Ligation of B7-1/B7-2 by human CD4+ T cells triggers indoleamine 2,3-dioxygenase activity in dendritic cells

Regulatory dendritic cell therapy in organ transplantation

Dendritic cells (DCs) are uniquely well equipped antigen (Ag)-presenting cells. Their classic function was thought to be that of potent initiators of innate and adaptive immunity to infectious organisms and other Ags (including transplanted organs). Evidence has emerged, however, that DCs have a central and crucial role in determining the fate of immune responses toward either immunity or tolerance. This dichotomous function of DCs, coupled with their remarkable plasticity, renders them attractive therapeutic targets for immune modulation. In transplantation, much recent work has focused on the ability of DCs to silence immune reactivity in an Ag-specific manner in the hope of preventing rejection and diminishing reliance on potentially harmful immunosuppressive agents. Experimental strategies have included in vivo targeting of DCs, as well as ex vivo generation of regulatory (or tolerogenic) DCs with subsequent reinfusion (i.e. cell therapy). Different approaches to 'program' DC toward tolerogenic properties include genetic (transgene insertion), biologic (differential culture conditions, anti-inflammatory cytokine exposure) and pharmacologic manipulation. Recent data suggest a promising role for pharmacologic treatment as a means of generating potent regulatory DCs and have further stimulated speculation regarding their potential clinical application. Herein, we discuss evidence that the potential of regulatory DC therapy is considerable and that there are compelling reasons to evaluate it in the setting of organ transplantation in the near future.

The role of regulatory T cells in alloantigen tolerance

The diversification mechanism used by the adaptive immune system to maximize the recognition of foreign antigens has the side effect of generating autoreactivity. This effect is counteracted by deletion of cells expressing receptors with high affinity to self (central tolerance) and suppression of autoreactive cells by regulatory T cells (Tregs; peripheral tolerance). This understanding led to the notion that Tregs represent a specialized subset of autoreactive T cells with inhibitory function. The process of generating a diverse repertoire of receptors recognizing antigen presented by major histocompatibility complex (MHC) intrinsically leads to the generation of cells recognizing foreign MHC (alloantigen). The precursor frequency of T cells responding to alloantigen is substantially higher than that responding to any exogenous antigen. The only physiological context in which this becomes a problem is placental viviparity. Although the maternal immune system has no intrinsic mechanism to distinguish between a pathogen and paternally derived fetal alloantigen, it has to neutralize the former and tolerate the latter. We review the function of Tregs from this perspective and propose that they may have evolved to promote tolerance to alloantigen in the context of pregnancy.

Indoleamine-2,3-dioxygenase modulation of allergic immune responses

Induction of immunologic tolerance is highly desirable in the treatment and prevention of allergy and other immune disease states in which the immune response to foreign or self antigens has become overactive. Indoleamine-2,3-dioxygenase (IDO), an enzyme classically known for its role in the tryptophan degradation pathway, has recently emerged as an important immunomodulator of T-cell function and inducer of tolerance. The induced expression of IDO by dendritic cells may suppress T-cell responses and promote tolerance either through direct effects on T cells (mediated by tryptophan depletion or tryptophan metabolites) or through effects of IDO on the dendritic cell. In addition to the potential role of IDO in promoting tolerance in pregnancy, transplantation, and autoimmunity, its role in modulating allergic responses has more recently been investigated, raising the possibility that IDO and its metabolites may be novel targets for immunomodulation in allergy and asthma.

Regulatory T cell-mediated transplantation tolerance

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

Lung dendritic cells and the inflammatory response

OBJECTIVE

To discuss the role of conventional and plasmacytoid dendritic cells in inducing and modulating immune responses in the lung.

DATA SOURCES

The primary literature and selected review articles studying the role of dendritic cells in both rodent and human lungs as identified via a PubMed/MEDLINE search using the keywords dendritic cell, antigen-presenting cell, viral airway disease, asthma, allergy, and atopy.

STUDY SELECTION

The author's knowledge of the field was used to identify studies that were relevant to the stated objective.

RESULTS

Dendritic cells are well positioned in the respiratory tract and other mucosal surfaces to respond to any foreign protein. These cells are crucial to the initiation of the adaptive immune response through induction of antigen specific T-cell responses. These cells also play an important role in the regulation of developing and ongoing immune responses, an area that is currently under intense investigation. This review discusses the various subsets of human and rodent dendritic cells and the pathways involved in antigen processing and subsequent immune regulation by dendritic cells in the lung using both viral and nonviral allergenic protein exposure as examples.

CONCLUSIONS

Conventional and plasmacytoid dendritic cells are uniquely situated in the immune cascade to not only initiate but also modulate immune responses. Therapeutic interventions in allergic and asthmatic diseases will likely be developed to take advantage of this exclusive position of the dendritic cell.

Therapeutic potential of adenovirus as a vaccine vector for chronic virus infections

Therapeutic vaccines for chronic infections and cancer are needed. Challenges faced by therapeutic vaccines differ from those of preventative vaccines. Whereas the latter target a naive immune system, the former have to readjust an antigen-experienced immune system that is subverted due to sustained exposure to antigen. E1-deleted adenoviral vectors have succeeded preclinically as preventative vaccines and are now in clinical trials. Their potential as therapeutic vaccines for diseases caused by chronic virus infections or virus-associated malignancies remains to be explored in more depth and may require modifications to circumvent negative immunoregulatory pathways that develop following chronic infections or during tumour progression.

A new look at blockade of T-cell costimulation: a therapeutic strategy for long-term maintenance immunosuppression

Activated T cells orchestrate the immune response that results in graft rejection; therefore, a common goal among current immunosuppressive therapies is to block T-cell activation, proliferation and function. Current immunosuppressive regimens that inhibit T cells and immune cells have greatly reduced the incidence of acute rejection following solid-organ transplant. However, the expected improvements in long-term outcomes have not been realized. This may be related to the non-immune side effects of current maintenance immunosuppressants, which target ubiquitously expressed molecules. The focus in transplantation research is shifting in search of maintenance immunosuppressive regimens that might offer improved long-term outcomes by providing efficacy in prevention of acute rejection combined with reduced toxicities. An emerging therapeutic strategy involves an immunoselective maintenance immunosuppressant that inhibits full T-cell activation by blocking the interaction between costimulatory receptor-ligand pairs. This review describes costimulatory pathways and the development of molecules, which inhibit them in the context of transplantation research. Recent clinical data using the selective costimulation blocker, belatacept (LEA29Y), as a part of a CNI-free maintenance immunosuppressive regimen in renal transplantation is highlighted.

