The immune regulatory function of lymphoproliferative double negative T cells in vitro and in vivo

Characterization of the immunoregulatory function of human TCR-αβ+ CD4- CD8- double-negative T cells

Regulatory T cells (Tregs) play an important role in the maintenance of immune tolerance to self-antigens and are involved in modulating immune responses in autoimmunity, transplant rejection, and tumor immunity. Recently, a novel subset of TCR-αβ(+) CD4(-) CD8(-) (double negative, DN) T cells has been described to specifically suppress T-cell responses in mice. Here, we demonstrate that human DN T cells are highly potent suppressors of both CD4(+) and CD8(+) T-cell responses. In contrast to naturally occurring CD4(+) CD25(+) Tregs, DN T cells have to be activated by antigen-presenting cells (APCs) to induce their regulatory potential. The suppressive activity of DN T cells is neither mediated indirectly by modulation of APCs nor by competition for T-cell growth factors. Furthermore, DN T-cell-mediated suppression toward responder T cells is TCR dependent and requires novel protein synthesis. In contrast to murine DN T cells, which eliminate effector T cells via Fas/FasL or perforin/granzyme, human DN T cells suppress proliferation of responder T cells by cell contact-dependent mechanisms. Taken together, our data indicate that human DN T cells exert strong immunosuppressive effects on both CD4(+) and CD8(+) T cells and may serve as a new therapeutic approach to treat autoimmunity and transplant rejection.

Adoptive transfer of DNT cells induces long-term cardiac allograft survival and augments recipient CD4(+)Foxp3(+) Treg cell accumulation

Regulatory T (Treg) cells play an important role in the regulation of immune responses but whether Treg will induce tolerance in transplant recipients in the clinic remains unknown. Our previous studies have shown that TCRαβ(+)CD3(+)CD4⁻CD8⁻NK1.1⁻ (double negative, DN) T cells suppress T cell responses and prolong allograft survival in a single locus MHC-mismatched mouse model. In this study, we investigated the role of DNT cells in a more robust, fully MHC-mismatched BALB/c to C57BL/6 transplantation model, which may be more clinically relevant. Adoptive transfer of DNT cells in combination with short-term rapamycin treatment (days 1-9) induced long-term heart allograft survival (101±31 vs. 39±13 days rapamycin alone, p<0.01). Furthermore adoptive transfer DNT cells augmented CD4+Foxp3+ Treg cells accumulation in transplant recipients while depletion of CD4(+) Treg cells by anti-CD25 inhibited the effect of DNT cells on long-term graft survival (48±12 days vs. 101±31 days, p<0.001). In conclusion, DNT cells combined with short-term immunosuppression can prolong allograft survival, which may be through the accumulation of CD4(+)Foxp3(+) Treg cells in the recipient. Our result suggests that allograft tolerance may require the co-existence of different type Treg cell phenotypes which are affected by current immunosuppression.

Regulatory CD4(-)CD8(-) double negative T cells

Peripheral αβTCR(+)CD3(+)CD4(-)CD8(-) NK1.1/CD56(-) double-negative (DN) Treg cells are a relatively rare subset of regulatory cells found in both humans and mice, typically comprising less than 5% of the total peripheral T-cell pool. Numerous studies have shown that DN Tregs can inhibit CD4(+) and CD8(+) T-cell responses in vitro and in vivo using a variety of model systems [Zhang et al., Nature Medicine 6:782, 2000; Young et al., Blood 100:3408, 2002; Ford et al., Experimental Medicine 196:261, 2002; Young et al., Journal of Immunology 171:134, 2003; Ford et al., European Journal of Immunology 37:2234, 2007; Zhang et al., Blood 109:4071, 2007; Fischer et al., Blood 105:2828, 2005]. This chapter describes published methods for the phenotypic identification of DN Tregs, their isolation from secondary lymphoid organs of mice or human peripheral blood, activation and expansion, and assays for their ability to suppress T-cell proliferation, induce apoptosis, and promote tolerance to allografts in vivo.

