Identification of a previously unknown antigen-specific regulatory T cell and its mechanism of suppression

Targeting LFA-1 and cd154 suppresses the in vivo activation and development of cytolytic (cd4-Independent) CD8+ T cells

Short-term immunotherapy targeting both LFA-1 and CD40/CD154 costimulation produces synergistic effects such that long-term allograft survival is achieved in the majority of recipients. This immunotherapeutic strategy has been reported to induce the development of CD4+ regulatory T cells. In the current study, the mechanisms by which this immunotherapeutic strategy prevents CD8+ T cell-dependent hepatocyte rejection in CD4 knockout mice were examined. Combined blockade of LFA-1 and CD40/CD154 costimulation did not influence the overall number or composition of inflammatory cells infiltrating the liver where transplanted hepatocytes engraft. Expression of T cell activation markers CD43, CD69, and adhesion molecule CD103 by liver-infiltrating cells was suppressed in treated mice with long-term hepatocellular allograft survival compared to liver-infiltrating cells of untreated rejector mice. Short-term immunotherapy with anti-LFA-1 and anti-CD154 mAb also abrogated the in vivo development of alloreactive CD8+ cytotoxic T cell effectors. Treated mice with long-term hepatocyte allograft survival did not reject hepatocellular allografts despite adoptive transfer of naive CD8+ T cells. Unexpectedly, treated mice with long-term hepatocellular allograft survival demonstrated prominent donor-reactive delayed-type hypersensitivity responses, which were increased in comparison to untreated hepatocyte rejectors. Collectively, these findings support the conclusion that short-term immunotherapy with anti-LFA-1 and anti-CD154 mAbs induces long-term survival of hepatocellular allografts by interfering with CD8+ T cell activation and development of CTL effector function. In addition, these recipients with long-term hepatocellular allograft acceptance show evidence of immunoregulation which is not due to immune deletion or ignorance and is associated with early development of a novel CD8+CD25high cell population in the liver.

Low-avidity CD8lo T cells induced by incomplete antigen stimulationin vivo regulate naive higher avidity CD8hi T cell responses to the same antigen

We have previously reported that multiple injections of soluble MHC class I tetramers assembled with wild-type HY peptide induces unresponsiveness to male skin grafts in naive female C57BL/6 (B6) mice. Induction of unresponsiveness is dependent on a population of unresponsive allospecific CD8(lo )T cells. Reduced expression of CD8 acts to limit a T cell response to HY peptide by limiting the avidity window of effective signal transduction. We and others have demonstrated that CD8(lo) T cells are an alternative stable phenotype of CD8alphabeta(+) T cells in vitro and in vivo after antigen stimulation. We show here that CD8(lo) T cells can suppress naive CD8(+) T cell responses to HY antigen in vitro and male skin graft rejection in vivo after adoptive transfer into female recipients. These novel regulatory T cells express surface TGF-beta1 and secrete T cytotoxic 2 cytokines after antigen-specific stimulation. Anti-TGF-beta antibody and latency-associated peptide inhibit the suppressive effects in vitro. We also show that HY-specific memory CD8(+) T cells overcome regulation by CD8(lo) T cells. These data define a novel peripheral regulatory CD8(+ )T cell population that arises after repeated antigen encounter in vivo. These cells have implications in the maintenance of tolerance and memory.

Transplant Tolerance: Converging on a Moving Target

Enthusiasm for tolerance induction has been tempered by the realization that it is more difficult to achieve clinically than was predicted by experimental models. Unlike the view that the immune response to an allograft is ordered and thus predictable, we view alloimmunity as highly plastic and molded by previous and ongoing experiences with allogeneic and environmental antigens. This implies that an individual's response to an allograft changes over time and that responses of seemingly similar individuals may vary greatly. This variability highlights the need to develop assays for monitoring the recipient immune response as well as individualized methods for therapeutic immune modulation.

CD4+CD25+ Regulatory T Cells in Hematopoietic Stem Cell Transplantation

Allogeneic hematopoietic stem cell transplantation (SCT) is a well-established treatment modality for malignant and nonmalignant hematologic diseases. High-dose radio- and/or chemotherapy eradicate the hematopoietic system of the patient and induce sufficient immunosuppression to enable donor stem cell engraftment. The replacement of the recipient's immune system with that of the donor significantly contributes to the success of this treatment, since donor immune cells facilitate stem cell engraftment, provide protection from infections, and eliminate residual malignant or nonmalignant host hematopoiesis, thereby protecting from disease relapse in patients transplanted for leukemia or lymphoma (graft-versus-leukemia effect, GVL). Mediators of these beneficial effects are mature T cells within the stem cell graft. However, donor T cells can also attack host tissues and induce a life-threatening syndrome called graft-versus-host disease (GVHD). The challenge of allogeneic SCT is to find a balance between beneficial and harmful T cell effects, which at present is only insufficiently achieved by the use of immunosuppressive drugs. In the future, it might be possible to replace or support such medications by using the intrinsic regulatory capacity of the transplanted immune system, as represented by T cell subpopulations with suppressive activity, such as CD4+ CD25+ regulatory T (Treg) cells. In various mouse model systems, these cells have been shown to suppress GVHD while preserving the GVL effect. As the characterization of their human counterparts is rapidly progressing, their application in allogeneic SCT might soon be explored in clinical trials.

Molecular signature of recent thymic selection events on effector and regulatory CD4+ T lymphocytes

Natural CD4+CD25+ regulatory T lymphocytes (Treg) are key protagonists in the induction and maintenance of peripheral T cell tolerance. Their thymic origin and biased repertoire continue to raise important questions about the signals that mediate their development. We validated analysis of MHC class II capture by developing thymocytes from thymic stroma as a tool to study quantitative and qualitative aspects of the cellular interactions involved in thymic T cell development and used it to analyze Treg differentiation in wild-type mice. Our data indicate that APCs of bone marrow origin, but, surprisingly and importantly, not thymic epithelial cells, induce significant negative selection among the very autoreactive Treg precursors. This fundamental difference between thymic development of regulatory and effector T lymphocytes leads to the development of a Treg repertoire enriched in cells specific for a selected subpopulation of self-Ags, i.e., those specifically expressed by thymic epithelial cells.

Epithelial proliferation induces novel changes in APC expression

The role of wild-type adenomatous polyposis coli (APC) protein in native epithelia is poorly understood. The present study examined the relationships between wild-type APC and beta-catenin expression in an established model of hyperproliferation, transmissible murine colonic hyperplasia (TMCH). Distal colonic crypts isolated from normal or TMCH mice were: (i) fractionated into cytosolic and nuclear components for Western blotting and immunoprecipitation (IP), (ii) extracted for total RNA isolation for Northern blotting and, (iii) analysed immunohistochemically by confocal microscopy. Western blots performed sequentially through day 12 TMCH with N-terminal APC antibodies revealed increased abundance of approximately 312 kDa (p312) protein by day 6 (4.0 +/- 0.75-fold, n = 6) that peaked by day 9, before declining by day 12. A approximately 130 kDa (p130) band appeared at day 9 and increased by day 12 (1.5 +/- 0.11-fold, n = 6). A C-terminal antibody detected only p312. APC mRNA level did not change during TMCH and appearance of p130 was not due to alternative splicing. Co-IP with N-terminal anti-APC antibodies, revealed APC's association with beta-catenin both at day 6 and day 12. p130, but not p312, associated predominantly with beta-catenin at day 12 during co-IP with anti-beta-catenin. p130 also selectively accumulated in the nucleus, bound to nuclear beta-catenin at day 12. Immunocytochemistry with N-terminal antibodies revealed an increasing crypt base : surface gradient of APC within the apical pole/apical-lateral membranes at day 6. At day 12, intense apical/cytoplasmic and occasional nuclear staining along the longitudinal crypt axis was observed. Full-length APC increases during epithelial hyperproliferation and may represent a homoeostatic response. The dramatic increase in cytoplasmic and sporadic nuclear APC staining at day 12 with N-terminal antibodies may represent p130. The nuclear accumulation of p130 may be a novel mechanism regulating nuclear beta-catenin function during TMCH.

