Exogenous Ags Research Articles

Membrane Transfer from Tumor Cells Overcomes Deficient Phagocytic Ability of Plasmacytoid Dendritic Cells for the Acquisition and Presentation of Tumor Antigens

The potential contribution of plasmacytoid dendritic cells (pDCs) in the presentation of tumor cell Ags remains unclear, and some controversies exist with regard to the ability of pDCs to phagocytose cell-derived particulate Ags and cross-present them to MHC class I-restricted T lymphocytes. In this study, we show that human pDCs, although inefficient in the internalization of cell membrane fragments by phagocytosis, can efficiently acquire membrane patches and associated molecules from cancer cells of different histotypes. The transfer of membrane patches to pDCs occurred in a very short time and required cell-to-cell contact. Membrane transfer also included intact HLA complexes, and the acquired Ags could be efficiently recognized on pDCs by tumor-specific CD8(+) T cells. Remarkably, pDCs isolated from human colon cancer tissues displayed a strong surface expression of epithelial cell adhesion molecule, indicating that the exchange of exogenous Ags between pDCs and tumor cells also can occur in vivo. These data demonstrate that pDCs are well suited to acquire membrane patches from contiguous tumor cells by a cell-to-cell contact-dependent mechanism that closely resembles "trogocytosis." This phenomenon may allow pDCs to proficiently present tumor cell-derived Ags, despite limited properties of endophagocytosis.

T-Cell-Mediated Suppression: From Bench to Bedside

T-Cell-Mediated Suppression: From Bench to Bedside

Targeting Uptake Receptors on Human Plasmacytoid Dendritic Cells Triggers Antigen Cross-Presentation and Robust Type I IFN Secretion

Plasmacytoid dendritic cells (pDCs) play a crucial role in initiating immune responses by secreting large amounts of type I IFNs. Currently, the role for human pDCs as professional APCs in the cross-presentation of exogenous Ags is being re-evaluated. Human pDCs are equipped with a broad repertoire of Ag uptake receptors and an efficient Ag-processing machinery. In this study, we set out to investigate which receptor can best be deployed to deliver Ag to pDCs for Ag (cross-)presentation. We show that targeting nanoparticles to pDCs via the C-type lectins DEC-205, DC immunoreceptor, blood DC Ag-2, or the FcR CD32 led to uptake, processing, and (cross-) presentation of encapsulated Ag to both CD4(+) and CD8(+) T cells. This makes these receptors good candidates for potential in vivo targeting of pDCs by nanocarriers. Notably, the coencapsulated TLR7 agonist R848 efficiently activated pDCs, resulting in phenotypical maturation as well as robust IFN-α and TNF-α production. Taken together, their cross-presentation capacity and type I IFN production to further activate components of both the innate and adaptive immune system mark pDCs as inducers of potent antitumor responses. These findings pave the way to actively recruit human pDCs for cellular cancer immunotherapy.

A novel nanoparticle containing MOG peptide with BTLA induces T cell tolerance and prevents multiple sclerosis

Accumulative evidence demonstrates that multiple sclerosis (MS) is caused by activation of myelin Ag-reactive CD4+ T cells. Therefore, the CD4+ T cells specific for myelin Ag may be the important therapeutical target of MS. The novel coinhibitory receptor B and T lymphocyte attenuator (BTLA) may have a regulatory role in maintaining peripheral tolerance, however, its role in MS is still unknown. In this study, a novel nanoparticle containing MOG peptide with BTLA was designed and transduced into dendritic cells (DCs), and MOG peptide-induced EAE mice were adminstrated with the genetically modified DCs in vivo. The results demonstrated that modified DCs significantly enhanced the proportion of Foxp3+ CD4+ regulatory T cells, increased IL-10 and TGF-β cytokine secretion, while decreased IL-2 and IFN-γ cytokine secretion. Furthermore, modified DCs supressed the CD4+ T cell response to MOG, cell infiltration into spinal cord, and the severity of EAE. In contrast, immune response to irrelevant exogenous Ag was not impaired by treatment with modified DCs. These findings suggested that DCs transduced with nanoparticle could induce specific CD4+ T-cells tolerance, which provided a promising therapeutic means to negatively manipulate immune response for autoimmune diseases without inhibition of the immune response to irrelevant Ag.