Function of indoleamine 2,3-dioxygenase in corneal allograft rejection and prolongation of allograft survival by over-expression

Indoleamine 2,3-dioxygenase (IDO) suppresses T cell responses by its action in catabolising tryptophan. It is important in maintenance of immune privilege in the placenta. We investigated the activity of IDO in the cornea, following corneal transplantation and the effect of IDO over-expression in donor corneal endothelium on the survival of corneal allografts. IDO expression was analysed and functional activity was quantified in normal murine cornea and in corneas following transplantation as allografts. Low levels of IDO, at both mRNA and protein levels, was detected in the normal cornea, up-regulated by IFN-gamma and TNF. Expression of IDO in cornea was significantly increased following corneal transplantation. However, inhibition of IDO activity in vivo had no effect on graft survival. Following IDO cDNA transfer, murine corneal endothelial cells expressed functional IDO, which was effective at inhibiting allogeneic T cell proliferation. Over-expression of IDO in donor corneal allografts resulted in prolonged graft survival. While, on one hand, our data indicate that IDO may augment corneal immune privilege, up-regulated IDO activity following cytokine stimulation may serve to inhibit inflammatory cellular responses. While increasing IDO mRNA expression was found in allogeneic corneas at rejection, over-expression in donor cornea was found to significantly extend survival of allografts.

B7-1 and B7-2: Similar costimulatory ligands with different biochemical, oligomeric and signaling properties

B7-1 and B7-2 are homologous costimulatory ligands expressed on the surface of antigen presenting cells (APCs). Binding of these molecules to the T cell costimulatory receptors, CD28 and CTLA-4, is essential for the activation and regulation of T cell immunity. Despite strong structural similarities, B7-1 and B7-2 exhibit different biochemical features, and their binding to the costimulatory receptors results in distinct T cell functional outcomes. Using photobleaching based fluorescence resonance energy transfer (FRET), our previous studies have demonstrated that B7-1 and B7-2 have different cell surface oligomeric states. While B7-1 is present as a dimer, B7-2 exists as a monomer on the cell surface suggesting that the unique cell surface oligomeric states of the costimulatory ligands may play a key role in the regulation of T cell responses. Moreover, signaling via B7-1 and B7-2 in dendritic cells has been reported to be dependent on their simultaneous expression, raising the possibility that their direct interaction or their involvement in synergistic signaling pathways may play a role in the function of antigen presenting cells. We discuss physiological relevance of distinct oligomeric states of B7-1 and B7-2 and address whether these molecules can associate with one another on the cell surface to form hetero-oligomers. Our findings suggest that B7-1 and B7-2 do not form hetero-oligomers, underscoring the biological relevance of dimeric and monomeric state of B7-1 and B7-2, respectively.

Toward the identification of a tolerogenic signature in IDO-competent dendritic cells

Although much is known about the transcriptional profiles of dendritic cells (DCs) during maturation, the molecular switches critical for the induction of a tolerogenic program in DC subsets are still obscure. We examined the gene-expression profiles of murine splenic CD8+ DCs rendered highly tolerogenic by interferon-γ (IFN-γ), which activates the enzyme indoleamine 2,3-dioxygenase (IDO, encoded by Indo) and thus initiates the immunosuppressive pathway of tryptophan catabolism. By examining the expression of a series of relevant genes in IDO+ compared with IDO- DCs, we found consistent and selective association of the IDO-competent phenotype with down-modulation of the Tyrobp gene, encoding the signaling adapter DAP12, which typically associates with activating receptors. Down-modulation of Tyrobp involved IFN consensus sequence binding protein (ICSBP), a transcription factor also known as IRF-8. In murine and human monocyte-derived DCs, silencing DAP12 expression imparted IDO functional competence to IDO- cells, whereas silencing IRF-8 in IDO+ counterparts abolished IDO expression and function. Thus, IRF-8 is required in tolerogenic DCs for the positive regulation of Indo and the negative regulation of Tyrobp. Overall, these studies reveal the occurrence of a simple and evolutionarily conserved code in the control of tolerance by an ancestral metabolic enzyme.

CD80 cytoplasmic domain controls localization of CD28, CTLA-4, and protein kinase Ctheta in the immunological synapse

The binding of costimulatory ligand CD80 to CD28 or CTLA-4 on T cells plays an important role in the regulation of the T cell response. We have examined the role of the cytoplasmic domain of CD80 in murine T cell costimulation and its organization in the immunological synapse (IS). Removal of CD80 cytoplasmic tail decreased its effectiveness in costimulating T cell proliferative response and early IL-2 production in response to agonist MHC-peptide complexes. Immunofluorescent study showed a decreased tailless CD80 accumulation in the IS of naive T cells. The two forms of CD80 accumulated differently at the IS; the tailless CD80 was colocalized with the TCR whereas the full-length CD80 was segregated from the TCR. In addition, we showed that CD80, CD28, and protein kinase Ctheta colocalized in the presence or absence of the CD80 cytoplasmic tail. Thus, the cytoplasmic tail of CD80 regulates its spatial localization at the IS and that of its receptors and T cell signaling molecules such as protein kinase Ctheta, and thereby facilitates full T cell activation.

Inhibition of indoleamine 2,3 dioxygenase activity by H2O2

Indoleamine 2,3-dioxygenase is the first and rate limiting enzyme of the kynurenine pathway of tryptophan metabolism, has potent effects on cell proliferation and mediates antimicrobial, antitumorogenic, and immunosuppressive effects. As a potent cytotoxic effector, the mechanisms of indoleamine 2,3-dioxygenase inhibition deserve greater attention. The work presented here represents the first systematic study exploring the mechanisms by which low levels of hydrogen peroxide (10-100 microM) inhibit indoleamine 2,3-dioxygenase in vitro. Following brief peroxide exposure both enzyme inhibition and structural changes were observed. Loss of catalysis was accompanied by oxidation of several cysteine residues to sulfinic and sulfonic acids, observed by electrospray and MALDI mass spectrometry. Enzyme activity could in part be preserved in the presence of sulfhydryl containing compounds, particularly DTT and methionine. However, these structural alterations did not prevent substrate (l-tryptophan) binding. Some enzyme activity could be recovered in the presence of thioredoxin, indicating that the inhibitory effect of H(2)O(2) is at least partially reversible in vitro. We present evidence that cysteine oxidation represents one mechanism of indoleamine 2,3-dioxygenase inhibition.

Dominant Tolerance to Kidney Allografts Induced by Anti-Donor MHC Class II Antibodies: Cooperation between T and Non-T CD103+Cells

Allograft acceptance can be induced in the rat by pretransplant infusion of donor blood or spleen cells. Although promoting long-term acceptance, this treatment is also associated with chronic rejection. In this study, we show that a single administration of anti-donor MHC class II alloimmune serum on the day of transplantation results in indefinite survival of a MHC-mismatched kidney graft. Long-term recipients accept a donor-type skin graft and display no histological evidence of chronic rejection. The kidney grafts of tolerant animals display an accumulation of TCR Cbeta, FoxP3, and IDO transcripts. Moreover, as compared with syngeneic recipients, tolerant recipients harbor a large infiltrate of MHC class II(+) cells and CD103(+) cells. In vitro, splenocytes from tolerant recipients exhibit decreased donor-specific proliferation, which is restored by depletion of non-T cells and partially restored by the blockade of IDO. Finally, splenocytes from tolerant recipients, but not purified T cell splenocytes, transfer donor-specific infectious tolerance without chronic rejection, after infusion into naive recipients, over two generations. However, splenocytes depleted of T cells or splenocytes depleted of CD103(+) cells fail to transfer tolerance. Collectively, these data show that a single administration of anti-donor MHC class II alloimmune serum induces a tolerant state characterized by an infiltration of the kidney graft by regulatory T cells and CD103(+) cells. These data also show that the transfer of tolerance requires the presence of both T cells and CD103(+) dendritic cells. The precise mechanism of cooperation of these two cell subsets remains to be defined.