Characterization of gene expression profiles of T cells during anti-tumor response

BACKGROUND AND AIMS

T cells of tumor-bearing mice or cancer patients exhibit an immune dysfunction, enabling the tumor to escape immune surveillance.

METHODS

The experiments are based on EL4 thymoma cells that were transfected with costimulatory ligands B7-1, B7-2, or both at the same time. We used oligonucleotide-based DNA chip microarrays to characterize the genomic expression profile of peripheral T cells according to their anti-tumor immune response in vivo. These murine T cells were also characterized by ELISA, FACS analysis, and co-stimulatory assays.

RESULTS

Using commonly established methods, such as FACS analysis or the analysis of the cytokine profile by ELISA, it was not possible to determine functional differences in the in vivo activity of T lymphocytes against tumor cells. EL4 tumor cells induced multiple anti-tumor immune responses in vivo depending on their B7 expression. We successfully used microarray analysis to identify genes that were differentially expressed in the dysfunctional T cells, which were unable to reject tumors in vivo. Although Th1 and Th2 cytokine expression was not affected, we observed differential expression of genes involved in the regulation of an innate immune response.

CONCLUSION

Our results provide evidence that the anti-tumor response can be identified by the "gene profile" of T cells. Genomic scale analysis offers the opportunity to identify subtle changes in gene expression in T cells reflecting a distinct biological behavior in vivo.

CD4-veCD8-ve CD30+ve T cells are detectable in human lung transplant patients and their proportion of the lymphocyte population after in vitro stimulation with donor spleen cells correlates with preservation of lung physiology

INTRODUCTION

Survival following lung transplantation is less than 50% at 5 years, mainly due to immune-mediated chronic rejection. Recently a novel subset of T cells, CD4-veCD8-ve CD30+ve, so-called double negative (DN) CD30+ve T cells, has been described and shown to be responsible for tolerance in an animal model of skin transplantation.

METHODS

We investigated 18 lung transplant recipients for the presence of DN CD30+ve T cells in resting peripheral blood and also following in vitro stimulation of recipient peripheral blood mononuclear cells (PBMCs) with donor spleen cells.

RESULTS

Small percentages (0.2% to 6%) of DN T cells are detectable in resting PBMCs of human transplant patients (n = 18), but these did not correlate with allograft function, acute rejection episodes, HLA mismatch, or CMV status. On repeated stimulation of recipient PBMCs (two exposures) in vitro by donor spleen cells (2:1 ratio stimulators to responders) the percentage of DN CD30+ve T cells within the lymphocyte pool correlated with preservation of allograft lung function (both for FEV(1), P = .009, and FEF(25-75), P = .036) and was inversely correlated with grade of chronic rejection. On repeated exposure of recipient PBMCs to donor spleen cells with a 1:1 ratio the percentage of DN CD30+ve T cells correlated with the number of acute rejection episodes of grade 2 or greater. The total number of HLA mismatches correlated with the percentage DN CD30+ve T cells present after primary stimulation of recipient PBMCs with donor spleen cells (1:1 ratio). The number of mismatches at the B locus inversely correlated with the percentage of DN CD30+ve T cells after primary stimulation of recipient PBMCs with donor spleen cells (1:1 ratio; P = .031, n = 18).

CONCLUSION

Percentages of DN CD30+ve T cells present following repeated stimulation of recipient PBMCs by donor spleen cells correlated with preservation of graft function following lung transplantation.

Overlap between molecular markers expressed by naturally occurring CD4+CD25+ regulatory T cells and antigen specific CD4+CD25+ and CD8+CD28- T suppressor cells

Alloantigen specific CD8+CD28- T suppressor (TS) cells differ from naturally occurring CD4+CD25+ T-regulatory (natural TR) cells not only by their phenotype but also by their mechanism of action. Natural TR have been extensively studied, leading to the identification of characteristic "molecular markers" such as Forkhead box P3 (FOXP3), glucocorticoid-induced tumor necrosis factor receptor-related protein (GITR) and cytotoxic T lymphocyte-associated antigen 4 (CTLA-4). We have investigated the expression of these genes in alloantigen specific TS and CD4+CD25+ T regulatory (TR) cells and found that they are expressed at levels similar to those observed in natural TR. Furthermore, similar to natural CD4+CD25+ TR, antigen-specific CD8+CD28-CD62L+ TS cells have more suppressive capacity than CD8+CD28-CD62L- TS cells. In spite of these similarities, natural TR are not antigen-specific and inhibit other T cells by T cell-to-T cell interaction, whereas TS are antigen-specific and exert their inhibitory function by interacting with antigen-presenting cells and render them tolerogenic to other T cells. The molecular characterization of TS cells may contribute to a better understanding of mechanisms involved in inhibition of immune responses in autoimmunity, transplantation, and chronic viral infection.

Early Presence of Regulatory Cells in Transplanted Rats Rendered Tolerant by Donor-Specific Blood Transfusion

Mechanisms by which donor-specific blood transfusion (DSBT) promotes organ allograft acceptance are unclear. In a rat fully mismatched cardiac allograft model, we found that DSBT alone (without immunotherapy) induces the development of regulatory T cells (DSBT-Tregs) posttransplant, thereby shedding new light in the mechanisms of the transfusion effect. Compartments and timing of expansion, requirements, and phenotype of DSBT-Tregs are unknown. It is generally assumed that some time is necessary before Tregs develop. However, we show-by adoptive transfer from DSBT-tolerant into naive recipients: 1) the presence of DSBT-Tregs at 5 days posttransplant in spleen and lymph nodes; 2) their gradual expansion in these compartments; and 3) their presence in the graft 14 of 30 days posttransplant. DSBT-Tregs are donor specific and do not protect third-party allografts. Splenocytes from DSBT-treated nontransplanted recipients or from transplanted DSBT-untreated (rejecting) recipients do not transfer tolerance, indicating that both DSBT and graft are required for sufficient numbers of DSBT-Tregs to develop. Thymectomy (or splenectomy) before DSBT (not at transplantation) abrogate DSBT-Tregs generation and tolerance, showing that thymus (and spleen) are required for DSBT-Tregs generation (not for expansion/maintenance). In contrast with other Tregs models, DSBT-Tregs activity is not restricted to CD4(+)CD25(+) but to CD4(+)CD45RC(-) cells, whereas CD4(+)CD45RC(+) cells act as effector cells and accelerate rejection. In conclusion, DSBT alone induces-rapidly posttransplant-the development of alloantigen-specific Tregs in lymphoid tissues and in the graft. DSBT, graft, thymus, and spleen are required for DSBT-Tregs generation. DSBT-Tregs in this model are CD4(+)CD45RC(-) (identical to Tregs protecting from autoimmunity in rats).