Double negative (CD3+ 4- 8-) TCR alphabeta splenic cells from young NOD mice provide long-lasting protection against type 1 diabetes

BACKGROUND

Double negative CD3(+)4(-)8(-) TCR alphabeta splenic cells (DNCD3) can suppress the immune responses to allo and xenografts, infectious agents, tumors, and some autoimmune disorders. However, little is known about their role in autoimmune diabetes, a disease characterized by the reduction of insulin production subsequent to destruction of pancreatic beta-cells by a polyclonal population of self-reactive T-cells. Herein, we analyzed the function and phenotype of DNCD3 splenic cells in young NOD mice predisposed to several autoimmune disorders among which, the human-like autoimmune diabetes.

METHODOLOGY/PRINCIPAL FINDINGS

DNCD3 splenic cells from young NOD mice (1) provided long-lasting protection against diabetes transfer in NOD/Scid immunodeficient mice, (2) proliferated and differentiated in the spleen and pancreas of NOD/Scid mice and pre-diabetic NOD mice into IL-10-secreting T(R)-1 like cells in a Th2-like environment, and (3) their anti-diabetogenic phenotype is CD3(+)(CD4(-)CD8(-))CD28(+)CD69(+)CD25(low) Foxp3(-) iCTLA-4(-)TCR alphabeta(+) with a predominant Vbeta13 gene usage.

CONCLUSIONS/SIGNIFICANCE

These findings delineate a new T regulatory component in autoimmune diabetes apart from that of NKT and CD4(+)CD25(high) Foxp3(+)T-regulatory cells. DNCD3 splenic cells could be potentially manipulated towards the development of autologous cell therapies in autoimmune diabetes.

The effect of murine anti-thymocyte globulin on experimental kidney warm ischemia-reperfusion injury in mice

Kidney ischemia-reperfusion injury (IRI) is an important contributor to delayed graft function (DGF) and poor outcome of allografts. Small clinical studies suggest a beneficial role for human anti-thymocyte globulin (ATG) in DGF. We investigated the short-term effect of mouse anti-thymocyte globulin (mATG) on kidney warm IRI in mice. We administered either mATG, rabbit immunoglobulin (RIgG), or saline with different dosing schedules in three different IRI models: 30 min bilateral, 60 min bilateral, and 45min unilateral IRI. mATG effectively depleted circulating T cells but had less effect on kidney-infiltrating T cells. There was no difference in serum creatinine levels between groups in each study. Scoring of renal tubular damage and regenerating tubules revealed no difference between groups. The percentage of CD3(+)CD4(-)CD8(-) double-negative (DN) T cells, which were reported to contribute to the pathogenesis of lupus nephritis, increased and the percentages of regulatory T cells and NK cells decreased in the post-ischemic kidneys of mATG treated mice. mATG did not alter the expression of pro-inflammatory cytokines such as IFN-gamma or anti-inflammatory cytokines such as IL-10 in post-ischemic kidneys. mATG treatment, whether initiated before ischemia or immediately after reperfusion, had minimal effects on renal injury following warm IRI in mice.

Apolipoprotein A-I and its role in lymphocyte cholesterol homeostasis and autoimmunity

OBJECTIVE

The purpose of this study was to determine the effects of an atherogenic diet on immune function in LDLr(-/-), ApoA-I(-/-) mice.

