FTOC) and kidney capsule transplantation. Our results demonstrated that Th2 cells were found to play a fundamental role in the inhibition of embryonic thymocyte development since a very low concentration of Th2 cells could obviously reduce the total number of thymocytes. And this effect was not tenable in other Th cell type. Notably, IL-4, the major cytokine secreted by Th2 cells, was suggested the key factor playing the inhibition role. In addition to reduced cell population, the proportion of double positive (DP) T cells was also heavily decreased. Furthermore, we demonstrated that it was the downstream effector signal transducer and activator of transcription 6 (STAT6) of IL-4 partially manipulate this inhibition. Together, these findings reveal a novel influence of Th2 cells re-entering the thymus on thymic involution.

Thymic homing of activated CD4+ T cells induces degeneration of the thymic epithelium through excessive RANK signaling

Activated T cells have been shown to be able to recirculate into the thymus from the periphery. The present study was aimed to elucidate the functional consequences of thymic homing of activated T cells upon developing thymocytes and thymic epithelial cells (TEC). In the presence of activated T cells, especially CD4⁺ T cells, T cell development was found to be inhibited in thymic organ cultures with markedly reduced cellularity. Thymic transplantation demonstrated that the inhibitory effect was most likely due to a defective microenvironment. As the major component of the thymic stroma, the TEC compartment was severely disturbed after prolonged exposure to the activated T cells. In addition to reduced cell proliferation, TEC differentiation was heavily skewed to the mTEC lineage. Furthermore, we demonstrated that RANKL highly expressed by activated CD4⁺ T cells was primarily responsible for the detrimental effects. Presumably, excessive RANK signaling drove overproduction of mTECs and possibly exhaustion of epithelial progenitors, thereby facilitating the deterioration of the epithelial structures. These findings not only reveal a novel activity of activated T cells re-entering the thymus, but also provide a new perspective for understanding the mechanism underlying thymic involution.

Stable interactions and sustained TCR signaling characterize thymocyte-thymocyte interactions that support negative selection

Negative selection is one of the primary mechanisms that render T cells tolerant to self. Thymic dendritic cells play an important role in negative selection, in line with their ability to induce migratory arrest and sustained TCR signals. Thymocytes themselves display self-peptide/MHC class I complexes, and although there is evidence that they can support clonal deletion, it is not clear whether they do so directly via stable cell-cell contacts and sustained TCR signals. In this study, we show that murine thymocytes can support surprisingly efficient negative selection of Ag-specific thymocytes. Furthermore, we observe that agonist-dependent thymocyte-thymocyte interactions occurred as stable, motile conjugates led by the peptide-presenting thymocyte and in which the trailing peptide-specific thymocyte exhibited persistent elevations in intracellular calcium concentration. These data confirm that self-Ag presentation by thymocytes is an additional mechanism to ensure T cell tolerance and further strengthen the correlation between stable cellular contacts, sustained TCR signals, and efficient negative selection.

Thymus-resident memory CD8+ T cells mediate local immunity

The thymus is a primary lymphoid organ responsible for production and selection of T cells. Nonetheless, mature T cells and in particular activated T cells can reenter the thymus. Here, we identified memory CD8(+) T cells specific for lymphocytic choriomeningitis virus or vaccinia virus in the thymus of mice long-time after the infection. CD8(+) T cells were mainly located in the thymic medulla, but also in the cortical areas. Interestingly, virus-specific memory CD8(+) T cells in the thymus expressed the cell surface markers CD69 and CD103 that are characteristic of tissue-resident memory T cells in a time-dependent manner. Kinetic analyses and selective depletion of peripheral CD8(+) T cells by antibodies further revealed that thymic virus-specific memory CD8(+) T cells did not belong to the circulating pool of lymphocytes. Finally, we demonstrate that these thymus-resident virus-specific memory CD8(+) T cells efficiently mounted a secondary proliferative response, exhibited immediate effector functions and were able to protect the thymus from lymphocytic choriomeningitis virus reinfection. In conclusion, the present study not only describes for the first time virus-specific memory CD8(+) T cells with characteristics of tissue-resident memory T (T(RM)) cells in a primary lymphoid organ but also extends our knowledge about local T-cell immunity in the thymus.

Peripheral T cells re-enter the thymus and interfere with central tolerance induction

The thymus mainly contains developing thymocytes that undergo thymic selection. In addition, some mature activated peripheral T cells can re-enter the thymus. We demonstrated in this study that adoptively transferred syngeneic Ag-specific T cells can enter the thymus of lymphopenic mice, where they delete thymic dendritic cells and medullary thymic epithelial cells in an Ag-specific fashion, without altering general thymic functions. This induced sustained thymic release of autoreactive self-Ag-specific T cells suggested that adoptively transferred activated T cells can specifically alter the endogenous T cell repertoire by erasing negative selection of their own specificities. Especially in clinical settings in which adoptively transferred T cells cause graft-versus-host disease or graft-versus-leukemia, as well as in adoptive tumor therapies, these findings might be of importance, because the endogenous T cell repertoire might be skewed to contribute to both manifestations.

Induction of transplantation tolerance to fully mismatched cardiac allografts by T cell mediated delivery of alloantigen

Induction of transplantation tolerance has the potential to allow for allograft acceptance without the need for life-long immunosuppression. Here we describe a novel approach that uses delivery of alloantigen by mature T cells to induce tolerance to fully allogeneic cardiac grafts. Adoptive transfer of mature alloantigen-expressing T cells into myeloablatively conditioned mice results in long-term acceptance of fully allogeneic heart transplants without evidence of chronic rejection. Since myeloablative conditioning is clinically undesirable we further demonstrated that adoptive transfer of mature alloantigen-expressing T cells alone into mice receiving non-myeloablative conditioning resulted in long-term acceptance of fully allogeneic heart allografts with minimal evidence of chronic rejection. Mechanistically, tolerance induction involved both deletion of donor-reactive host T cells and the development of regulatory T cells. Thus, delivery of alloantigen by mature T cells induces tolerance to fully allogeneic organ allografts in non-myeloablatively conditioned recipients, representing a novel approach for tolerance induction in transplantation.

Mls: A Link Between Immunology and Retrovirology

The nature of the mysterious minor lymphocyte stimulating (Mls) antigens has recently been clarified. These molecules which were key elements for our current understanding of immune tolerance, have a strong influence on the mouse immune system and are encoded by the open reading frame (orf) of endogenous and exogenous mouse mammary tumor viruses (MMTV's). The knowledge that these antigens are encoded by cancerogenic retroviruses opens an interdisciplinary approach for understanding the mechanisms of immune responses and immune tolerance, retroviral carcinogenesis, and retroviral strategies for infection.