CD4-CD8- T cells control intracellular bacterial infections both in vitro and in vivo

Memory T cells, including the well-known CD4⁺ and CD8⁺ T cells, are central components of the acquired immune system and are the basis for successful vaccination. After infection, CD4⁺ and CD8⁺ T cells expand into effector cells, and then differentiate into long-lived memory cells. We show that a rare population of CD4⁻CD8⁻CD3⁺αβ⁺γδ⁻NK1.1⁻ T cells has similar functions. These cells potently and specifically inhibit the growth of the intracellular bacteria Mycobacterium tuberculosis (M. tb.) or Francisella tularensis Live Vaccine Strain (LVS) in macrophages in vitro, promote survival of mice infected with these organisms in vivo, and adoptively transfer immunity to F. tularensis LVS. Furthermore, these cells expand in the spleens of mice infected with M. tb. or F. tularensis LVS, and then acquire a memory cell phenotype. Thus, CD4⁻CD8⁻ T cells have a role in the control of intracellular infection and may contribute to successful vaccination.

Donor Lymphocyte Infusion Induces Long-Term Donor-Specific Cardiac Xenograft Survival through Activation of Recipient Double-Negative Regulatory T Cells

Previous studies have shown that pretransplant donor lymphocyte infusion (DLI) can enhance xenograft survival. However, the mechanism by which DLI induces xenograft survival remains obscure. Using T cell subset-deficient mice as recipients we show that CD4+, but not CD8+, T cells are necessary to mediate the rejection of concordant cardiac xenografts. Adoptive transfer of naive CD4+ T cells induces rejection of accepted cardiac xenografts in CD4-/- mice. This rejection can be prevented by pretransplant DLI in the absence of any other treatment. Furthermore, we demonstrate that DLI activates alphabeta-TCR+CD3+CD4-CD8- double-negative (DN) regulatory T (Treg) cells in xenograft recipients, and that DLI-activated DN Treg cells can inhibit the proliferation of donor-specific xenoreactive CD4+ T cells in vitro. More importantly, adoptive transfer of DLI-activated DN Treg cells from xenograft recipients can suppress the proliferation of xenoreactive CD4+ T cells and their ability to produce IL-2 and IFN-gamma in vivo. Adoptive transfer of DLI-activated DN Treg cells also prevents CD4+ T cell-mediated cardiac xenograft rejection in an Ag-specific fashion. These data provide direct evidence that DLI can activate recipient DN Treg cells, which can induce donor-specific long-term cardiac xenograft survival by suppressing the proliferation and function of donor-specific CD4+ T cells in vivo.

Induction of transplantation tolerance—the potential of regulatory T cells

Solid organ transplantation is widely accepted as an effective treatment for end organ failure. Although the treatment with immunosuppressive drugs has undoubtedly greatly improved graft survival, chronic rejection still has considerable impact on long term outcome. This, together with the undesirable side effects associated with life long treatment with immunosuppressive drugs, have significant implications for long term outcomes. In a small number of patients, drug non-compliance as well as controlled reduction or removal of maintenance immune suppressive drug therapy has led to the uncovering of a tolerant state. The challenge of achieving improved monitoring of all transplant patients may allow tailoring of immunosupression in a proportion of recipients thereby increasing the opportunities for the induction of specific unresponsiveness to donor alloantigens in the future. The immune system using several mechanisms to both induce and maintain tolerance to alloantigens, including the deletion of allo-reactive T cells, the induction of anergy, clonal exhaustion, ignorance and active suppression (immunoregulation) of allo-responses. A minor subpopulation of CD4+ T cells, regulatory or suppressor CD4+ T cells that co-express the cell-surface molecule CD25 (IL2 alpha subunit) at a high level may play a major role in the maintenance of specific unresponsiveness and operational tolerance to donor antigens in vivo. Intensive investigation of these cells in recent years has started to uncover the mechanisms of active suppression by regulatory T cells in this setting.

T cell activation correlates with an increasedproportion of antigen among the materials acquiredfrom target cells

We have investigated the density of peptides required to elicit different biological responses in cytotoxic T lymphocytes (CTL), including trogocytosis (i.e., the phenomenon whereby the lymphocytes actively capture fragments of plasma membrane from those cells with which they establish an immune synapse). We have used two separate mouse models of CTL recognising defined peptides presented by MHC class I molecules. In both systems, triggering of cytotoxicity and capture of membrane components reached saturation with low densities of ligand. On the other hand, down-modulation of cell-surface levels of TCR, induction of IFN-gamma production and detection of peptide captured required much higher ligand densities. Interestingly, fratricide (i.e., killing between CTL sharing the same specificity), a mechanism proposed to account for CTL exhaustion, was detected only at antigen concentrations still well above that second threshold leading to full blown activation. Taken together, our results show that the different thresholds that govern the elicitation of different CTL functions correlate with different proportions of antigen among the target cell components being captured via trogocytosis.

Regulatory T cells suppress in vitro proliferation of virus-specific CD8+ T cells during persistent hepatitis C virus infection

The basis of chronic infection following exposure to hepatitis C virus (HCV) infection is unexplained. One factor may be the low frequency and immature phenotype of virus-specific CD8(+) T cells. The role of CD4(+)CD25(+) T regulatory (T(reg)) cells in priming and expanding virus-specific CD8(+) T cells was investigated. Twenty HLA-A2-positive patients with persistent HCV infection and 46 healthy controls were studied. Virus-specific CD8(+) T-cell proliferation and gamma interferon (IFN-gamma) frequency were analyzed with/without depletion of T(reg) cells, using peptides derived from HCV, Epstein-Barr virus (EBV), and cytomegalovirus (CMV). CD4(+)CD25(+) T(reg) cells inhibited anti-CD3/CD28 CD8(+) T-cell proliferation and perforin expression. Depletion of CD4(+)CD25(+) T(reg) cells from chronic HCV patients in vitro increased HCV and EBV peptide-driven expansion (P = 0.0005 and P = 0.002, respectively) and also the number of HCV- and EBV-specific IFN-gamma-expressing CD8(+) T cells. Although stimulated CD8(+) T cells expressed receptors for transforming growth factor beta and interleukin-10, the presence of antibody to transforming growth factor beta and interleukin-10 had no effect on the suppressive effect of CD4(+)CD25(+) regulatory T cells on CD8(+) T-cell proliferation. In conclusion, marked CD4(+)CD25(+) regulatory T-cell activity is present in patients with chronic HCV infection, which may contribute to weak HCV-specific CD8(+) T-cell responses and viral persistence.

The Pathophysiology of Acute Graft-Versus-Host Disease

Despite improvements in allogeneic stem cell transplantation, acute graft-versus-host disease (GVHD) remains a significant problem after transplantation, and it is still a major cause of post-transplant mortality. Disease progression is characterized by the differentiation of alloreactive T cells to effector cells leading to tissue damage, recruitment of additional inflammatory cell populations and further cytokine dysregulation. To make the complex process of acute GVHD more explicit, the pathophysiology of acute GVHD is often divided into three different phases. This review summarizes the mechanisms involved in the three phases of acute GVHD.

