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

Cutting edge: in vivo trogocytosis as a mechanism of double negative regulatory T cell-mediated antigen-specific suppression

Recent data have demonstrated that treatment with alphabeta-TCR(+)CD3(+)CD4(-)CD8(-)NK1.1(-) double negative (DN) regulatory T cells (Tregs) inhibits autoimmune diabetes and enhances allotransplant and xenotransplant survival in an Ag-specific fashion. However, the mechanisms whereby DN Tregs suppress Ag-specific immune responses remain largely unknown. In this study, we demonstrate that murine DN Tregs acquire alloantigen in vivo via trogocytosis and express it on their cell surface. Trogocytosis requires specific interaction of MHC-peptide on APCs and Ag-specific TCR on DN Tregs, as blocking this interaction prevents DN Treg-mediated trogocytosis. Acquisition of alloantigen by DN Tregs was required for their ability to kill syngeneic CD8(+) T cells. Importantly, DN Tregs that had acquired alloantigen were cytotoxic toward Ag-specific, but not Ag-nonspecific, syngeneic CD8(+) T cells. These data provide new insight into how Tregs mediate Ag-specific T cell suppression and may enhance our ability to use DN Tregs as a therapy for transplant rejection and autoimmune diseases.

Pathways of major histocompatibility complex allorecognition

PURPOSE OF REVIEW

Here, we review the pathways of allorecognition and their potential relevance to the balance between regulatory and effector responses following transplantation.

RECENT FINDINGS

Transplantation between nonidentical members of the same species elicits an immune response that manifests as graft rejection or persistence. Presentation of foreign antigen to recipient T cells can occur via three nonmutually exclusive routes, the direct, indirect and semi-direct pathways. Allospecific T cells can have effector or regulatory functions, and the relative proportions of the two populations activated following alloantigen presentation are two of the factors that determine the clinical outcome. Regulatory T cells have been the subject of significant research, and there is now greater understanding of their recruitment and function in the context of allorecognition.

SUMMARY

A greater understanding of the mechanisms underlying allorecognition may be fundamental to appreciating how these different populations are recruited and could in turn inform novel strategies for immunomodulation.

Induction of tolerance to cardiac allografts using donor splenocytes engineered to display on their surface an exogenous fas ligand protein

The critical role played by Fas ligand (FasL) in immune homeostasis renders this molecule an attractive target for immunomodulation to achieve tolerance to auto- and transplantation Ags. Immunomodulation with genetically modified cells expressing FasL was shown to induce tolerance to alloantigens. However, genetic modification of primary cells in a rapid, efficient, and clinically applicable manner proved challenging. Therefore, we tested the efficacy of donor splenocytes rapidly and efficiently engineered to display on their surface a chimeric form of FasL protein (SA-FasL) for tolerance induction to cardiac allografts. The i.p. injection of ACI rats with Wistar-Furth rat splenocytes displaying SA-FasL on their surface resulted in tolerance to donor, but not F344 third-party cardiac allografts. Tolerance was associated with apoptosis of donor reactive T effector cells and induction/expansion of CD4(+)CD25(+)FoxP3(+) T regulatory (Treg) cells. Treg cells played a critical role in the observed tolerance as adoptive transfer of sorted Treg cells from long-term graft recipients into naive unmanipulated ACI rats resulted in indefinite survival of secondary Wistar-Furth grafts. Immunomodulation with allogeneic cells rapidly and efficiently engineered to display on their surface SA-FasL protein provides an effective and clinically applicable means of cell-based therapy with potential application to regenerative medicine, transplantation, and autoimmunity.

Regulatory T cells after organ transplantation: where does their action take place?

Regulatory T cells are considered to be pivotal for the induction of tolerance to donor antigens. In the past decades, several regulatory T-cell subsets have been identified, such as CD4(+)CD25(+) regulatory T cells and the CD8(+)CD28(-) suppressor T cells. Although many studies have investigated the role of these regulators in transplant tolerance, relatively little attention has focused on the exact place where these cells suppress immune responses directed to donor antigens. The localization of regulatory T cells may influence their effect on allogeneic immune responses. More insight into the localization and migration of regulatory T cells in transplant recipients is therefore important, especially when these cells are to be used for monitoring purposes and for cellular immune therapy. In the present review we summarize current knowledge about the presence of functional donor-directed regulatory T cells in the secondary lymphoid organs, peripheral blood, and the transplanted organ itself. In addition, we discuss the importance of the appropriate localization for the control of anti-donor immune reactivity.

In vivo-activated CD103+CD4+ regulatory T cells ameliorate ongoing chronic graft-versus-host disease

CD103 (alphaEbeta7) has been shown to be an excellent marker for identifying in vivo-activated FoxP3(+)CD4(+) regulatory T (Treg) cells. It is unknown whether reinfusion of in vivo-activated donor-type CD103(+) Treg cells from recipient can ameliorate ongoing chronic graft-versus-host disease (GVHD). Here, we showed that, in a chronic GVHD model of DBA/2 (H-2(d)) donor to BALB/c (H-2(d)) recipient, donor-type CD103(+) Treg cells from recipients were much more potent than CD25(hi) natural Treg cells from donors in reversing clinical signs of GVHD and tissue damage. Furthermore, in contrast to CD25(hi) natural Treg cells, CD103(+) Treg cells expressed high levels of CCR5 but low levels of CD62L and directly migrated to GVHD target tissues. In addition, the CD103(+) Treg cells strongly suppressed donor CD4(+) T-cell proliferation; they also induced apoptosis of in vivo-activated CD4(+) T and B cells and significantly reduced pathogenic T and B cells in GVHD target tissues. These results indicate that CD103(+) Treg cells from chronic GVHD recipients are functional, and reinfusion of the CD103(+) Treg cells can shift the balance between Treg cells and pathogenic T cells in chronic GVHD recipients and ameliorate ongoing disease.

Bystander central memory but not effector memory CD8+ T cells suppress allograft rejection

Memory T cells respond faster and more vigorously than their naive counterparts and are critical for adaptive immunity. However, it is unknown whether and how memory T cells react in the face of irrelevant Ags. It is generally accepted that bystander memory T cells are neutral in immune responsiveness. In this study, we present the first evidence that bystander central memory (TCM), but not effector memory (TEM), CD8+ T cells suppress allograft rejection as well as T cell proliferation in the draining lymph nodes (DLN) of recipient mice. Both bystander TCM and naive T cells, but fewer TEM cells, migrated to DLN, whereas TCM cells exhibited faster turnover than their naive counterparts, suggesting that bystander TCM cells have an advantage over their naive counterparts in suppression. However, bystander TEM cells migrated to inflammatory graft sites, but not DLN, and yet failed to exert their suppression. These findings indicate that bystander memory T cells need to migrate to lymph nodes to exert their suppression by inhibiting responder T cell activation or homeostatic proliferation. Moreover, the suppression mediated by bystander TCM cells was largely dependent on IL-15, as IL-15 was required for their homeostatic proliferation and TCM-mediated suppression of allograft rejection. This suppression also required the presence of TGFbeta1, as TCM cells expressed TGFbeta1 while neutralizing TGFbeta1 abolished their suppression. Thus, bystander TCM, but not TEM, CD8+ T cells are potent suppressors rather than bystanders. This new finding will have an impact on cellular immunology and may have clinic implications for tolerance induction.