METHODS AND RESULTS

When LDLr(-/-), ApoA-I(-/-) (DKO), and LDLr(-/-) (SKO) mice were fed an atherogenic diet, DKO had larger peripheral lymph nodes (LNs) and spleens compared to SKO mice. LNs were enriched in cholesterol and contain expanded populations of T, B, dendritic cells, and macrophages. Expansion of all classes of LN cells was accompanied by a approximately 1.5-fold increase in T cell proliferation and activation. Plasma antibodies to dsDNA, beta2-glycoprotein I, and oxidized LDL were increased in DKO, similar to levels in diet-fed Fas(lpr/lpr) mice, suggesting the development of an autoimmune phenotype. Both LN enlargement and cellular cholesterol expansion were "prevented" when diet-fed DKO mice were treated with helper dependent adenovirus expressing apoA-I. Independent of the amount of dietary cholesterol, DKO mice consistently showed lower plasma cholesterol than SKO mice, yet greater aortic cholesterol deposition and inflammation.

CONCLUSIONS

ApoA-I prevented cholesterol-associated lymphocyte activation and proliferation in peripheral LN of diet-fed DKO mice. A approximately 1.5-fold increase in T cell activation and proliferation was associated with a approximately 3-fold increase in concentrations of circulating autoantibodies and approximately 2-fold increase in the severity of atherosclerosis suggesting a common link between plasma apoA-I, inflammation, and atherosclerosis.

Lymphotoxin beta receptor signaling promotes tertiary lymphoid organogenesis in the aorta adventitia of aged ApoE-/- mice

Atherosclerosis involves a macrophage-rich inflammation in the aortic intima. It is increasingly recognized that this intimal inflammation is paralleled over time by a distinct inflammatory reaction in adjacent adventitia. Though cross talk between the coordinated inflammatory foci in the intima and the adventitia seems implicit, the mechanism(s) underlying their communication is unclear. Here, using detailed imaging analysis, microarray analyses, laser-capture microdissection, adoptive lymphocyte transfers, and functional blocking studies, we undertook to identify this mechanism. We show that in aged apoE^−/− mice, medial smooth muscle cells (SMCs) beneath intimal plaques in abdominal aortae become activated through lymphotoxin β receptor (LTβR) to express the lymphorganogenic chemokines CXCL13 and CCL21. These signals in turn trigger the development of elaborate bona fide adventitial aortic tertiary lymphoid organs (ATLOs) containing functional conduit meshworks, germinal centers within B cell follicles, clusters of plasma cells, high endothelial venules (HEVs) in T cell areas, and a high proportion of T regulatory cells. Treatment of apoE^−/− mice with LTβR-Ig to interrupt LTβR signaling in SMCs strongly reduced HEV abundance, CXCL13, and CCL21 expression, and disrupted the structure and maintenance of ATLOs. Thus, the LTβR pathway has a major role in shaping the immunological characteristics and overall integrity of the arterial wall.

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.

B cell depletion with anti-CD79 mAbs ameliorates autoimmune disease in MRL/lpr mice

MRL/lpr mice develop a spontaneous systemic lupus erythematosus-like autoimmune syndrome due to a dysfunctional Fas receptor, with contributions from other less well-defined genetic loci. The removal of B cells by genetic manipulation not only prevents autoantibody formation, but it also results in substantially reduced T cell activation and kidney inflammation. To determine whether B cell depletion by administration of Abs is effective in lupus mice with an intact immune system and established disease, we screened several B cell-specific mAbs and found that a combination of anti-CD79alpha and anti-CD79beta Abs was most effective at depleting B cells in vivo. Anti-CD79 therapy started at 4-5 mo of age in MRL/lpr mice significantly decreased B cells (B220(+)CD19(+)) in peripheral blood, bone marrow, and spleens. Treated mice also had a significant increase in the number of both double-negative T cells and naive CD4(+) T cells, and a decreased relative abundance of CD4(+) memory cells. Serum anti-chromatin IgG levels were significantly decreased compared with controls, whereas serum anti-dsDNA IgG, total IgG, or total IgM were unaffected. Overall, survival was improved with lower mean skin scores and significantly fewer focal inflammatory infiltrates in submandibular salivary glands and kidneys. Anti-CD79 mAbs show promise as a potential treatment for systemic lupus erythematosus and as a model for B cell depletion in vivo.