Expression profiling of murine double-negative regulatory T cells suggest mechanisms for prolonged cardiac allograft survival

Recent studies have demonstrated that both mouse and human alpha beta TCR(+)CD3(+)NK1.1(-)CD4(-)CD8- double-negative regulatory T (DN Treg) cells can suppress Ag-specific immune responses mediated by CD8+ and CD4+ T cells. To identify molecules involved in DN Treg cell function, we generated a panel of murine DN Treg clones, which specifically kill activated syngeneic CD8+ T cells. Through serial cultivation of DN Treg clones, mutant clones arose that lost regulatory capacity in vitro and in vivo. Although all allogeneic cardiac grafts in animals preinfused with tolerant CD4/CD8 negative 12 DN Treg clones survived over 100 days, allograft survival is unchanged following infusion of mutant clones (19.5 +/- 11.1 days) compared with untreated controls (22.8 +/- 10.5 days; p < 0.001). Global gene expression differences between functional DN Treg cells and nonfunctional mutants were compared. We found 1099 differentially expressed genes (q < 0.025%), suggesting increased cell proliferation and survival, immune regulation, and chemotaxis, together with decreased expression of genes for Ag presentation, apoptosis, and protein phosphatases involved in signal transduction. Expression of 33 overexpressed and 24 underexpressed genes were confirmed using quantitative real-time PCR. Protein expression of several genes, including Fc epsilon RI gamma subunit and CXCR5, which are >50-fold higher, was also confirmed using FACS. These findings shed light on the mechanisms by which DN Treg cells down-regulate immune responses and prolong cardiac allograft survival.

[Immunological tolerance and liver transplantation]

The induction of tolerance to allografts has traditionally been one of the basic aims of transplantation research. Multiple data obtained in experimental models indicate that the outcome of transplantation (rejection versus acceptance/tolerance) depends on the balance between allo-reactive cytopathic lymphocytes and immunoregulatory lymphocytes. Thus, most tolerance-inducing treatments aim to reduce the number of allo-aggressive lymphocytes and, at the same time, to increase the population of regulatory lymphocytes, which ensure graft viability once drug therapy has been withdrawn. Liver allografts are singular in that they are accepted without the need for treatment in most experimental models. Likewise, in humans, liver grafts also show a lower susceptibility to rejection than any other organ and immunosuppressive treatment can be completely eliminated in approximately 25% of recipients. Many mechanisms have been proposed to explain the tolerogenic properties of the liver. Notable among these are the effects derived from the large number of passing leukocytes present in the liver and its peculiar anatomy that maximizes contact among blood lymphocytes and liver cells with tolerogenic potential. Although there are many cases of tolerance in human allograft recipients, therapeutic strategies that would allow predictable tolerance induction and without a high risk of adverse affects are still lacking. Therefore, most studies in humans have traditionally aimed to minimize doses of immunosuppressive drugs rather than eliminate them. However, recent results in preclinical models and pilot studies indicate that therapeutic protocols for tolerance induction may become available in the not too distant future.

Progressive differentiation and commitment of CD8+ T cells to a poorly cytolytic CD8low phenotype in the presence of IL-4

Exposure to IL-4 during activation of naive murine CD8+ T cells leads to generation of IL-4-producing effector cells with reduced surface CD8, low perforin, granzyme B and granzyme C mRNA, and poor cytolytic function. We show in this study that maximal development of these cells depended on exposure to IL-4 for the first 5 days of activation. Although IL-4 was not required at later times, CD8 T cell clones continued to lose surface CD8 expression with prolonged culture, suggesting commitment to the CD8low phenotype. This state was reversible in early differentiation. When single CD8low cells from 4-day cultures were cultured without IL-4, 65% gave rise to clones that partly or wholly comprised CD8high cells; the proportion of reverted clones was reduced or increased when the cells were cloned in the presence of IL-4 or anti-IL-4 Ab, respectively. CD8 expression positively correlated with perforin and granzyme A, B, and C mRNA, and negatively correlated with IL-4 mRNA levels among these clones. By contrast, most CD8low cells isolated at later time points maintained their phenotype, produced IL-4, and exhibited poor cytolytic function after many weeks in the absence of exogenous IL-4. We conclude that IL-4-dependent down-regulation of CD8 is associated with progressive differentiation and commitment to yield IL-4-producing cells with little cytolytic activity. These data suggest that the CD4-CD8- cells identified in some disease states may be the product of a previously unrecognized pathway of effector differentiation from conventional CD8+ T cells.

Impaired recall of CD8 memory T cells in immunologically privileged tissue

Foreign Ags that enter immunologically privileged sites such as the eye, brain, and testis persist for an extended period of time, whereas the same Ags are rapidly eliminated at conventional sites. Immune privilege, therefore, provides unwanted refuge for pathogens and tumor cells but is beneficial for the survival of allogeneic grafts. In this study, we asked whether memory T cells can eliminate foreign Ags deposited at an immunologically privileged site by studying CD8 memory T cell-mediated rejection of pancreatic islet allografts placed either in the testis (a privileged organ) or under the kidney capsule (a nonprivileged site) of diabetic mice. We found that CD8 memory T cells reject intratesticular grafts at a significantly slower rate than the rejection of intrarenal grafts. Delayed graft rejection in the testis was not due to reduced homing or proliferation of memory T cells but due to their increased apoptosis at that site. Apoptosis was mediated by the combined actions of two TNFR family members that are up-regulated on activated memory T cells, Fas, and CD30. Therefore, memory T cells survey immunologically privileged tissues but are subject to the immunosuppressive mechanisms present at these sites.

Augmentation of antitumor immune responses after adoptive transfer of bone marrow derived from donors immunized with tumor lysate-pulsed dendritic cells

We demonstrated previously that tumor lysate-pulsed dendritic cells (TP-DC) could mediate a specific and long-lasting antitumor immune response against a weakly immunogenic breast tumor during early lymphoid reconstitution. The purpose of this study was to examine the potential therapeutic efficacy of bone marrow transplants from TP-DC-vaccinated donors. In 2 aggressive metastatic models, bone marrow transplantation with donor bone marrow cells from TP-DC-immunized mice mediated a tumor-specific immune response in the recipient, and this caused regressions of preexisting tumor metastases. After vaccination with TP-DC, donors harbored increased numbers of both activated CD4+ and CD8+ T-cell populations in the bone marrow. Adoptive transfer of T cells purified from the bone marrow of TP-DC-vaccinated mice led to a reduction in preestablished lung metastases, whereas depletion of T cells from bone marrow abolished this effect. By using T cells derived from the bone marrow of TP-DC-vaccinated major histocompatibility complex class I and class II knockout mice, the effector cells required for the observed antitumor effect were determined to be major histocompatibility complex class I-restricted CD8+ T cells. Additionally, the tumor burden in TP-DC-immunized transplant recipients could be reduced further by repetitive TP-DC immunizations after bone marrow transplantation. Collectively, these results demonstrate an important therapeutic role of bone marrow from TP-DC-immunized donors and raise the potential for this approach in patients with advanced cancer.