In vivo activation of transferred regulatory T cells specific for third‐party exogenous antigen controls GVH disease in mice

Treg cells hold enormous promise for therapeutic application in GVH disease, a lethal complication of allogeneic HSC transplantation. Mouse studies showed that donor‐derived recipient‐specific Treg (rsTreg) cells are far more efficient than polyclonal Treg cells in suppressing GVH disease. However, clinical grade preparations of rsTreg cells carries the risk of containing significant numbers of highly pathogenic recipient‐specific effector T cells. We hypothesized that an alternative approach using Treg cells specific for an exogenous (i.e. nondonor, nonrecipient) Ag (exoTreg cells) can overcome this risk by taking advantage of the bystander suppressive effect of Treg cells. For this, we used a murine model for aggressive GVH disease. We expanded ex vivo exoTreg cells that are primed against the HY Ag, which is only expressed in males. ExoTreg cells supressed GVH disease as efficiently as rsTreg cells in recipient male mice. We also applied this strategy in female mice that do not express this Ag. While exoTreg cells were not effective in female recipients when applied alone, providing the cognate HY Ag in vivo along side effectively activated exoTreg cells and completely abrogated GVH disease, establishing a targeted on/off system to provide a suppressive effect on alloreactive effector T cells.

SWAP-70 Restricts Spontaneous Maturation of Dendritic Cells

Spontaneous maturation observed in dendritic cell (DC) cultures has been linked to their capacity to induce immune responses. Despite several recent studies, the mechanisms and signals triggering spontaneous maturation of DCs are largely unknown. We found that the absence of SWAP-70 causes spontaneous maturation of spleen- and bone marrow-derived DCs and, in vivo, of spleen-resident CD11c(+)CD11b(+)CD8α(-) DCs. Activation markers, cross-presentation of exogenous Ags, and activation of CD8(+) T cells are much increased in Swap-70(-/-) DCs. Spontaneous maturation of Swap-70(-/-) DCs depends on cell-cell contact and does not involve β-catenin signaling. SWAP-70 is known to regulate integrin activity. Signaling through the integrin CD11b (αM) subunit increases spontaneous maturation of wild-type (wt), but not of Swap-70(-/-) DCs. Signaling through the CD18 (β2) subunit decreases spontaneous maturation of wt and Swap-70(-/-) DCs. Constitutive activation of RhoA in Swap-70(-/-) DCs was determined as a key mechanism causing the increased spontaneous maturation. Inhibition of RhoA early, but not late, in the activation process reduces spontaneous maturation in Swap-70(-/-) DCs to wt levels. Inhibition of RhoA activation during CD11b integrin activation had a significant effect only in Swap-70(-/-) but not in wt DCs. Together, our data suggest that integrin-mediated spontaneous maturation of wt DCs does not depend on active RhoA, whereas the increase in spontaneous maturation of Swap-70(-/-) DCs is supported by integrin CD11b and by hyperactive RhoA. Thus, SWAP-70 deficiency reveals two pathways that contribute to spontaneous maturation of DCs.

Immunodeficiency and autoimmunity in H2-O deficient mice (P5016)

Abstract HLA-DO/H2-O is a highly conserved non-polymorphic MHC class II-like molecule whose physiological function remains unknown. The cell maturation-dependent DO expression in B-lymphocytes and DCs suggests a role H2-O in promoting presentation of exogenous Ag by attenuating presentation of endogenous self-peptides. We report herein that H2-O-/- mice spontaneously develop high titers of IgG2a/c antinuclear antibodies (ANA) with specificity for dsDNA, ssDNA, and histones. Reconstitution of RAG1-/- mice with T and B cells from H2-O-/- or WT mice demonstrated that production of ANAs requires participation of CD4+ T cells from H2-O-/- mice. Bone marrow chimeras demonstrated that loss of H2-O expression in thymic epithelial cells did not induce ANAs, and that lack of H2-O expression in bone marrow-derived cells was sufficient to induce the autoimmune phenotype. Despite production of high titers of autoantibodies, H2-O-/- mice exhibit a delayed generation of humoral immunity to model antigens (OVA and KLH), affecting all major T-dependent Ig classes including IgG2a/c. Consistently, presentation of all the natural Ag epitopes by H2-O-/- APC was inefficient as compared to WT APC. Thus, H2-O promotes immunity towards exogenous Ag while preventing autoimmunity. We propose that H2-O, through spatially and temporally inhibiting H2-M, enhances presentation of exogenous antigen by preventing nascent/newly-synthesized class II molecules from forming stable complexes with endogenous self-peptides.