Double-negative regulatory T cells induce allotolerance when expanded after allogeneic haematopoietic stem cell transplantation

Double-negative (DN) regulatory T cells (Tregs) are specialized T lymphocytes involved in the down-modulation of immune responses, resulting in allotolerance after allogeneic haematopoietic stem cell transplantation (HSCT). Most of the properties of DN Tregs were identified in murine models, including the unique ability to suppress alloreactive syngeneic effector T cells in an antigen-specific manner via Fas/Fas-ligand interactions. We investigated the behaviour of DN Tregs following human allogeneic HSCT with regard to occurrence of graft-versus-host disease (GvHD) and restoration of T-cell receptor repertoire in a cohort of 40 patients. The frequency of DN Tregs and CD4/CD8 TCR repertoire was measured serially and at the time of diagnosis of GvHD by flow cytometry. Analysis demonstrated a positive correlation between degree of alloreactivity, as measured by grade of GvHD, and the number of variable beta chain (Vbeta) family expansions in both T-cell populations. We also found that a deficiency of DN Tregs was associated with an increased number of Vbeta family expansions, and most importantly, with the occurrence of GvHD. All individuals who demonstrated more than 1% DN Tregs did not develop GvHD, providing evidence that DN Tregs participate in peripheral tolerance to prevent GvHD when expanded after allogeneic HSCT.

The Foxp3+ regulatory T cell: a jack of all trades, master of regulation

The function of regulatory T cells (T(reg) cells) has been attributed to a growing number of diverse pathways, molecules and processes. Seemingly contradictory conclusions regarding the mechanisms underlying T(reg) cell suppressive activity have revitalized skeptics in the field who challenge the core validity of the idea of T(reg) cells as central immune regulators. However, we note that a consensus may be emerging from the data: that multiple T(reg) cell functions act either directly or indirectly at the site of antigen presentation to create a regulatory milieu that promotes bystander suppression and infectious tolerance. Thus, the versatility and adaptability of the Foxp3+ T(reg) cells may in fact be the best argument that these cells are 'multitalented masters of immune regulation'.

Adoptive transfer of double negative T regulatory cells induces B-cell death in vivo and alters rejection pattern of rat-to-mouse heart transplantation

BACKGROUND

Antibody-mediated hyperacute and acute graft rejection are major obstacles in achieving long-term graft survival in xenotransplantation. It is well documented that regulatory T (Treg) cells play a very important role in regulating immune responses to self and non-self antigens. Our previous studies have shown that TCRalphabeta+CD3+CD4-CD8- (double negative, DN)-Treg cells can suppress anti-donor T-cell responses and prolong graft survival in allo- and xenotransplantation models. We have demonstrated that DN-Treg cells can induce B-cell apoptosis in vitro through a perforin-dependent pathway.

METHODS

B6 mice received rat heart grafts, followed by 14 days of LF15-0195 treatment. Some mice received Lewis rat cell activated DN-Treg cells after LF treatment. DN-Treg cells, purified from perforin-/- mice and from B6 mice pre-immunized with third party rat cells, were used as controls.

RESULTS

In this study, we investigated the possibility that adoptive transfer of xenoreactive DN-Treg cells could suppress B cells in vivo, thus prolonging xenograft survival. We found that apoptotic death of B cells significantly increased after adoptive transfer of DN-Treg cells. In addition, anti-donor IgG subtypes were significantly inhibited in the DN-Treg cell-treated group, in which the rejection pattern was altered towards cellular-mediated rejection rather than antibody-mediated acute vascular rejection. However, perforin-deficient DN-Treg cells failed to induce B-cell death and to prolong heart graft survival, indicating a perforin-dependent mechanism contributes to B-cell death in vivo.

CONCLUSIONS

This study suggests that adoptive transfer of xenoreactive DN-Treg cells can inhibit B-cell responses in vivo. DN-Treg cells may be valuable in controlling B-cell responses in xenotransplantation.

The effects of antibody treatment on regulatory CD4(+)CD25(+) T cells

Current therapeutic antibodies, at least some, possess the capacity to induce immune tolerance in experimental models with allo-grafts or autoimmune diseases. Clinical application of humanized or chimeric antibodies to treat graft rejection or autoimmune diseases is presently underway. It is now becoming clear that immune tolerance can be acquired in some cases due to the action of regulatory T cells (Tregs), especially CD4(+)CD25(+) Tregs. In addition to their inhibition on immune response, some antibodies could promote tolerance induction in organ transplantation and autoimmune diseases essentially through the induction of Tregs. In this manuscript, we review the recent progress on the effects of therapeutic antibodies on the development, phenotypic changes and functions of CD4(+)CD25(+) Tregs.

Donor double-negative Treg promote allogeneic mixed chimerism and tolerance

Bone marrow (BM) transplantation is an efficient approach to develop donor-specific tolerance and prevent chronic rejection. Allogeneic BM transplantation is limited by donor T cell-mediated graft-versus-host disease, requirement of cytoreduction and high numbers of BM cells. In addition of these drawbacks, recent studies demonstrate that not only T cells, but also NK cells can mediate BM rejection, and long-term mixed chimerism depends on NK cell tolerance. Thus, NK cell is another potential barrier against engraftment of BM and an important target in efforts to induce transplant tolerance. We have previously identified a novel type of Treg with the phenotype TCRalphabeta+CD3+CD4-CD8- (double-negative, DN). We and others have demonstrated that DN-Treg can effectively suppress anti-donor T cell responses. In this study, we found that donor-derived DN-Treg can suppress NK cell-mediated allogeneic BM graft rejection in both parent-to-F1 and fully MHC-mismatched BM transplantation models. Perforin and FasL in DN-Treg play important roles in the suppression of NK cells. Furthermore, adoptive transfer of DN-Treg can promote a stable mixed chimerism and donor-specific tolerance without inducing graft-versus-host disease. These results demonstrate a potential approach to control innate immune responses and promote allogeneic BM engraftment.

Clinical-scale single-step CD4(+) and CD8(+) cell depletion for donor innate lymphocyte infusion (DILI)

The ability to selectively deplete or enrich cells of specific phenotype by immunomagnetic selection to reduce the risk of GVHD holds significant promise for application in adoptive immunotherapy. Current clinical-scale approaches for T-cell depletion (e.g., CD34(+) selection, CD3(+) depletion), usually deplete gammadelta T cells, which may be advantageous in mediating graft-versus-tumor (GVT) effects and augmenting the innate immune response against infections. Here, we present a new method for depletion of T cells with potential GVHD reactivity by using a single-step immunomagnetic protocol, which efficiently depletes CD4(+) and CD8(+) alphabeta T cells under good manufacturing practice (GMP) conditions. Depletion from unstimulated leukapheresis products (n=6) containing up to 2.0 x 10(10) cells showed high efficiency (mean log depletion of CD4(+) cells: 4.12, CD8(+) cells: 3.77). In addition, immunomagnetic CD4/CD8 depletion resulted in passive enrichment of innate lymphocytes (mean recovery of natural killer (NK) cells: 38%, gammadelta T cells: 50%). We demonstrated that gammadelta/NK cells preserved their proliferative and cytotoxic capacity and conclude that simultaneous large-scale depletion of CD4(+)/CD8(+) T cells is feasible and can be performed under GMP conditions with high-depletion efficacy for alphabeta T cells and recovery of functionally intact innate effector lymphocytes for potential use in adoptive immunotherapy studies.