Human TCR alpha/beta+ CD4-CD8- double-negative T cells in patients with autoimmune lymphoproliferative syndrome express restricted Vbeta TCR diversity and are clonally related to CD8+ T cells

The peripheral expansion of alpha/beta+-CD4-CD8- double negative (DN) T cells in patients with autoimmune lymphoproliferative syndrome (ALPS) is a consistent feature of this disease, and part of the diagnostic criteria of ALPS. The origin of these cells remains undetermined. They could derive from mature T cells that have lost coreceptor expression, or represent a special minor cell lineage. To investigate relationship of DN and single positive (SP) T cells in ALPS, we used Immunoscope technology to analyze the TCRVbeta repertoire diversity of sorted DN and SP T cells, and we performed CDR3 sequence analyses of matching clonotypes. We show that DN T cells express all the Vbeta gene families that are used by their SP counterparts, though they dominantly use some Vbeta genes. Analysis of CDR3 length distribution revealed a diverse polyclonal TCR repertoire for sorted CD4+ T cells, whereas both DN and CD8+ T cells showed a skewed TCR repertoire with oligoclonal expansions throughout most of the Vbeta families. CDR3 sequencing of matching clonotypes revealed a significant sharing of CDR3 sequences from selected Vbeta-Jbeta transcripts between DN and CD8+ T cells. Altogether, these data strongly argue for a CD8 origin of DN T cells in ALPS.

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.

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.

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.

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.

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.

OBF-1 is essential for the generation of antibody-secreting cells and the development of autoimmunity in MRL-lpr mice

As reported previously, the lack of the transcriptional co-activator OBF-1 prevented development of autoimmunity in Aiolos knockout mice. To further investigate the role and mechanism of OBF-1 in autoimmunity, we crossed OBF-1 null mice with MRL-lpr mice and generated OBF-1-deficent MRL-lpr mice. OBF-1 deletion abrogated all autoantibodies in the MRL-lpr mice, including anti-dsDNA Ab and anti-Sm Ab. The failure to produce autoantibodies was not related to development of immature or mature B cells, but correlated with severely reduced antibody-secreting cells (ASCs). The loss of OBF-1 protected against hypergammaglobulinemia, immune complex deposition, glomerulonephritis, and early mortality in MRL-lpr mice. In addition, accumulation of CD4(-)CD8(-)B220(+)CD3(+) T cells that characteristically develop in Fas mutation mice were markedly reduced in MRL-lpr mice without OBF-1. These results identify OBF-1 as a critical gene in the development of autoantibodies and reveal an essential role for OBF-1 in the generation of antibody/autoantibody-secreting cells in vivo.

Treatment of autoimmune diseases in MRL/lpr mice by allogenic bone marrow transplantation plus adult thymus transplantation

MRL/lpr mice (H-2(k)) with Fas gene mutation develop severe autoimmune diseases, and their haematolymphoid cells such as bone marrow and spleen cells showed a low apoptotic activity by irradiation. Therefore, conventional bone marrow transplantation (BMT) cannot be used to treat autoimmune diseases in these mice (chimeric resistance). In the present study, we examine the effects of additional adult thymus transplantation (TT) from the same donor on successful BMT. When the MRL/lpr mice were lethally irradiated (9 x 5Gy) and reconstituted with 3 x 10(7) of C57BL/6 mouse (H-2b) bone marrow cells (BMCs) in conjunction with TT, the mice significantly survived long term and showed a high donor-derived chimerism in comparison with those treated with BMT alone. Interestingly, the numbers of not only donor-derived T cells but also B cells increased significantly in the mice treated with BMT plus TT, even at the early phase of BMT. The number of aberrant CD3+B220+ cells decreased significantly, and the numbers of lymphocyte subsets were also normalized 4 weeks after the treatment. Finally, the autoimmune diseases in MRL/lpr mice could be cured by BMT with TT. These results indicate that the combination of BMT plus TT can overcome the chimeric resistance and treat the autoimmune diseases in MRL/lpr mice.