Re-entry of mature T cells to the thymus: an epiphenomenon?

In certain situations mature post-thymic T cells are able to leave their residence in the secondary lymphoid tissues and re-enter the thymus. The physiological significance of this phenomenon is discussed.

MHC class II-dependent T-T interactions create a diverse, functional and immunoregulatory reaction circle

Unlike conventional T cells, innate-like T cells such as natural killer (NK) T cells are selected by homotypic T-cell interactions. Recently, a few reports have shown that T-T CD4(+) T cells can be generated in a similar manner to that for NKT cells. These two types of cells share common functional properties such as rapid response to antigenic encounters and the potential for a panoply of cytokine secretion. However, T-T CD4(+) T cells differ from NKT cells in that they are restricted by highly polymorphic major histocompatibility complex (MHC) II molecules and have a diverse T-cell receptor repertoire. Additional example of T-T interactions was recently reported in which peripheral T cells re-circulate to the thymus and participate in the thymocyte selection process. In this review, we dissect the cellular mechanisms underlying the production of T-T CD4(+) and NKT cells, with particular emphasis on the differences between these two T-cell prototypes. Finally, we propose that T-T CD4(+) T cells serve two major functions: one as an acute-phase reactant against viral infection and the other is the generation of anti-ergotypic CD4(+) T cells for regulatory purposes. All of these features make it possible to create a diverse set of functional cells through MHC class II-restricted T-T interactions.

Back to the thymus: peripheral T cells come home

The thymus has long been known as the generative organ for the T-cell arm of the immune system. To perform this role, the thymus was thought to require protection from antigenic and cellular insult from the 'outside world', with the notable exception of the continual influx of progenitor cells required to initiate the complicated process of T-cell differentiation. Overwhelming evidence that mature T cells can recirculate and persist in the thymus has required us to revamp this earlier view of the thymus as detached from outside influence. In this review, we consider the evidence for T-cell recirculation into the thymus, discuss the likely means and location of mature T-cell entry, and speculate on the potential consequences of such close apposition between differentiating thymocytes and mature recirculating lymphocytes.

Peripheral T lymphocytes recirculating back into the thymus can mediate thymocyte positive selection

The thymus continuously produces T lymphocytes that contribute to the maintenance of the peripheral T cell pool. Since peripheral recirculating T cells represent a very minor population among total thymocytes in normal animals, the relationship between the thymus and secondary lymphoid organs is generally considered unidirectional. Recently, several reports have described the presence of recirculating T cells in the thymus, raising issues regarding their possible function. In this article, we show that the niche for recirculating T cells in the thymus, i.e., their absolute number, is the same in lymphopenic and normal mice. Using a novel combination of TCR-transgenic mice in which the ligand necessary for positive selection of host T cells is only expressed by transferred donor T cells, we show that mature T cells recirculating back to the thymus can mediate positive selection.

Homeostatic expansion permits T cells to re-enter the thymus and deliver antigen in a tolerogenic fashion

We previously have shown that delivery of alloantigen on T cells can be used to induce tolerance through central deletion. Here, we analyzed the requirements for tolerance induced by T cells. Adoptively transferred allogeneic T cells undergo extensive homeostatic proliferation in the periphery of lethally irradiated hosts receiving a syngeneic bone marrow transplant, and acquire a memory-like cell surface phenotype. Analysis of the kinetics of thymic re-entry of transferred T cells revealed that T cells undergo homeostatic proliferation in the periphery prior to re-entry into the thymus. Prevention of homeostatic proliferation results in a failure of transferred T cells to re-enter the thymus. In the absence of homeostatic proliferation, adoptively transferred T cells were unable to induce tolerance. These date suggest that homeostatic proliferation of T cells resulting in an activated cell surface phenotype is required for thymic re-entry and is mechanistically linked to the ability of T cells to induce tolerance.

Microchimerism maintains deletion of the donor cell-specific CD8+ T cell repertoire

Rare cases of stable allograft acceptance after discontinuation of immunosuppression are often accompanied by macrochimerism (> 1% donor cells in blood) or microchimerism (< 1% donor cells in blood). Here, we have investigated whether persistence of donor cells is the cause or the consequence of long-lasting CTL unresponsiveness. We found that engraftment of splenocytes bearing a single foreign MHC class I-restricted epitope resulted in lifelong donor cell microchimerism and specific CTL unresponsiveness. This status was reversed in a strictly time- and thymus-dependent fashion when the engrafted cells were experimentally removed. The results presented herein show that microchimerism actively maintains CTL unresponsiveness toward a minor histocompatibility antigen by deleting the specific repertoire and thus excluding dominant, T cell extrinsic mechanisms of CTL unresponsiveness independent of systemically persisting donor cell antigen.

Nonlymphoid peritoneal cells suppress the T cell response to Mls

Comparative analyses of the ability of lymphoid tissue to present the minor lymphocyte stimulatory (Mls) superantigen Mls-1a in vitro revealed that all tissues containing mature B cells, except peritoneal cavity (PerC) cells, induced Mls-1a-specific T cell activation. Irradiation and mitomycin C treatment, addition of IL-2 and IL-12, and neutralization of IL-10 and TGF-beta did not restore Mls-1a antigen presentation by PerC cells. Co-culture studies revealed that PerC cells actively suppress the T cell response to Mls-1a. PerC cells from severe-combined immune-defective (SCID) mice also suppressed this response indicating that nonlymphoid cells mediate this effect. These results suggest that in addition to antigen processing and presentation, resident peritoneal cavity cells may temper lymphocyte activation.

Mls presentation by peritoneal cavity B cells

DBA/2J spleen and peritoneal cells were compared for their ability to present the minor lymphocyte stimulatory superantigen Mls-1a. Although capable of Mls presentation in vivo, peritoneal cells were less effective than spleen cells in vitro. This difference was not due to cell concentration or culture duration. Flow cytometric comparison of spleen and peritoneal B cells revealed no significant differences in cell surface markers needed for cognate interaction with T cells. Resolution of peritoneal B cell subsets by cell sorting revealed that even though B-1 cells were capable of Mls presentation, they were less effective than B-2 cells. Mixing experiments showed that B-1 cells did not inhibit B-2 cell presentation of Mls. In contrast, total peritoneal cells inhibited T cell responses to Mls presented by spleen cells. The peritoneal cavity harbors B cells that can present Mls as well as other cells that can suppress this response.

