IFN-gamma production by alloantigen-reactive regulatory T cells is important for their regulatory function in vivo

IDO and regulatory T cells: a role for reverse signalling and non-canonical NF-kappaB activation

The immunoregulatory enzyme indoleamine 2,3-dioxygenase (IDO) suppresses T-cell responses and promotes immune tolerance in mammalian pregnancy, tumour resistance, chronic infection, autoimmunity and allergic inflammation. 'Reverse signalling' and 'non-canonical activation' of the transcription factor nuclear factor-kappaB (NF-kappaB) characterize the peculiar events that occur in dendritic cells when T-cell-engaged ligands work as signalling receptors and culminate in the induction of IDO expression by dendritic cells in an inhibitor of NF-kappaB (IkappaB) kinase-alpha (IKKalpha)-dependent manner. In this Opinion article, we propose that IDO acts as a bridge between dendritic cells and CD4+ regulatory T cells, and that regulatory T cells use reverse signalling and non-canonical NF-kappaB activation for effector function and self-propagation. This mechanism may also underlie the protective function of glucocorticoids in pathological conditions.

A synthetic triacylated pseudo-dipeptide molecule promotes Th1/TReg immune responses and enhances tolerance induction via the sublingual route

In this study, we tested two triacylated pseudo-dipeptidic molecules, OM-197-MP-AC and OM-294-BA-MP as candidate adjuvants for allergy vaccines. Both molecules induce human dendritic cell (h-DC) maturation and polarize naïve T cells toward the Th1 type with IFNgamma production. Only OM-294-BA-MP induces IL10 gene expression both in monocyte-derived DCs and CD4+ naïve T cells. Sublingual administration of OM-294-BA-MP plus the antigen enhances tolerance induction in BALB/c mice with established asthma to ovalbumin with an impact on both airways hyperresponsiveness and lung inflammation. Given its Th1/Treg polarizing properties, OM-294-BA-MP is a valid candidate for sublingual allergy vaccines.

Role of IFNgamma in allograft tolerance mediated by CD4+CD25+ regulatory T cells by induction of IDO in endothelial cells

Regulatory T cells have been described to specifically accumulate at the site of regulation together with effector T cells and antigen-presenting cells, establishing a state of local immune privilege. However the mechanisms of this interplay remain to be defined. We previously demonstrated, in a fully MHC mismatched rat cardiac allograft combination, that a short-term treatment with a deoxyspergualine analogue, LF15-0195, induces long-term allograft tolerance with a specific expansion of regulatory CD4+CD25+T cells that accumulate within the graft. In this study, we show that following transfer of regulatory CD4+T cells to a secondary irradiated recipient, regulatory CD25+Foxp3+ and CD25+Foxp3(-) CD4+T cells accumulate at the graft site and induce graft endothelial cell expression of Indoleamine 2, 3-dioxygenase (IDO) by an IFNgamma-dependent mechanism. Moreover, in vivo transfer of tolerance can be abrogated by blocking IFNgamma or IDO, and anti-IFNgamma reduces the survival/expansion of alloantigen-induced regulatory Foxp3+CD4+T cells. Together, our results demonstrate interrelated mechanisms between regulatory CD4+CD25+T cells and the graft endothelial cells in this local immune privilege, and a key role for IFNgamma and IDO in this process.

Host-derived interleukin-18 differentially impacts regulatory and conventional T cell expansion during acute graft-versus-host disease

Interleukin (IL)-18 stimulates T helper 1 (Th1) and Th2-mediated immune responses, and has been shown to modulate acute graft-versus-host disease (aGVHD). It is still unknown whether increased IL-18 levels during aGVHD are of host or donor origin, and how the absence of IL-18 has an impact on migration and expansion of conventional CD4(+)CD25(-)(Tconv) and CD4(+)CD25(+) regulatory (Treg) T cells in vivo. By utilizing IL-18 gene-deficient donor versus recipient animals we found that the major cytokine production during the early phase of aGVHD induction was recipient derived, whereas donor hematopoietic cells contributed significantly less. By generating IL-18(-/-) luciferase transgenic mice we were able to investigate the impact of IL-18 on Tconv and Treg expansion and trafficking with in vivo bioluminescence imaging. Although migration to secondary lymphoid organs did not have a significantly impact from the absence of host IL-18, Tconv but not Treg expansion increased significantly. Absence of host IL-18 production translated into lower IFN-gamma levels in the early phase after transplantation. We conclude that host-derived IL-18 is a major factor for IFN-gamma production that may have a protective effect on CD4(+)-mediated aGVHD, but is nonessential for Treg expansion in an allogeneic environment.

Induction of FoxP3+CD4+25+ regulatory T cells following hemopoietic stem cell transplantation: role of bone marrow-derived facilitating cells

The establishment of donor cell lineages following allogeneic bone marrow transplantation is frequently associated with the development of graft-vs-host disease (GVHD). The identification of cell populations that are capable of supporting allogeneic stem cell (SC) engraftment and the induction of tolerance without inducing GVHD could expand the use of this therapy. CD8(+)TCR(-) facilitating cells (FC) have been shown to promote allogeneic SC engraftment with resulting transplantation tolerance across complete MHC barriers without inducing GVHD. Although donor reconstitution in SC plus FC recipients is associated with the induction of regulatory T cell-associated factors, it is not known whether an induction of regulatory T cells and subsequent tolerance is a direct effect of the FC. The current study demonstrates that 1) SC plus FC transplantation results in the induction of donor CD4(+)25(+) regulatory T cells and that FC are present in the spleen of recipients before the induction of these cells, 2) activation of FC with CpG-oligodeoxynucleotide promotes CD4(+)25(-) T cell differentiation into CD4(+)25(+) regulatory T cells in vitro, as demonstrated by cytokine and forkhead/winged helix transcription factor (FoxP3) gene and protein expression, and 3) direct contact between FC and CD4(+)25(-) T cells is required for FoxP3(+)CD4(+)25(+) regulatory T cell induction and is dependent on CD86 expression on FC. This is the first report to demonstrate a mechanism for FC in the induction of regulatory T cells following allogeneic SC plus FC transplantation. The transplantation of donor FC may provide an alternative approach to permit clinical SC engraftment and induction of transplantation tolerance in the future.

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.