The critical role of Fas-Fas ligand interaction in donor-specific transfusion-induced tolerance to H-Y antigen

BACKGROUND

Donor-specific transfusion (DST) has been clinically used to enhance the survival of transplanted organs, and it has been shown in mice to induce tolerance to male (H-Y) antigen (Ag). Although the biologic mechanisms that initiate and maintain DST-induced tolerance involve clonal deletion, induction of anergy, and generation of regulatory cells, the molecules essential to tolerance induction are still unclear. In this study, we investigated the role of Fas-FasL interaction in DST-induced tolerance to H-Y Ag.

METHODS

C57BL/6 (B6) or B6-Fas(lpr) (lpr) female mice were intravenously injected with B6, lpr, or B6-FasL(gld) (gld) male spleen cells (SC). B6 male skin grafts, mixed lymphocyte reaction (MLR) assay, and cytotoxicity assay (CTL) were performed 7 days after DST. In some experiments, purified B-cells were used as transfused cells.

RESULTS

B6 female mice treated with B6 male SC permanently accepted B6 male skins, whereas untreated B6 or lpr female mice rejected B6 male skins. On the other hand, B6 female mice treated with gld male SC acceleratingly rejected male skin, as did lpr female mice treated with B6 or gld male SC. The recipient mice in the experimental groups, in which DST resulted in the accelerated rejection of the skin grafts, had strong allo-responses to H-Y Ag in MLR and CTL. Further, B6 female mice treated with gld male B-cells acceleratingly rejected male skins, whereas B6 female mice treated with B6 or lpr male B-cells from mice accepted male skins.

CONCLUSIONS

These findings suggest that the interaction between FasL upon infused SC, especially upon B-cells and Fas in a recipient, is essential in DST-induced tolerance to H-Y Ag.

Isolation and characterization of human antigen-specific TCR alpha beta+ CD4(-)CD8- double-negative regulatory T cells

Down-regulation of immune responses by regulatory T (Treg) cells is an important mechanism involved in the induction of tolerance to allo-antigens (Ags). Recently, a novel subset of Ag-specific T-cell receptor (TCR)alpha beta+ CD4(-)CD8- (double-negative [DN]) Treg cells has been found to be able to prevent the rejection of skin and heart allografts by specifically inhibiting the function of antigraft-specific CD8+ T cells. Here we demonstrate that peripheral DN Treg cells are present in humans, where they constitute about 1% of total CD3+ T cells, and consist of both naive and Ag-experienced cells. Similar to murine DN Treg cells, human DN Treg cells are able to acquire peptide-HLA-A2 complexes from antigen-presenting cells by cell contact-dependent mechanisms. Furthermore, such acquired peptide-HLA complexes appear to be functionally active, in that CD8+ T cells specific for the HLA-A2-restricted self-peptide, Melan-A, became sensitive to apoptosis by neighboring DN T cells after acquisition of Melan-A-HLA-A2 complexes and revealed a reduced proliferative response. These results demonstrate for the first time that a sizable population of peripheral DN Treg cells, which are able to suppress Ag-specific T cells, exists in humans. DN Treg cells may serve to limit clonal expansion of allo-Ag-specific T cells after transplantation.

Paxillin selectively associates with constitutive and chemoattractant-induced high-affinity α4β1 integrins: implications for integrin signaling

Leukocyte α4β1 integrins regulate hematopoietic and lymphoid development, as well as the emigration of circulating cells to sites of inflammation. Because vascular cell adhesion molecule-1 (VCAM-1) binding to high-affinity α4β1 is stable, these integrins can be detected and selectively precipitated from cell lysates using VCAM-1/Fc. With this approach, high-affinity α4β1 integrin expression was demonstrated on lymphocytes in the bone marrow, thymus, spleen, and the peritoneal cavity of normal mice, but not in peripheral lymph nodes. Immature lymphocytes preferentially expressed high-affinity α4β1 in the bone marrow and thymus. Paxillin is a cytoplasmic adaptor molecule that can bind to the α4 tail and initiate signaling. Paxillin was associated selectively with high-affinity integrins that were isolated from human Jurkat T cells or from murine tissues, and blotting with a phospho-specific antibody demonstrated that Ser988 in the α4 cytoplasmic tail was dephosphorylated in high-affinity but not low-affinity integrins. A rapid and transient α4β1 affinity up-regulation in formyl peptide receptor-transfected U937 cells stimulated with N-formyl-methyonyl-leucyl-phenylalanine (fMLP) correlated temporally with induced paxillin binding to α4 integrins. These data suggest that ligand binding to high-affinity α4β1 integrins may initiate outside-in signaling cascades through paxillin that regulate leukocyte maturation and emigration. (Blood. 2004;104:2818-2824)

Tregs and transplantation tolerance

The induction and maintenance of immune tolerance to transplanted tissues constitute an active process involving multiple mechanisms that work cooperatively to prevent graft rejection. These mechanisms are similar to inherent tolerance toward self antigens and have a requirement for active immunoregulation, largely T cell mediated, that promotes specific unresponsiveness to donor alloantigens. This review outlines our current understanding of the Treg subsets that contribute to allotolerance and the mechanisms by which these cells exert their effects as well as their potential for therapy.

Are genetic determinants of asymmetric stem cell division active in hematopoietic stem cells?

Stem cells have acquired a golden glow in the past few years as they represent possible tools for reversing the damage wreak on organs. These cells are found not only in major regenerative tissues, such as the epithelia, blood and testes, but also in 'static tissues', such as the nervous system and liver, where they play a central role in tissue growth and maintenance. The mechanism by which stem cells maintain populations of highly differentiated, short-lived cells seems to involve a critical balance between alternate fates: daughter cells either maintain stem cell identity or initiate differentiation. Recent studies in lower organisms have unveiled the regulatory mechanisms of asymmetric stem cell divisions. In these models, the surrounding environment likely provides key instructive signals for the cells to choose one fate over another. Our understanding now extends to the intrinsic mechanisms of cell polarity that influence asymmetrical stem cell divisions. This article focuses on the genetic determinants of asymmetric stem cell divisions in lower organisms as a model for studying the process of self-renewal of mammalian hematopoietic stem cells.

Genes of tolerance

Activation-induced cell death, anergy and/or immune response modulation by T-regulatory cells (T(Reg)) are essential mechanisms of peripheral T-cell tolerance. There is growing evidence that anergy, tolerance and active suppression are not entirely distinct, but rather, represent linked mechanisms possibly involving the same cells and multiple suppressor mechanisms. Skewing of allergen-specific effector T cells to T(Reg) cells appears as a crucial event in the control of healthy immune response to allergens and successful allergen-specific immunotherapy. The T(Reg) cell response is characterized by abolished allergen-induced specific T-cell proliferation and suppressed T helper 1 (Th1)- and Th2-type cytokine secretion. In addition, mediators of allergic inflammation that trigger cAMP-associated G-protein coupled receptors, such as histamine receptor 2 may contribute to peripheral tolerance mechanisms. The increased levels of interleukin-10 (IL-10) and transforming growth factor-beta (TGF-beta) that are produced by T(Reg) cells potently suppress immunoglobulin E (IgE) production, while simultaneously increasing production of noninflammatory isotypes IgG4 and IgA, respectively. In addition, T(Reg) cells directly or indirectly suppress effector cells of allergic inflammation such as mast cells, basophils and eosinophils. In conclusion, peripheral tolerance to allergens is controlled by multiple active suppression mechanisms. It is associated with regulation of antibody isotypes and effector cells to the direction of a healthy immune response and opens a window for novel therapies of allergic diseases.

