The signaling function of IDO1 incites the malignant progression of mouse B16 melanoma.

Indoleamine 2,3 dioxygenase 1 (IDO1), a leader tryptophan-degrading enzyme, represents a recognized immune checkpoint molecule. In neoplasia, IDO1 is often highly expressed in dendritic cells infiltrating the tumor and/or in tumor cells themselves, particularly in human melanoma. In dendritic cells, IDO1 does not merely metabolize tryptophan into kynurenine but, after phosphorylation of critical tyrosine residues in the non-catalytic small domain, it triggers a signaling pathway prolonging its immunoregulatory effects by a feed-forward mechanism. We here investigated whether the non-enzymatic function of IDO1 could also play a role in tumor cells by using B16-F10 mouse melanoma cells transfected with either the wild-type Ido1 gene (Ido1WT ) or a mutated variant lacking the catalytic, but not signaling activity (Ido1H350A ). As compared to the Ido1WT -transfected counterpart (B16WT), B16-F10 cells expressing Ido1H350A (B16H350A) were characterized by an in vitro accelerated growth mediated by increased Ras and Erk activities. Faster growth and malignant progression of B16H350A cells, also detectable in vivo, were found to be accompanied by a reduction in tumor-infiltrating CD8+ T cells and an increase in Foxp3+ regulatory T cells. Our data, therefore, suggest that the IDO1 signaling function can also occur in tumor cells and that alternative therapeutic approach strategies should be undertaken to effectively tackle this important immune checkpoint molecule.

Aryl Hydrocarbon Receptor-Dependent Pathways in Immune Regulation.

The idea of possible involvement of the aryl hydrocarbon receptor (AhR) in transplant tolerance can be traced back >30 years, when very low doses of dioxin-the most potent AhR ligand-were found to markedly reduce the generation of cytotoxic T lymphocytes in response to alloantigen challenge in vivo. AhR is a ligand-activated transcription factor that is activated by dioxins and other environmental pollutants. We now know that AhR can bind a broad variety of activating ligands that are disparate in nature, including endogenous molecules and those formed in the gut from food and bacterial products. Consequently, in addition to its classical role as a toxicological signal mediator, AhR is emerging as a transcription factor involved in the regulation of both innate and adaptive immune responses in various immune cell types, including lymphocytes and antigen-presenting cells (APCs). Allograft rejection is mostly a T cell-mediated alloimmune response initiated by the recognition of alloantigens presented by donor and recipient APCs to recipient CD4(+) and CD8(+) T cells. Based on those findings, AhR may function as a critical sensor of outside and inside environments, leading to changes in the immune system that may have relevance in transplantation.

Cytotoxic T lymphocyte antigen 4-immunoglobulin G is a potent adjuvant for experimental allergen immunotherapy.

Allergen-specific immunotherapy (SIT) is the only treatment for allergic diseases that targets allergen-specific T helper type 2 (Th2) cells, which are the cause of the disease. There is an unmet requirement for adjuvants that increase the clinical efficacy of SIT allowing application of lower doses of the allergen, thereby reducing the risk of anaphylactic reactions. Cytotoxic T lymphocyte antigen 4-immunoglobulin (CTLA-4-Ig) has been shown to induce immunological tolerance in autoimmunity and allograft transplantation by blocking T cell co-stimulation and induction of the immunoregulatory enzyme indoleamine 2,3 dioxygenase (IDO). Previously, we showed that CTLA-4-Ig treatment at the time of allergen inhalation induced tolerance to subsequent allergen exposure in a mouse model of asthma. In this study, we test the hypothesis that CTLA-4-Ig acts as an adjuvant for experimental SIT. We evaluated the adjuvant effects of CTLA-4-Ig on SIT in a mouse model of ovalbumin-driven asthma. We used both wild-type and IDO-deficient mice to assess the role of IDO in the adjuvant effects of CTLA-4-Ig. Co-administration of CTLA-4-Ig strongly increased SIT-induced suppression of airway hyperreactivity (AHR), specific IgE in serum, airway eosinophilia and Th2 cytokine levels. Moreover, we found that CTLA-4-Ig, as an adjuvant for SIT, is equally effective in IDO-deficient and wild-type mice, demonstrating that the effect of CTLA-4-Ig is independent of IDO expression. We show that CTLA-4-Ig acts as a potent adjuvant to augment the therapeutic effects of SIT. As the adjuvant activity of CTLA-4-Ig is independent of IDO, we conclude that it acts by blocking CD28-mediated T cell co-stimulation.