Transfusion of apoptotic beta-cells induces immune tolerance to beta-cell antigens and prevents type 1 diabetes in NOD mice

In vivo induction of beta-cell apoptosis has been demonstrated to be effective in preventing type 1 diabetes in NOD mice. Based on the notion that steady-state cell apoptosis is associated with self-tolerance and the need for developing a more practical approach using apoptotic beta-cells to prevent type 1 diabetes, the current study was designed to investigate apoptotic beta-cells induced ex vivo in preventing type 1 diabetes. The NIT-1 cell line serves as a source of beta-cells. Apoptotic NIT-1 cells were prepared by ultraviolet B (UVB) irradiation. Three weekly transfusions of UVB-irradiated NIT-1 cells (1 x 10(5)/mouse) or PBS were used to determine whether transfusions of UVB-irradiated NIT-1 cells induce immune tolerance to beta-cell antigens in vivo and prevent type 1 diabetes. The suppression of anti-beta-cell antibodies, polarization of T-helper (Th) cells, and induction of regulatory T-cells by UVB-irradiated NIT-1 cell treatment were investigated. The transfusions of apoptotic NIT-1 cells suppress anti-beta-cell antibody development and induce Th2 responses and interleukin-10-producing regulatory type 1 cells. Importantly, this treatment significantly delays and prevents the onset of diabetes when 10-week-old NOD mice are treated. Adoptive transfer of splenocytes from UVB-irradiated NIT-1 cell-treated mice prevents diabetes caused by simultaneously injected diabetogenic splenocytes in NOD-Rag(-/-) mice. Moreover, the proliferation of adoptively transferred carboxyfluorescein diacetate succinimidyl ester-labeled beta-cell antigen-specific T-cell receptor-transgenic T-cells in UVB-irradiated NIT-1-cell treated mice is markedly suppressed. The transfusion of apoptotic beta-cells effectively protects against type 1 diabetes in NOD mice by inducing immune tolerance to beta-cell antigens. This approach has great potential for immune intervention for human type 1 diabetes.

Infusion of Stably Immature Monocyte-Derived Dendritic Cells Plus CTLA4Ig Modulates Alloimmune Reactivity in Rhesus Macaques

BACKGROUND

Immature dendritic cells (DC) can promote long-term transplant survival in rodents. We assessed the impact of stably immature, donor-derived DC on alloimmune reactivity in rhesus macaques.

METHODS

CD14 monocytes isolated from leukapheresis products of Macacca mulatta were cultured in granulocyte-macrophage colony stimulating factor plus interleukin (IL)-4+/-vitamin (vit) D3, and IL-10. Major histocompatibility complex class II and cosignaling molecule expression was determined on CD11c cells by flow cytometry. T-cell allostimulatory capacity of the DC, including DC exposed to proinflammatory cytokines, was determined in mixed leukocyte reaction. To test their influence in vivo, purified DC were infused intravenously into allogeneic recipients, either alone or followed by CTLA4Ig, 24 hr later. Proliferative responses of recipient CFSE-labeled T cells to donor or third party DC, cytokine production by stimulated T cells, and circulating alloantibody levels were determined by flow cytometry, up to 100 days postinfusion.

RESULTS

VitD3/IL-10-conditioned, monocyte-derived DC were resistant to maturation and failed to induce allogeneic T cell proliferation in vitro. After their infusion, an increase in anti-donor and anti-third party T-cell reactivity was observed, that subsequently subsided to fall significantly below pretreatment levels (by day 56) only in animals also given CTLA4Ig. No increase in circulating immunoglobulin (Ig) M or IgG anti-donor alloantibody titers compared with pretreatment values was detected. With DC+CTLA4Ig infusion, alloreactive IL-10-producing T cells were prevalent in the circulation after day 28.

CONCLUSIONS

Maturation-resistant rhesus DC infusion is well-tolerated. DC+CTLA4Ig infusion modulates allogeneic T-cell responses and results in hyporesponsiveness to donor and third party alloantigens.

Frontiers in Nephrology: The Varied Faces of Natural Killer Cells in Transplantation—Contributions to Both Allograft Immunity and Tolerance

Natural killer (NK) cells are recognized for providing an important early innate immune response to viral and bacterial pathogens and for the surveillance of stressed and transformed autologous cells. However, with the exception of a pronounced role in allogeneic hematopoietic stem cell rejection, it has been challenging to ascribe the precise roles for NK cells in reactivity to tissue and solid-organ transplants. In general, NK cells initiate a rapid, proinflammatory environment that is conducive to many forms of effective immune host defense. This reactivity is often considered deleterious to allograft survival because NK cells are implicated in promoting both acute and chronic graft injury. However, more recent findings indicate that NK cells can also play a surprisingly profound role in allograft tolerance induction. This duality of function requires a reconsideration of the nature and consequence of NK cell reactivity during the allograft response. This review focuses on the differing "faces" of NK cells, especially the unexpected role of NK cells in allograft tolerance induction.

Interferon Gamma: Friend or Foe?

Interferon gamma (IFN-gamma) can elicit an inflammatory TH1-driven immune response but has also been found to be necessary for long-term allograft survival induced by costimulation blockade. Recently, we have found that regulatory T cells rapidly and transiently produce IFN-gamma creating a microenvironment that can influence the function of antigen presenting cells (APCs), T-cell proliferation and activation as well as T-cell effector mechanisms, thereby controlling immune responses locally. Moreover, addition of IFN-gamma to cocultures of T cells and APCs can drive the generation of T cells with regulatory activity. Thus, the influence of IFN-gamma on the immune response to a transplant is likely to be context dependent.

C57BL/6 mice genetically deficient in IL-12/IL-23 and IFN-gamma are susceptible to experimental autoimmune myasthenia gravis, suggesting a pathogenic role of non-Th1 cells

Immunization with Torpedo acetylcholine receptor (TAChR) induces experimental autoimmune myasthenia gravis (EAMG) in C57BL/6 (B6) mice. EAMG development needs IL-12, which drives differentiation of Th1 cells. The role of IFN-gamma, an important Th1 effector, is not clear and that of IL-17, a proinflammatory cytokine produced by Th17 cells, is unknown. In this study, we examined the effect of simultaneous absence of IL-12 and IFN-gamma on EAMG susceptibility, using null mutant B6 mice for the genes of both the IL-12/IL-23 p40 subunit and IFN-gamma (dKO mice). Wild-type (WT) B6 mice served as control for EAMG induction. All mice were immunized with TAChR in Freund's adjuvant. dKO mice developed weaker anti-TAChR CD4(+)T cells and Ab responses than WT mice. Yet, they developed EAMG symptoms, anti-mouse acetylcholine receptor (AChR) Ab, and CD4(+) T cell responses against mouse AChR sequences similar to those of WT mice. dKO and WT mice had similarly reduced AChR content in their muscles, and IgG and complement at the neuromuscular junction. Naive dKO mice had significantly fewer NK, NKT, and CD4(+)CD25(+)Foxp3(+) T regulatory (Treg) cells than naive WT mice. Treg cells from TAChR-immunized dKO mice had significantly less suppressive activity in vitro than Treg cells from TAChR-immunized WT mice. In contrast, TAChR-specific CD4(+) T cells from TAChR-immunized dKO and WT mice secreted comparable amounts of IL-17 after stimulation in vitro with TAChR. The susceptibility of dKO mice to EAMG may be due to reduced Treg function, in the presence of a normal function of pathogenic Th17 cells.