CD25 and indoleamine 2,3-dioxygenase are up-regulated by prostaglandin E2 and expressed by tumor-associated dendritic cells in vivo: additional mechanisms of T-cell inhibition

Immune tolerance is a central mechanism counteracting tumor-specific immunity and preventing effective anticancer immunotherapy. Induction of tolerance requires a specific environment in which tolerogenic dendritic cells (DCs) play an essential role deviating the immune response away from effective immunity. It was recently shown that maturation of DCs in the presence of PGE2 results in upregulation of indoleamine 2,3-dioxygenase (IDO) providing a potential mechanism for the development of DC-mediated Tcell tolerance. Here, we extend these findings, demonstrating a concomitant induction of IDO and secretion of soluble CD25 after DC maturation in the presence of PGE2. While maturation of DCs induced IDO expression on transcriptional level, only integration of PGE2 signaling led to up-regulation of functional IDO protein as well as significant expression of cell-surface and soluble CD25 protein. As a consequence, T-cell proliferation and cytokine production were significantly inhibited, which was mediated mainly by IDO-induced tryptophan depletion. Of importance, we demonstrate that different carcinoma entities associated with elevated levels of PGE2 coexpress CD25 and IDO in peritumoral dendritic cells, suggesting that PGE2 might influence IDO expression in human DCs in the tumor environment. We therefore suggest PGE2 to be a mediator of early events during induction of immune tolerance in cancer.

Impaired maturation and altered regulatory function of plasmacytoid dendritic cells in multiple sclerosis

Plasmacytoid dendritic cells (pDCs) represent a DC subtype that exerts divergent functions in innate and adoptive immunity including the immediate reaction to microbial factors and the induction of immunoregulatory responses. It is thought that different DC subtypes may be critically involved in the pathogenesis of multiple sclerosis (MS). In our study we assessed the phenotype, maturation and functional properties of peripheral blood pDCs from 35 clinically stable, untreated multiple sclerosis patients, 30 healthy controls and 9 patients with pneumonia, which was used as a non-specific inflammatory condition (NIC). Ex vivo expression of CD86 and 4-1BBL was significantly lower on pDCs from multiple sclerosis patients than from controls and patients with NIC (22 versus 47 versus 41% and 12 versus 35 versus 32%, respectively). When stimulated with IL-3 and CD40L, pDCs of multiple sclerosis patients showed inefficient maturation as demonstrated by significantly lower or delayed upregulation of CD86, 4-1BBL, CD40 and CD83. Additionally, in multiple sclerosis, stimulation of pDCs by unmethylated cytosine-phosphate-guanosine oligodeoxynucleotides (CpG ODN) resulted in a significantly lower interferon (IFN) alpha secretion than in controls. In multiple sclerosis, but not in controls, pDCs failed to upregulate proliferative responses and IFN-gamma secretion of autologous peripheral blood mononuclear cells (PBMC) in a co-culture system. Moreover, depletion of pDCs in multiple sclerosis patients, but not in controls, had no effect on generation of CD4+Foxp3+ regulatory T cells. We also provide data showing that glatiramer acetate (GA) treatment partially restores phenotype and function of pDCs in multiple sclerosis patients. These findings suggest functional abnormalities of pDCs in these patients, which might be of importance in the understanding of the development of immune dysregulation in this disease.

Indoleamine 2,3-dioxygenase, tumor-induced tolerance and counter-regulation

Tumors create an abnormal state of tolerance toward themselves and their antigens. One mechanism that might contribute to this tolerance is the immunoregulatory enzyme indoleamine 2,3-dioxygenase (IDO). IDO-expressing antigen-presenting cells are found in tumor-draining lymph nodes, where they can create a tolerogenic microenvironment. IDO can also be expressed within the tumor itself, by tumor cells or host stromal cells, where it can inhibit the effector phase the immune response. Finally, emerging evidence suggests that IDO might also constitute a significant counter-regulatory mechanism, induced by clinically relevant pro-inflammatory signals, such as IFN-gamma, IFN-alpha, CpG oligodeoxynucleotides, and 4-1BB ligation. Strategies to inhibit the IDO pathway may thus assist in breaking tolerance to tumors, and might enhance the efficacy of other immunotherapy strategies by removing unwanted counter-regulation.

CD4+ regulatory cells as a potential immunotherapy

CD4(+) regulatory T (T(R)) cells represent a unique lineage of thymically generated lymphocytes capable of powerfully suppressing immune responses. A large body of experimental data has now confirmed the key role played by these cells in the maintenance of self-tolerance. Increasingly, the importance of these cells is also being recognized in a host of other clinically relevant areas such as transplantation, tumour immunity, allergy and microbial immunity. Additionally, it is also possible to generate T(R) cells by using a variety of ex vivo experimental approaches. We will focus here on harnessing the suppressive abilities of both these families of regulatory cells and how this should give us access to a potent cell-based immunotherapy appropriate for clinical application.

The immunoregulatory role of IDO-producing human dendritic cells revisited

Following the finding that indoleamine 2,3-dioxygenase (IDO), an enzyme expressed in the placenta, prevents rejection of allogeneic fetuses in mice, many studies have focused on the role of IDO in the regulation of the immune response. Most arguments for an immunoregulatory role of IDO in vivo are based on observations in mice. Here, we critically examine the arguments for and against a function of IDO-expressing human dendritic cells (DCs) and conclude that proof for an immunoregulatory role in vivo is still lacking.

IDO expression on decidual and peripheral blood dendritic cells and monocytes/macrophages after treatment with CTLA-4 or interferon-gamma increase in normal pregnancy but decrease in spontaneous abortion

Recent data demonstrated that CD4+CD25+ regulatory T (Treg) cells and an enzyme called indoleamine 2,3-dioxygenase (IDO) mediate maternal tolerance to the fetus. Interestingly, Treg cells express the CTLA-4 molecule on their surface, and B7 (CD80/86) ligation by CTLA-4 enhanced IDO activity of dendritic cells (DCs) and monocytes by the induction of interferon gamma (IFN-gamma) production. In this study, we studied the IDO expression on peripheral blood monocytes and decidual monocytes or DCs after treatment with CTLA-4/Fc fusion protein or IFN-gamma using flow cytometry. IDO expressions on both peripheral blood DC and decidual DC and monocytes were up-regulated during normal pregnancy. On the other hand, both IDO expression on DC and monocytes after IFN-gamma treatment or CTLA-4 treatment were decreased in spontaneous abortion cases. The expression of CD86 on peripheral blood and decidual monocytes and DC in spontaneous abortion cases was lower compared with those in normal pregnancy subjects. Also, IFN-gamma production by decidual and peripheral blood mononuclear cells after CTLA-4/Fc treatment in spontaneous abortion cases was significantly lower than those in normal pregnancy subjects. These data suggest that CTLA-4 on Treg cells up-regulates IDO expression on decidual and peripheral blood DC and monocytes by the induction of IFN-gamma production.