Induction of central tolerance by mature T cells

Induction of immunological tolerance is highly desirable for the treatment and prevention of autoimmunity, allergy, and organ transplant rejection. Adoptive transfer of MHC class I disparate mature T cells at the time of reconstitution of mice with syngeneic bone marrow resulted in specific tolerance to allogeneic skin grafts that were matched to the T cell donor strain. Mature allogeneic T cells survived long-term in reconstituted hosts and were able to re-enter the thymus. Analysis of T cell development using transgenic mice expressing an alloantigen-reactive TCR revealed that expression of allogeneic MHC class I on adoptively transferred mature T cells mediated negative selection of developing alloreactive T cells in the thymus. Thus, mature allogeneic T cells are able to mediate central deletion of alloreactive cells and induce transplantation tolerance without the requirement for any other alloantigen-expressing cell type.

Role of reentry of in vivo alloMHC peptide-activated T cells into the adult thymus in acquired systemic tolerance

BACKGROUND

T cell recognition of alloMHC peptide presented by self dendritic cells via the indirect pathway of allorecognition in the thymus induces T cell tolerance. Most recently we have shown that the i.v. administration of immunodominant Wistar Furth MHC class I (RT1.Au) peptide 5- (P5) pulsed myeloid or lymphoid dendritic cells induces operational tolerance to a fully MHC-mismatched cardiac allograft. This finding led us to hypothesize that circulation of peripheral P5-activated T cells to the thymus plays an important role in the induction of acquired tolerance.

METHODS

We used the adoptive transfer of 111Indium-oxine- (111In-oxine) labeled P5-pulsed syngeneic dendritic cells and in vivo P5-activated syngeneic T cells to study the role of their circulation to the thymus in the induction of transplantation tolerance.

RESULTS

Intravenously administered 111In-oxine-labeled naïve DC actively migrated to and localized in the liver and spleen but did not enter the lymph nodes, bone marrow, and thymus. In vitro peptide-pulsed dendritic cells had a similar pattern of tissue localization except for a modest number of myeloid but not lymphoid DC entering the thymus. The demonstration that adoptive transfer of in vivo peptide-primed T cells induces permanent graft survival in antilymphocyte serum transiently immunosuppressed syngeneic secondary hosts led us to examine the traffic of in vivo activated T cells. Whereas naïve syngeneic T cells preferentially homed to the peripheral lymphoid organs, they did not reenter the thymus. In contrast, in vivo peptide-activated peripheral T cells migrated to and accumulated in the thymus, thus confirming that reentry of T cells to the thymus is restricted to in vivo activated T cells. Although antilymphocyte serum immunosuppression significantly reduced circulation of primed T cells to the thymus, it did not completely abolish it, as seen with gamma-irradiated primed T cells.

CONCLUSION

These findings provide the first formal evidence directly linking reentry of in vivo alloMHC peptide-activated T cells to the thymus with the induction and possibly maintenance of acquired antigen-specific tolerance. Our results suggest that the thymus is open to a two-way traffic with the periphery and may function as a repository of immunological memory.

Requirement for a complex array of costimulators in the negative selection of autoreactive thymocytes in vivo

Autoreactive thymocytes can be deleted at an immature stage of their development by Ag-induced apoptosis or negative selection. In addition to Ag, negative selection also requires costimulatory signals from APC. We recently used a fetal thymus organ culture system to show that CD5, CD28, and TNF cooperatively regulate deletion of autoreactive thymocytes. Although these experiments provided strong evidence for the action of several costimulators in negative selection, we wished to demonstrate a role for these molecules in a physiologically natural model where thymocytes are deleted in vivo by endogenously expressed AGS: Accordingly, we examined thymocyte deletion in costimulator-null mice in three models of autoantigen-induced negative selection. We compared CD5(-/-) CD28(-/-) mice to CD40L(-/-) mice, which exhibited a profound block in negative selection in all three systems. Surprisingly, only one of the three models revealed a requirement for the CD5 and CD28 costimulators in autoantigen-induced deletion. These results suggest that an extraordinarily complex array of costimulators is involved in negative selection. We predict that different sets of costimulators will be required depending on the timing of negative selection, the Ag, the signal strength, the APC, and whether Ag presentation occurs on class I or class II MHC molecules.

CD4 T cell-mediated alloresistance to fully MHC-mismatched allogeneic bone marrow engraftment is dependent on CD40-CD40 ligand interactions, and lasting T cell tolerance is induced by bone marrow transplantation with initial blockade of this pathway

Costimulatory blockade can be used to promote allogeneic marrow engraftment and tolerance induction, but on its own is not 100% reliable. We sought to determine whether one or the other of the CD4 or CD8 T cell subsets of the recipient was primarily responsible for resistance to allogeneic marrow engraftment in mice receiving costimulatory blockade, and to use this information to develop a more reliable, minimal conditioning regimen for induction of mixed chimerism and transplantation tolerance. We demonstrate that a single anti-CD40 ligand mAb treatment is sufficient to completely overcome CD4 cell-mediated resistance to allogeneic marrow engraftment and rapidly induce CD4 cell tolerance, but does not reliably overcome CD8 CTL-mediated alloresistance. The data suggest that costimulation, which activates alloreactive CTL, is insufficient to activate alloreactive CD4 cells when the CD40 pathway is blocked. The addition of host CD8 T cell depletion to anti-CD40 ligand treatment reliably allows the induction of mixed chimerism and donor-specific skin graft tolerance in 3 Gy-irradiated mice receiving fully MHC-mismatched bone marrow grafts. Thus, despite the existence of multiple costimulatory pathways and pathways of APC activation, our studies demonstrate an absolute dependence on CD40-mediated events for CD4 cell-mediated rejection of allogeneic marrow. Exposure to donor bone marrow allows rapid tolerization of alloreactive CD4 cells when the CD40 pathway is blocked, leading to permanent marrow engraftment and intrathymic tolerization of T cells that develop subsequently.

Superantigen disruption of CD8+ T and B lymphocyte homeostasis

Superantigens (SAgs) activate TH cells and induce their differentiation into cytokine-producing effector cells. Supranormal cytokine production is characteristic of SAg-induced polyclonal TH activation. Study of this interaction has focused upon TH cell function to the relative exclusion of other lymphocyte populations. SAgs also impact cells dependent upon TH cells for their differentiation and disrupt the normal homeostasis of the immune system. In this report, several changes in lymphocyte biology that result from SAg activation of TH cells are described. SCID mice, reconstituted with the SAg-expressing cells of DBA/2J mice, were employed as secondary recipients of SAg-reactive TH cells. Significant increases in serum IgM and IgG2a production were noted after the transfer of SAg-reactive It cells. Both B and CD8 T lymphocyte numbers increased with those of CD8 T cells surpassing levels found in normal mice. These results illustrate the ability of the TH-SAg interaction to disrupt B and CD8+ T lymphocyte homeostasis.