Evidence for IFN-? up- and IL-4 downregulation late post-transplant in patients with good kidney graft outcome

We found recently that patients with good graft outcome showed higher IFN-gamma and IL-2, and lower IL-10 plasma levels late post-transplant than early post-transplant. In this retrospective study, we compared cytokine plasma levels in 33 symptom-free outpatients with those of 33 renal transplant recipients with early acute rejection (EAR), 29 with chronic rejection (CR), and 34 healthy controls (HC) to assess whether there is evidence for Th1 activation late post-transplant in patients with good graft outcome. Cytokines were measured pre-transplant, one wk, one month, six months, one yr, and two yr after transplantation. Twelve and 24 months post-transplant, IFN-gamma plasma levels were significantly higher (p = 0.001; p = 0.001, respectively) and IL-4 plasma levels significantly lower (p = 0.028; p = 0.003, respectively) in patients with stable graft function than those in controls. Six, 12, and 24 months post-transplant, patients with stable graft function had similar IFN-gamma and IL-4 plasma levels as patients with successfully treated EAR (p = n.s.), and higher IFN-gamma (p = 0.013; p = 0.001; p = 0.0005, respectively) and lower IL-4 (p = 0.007; p = 0.417; p = 0.0001, respectively) plasma levels than patients with CR. These data suggest that increased plasma IFN-gamma and decreased plasma IL-4 late post-transplant might be involved in the induction of mechanisms that facilitate good long-term graft outcome.

CD40Ig treatment results in allograft acceptance mediated by CD8CD45RC T cells, IFN-gamma, and indoleamine 2,3-dioxygenase

Treatment with CD40Ig results in indefinite allograft survival in a complete MHC-mismatched heart allograft model in the rat. Here we show that serial second, third, and fourth adoptive transfers of total splenocytes from CD40Ig-treated recipients into secondary recipients led to indefinite donor-specific allograft acceptance. Purification of splenocyte subpopulations from CD40Ig-treated recipients demonstrated that only the adoptively transferred CD8(+)CD45RC(low) subset resulted in donor-specific long-term survival, whereas CD8(+)CD45RC(low) T cells from naive animals did not. Accepted grafts displayed increased indoleamine 2,3-dioxygenase (IDO) expression restricted in the graft to ECs. Coculture of donor ECs with CD8(+)CD45RC(low) T cells purified from CD40Ig-treated animals resulted in donor-specific IDO expression dependent on IFN-gamma. Neutralization of IFN-gamma or IDO triggered acute allograft rejection in both CD40Ig-treated and adoptively transferred recipients. This study demonstrates for what we believe to be the first time that interference in CD40-CD40 ligand (CD40-CD40L) interactions induces allospecific CD8(+) Tregs that maintain allograft survival. CD8(+)CD45RC(low) T cells act through IFN-gamma production, which in turn induces IDO expression by graft ECs. Thus, donor alloantigen-specific CD8(+) Tregs may promote local graft immune privilege through IDO expression.

Yin and yang interplay of IFN-gamma in inflammation and autoimmune disease

IFN-gamma has long been recognized as a signature proinflammatory cytokine that plays a central role in inflammation and autoimmune disease. There is now emerging evidence indicating that IFN-gamma possesses unexpected properties as a master regulator of immune responses and inflammation. In this issue of the JCI, Guillonneau et al. show that indefinite allograft survival induced by CD40Ig treatment is mediated by CD8(+)CD45RC(low) T cells through the production of IFN-gamma (see the related article beginning on page 1096), supporting the emerging view that IFN-gamma is critical in the self-regulation of inflammation. These contradictory roles of IFN-gamma, perhaps best understood by the principle of yin and yang, represent one of nature's paradoxes, whereby the same cytokine functions as an inducer as well as a regulator for inflammation. Understanding this complex process of IFN-gamma signaling is essential, as it has therapeutic implications.

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.

Transfer of Allograft Specific Tolerance Requires CD4+CD25+T Cells but Not Interleukin-4 or Transforming Growth Factor–β and Cannot Induce Tolerance to Linked Antigens

BACKGROUND

The mechanisms by which CD4+T cells, especially CD4+ CD25+T cells, transfer allograft specific tolerance are poorly defined. The role of cytokines and the effect on antigen-presenting cells is not resolved.

METHODS

Anti-CD3 monoclonal antibody (mAb) therapy induced tolerance to PVG heterotopic cardiac transplantation in DA rats. Peripheral CD4+T cells or CD4+ CD25+ and CD4+ CD25-T cell subsets were adoptively transferred to irradiated DA hosts grafted with PVG heart grafts. For specificity studies, tolerant CD4+T cells were transferred to hosts with Lewis or (PVGxLewis)F1 heart grafts. Cytokine mRNA induction and the requirement for interleukin (IL)-4 and transforming growth factor (TGF)-beta in the transfer of tolerance was assessed.

RESULTS

CD4+T cells transferred specific tolerance and suppressed naïve CD4+T cells capacity to effect rejection of PVG but not Lewis grafts. (PVGxLewis)F1 grafts had a major rejection episode but recovered. Later these hosts accepted PVG but not Lewis skin grafts. Adoptive hosts restored with tolerant or naïve cells had similar levels of mRNA expression for all Th1 and Th2 cytokines and effector molecules assayed. Transfer of tolerance by CD4+T cells was not blocked by mAb to IL-4 or TGF-beta. CD4+ CD25-T cells from either naïve or tolerant hosts effected rejection. In contrast neither tolerant nor naïve CD4+ CD25+T cells restored rejection.

CONCLUSIONS

Specific tolerance transfer required CD4+ containing CD4+ CD25+T cells. An inflammatory response with induction of mRNA for Th1 and Th2 cytokines plus cytotoxic effector molecules occurred, but IL-4 and TGF-beta were not essential. Inhibition of antigen presenting cells was not the sole mechanism as there was no linked tolerance.

Addition of Cyclophosphamide to T-cell Depletion–Based Nonmyeloablative Conditioning Allows Donor T-cell Engraftment and Clonal Deletion of Alloreactive Host T-cells After Bone Marrow Transplantation

BACKGROUND

Bone marrow (BM) chimerism has been shown to have a beneficial effect on allograft survival. We recently found that production of donor T-cells was highly correlated with induction of tolerance in minimally conditioned chimeras. In the present studies, we demonstrate that nonmyeloablative conditioning and BM cell infusion modulate innate and adaptive host immune responses.

METHODS

Chimeras were generated by bone marrow transplantation (B10.BR to B10). Recipients were preconditioned with T-cell depleting antibodies and total body irradiation with or without cyclophosphamide. Donor-specific tolerance was tested by skin grafting.