Targeting metabotropic glutamate receptors in neuroimmune communication.

L-Glutamate (L-Glu) is the principal excitatory neurotransmitter in the Central Nervous System (CNS), where it regulates cellular and synaptic activity, neuronal plasticity, cell survival and other relevant functions. Glutamatergic neurotransmission is complex and involves both ionotropic (ligand-gated ion channels; iGluRs) and metabotropic receptors (G-protein coupled receptors). Recent evidence suggests that glutamatergic receptors are also expressed by immune cells, regulating the degree of cell activation. In this review we primarily focus on mGluRs and their role in the crosstalk between the central nervous and immune systems during neuroinflammation.

Using an ancient tool for igniting and propagating immune tolerance: IDO as an inducer and amplifier of regulatory T cell functions.

Although most CD4(+)CD25(+) regulatory T (T(reg)) cells develop in the thymus (i.e., natural T(reg) or nT(reg)), accumulating evidence suggests that they can also develop in the periphery (adaptive/induced T(reg) or iT(reg)). Both types of cells are functionally associated with the expression of Foxp3, a transcription factor that is constitutively expressed in nT(reg) cells and inducible during iT(reg) cell generation from CD4+CD25- T lymphocytes. Multiple factors are involved in the generation and function of T(reg) cells, but a major role seems to be played by indoleamine 2,3-dioxygenase (IDO). IDO can both deplete tryptophan in local tissue microenvironments and generate immunoregulatory catabolites, known as kynurenines. Tryptophan starvation and presence of kynurenines can induce the conversion of naïve CD4(+)CD25(-) T cells into highly suppressive CD4(+)CD25(+)Foxp3(+) T(reg) cells. In turn, T(reg) cells induce IDO in dendritic cells (DCs) and convert inflammatory into regulatory DCs, which can further expand the T(reg) cell compartment by tryptophan catabolism. Evidence suggests that the modulation of IDO activity favors the interconversion between T(reg) cells and T helper type 17 (T(H)17) inflammatory cells. Thus, in the periphery, tolerogenic immune responses mediated by T(reg) cells can be induced and amplified by IDO, a tryptophan catabolizing enzyme that also contributes to the plasticity of the T(reg) cell lineage.

IL-22 defines a novel immune pathway of antifungal resistance.

The role of IL-17 and Th17 cells in immunity vs. pathology associated with the human commensal Candida albicans remains controversial. Both positive and negative effects on immune resistance have been attributed to IL-17/Th17 in experimental candidiasis. In this study, we provide evidence that IL-22, which is also produced by Th17 cells, has a critical, first-line defense in candidiasis by controlling the growth of infecting yeasts as well as by contributing to the host's epithelial integrity in the absence of acquired Th1-type immunity. The two pathways are reciprocally regulated, and IL-22 is upregulated under Th1 deficiency conditions and vice versa. Whereas both IL-17A and F are dispensable for antifungal resistance, IL-22 mediates protection in IL-17RA-deficient mice, in which IL-17A contributes to disease susceptibility. Thus, our findings suggest that protective immunity to candidiasis is made up of a staged response involving an early, IL-22-dominated response followed by Th1/Treg reactivity that will prevent fungal dissemination and supply memory.