Capture of membrane components via trogocytosis occurs in vivo during both dendritic cells and target cells encounter by CD8(+) T cells

Cytotoxic T lymphocytes recently stimulated by antigen-presenting cells (APC) display major histocompatibility class (MHC) I and II molecules inherited from APC. We have previously reported that, in vitro, transfer of MHC molecules and several other proteins occurs through trogocytosis, i.e. the active acquisition of target cell membrane fragments by T lymphocytes. Here, using the model of viral antigen LCMVgp33-41 recognition in transgenic P14 mice, we show that CD8(+) T cells perform trogocytosis in vivo, as detected by the capture of biotin- or fluorescence-labeled components of the APC surface. Trogocytosis occurs during interactions of CD8(+) T cells with at least two kinds of cells: target cells and dendritic cells (DC). In lymph nodes, CD8(+) T cells having performed trogocytosis with DC express the CD69 activation marker indicating that trogocytosis detects recently activated cells. Taken together, our findings suggest that trogocytosis may be a new in vivo marker of the recent interaction between a CD8(+) T cell and its cellular partners or targets.

OX40 costimulation turns off Foxp3+ Tregs

OX40 is a recently identified T-cell costimulatory molecule that belongs to the TNF/TNFR superfamily. OX40 can be expressed by both activated T effector cells and Foxp3(+) Tregs. It is well known that OX40 delivers a potent costimulatory signal to T effector cells, but very little is known about the role of OX40 in regulating the suppressor properties of Foxp3(+) Tregs and the de novo generation of new inducible Foxp3(+) Tregs from T effector cells. In the present study, we found, by using a newly created foxp3gfp knockin model, that OX40 was dispensable for the genesis and suppressor functions of naturally arising CD4(+)Foxp3(+) Tregs, but stimulating OX40 on the Foxp3(+) Tregs abrogated their ability to suppress T effector cell proliferation, IFN-gamma production, and T effector cell-mediated allograft rejection. OX40 costimulation did not significantly affect proliferation and survival of the naturally arising Foxp3(+) Tregs, but profoundly inhibited Foxp3 gene expression. Importantly, OX40 costimulation to T effector cells prevented the induction of new inducible Foxp3(+) Tregs from T effector cells. Our study identified OX40 as a key negative regulator of Foxp3(+) Tregs and may have important clinical implications in models of transplantation and autoimmunity.

Peptide-activated double-negative T cells can prevent autoimmune type-1 diabetes development

Autoimmune diseases may develop because of defective maturation, activation, differentiation and function of regulatory T cells. Previous studies have shown that exposure to donor antigen activates peripheral TCRalphabeta+CD3+CD4-CD8-NK1.1-, double-negative (DN) T cells, which specifically suppress anti-donor T cells and enhance survival of skin and heart grafts from allogeneic and xenogeneic donors. However, the role of DN T cells in preventing T cell-mediated autoimmune disease is unknown. Here, we analyzed the ability of DN T cells to recognize peptides expressed on self MHC and to suppress peptide-reactive CD8+ T cells, using the P14 mouse model that expresses a transgenic TCR specific for gp33 peptide presented on self MHC class I-Db. We found that injection of gp33 peptide resulted in increased DN and decreased CD8+ T cell numbers in the lymph nodes when compared to untreated mice. Injection of gp33, but not TCR-non-specific AV peptide, increased expression of T cell activation markers on DN T cells. Moreover, gp33-activated DN T cells suppressed proliferation of syngeneic CD8+ T cells via killing activated CD8+ T cells in an antigen-specific fashion in vitro. Furthermore, transferring gp33-activated DN T cells inhibited the development of autoimmune diabetes, suggesting that DN T cells may provide a novel therapy for T cell-mediated autoimmune diseases.

Immune tolerance: mechanisms and application in clinical transplantation

The achievement of immune tolerance, a state of specific unresponsiveness to the donor graft, has the potential to overcome the current major limitations to progress in organ transplantation, namely late graft loss, organ shortage and the toxicities of chronic nonspecific immumnosuppressive therapy. Advances in our understanding of immunological processes, mechanisms of rejection and tolerance have led to encouraging developments in animal models, which are just beginning to be translated into clinical pilot studies. These advances are reviewed here and the appropriate timing for clinical trials is discussed.

Autologous designer antigen-presenting cells by gene modification of T lymphocyte blasts with IL-7 and IL-12

An effective immune response to antigen requires professional antigen-presenting cell (APC), which not only present antigen, but also provide costimulation and cytokines (eg, IL-12) that drive T cell differentiation down the appropriate effector pathway (Tc1/TH1). For T cell-based immunotherapy protocols, the availability of large numbers of autologous professional APC is a major limitation because professional APC do not proliferate in vitro. T cells themselves can proliferate exponentially in vitro and have the ability to present antigen. They can also express costimulatory molecules after activation. Therefore, we hypothesized that if activated T cells were genetically modified to express proinflammatory cytokines required to polarize T cells toward a Tc1 response, they could fulfill the requirements for an abundant, autologous APC. To test this potential, T cells were activated by CD3/CD28 antibodies and pulsed with model HLA-A2+ peptides derived from CMVpp65, MAGE-3, and MART-1. Activated T-APC readily reactivated CD8 pp65 memory T cells from healthy CMV seropositive donors; however, the activation of MAGE-3 and MART-1-specific CD8 T cells required both IL-7 and IL-12, which could be provided either exogenously or by genetic modification of the T-APC. Responder T cells could be expanded to large numbers with subsequent stimulations using activated, peptide-pulsed T-APC and IL-2. Tumor antigen-specific T cell lines killed both peptide-pulsed target cells and tumor cell lines. Thus, T cells provide a platform for the generation of autologous APC that can be customized to express both antigens and therapeutic molecules for the induction of antigen-specific T cell immunity.

Both Infiltrating Regulatory T Cells and Insufficient Antigen Presentation Are Involved in Long-Term Cardiac Xenograft Survival

We have previously shown that pretransplant donor lymphocyte infusion (DLI) together with transient depletion of CD4(+) T cells could induce permanent rat-to-mouse heart graft survival, whereas depleting CD4(+) T cells alone failed to do so. In this study, we investigated the mechanism leading to long-term xenograft survival. We found that peripheral CD4(+) T cells from DLI/anti-CD4-treated mice could mount rat heart graft rejection after adoptive transfer into B6 CD4(-/-) mice. Infusing donor-Ag-loaded mature dendritic cells (DCs) could break long-term cardiac xenograft survival in DLI/anti-CD4-treated mice. Interestingly, when the number and phenotype of graft-infiltrating cells were compared between anti-CD4- and DLI/anti-CD4-treated groups, we observed a significant increase in both the number and suppressive activity of alphabeta-TCR(+)CD3(+)CD4(-)CD8(-) double negative regulatory T cells and decrease in the numbers of CD4(+) and CD8(+) T cells in the xenografts of DLI/anti-CD4-treated mice. Moreover, there was a significant reduction in MHC class II-high DCs within the xenografts of DLI/anti-CD4-treated recipients. DCs isolated from the xenografts of anti-CD4- but not DLI/anti-CD4-treated recipients could stimulate CD4(+) T cell proliferation. Our data indicate that functional anti-donor T cells are present in the secondary lymphoid organs of the mice that permanently accepted cardiac xenografts. Their failure to reject xenografts is associated with an increase in double negative regulatory T cells as well as a reduction in Ag stimulation by DCs found within grafts. These findings suggest that local regulatory mechanisms need to be taken into account to control anti-xenograft T cell responses.