Intercellular transfer of a soluble viral superantigen

Mouse mammary tumor virus (MMTV) superantigens (vSAgs) can undergo intercellular transfer in vivo and in vitro such that a vSAg can be presented to T cells by major histocompatibility complex (MHC) class II proteins on antigen-presenting cells (APCs) that do not express the superantigen. This process may allow T-cell activation to occur prior to viral infection. Consistent with these findings, vSAg produced by Chinese hamster ovary (CHO) cells was readily transferred to class II IE and IA (H-2(k) and H-2(d)) proteins on a B-cell lymphoma or mouse splenocytes. Fixed class II-expressing acceptor cells were used to demonstrate that the vSAg, but not the class II proteins, underwent intercellular transfer, indicating that vSAg binding to class II MHC could occur directly at the cell surface. Intercellular transfer also occurred efficiently to splenocytes from endogenous retrovirus-free mice, indicating that other proviral proteins were not involved. Presentation of vSAg7 produced by a class II-negative, furin protease-deficient CHO variant (FD11) was unsuccessful, indicating that proteolytic processing was a requisite event and that proteolytic activity could not be provided by an endoprotease on the acceptor APC. Furthermore, vSAg presentation was effected using cell-free supernatant from class II-negative, vSAg-positive cells, indicating that a soluble molecule, most likely produced by proteolytic processing, was sufficient to stimulate T cells. Because the membrane-proximal endoproteolytic cleavage site in the vSAg (residues 68 to 71) was not necessary for intercellular transfer, the data support the notion that the carboxy-terminal endoproteolytic cleavage product is an active vSAg moiety.

Failure to remove autoreactive Vbeta6+ T cells in Mls-1 newborn mice attributed to the delayed development of B cells in the thymus

Clonal deletion of autoreactive T cells in the thymus is one of the major mechanisms for establishing tolerance to self-antigens, and self-reactive T cells bearing Vbeta6 T-cell receptors are usually deleted before their maturation in Mls-1a mice. However, these T cells develop transiently in the neonatal thymus, and migrate to the periphery. In order to understand the mechanisms which permit these potentially auto-toxic T cells to generate, we investigated in vivo the physiological or functional properties of the elements involved, such as neonatal T cells, antigens and antigen-presenting cells (APC). Confirming the previous findings that each of these elements per se is already completed in function in neonates, we investigated the possibility of the absence or immaturity of particular APC with Mls antigens of their own products in the neonatal thymus. In the search for the cellular and histological changes occurring in the newborn thymus, we found that the elimination of Vbeta6+ T cells progressed in parallel with the development of thymic B cells. Involvement of B cells in purging the autoreactive T cells from the newborn thymus was shown by prevention of the deletion of Vbeta6+ T cells after the removal of B cells by the treatment of neonates with anti-immunoglobulin M antibodies. The restricted and stable expression of CD5 on the thymic B cells, but not on the splenic cells, suggests that these B cells are not postnatal immigrants from the periphery. Finally, it is concluded that the deficiency in the deletion of self-reactive T cells in the thymus of Mls-1a neonates is due to the delayed development of B cells.

DNA vaccination and the immune responsiveness of neonates

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

Alternative proteolytic processing of mouse mammary tumor virus superantigens

Mouse mammary tumor viruses express a superantigen essential for their life cycle. It has been proposed that viral superantigens (vSags) require processing by prohormone convertases (PCs) for activity. We now observe, using a panel of mutant forms of potential PC cleavage sites and in vitro cleavage assays, that only the CS1 (position 68 to 71) and CS2 (position 169 to 172) sites are utilized by furin and PC5. Other members of the convertase family that are expressed in lymphocytes are not endowed with this activity. Furthermore, mutant forms of two different viral superantigens, vSag7 and vSag9, which completely abrogated in vitro processing by convertases, were efficient in functional presentation to responsive T-cell hybridomas. This effect was observed in both endogenous presentation and paracrine transfer of the vSag. Processing by convertases thus appears not to be essential for vSag function. Finally, we have identified the purified endosomal protease cathepsin L as another protease that is able to cleave convertase mutant vSag in vitro, yielding fragments similar to those detected in vivo, thus suggesting that proteases other than convertases are involved in the activation of vSags.

Superantigens: mechanisms by which they may induce, exacerbate and control autoimmune diseases

Superantigens are polypeptide molecules produced by a broad range of infectious microorganisms which elicit excessive and toxic T-cell responses in mammalian hosts. In light of this property and the fact that autoimmune diseases are frequently the sequelae of microbial infections, it has been suggested that superantigens may be etiologic agents of autoreactive immunological responses resulting in initiation, exacerbation or relapse of autoimmune diseases. This article relates the biology of superantigens to possible mechanisms by which they may exert these activities and reviews the evidence for their roles in various human and animal models of autoimmune disease. Finally, a mechanism of active suppression by superantigen-activated CD4+ T-cells that could be exploited for therapy as well as prophylaxis of human autoimmune diseases is proposed.

Purified donor T cells alone activate transplantation immunity to the male antigen but induce tolerance in combination with Mac-1+ donor cells

BACKGROUND

In most experimental systems examined, "professional" antigen-presenting cells (APCs), such as dendritic cells, have been found to activate T cells, whereas "nonprofessional" antigen-bearing cells (nonAPC) may induce tolerance. Some recent studies have suggested that nonAPCs may under certain conditions prime a T-cell immune response. We have attempted to separate the roles of transplanted T cells and monocytic/dendritic cells in activating or tolerizing antigen-specific T cells in vivo, by examining the consequences of parenteral exposure to male antigen in anti-male TCR transgenic female mice.

METHODS

Qualitative and quantitative changes in the large population of male-reactive transgenic T cells to various male donor cell populations in transgenic female mice were followed after injections of highly purified male lymphoid cells. Changes in male-reactive T cells with time and the long-term outcome of male skin grafts were measured.

RESULTS

When a nonAPC population consisting of highly purified male T cells alone was injected intravenously into H-Y antigen-specific TCR transgenic female mice, the number of host transgenic T cells was sustainably increased, and male graft rejection was accelerated. Injection of a combination of purified T cells and purified Mac-l+ cells induced massive and permanent deletion of the host male-reactive T-cell population and permanent graft tolerance. Mac-l+ cells alone gave no appreciable change in responsive T cells or graft rejection times.

CONCLUSIONS

The data indicate that highly purified T cells engrafted alone induce rapid sensitization toward the male antigen. They also show that both male donor T cells and a population of male monocytic/ dendritic cells are required to induce peripheral tolerance toward this antigen and that this tolerance is related to permanent peripheral deletion of male-reactive T cells.