The many sounds of T lymphocyte silence

It is not unusual for antigens and potentially responsive T cells to co-exist in the same organism while these T cells remain silent and do not mount life-threatening immune responses. A rich array of mechanisms has been proposed to explain these observations. T cell silencing is controlled in multiple levels. Initially, dendritic cells and regulatory T cells appear to play critical roles. In addition, T cell immunity is tightly regulated by a molecular network of cytokines and cell receptor interactions by the opposed surfaces of antigen-presenting cells and T cells. Recognition of a specific antigen is therefore shaped and tuned by co-stimulatory and co-inhibitory receptor-ligand pairs. At last, immunologists are beginning to exploit the rules governing these assorted sounds of T cell silence.

CTLA-4 and PD-1 receptors inhibit T-cell activation by distinct mechanisms

CTLA-4 and PD-1 are receptors that negatively regulate T-cell activation. Ligation of both CTLA-4 and PD-1 blocked CD3/CD28-mediated upregulation of glucose metabolism and Akt activity, but each accomplished this regulation using separate mechanisms. CTLA-4-mediated inhibition of Akt phosphorylation is sensitive to okadaic acid, providing direct evidence that PP2A plays a prominent role in mediating CTLA-4 suppression of T-cell activation. In contrast, PD-1 signaling inhibits Akt phosphorylation by preventing CD28-mediated activation of phosphatidylinositol 3-kinase (PI3K). The ability of PD-1 to suppress PI3K/AKT activation was dependent upon the immunoreceptor tyrosine-based switch motif located in its cytoplasmic tail, adding further importance to this domain in mediating PD-1 signal transduction. Lastly, PD-1 ligation is more effective in suppressing CD3/CD28-induced changes in the T-cell transcriptional profile, suggesting that differential regulation of PI3K activation by PD-1 and CTLA-4 ligation results in distinct cellular phenotypes. Together, these data suggest that CTLA-4 and PD-1 inhibit T-cell activation through distinct and potentially synergistic mechanisms.

Indoleamine 2,3 dioxygenase and human leucocyte antigen-G inhibit the T-cell alloproliferative response through two independent pathways

Both human leucocyte antigen (HLA)-G and indoleamine 2,3 dioxygenase (IDO) are key molecules involved in immune tolerance. HLA-G is a non-classical HLA class I molecule that can be expressed in both membrane-bound (HLA-G1) and soluble (HLA-G5) forms, both of which exhibit tolerogenic properties via interaction with inhibitory receptors present on natural killer (NK) cells, T cells and antigen-presenting cells (APC). IDO is an enzyme that acts by depleting the surrounding microenvironment of the essential amino acid, tryptophan, thereby inhibiting T-cell proliferation. Our present study was aimed at analysing the potential link that may exist between IDO and HLA-G. Our results showed that during allogeneic reactions, soluble HLA-G expression was not regulated by the addition of IDO substrate (i.e. tryptophan), metabolite (i.e. kynurenine) or inhibitor (i.e. 1-methyl-tryptophan), that IDO activity was not altered by HLA-G5 treatment, and that HLA-G5-mediated inhibition of the T-cell alloproliferative response was neither affected by the presence of tryptophan and kynurenine nor reversed after IDO activity blockage, demonstrating that HLA-G5 can exert its function in the absence of functional IDO. Similarly, inhibition of the T-cell alloresponse, induced by HLA-G1-expressing antigen-presenting cells, was not altered by IDO metabolites or inhibitor. Taken together, these findings show that the function and expression of IDO and HLA-G5 are not mutually influenced, but rather inhibit the T-cell alloproliferative response through two independent pathways. IDO and HLA-G are thus complementary for inducing and maintaining immune tolerance in physiological (pregnancy) and pathological (tumour and allograft) situations.

Fuel feeds function: energy metabolism and the T-cell response

Ligation of antigen receptors at the surface of lymphocytes initiates a transcriptional and translational response that is required for cellular proliferation and effector function. By contrast, co-stimulatory-molecule ligation contributes to the immune response by allowing the uptake and utilization of extracellular nutrients to provide energy for cellular proliferation and effector functions. Growth factors also potentiate the ability of lymphocytes to metabolically switch between resting and proliferative states. Lymphocytes that do not receive these signals fail to increase their metabolism to meet the higher bioenergetic demands of cell growth and are either deleted or rendered unresponsive to mitogenic signals. In this Review, we describe how T cells actively acquire metabolic substrates from their environment to meet these energy demands and respond appropriately to pathogens.

Indoleamine 2,3-dioxygenase protects corneal endothelial cells from UV mediated damage

Indoleamine-2,3-dioxygenase (IDO) is an intracellular enzyme present in dendritic cells and macrophages. It is a known modulator of T-cell response and contributes to the UV protection of the lens. There yet is no information on IDO activity in the corneal endothelium, protecting the endothelial cells from light mediated damage. We exposed murine corneal endothelial cells (MCEC) with different doses of UV-B light 280-320 nm, probed for IDO mRNA (real-time PCR) and assessed apoptosis rate (flow cytometry) and caspase-3-activity in the cells. The metabolites of the IDO catalysed reaction, l-kynurenine, was also measured. Malondialdehyde was detected for quantification of UV-B-induced oxidative stress. To investigate specificity, IDO effects were blocked by 1-methyl-tryptophan. The effects of IDO overexpression in the MCEC were assessed by transfection of an expression vector. MCEC consistently express IDO at low levels. Exposure to UV-B light led to a dose-responding upregulation of IDO; IDO was found competent converting l-tryptophan into l-kynurenine. Irradiation led to increased apoptosis and caspase-3-activity of MCEC. Supplementation of l-kynurenine or overexpression of IDO in the MCEC could reduce apoptosis significantly following UV-B irradiation. Inhibition of IDO by 1-MT was potent to reverse this effect. IDO and its metabolite l-kynurenine can protect corneal endothelial cells from UV-B-induced oxidative stress and apoptosis. It may be an active protection mechanism against corneal endothelial damage.