Natural autoantibodies protect from autoimmune disease (P4050)

Abstract Natural autoantibodies (NAAs) arise naturally without exogenous Ag stimulation. They constitute a substantial proportion of the normal Ab repertoire. They are mainly of the IgM class, unmutated, and typically polyreactive. It remains controversial whether NAA contribute to or protect from autoimmune diseases. Using site-directed transgenic mice expressing a prototypic NAA, named ppc1-5, we found that the expression of NAA in the autoimmune MRL-lpr mice protect from autoimmune manifestations. To demonstrate that the protection is, at least in part, via the secreted NAA per se, we administered purified ppc1-5 IgM to the wt MRL-lpr mice. We observed substantial reduction in proteinuria and kidney IC deposition, and prolonged survival in mice injected with ppc1-5 IgM as compared to those that received either PBS or control IgM. The ppc1-5 IgM injected mice had decreased levels of anti-Hep2 ANA and anti-dsDNA of the IgG3 subclass, although the total IgM and IgG levels did not alter significantly. We demonstrated that the ppc1-5 NAA, but not the control IgM, bound to apoptotic cells with a high capacity. Examination of cytokine production showed that the administration of ppc1-5 NAA significantly reduced the production of IFNgamma and IL-10 by CD4+ T cells. Taken together, our data demonstrate that the low-affinity, self-reactive NAAs can protect from lupus nephritis, and suggest that they may do so by promoting removal of apoptotic cells, and by regulating T-cell function.

Efficiently stimulated adult microglia cross‐prime naive CD8+T cells injected in the brain

Microglia are the major myeloid-immune cells of the brain parenchyma. In a steady state, microglia monitor their environment for pathogens or damaged cells. In response to neural injury or inflammation, microglia become competent APCs able to prime CD4(+) and CD8(+) T lymphocytes. We previously demonstrated that neonatal and adult microglia cross-present exogenous soluble Ags in vitro. However, whether microglia are able to cross-present Ag to naive CD8(+) T cells in vivo, within the brain microenvironment, remains undetermined. Here, we have designed an original protocol in order to exclude the involvement in cross-presentation activity of peripheral migrating APCs and of CNS-associated APCs. In C57Bl/6 mice, in which the body but not the head has been properly irradiated, we analyzed the ability of resident microglia to stimulate intracerebrally injected CD8(+) T cells in vivo. This study demonstrates for the first time that adult microglia cross-present Ag to naive CD8(+) T cells in vivo and that full microglia activation is required to overcome the inhibitory constrains of the brain and to render microglia able to cross-prime naive CD8(+) T cells injected in the brain. These observations offer new insights in brain-tumor immunotherapy based on the induction of cytotoxic antitumoral T cells.

TLR7 Triggering with Polyuridylic Acid Promotes Cross-Presentation in CD8α+ Conventional Dendritic Cells by Enhancing Antigen Preservation and MHC Class I Antigen Permanence on the Dendritic Cell Surface

ssRNA can interact with dendritic cells (DCs) through binding to TLR7, inducing secretion of proinflammatory cytokines and type I IFN. Triggering TLR7 enhances cross-priming of CD8(+) T cells, which requires cross-presentation of exogenous Ag to DCs. However, how TLR triggering can affect Ag cross-presentation is still not clear. Using OVA as an Ag model, we observed that stimulation of TLR7 in DCs by polyuridylic acid (polyU), a synthetic ssRNA analog, generates a strong specific cytotoxic response in C57BL/6 mice. PolyU stimulate CD8α(+) DCs to cross-prime naive CD8(+) T cells in a type I IFN-dependent fashion. This enhanced cross-priming is accompanied by a higher density of OVA(256-264)/H-2K(b) complexes on CD8α(+) DCs treated with polyU, as well as by upregulation of costimulatory molecules and increased secretion of proinflammatory cytokines by DCs. Cross-priming of CD8(+) T cells by DCs treated with polyU requires proteasome and Ag translocation to cytosol through the Sec61 channel in DCs. The observed enhancement in OVA cross-presentation with polyU in DCs could be mediated by a limited Ag degradation in endophagosomal compartments and a higher permanence of OVA peptide/MHC class I complexes on DCs. These observations clearly reveal that key steps of Ag processing for cross-presentation can be modulated by TLR ligands, opening new avenues for understanding their mechanisms as adjuvants of the immune response.