Human γδ T lymphocytes strip and kill tumor cells simultaneously

When human gammadelta lymphocytes bind to tumor cells for killing, they also strip their membrane for unknown reasons. Here we investigated this topic using the model of human gammadelta lymphocytes co-incubated with anaplastic large cell lymphomas, a group of tumors with cytolytic T or null lineage. By using flow cytometry and live cell imaging, we show that as soon as both cells were in contact, the TCR-mediated activation of gammadelta lymphocytes simultaneously triggered their secretion of lytic granules and stripping of lymphoma cell membranes, and both activities continued even after their cell death. However reciprocally in such conjugates, resistant lymphoma failed to strip gammadelta cells and to kill them by untargeted secretion of their own lytic granules. This indicated that secretion of lytic granules and target membrane stripping are associated in lytic cell conjugates, and that gammadelta T lymphocytes strip and kill their targets simultaneously.

The role of T helper 17 (Th17) and regulatory T cells (Treg) in human organ transplantation and autoimmune disease

Uncommitted (naive) murine CD4+ T helper cells (Thp) can be induced to differentiate towards T helper 1 (Th1), Th2, Th17 and regulatory (Treg) phenotypes according to the local cytokine milieu. This can be demonstrated most readily both in vitro and in vivo in murine CD4+ T cells. The presence of interleukin (IL)-12 [signalling through signal transduction and activator of transcription (STAT)-4] skews towards Th1, IL-4 (signalling through STAT-6) towards Th2, transforming growth factor (TGF)-beta towards Treg and IL-6 and TGF-beta towards Th17. The committed cells are characterized by expression of specific transcription factors, T-bet for Th1, GATA-3 for Th2, forkhead box P3 (FoxP3) for Tregs and RORgammat for Th17 cells. Recently, it has been demonstrated that the skewing of murine Thp towards Th17 and Treg is mutually exclusive. Although human Thp can also be skewed towards Th1 and Th2 phenotypes there is as yet no direct evidence for the existence of discrete Th17 cells in humans nor of mutually antagonistic development of Th17 cells and Tregs. There is considerable evidence, however, both in humans and in mice for the importance of interferon (IFN)-gamma and IL-17 in the development and progression of inflammatory and autoimmune diseases (AD). Unexpectedly, some models of autoimmunity thought traditionally to be solely Th1-dependent have been demonstrated subsequently to have a non-redundant requirement for Th17 cells, notably experimental allergic encephalomyelitis and collagen-induced arthritis. In contrast, Tregs have anti-inflammatory properties and can cause quiescence of autoimmune diseases and prolongation of transplant function. As a result, it can be proposed that skewing of responses towards Th17 or Th1 and away from Treg may be responsible for the development and/or progression of AD or acute transplant rejection in humans. Blocking critical cytokines in vivo, notably IL-6, may result in a shift from a Th17 towards a regulatory phenotype and induce quiescence of AD or prevent transplant rejection. In this paper we review Th17/IL-17 and Treg biology and expand on this hypothesis.

Long-Term Cell Monitoring of Kidney Recipients After an Antilymphocyte Globulin Induction With and Without Steroids

BACKGROUND

Because of several side effects, the corticosteroid usage has been minimized in kidney transplantation. The increased acute rejection episodes associated with their withdrawal may counterbalance with induction treatment using polyclonal antilymphocyte globulin (ALG). The effects of ALG on blood cell phenotype have already been the subject of several reports. However, to date, no data are available concerning the comparison of blood phenotype when ALG is given with or without steroids and no gene profiling study has been performed.

METHODS

We report here on a longitudinal blood cell analysis of a selected cohort of kidney recipients enrolled in a randomized study of steroid avoidance or withdrawal (during 6 months) during ALG induction.

RESULTS

In the two groups, ALG quickly and massively depleted all the T cells and natural killer cells, but not B cells. Interestingly, the lymphopenia-driven homeostatic proliferation of CD4 and CD8T cells strongly differed with persistent low CD4 (including CD25CD4) T-cell counts. Effector memory CD8T cells reappeared rapidly. ALG induced apoptosis-associated molecules and increased myeloid cell genes. However, few genes were found differentially expressed with a low fold ratio between the two groups during and at distance of corticotherapy.

CONCLUSION

Thus initial steroid avoidance or withdrawal associated with ALG induction has a weak influence on phenotype and transcriptional pattern of blood leukocytes. In contrast, ALG therapy induces an early and strong depletion of all T-cell subsets with contrasted long-lasting homeostatic regulation.

Abrogation of antibody-mediated allograft rejection by regulatory CD4 T cells with indirect allospecificity

Alloantibody is an important effector mechanism for allograft rejection. In this study, we tested the hypothesis that regulatory T cells with indirect allospecificity can prevent humoral rejection by using a rat transplant model in which acute rejection of MHC class I-disparate PVG.R8 heart grafts by PVG.RT1(u) recipients is mediated by alloantibody and is dependent upon help from CD4 T cells that can recognize the disparate MHC alloantigen only via the indirect pathway. Pretransplant treatment of PVG.RT1(u) recipients with anti-CD4 mAb plus donor-specific transfusion abrogated alloantibody production and prolonged PVG.R8 graft survival indefinitely. Naive syngeneic splenocytes injected into tolerant animals did not effect heart graft rejection, suggesting the presence of regulatory mechanisms. Adoptive transfer experiments into CD4 T cell-reconstituted, congenitally athymic recipients confirmed that regulation was mediated by CD4 T cells and was alloantigen-specific. CD4 T cell regulation could be broken in tolerant animals either by immunizing with an immunodominant linear allopeptide or by depleting tolerant CD4 T cells, but surprisingly this resulted in neither alloantibody generation nor graft rejection. These findings demonstrate that anti-CD4 plus donor-specific transfusion treatment results in the development of CD4 regulatory T cells that recognize alloantigens via the indirect pathway and act in an Ag-specific manner to prevent alloantibody-mediated rejection. Their development is associated with intrinsic tolerance within the alloantigen-specific B cell compartment that persists after T cell help is made available.

An eye's view of T regulatory cells

T regulatory (Treg) cells have been studied for more than 30 years. Recently, changing technology and attitudes have led to new interest in T cell regulation of the immune responses. The eye is an immune-privileged site with unique mechanisms for the prevention of damaging immune inflammation. The eye fashions its Treg cells in novel ways to prevent immune inflammation locally and systemically. The purpose of this mini-review is to condense and summarize reports of Treg cells dependent on the eye in the context of the Treg literature in general.

A 20-Mb region of chromosome 4 controls TNF-alpha-mediated CD8+ T cell aggression toward beta cells in type 1 diabetes

Identification of candidate genes and their immunological mechanisms that control autoaggressive T cells in inflamed environments, may lead to novel therapies for autoimmune diseases, like type 1 diabetes (T1D). In this study, we used transgenic NOD mice that constitutively express TNF-alpha in their islets from neonatal life (TNF-alpha-NOD) to identify protective alleles that control T1D in the presence of a proinflammatory environment. We show that TNF-alpha-mediated breakdown in T cell tolerance requires recessive NOD alleles. To identify some of these recessive alleles, we crossed TNF-alpha-NOD mice to diabetes-resistant congenic NOD mice having protective alleles at insulin-dependent diabetes (Idd) loci that control spontaneous T1D at either the preinsulitis (Idd3.Idd5) or postinsulitis (Idd9) phases. No protection from TNF-alpha-accelerated T1D was afforded by resistance alleles at Idd3.Idd5. Lack of protection was not at the level of T cell priming, the efficacy of islet-infiltrating APCs to present islet peptides, nor the ability of high levels of CD4+ Foxp3+ T cells to accumulate in the islets. In contrast, protective alleles at Idd9 significantly increased the age at which TNF-alpha-NOD mice developed T1D. Disease delay was associated with a decreased ability of CD8+ T cells to respond to islet Ags presented by islet-infiltrating APCs. Finally, we demonstrate that the protective region on chromosome 4 that controls T1D in TNF-alpha-Idd9 mice is restricted to the Idd9.1 region. These data provide new evidence of the mechanisms by which selective genetic loci control autoimmune diseases in the presence of a strong inflammatory assault.