Intrathymic deletion of MHC class I-restricted cytotoxic T cell precursors by constitutive cross-presentation of exogenous antigen

Cross-priming of cytotoxic T lymphocytes by professional antigen-presenting cells (APC) is a potential hazard to self tolerance because it exposes naive T cells to tissue-specific self antigens in the context of co-stimulatory signals. Here we show that cross-presentation of exogenous material occurs constitutively within the thymus. Although efficient cross-presentation is a property of relatively few APC it results in thymocyte deletion both in vitro and in vivo, suggesting that intrathymic cross-presentation can operate as an effective component of tolerance to circulating self antigens. The capacity of minor cell populations to mediate thymocyte deletion but not positive selection reflects an underlying difference in the biology of these two processes.

Ontogeny of T cell tolerance to peripherally expressed antigens

Transgenic expression of the influenza virus hemagglutinin (HA) in the pancreatic islet beta cells of InsHA mice leads to peripheral tolerance of HA-specific T cells. To examine the onset of tolerance, InsHA mice were immunized with influenza virus A/PR/8 at different ages, and the presence of nontolerant T cells was determined by the induction of autoimmune diabetes. The data revealed a neonatal period wherein T cells were not tolerant and influenza virus infection led to HA-specific beta cell destruction and autoimmune diabetes. The ability to induce autoimmunity gradually waned, such that adult mice were profoundly tolerant to viral HA and were protected from diabetes. Because cross-presentation of islet antigens by professional antigen-presenting cells had been reported to induce peripheral tolerance, the temporal relationship between tolerance induction and activation of HA-specific T cells in the lymph nodes draining the pancreas was examined. In tolerant adult mice, but not in 1-week-old neonates, activation and proliferation of HA-specific CD8(+) T cells occurred in the pancreatic lymph nodes. Thus, lack of tolerance in the perinatal period correlated with lack of activation of antigen-specific CD8(+) T cells. This work provides evidence for the developmental regulation of peripheral tolerance induction.

Sphingolipids in atherosclerosis and vascular biology

Sphingolipids and their metabolic products are now known to have second-messenger functions in a variety of cellular signaling pathways. Lactosylceramide (LacCer), a glycosphingolipid (GSL) present in vascular cells such as endothelial cells, smooth muscle cells, macrophages, neutrophils, platelets, and monocytes, contributes to atherosclerosis. Large amounts of LacCer accumulate in fatty streaks, intimal plaque, and calcified intimal plaque, along with oxidized low density lipoproteins (Ox-LDLs), growth factors, and proinflammatory cytokines. A possible role for LacCer in vascular cell biology was suggested when this GSL was found to stimulate the proliferation in vitro of aortic smooth muscle cells (ASMCs). A further link of LacCer in atherosclerosis was uncovered by the finding that Ox-LDLs stimulated specifically the biosynthesis of LacCer. Ox-LDL-stimulated endogenous synthesis of LacCer by activation of UDP-Gal:GlcCer,beta1-4galtransferase (GalT-2) is an early step in this signaling pathway. In turn, LacCer serves as a lipid second messenger that orchestrates a signal transduction pathway, ultimately leading to cell proliferation. This signaling pathway includes LacCer-mediated activation of NADPH oxidase that produces superoxide. Such superoxide molecules stimulate the GTP loading of p21(ras). Subsequently, the kinase cascade (Raf-1, Mek2, and p44MAPK [mitogen-activated protein kinase]) is activated. The phosphorylated form of p44MAPK translocates from the cytoplasm to the nucleus and engages in c-fos expression, proliferating cell nuclear antigen (PCNA) such as cyclin activation, and cell proliferation takes place. Interestingly, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), an inhibitor of GalT-2, can abrogate the Ox-LDL-mediated activation of GalT-2, the signal kinase cascade noted above, as well as cell proliferation. Additional studies have revealed that LacCer mediates the tumor necrosis factor-alpha (TNF-alpha)-induced nuclear factor-kappaB expression and intercellular adhesion molecule (ICAM-1) expression in vascular endothelial cells via the redox-dependent transcriptional pathway. LacCer also stimulates the expression of CD11/CD8, or Mac-1, on the surface of human neutrophils. Collectively, this phenomenon may contribute to the adhesion of neutrophils or monocytes to the endothelial cell surface and thus initiate the process of atherosclerosis. In addition, the LacCer-mediated proliferation of ASMCs may contribute to the progression of atherosclerosis. On the other hand, programmed cell death (apoptosis) by proinflammatory cytokines such as TNF-alpha, interleukin-1, and high concentrations of Ox-LDL occur via activation of a cell membrane-associated neutral sphingomyelinase (N-SMase). N-SMase hydrolyzes sphingomyelin into ceramide and phosphocholine. In turn, ceramide or a homologue serves as an important stress-signaling molecule. Interestingly, an antibody against N-SMase can abrogate Ox-LDL- and TNF-alpha-induced apoptosis and therefore may be useful for in vivo studies of apoptosis in experimental animals. Because plaque stability is an integral aspect of atherosclerosis management, activation of N-SMase and subsequent apoptosis may be vital events in the onset of plaque rupture, stroke, or heart failure. Interestingly, in human liver cells, N-SMase action mediates the TNF-alpha-induced maturation of the sterol regulatory-element binding protein. Moreover, a cell-permeable ceramide can reconstitute the phenomenon above in a sterol-independent fashion. Such findings may provide new avenues for therapy for patients with atherosclerosis. The findings described here indicate an important role for sphingolipids in vascular biology and provide an exciting opportunity for further research in vascular disease and atherosclerosis.

Altered antigen presentation in mice lacking H2-O

HLA-DM catalyzes the release of MHC class II-associated invariant chain-derived peptides (CLIP) from class II molecules. Recent evidence has suggested that HLA-DO is a negative regulator of HLA-DM in B cells, but the physiological function of HLA-DO remains unclear. Analysis of antigen presentation by B cells from mice lacking H2-O (the mouse equivalent of HLA-DO), together with biochemical analysis using purified HLA-DO and HLA-DM molecules, suggests that HLA-DO/H2-O influences the peptide loading of class II molecules by limiting the pH range in which HLA-DM is active. This effect may serve to decrease the presentation of antigens internalized by fluid-phase endocytosis, thus concentrating the B cell-mediated antigen presentation to antigens internalized by membrane immunoglobulin.