Acceptance of related and unrelated cardiac allografts in neonatally tolerized mice is cardio-specific and transferable by regulatory CD4+ T cells

BACKGROUND

Non-donor-specific cardiac allograft acceptance is induced in C3H/He (C3H; H-2k) recipients injected as neonates with allogeneic BALB/c (BALB; H-2d) fetal liver cells (FLC). This occurs despite intact reactivity to donor-type and third-party alloantigens in in vitro assays and skin transplants. To investigate a role for regulatory T cells, we performed adoptive transfer studies and specifically assessed CD4+ and CD4- T cells.

METHODS

Three cell populations (splenocytes, CD4+, CD4-) derived from neonatally-treated mice with accepted C57BL/6 (B6; H-2b) third-party cardiac grafts were adoptively transferred into sub-lethally-irradiated C3H mice. Reconstituted mice were challenged with B6 cardiac grafts, B6 skin grafts, or unrelated cardiac grafts. Separated cells were assessed in vitro.

RESULTS

B6, BALB, and NZW (H-2z) graft acceptance was transferred by unfractionated splenocytes. CD4+ cells transferred B6 graft acceptance (85% survival > 100 days). CD4- cells, unfractionated cells from naive or only irradiated mice, and unfractionated cells from neonatally-treated non-transplanted C3H mice rejected grafts within 35 days. No inoculum induced skin graft acceptance. Co-cultured assays confirmed the suppressive function of CD4+ cells in vitro.

CONCLUSIONS

Cardiac allograft acceptance in our model is regulated by CD4+ cells. The regulatory cell population is induced by the cardiac graft itself and mediates in vivo cardiac graft acceptance in a tissue-specific but not donor-strain-specific manner.

Infusion of in vitro-generated DN T regulatory cells induces permanent cardiac allograft survival in mice

Previously, we have demonstrated that pretransplant donor lymphocyte infusion (DLI) can activate recipient-derived CD3+CD4-CD8- double negative T regulatory (DN Tr) cells which have a potent immune regulatory function in vitro and in vivo. Here we studied the regulatory ability of DN T cell clones generated from the spleens of nai;ve anti-L(d) transgenic TCR+ (2C x dm2)F1 mice. We were able to identify subsets of DN T cell clones that were able to kill anti-Ld CD8+ T cells, and therefore had regulatory properties, and DN T cells with no regulatory properties. Next, we investigated the ability of these in vitro generated DN T cell clones to enhance cardiac allograft survival. (2C x dm2)F1 transgenic mice were infused with either regulatory or non-regulatory DN T cell clones, or left untreated one day before receiving an Ld-mismatched cardiac grafts from (C57BL/6 x Balb/c)F1 mice. Injection of non-regulatory DN T clone cells did not prolong cardiac graft survival in (2C x dm2)F1 mice when compare to untreated controls. In contrast, all of the cardiac grafts survived more than 100 days in mice that received DN Tr clone cells prior to transplantation. These results demonstrate that DN Tr cells can be generated in vitro and protect cardiac allograft from rejection when infused into recipients prior to transplantation. They also suggest that DN Tr cells may provide a novel therapy for the treatment of allograft rejection.

Role of autoimmunity in organ allograft rejection: a focus on immunity to type V collagen in the pathogenesis of lung transplant rejection

Lung transplantation is the only definitive treatment modality for many forms of end-stage lung disease. However, the lung is rejected more often than any other type of solid organ allograft due to chronic rejection known as bronchiolitis obliterans (BO). Indeed, BO is the primary reason why the 5- and 7-yr survival rates are worse for the lung than for any other transplanted organ. Alloimmunity to donor antigens is established as the primary mechanism that mediates rejection responses. However, newer immunosuppressive regimens designed to abrogate alloimmune activation have not improved survival. Therefore, these data suggest that other antigens, unrelated to donor transplantation antigens, are involved in rejection. Utilizing human and rodent studies of lung transplantation, our laboratory has documented that a native collagen, type V collagen [col(V)], is a target of the rejection response. Col(V) is highly conserved; therefore, these data indicate that transplant rejection involves both alloimmune and autoimmune responses. The role of col(V) in lung transplant rejection is described in this review article. In addition, the potential role of regulatory T cells that are crucial to modulating autoimmunity and alloimmunity is also discussed.

Haematopoietic stem cell transplantation for autoimmune disease: limits and future potential

Stem cell transplantation (SCT) for autoimmune disease is handicapped by a lack of definitive clinical trials able to demonstrate an overall benefit. This deficiency will become more problematic as the impetus grows to introduce and evaluate additional technologies intended to improve the safety and efficacy of the procedure. The development of effective surrogate analyses to predict outcome by measuring resurgent autoimmune clones or by genomic- and proteomic-based technologies to detect early disease recurrence may be of value in assessing the benefits of these modifications without the need for full-scale, long-term, randomized trials. The introduction of safer allogeneic transplantation techniques may increase the effectiveness of the procedure, while work on marrow stem cell plasticity and/or fusion suggests that SCT may serve not simply to halt the autoimmune process, but also to contribute cells capable of healing or regenerating diseased organs. Finally, the introduction of therapeutic transgenes into transplanted cells may further increase the effectiveness of SCT, although the regulatory complexities of gene therapy trials will probably delay this process. All these innovations will ensure that the next decade will see major changes in the practice and purpose of SCT for autoimmune disease.

Both T and non-T cells with proliferating potentials are effective in inducing suppression of allograft responses by alloantigen-specific intravenous presensitization combined with suboptimal doses of 15-deoxyspergualin

In an MHC class I-disparate combination of mouse strains, a single intravenous injection of donor spleen cells combined with 10 suboptimal doses of 15-deoxyspergualin (DSG) administration was effective in inducing donor-specific suppression of cytotoxic T-lymphocyte (CTL) responses and prolonged survival of the relevant skin allograft. Proliferative potentials of the donor spleen cells were requirement for the induction of suppressed allospecific responses, but both highly purified T cells and non-T cells were equally effective to induce the suppression of CTL responses by intravenous injection. These results have shown that, although working on different mechanisms, DSG is as effective as FK506 or rapamycin in inducing allograft tolerance when used at suboptimal doses along with the donor-specific intravenous presensitization, and an immune mechanism other than well-characterized veto T cells is working in this model in suppressing alloreactive CTL precursors.