IL-2-independent generation of FOXP3+CD4+CD8+CD25+ cytotoxic regulatory T cell lines from human umbilical cord blood

OBJECTIVE

Since the existence of mouse naturally occurring CD4(+)CD25(+) T regulatory (Treg) cells was demonstrated, a variety of human Treg subsets have been identified as distinct T cell populations. Here we show the establishment of novel Treg cell lines possessing unique characteristics.

METHODS

Novel Treg cell lines, designated HOZOT, were generated by coculturing human umbilical cord blood cells with mouse stromal cell lines in the absence of exogenous IL-2 or other cytokines. HOZOT were characterized and compared with CD4(+)CD25(+) Treg cells in terms of the CD phenotype, FOXP3 expression, suppressor activity against allogeneic MLR, anergy property, and IL-10 production.

RESULTS

HOZOT were generated and expanded as normal lymphoblastoid cells with cytotoxic activity against the cocultured stromal cells. HOZOT consisted of three subpopulations as defined by phenotype: CD4(+)CD8(+), CD4(+)CD8(dim), and CD4(-)CD8(+). All three subpopulations showed both suppressor and cytotoxic activities. While HOZOT's expression of FOXP3, CD25, GITR, and cytoplasmic CTLA-4 implied a similarity to naturally occurring CD4(+)CD25(+) Treg cells, these two Treg cells differed in IL-2 responsiveness and IL-10 production.

CONCLUSIONS

Our studies introduce a new method of generating Treg cells in an IL-2-independent manner and highlight a unique Treg cell type with cytotoxic activity and a phenotype of FOXP3(+)CD4(+)CD8(+)CD25(+).

Lentivirally transduced recipient-derived dendritic cells serve to ex vivo expand functional FcRgamma-sufficient double-negative regulatory T cells

alphabetaTCR(+)CD4(-)CD8(-) double-negative (DN) T regulatory (Treg) cells have recently been shown to suppress antigen-specific immune responses mediated by CD8(+) and CD4(+) T cells in mice and humans. In this study, we developed a system to expand DN Treg cells for transplantation therapy that exclusively uses recipient-derived immune cells and confers a high degree of safety as the protocol does not involve the direct injection of lentiviral vectors. Recipient-derived dendritic cells (DCs) were transduced with lentiviral vectors that express major histocompatibility complex class I L(d) antigen (LV-L(d)), which is expressed by the donor graft but is allogeneic to the graft recipient. LV-L(d)-transduced mature DCs (mDCs) were able to expand effectively both FcRgamma(-/-) and FcRgamma(+/+) DN T cells. After expansion with LV-L(d)-transduced mDCs, only the FcRgamma(+/+) DN Treg cells maintained their ability to suppress CD8(+) T cells in vitro. In addition, adoptive transfer of the FcRgamma(+/+) ex vivo expanded DN Treg cells significantly prolonged the survival of L(d+) skin grafts. This study is the first description of successful ex vivo expansion of antigen-specific DN Treg cells using genetically modified syngeneic DCs for adoptive immunotherapy and demonstrates that although FcRgamma(-/-) DN T cells can be expanded, they do not gain regulatory ability.

The histopathology of fatal untreated human respiratory syncytial virus infection

The pathology of respiratory syncytial virus (RSV) infection was evaluated 1 day after an outpatient diagnosis of RSV in a child who died in a motor vehicle accident. We then identified 11 children with bronchiolitis from the Vanderbilt University autopsy log between 1925 and 1959 who met criteria for possible RSV infection in the preintensivist era. Their tissue was re-embedded and evaluated by routine hematoxylin and eosin and PAS staining and immunostaining with RSV-specific antibodies. Tissue from three cases was immunostain-positive for RSV antigen and was examined in detail. Small bronchiole epithelium was circumferentially infected, but basal cells were spared. Both type 1 and 2 alveolar pneumocytes were also infected. Although, not possible for archival cases, tissue from the index case was evaluated by immunostaining with antibodies to define the cellular components of the inflammatory response. Inflammatory infiltrates were centered on bronchial and pulmonary arterioles and consisted of primarily CD69+ monocytes, CD3+ double-negative T cells, CD8+ T cells, and neutrophils. The neutrophil distribution was predominantly between arterioles and airways, while the mononuclear cell distribution was in both airways and lung parenchyma. Most inflammatory cells were concentrated submuscular to the airway, but many cells traversed the smooth muscle into the airway epithelium and lumen. Airway obstruction was a prominent feature in all cases attributed to epithelial and inflammatory cell debris mixed with fibrin, mucus, and edema, and compounded by compression from hyperplastic lymphoid follicles. These findings inform our understanding of RSV pathogenesis and may facilitate the development of new approaches for prevention and treatment.

New differentiation pathway for double-negative regulatory T cells that regulates the magnitude of immune responses

Recent studies have demonstrated that in peripheral lymphoid tissues of normal mice and healthy humans, 1% to 5% of alphabeta T-cell receptor-positive (TCR(+)) T cells are CD4(-)CD8(-) (double-negative [DN]) T cells, capable of down-regulating immune responses. However, the origin and developmental pathway of DN T cells is still not clear. In this study, by monitoring CD4 expression during T-cell proliferation and differentiation, we identified a new differentiation pathway for the conversion of CD4(+) T cells to DN regulatory T cells. We showed that the converted DN T cells retained a stable phenotype after restimulation and that furthermore, the disappearance of cell-surface CD4 molecules on converted DN T cells was a result of CD4 gene silencing. The converted DN T cells were resistant to activation-induced cell death (AICD) and expressed a unique set of cell-surface markers and gene profiles. These cells were highly potent in suppressing alloimmune responses both in vitro and in vivo in an antigen-specific manner. Perforin was highly expressed by the converted DN regulatory T cells and played a role in DN T-cell-mediated suppression. Our findings thus identify a new differentiation pathway for DN regulatory T cells and uncover a new intrinsic homeostatic mechanism that regulates the magnitude of immune responses. This pathway provides a novel, cell-based, therapeutic approach for preventing allograft rejection.