Intranasally delivered siRNA targeting PI3K/Akt/mTOR inflammatory pathways protects from aspergillosis.

Innate responses combine with adaptive immunity to generate the most effective form of anti-Aspergillus immune resistance. Although some degree of inflammation is required for protection, progressive inflammation may worsen disease and ultimately prevents pathogen eradication. To define molecular pathways leading to or diverting from pathogenic inflammation in infection, we resorted to dendritic cells (DCs), known to activate distinct signaling pathways in response to pathogens. We found that distinct intracellular pathways mediated the sensing of conidia and hyphae by lung DCs in vitro, which translate in vivo in the activation of protective Th1/Treg responses by conidia or inflammatory Th2/Th17 responses by hyphae. In vivo targeting inflammatory (PI3K/Akt/mTOR) or anti-inflammatory (STAT3/IDO) DC pathways by intranasally delivered small interfering RNA (siRNA) accordingly modified inflammation and immunity to infection. Thus, the screening of signaling pathways in DCs through a systems biology approach may be exploited for the development of siRNA therapeutics to attenuate inflammation in respiratory fungal infections and diseases.

Balancing inflammation and tolerance in vivo through dendritic cells by the commensal Candida albicans.

We analyzed the contribution of intracellular signaling to the functional plasticity of dendritic cells (DCs) presenting Candida albicans, a human commensal associated with severe diseases. Distinct intracellular pathways were activated by recognition of different fungal morphotypes in distinct DC subsets and in Peyer's patches DCs. Inflammatory DCs initiated Th17/Th2 responses to yeasts through the adaptor myeloid differentiation factor-88 (MyD88), whereas tolerogenic DCs activate Th1/T regulatory cell (Treg) differentiation programs to hyphae involving Toll/IL-1 receptor domain-containing adaptor inducing IFN-beta (TRIF) as an intermediary of signaling. In addition, signal transducer and activator of transcription 3 (STAT3), affecting the balance between canonical and non-canonical activation of nuclear factor-kappaB (NF-kappaB) and 2,3 indoleamine dioxygenase (IDO), pivotally contributed to DC plasticity and functional specialization. As Candida-induced tolerogenic DCs ameliorated experimental colitis, our data qualify Candida as a commensal with immunoregulatory activity, resulting from the orchestrated usage of multiple, yet functionally distinct, receptor-signaling pathways in DCs. Ultimately, affecting the local Th17/Treg balance might likely be exploited by the fungus for either commensalism or pathogenicity.

Mechanisms of CTLA-4-Ig in tolerance induction.

The size of the peripheral T lymphocyte pool remains relatively constant throughout adult life, but individual populations undergo expansion and contraction upon antigen encounter due to signals delivered by members of the B7-CD28 family of costimulatory molecules. This family includes receptors on T cells that can provide either activating or inhibitory signals. In general, activation occurs in response to pathogens, when lymphocyte expansion and acquisition of effector functions is appropriate. Conversely inhibitory receptors provide down-modulating signals that help terminate immune responses and maintain self-tolerance. The activating receptor CD28 engages the same B7-1 and B7-2 molecules as the inhibitory receptor cytotoxic T lymphocyte antigen 4 (CTLA-4), although with reduced affinity than CTLA-4. In addition to this direct competitive mechanism, CTLA-4 can directly inhibit T cell receptor (TCR) signals independently of CD28 expression and recent findings indicate that CTLA-4 may also operate through reverse signaling on ligand-expressing cells. Fusion proteins between the extracellular domain of CTLA-4 and an immunoglobulin Fc portion have been created that have potent immunosuppressive properties in animal models of transplantation and autoimmunity and that show great promise in clinical trials. Like CTLA-4, CTLA-Ig, is thought to selectively prevent activation of CD28 by interacting with B7-1 and B7-2. In addition, CTLA-4-Ig can bind to B7 molecules expressed on dendritic cells and activate a pathway of tryptophan catabolism that can lead to indirect inhibition of lymphocyte activation and T cell death. In this review, we will focus on the current knowledge of the mechanisms of action of CTLA-4 and CTLA-4-Ig.