Human plasmacytoid dendritic cells efficiently cross-present exogenous Ags to CD8+ T cells despite lower Ag uptake than myeloid dendritic cell subsets

AbstractIn human peripheral blood, 4 populations of dendritic cells (DCs) can be distinguished, plasmacytoid dendritic cells (pDCs) and CD16+, CD1c+, and BDCA-3+ myeloid DCs (mDCs), each with distinct functional characteristics. DCs have the unique capacity to cross-present exogenously encountered antigens (Ags) to CD8+ T cells. Here we studied the ability of all 4 blood DC subsets to take up, process, and present tumor Ags to T cells. Although pDCs take up less Ags than CD1c+ and BDCA3+ mDCs, pDCs induce potent Ag-specific CD4+ and CD8+ T-cell responses. We show that pDCs can preserve Ags for prolonged periods of time and on stimulation show strong induction of both MHC class I and II, which explains their efficient activation of both CD4+ and CD8+ T cells. Furthermore, pDCs cross-present soluble and cell-associated tumor Ags to cytotoxic T lymphocytes equally well as BDCA3+ mDCs. These findings, and the fact that pDCs outnumber BDCA3+ mDCs, both in peripheral blood and lymph nodes, together with their potent IFN-I production, known to activate both components of the innate and adaptive immune system, put human pDCs forward as potent activators of CD8+ T cells in antitumor responses. Our findings may therefore have important consequences for the development of antitumor immunotherapy.

Immunodeficiency and Autoimmunity in H2-O–Deficient Mice

HLA-DO/H2-O is a highly conserved, nonpolymorphic MHC class II-like molecule expressed in association with H2-M in thymic epithelial cells, B lymphocytes, and primary dendritic cells. The physiological function of DO remains unknown. The finding of cell maturation-dependent DO expression in B lymphocytes and dendritic cells suggests the possibility that H2-O functions to promote the presentation of exogenous Ag by attenuating presentation of endogenous self-peptides. In the current study, we report that H2-O(-/-) mice spontaneously develop high titers of IgG2a/c antinuclear Abs (ANAs) with specificity for dsDNA, ssDNA, and histones. Reconstitution of RAG1(-)(/)(-) mice with T and B cells from H2-O(-)(/)(-) or wild-type mice demonstrated that production of ANAs requires participation of CD4(+) T cells from H2-O(-)(/)(-) mice. Bone marrow chimeras demonstrated that loss of H2-O expression in thymic epithelial cells did not induce ANAs, and that lack of H2-O expression in bone marrow-derived cells was sufficient to induce the autoimmune phenotype. Despite production of high titers of autoantibodies, H2-O(-/-) mice exhibit a delayed generation of humoral immunity to model Ags (OVA and keyhole limpet hemocyanin), affecting all major T-dependent Ig classes, including IgG2a/c. Ag presentation experiments demonstrated that presentation of exogenous Ag by H2-O(-/-) APC was inefficient as compared with wild-type APC. Thus, H2-O promotes immunity toward exogenous Ags while inhibiting autoimmunity. We suggest that H2-O, through spatially or temporally inhibiting H2-M, may enhance presentation of exogenous Ag by limiting newly generated MHC class II molecules from forming stable complexes with endogenous self-peptides.

An Anti-Nucleic Acid Antibody Delivers Antigen to the Cross-Presentation Pathway in Dendritic Cells and Potentiates Therapeutic Antitumor Effects

Cross-presentation is important for initiating CTL responses against tumors. Delivery of exogenous Ags to the cross-presentation pathway in dendritic cells (DCs), using a number of different carriers, has been attempted to further understand the mechanisms underlying cross-presentation and to develop therapeutic tumor vaccines. The present study reports a new antigenic carrier molecule: a single-chain V region fragment (scFv) of a nucleic acid-hydrolyzing Ab, 3D8. A fusion protein comprising 3D8 scFv and the CTL epitope OVA(250-264) (chicken OVA aa 250-264) was internalized by DC2.4 DCs and processed via a proteasome-dependent, brefeldin- and cycloheximide-sensitive, chloroquine- and primaquine-insensitive pathway, resulting in loading of the CTL epitope onto H-2K(b). In vivo cross-presentation and cross-priming were efficient, even without adjuvant; injection of mice with 3D8 scFv-OVA(250-264) induced cross-presentation of the CTL epitope by draining lymph node CD11c(+) B7.1(+) MHC class II(high) DCs, elicited a CTL response, and suppressed the growth of tumors expressing the OVA epitope. This report shows that an anti-nucleic acid Ab is used to deliver exogenous Ag to the cross-presentation pathway and inhibit in vivo tumor growth.