Neutralizing IL-7 promotes long-term allograft survival induced by CD40/CD40L costimulatory blockade

Memory T cells are somewhat resistant to immunosuppresion. They therefore pose a threat to inducing long-term allograft survival. IL-7 is essential for memory T-cell generation. Here, we investigated whether neutralizing IL-7 promotes allograft survival. We found that neutralizing IL-7 alone did not significantly prolong allograft survival. However, blocking both IL-7 and CD154 signaling synergistically prolonged allograft survival. In contrast, neutralizing IL-2 failed to further prolong allograft survival induced by CD40/CD154 costimulatory blockade. Allospecific memory CD8+ T-cell generation was severely impaired under the treatment of anti-IL-7 plus anti-CD154 Ab while administering recombinant IL-7 enhanced CD8+ memory generation even under donor-specific transfusion plus anti-CD154 Ab treatment. Neutralizing IL-7, but not IL-2, together with blocking CD154 synergistically suppressed the proliferation of naïve/effector CD8+ T cells infiltrating grafts. Nevertheless, neutralizing IL-7 did not alter regulatory T-cell generation while neutralizing IL-2 suppressed their generation. Hence, targeting IL-7 represents a new strategy to prolong allograft survival by acting on both naïve and memory T cells. Long-term allograft survival may be achieved by neutralizing IL-7 plus CD40/CD154 blockade, since CD40/CD154 costimulatory blockade prevents acute rejection while neutralizing IL-7 suppresses the generation of memory T cells that persist and mediate late or chronic rejection.

Double-negative T cells, activated by xenoantigen, lyse autologous B and T cells using a perforin/granzyme-dependent, Fas-Fas ligand-independent pathway

The ability to control the response of B cells is of particular interest in xenotransplantation as Ab-mediated hyperacute and acute xenograft rejection are major obstacles in achieving long-term graft survival. Regulatory T cells have been proven to play a very important role in the regulation of immune responses to self or non-self Ags. Previous studies have shown that TCRalphabeta+CD3+CD4-CD8- (double-negative (DN)) T cells possess an immune regulatory function, capable of controlling antidonor T cell responses in allo- and xenotransplantation through Fas-Fas ligand interaction. In this study, we investigated the possibility that xenoreactive DNT cells suppress B cells. We found that DNT cells generated from wild-type C57BL/6 mice expressed B220 and CD25 after rat Ag stimulation. These xenoreactive B220+CD25+ DNT cells lysed activated, but not naive, B and T cells. This killing, which took place through cell-cell contact, required participation of adhesion molecules. Our results indicate that Fas ligand, TGF-beta, TNF-alpha, and TCR-MHC recognition was not involved in DNT cell-mediated syngenic cell killing, but instead this killing was mediated by perforin and granzymes. The xenoreactive DNT cells expressed high levels of granzymes in comparison to allo- or xenoreactive CD8+ T cells. Adoptive transfer of DNT cells in combination with early immune suppression by immunosuppressive analog of 15-deoxyspergualin, LF15-0195, significantly prolonged rat heart graft survival to 62.1 +/- 13.9 days in mice recipients. In conclusion, this study suggests that xenoreactive DNT cells can control B and T cell responses in perforin/granzyme-dependent mechanisms. DNT cells may be valuable in controlling B and T cell responses in xenotransplantation.

Early CD30 signaling is critical for adoptively transferred CD4+CD25+ regulatory T cells in prevention of acute graft-versus-host disease

Murine CD4+CD25+ regulatory T cells (Treg cells) reduce acute graft-versus-host disease (aGvHD). However, surface molecules critical for suppression are unclear. Deficiency of CD30 (CD30-/-) leads to impaired thymic negative selection and augmented T-cell autoreactivity. Therefore, we investigated the role of CD30 signaling in Treg-cell function during aGvHD. Treg cells derived from CD30-/- animals were significantly less effective in preventing aGvHD lethality. Early blockade of the CD30/CD153 pathway with a neutralizing anti-CD153 mAb reduced Treg-mediated protection from proinflammatory cytokine accumulation and donor-type T-cell apoptosis. In vivo bioluminescence imaging demonstrated intact homing but reduced expansion of luciferase-expressing Treg cells when CD153 was blocked during the early phase after adoptive transfer. CD30 surface expression on Treg cells increased with alloantigen exposure, and CD153 expression on recipient-type dendritic cells increased in the presence of a proinflammatory environment. These data demonstrate that early CD30 signaling is critical for Treg-mediated aGvHD protection after major MHC-mismatch bone marrow transplantation.

Double-negative regulatory T cells: non-conventional regulators

The crucial role of regulatory T (Treg) cells in self-tolerance and downregulating immune responses has been clearly established. Numerous different Treg subsets have been identified that possess distinct phenotypes and functions in various disease models. Among these subsets, alphabeta-TCR+CD3+CD4-CD8- double-negative (DN) Treg cells have been shown to be able to inhibit a variety of immune responses in part via direct killing of effector T cells in an antigenspecific manner in both mice and humans. This was shown to occur at least partially by acquisition of MHC-peptide complexes from antigen-presenting cells (APCs) and subsequent Fas/Fas-ligand interactions. In addition, DN Treg cells have been shown to express several molecules uncommon to other Treg cell subsets, such as IFN-gamma, TNF-alpha, Ly6A, FcRgamma, and CXCR5, which may contribute to their unique regulatory ability. Understanding the development and regulatory functions of DN Treg cells may elucidate the etiology for loss of self-tolerance and serve as a therapeutic modality for various diseases. This review will summarize the characteristics, developmental pathways, and mechanisms of action of DN Treg cells, as well as their role in transplant tolerance, autoimmunity, and anticancer immunity.

CD8+ regulatory T cells generated by neonatal recognition of peripheral self-antigen

In comparison with CD4+ regulatory T cells, the generation and function of immunomodulatory CD8+T cells is less well defined. Here we describe the existence of regulatory anti-Kb-specific CD8+ T cells that are rendered tolerant during neonatal life via antigen contact exclusively on keratinocytes. These regulatory T cells maintain tolerance during adulthood as they prevent Kb-specific graft rejection by naïve CD8+ T cells. Third-party immune responses remain unaffected. Up-regulation of TGF-beta1 and granzyme B in the regulatory CD8+ T cell population suggests the involvement of these molecules in common suppressive pathways shared with CD4+ regulatory T cells. In summary, CD8+ regulatory T cells can be induced extrathymically through antigen contact on neonatally accessible parenchymal cells and maintain tolerance throughout adult life.

Role and mechanisms of CD4+CD25+ regulatory T cells in the induction and maintenance of transplantation tolerance

To gain transplantation tolerance between donor organs and hosts is the ultimate goal of all sorts of organ transplantations. Induction of regulatory T cells has been demonstrated to lead to transplantation tolerance. This paper will review subsets of regulatory T cells, the role and mechanisms of CD4(+)CD25(+) regulatory T cells (Tregs) in graft rejection and tolerance, pathway used by Tregs to recognized alloantigens, pathways of Tregs homing into the graft and effects of immunosuppression on Tregs. It was well known that Tregs play a pivotal role in transplantation tolerance. The mechanisms by which Tregs exert their regulatory effect in the induction and maintenance of transplantation tolerance, anthropogenically, consist of physical cell-to-cell contact with potential target cells, autocrine and paracrine properties. ICAM-1, TGF-beta, CTLA-4, GITR and OX40 (CD134), etc. are involved in the regulatory function of Tregs through cell-to-cell contact mechanism. IL-10 and TGF-beta are two important soluble mediators involved in the autocrine mechanism by which Tregs exert their regulatory function. Paracrine properties refer to re-educate potentially destructive alloresponsive T cells to gain regulatory function. All that discussed above could illustrate, at least partially, how naturally occurring Tregs exert their regulatory function in vivo as they constitute only 5-10% of peripheral CD4(+) T cells.