Influence of graft versus host reaction on the T cell repertoire differentiating from bone marrow precursors following allogeneic bone marrow transplantation

When lethally irradiated AKR (Mls-1a) mice were reconstituted with bone marrow (BM) cells plus a small number (0.5%) of mature T cells from allogeneic B10.AQR or B10 (Mls-1b) mice and minor GVHR was induced in the recipients, almost complete donor chimerism was accomplished in the early stages after reconstitution. By contrast, in irradiated AKR mice reconstituted with T cell-depleted BM cells alone from B10 or B10.AQR mice, radio-resistant T cells of recipient origin persisted for a relatively long period in peripheral lymphoid tissues. In this paper the influence of residual T cells in the chimeric mice on generation of the T cell repertoire derived from donor BM is discussed. It will be demonstrated that the recipient (AKR) T cells are capable of producing Mls-1a antigens (Ag) after lethal irradiation in vivo. These recipient T cells eventually induce clonal elimination of Mls-1a reactive V beta 6+, V beta 8.1+ and V beta 9+ T cells derived from developing thymocytes of donor BM origin. The Mls-1a reactive T cells are not eliminated in GVHR chimeras in which recipient T cells are absent. However, V beta 5+ T cells reactive to I-E plus Etc-1 Ag are deleted in the chimeras undergoing GVHR. These results indicate that recipient cells which produce tissue-specific antigens (tolerogens) should be taken into consideration when generation of the T cell repertoire of donor origin following allogeneic BM transplantation is investigated.

Paracrine transfer of mouse mammary tumor virus superantigen

Transfer of vSAG7, the endogenous superantigen encoded in the Mtv7 locus, from MHC class II to MHC class II+ cells has been suggested to occur both in vivo and in vitro. This transfer usually leads to the activation and deletion of T cells expressing responsive V beta s. However, there is no direct molecular evidence for such a transfer. We have developed an in vitro system which confirms this property of vSAGs. vSAG7 was transfected into a class II murine fibroblastic line. Coculture of these cells with class II+ cells and murine T cell hybridomas expressing the specific V beta s led to high levels of IL-2 production which was specifically inhibited by vSAG7- and MHC class II-specific mAbs. Moreover, injection of vSAG7+ class II+ cells in mice led to expansion of V beta 6+ CD4+ cells. We show that this transfer activity is paracrine but does not require cell-to-cell contact. Indeed, vSAG7 was transferred across semi-permeable membranes. Transfer can occur both from class II+ and class II+ cells, indicating that MHC class II does not sequester vSAG7. Finally, competition experiments using bacterial toxins with well defined binding sites showed that the transferred vSAG7 fragment binds to the alpha 1 domain of HLA-DR.

Both activated and nonactivated leukocytes from the periphery continuously enter the thymic medulla of adult rats: phenotypes, sources and magnitude of traffic

Although the thymus is primarily noted for the export of T cells to the periphery, a small influx of cells has also been observed. It is still a matter of debate whether entry into the thymus depends on prior activation. The phenotypes, sources and degree of immigration are largely unknown. We monitored by quantitative immunohistochemistry the entry of cells from the periphery into the rat thymus in three experimental models. We injected i.v. recirculating, small, nonactivated CD4+ T cell subsets, often referred to as naive (CD45RC+) and memory or antigen-experienced (CD45RC-) cells, purified from thoracic duct lymph of allotype-marked donors, allotype-marked leukocytes released from spleen or lung transplants, or leukocytes labeled in the periphery for 12 weeks during the S-phase of the cell cycle by oral application of 5-bromo-2-deoxyuridine (BrdUrd). Early after i.v. injection (0.5 h), significantly more antigen-experienced (CD45RC-) CD4+ T cells entered the thymus, and by 24 h four times as many cells from the CD45RC- subset as from the CD45RC+ subset had entered the thymus and localized to the medulla. None of the thymic entrants expressed the interleukin (IL)-2 receptor. Following spleen transplantation approximately 40% of donor cells entering the thymic medulla were T cells and approximately 55% were B cells. In contrast, from a lung transplant, approximately 85% of peripheral immigrants were T cells and approximately 10% were B cells. After both procedures, a small number of NK cells and monocytes/macrophages were found among the immigrants (< 5%). Rats were fed BrdUrd continuously for 12 weeks, a procedure which labeled approximately 30% of peripheral lymphocytes but not cortical thymocytes. BrdUrd-labeled cells were localized almost exclusively to the thymic medulla and represented approximately 10% of medullary cells. Of the thymic immigrants approximately 50% were T cells, approximately 30% were B cells (including approximately 15% IgD+ cells), approximately 15% were NK cells and the remainder (approximately 5%) were monocytes/macrophages. Only a quarter of BrdUrd-labeled cells expressed the IL-2 receptor. The thymus is continuously infiltrated by both activated and nonactivated leukocytes from the periphery, including T cells, B cells, NK cells and monocytes. These immigrants are supplied by lymphoid and nonlymphoid organs in a characteristic subset composition. Their entry is facilitated by prior antigen experience or activation. Thus, the participation of the thymic medulla in general leukocyte traffic suggests a mechanism by which the T cell repertoire could potentially be modulated by the peripheral tissues.

Tracing interactions of thymocytes with individual stromal cell partners

Cellular interactions in T cell development can be analyzed using thymus chimeras prepared in vitro, in which stromal cells and T cell precursors are manipulated separately. In an earlier study, we showed that for optimal T cell maturation most--if not all--stromal cells must display appropriate (selecting) major histocompatibility complex (MHC) molecules: the substitution of selecting by nonselecting stromal cells leads to a proportional decrease in mature T cell production. These data imply that the availability of selecting stromal micro-environments is rate limiting for positive selection, and that in positive selection each thymocyte engages only one (rather than multiple) stromal cell partners. To test this hypothesis, we developed a tracing system for thymocyte/stromal cell interactions, based on the acquisition by thymocytes of stroma-derived MHC class II determinants. When MHC class II-deficient precursors are placed in H-2b x k F1 environments (where all stromal cells co-express H-2b and H-2k), individual thymocytes acquire class II determinants of both haplotypes. In striking contrast, when placed in mosaic stromal environments (where stromal cells express either H-2b or H-2k evenly interspersed), individual thymocytes preferentially acquire MHC class II determinants of one or the other haplotypes, but rarely both. This provides strong evidence that thymocytes have intimate interactions with individual stromal cells: having engaged one stromal cell niche, thymocytes do not (or only rarely) have promiscuous liaisons with others.

Antigen presentation and T cell development in H2-M-deficient mice

HLA-DM (DM) facilitates peptide loading of major histocompatibility complex class II molecules in human cell lines. Mice lacking functional H2-M, the mouse equivalent of DM, have normal amounts of class II molecules at the cell surface, but most of these are associated with invariant chain-derived CLIP peptides. These mice contain large numbers of CD4+ T cells, which is indicative of positive selection in the thymus. Their CD4+ cells were unresponsive to self H2-M-deficient antigen-presenting cells (APCs) but were hyperreactive to wild-type APCs. H2-M-deficient APCs failed to elicit proliferative responses from wild-type T cells.