RESULTS

Transfer of tolerant splenocytes to immunocompetent secondary recipients did not transfer tolerance, nor did infusion of tolerant CD4+/CD25+ T-cells into chimeras without donor T-cell production, demonstrating that linked suppression is an unlikely mechanism in tolerance induction in the context of BM cell infusion. The addition of a single dose of cyclophosphamide to the conditioning enhanced engraftment and tolerance. This was associated with production of donor T-cells and effective clonal deletion, and a significant reduction in activated recipient plasmacytoid dendritic cells (pDC) and natural killer (NK) cells. Chimeras without donor T-cell production that eventually lost their chimerism did not generate an antidonor humoral response, whereas unconditioned controls infused with similar numbers of BM cells did, indicating that infusion of donor BM cells into conditioned recipients induced immune deviation for adaptive B-cell immunity, preventing sensitization to major histocompatibility complex (MHC) alloantigens.

CONCLUSIONS

These results demonstrate that recipient T-cells, pDC, and NK cells contribute to the host barrier for establishing chimerism, implicate deletional tolerance as the mechanism for total body irradiation-based nonmyeloablative conditioning for BM transplantation, and show a beneficial effect of BM cells in preventing sensitization to MHC alloantigens.

Pathological and clinical correlates of FOXP3+ cells in renal allografts during acute rejection

The localization and significance of regulatory T cells (Treg) in allograft rejection is of considerable clinical and immunological interest. We analyzed 80 human renal transplant biopsies (including seven donor biopsies) with a double immunohistochemical marker for the Treg transcription factor FOXP3, combined with a second marker for CD4 or CD8. Quantitative FOXP3 cell counts were performed and analyzed for clinical and pathologic correlates. FOXP3(+) cells were present in the interstitium in acute cellular rejection (ACR) type I and II, at a greater density than in acute humoral rejection or CNI toxicity (p < 0.01). Most FOXP3(+) cells were CD4(+) (96%); a minority expressed CD8. FOXP3(+)CD4(+) cells were concentrated in the tubules (p < 0.001), suggesting a selective attraction or generation at that site. Considering only patients with ACR, a higher density of FOXP3(+) correlated with HLA class II match (p = 0.03), but paradoxically with worse graft survival. We conclude that infiltration of FOXP3(+) cells occurs in ACR to a greater degree than in humoral rejection, however, within the ACR group, no beneficial effect on outcome was evident. Tregs concentrate in tubules, probably contributing to FOXP3 mRNA in urine; the significance and pathogenesis of 'Treg tubulitis' remains to be determined.

An essential role for IFN-gamma in regulation of alloreactive CD8 T cells following allogeneic hematopoietic cell transplantation

We previously found that CD8 T cells from IFN-gamma gene knockout (GKO) donors induce more severe lethal GVHD compared with CD8 T cells from wild-type (WT) donors in fully MHC-mismatched strain combinations. In this study, we investigated the mechanisms by which IFN-gamma inhibits GVHD in a parent --> F1 (B6 --> B6D2F1) allogeneic HCT (allo-HCT) model. IFN-gamma was strongly protective against GVHD in this parent --> F1 haplotype-mismatched allo-HCT model. Irradiated B6D2F1 mice that received GKO B6 CD4-depleted splenocytes developed lethal GVHD with severe lung and liver injury, whereas those receiving a similar cell population from WT B6 donors survived long term. Donor CD8 cells showed rapid activation, accelerated cell division, and reduced/delayed activation-induced cell death in allogeneic recipients in which donor cells were incapable of producing IFN-gamma. In consequence, the numbers of activated/effector (ie, CD25+, CD62L-, and CD44(high)) donor CD8 T cells in the recipients of GKO allo-HCT significantly exceeded those in mice receiving WT allo-HCT. These data show that IFN-gamma negatively regulates the CD8 T cell response by inhibiting cell division and promoting cell death and suggest that blockade of IFN-gamma could augment the severity of GVHD in patients undergoing allo-HCT.

Conventional T-bet(+)Foxp3(-) Th1 cells are the major source of host-protective regulatory IL-10 during intracellular protozoan infection

Although interferon γ (IFN-γ) secretion is essential for control of most intracellular pathogens, host survival often also depends on the expression of interleukin 10 (IL-10), a cytokine known to counteract IFN-γ effector functions. We analyzed the source of regulatory IL-10 in mice infected with the protozoan parasite Toxoplasma gondii. Unexpectedly, IFN-γ–secreting T-bet⁺Foxp3⁻ T helper type 1 (Th1) cells were found to be the major producers of IL-10 in these animals. Further analysis revealed that the same IL-10⁺IFN-γ^γ population displayed potent effector function against the parasite while, paradoxically, also inducing profound suppression of IL-12 production by antigen-presenting cells. Although at any given time point only a fraction of the cells appeared to simultaneously produce IL-10 and IFN-γ, IL-10 production could be stimulated in IL-10⁻IFN-γ⁺ cells by further activation in vitro. In addition, experiments with T. gondii–specific IL-10⁺IFN-γ⁺ CD4 clones revealed that although IFN-γ expression is imprinted and triggered with similar kinetics regardless of the state of Th1 cell activation, IL-10 secretion is induced more rapidly from recently activated than from resting cells. These findings indicate that IL-10 production by CD4⁺ T lymphocytes need not involve a distinct regulatory Th cell subset but can be generated in Th1 cells as part of the effector response to intracellular pathogens.

Mechanisms of prolongation of allograft survival by HLA-G/ILT4-modified dendritic cells

Engagement of inhibitory receptors on dendritic cells (DCs) is a powerful way to modulate their functions to achieve hyporesponsiveness or tolerance induction. Transgenic mice expressing human ILT4 receptor exclusively on DCs and triggered by HLA-G1 developed long-term survival of allogeneic skin transplant. Here we identify the cellular and molecular mechanisms responsible for that induction of hyporesponsiveness to alloantigen in vivo. Engagement of ILT4 receptor by HLA-G1 resulted in down-regulation of expression of MHC class II and costimulatory molecules, and modulation of cytokine production on DCs. HLA-G-modified DCs from ILT4 transgenic mice promote long-term survival of allografts by mechanisms involving both the induction of regulatory T cells and T-cell anergy. A novel feature of our research was to establish a model for the study of the prospective mechanisms of regulation of alloimmune responses by inhibitory receptors in vivo and analysis of the potential of HLA-G and its inhibitory receptors in modulation of DCs and T-cell function.

Targeting Acute Allograft Rejection by Immunotherapy With Ex Vivo-Expanded Natural CD4+CD25+ Regulatory T Cells

BACKGROUND

Natural CD4CD25 regulatory T (Treg) cells have been implicated in suppressing alloreactivity in vitro and in vivo. We hypothesized that immunotherapy using ex vivo-expanded natural Treg could prevent acute allograft rejection in mice.

METHODS

Natural CD4+ CD25+ Treg were freshly purified from naive mice via automated magnetic cell sorter and expanded ex vivo by anti-CD3/CD28 monoclonal antibody (mAb)-coated Dynabeads. Suppression was assayed in vitro by mixed lymphocyte reaction and in vivo by targeting cardiac allograft rejection. Survival of Treg or effector T (Teff) cells after adoptive transfer in vivo was tracked by flow cytometry and all allografts were examined by histology and immunohistochemistry.