Regulatory T Cells and Transplantation Tolerance

In the past decade, several types of regulatory T cells (Tregs) have been identified to play a pivotal role in the control of autoimmunity and transplantation tolerance in rodents and in human beings, including innate regulatory NKT cells and gammadelta T cells, naturally occurring FoxP3 expressing CD4(+)CD25(+) T cells, and in-vitro induced Tregs including interleuking-10 (IL-10)-secreting Tr1 CD4(+) T cells, TGF-beta-producing Th3 CD4(+) T cells, anergic CD4(+) T cells, CD8(+)CD28(-) and CD3(+)CD4(-)CD8(-) T cells. Recent studies have shown that innate and adaptive Tregs may be linked and act in concert to mediate immunosuppression. As our understanding of regulatory T cell populations has substantially advanced, compelling evidence support the prospect that in-vitro expanded, patient-tailored Tregs with indirect anti-donor allospecificity could be potential reagents as adoptive cell therapy for individualized medicine to promote clinical transplantation tolerance.

FcR gamma presence in TCR complex of double-negative T cells is critical for their regulatory function

TCRalphabeta+CD4-CD8- double-negative (DN) T regulatory (Treg) cells have recently been shown to suppress Ag-specific immune responses mediated by CD8+ and CD4+ T cells in humans and mice. Our previous study using cDNA microarray analysis of global gene expression showed that FcRgamma was the most highly overexpressed gene in functional DN Treg cell clones compared with nonfunctional mutant clones. In this study, we demonstrate that FcRgamma-deficient DN T cells display markedly reduced suppressive activity in vitro. In addition, unlike FcRgamma-sufficient DN T cells, FcRgamma-deficient DN T cells were unable to prolong donor-specific allograft survival when adoptively transferred to recipient mice. Protein analyses indicate that in addition to FcRgamma, DN Treg cell clones also express higher levels of TCRbeta, while mutant clones expressed higher levels of Zap70 and Lck. Within DN Treg cells, we found that FcRgamma associates with the TCR complex and that both FcRgamma and Syk are phosphorylated in response to TCR cross-linking. Inhibition of Syk signaling and FcRgamma expression were both found to reduce the suppressive function of DN Treg cells in vitro. These results indicate that FcRgamma deficiency significantly impairs the ability of DN Treg cells to down-regulate allogeneic immune responses both in vitro and in vivo, and that FcRgamma plays a role in mediating TCR signaling in DN Treg cells.

Dendritic cells can turn CD4+ T lymphocytes into vascular endothelial growth factor-carrying cells by intercellular neuropilin-1 transfer

Neuropilin-1 (NRP1) is a transmembrane protein expressed on neuronal and endothelial cells where it plays a crucial role in guiding axons and regulating angiogenesis. We have recently shown that NRP1 also is expressed on dendritic cells (DC) in the human immune system and have proposed a role for NRP1 in the first stages of the immune response. In these studies, we show that NRP1 can be transferred with a high efficiency from human DC to T lymphocytes by trogocytosis. The NRP1 transfer can occur independently of T lymphocyte activation; the amount of NRP1 transferred depends on the NRP1 expression level on APC and is enhanced when T cells are activated through the TCR. Moreover, the NRP1 transfer occurs between specific donor and recipient cells, because no NRP1 transfer is observed between endothelial cells and T lymphocytes or between APCs and CD34(+) hemopoietic cells. Finally, we show that a major NRP1 ligand, vascular endothelial growth factor (VEGF)(165), is secreted by mature human DCs and binds to NRP1 captured by T lymphocytes. These results show that NRP1 transfer to T lymphocytes during the immune synapse can convert T lymphocytes into VEGF(165)-carrying cells. Together with the enhanced signaling of VEGF-R2 on endothelial cells in the presence, in trans, of the NRP1-VEGF(165) complex, our results suggest that the intercellular transfer of NRP1 might participate in the Ag-independent remodelling of the endothelial vessels in secondary lymphoid organs during inflammation.

Double-negative T regulatory cells can develop outside the thymus and do not mature from CD8+ T cell precursors

Recent studies have demonstrated that activated peripheral alphabeta TCR+ CD3+ CD4- CD8- NK1.1- (double-negative, DN) regulatory T cells (Tregs) from both mice and humans are able to down-regulate immune responses in vitro and in vivo. However, the origin and developmental requirements of functional DN Tregs remain unclear. In this study, we investigated the requirement for CD8 expression as well as the presence of a thymus for the development of functional DN Tregs. We demonstrate that DN Tregs exist in CD8-deficient mice and that stimulation of CD8+ T cells in vivo with TCR-specific Ag does not convert CD8+ T cells into DN Tregs. In addition, we found that DN T cells are present in the spleens and lymph nodes of thymectomized mice that are irradiated and reconstituted with T cell-depleted bone marrow cells. Interestingly, DN Tregs that develop in thymectomized mice can suppress syngeneic CD8+ T cells more effectively than those that develop in sham-thymectomized mice. Taken together, our data suggest that DN Tregs are not derived from CD8+ T cell precursors and that functional DN Tregs may preferentially develop outside of the thymus. These data suggest that DN Tregs may represent a developmentally and functionally unique cell population.

Mechanisms of allergen-specific immunotherapy: T-regulatory cells and more

Activation-induced cell death, anergy, or immune response modulation by regulatory T cells (Treg cells) are essential mechanisms of peripheral T-cell tolerance. Genetic predisposition and environmental instructions tune thresholds for the activation of T cells, other inflammatory cells, and resident tissue cells in allergic diseases. Skewing allergen-specific effector T cells to a Treg-cell phenotype seems to be crucial in maintaining a healthy immune response to allergens and successful allergen-specific immunotherapy. The Treg-cell response is characterized by an abolished allergen-specific T-cell proliferation and the suppressed secretion of T-helper 1- and T-helper 2-type cytokines. Suppressed proliferative and cytokine responses against allergens are induced by multiple suppressor factors, including cytokines such as interleukin-10 (IL-10) and transforming growth factor beta (TGF-beta), and cell surface molecules such as cytotoxic T-lymphocyte antigen-4, programmed death-1, and histamine receptor 2. The increased levels of IL-10 and TGF-beta produced by Treg cells potently suppress IgE production while simultaneously increasing the production of noninflammatory isotypes IgG4 and IgA, respectively. In addition, Treg cells directly or indirectly suppress the activity of 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 on T cells, regulation of antibody isotypes, and suppression of effector cells. The application of current knowledge of Treg cells and related mechanisms of peripheral tolerance may soon lead to more rational and safer approaches to the prevention and cure of allergic disease.

Deciphering CD30 ligand biology and its role in humoral immunity

Ligands and receptors in the tumour necrosis factor (TNF) and tumour necrosis factor receptor (TNFR) superfamilies have been the subject of extensive investigation over the past 10-15 years. For certain TNFR family members, such as Fas and CD40, some of the consequences of receptor ligation were predicted before the identification and cloning of their corresponding ligands through in vitro functional studies using agonistic receptor-specific antibodies. For other members of the TNFR family, including CD30, cross-linking the receptor with specific antibodies failed to yield many clues about the functional significance of the relevant ligand-receptor interactions. In many instances, the subsequent availability of TNF family ligands in the form of recombinant protein facilitated the determination of biological consequences of interactions with their relevant receptor in both in vitro and in vivo settings. In the case of CD30 ligand (CD30L; CD153), definition of its biological role remained frustratingly elusive. Early functional studies using CD30L+ cells or agonistic CD30-specific antibodies logically focused attention on cell types that had been shown to express CD30, namely certain lymphoid malignancies and subsets of activated T cells. However, it was not immediately clear how the reported activities from these in vitro studies relate to the biological activity of CD30L in the more complex whole animal setting. Recently, results from in vivo models involving CD30 or CD30L gene disruption, CD30L overexpression, or pharmacological blockade of CD30/CD30L interactions have begun to provide clues about the role played by CD30L in immunological processes. In this review we consider the reported biology of CD30L and focus on results from several recent studies that point to an important role for CD30/CD30L interactions in humoral immune responses.