T cell apoptosis by tryptophan catabolism.

Indoleamine 2,3-dioxygenase (IDO) is a tryptophan-catabolizing enzyme that, expressed by different cell types, has regulatory effects on T cells resulting from tryptophan depletion in specific local tissue microenvironments. Different mechanisms, however, might contribute to IDO-dependent immune regulation. We show here that tryptophan metabolites in the kynurenine pathway, such as 3-hydroxyanthranilic and quinolinic acids, will induce the selective apoptosis in vitro of murine thymocytes and of Th1 but not Th2 cells. T cell apoptosis was observed at relatively low concentrations of kynurenines, did not require Fas/Fas ligand interactions, and was associated with the activation of caspase-8 and the release of cytochrome c from mitochondria. When administered in vivo, the two kynurenines caused depletion of specific thymocyte subsets in a fashion qualitatively similar to dexamethasone. These data suggest that the selective deletion of T lymphocytes may be a major mechanism whereby tryptophan metabolism affects immunity under physiopathologic conditions.

IL-6 inhibits the tolerogenic function of CD8 alpha+ dendritic cells expressing indoleamine 2,3-dioxygenase.

The outcome of dendritic cell (DC) presentation of tumor and/or self peptides, including P815AB (a tumor peptide of murine mastocytoma cells) and NRP-A7 (a synthetic peptide mimotope recognized by diabetogenic T cells), may depend on a balance between the activities of immunogenic (CD8alpha(-)) and tolerogenic (CD8alpha(+)) DC. By virtue of their respective actions on CD8(-) and CD8(+) DC, IL-12 and IFN-gamma have functionally opposing effects on peptide presentation by the CD8(-) DC subset, and IFN-gamma-activated CD8(+) DC mediate tolerogenic effects that prevail over the adjuvant activity of IL-12 on CD8(-) DC. We have previously shown that CD40 ligation abrogates the tolerogenic potential of CD8(+) DC, an effect associated with an impaired capacity of the CD40-modulated and IFN-gamma-treated DC to degrade tryptophan and initiate T cell apoptosis in vitro. We report here that IL-6 may both replace (upon administration of the recombinant cytokine) and mediate (as assessed by the use of neutralizing Abs) the effect of CD40 ligation in ablating the tolerogenic activity of CD8(+) DC. The activity of IL-6 includes down-regulation of IFN-gammaR expression in the CD8(+) DC subset and correlates to a reduced ability of these cells to metabolize tryptophan and initiate T cell apoptosis in vitro.

Positive regulatory role of IL-12 in macrophages and modulation by IFN-gamma.

Similar to myeloid dendritic cells, murine macrophages and macrophage cell lines were found to express a surface receptor for IL-12. As a result, peritoneal macrophages could be primed by IL-12 to present an otherwise poorly immunogenic tumor peptide in vivo. Using binding analysis and RNase protection assay, we detected a single class of high affinity IL-12 binding sites (K(d) of approximately 35 pM) whose number per cell was increased by IFN-gamma via up-regulation of receptor subunit expression. Autocrine production of IL-12 was suggested to be a major effect of IL-12 on macrophages when the cytokine was tested alone or after priming with IFN-gamma in vitro. In vivo, combined treatment of macrophages with IFN-gamma and IL-12 resulted in synergistic effects on tumor peptide presentation. Therefore, our findings suggest a general and critical role of IL-12 in potentiating the accessory function of myeloid APC.

Immunization of HLA-A2+ melanoma patients with MAGE-3 or MelanA peptide-pulsed autologous peripheral blood mononuclear cells plus recombinant human interleukin 12.