Distinct and overlapping roles of interleukin-10 and CD25+ regulatory T cells in the inhibition of antitumor CD8 T-cell responses

Lack of antitumor immunity is often related to impaired CD8 T-cell responses that could result from a poor priming capacity by tumor-infiltrating dendritic cells (TIDC) and/or further inhibition by regulatory T cells (T(reg)). Interleukin-10 (IL-10) has been implicated in the inhibition of TIDC as well as in the generation and functions of T(reg). Here, we address some of the respective and possibly overlapping roles of IL-10 and CD25+ T(reg) in CD8 antitumor immunity. Whereas tumor antigen-specific CD8 T cells proliferated in vivo in the presence of IL-10 or T(reg), optimal effector functions were observed in mice lacking both IL-10 and T(reg). Indeed, tumors grown in normal but not in IL-10-deficient or CD25-depleted mice induced tumor antigen-specific CD8 suppressor T cells. Suppression involved transforming growth factor-beta. Similarly, both IL-10 and T(reg) were responsible for impaired CD8 T cell priming by TIDCs, but IL-12 production by TIDCs was prevented only by T(reg)-independent IL-10. Subsequently, IL-10 defect and T(reg) depletion were required to achieve optimal induction of CD8 T-cell effectors by TIDC following CpG activation. Our results point out major redundant and nonredundant roles for IL-10 and T(reg) in the inhibition of TIDC-mediated generation of antitumor CD8 T-cell response.

Tumor-derived lactic acid modulates dendritic cell activation and antigen expression

The tumor milieu can influence dendritic cell (DC) differentiation. We analyzed DC differentiation in a 3-dimensional tumor model and propose a new mechanism of DC modulation by the tumor environment. Monocytes were cultured in the presence of IL-4 and GM-CSF within multicellular tumor spheroids (MCTSs) generated from different tumor cell lines. Monocytes invaded the MCTSs and differentiated into tumor-associated dendritic cells (TADCs). The antigen expression was altered on TADCs independent of the culture conditions (immature/mature DCs, Langerhans cells) and IL-12 secretion was reduced. Supernatants of MCTSs could partially transfer the suppressive effect. Conditioned media from urothelial carcinoma cell lines contained high levels of M-CSF and IL-6, both cytokines known to modulate DC differentiation. In contrast, melanoma and prostate carcinoma MCTS cocultures produced little M-CSF and IL-6, but high levels of lactic acid. Indeed, addition of lactic acid during DC differentiation in vitro induced a phenotype comparable with TADCs generated within melanoma and prostate carcinoma MCTSs. Blocking of lactic acid production in melanoma MCTS cocultures reverted the TADC phenotype to normal. We therefore conclude that tumor-derived lactic acid is an important factor modulating the DC phenotype in the tumor environment, which may critically contribute to tumor escape mechanisms.

Different cell surface oligomeric states of B7-1 and B7-2: implications for signaling

The costimulatory ligands B7-1 and B7-2 are expressed on the surface of antigen-presenting cells and interact with the costimulatory receptors CD28 and cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) expressed on T cells. Although B7-1 and B7-2 are homologous ligands having common receptors, they exhibit distinct biochemical features and roles in immune regulation. Several biochemical and structural studies have indicated differences in the oligomeric state of B7-1 and B7-2. However, the organization of B7 ligands on the cell surface has not been examined. By using photobleaching-based FRET (pbFRET), we demonstrate that B7-1 and B7-2 adopt different oligomeric states on the cell surface. Our study shows that B7-2 exists as a monomer on the cell surface whereas B7-1 exists predominantly as dimers on the cell surface. A series of mutations in B7-1 result in the expression of a predominantly monomeric species on the cell surface and validate the dimer interface proposed by prior crystallographic analysis. The difference in the oligomeric states of B7-1 and B7-2 provides insight into the geometric organization of the costimulatory receptor-ligand complexes in the immunological synapse and suggests constraints on signal transduction mechanisms involved in T cell activation.

Cellular mechanisms of the adjuvant activity of the flagellin component FljB of Salmonella enterica Serovar Typhimurium to potentiate mucosal and systemic responses

An expanding area of interest is the utilization of microbe-based components to augment mucosal and systemic immune responses to target antigens. Thus, the aim of the present study was to assess if the flagellin component FljB from Salmonella enterica serovar Typhimurium could act as a mucosal adjuvant and then to determine the cellular mechanism(s) by which FljB mediates its adjuvant properties. To determine if FljB could act as a mucosal adjuvant, mice were immunized by the intranasal (i.n.) route with antigen alone or in conjunction with FljB. Additionally, we assessed how FljB affected the levels of the costimulatory molecules B7-1 and B7-2 on dendritic cells by flow cytometry and determined the functional role these costimulatory molecules played in the adjuvant properties of FljB in vivo. Mice immunized by the i.n. route with antigen and FljB exhibited significantly elevated levels of mucosal and systemic antibody and CD4(+)-T-cell responses compared to mice given antigen only. Stimulation of dendritic cells in vitro with FljB resulted in a pronounced increase in the surface expression of B7-1 and B7-2. The percentage of dendritic cells expressing B7-2 but not B7-1 increased significantly when stimulated with FljB over a concentration range of 10 to 10,000 ng/ml. Immunization of wild-type and B7-1, B7-2, and B7-1/2 knockout mice by the i.n. route revealed that the ability of FljB to increase B7-2 expression is largely responsible for its adjuvant effect in vivo. These findings demonstrate that FljB can act as an effective mucosal adjuvant and that its ability to enhance the level of B7-2 expression is predominantly responsible for its adjuvant properties.

CD40 ligation prevents onset of tolerogenic properties in human dendritic cells treated with CTLA-4-Ig

The synthetic immunomodulator cytotoxic T lymphocyte antigen 4-Ig (CTLA-4-Ig) initiates effects in human monocyte-derived dendritic cells (DC) that rely on immunosuppressive tryptophan catabolism. However, it is unable to induce suppressive properties in DC matured by CD40 engagement. Thus, CD40-driven events may physiologically set human DC free from restraint by regulatory cells expressing surface CTLA-4.

Thrombospondin-1 and indoleamine 2,3-dioxygenase are major targets of extracellular ATP in human dendritic cells

Extracellular adenosine triphosphate affects the maturation of human monocyte-derived dendritic cells (DCs), mainly by inhibiting T-helper 1 (Th1) cytokines, promoting Th2 cytokines, and modulating the expression of costimulatory molecules. In this study, we report that adenosine triphosphate (ATP) can induce immunosuppression through its action on DCs, defining a new role for extracellular nucleotides. Microarray analysis of ATP-stimulated human DCs revealed inter alia a drastic up-regulation of 2 genes encoding mediators involved in immunosuppression: thrombospondin-1 (TSP-1) and indoleamine 2,3-dioxygenase (IDO). The release of TSP-1 by DCs in response to ATP was confirmed at the protein level by enzyme-linked immunosorbent assay (ELISA), immunodetection, and mass spectrometry analysis, and has an antiproliferative effect on T CD4+ lymphocytes through TSP-1/CD47 interaction. Our pharmacologic data support the involvement of purinergic receptor P2Y11 in this ATP-mediated TSP-1 secretion. We demonstrate also that ATP significantly potentiates the up-regulation of IDO--a negative regulator of T lymphocyte proliferation--and kynurenine production initiated by interferon-gamma (IFN-gamma) in human DCs. Thus, extracellular ATP released from damaged cells and previously considered as a danger signal is also a potent regulator of mediators playing key roles in immune tolerance. Consequently, nucleotides' derivatives may be considered as useful tools for DC-based immunotherapies.