Regulatory T cell-mediated transplantation tolerance

The existence of naturally occurring regulatory T cells in normal hosts and their pivotal role in maintaining both auto- and allo-tolerance have direct implications on the therapy of autoimmune disorders and for achieving immunosuppression-free allotransplantation. Among the various forms of regulatory T cells described, CD4(+)CD25(+) T cells have emerged as one of the most potent tolerogenic subsets. In this review, we discuss the molecular basis of development and function of these regulatory T cells and their potential role in the context of chronic lung allograft rejection.

Dendritic cells as promoters of transplant tolerance

Dendritic cells (DCs) play a crucial role during the initiation of immune responses against non-self antigens. Following organ transplantation, activated donor- and recipient-derived DCs participate actively in graft rejection by sensitising recipient T cells via the direct or indirect pathways of allorecognition, respectively. There is increasing evidence that immature/semi-mature DCs induce antigen-specific unresponsiveness or tolerance to self antigens, both in central lymphoid tissue and in the periphery, through a variety of mechanisms (deletion, anergy and regulation). In the past few years, DC-based therapy of experimental allograft rejection has focused on ex vivo biological, pharmacological and genetic engineering of DCs to mimic/enhance their natural tolerogenicity. Successful outcomes in rodent models have built the case that DC-based therapy may provide a novel approach to transplant tolerance. Ongoing research into the role that DCs play in the induction of tolerance should allow for its clinical application in the near future.

CXCR5/CXCL13 interaction is important for double-negative regulatory T cell homing to cardiac allografts

Accumulating evidence indicates that regulatory T (Treg) cells control development of various diseases both systemically and locally. However, molecular mechanisms involved in Treg cell homing remain elusive. We have shown previously that alphabetaTCR(+)CD3(+)CD4(-)CD8(-) double-negative (DN) Treg cells selectively accumulate in tolerant allografts to maintain localized immune regulation. However, the molecular mechanism leading to the accumulation of DN Treg cells in tolerant grafts was not known. Our cDNA microarray analysis revealed significant up-regulation of chemokine receptor CXCR5 mRNA in DN Treg clones compared with nonregulatory clones. In this study, we examined the importance of CXCR5 in mediating DN Treg migration. Compared with CD4 and CD8 T cells, both primary DN Treg cells and clones constitutively express high levels of CXCR5 protein, enabling them to migrate toward increasing CXCL13 gradients in vitro. After infusion into recipient mice, CXCR5(+) DN Treg clones, but not their CXCR5(-) mutants, preferentially accumulated in cardiac allografts and could prevent graft rejection. Furthermore, we found that allogeneic cardiac allografts express high levels of CXCL13 mRNA compared with either recipient native hearts or nontransplanted donor hearts. Ab neutralization of CXCL13 abrogated DN Treg cell migration in vitro and prevented in vivo homing of DN Treg clones into allografts. These data demonstrate that DN Treg cells preferentially express CXCR5, and interaction of this chemokine receptor with its ligand CXCL13 plays an important role in DN Treg cell migration both in vitro and in vivo.

Immunological analogy between allograft rejection, recurrent abortion and pre-eclampsia - the same basic mechanism?

There are still controversies concerning the role of immunological mechanisms engaged both in recurrent abortions (RA) and pre-eclampsia (PE). According to some opinions, recurrent miscarriage is comparable to organ-specific autoimmune disease. Analysis of immune reactions shows that graft rejection shares many similar mechanisms with RA and PE. This fact allows us to conclude that rejection of transplanted alloantigenic organs and pregnancy loss have probably the same evolutionary origin. Subsets and functions of immunocompetent cells (T CD4, suppressor gammadeltaT, cytotoxic T CD8, Treg, Tr1, uterine NK cells), over-activation of innate immunity (activation of NK cytotoxic cells, macrophages, neutrophils and complement), changes of Th1/Th2 cytokine balance (IL-2, IL-12, IL-15, IL-18, IFNgamma, TNFalpha vs. IL-4, IL-10, TGFbeta), importance of HLA-G molecule, CD200/CD200R interaction, over-expression of adhesion molecules, fgl2 prothrombinase activation and stimulation of IDO and HO expression, all suggest that RA and PE are syndromes of fetal allograft rejection, and not organ-specific autoimmune diseases. According to that supposition, an analogy might exist between acute graft rejection and recurrent abortion, and between chronic graft rejection and pre-eclampsia.

CD4+ T Regulatory Cell Induction and Function in Transplant Recipients after CD154 Blockade Is TLR4 Independent

Although the role of CD4(+) T regulatory cells (Treg) in transplantation tolerance has been established, putative mechanisms of Treg induction and function in vivo remain unclear. TLR4 signaling has been implicated in the regulation of CD4(+)CD25(+) Treg functions recently. In this study, we first examined the role of recipient TLR4 in the acquisition of operational CD4(+) Treg following CD154 blockade in a murine cardiac transplant model. Then, we determined whether TLR4 activation in allograft tolerant recipients would reverse alloimmune suppression mediated by CD4(+) Treg. We document that donor-specific immune tolerance was readily induced in TLR4-deficient recipients by a single dose of anti-CD154 mAb, similar to wild-type counterparts. The function and phenotype of CD4(+) Treg in both wild-type and TLR4 knockout long-term hosts was demonstrated by a series of depletion experiments examining their ability to suppress the rejection of secondary donor-type test skin grafts and to inhibit alloreactive CD8(+) T cell activation in vivo. Furthermore, TLR4 activation in tolerant recipients following exogenous LPS infusion in conjunction with donor-type skin graft challenge, failed to break Treg-mediated immune suppression. In conclusion, our data reveals a distinctive property of CD4(+) Treg in tolerant allograft recipients, whose induction and function are independent of TLR4 signaling.

Contrasting CD25hiCD4+T cells/FOXP3 patterns in chronic rejection and operational drug-free tolerance

BACKGROUND

Although immunosuppression withdrawal in kidney recipients usually leads to rejection, in some patients it does not, leading to a state of clinical operational tolerance.

METHODS

We compared these highly contrasted situations by analyzing blood cell phenotype and transcriptional patterns in drug-free spontaneously tolerant kidney recipients, recipients with chronic rejection, recipients with stable graft function under standard or minimal immunosuppression and healthy individuals

RESULTS

The blood cell phenotype of clinically tolerant patients did not differ from that of healthy individuals. In contrast, recipients with chronic rejection had significantly less CD25hiCD4+T cells and lower levels of FOXP3 transcripts compared with clinically tolerant recipients. Patients with chronic rejection also displayed CD25-CD4+T cells expressing NKG2D+CD94+ and CD57+CD27-CD28- cytotoxic-associated markers (P<0.05).

CONCLUSION

These data show that whereas clinically tolerant recipients displayed normal levels of CD25hiCD4+T cells and FOXP3 transcripts, chronic rejection is associated with a decrease in CD25hiCD4+T cells and FOXP3 transcripts, suggesting that clinically "operational tolerance" may be due to a maintained phenomenon of natural tolerance that is lacking in patients with chronic rejection.