Cutting Edge: NADPH Oxidase Modulates MHC Class II Antigen Presentation by B Cells

Phagocyte NADPH oxidase plays a key role in pathogen clearance via reactive oxygen species (ROS) production. Defects in oxidase function result in chronic granulomatous disease with hallmark recurrent microbial infections and inflammation. The oxidase's role in the adaptive immune response is not well understood. Class II presentation of cytoplasmic and exogenous Ag to CD4(+) T cells was impaired in human B cells with reduced oxidase p40(phox) subunit expression. Naturally arising mutations, which compromise p40(phox) function in a chronic granulomatous disease patient, also perturbed class II Ag presentation and intracellular ROS production. Reconstitution of patient B cells with a wild-type, but not a mutant, p40(phox) allele restored exogenous Ag presentation and intracellular ROS generation. Remarkably, class II presentation of epitopes from membrane Ag was robust in p40(phox)-deficient B cells. These studies reveal a role for NADPH oxidase and p40(phox) in skewing epitope selection and T cell recognition of self Ag.

Comprehensive Analysis of CD4+ T Cells in the Decision between Tolerance and Immunity In Vivo Reveals a Pivotal Role for ICOS

We have established a comprehensive in vivo mouse model for the CD4(+) T cell response to an "innocuous" versus "dangerous" exogenous Ag and developed an in vivo test for tolerance. In this model, specific gene-expression signatures, distinctive upregulation of early T cell-communication molecules, and differential expansion of effector T cells (Teff) and regulatory T cells (Treg) were identified as central correlates of T cell tolerance and T cell immunity. Different from essentially all other T cell-activation molecules, ICOS was found to be induced in the immunity response and not by T cells activated under tolerogenic conditions. If expressed, ICOS did not act as a general T cell costimulator but selectively caused a massive expansion of effector CD4(+) T cells, leaving the regulatory CD4(+) T cell compartment largely undisturbed. Thus, ICOS strongly contributed to the dramatic change in the balance between Ag-specific Teff and Treg from ∼1:1 at steady state to 21:1 at the height of the immune response. This newly defined role for the balance of Teff to Treg, together with its known key function in T cell help for B cells, establishes ICOS as a central mediator of immunity. Given its exceptionally selective induction on CD4(+) T cells under inflammatory, but not tolerogenic, conditions, ICOS emerges as a pivotal effector molecule in the early decision between tolerance and immunity to exogenous Ag.

Profound Depletion of Host Conventional Dendritic Cells, Plasmacytoid Dendritic Cells, and B Cells Does Not Prevent Graft-versus-Host Disease Induction

The efficacy of allogeneic hematopoietic stem cell transplantation is limited by graft-versus-host disease (GVHD). Host hematopoietic APCs are important initiators of GVHD, making them logical targets for GVHD prevention. Conventional dendritic cells (DCs) are key APCs for T cell responses in other models of T cell immunity, and they are sufficient for GVHD induction. However, we report in this article that in two polyclonal GVHD models in which host hematopoietic APCs are essential, GVHD was not decreased when recipient conventional DCs were inducibly or constitutively deleted. Additional profound depletion of plasmacytoid DCs and B cells, with or without partial depletion of CD11b(+) cells, also did not ameliorate GVHD. These data indicate that, in contrast with pathogen models, there is a surprising redundancy as to which host cells can initiate GVHD. Alternatively, very low numbers of targeted APCs were sufficient. We hypothesize the difference in APC requirements in pathogen and GVHD models relates to the availability of target Ags. In antipathogen responses, specialized APCs are uniquely equipped to acquire and present exogenous Ags, whereas in GVHD, all host cells directly present alloantigens. These studies make it unlikely that reagent-based host APC depletion will prevent GVHD in the clinic.