In vitro preparation and functional assessment of human monocyte-derived dendritic cells—potential antigen-specific modulators of in vivo immune responses

Dendritic cells (DCs) are highly specialized professional antigen presenting cells that have a potent capacity for stimulating naïve, memory and effector T-cells. They are located in lymphoid organs as well as in almost all nonlymphoid tissues. Immature DCs, residing in the host microenvironment, respond to danger signals with maturation, a differentiation process along which they acquire the ability to direct the extent and the type of primary immune responses according to the type of danger perceived. In this review we present some of our approaches and experiences regarding the isolation of human monocytes from peripheral blood and the in vitro preparation of, first, immature and then mature DCs by applying several maturation factors: bacterial lipopolysaccharide (LPS), a defined mixture of recombinant pro-inflammatory cytokines, monocyte conditioned medium (MCM) and TNF-alpha alone. The assessment of DC phenotypes and their functional capabilities as well as some of the techniques used for tumour associated antigen loading are also presented. The results of such studies represent a basis for optimal in vitro preparation of DCs, which could be clinically used to modulate immune responses in cancer, autoimmune diseases and in the planned onset of tolerance to disparate major histocompatibilty complex (MHC) antigens prior to tissue or organ transplantation.

Immunoregulation of dendritic cells

The paradigm of tolerogenic/immature versus inflammatory/mature dendritic cells has dominated the recent literature regarding the role of these antigen-presenting cells in mediating immune homeostasis or self-tolerance and response to pathogens, respectively. This issue is further complicated by the identification of distinct subtypes of dendritic cells that exhibit different antigen-presenting cell effector functions. The discovery of pathogen-associated molecular patterns and toll-like receptors provides the mechanistic basis for dendritic cell recognition of specific pathogens and induction of appropriate innate and adaptive immune responses. Only recently has insight been gained into how dendritic cells contribute to establishing and/or maintaining immunological tolerance to self. Soluble and cellular mediators have been reported to effectively regulate the function of dendritic cells by inducing several outcomes ranging from non-inflammatory dendritic cells that lack the ability to induce T lymphocyte activation to dendritic cells that actively suppress T lymphocyte responses. A thorough discussion of these stimuli and their outcomes is essential to understanding the potential for modulating dendritic cell function in the treatment of inflammatory disease conditions.

Regulation of immune responses by L-arginine metabolism

L-Arginine is an essential amino acid for birds and young mammals, and it is a conditionally essential amino acid for adult mammals, as it is important in situations in which requirements exceed production, such as pregnancy. Recent findings indicate that increased metabolism of L-arginine by myeloid cells can result in the impairment of lymphocyte responses to antigen during immune responses and tumour growth. Two enzymes that compete for L-arginine as a substrate - arginase and nitric-oxide synthase - are crucial components of this lymphocyte-suppression pathway, and the metabolic products of these enzymes are important moderators of T-cell function. This Review article focuses on the relevance of L-arginine metabolism by myeloid cells for immunity under physiological and pathological conditions.

The B7 family revisited

The discovery of new functions for the original B7 family members, together with the identification of additional B7 and CD28 family members, have revealed new ways in which the B7:CD28 family regulates T cell activation and tolerance. B7-1/B7-2:CD28 interactions not only promote initial T cell activation but also regulate self-tolerance by supporting CD4+CD25+ T regulatory cell homeostasis. CTLA-4 can exert its inhibitory effects in both B7-1/B7-2 dependent and independent fashions. B7-1 and B7-2 can signal bidirectionally by engaging CD28 and CTLA-4 on T cells and by delivering signals into B7-expressing cells. The five new B7 family members, ICOS ligand, PD-L1 (B7-H1), PD-L2 (B7-DC), B7-H3, and B7-H4 (B7x/B7-S1) are expressed on professional antigen-presenting cells as well as on cells within nonlymphoid organs, providing new means for regulating T cell activation and tolerance in peripheral tissues. The new CD28 families members, ICOS, PD-1, and BTLA, are inducibly expressed on T cells, and they have important roles in regulating previously activated T cells. PD-1 and BTLA also are expressed on B cells and may have broader immunoregulatory functions. The ICOS:ICOSL pathway appears to be particularly important for stimulating effector T cell responses and T cell-dependent B cell responses, but it also has an important role in regulating T cell tolerance. In addition, the PD-1:PD-L1/PD-L2 pathway plays a critical role in regulating T cell activation and tolerance. In this review, we revisit the roles of the B7:CD28 family members in regulating immune responses, and we discuss their therapeutic potential.

GCN2 kinase in T cells mediates proliferative arrest and anergy induction in response to indoleamine 2,3-dioxygenase

Indoleamine 2,3 dioxygenase (IDO) catabolizes the amino acid tryptophan. IDO-expressing immunoregulatory dendritic cells (DCs) have been implicated in settings including tumors, autoimmunity, and transplant tolerance. However, the downstream molecular mechanisms by which IDO functions to regulate T cell responses remain unknown. We now show that IDO-expressing plasmacytoid DCs activate the GCN2 kinase pathway in responding T cells. GCN2 is a stress-response kinase that is activated by elevations in uncharged tRNA. T cells with a targeted disruption of GCN2 were not susceptible to IDO-mediated suppression of proliferation in vitro. In vivo, proliferation of GCN2-knockout T cells was not inhibited by IDO-expressing DCs from tumor-draining lymph nodes. IDO induced profound anergy in responding wild-type T cells, but GCN2-knockout cells were refractory to IDO-induced anergy. We hypothesize that GCN2 acts as a molecular sensor in T cells, allowing them to detect and respond to conditions created by IDO.