Donor treatment with pegylated G-CSF augments the generation of IL-10-producing regulatory T cells and promotes transplantation tolerance

We investigated whether the protection from graft-versus-host disease (GVHD) afforded by donor treatment with granulocyte colony-stimulating factor (G-CSF) could be enhanced by dose escalation. Donor treatment with human G-CSF prevented GVHD in the B6 → B6D2F1 murine model in a dose-dependent fashion, and murine G-CSF provided equivalent protection from GVHD at 10-fold lower doses. Donor pretreatment with a single dose of pegylated G-CSF (peg-G-CSF) prevented GVHD to a significantly greater extent than standard G-CSF (survival, 75% versus 11%, P &lt; .001). Donor T cells from peg-G-CSF-treated donors failed to proliferate to alloantigen and inhibited the responses of control T cells in an interleukin 10 (IL-10)-dependent fashion in vitro. T cells from peg-G-CSF-treated IL-10-/- donors induced lethal GVHD; T cells from peg-G-CSF-treated wild-type (wt) donors promoted long-term survival. Whereas T cells from peg-G-CSF wt donors were able to regulate GVHD induced by T cells from control-treated donors, T cells from G-CSF-treated wt donors and peg-G-CSF-treated IL-10-/- donors did not prevent mortality. Thus, peg-G-CSF is markedly superior to standard G-CSF for the prevention of GVHD following allogeneic stem cell transplantation (SCT), due to the generation of IL-10-producing regulatory T cells. These data support prospective clinical trials of peg-G-CSF-mobilized allogeneic blood SCT. (Blood. 2004;103:3573-3581)

Revival of the regulatory T cell: new targets for drug development

Compelling new evidence supports the idea that regulatory T cells play a major role in our immune system. Several subsets of these regulators have been identified recently. Differences in the phenotypical and functional characteristics of these subsets have immunological implications. From our growing knowledge of the field of immunology, we could potentially generate a new class of therapeutic agents that target immune-related diseases.

Antigen-induced regulatory T cells

Regulatory T cells participate in immunologic homeostasis by active suppression of inappropriate immune responses. Regulatory T lymphocytes expressing CD4 and CD25 antigens and naturally present in the peripheral blood were the first to be phenotypically characterized. However, their small number and antigen nonspecific suppression has prompted efforts to identify and dissect antigen-specific regulatory T cells. In this review we discuss how antigen-specific regulatory T cells can be identified, the cellular and molecular mechanisms underlying their induction and activity, and the challenges facing their potential clinical application.

Antibody-induced transplantation tolerance: the role of dominant regulation

A short-treatment with nondepleting antibodies, such as those targeting CD4 or CD154 (CD40 ligand), allows long-term graft survival without the need for continuous immunosuppression. This state of immune tolerance is maintained by regulatory CD4+ T cells present within both the lymphoid tissue and the tolerated graft. The nature of such regulatory T cells, their relationship to CD4+CD25+ T cells, and their mode of action have all been the subjects of much attention recently. Here, we review recent progress on understanding the nature, specificity, and mechanisms of action of T cells mediating dominant tolerance brought about by antibody therapy.

Absence of programmed death receptor 1 alters thymic development and enhances generation of CD4/CD8 double-negative TCR-transgenic T cells

Programmed death receptor 1 (PD-1) is expressed on thymocytes in addition to activated lymphocyte cells. Its ligation is thought to negatively regulate T cell activation, and PD-1(-/-) mice develop autoimmunity. To study the role of PD-1 on the development and function of a monoclonal CD8(+) T cell population, 2C TCR-transgenic/recombination-activating gene 2(-/-)/PD-1(-/-) mice were generated. Unexpectedly, approximately 30% of peripheral T cells in these mice were CD4/CD8 double negative (DN). Although the DN cells were not activated by Ag-expressing APCs, they functioned normally in response to anti-CD3/anti-CD28. These cells had a naive surface phenotype and lacked expression of NK1.1, B220, and gammadelta TCR; and the majority did not up-regulate CD8alphaalpha expression upon activation, arguing that they are not predominantly diverted gammadelta-lineage cells. The thymus was studied in detail to infer the mechanism of generation of DN peripheral T cells. Total thymus cellularity was reduced in 2C TCR-transgenic/recombination-activating gene 2(-/-)/PD-1(-/-) mice, and a relative increase in DN cells and decrease in double-positive (DP) cells were observed. Increased annexin V(+) cells among the DP population argued for augmented negative selection in PD-1(-/-) mice. In addition, an increased fraction of the DN thymocytes was HSA negative, suggesting that they had undergone positive selection. This possibility was supported by decreased emergence of DN PD-1(-/-) 2C cells in H-2(k) bone marrow chimera recipients. Our results are consistent with a model in which absence of PD-1 leads to greater negative selection of strongly interacting DP cells as well as increased emergence of DN alphabeta peripheral T cells.

IL-10 induces regulatory T cell apoptosis by up-regulation of the membrane form of TNF-alpha

Numerous studies have demonstrated the role of regulatory T (Treg) cells in peripheral tolerance. Nevertheless, how the survival and death of Treg cells is controlled is largely unknown. In this study, we investigated the mechanisms involved in regulating the homeostasis of a subset of Ag-specific alphabetaTCR+ CD4-CD8- double negative (DN) Treg cells. We demonstrate that DN Treg cells are naturally resistant to TCR cross-linking-induced apoptosis. Administration of exogenous IL-10 renders DN Treg cells susceptible to apoptosis, and abolishes their suppressive function. Furthermore, TCR cross-linking of DN Treg cells in the presence of IL-10 leads to the up-regulation of the membrane-bound but not the soluble form of TNF-alpha. Interaction of membrane bound TNF-alpha with TNFR2 sends death signals to DN Treg cells. Blocking their interaction can reverse the effects of IL-10 on DN Treg cells. These results provide insights into the mechanisms that regulate the function and homeostasis of DN Treg cells.

CD4+CD25+ regulatory T cells suppress allograft rejection mediated by memory CD8+ T cells via a CD30-dependent mechanism

CD4(+)CD25(+) regulatory T (Treg) cells suppress naive T cell responses, prevent autoimmunity, and delay allograft rejection. It is not known, however, whether Treg cells suppress allograft rejection mediated by memory T cells, as the latter mount faster and stronger immune responses than their naive counterparts. Here we show that antigen-induced, but not naive, Treg cells suppress allograft rejection mediated by memory CD8(+) T cells. Suppression was allospecific, as Treg cells induced by third-party antigens did not delay allograft rejection. In vivo and in vitro analyses revealed that the apoptosis of allospecific memory CD8(+) T cells is significantly increased in the presence of antigen-induced Treg cells, while their proliferation remains unaffected. Importantly, neither suppression of allograft rejection nor enhanced apoptosis of memory CD8(+) T cells was observed when Treg cells lacked CD30 or when CD30 ligand-CD30 interaction was blocked with anti-CD30 ligand Ab. This study therefore provides direct evidence that pathogenic memory T cells are amenable to suppression in an antigen-specific manner and identifies CD30 as a molecule that is critical for the regulation of memory T cell responses.

The protean immune cell synapse: a supramolecular structure with many functions

Heterogeneity in the supramolecular organization of immunological synapses arises from the involvement of different cells, distinct environmental stimuli, and varying levels of protein expression. There may also be heterogeneity in the types and amounts of cell surface proteins and lipids that transfer between lymphocytes during immune surveillance. In addition, immune cells can be involved in the assembly of a 'viral synapse', such that micrometer-scale organization of proteins at intercellular contacts occurs during transmission of a virus between T cells. Thus, while there may be unity in molecular mechanisms underlying the organization of cell surface receptors at immune cell synapses, there is diversity in their function.