Vaccination with dendritic cells (DCs) pulsed with tumor antigen peptides has shown promise in the treatment of melanoma. Interleukin (IL)-12 production by DCs is a key component for their efficacy. Murine studies have shown that IL-12 promotes potent antitumor immunization when coadministered with peptides loaded onto other class I MHC+ cells, thus bypassing the need to use DCs. The easiest cell source to obtain in large quantity from human patients is peripheral blood mononuclear cells (PBMCs). A Phase I clinical trial was thus performed in patients with metastatic melanoma using immunization with autologous PBMCs pulsed with a MAGE-3 or a MelanA peptide, coadministered with various doses of recombinant human (rh)IL-12. Patients receiving low-to-moderate doses of rhIL-12 developed increased specific CD8+ T-cell responses. Of the eight patients showing increased immunity, six had evidence of clinical activity, with one complete, one partial, one minor, and three mixed responses observed. In two patients with mixed responses, growing tumors were found to lack expression of the antigen used to immunize. Thus, vaccination with peptide-pulsed PBMCs plus rhIL-12 induces specific immunity and has clinical activity, without the need to generate DCs. Outgrowth of antigen-negative tumors argues for the future development of polyepitope vaccines.

Epitope spreading upon P815 tumor rejection triggered by vaccination with the single class I MHC-restricted peptide P1A.

Epitope spreading has been best characterized as an exacerbating factor in CD4(+) T cell-dependent autoimmune disease models and is believed to occur via presentation of antigens liberated by tissue destruction initiated by CD4(+) T cells specific for a primary epitope. The growing evidence that exogenous antigens can also be processed and presented by class I MHC molecules has suggested that epitope spreading could occur for CD8(+) cytotoxic T lymphocyte (CTL) responses as well. In the context of anti-tumor immunity, expansion of a CTL response to include secondary epitopes could improve the efficacy of therapeutic vaccines. To determine directly whether epitope spreading can occur during an anti-tumor immune response, two defined class I MHC-binding peptides in the P815 tumor model were utilized. We observed that immunization against the single tumor peptide, P1A, followed by rejection of a P1A(+) tumor, subsequently yielded CTL activity and tumor protection against a P1A(-) tumor variant. P1A immunized mice that subsequently rejected tumor challenge developed CTL against a second defined epitope, P1E. These results indicate that, as for class II-restricted peptides in autoimmune disease, epitope spreading can occur for class I-restricted peptides during tumor rejection. A broadened CTL response may help eliminate outgrowth of antigen-negative tumor variants.

Absence of CTLA-4 lowers the activation threshold of primed CD8+ TCR-transgenic T cells: lack of correlation with Src homology domain 2-containing protein tyrosine phosphatase.

To examine the role of CTLA-4 in controlling Ag-specific CD8(+) T cell activation, TCR-transgenic/CTLA-4 wild-type or -deficient mice were generated in a recombination-activating gene 2-deficient background. Naive T cells from these mice responded comparably whether or not CTLA-4 was expressed. In contrast, primed T cells responded more vigorously if they lacked CTLA-4 expression. We took advantage of the difference between naive and primed T cell responses to approach the mechanism of CTLA-4 function. Single-cell analyses demonstrated that a greater fraction of CTLA-4-deficient cells responded to a fixed dose of Ag compared with CTLA-4-expressing cells, whereas the magnitude of response per cell was comparable. A shift in the dose-response curve to APCs was also observed such that fewer APCs were required to activate CTLA-4-deficient T cells to produce intracellular IFN-gamma and to proliferate. These results suggest that CTLA-4 controls the threshold of productive TCR signaling. Biochemical analysis comparing stimulated naive and primed TCR-transgenic cells revealed no obvious differences in expression of total CTLA-4, tyrosine-phosphorylated CTLA-4, and associated Src homology domain 2-containing protein tyrosine phosphatase. Thus, the biochemical mechanism explaining the differential inhibitory effect of CTLA-4 on naive and primed CD8(+) T cells remains unclear.