An important role of CD80/CD86-CTLA-4 signaling during photocarcinogenesis in mice

Although previous studies have shown that altered B7 costimulation plays a critical role in UV irradiation-induced regulation of immunity, the individual roles of the B7 receptors (CD28 and CTLA-4) or the B7 family members (CD80 and CD86) have not been explored. Thus, we investigated CTLA-4 signaling during photocarcinogenesis of chronically UV-B-exposed mice using an antagonistic anti-CTLA-4 Ab. Anti-CTLA-4-treated mice developed significantly fewer UV-induced tumors. Moreover, anti-CTLA-4 treatment induced long-lasting protective immunity because progressively growing UV tumors inoculated into anti-CTLA-4- and UV-treated mice that had not developed tumors were rejected. Next, we used mice deficient for CD80, CD86, or both in photocarcinogenesis studies to assess the relative contributions of these CTLA-4 ligands. Double-deficient mice showed significantly reduced UV-induced skin tumor development, whereas CD86(-/-) mice produced skin cancer earlier compared with CD80(-/-) and control mice. The growth of UV-induced tumors appears to be controlled by UV-induced suppressor T cells, because CD80(-/-)/CD86(-/-) mice had strongly reduced numbers of UV-induced CD4(+)CD25(+) suppressor T cells. In vitro, CTLA-4 blockade inhibited the suppressor activity of UV-induced CD4(+)CD25(+) T cells, suggesting that reduced photocarcinogenesis might be due to decreased numbers or function of suppressor T cells. Together, these data indicate that blocking CD80/86-CTLA-4 signaling induced immune protection against the development of UV-induced skin tumors. Furthermore, CD86-mediated costimulation appears to play a more critical role in the protection against photocarcinogenesis than CD80.

Inhibition of indoleamine 2,3-dioxygenase (IDO) enhances elimination of virus-infected macrophages in an animal model of HIV-1 encephalitis

Indoleamine 2,3-dioxygenase (IDO) is the rate-limiting enzyme in the kynurenine pathway of tryptophan metabolism. IDO activity is linked with immunosuppression by its ability to inhibit lymphocyte proliferation, and with neurotoxicity through the generation of quinolinic acid and other toxins. IDO is induced in macrophages by HIV-1 infection, and it is up regulated in macrophages in human brain tissue with HIV-1 encephalitis (HIVE). Using a model of HIVE, we investigated whether IDO inhibitor 1-methyl-d-tryptophan (1-MT) could affect the generation of cytotoxic T lymphocytes (CTLs) and clearance of virus-infected macrophages from the brain. Severe combined immunodeficient mice were reconstituted with human peripheral blood lymphocytes, and encephalitis was induced by intracranial injection of autologous HIV-1-infected monocyte-derived macrophages (MDMs). Animals treated with 1-MT demonstrated increased numbers of human CD3+, CD8+, CD8+/interferon-gamma+ T cells, and HIV-1(gag/pol)-specific CTLs in peripheral blood compared with controls. At week 2 after MDM injection in the basal ganglia, mice treated with 1-MT showed a 2-fold increase in CD8+ T lymphocytes in the areas of the brain containing HIV-1-infected MDMs compared with untreated controls. By week 3, 1-MT-treated mice showed 89% reduction in HIV-infected MDMs in brain as compared with controls. Thus, manipulation of immunosuppressive IDO activity in HIVE may enhance the generation of HIV-1-specific CTLs, leading to elimination of HIV-1-infected macrophages in brain.

A two-step induction of indoleamine 2,3 dioxygenase (IDO) activity during dendritic-cell maturation

Prostaglandins, a family of lipidic molecules released during inflammation, display immunomodulatory properties in several models. One use includes exposure of monocyte-derived dendritic cells (DCs) to a cocktail of cytokines that contains prostaglandin E2 (PGE2) for purposes of maturation; such cells are currently being used for cancer immunotherapy trials. Our analysis of the transcription profile of DCs matured in the presence of tumor necrosis factor alpha (TNFalpha) and PGE2 revealed a strong up-regulation of indoleamine 2-3 dioxygenase (IDO), an enzyme involved in tryptophan catabolism and implicated in both maternal and T-cell tolerance. Using quantitative assays to monitor levels of IDO mRNA, protein expression, and enzyme activity, we report that PGE2 induces mRNA expression of IDO; however, a second signal through TNF receptor (TNF-R) or a Toll-like receptor (TLR) is necessary to activate the enzyme. Interestingly, use of TNFalpha, lipopolysaccharide, or Staphylococcus aureus Cowan I strain (SAC) alone does not induce IDO. The effect of PGE2 is mediated by activation of adenylate cyclase via the Gs-protein-coupled receptor E prostanoid-2 (EP2). A better understanding of these regulatory mechanisms and the crosstalk between TNF-R/TLR and EP2 signaling pathways will provide insight into the regulation of T-cell activation by DCs and may help to improve existing immunotherapy protocols.

Organ transplantation—how much of the promise has been realized?

Since the introduction of organ transplantation into medical practice, progress and optimism have been abundant. Improvements in immunosuppressive drugs and ancillary care have led to outstanding short-term (1--3-year) patient and graft survival rates. This success is mitigated by several problems, including poor long-term (>5-year) graft survival rates, the need for continual immunosuppressive medication and the discrepancy between the demand for organs and the supply. Developing methods to induce transplant tolerance, as a means to improve graft outcomes and eliminate the requirement for immunosuppression, and expanding the pool of organs for transplantation are the major challenges of the field.

Naturally arising Foxp3-expressing CD25+CD4+ regulatory T cells in immunological tolerance to self and non-self

Naturally arising CD25(+)CD4(+) regulatory T cells actively maintain immunological self-tolerance. Deficiency in or dysfunction of these cells can be a cause of autoimmune disease. A reduction in their number or function can also elicit tumor immunity, whereas their antigen-specific population expansion can establish transplantation tolerance. They are therefore a good target for designing ways to induce or abrogate immunological tolerance to self and non-self antigens.

Cutting Edge: The Prevalence of Regulatory T Cells in Lymphoid Tissue Is Correlated with Viral Load in HIV-Infected Patients

Inadequate local cell-mediated immunity appears crucial for the establishment of chronic HIV infection. Accumulation of regulatory T cells (Treg) at the site of HIV replication, the lymphoid organs, may influence the outcome of HIV infection. Our data provide the first evidence that chronic HIV infection changes Treg tissue distribution. Several molecules characteristics of Treg (FoxP3, CTLA-4, glucocorticoid-induced TNFR family-related receptor, and CD25) were expressed more in tonsils of untreated patients compared with antiretroviral-treated patients. Importantly, most FoxP3+ cells expressed CTLA-4, but not CD69. Furthermore, a direct correlation between FoxP3 levels and viral load was evident. In contrast, FoxP3 expression was decreased in circulating T cells from untreated patients, but normalized after initiation of treatment. Functional markers of Treg activity (indoleamine 2,3-dioxygenase, TGF-beta, and CD80) were markedly increased in the tonsils of untreated patients. Our data could provide a new basis for immune-based therapies that counteract in vivo Treg and thereby reinforce appropriate antiviral immunity.