The role of superantigens in virus infection

Murine mammary tumor viruses are retroviruses which encode superantigens capable of stimulating T cells via superantigen-reactive T-cell receptor V beta chains. Murine mammary tumor viruses are transmitted to the suckling offspring through the milk. We have established that B cell-deficient pups which were foster-nursed by virus-secreting mice do not transfer infectious murine mammary tumor viruses to their offspring. No murine mammary tumor virus proviruses could be detected in the spleen and mammary tissue of these mic. We conclude that B cells are essential for the completion of the viral life cycle in vivo. This indicates that B cells are infected first and that viral amplification takes place only if infected B cells present the murine mammary tumor virus superantigen on their surface, which, in turn, results in activation of T cells expressing the appropriate T-cell receptor B beta chains. These activated T cells secrete factors which stimulate B cells, enabling viral replication.

In vivo effects of superantigens

Superantigens are potent immunostimulatory molecules that activate both T cells and antigen presenting cells. The consequences of superantigen exposure range from induction of T cell proliferation, massive cytokine release and systemic shock to immunosuppression and tolerance. Superantigens have been directly implicated in a number of human conditions including food poisoning and toxic shock. In addition, there is evidence to suggest that superantigens are involved in the initiation of autoimmunity, and the immune dysfunction associated with HIV infection. Because of their possible role in human disease, and their potential use in immune therapy, it is important that we more completely understand the in vivo effects of superantigens.

Production of minor lymphocyte stimulatory-1a antigens from T cell subsets

T cell subsets that produce minor lymphocyte stimulatory (Mls) antigens were analyzed using mixed lymphocyte reaction (MLR) in vitro or clonal elimination assay in vivo. When lymph node T cells from B10.BR(Mls-1b) mice were stimulated with various T cell subsets from AKR (Mls-1a) mice in the presence of B10.BR antigen presenting cells (APC), proportions of Mls-1a reactive T cell blasts (V beta 6+, V beta 8.1+) increased. The stimulatory potency of CD8+ T cells was higher than that of CD4+ T cells. Furthermore, among either CD8+ or CD4+ T cell subset, CD44+ T cells appeared to produce larger amounts of Mls-1a antigens than CD44- T cells. More marked difference was demonstrated, when stimulator AKR T cells were being activated by immobilized anti-T cell antigen receptor (TCR) antibody during MLR. Thus, AKR T cells appeared to produce large amounts of Mls-1a antigens on appropriate stimulations. These findings were confirmed by the semiquantitative analysis of mRNA levels of MTV-7 in the AKR T cell subsets. When CD8+CD44+ T cells from (AKR x B10.BR)F1 mice were injected intravenously into [B10.BR-->B10.BR] syngeneic bone marrow (BM) chimeras 1 week after BM reconstitution and proportions of V beta 6+ T cells were quantitated 7 weeks later, significant clonal elimination of V beta 6+ T cells was induced among both thymocyte population and lymph node T cell population in a dose-dependent manner of the inoculated F1 T cells. Inoculation of CD8+CD44-F1 T cells eliminated V beta 6+ T cells less efficiently from lymph node T cells and inoculation of CD4+F1 T cells induced no significant clonal elimination of the V beta 6+ T cells. The present findings demonstrate clearly that CD8+CD44+ T cells represent the cells producing large amounts of Mls-1a antigens and inducing clonal elimination of V beta 6+ T cells in vivo.

A quantitative analysis of antigen-presenting cell function: activated B cells stimulate naive CD4 T cells but are inferior to dendritic cells in providing costimulation

Ligation of CD28 on CD4 Th1 clones and freshly isolated mixtures of naive and memory CD4 T cells triggered their T cell receptors (TCR) is sufficient to induce the costimulatory signals necessary for interleukin 2 (IL-2) production by these cells. CTLA-4-reactive ligands expressed on antigen-presenting cells (APC) are critical in providing costimulatory signals to these T cell populations. We demonstrate that these activation characteristics apply equally to purified naive CD4 T cells. Because B cell blasts express CTLA-4-reactive ligands and high levels of adhesion and major histocompatibility complex class II molecules, they would be expected to engage both the TCR and CD28 and consequently stimulate IL-2 production by naive CD4 T cells. Using purified populations of cells in limiting dilution cultures, we have carried out a quantitative analysis of the interaction between naive CD4 T cells and either activated B or dendritic cells. We demonstrate that B cell blasts stimulate a high frequency of naive CD4 T cells. Slight differences in TCR signaling efficiency between the two APC types were observed. Even at optimal peptide concentrations, however, the amount of IL-2 made by individual T cells was fourfold lower in response to B cell blasts than to dendritic cells. This relative deficiency of activated B cells was due to their inability to optimally costimulate naive CD4 T cells.

B cells are essential for murine mammary tumor virus transmission, but not for presentation of endogenous superantigens

Murine mammary tumor viruses (MMTVs) are retroviruses that encode superantigens capable of stimulating T cells via superantigen-reactive T cell receptor V beta chains. MMTVs are transmitted to the suckling offspring through milk. Here we show that B cell-deficient mice foster nursed by virus-secreting mice do not transfer infectious MMTVs to their offspring. No MMTV proviruses could be detected in the spleen and mammary tissue of these mice, and no deletion of MMTV superantigen-reactive T cells occurred. By contrast, T cell deletion and positive selection due to endogenous MMTV superantigens occurred in B cell-deficient mice. We conclude that B cells are essential for the completion of the viral life cycle in vivo, but that endogenous MMTV superantigens can be presented by cell types other than B cells.

Functional studies on MEL-14+ and MEL-14- T cells in peripheral lymphoid tissues

Functions of MEL-14+ T cells and MEL-14- T cells in peripheral lymphoid tissues were analyzed and compared. The MEL-14- T cells, representing a minor subpopulation of spleen and lymph node T cells, generated considerably higher mixed lymphocyte reaction and mitogen responses than the MEL-14+ T cells in any lymphoid tissues studied. Furthermore, upon stimulation with ConA the MEL-14- CD8+ T cells produced significantly larger amounts of IL-2 and IFN-gamma than MEL-14+ CD8+ T cells did. A similar but less marked observation was obtained with the CD4+ T cell population. Furthermore, when B10.BR mice were immunized with AKR (Mls-1a) spleen cells, the proportion of the Mls-1a reactive V beta 6+ T cells from draining lymph nodes increased and a substantial proportion of the increasing V beta 6+ T cells was shown to be MEL-14-. The present findings on the whole indicate that MEL-14- T cells in the peripheral lymphoid tissues are at functionally high levels and may represent memory cells which have been previously stimulated in vivo.