RESULTS

By day nine in culture, 26.6+/-5.3-fold of expansion was achieved by co-culture of fresh natural Treg with anti-CD3/CD28 mAb-coated Dynabeads and interleukin-2. Ex vivo-expanded Treg exerted stronger suppression than fresh ones towards alloantigens in vitro and prevented CD4 Teff-mediated but only delayed CD4+/CD8+ Teff-mediated heart allograft rejection in Rag-/- mice. Long-term surviving allografts showed no signs of acute or chronic rejection with graft-infiltrating Treg expressing CD25 and FoxP3. Infused Treg persisted and expanded long-term in vivo and trafficked through the peripheral lymphoid tissues. CD25 expression was dynamic in vivo: maintained CD25 expression on Treg was indicative for the preservation of allosuppression, while significantly enhanced CD25 expression on CD4+ effector T cells was most likely associated with T-cell expansion and graft rejection.

CONCLUSIONS

Therapeutic use of ex vivo-expanded natural CD4+ CD25+ Treg may be a feasible and nontoxic modality for controlling allograft rejection or perhaps inducing allograft tolerance.

The Transplantation Society--new key opinion leaders of tomorrow 2006

One of the new initiatives taken by the immediate past president of The Transplantation Society, KJ Wood, is to introduce a new series of meetings named 'New Key Opinion Leaders of Tomorrow'. The Transplantation Society invites the leading young investigators in the field of transplantation to participate in this meeting together with the present leaders of the field. The key objective of this meeting is to identify the potential key opinion leaders of tomorrow. Each presentation during the meeting was jointly given by the senior and junior investigators. The meeting in Buenos Aires covered six main topics on clinical, translational and basic transplantation. These included: innate immunity and inflammation; transplant infectious diseases; tumour and transplantation; medical and ethical aspects of transplantation; dendritic cells (stimulating and manipulating the immune response to a transplant); and ABO incompatible donor recipient pairs. This report focuses on the emerging biological therapeutics and their ramifications on clinical transplantation.

Aspirin modified dendritic cells are potent inducers of allo-specific regulatory T-cells

Pods of Acacia concinna (Leguminosae) contain several saponins. In this study, four saponin fractions which were acetone fraction (AAC), aqueous fraction (WAC), hydromethanolic fraction (HAC) and methanolic fraction (MAC) were generated and their haemolytic activities and surface activities were determined in comparison with quillaja saponin (QS). There were no significant differences between the haemolytic activities of MAC and QS. However, the surface tensions of MAC was significantly lower than QS (p < 0.001). Furthermore, the immunomodulatory effect and the adjuvant potential of MAC on the cellular and humoral immune response of BALB/c mice against ovalbumin were investigated. The splenocyte proliferations induced by MAC were significantly higher than QS at the concentrations of 200, 400, 800 and 1000 microg/ml (p < 0.05). BALB/c mice were immunized subcutaneously either with OVA 20 microg alone or with OVA 20 microg combining with QS (10 microg) or MAC (10 and 40 microg). Ten days after the second immunization, concanavalin A (Con A)-, pokeweed mitogen (PWM)-, and OVA-stimulated splenocyte proliferation and OVA-specific antibodies in serum were measured. The results suggested that MAC (40 microg) could activate T and B cells. In addition, OVA-specific IgG, IgG1 IgG2a and IgG2b antibody levels in serum were significantly enhanced by MAC (40 microg) as compared with OVA control group (p < 0.001). This finding suggested that MAC might be effect on Th1 and Th2 helper T cells. In conclusion, the results indicated that MAC at a dose of 40 microg could be used as vaccine adjuvant to increase immune responses.

Role of IL-17 and regulatory T lymphocytes in a systemic autoimmune disease

To explore the interactions between regulatory T cells and pathogenic effector cytokines, we have developed a model of a T cell–mediated systemic autoimmune disorder resembling graft-versus-host disease. The cytokine responsible for tissue inflammation in this disorder is interleukin (IL)-17, whereas interferon (IFN)-γ produced by Th1 cells has a protective effect in this setting. Because of the interest in potential therapeutic approaches utilizing transfer of regulatory T cells and inhibition of the IL-2 pathway, we have explored the roles of these in the systemic disease. We demonstrate that the production of IL-17 and tissue infiltration by IL-17–producing cells occur and are even enhanced in the absence of IL-2. Regulatory T cells favor IL-17 production but prevent the disease when administered early in the course by suppressing expansion of T cells. Thus, the pathogenic or protective effects of cytokines and the therapeutic capacity of regulatory T cells are crucially dependent on the timing and the nature of the disease.

Interleukin-12p70 Prolongs Allograft Survival by Induction of Interferon Gamma and Nitric Oxide Production

BACKGROUND

Interleukin (IL)-12p70, a heterodimeric cytokine has been considered central to induction of Th1 responses with the assistance of IL-18 and IL-27. It was predicted IL-12p70 treatment would promote allograft rejection. In these studies, IL-12p70 delayed rejection.

METHODS

We compared Piebald Virol Glaxo (PVG) neonatal heart graft survival in fully allogeneic Dark Agoutti (DA) rats treated with IL-12p70 alone or in combination with other cytokines. The mechanism by which IL-12p70 induced delayed rejection was examined by reverse transcription polymerase chain reaction of cytokine mRNA and studying the role of interferon (IFN)-gamma and inducible nitric oxide synthase (iNOS) that were induced by IL-12.

RESULTS

IL-12p70 treatment significantly delayed PVG neonatal heart graft rejection compared to normal rejection control and other control groups treated with supernatant from Chinese hamster ovary (CHO)-K1 cells transfected with IL-12p35, IL-12p40, or no cytokine gene. IL-12p70 had no effect on alloantibody response. IFN-gamma and iNOS mRNA expression was increased in heart graft and regional lymph node compared to normal rejection and other treatment groups, consistent with Th1 response induction. IL-12p35 mRNA expression decreased in IL-12p70 treated rats but there was no difference in IL-12p40, Th2, or Tr1 cytokine mRNA expression. Coadministration of an iNOS inhibitor, L-NIL, or a monoclonal antibody (mAb) that blocks IFN-gamma, inhibited IL-12p70's ability to prolong allograft survival; as did co-treatment with IL-4 but not IL-13.

CONCLUSIONS

IL-12p70 treatment may inhibit rejection by hyperinduction of Th1 responses, especially production of IFN-gamma and nitric oxide. These effects may be by enhancing regulatory T-cell responses or by the activation of iNOS in macrophages to produce excessive nitric oxide that in turn inhibits alloimmune responses.