Indoleamine 2, 3-dioxygenase (IDO) is essential for dendritic cell activation and chemotactic responsiveness to chemokines

Indoleamine 2, 3-dioxygenase (IDO) is a rate-limiting enzyme for the tryptophan catabolism. In human and murine cells, IDO inhibits antigen-specific T cell proliferation in vitro and suppresses T cell responses to fetal alloantigens during murine pregnancy. In mice, IDO expression is an inducible feature of specific subsets of dendritic cells (DCs), and is important for T cell regulatory properties. However, the effect of IDO and tryptophan deprivation on DC functions remains unknown. We report here that when tryptophan utilization was prevented by a pharmacological inhibitor of IDO, 1-methyl tryptophan (1MT), DC activation induced by pathogenic stimulus lipopolysaccharide (LPS) or inflammatory cytokine TNF-alpha was inhibited both phenotypically and functionally. Such an effect was less remarkable when DC was stimulated by a physiological stimulus, CD40 ligand. Tryptophan deprivation during DC activation also regulated the expression of CCR5 and CXCR4, as well as DC responsiveness to chemokines. These results suggest that tryptophan usage in the microenvironment is essential for DC maturation, and may also play a role in the regulation of DC migratory behaviors.

A Crucial Role for Tryptophan Catabolism at the Host/Candida albicansInterface

By mediating tryptophan catabolism, the enzyme indoleamine 2,3-dioxygenase (IDO) has a complex role in immunoregulation in infection, pregnancy, autoimmunity, transplantation, and neoplasia. We hypothesized that IDO might affect the outcome of the infection in mice infected with Candida albicans by virtue of its potent regulatory effects on inflammatory and T cell responses. IDO expression was examined in mice challenged with the fungus along with the consequences of its blockade by in vivo treatment with an enzyme inhibitor. We found that IDO activity was induced at sites of infection as well as in dendritic cells and effector neutrophils via IFN-gamma- and CTLA-4-dependent mechanisms. IDO inhibition greatly exacerbated infection and associated inflammatory pathology as a result of deregulated innate and adaptive/regulatory immune responses. However, a role for tryptophan catabolism was also demonstrated in a fungus-autonomous fashion; its blockade in vitro promoted yeast-to-hyphal transition. These results provide novel mechanistic insights into complex events that, occurring at the fungus/pathogen interface, relate to the dynamics of host adaptation to the fungus. The production of IFN-gamma may be squarely placed at this interface, where IDO activation probably exerts a fine control over fungal morphology as well as inflammatory and adaptive antifungal responses.

Monocyte-mediated T-cell suppression and augmented monocyte tryptophan catabolism after human hematopoietic stem-cell transplantation

T-cell dysfunction after human hematopoietic stem-cell transplantation (HSCT) is generally attributed to intrinsic T-cell defects. Here we show that the characteristic impaired proliferative responses to polyclonal stimulation of post-HSCT peripheral blood mononuclear cells (PB-MCs) were markedly (4-fold) improved by T-cell enrichment. Conversely, addback of post-HSCT monocytes to these enriched T cells dampened their proliferative responses, suggesting that post-HSCT monocytes effectively mediate T-cell suppression. As a mechanism possibly contributing to monocyte-mediated T-cell suppression, we investigated monocyte tryptophan catabolism by indoleamine 2,3-dioxygenase into kynurenine, which has been implicated in regulating T-cell responses. Compared with controls, all post-HSCT monocyte-containing cell cultures (total PBMCs, monocytes, and monocyte/T-cell cocultures), but not monocyte-depleted populations, secreted elevated amounts of kynurenine. Blockade of tryptophan catabolism improved the proliferative responses. The slightly increased kynurenine release and substantial release of neopterin by unstimulated post-HSCT monocytes suggests that they were in a state of continuous activation. Superimposed on this state, stimulation of these cells caused a striking, additional increase (10-fold) in kynurenine release, and they triggered marked apoptosis of autologous post-HSCT T cells. We conclude that the amplified kynurenine release by post-HSCT monocytes, particularly induced upon stimulation, may underlie their suppressor activity, which in turn may contribute to the depressed T-cell immune responses after HSCT.

CD4+ Tregs and immune control

Recent years have seen Tregs become a popular subject of immunological research. Abundant experimental data have now confirmed that naturally occurring CD25+CD4+ Tregs in particular play a key role in the maintenance of self tolerance, with their dysfunction leading to severe or even fatal immunopathology. The sphere of influence of Tregs is now known to extend well beyond just the maintenance of immunological tolerance and to impinge on a host of clinically important areas from cancer to infectious diseases. The identification of specific molecular markers in both human and murine immune systems has enabled the unprecedented investigation of these cells and should prove key to ultimately unlocking their clinical potential.

IDO expression by dendritic cells: tolerance and tryptophan catabolism

Indoleamine 2,3-dioxygenase (IDO) is an enzyme that degrades the essential amino acid tryptophan. The concept that cells expressing IDO can suppress T-cell responses and promote tolerance is a relatively new paradigm in immunology. Considerable evidence now supports this hypothesis, including studies of mammalian pregnancy, tumour resistance, chronic infections and autoimmune diseases. In this review, we summarize key recent developments and propose a unifying model for the role of IDO in tolerance induction.

CD4+ Tregs and immune control

Recent years have seen Tregs become a popular subject of immunological research. Abundant experimental data have now confirmed that naturally occurring CD25+CD4+ Tregs in particular play a key role in the maintenance of self tolerance, with their dysfunction leading to severe or even fatal immunopathology. The sphere of influence of Tregs is now known to extend well beyond just the maintenance of immunological tolerance and to impinge on a host of clinically important areas from cancer to infectious diseases. The identification of specific molecular markers in both human and murine immune systems has enabled the unprecedented investigation of these cells and should prove key to ultimately unlocking their clinical potential.

Regulation of indoleamine 2,3-dioxygenase and tryptophanyl-tRNA-synthetase by CTLA-4-Fc in human CD4+ T cells

Indoleamine-2,3-dioxygenase (IDO) and tryptophanyl-tRNA-synthetase (TTS) are interferon-gamma (IFN-gamma)-inducible enzymes that are responsible for tryptophan degradation and for its use in protein synthesis, respectively. IFN-gamma-induced IDO has immunomodulatory properties in murine and human models. A concomitant increase of TTS has been postulated to protect the IDO-expressing cells from tryptophan catabolism. IDO can be induced in dendritic cells (DCs) by recombinant soluble cytotoxic T lymphocyte antigen-4 (CTLA-4-Fc). We investigated the effects of CTLA-4-Fc on IDO and TTS mRNA expression in human peripheral blood mononuclear cells (PBMCs) and isolated leukocyte subsets. CTLA-4-Fc exposure induced increased IDO and TTS expression in unseparated PBMCs, as well as in monocyte-derived mature DCs. CD4(+) T cells isolated from CTLA-4-Fc-treated PBMCs showed increased IDO and TTS compared with untreated cells. CD8(+) T cells from CTLA-4-Fc-treated PBMCs expressed increased levels of TTS but not IDO. Pretreatment of PBMCs with CTLA-4-Fc inhibited the activation of CD4(+) T cells induced by influenza A virus (Flu) or phytohemagglutinin A (PHA), but had no effect on CD8(+) T cells. This is the first report of IDO and TTS regulation by the CTLA-4-B7 system in human CD4(+) and CD8(+) T cells, and raises the possibility that these 2 tryptophan-modulating enzymes provide an important mechanism for regulating immune responses.