Development of T cells in SCID mice grafted with fetal thymus from AKR mice or F344 rats

To examine the development of T cells within an allogeneic or xenogeneic environment, we engrafted the fetal thymus from AKR mice or F344 rats under the kidney capsule of SCID mice (mTG and rTG mice). T lymphopoiesis developed in SCID mice 2 months after transplantation, although the ratio of CD4/CD8 in both experimental groups was different from that of normal control. T cells in mTG mice did not show in vitro proliferation or cytotoxicity against either host-type C.B-17 (H-2d) or donor-type AKR (H-2k) cells, while they exerted potent activities against third-party B10 (H-2b) cells. In contrast, T cells in rTG mice exhibited proliferation against both host-type C.B-17 and donor-type F344 rat cells. Consistently, graft-vs.-host disease symptoms developed in these mice and histological examination showed impressive infiltration of lymphocytes into the skin or into the mucosal layers of the stomach. Activated state of T cells in rTG mice was also evidence by the positive expression of interleukin-2 receptor. Taken together, fetal thymus appears to contain progenitor cells which are sufficient for in vivo reconstitution of T lymphopoiesis, but species-specific environment is important for the induction of tolerance. In mTG mice, V beta 6+ T cells reactive to donor Mlsa determinants and V beta 3+ T cells reactive to host Mlsc determinants were deleted, suggesting that tolerance was regulated mainly by clonal deletion. By contrast, V beta 11+ T cells reactive to Mlsf determinants were not deleted possibly due to the lack of their ligands.

Influence of a small number of mature T cells in donor bone marrow inocula on reconstitution of lymphoid tissues and negative selection of a T cell repertoire in the recipient

Allo-chimerism and clonal elimination of self antigen (Ag) (Ia + Mls-1a) reactive V beta 6+ T cells were analyzed and compared between allogeneic bone marrow (BM) chimeras reconstituted with BM cells which had been treated with anti-Thy-1 monoclonal antibody (mAb) plus complement (C) (T- chimeras) and BM chimeras which had been reconstituted with BM cells pretreated with anti-Thy-1 mAb alone (T+ chimeras). When lethally irradiated AKR (Mls-1a) mice were reconstituted with BM cells from B10 or B10 H-2 congenic mice, both T+ and T- chimeras were entirely free of signs of graft-versus-host reaction (GVHR). However, complete replacement of the AKR lymphoid tissues by donor BM cells was accomplished at an early stage in T+ chimeras but not in T- chimeras. On the other hand, clonal elimination of V beta 6+ T cells reactive to the recipient Ag (Mls-1a) was abolished in T+ chimeras but successfully induced in T- chimeras. The V beta 6+ T cells not eliminated in T+ chimeras showed depressed responses against Mls-1a antigens. The findings herein demonstrate that T cells which contaminate a BM inoculum survive in recipient mice after treatment with anti-Thy-1 mAb without C in vitro followed by BMT. The surviving T cells have been estimated to represent fewer than 0.5% of the BM cells inoculated. These cells appear to accelerate the full replacement of recipient lymphoid tissues by donor cells. Furthermore, the T cells which survive in the marrow inoculum influence eventually the development of a tolerant state in the T cell repertoire of the donor.

Autoimmune syndrome after induction of neonatal tolerance to I-E antigens

Neonatal injection of semiallogeneic spleen cells induces a state of specific tolerance to the parental alloantigens, but also the development of an autoimmune syndrome known as host-versus-graft disease (HVGD). The autoimmune features are a consequence of the allogeneic cooperation between persisting alloreactive host T helper type 2 (TH2) cells and donor semiallogeneic B cells. It has been established that I-A alloantigens play a central role in the triggering of this HVGD. Here it was investigated if I-E antigens, which have shown functional differences, regarding autoimmunity and alloreactivity, with respect to I-A antigens, are also able to trigger this autoimmune syndrome. The injection of spleen cells from [B10.A(4R) x B10.A(2R)]F1 (I-E+) hybrid mice into newborn B10.A(4R) (I-E-) mice was accompanied by the establishment of chimerism and also by the development of a characteristic, but moderated, HVGD. The weak intensity of this HVGD is likely due to the moderation of the alloreactive responses induced against I-E molecules. Moreover, the marked increase in the levels of IgE and in the titers of anti-DNA IgG1 antibodies strongly suggest that alloreactive TH2 cells play also a main role in the autoimmune syndrome following tolerization to I-E antigens. Therefore, it is concluded that the I-E and I-A isotypes are functionally similar with respect to the allogeneic cellular interactions that account for the HVGD.

The role of thymic epithelium in the establishment of transplantation tolerance

From experimental observations on induction of transplantation tolerance, we discuss a model that accounts for tissue-specific tolerance to antigens not expressed inside the thymus. It is postulated that antigens presented to differentiating T cells by thymic epithelium (or at large within the thymic environment) positively select and activate self-reactive T cells. A developmental program and/or prevalent conditions in the thymic environment restrict the proliferative potential and the class of effector functions that can be exerted by differentiating T cells activated in the thymus. These do not mediate inflammatory or cytolytic activities, but instead will produce the appropriate mediators to inhibit aggressive effector activities by other T cells activated in their proximity. Such "regulatory" functions will be locally expressed at the periphery upon recognition of tissue antigens shared with the thymus, towards newly formed thymic emigrants directed at tissue-specific antigens expressed by the same "target" cells. This mechanism imposes "dominant tolerance", based on specific self-recognition and predominantly established in the embryonic and neonatal period. Throughout life, the process of thymic positive selection results in all newly-formed T cells being susceptible to such suppressive mechanisms, but becoming increasingly refractory with time in the resting, post-differentiative stage. Absence of antigen (nonself) in the embryonic and neonatal life therefore allows for the accumulation of such "suppression-resistant" antigen-reactive T cells that will mount aggressive responses upon antigenic exposure. Tolerance or immunity thus represent two classes of specific immune responses, the relative predominance of which is determined by the frequency of each type of effector T cell, representing the antigenic overlap between thymic and peripheral tissues, as well as the frequency of tissue-specific T-cell generation, and the kinetics of peripheral antigenic exposure. Tolerance induced by hemopoietic cells to all other tissues is also "dominant" and based on thymic colonization and persistence of antigenic cells, with the consequent positive selection of regulatory T cells and peripheral conditions for the establishment of suppression. Upon this simple model, that ensures "interclonal class regulation" by "bridging" regulatory and effector T cells through the recognition of different antigens on the same target cell, other mechanisms which are based on V-region interactions among T cells (Ben-Nun et al. 1981, Pereira et al. 1989, Webb & Sprent 1990, Gaur et al. 1993) might well operate to ensure "dominant tolerance" by self-reactivity and class regulation.(ABSTRACT TRUNCATED AT 400 WORDS)