Altered activation of AKT is required for the suppressive function of human CD4+CD25+ T regulatory cells

Suppression by T regulatory cells (Treg cells) is a major mechanism by which the immune system controls responses to self and nonharmful foreign proteins. Although there are many different types of Treg cells, the best characterized are those that constitutively express cell-surface IL-2Ralpha (CD25). We investigated whether altered T-cell-receptor (TCR)-mediated signaling in pure populations of ex vivo human CD4+CD25+ Treg cells might underlie their unique phenotype, including hyporesponsiveness to TCR-mediated activation and lack of cytokine production. CD4+CD25+ Treg cells displayed a consistent defect in phosphorylation of AKT at serine 473 and reduced phosphorylation of the AKT substrates FOXO and S6. Restoration of AKT activity via lentiviral-mediated expression of an inducibly active form of the kinase revealed that reduced activity of this pathway was necessary for the suppressive function of CD4+CD25+ Treg cells. These data represent the first demonstration of a causal association between altered signaling and the function of CD4+CD25+ Treg cells. Moreover, we have created the first system allowing inducible abrogation of suppression through manipulation of the suppressor cells. This system will be a powerful tool to further study the mechanism(s) of suppression by CD4+CD25+ Treg cells.

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.

The suppression of delayed-type hypersensitivity by CD8+ regulatory T cells requires interferon-gamma

CD8(+) regulatory (suppressor) T cells are induced by complex cellular pathways in the spleens of mice that have received an injection of antigen into the anterior chamber (AC) of an eye, an immune-privileged site. Although these CD8(+) regulatory T cells perform an antigen-specific regulatory function for an immune response to self and non-self antigens, the mechanisms of the activation or function of these regulatory cells are not clear. Here, we describe a novel mechanism for the activation of splenic CD8(+) regulatory T cells induced by injection of antigen into the AC. Immunization of mice with trinitrophenyl and bovine serum albumin (TNP-BSA) amplified AC-induced splenic CD8(+) regulatory T cells that suppressed the initiation of contact sensitivity when transferred to immunized, challenged mice. These CD8(+) regulatory T cells were produced independently of perforin, indicating that they are not canonical cytotoxic T cells. Fas ligand (FasL)-deficient CD8(+) regulatory T-cell function was rescued by inclusion of exogenous interferon-gamma (IFN-gamma), demonstrating that the expression of FasL by CD8(+) regulatory T cells was dispensable, but IFN-gamma was not. Ultimately, we demonstrated that the generation of these CD8(+) regulatory T cells occurred independently of IFN-gamma, but their suppressor function required IFN-gamma receptor stimulation.

Regulatory T cells in the periphery

Recognition of a systemic antigen by CD4+ T cells in a lymphopenic host leads to the sequential generation of pathogenic effector cells and protective CD25+ forkhead box protein (Foxp3+) regulatory T cells (Tregs) in the periphery. Such an experimental model is potentially valuable for defining the stimuli that determine the balance of effector and regulatory T cells. Our studies have shown that interleukin-2 (IL-2) enhances the development of effector cells and is essential for the peripheral generation of regulatory cells. Other models of peripheral Treg generation suggest that the concentration of antigen, the nature of the antigen-presenting cells, and cytokines such as transforming growth factor-beta and IL-10 may all influence the peripheral generation of Tregs.

World Transplant Congress. Boston, Massachusetts, USA, 22-27 July 2006

The World Transplant Congress (WTC) is the first joint international transplant meeting co-organised by the Transplantation Society, the American Society of Transplantation (AST) and the American Society of Transplant Surgeons (ASTS). It was the first meeting of this calibre that was being held by three world-renowned authorities in the field of transplantation. Under the direction of the congress president, AB Cosimi, the attendance of this meeting is a 'must' for all working in the field of transplantation. The congress had provided an important forum for the exchange of new scientific and clinical information that is relevant to solid organ and tissue transplantation. In addition, it also created an arena for the interchange of ideas regarding the care and management of transplant recipients. There were many sessions that were engineered in order to facilitate the discussion of socioeconomic, ethical and regulatory issues related transplantation. On the whole, the meeting has set a very high standard for any future transplant congresses to match-providing the delegates with updates on transplantation with the new scientific prospectives in T cell costimulation, innate immunity (dendritic cells), anatomy of immune response and regulatory T cell biology. More importantly, it has highlighted the potentials of harnessing these new findings for future biological therapies in preventing rejection and inducing tolerance.

Rapid quantification of naive alloreactive T cells by TNF-alpha production and correlation with allograft rejection in mice

Allograft transplantation requires chronic immunosuppression, but there is no effective strategy to evaluate the long-term maintenance of immunosuppression other than assessment of graft function. The ability to monitor naive alloreactive T cells would provide an alternative guide for drug therapy at early, preclinical stages of graft rejection and for evaluating tolerance-inducing protocols. To detect and quantify naive alloreactive T cells directly ex vivo, we used the unique ability of naive T cells to rapidly produce TNF-alpha but not IFN-gamma. Naive alloreactive T cells were identified by the production of TNF-alpha after a 5-hour in vitro stimulation with alloantigen and were distinguished from effector/memory alloreactive T cells by the inability to produce IFN-gamma. Moreover, naive alloreactive T cells were not detected in mice tolerized against specific alloantigens. The frequency of TNF-alpha-producing cells was predictive for rejection in an in vivo cytotoxicity assay and correlated with skin allograft rejection. Naive alloreactive T cells were also detected in humans, suggesting clinical relevance. We conclude that rapid production of TNF-alpha can be used to quantify naive alloreactive T cells, that it is abrogated after the induction of tolerance, and that it is a potential tool to predict allograft rejection.

Aspirin-treated human DCs up-regulate ILT-3 and induce hyporesponsiveness and regulatory activity in responder T cells

Mature dendritic cells (mDCs) are potent antigen presenting cells, but immature DCs (iDCs) have been shown to have reduced antigen stimulatory capacity. Different strategies have been investigated to augment the tolerogenic capacity of dendritic cells (DCs). We demonstrate that in aspirin-treated human DCs, there is reduced expression of CD1a, HLA-DR and CD86, up-regulation of ILT-3 expression and marginal increases in PDL-1. Aspirin-treated DCs are partially resistant to phenotypic changes following maturational stimuli, such as lipopolysaccharide (LPS) or TNFalpha, IL-1alpha and PGE2. Aspirin-treated DCs demonstrate normal endocytic function, but have a reduced ability to stimulate allogeneic T cells, which is comparable to iDCs. Furthermore, they induce hyporesponsiveness and regulatory activity in responder naïve and memory T cells; for naïve T cells this is achieved more quickly and efficiently than with iDCs. We investigated the mechanism of this regulatory activity and found that both cell-cell contact and inhibitory cytokine activity are involved, although no one cytokine predominates in importance. Blocking ILT-3 or IL-12 does not diminish the capacity of these DCs to induce regulation or Foxp3 expression on the regulatory T cells. Results demonstrate that aspirin-treated DCs display tolerogenic potential, which is of interest in their therapeutic potential in reducing chronic allograft rejection.