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.

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.

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.

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.

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.

The E delta enhancer controls the generation of CD4- CD8- alphabetaTCR-expressing T cells that can give rise to different lineages of alphabeta T cells

It is well established that the pre-T cell receptor for antigen (TCR) is responsible for efficient expansion and differentiation of thymocytes with productive TCRbeta rearrangements. However, Ptcra- as well as Tcra-targeting experiments have suggested that the early expression of Tcra in CD4- CD8- cells can partially rescue the development of alphabeta CD4+ CD8+ cells in Ptcra-deficient mice. In this study, we show that the TCR E delta but not E alpha enhancer function is required for the cell surface expression of alphabetaTCR on immature CD4- CD8- T cell precursors, which play a crucial role in promoting alphabeta T cell development in the absence of pre-TCR. Thus, alphabetaTCR expression by CD4- CD8- thymocytes not only represents a transgenic artifact but occurs under physiological conditions.

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.

Oral vaccination against Helicobacter pylori infection is not effective in mice with Fas ligand deficiency

The aim of this study was to delineate the role of the Fas pathway in vaccination against Helicobacter pylori. C57BL/6 and Fas ligand-deficient (gld) mice were divided into 3 groups: control, H. pylori infected, and orally vaccinated (H. pylori whole cell sonicate and cholera toxin adjuvant). Oral vaccination prevented H. pylori colonization in 78% of C57BL/6 mice compared to only 18% of gld mice. Vaccination did not alter the degree of apoptosis in either strain of mice. Vaccination led to significant increase in interleukin (IL)-5 and IL-10 in C57BL/6 but not gld mice. H. pylori infection increased interferon (IFN)-gamma levels in C57BL/6 but not in gld mice while vaccination had no effect on IFN-gamma levels in either strain. Oral vaccination is not effective in Fas ligand-deficient mice likely owing to lack of effective cytokine responses. This indicates that the Fas pathway plays a critical role in promoting an appropriate effector response following H. pylori vaccination.

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.

Identification of T-cell epitopes on U1A protein in MRL/lpr mice: double-negative T cells are the major responsive cells

Systemic lupus erythematosus (SLE) is characterized by the existence of a heterogeneous group of autoantibodies such as anti-DNA, chromatin, histone, and ribonucleoprotein antibodies (Abs). Although the B-cell antigenic determinants have been well characterized, very limited data about the T-cell epitopes of self-antigen (Ag) have been reported. In the present study, we analysed auto-T-cell epitopes using bone marrow-derived dendritic cells (BM-DCs) pulsed with murine U1A (mU1A) protein capable of activating autoreactive T cells from unprimed MRL/lpr mice in vitro. The data suggested that there are at least four T-cell epitopes on the U1A protein, U1A31-50, U1A61-80, U1A201-220 and U1A271-287, and U1A31-50 had the most significant T-cell proliferative response. In addition, the main responsive T cells are the CD4- CD8- double-negative subgroup of T cells. Furthermore, we also demonstrated that the activation of double-negative T cells is major histocompatibility complex class II restricted. The study here provides information on T-cell epitope analysis of the U1A antigen using BM-DCs as the effective antigen-presenting cells.