Dominant transplantation tolerance

Long-term allograft survival in the absence of continuous immunosuppression can be induced following a short treatment of nondepleting antibodies, such as those that target CD4 or CD154 (CD40 ligand). It is now established that this may involve dominant tolerance mechanisms that are maintained by CD4+ regulatory T cells present within the lymphoid tissue and the tolerated graft. The phenotype of these cells, their relationship to CD4+CD25+ T cells, and the mechanism of action are still controversial.

Induction of allopeptide-specific human CD4+CD25+ regulatory T cells ex vivo

Although CD4+CD25+ regulatory T cells are pivotal in the prevention of autoimmunity and appear to mediate transplantation tolerance, little is known concerning their antigen specificity. Here we describe the induction of a human CD4+CD25+ regulatory T-cell line specific for a defined peptide alloantigen (human leukocyte antigen A2 [HLA-A2] 138-170) by priming purified CD4+CD25+ cells ex vivo. The regulatory cells were anergic and retained their ability to suppress antigen-driven responses of CD4+CD25- cells. They inhibited not only interleukin 2 (IL-2) secretion by CD4+CD25- T cells specific for the same peptide but also direct alloresponse of naive CD4+CD25- T cells stimulated by semiallogeneic dendritic cells (DCs) in the presence of the peptide ("linked suppression"). They also suppressed the response of CD4+ T cells specific for viral and bacterial antigens. The suppressive T-cell line showed sustained high CD25 expression. These findings suggest that peripheral CD4+CD25+ regulatory cells are a precommitted cell lineage from which cells with specificity for non-self-peptides can be selected. This may pave the way for inducing and expanding peptide antigen-specific regulatory T cells ex vivo for cell therapy in transplantation, allergy, and autoimmune disease.

SUV39H1 interacts with AML1 and abrogates AML1 transactivity. AML1 is methylated in vivo

Acute myeloid leukemia 1 (AML1) belongs to a family of DNA-binding proteins highly conserved through evolution. AML1 regulates the expression of several hematopoietic genes and is essential for murine fetal liver hematopoiesis. We report here that the histone methyltransferase SUV39H1, a mammalian ortholog of the Drosophila melanogaster SU(VAR) 3-9, forms complex with AML1. SUV39H1 methylates lysine 9 of the histone protein H3 leading to the formation of the high-affinity binding site on chromatin for proteins of the heterochromatin protein 1 family (HP1). The interaction of AML1 with SUV39H1 requires the N-terminus of AML1 where the Runt domain is located. Binding of AML1 to SUV39H1 abrogates the transactivating and DNA-binding properties of AML1 and dissociates the net-like nuclear structure of AML1. It has been reported that AML1 is capable of interaction with histone acetyl transferases (CBP, p300, and MOZ) and with component of the histone deacetylase complex (Sin3), and that the interaction with these coregulators affects the strength of AML1 in promoter regulation. Our data suggest that other enzymes are also involved in gene regulation by AML1 activity by modulating the affinity of AML1 for DNA.

Xenotransplantation

The continued and growing success of lung allotransplantation has intensified the worldwide shortage of donor organs. Yet, xenotransplantation remains a daunting challenge. Additional molecular incompatibilities and unforeseen complications will continue to be discovered. Progress has been made, notably on the generation of alpha-Gal double knockout pigs. Progressive increases in organ survival times have been seen for most organs after significant investments of time and money. The lung continues to be an organ with the lowest supply of cadaveric donors and the least potential for expanded living donation or mechanical alternatives. As such, the impetus for xenotransplantation is strong. The lung appears to be exquisitely sensitive to xenograft rejection and resistant to strategies that have been moderately successful in other organs. A complex program involving genetically modified donor organs, recipient preparation for antibody removal or tolerance promotion, and multitargeted drug therapy will likely be required for successful clinical application.

p53RFP, a p53-inducible RING-finger protein, regulates the stability of p21WAF1

The mechanisms by which p53 prevents development of cancer are much more complicated than previously thought. Under normal conditions, p53 is involved in cell-cycle arrest, Q1apoptosis, DNA repair, and inhibition of angiogenesis; it also promotes degradation of proteins through transcriptional regulation of certain target genes. Here we report the isolation of a novel transcriptional target of p53, designated p53RFP (p53-inducible RING-finger protein), whose product has E3 ubiquitin ligase activity. Its expression was negatively correlated to that of p21(WAF1) protein; p53RFP is likely to play a role in the regulation of this protein, probably through interaction with, and ubiquitination of, p21(WAF1). p53RFP appears to represent the second known example, the first being MDM2, of an E3 ubiquitin ligase as a p53 target. Our results further suggest that p53 might regulate the stability of p21(WAF1) through transcriptional regulation of p53RFP, and this feature may represent a novel mechanism for a p53-dependent cell-cycle checkpoint.

Inhibition of graft-versus-host disease by double-negative regulatory T cells

Pretransplant infusion of lymphocytes that express a single allogeneic MHC class I Ag has been shown to induce tolerance to skin and heart allografts that express the same alloantigens. In this study, we demonstrate that reconstitution of immunoincompetent mice with spleen cells from MHC class I L(d)-mismatched donors does not cause graft-vs-host disease (GVHD). Recipient mice become tolerant to skin allografts of lymphocyte donor origin while retaining immunity to third-party alloantigens. The mechanism involves donor-derived CD3(+)CD4(-)CD8(-) double-negative T regulatory (DN Treg) cells, which greatly increase and form the majority of T lymphocytes in the spleen of recipient mice. DN Treg cells isolated from tolerant recipient mice can suppress the proliferation of syngeneic antihost CD8(+) T cells in vitro. Furthermore, we demonstrate that DN Treg cells can be generated in vitro by stimulating them with MHC class I L(d)-mismatched lymphocytes. These in vitro generated L(d)-specific DN Treg cells are able to down-regulate the activity of antihost CD8(+) T cells in vitro by directly killing activated CD8(+) T cells. Moreover, infusing in vitro generated L(d)-mismatched DN Treg cells prevented the development of GVHD caused by allogeneic CD8(+) T cells. Together these data demonstrate that infusion of single MHC class I locus-mismatched lymphocytes may induce donor-specific transplantation tolerance through activation of DN Treg cells, which can suppress antihost CD8(+) T cells and prevent the development of GVHD. This finding indicates that using single class I locus-mismatched grafts may be a viable alternative to using fully matched grafts in bone marrow transplantation.

Apoptosis and transplantation tolerance

Self-tolerance is maintained by several mechanisms including deletion (via apoptosis) and regulation. Acquired tolerance to allogeneic tissues and organs exploits similar strategies. One key difference between alloantigens and peptide antigens is the enormous number of T cells that are alloreactive. Accumulating evidence suggests that in the face of this large mass of potentially graft-destructive T cells, tolerance requires an initial wave of deletion. This creates a more level playing field in which a smaller number of regulatory T cells can then act to maintain an established tolerant state. Deletion of alloreactive T cells by apoptosis actively promotes immunoregulation as well, by interfering with proinflammatory maturation of antigen presenting cells. This article reviews the immune response to alloantigens, the development and use of both necrotic and apoptotic means of cell death during the evolution of the immune response, and the likely role and mechanisms by which apoptosis promotes, and may even be required for, transplantation tolerance.