Defective regulatory and effector T cell functions in patients with FOXP3 mutations

The autoimmune disease immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) is caused by mutations in the forkhead box protein P3 (FOXP3) gene. In the mouse model of FOXP3 deficiency, the lack of CD4+ CD25+ Tregs is responsible for lethal autoimmunity, indicating that FOXP3 is required for the differentiation of this Treg subset. We show that the number and phenotype of CD4+ CD25+ T cells from IPEX patients are comparable to those of normal donors. CD4+ CD25high T cells from IPEX patients who express FOXP3 protein suppressed the in vitro proliferation of effector T cells from normal donors, when activated by "weak" TCR stimuli. In contrast, the suppressive function of CD4+ CD25high T cells from IPEX patients who do not express FOXP3 protein was profoundly impaired. Importantly, CD4+ CD25high T cells from either FOXP3+ or FOXP3- IPEX patients showed altered suppression toward autologous effector T cells. Interestingly, IL-2 and IFN-gamma production by PBMCs from IPEX patients was significantly decreased. These findings indicate that FOXP3 mutations in IPEX patients result in heterogeneous biological abnormalities, leading not necessarily to a lack of differentiation of CD4+ CD25high Tregs but rather to a dysfunction in these cells and in effector T cells.

Infectious tolerance and the long-term acceptance of transplanted tissue

Short courses of antibody treatment aimed at blocking the coreceptors CD4 and CD8 and/or costimulatory molecules such as CD40L are able to bring about long-term acceptance and tolerance of allogeneic transplants. This tolerant state is operational, in that potential effector cells remain but are tightly regulated through the induction of antigen-specific CD4+ regulatory T cells (Tregs). CD4+ CD25+ FoxP3+ Tregs appear to play a prominent role, although other categories of Tregs have been documented. Transforming growth factor beta (TGFbeta) has been found to play a major role in the induction of the tolerant state with therapeutic antibodies as well as promoting the induction of FoxP3+ T cells from naïve populations. The observation that Tregs can be found in tolerated grafts has led to the idea that they may interact with the grafted tissue to establish a state of acquired privilege symmetrical with a similar privileged microenvironment around antigen-presenting cells in lymphoid tissues. Dampening of aggressive immune responses by Tregs allows antigen to persist and be presented in an innocuous way to promote tolerance in new cohorts of T cells throughout the life of the tolerated graft. Regulation may operate at many stages of an immune response, even as a censor at the terminal differentiation stages of effector function.

Tyk2 negatively regulates adaptive Th1 immunity by mediating IL-10 signaling and promoting IFN-gamma-dependent IL-10 reactivation

The Jak, Tyk2, is activated in response to IL-12 and IFN-alphabeta and promotes IFN-gamma production by Th1-type CD4 cells. Mice deficient in Tyk2 function have been previously shown to be resistant to autoimmune arthritis and septic shock but are acutely susceptible to opportunistic pathogens such as Toxoplasma gondii. In this study, we show that Tyk2, in addition to mediating the biological effects of IL-12 and IFN-alphabeta, is an important regulator for the signaling and expression of the immunosuppressive cytokine IL-10. In the absence of Tyk2, Ag-reactive CD4 cells exhibit impaired IL-10 synthesis following rechallenge of T. gondii vaccine-primed mice. The impaired IL-10 reactivation leads to unopposed antimicrobial effector mechanisms which results in a paradoxically superior protection of immune Tyk2(-/-) mice against virulent T. gondii challenge. We further demonstrate that Tyk2 indirectly controls CD4 IL-10 reactivation by signaling for maximal IFN-gamma secretion. The unexpected role of IFN-gamma in mediating IL-10 reactivation by Th1 cells provides compelling evidence that conditions driving Th1 responses establish a negative feedback loop, which will ultimately lead to its autoregulation. Thus, Tyk2 can be viewed as a dual-function Jak, mediating both pro and anti-inflammatory cytokine responses.

Glucocorticoid-induced TNF receptor family related gene activation overcomes tolerance/ignorance to melanoma differentiation antigens and enhances antitumor immunity

Glucocorticoid-induced TNF receptor family related protein (GITR) is present on many different cell types. Previous studies have shown that in vivo administration of an anti-GITR agonist mAb (DTA-1) inhibits regulatory T cells (Treg)-dependent suppression and enhances T cell responses. In this study, we show that administration of DTA-1 induces >85% tumor rejection in mice challenged with B16 melanoma. Rejection requires CD4+, CD8+, and NK1.1+ cells and is dependent on IFN-gamma and Fas ligand and independent of perforin. Depletion of Treg via anti-CD25 treatment does not induce B16 rejection, whereas 100% of the mice depleted of CD25+ cells and treated with DTA-1 reject tumors, indicating a predominant role of GITR on effector T cell costimulation rather than on Treg modulation. T cells isolated from DTA-1-treated mice challenged with B16 are specific against B16 and several melanoma differentiation Ags. These mice develop memory against B16, and a small proportion of them develop mild hypopigmentation. Consistent with previous studies showing that GITR stimulation increases Treg proliferation in vitro, we found in our model that GITR stimulation expanded the absolute number of FoxP3+ cells in vivo. Thus, we conclude that overall, GITR stimulation overcomes self-tolerance/ignorance and enhances T cell-mediated antitumor activity with minimal autoimmunity.

Kynurenine Pathway Enzymes in Dendritic Cells Initiate Tolerogenesis in the Absence of Functional IDO

Dendritic cell (DC) tryptophan catabolism has emerged in recent years as a major mechanism of peripheral tolerance. However, there are features of this mechanism, initiated by IDO, that are still unclear, including the role of enzymes that are downstream of IDO in the kynurenine pathway and the role of the associated production of kynurenines. In this study, we provide evidence that 1) murine DCs express all enzymes necessary for synthesis of the downstream product of tryptophan breakdown, quinolinate; 2) IFN-gamma enhances transcriptional expression of all of these enzymes, although posttranslational inactivation of IDO may prevent metabolic steps that are subsequent and consequent to IDO; 3) overcoming the IDO-dependent blockade by provision of a downstream quinolinate precursor activates the pathway and leads to the onset of suppressive properties; and 4) tolerogenic DCs can confer suppressive ability on otherwise immunogenic DCs across a Transwell in an IDO-dependent fashion. Altogether, these data indicate that kynurenine pathway enzymes downstream of IDO can initiate tolerogenesis by DCs independently of tryptophan deprivation. The paracrine production of kynurenines might be one mechanism used by IDO-competent cells to convert DCs lacking functional IDO to a tolerogenic phenotype within an IFN-gamma-rich environment.