Tolerization of dendritic cells by HLA-G

The expression of HLA-G at the fetal-maternal interface during pregnancy and in transplanted tissue makes this a key molecule in the acceptance of a semiallogeneic fetus and allogeneic transplant. Dendritic cells (DC) play a critical role in the control of innate and adaptive immune responses. DC are present in maternal decidua, but must be kept under tight control. Here we describe the mechanism of tolerization of DC by HLA-G through inhibitory receptor interactions. The HLA-G-ILT (immunoglobulin-like transcript) interaction leads to development of tolerogenic DC with the induction of anergic and immunosuppressive T cells. Using human monocyte-derived DC and ILT4-transgenic mice, we show that (i) HLA-G induces the development of tolerogenic DC with arrest maturation/activation of myeloid DC, (ii) HLA-G-modified DC induce differentiation of anergic and immunosuppressive CD4(+) and CD8(+) effector T cells, and (iii) the gene expression profile provides evidence that HLA-G induces tolerogenic DC by disruption of the MHC class II presentation pathway. Ligation of ILT4 receptor on DC from transgenic mice diminished peptide presentation by MHC class II molecules and significantly prolonged allograft survival. These findings provide support that HLA-G is an important tolerogenic molecule on DC for the acceptance of a semiallogeneic fetus and transplanted tissue/organ.

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.

Characterization of a B220+Lymphoid Cell Subpopulation with Immune Modulatory Functions in Nasal-Associated Lymphoid Tissues

Complex mechanisms operate on mucosal tissues to regulate immune responsiveness and tolerance. When the lymphocyte subpopulations from murine nasal-associated lymphoid tissues (NALT) were characterized, we observed an accumulation of B220(low)CD3(low)CD4(-)CD8(-)CD19(-)c-Kit(+) cells. TCR transgenic mice and athymic mice were used for monitoring T cell lineage and the presence of extrathymic T cell precursors. The majority of cells from NALT exhibited a T cell precursor phenotype (CD4(-)CD8(-)CD19(-)c-Kit(+)). Fas-independent apoptosis was their main mechanism of cell death. We also demonstrated that B220(low)CD4(-)CD8(-)CD19(-) cells from NALT exhibited the potential to down-regulate the activation of mature T cells. However, the innate immunity receptor TLR2 was also highly expressed by this cell subpopulation. Moreover, nasal stimulation with a TLR2/6 agonist resulted in a partial activation of the double-negative cells. These results suggest that the immune responses in NALT may be in part modulated by a cell subpopulation that maintains a tolerogenic milieu by its proapoptotic status and suppressive activity, which can be reverted through stimulation of a TLR signaling cascade.

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.

An Essential Role for IFN-α in the Overexpression of Fas Ligand on MRL/lpr Lymphocytes and on Their Spontaneous Fas-Mediated Cytotoxic Potential

Lymphocytes from aged autoimmune MRL/lpr mice overexpress Fas ligand (FasL), and are cytotoxic against Fas+ target cells. This cytotoxic potential is only partly due to FasL, as wild-type MRL+/+ lymphocytes are not able to kill Fas+ targets after induction of FasL. In addition, serum levels of interferon-alpha (IFN-alpha) increase in parallel with the Fas-dependent cytotoxic potential of lymphocytes from MRL/lpr mice as they age. To understand the mechanisms underlying these observations, combined suppression subtractive hybridization (SSH) and RT-PCR were used to study differential gene expression in splenocytes from MRL/lpr mice compared with splenocytes from MRL+/+ mice. Twenty-two genes were upregulated transcriptionally in MRL/lpr splenocytes compared with their MRL+/+ counterparts. Furthermore, 9 of these genes were also upregulated after treatment of MRL/lpr splenocytes with IFN-alpha, and 4 were strongly downregulated. MRL/lpr lymphocytes were also found to be hyperresponsive to IFN-alpha. Thus, MRL/lpr lymphocytes overexpressed mRNA for the IFN-alpha receptor (IFNAR-1 and IFNAR-2) chains of the IFN-alpha/beta receptor and exhibited high endogenous levels of both Stat1 and phosphorylated Stat1 proteins. Lymphocytes from young MRL/lpr mice, with low Fas-dependent cytotoxic activity, were found to become highly cytotoxic against Fas+ targets after treatment with IFN-alpha. These data suggest that IFN-alpha plays an important role in the physiopathology of the systemic lupus erythematosus (SLE)-like syndrome that occurs in MRL/lpr mice.