CD40 ligation ablates the tolerogenic potential of lymphoid dendritic cells.

The outcome of dendritic cell (DC) presentation of P815AB, a tolerogenic tumor/self peptide, depends on a balance between the respective immunogenic and tolerogenic properties of myeloid (CD8 alpha(-)) and lymphoid (CD8 alpha(+)) DC. We have previously shown that CD8(-) DC can be primed by IL-12 to overcome inhibition by the CD8(+) subset and initiate immunogenic presentation in vivo when the two types of peptide-pulsed DC are cotransferred into recipient hosts. IFN-gamma enhances the inhibitory activity of CD8(+) DC on Ag presentation by the other subset, blocking the ability of IL-12-treated CD8(-) DC to overcome suppression. We report here that CD40 ligation on lymphoid DC ablated their inhibitory function on Ag presentation as well as IFN-gamma potentiation of the effect. CD40 modulation of IFN-gamma action on lymphoid DC involved a reduction in IFN-gamma R expression and tryptophan-degrading ability. This effect was accompanied in vitro by an impaired capacity of the CD40-modulated and IFN-gamma-treated DC to initiate T cell apoptosis. In vivo, not only did CD40 triggering on lymphoid DC abrogate their tolerogenic activity, but it also induced the potential for immunogenic presentation of P815AB. Importantly, a pattern similar to P815AB as well as CD40 modulation of lymphoid DC function were observed on testing reactivity to NRP, a synthetic peptide mimotope recognized by diabetogenic CD8(+) T cells in nonobese diabetic mice.

Cutting edge: spontaneous rejection of poorly immunogenic P1.HTR tumors by Stat6-deficient mice.

Experimental evidence suggests that a type 1 T cell response may result in optimal tumor rejection in vivo. This phenotype is determined in part by cytokines that influence T cell differentiation. In transplantable tumor models such as P1.HTR, tumors grow progressively despite expression of defined tumor Ags. We hypothesized that this failure to reject may be due to poor generation of a type 1 phenotype, through a dominant influence of the type 2-promoting cytokines IL-4 and/or IL-13. This hypothesis was tested by implanting P1.HTR tumors into mice deficient in Stat6. In contrast to progressive growth of P1.HTR tumors in wild-type mice, and aggressive growth even of IL-12-transfected P1.HTR in Stat1(-/-) mice, P1.HTR was spontaneously rejected by Stat6(-/-) mice. Rejection was accompanied by augmented tumor-specific IFN-gamma production and CTL activity. These results suggest that pharmacologic inhibition of Stat6 signaling could potentiate anti-tumor immunity in vivo.

Th1 and Th2 cell clones to a poorly immunogenic tumor antigen initiate CD8+ T cell-dependent tumor eradication in vivo.

Although CD8(+) T cells play a central role as immune effectors, CD4(+) T cells act to control the activation and persistence of the CD8(+) T cell response in autoimmune disease, antiviral immunity, and experimental systems with immunogenic model tumor Ag. However, little information is available on the effects of CD4(+) T cells on the function of endogenous CD8(+) T lymphocytes recognizing authentic tumor rejection Ag with limited immunogenicity. We report here that the prophylactic or postchallenge administration of T helper Th1-type and Th2-type CD4(+) clones specific for an unmutated rejection Ag (murine P815AB, resembling tumor-specific shared Ag in humans) leads to the induction of P815AB-specific reactivity in vivo and concomitant tumor destruction, with quantitative rather than qualitative differences characterizing the antitumor activity of Th1 vs Th2 cells. Because the transferred CD4(+) cells lacked direct antitumor activity in vitro and required the de novo generation of P815AB-specific CD8(+) T cells in vivo, these findings suggest that CD4(+) lymphocytes can enhance the ability of host APC to initiate an endogenous CD8(+) T cell response to authentic, poorly immunogenic tumor rejection Ag.