Interferon gamma: a crucial role in the function of induced regulatory T cells in vivo

Interferon (IFN)gamma can have paradoxical functions, eliciting inflammatory T helper 1 (Th1)-driven immune responses in some circumstances, and enabling induced regulatory T (Treg) cells to control immune responses in others. Here, we propose a model in which IFNgamma produced rapidly and only transiently by induced Treg cells is crucial to their function in vivo. This early production of IFNgamma by induced Treg cells during an immune response can directly inhibit the activation and proliferation of IFNgammaR1- and IFNgammaR2-bearing T cells. Furthermore, it can indirectly prevent further T-cell activation by creating a microenvironment that influences the function of antigen-presenting cells (APCs) as a result of IFNgamma-induced inducible nitric oxide synthase (iNOS), indoleamine-2,3-dioxygenase (IDO) and heme oxygenase (HO)-1 expression.

Regulatory T cells in transplantation

Our ability to harness tolerance mechanisms will have a major impact in organ transplantation. It should enable drug minimization, and eventually, the elimination of all immunosuppressive drugs. An improved understanding of the biology of regulatory T cells will make it possible to replace current induction regimens with those favouring the selective vaccination of T cells that prevent graft rejection. Once regulation is established, the continued supply of graft antigens should empower T cell regulation to become the dominant natural mechanism to prevent graft rejection.

Functional Dynamics of Naturally Occurring Regulatory T Cells in Health and Autoimmunity

A network of regulatory T (Treg) cells exists to downregulate immune responses in various inflammatory circumstances and ultimately assure peripheral T cell tolerance. Naturally occurring CD4(+)CD25(+) Treg cell represents a major lymphocyte population engaged in the dominant control of self-reactive T responses and maintenance of tolerance within this network. CD4(+)CD25(+) Treg cells differentiate in the normal thymus as a functionally distinct subpopulation of T cells bearing a broad T cell receptor repertoire endowing these cells with the capacity to recognize a wide spectrum of self-Ag and non-self-Ag specificities. The development of CD4(+)CD25(+) Treg cells is genetically determined, influenced by Ag-specific and nonspecific signals, costimulation, and cytokines that control their activation, expansion, and suppressive activity. Functional abrogation of these cells in vivo, or genetic defects that affect their development or function, unequivocally predisposes animals and humans to the onset of autoimmune and other inflammatory diseases. Studies have shed light in our understanding of the cellular and molecular basis of CD4(+)CD25(+) Treg cell-mediated immune regulation. In this chapter, we discuss the contribution of naturally occurring CD4(+)CD25(+) Treg cells in the induction of immunologic self-tolerance in animal models and humans and attempt to provide a comprehensive overview of recent findings regarding the phenotype, functional dynamics, and effector mechanism of these cells in autoimmune diseases.

House dust mite-specific T cells in healthy non-atopic children

BACKGROUND

T-helper type 2 (Th2) cells play an important role in the pathogenesis of allergic diseases. Recent studies have demonstrated that allergen-specific T cells can also be found in the blood of healthy individuals. Both IL-10 and IFN-gamma might modulate the induction and maintenance of allergen-specific tolerance.

AIM

To study the phenotype and functional characteristics of allergen-specific T cells in healthy non-atopic children.

METHODS

Peripheral blood mononuclear cells (PBMC) from 13 symptomatic house dust mite (HDM)-allergic children and from nine matched healthy control children were stimulated with recombinant (r)Der p 2, a major allergen from HDMs.

RESULTS

Stimulation with rDer p 2 resulted in Th2 cytokine production in cultures of PBMC from allergic but not from healthy children. In contrast, IL-10 and IFN-gamma were induced in PBMC cultures from both healthy and HDM-allergic children. Intracellular staining revealed that IL-10 and IFN-gamma are largely produced by the same T cells. Stimulation of T cells from healthy children with rDer p 2 also induced expression of inducible costimulator (ICOS) on a small T cell subset.

CONCLUSION

Allergen-specific memory T cells from healthy non-atopic children produce IL-10 and IFN-gamma (but not Th2 cytokines) and express ICOS upon stimulation. These cells might be responsible for a normal immune balance after allergen encounter in non-atopics.

IFN-γ Controls the Generation/Activation of CD4+CD25+ Regulatory T Cells in Antitumor Immune Response

Immunization with serological identification of Ags by recombinant expression cloning (SEREX)-defined self-Ags leads to generation/activation of CD4+ CD25+ regulatory T cells with suppressive activities and enhanced expression of Foxp3. This is associated with increased susceptibility to pulmonary metastasis following challenge with syngeneic tumor cells and enhanced development of 3-methylcholanthrene-induced primary tumors. In contrast, coimmunization with the same SEREX-defined self-Ags mixed with a CTL epitope results in augmented CTL activity and heightened resistance to pulmonary metastasis, both of which depend on CD4+ Th cells. These active regulatory T cells and Th cells were derived from two distinct CD4+ T cell subsets, CD4+ CD25+ T cells and CD4+ CD25- T cells, respectively. In the present study, IFN-gamma was found to abrogate the generation/activation of CD4+ CD25+ regulatory T cells by immunization with SEREX-defined self-Ag. CD4+ CD25+ T cells from these IFN-gamma-treated mice failed to exhibit immunosuppressive activity as measured by 1) increased number of pulmonary metastasis, 2) enhanced development of 3-methylcholanthrene-induced primary tumors, 3) suppression of peptide-specific T cell proliferation, and 4) enhanced expression of Foxp3. The important role of IFN-gamma produced by CD8+ T cells was shown in experiments demonstrating that CD4+ CD25+ T cells cotransferred with CD8+ T cells from IFN-gamma(-/-) mice, but not from wild-type BALB/c mice, became immunosuppressive and enhanced pulmonary metastasis when recipient animals were subsequently immunized with a SEREX-defined self-Ag and a CTL epitope. These findings support the idea that IFN-gamma regulates the generation/activation of CD4+ CD25+ regulatory T cells.

Secretion of IFN-gamma and not IL-2 by anergic human T cells correlates with assembly of an immature immune synapse

We report differences in the supramolecular organization of the immunologic synapse (IS) formed by resting and anergic human T cells with agonist peptide-loaded antigen-presenting cells (APCs). T cells reactive to influenza A hemagglutinin peptide or Fel d 1 peptide 4 were rendered both anergic and regulatory by incubation with high doses of agonist peptide in the absence of APCs. At the IS between resting T cells and peptide-loaded APCs, both CD3epsilon and CD3zeta initially accumulate within a ring or arc before redistributing within 30 minutes to single or multiple foci more central to the contact. In contrast, at synapses formed by anergized T cells, CD3epsilon and CD3zeta remained organized within an arc or ring and failed to redistribute centrally. However, intercellular communication between anergic human T cells and agonist peptide-loaded APCs was not a null event, since it triggered secretion of T-cell interferon gamma (IFN-gamma) but not, for example, interleukin 2 (IL-2). Thus, distinct organizations of CD3 at the T-cell IS correlate with different cytokine profiles; the mature IS formed by resting T cells correlates with their production of both IFN-gamma and IL-2, whereas the immature IS formed by anergic T cells seems able to facilitate IFN-gamma but not IL-2 production.