A proinflammatory role for Fas in joints of mice with collagen-induced arthritis

Collagen-induced arthritis (CIA) is a chronic inflammatory disease bearing all the hallmarks of rheumatoid arthritis, e.g. polyarthritis, synovitis, and subsequent cartilage/bone erosions. One feature of the disease contributing to joint damage is synovial hyperplasia. The factors responsible for the hyperplasia are unknown; however, an imbalance between rates of cell proliferation and cell death (apoptosis) has been suggested. To evaluate the role of a major pathway of cell death – Fas (CD95)/FasL – in the pathogenesis of CIA, DBA/1J mice with a mutation of the Fas gene (lpr) were generated. The susceptibility of the mutant DBA-lpr/lpr mice to arthritis induced by collagen type II was evaluated. Contrary to expectations, the DBA-lpr/lpr mice developed significantly milder disease than the control littermates. The incidence of disease was also significantly lower in the lpr/lpr mice than in the controls (40% versus 81%; P < 0.05). However DBA-lpr/lpr mice mounted a robust immune response to collagen, and the expression of local proinflammatory cytokines such as, e.g., tumor necrosis factor α (TNF-α) and IL-6 were increased at the onset of disease. Since the contribution of synovial fibroblasts to inflammation and joint destruction is crucial, the potential activating effect of Fas on mouse fibroblast cell line NIH3T3 was investigated. On treatment with anti-Fas in vitro, the cell death of NIH3T3 fibroblasts was reduced and the expression of proinflammatory cytokines TNF-α and IL-6 was increased. These findings suggest that impairment of immune tolerance by increased T-cell reactivity does not lead to enhanced susceptibility to CIA and point to a role of Fas in joint destruction.

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.

MHC class II-independent CD25+ CD4+ CD8alpha beta+ alpha beta T cells attenuate CD4+ T cell-induced transfer colitis

CD4+ alpha beta T cell populations that develop in mice deficient in MHC class II (through 'knockout' of either the Aalpha, or the Abeta chain of the I-A(b) molecule) comprise a major 'single-positive' (SP) CD4+ CD8- subset (60-90%) and a minor 'double-positive' (DP) CD4+ CD8alpha beta+ subset (10-40%). Many DP T cells found in spleen, mesenteric lymph nodes (MLN) and colonic lamina propria (cLP) express CD25, CD103 and Foxp3. Adoptive transfer of SP but not DP T cells from Aalpha(-/-) or Abeta(-/-) B6 mice into congenic RAG(-/-) hosts induces colitis. Transfer of SP T cells repopulates the host with only SP T cells; transfer of DP T cells repopulates the host with DP and SP T cells. Anti-CD25 antibody treatment of mice transplanted with DP T cells induces severe, lethal colitis; anti-CD25 antibody treatment of mice transplanted with SP T cells further aggravates the course of severe colitis. Hence, regulatory CD25+ T cells within (or developing from) the DP T cell population of MHC class II-deficient mice control the colitogenic potential of CD25- CD4+ 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.

B220+ double-negative T cells suppress polyclonal T cell activation by a Fas-independent mechanism that involves inhibition of IL-2 production

Fas-mediated apoptosis is a key mechanism for elimination of autoreactive T cells, yet loss of function mutations in the Fas signaling pathway does not result in overt T cell-mediated autoimmunity. Furthermore, mice and humans with homozygous Fas(lpr) or Fas ligand(gld) mutations develop significant numbers of B220+ CD4- CD8- double-negative (DN) alphabeta T cells (hereafter referred to as B220+ DN T cells) of poorly understood function. In this study, we show that B220+ DN T cells, whether generated in vitro or isolated from mutant mice, can suppress the ability of activated T cells to proliferate or produce IL-2, IL-10, and IFN-gamma. B220+ DN T cells that were isolated from either lpr or gld mice were able to suppress proliferation of autologous and syngeneic CD4 T cells, showing that suppression is Fas independent. Furthermore, restoration of Fas/Fas ligand interaction did not enhance suppression. The mechanism of suppression involves inhibition of IL-2 production and its high affinity IL-2R alpha-chain (CD25). Suppression also requires cell/cell contact and TCR activation of B220+ DN T cells, but not soluble cytokines. These findings suggest that B220+ DN T cells may be involved in controlling autoreactive T cells in the absence of Fas-mediated peripheral tolerance.

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.

Regulatory T cells in the control of transplantation tolerance and autoimmunity

A role for immunoregulatory T cells in the maintenance of self-tolerance and in transplantation tolerance has long been suggested, but the identification of such cells has not been achieved until recently. With the characterisation of spontaneously occurring CD4+CD25+ and NK1.1+ T subpopulations of T cells as regulatory cells in rodents and in humans, together with several in vitro generated regulatory T-cell populations, it seems possible that 'customised' regulatory cells possessing antidonor specificity may become therapeutic tools in clinical transplantation tolerance.

Role of double-negative regulatory T cells in long-term cardiac xenograft survival

A novel subset of CD3(+)CD4(-)CD8(-) (double negative; DN) regulatory T cells has recently been shown to induce donor-specific skin allograft acceptance following donor lymphocyte infusion (DLI). In this study, we investigated the effect of DLI on rat to mouse cardiac xenotransplant survival and the ability of DN T cells to regulate xenoreactive T cells. B6 mice were given either DLI from Lewis rats, a short course of depleting anti-CD4 mAb, both DLI and anti-CD4 treatment together, or left untreated. DLI alone did not prolong graft survival when compared with untreated controls. Although anti-CD4-depleting mAb alone significantly prolonged graft survival, grafts were eventually rejected by all recipients. However, the combination of DLI and anti-CD4 treatment induced permanent cardiac xenograft survival. We demonstrate that recipients given both DLI and anti-CD4 treatment had a significant increase in the total number of DN T cells in their spleens when compared with all other treatment groups. Furthermore, DN T cells harvested from the spleens of DLI plus anti-CD4-treated mice could dose-dependently inhibit the proliferation of syngeneic antidonor T cells. Suppression mediated by these DN T cells was specific for antidonor T cells as T cells stimulated by third-party Ags were not suppressed. These results demonstrate for the first time that a combination of pretransplant DLI and anti-CD4-depleting mAb can induce permanent survival of rat to mouse cardiac xenografts and that DN T regulatory cells play an important role in preventing long-term concordant xenograft rejection through the specific suppression of antidonor T cells.

Donor-lymphocyte infusion induces transplantation tolerance by activating systemic and graft-infiltrating double-negative regulatory T cells

Donor-lymphocyte infusion (DLI) before transplantation can lead to specific tolerance to allografts in mice, nonhuman primates, and humans. We and others have demonstrated a role for regulatory T cells in DLI-induced, donor-specific transplantation tolerance, but it is not known how regulatory T cells are activated and where they execute their function. In this study, we observed, in both transgenic and normal mice, that DLI before transplantation is required for activation of alphabeta-T-cell-receptor-positive, CD3(+)CD4(-)CD8(-) double-negative (DN) regulatory T cells in the periphery of recipient mice. More interestingly, DLI induced DN regulatory T cells to migrate preferentially to donor-specific allogeneic skin grafts and to form a majority of graft-infiltrating T cells in accepted skin allografts. Furthermore, both recipient-derived peripheral and graft-infiltrating DN T cells were able to suppress and kill antidonor CD8(+) T cells in an antigen-specific manner. These data indicate that DLI may induce donor-specific transplantation tolerance by activating recipient DN regulatory T cells in the periphery and by promoting migration of regulatory T cells to donor-specific allogeneic skin grafts. Our results also show that DN regulatory T cells can eliminate antidonor T cells both systemically and locally, a finding suggesting that graft-infiltrating T cells can be beneficial to graft survival.