Conventional Dendritic Cells Require IRAP-Rab14 Endosomes for Efficient Cross-Presentation

Dendritic cells (DCs) use cellular pathways collectively referred to as cross-presentation to stimulate CD8(+) T cells with peptide Ags derived from internalized, exogenous Ags. We have recently reported that DCs rely on aminoterminal trimming of cross-presented peptides by insulin-responsive aminopeptidase (IRAP), an enzyme localized in a regulated endosomal storage compartment. Considering a report contending that this role is limited to inflammatory DCs (Segura et al. 2009. Proc. Natl. Acad. Sci. USA 106: 20377-20381), in this study, we examined the role of IRAP in steady-state DC subpopulations. Steady-state conventional DCs (cDCs) and plasmacytoid DCs expressed similar amounts of IRAP. IRAP colocalized with the endosomal markers Rab14 and syntaxin 6, both known to be associated with regulated endosomal storage compartments, in CD8(+) and CD8(-) cDCs-however, to a greater extent in the former population. Likewise, IRAP recruitment to phagosomes was significantly stronger in CD8(+) DCs. IRAP deficiency compromised cross-presentation of soluble and particulate Ag by both CD8(+) and CD8(-) cDCs, again with a stronger effect in the former population. Thus, the requirement of IRAP in cross-presentation extends to steady-state cDCs. Moreover, these data suggest that increased recruitment of an IRAP(+)/Rab14(+) compartment to Ag-containing vesicles contributes to the superior cross-presentation efficacy of CD8(+) cDCs.

MHC Class I Cross-Presentation by Dendritic Cells Counteracts Viral Immune Evasion

DCs very potently activate CD8+ T cells specific for viral peptides bound to MHC class I molecules. However, many viruses have evolved immune evasion mechanisms, which inactivate infected DCs and might reduce priming of T cells. Then MHC class I cross-presentation of exogenous viral Ag by non-infected DCs may become crucial to assure CD8+ T cell responses. Although many vital functions of infected DCs are inhibited in vitro by many different viruses, the contributions of cross-presentation to T cell immunity when confronted with viral immune inactivation in vivo has not been demonstrated up to now, and remains controversial. Here we show that priming of Herpes Simplex Virus (HSV)-, but not murine cytomegalovirus (mCMV)-specific CD8+ T cells was severely reduced in mice with a DC-specific cross-presentation deficiency. In contrast, while CD8+ T cell responses to mutant HSV, which lacks crucial inhibitory genes, also depended on CD8α+ DCs, they were independent of cross-presentation. Therefore HSV-specific CTL-responses entirely depend on the CD8α+ DC subset, which present via direct or cross-presentation mechanisms depending on the immune evasion equipment of virus. Our data establish the contribution of cross-presentation to counteract viral immune evasion mechanisms in some, but not all viruses.

The Actin Regulatory Protein HS1 Is Required for Antigen Uptake and Presentation by Dendritic Cells

The hematopoietic actin regulatory protein hematopoietic lineage cell-specific protein 1 (HS1) is required for cell spreading and signaling in lymphocytes, but the scope of HS1 function in Ag presentation has not been addressed. We show that dendritic cells (DCs) from HS1(-/-) mice differentiate normally and display normal LPS-induced upregulation of surface markers and cytokines. Consistent with their normal expression of MHC and costimulatory molecules, HS1(-/-) DCs present OVA peptide efficiently to CD4(+) T cells. However, presentation of OVA protein is defective. Similarly, MHC class I-dependent presentation of VSV8 peptide to CD8(+) T cells occurs normally, but cross-presentation of GRP94/VSV8 complexes is defective. Analysis of Ag uptake pathways shows that HS1 is required for receptor-mediated endocytosis, but not for phagocytosis or macropinocytosis. HS1 interacts with dynamin 2, a protein involved in scission of endocytic vesicles. However, HS1(-/-) DCs showed decreased numbers of endocytic invaginations, whereas dynamin-inhibited cells showed accumulation of these endocytic intermediates. Taken together, these studies show that HS1 promotes an early step in the endocytic pathway that is required for efficient Ag presentation of exogenous Ag by DCs.

Physiologic Control of IDO Competence in Splenic Dendritic Cells

Dendritic cells (DCs) competent to express the regulatory enzyme IDO in mice are a small but distinctive subset of DCs. Previously, we reported that a high-dose systemic CpG treatment to ligate TLR9 in vivo induced functional IDO exclusively in splenic CD19(+) DCs, which stimulated resting Foxp3-lineage regulatory T cells (Tregs) to rapidly acquire potent suppressor activity. In this paper, we show that IDO was induced in spleen and peripheral lymph nodes after CpG treatment in a dose-dependent manner. Induced IDO suppressed local T cell responses to exogenous Ags and inhibited proinflammatory cytokine expression in response to TLR9 ligation. IDO induction did not occur in T cell-deficient mice or in mice with defective B7 or programmed death (PD)-1 costimulatory pathways. Consistent with these findings, CTLA4 or PD-1/PD-ligand costimulatory blockade abrogated IDO induction and prevented Treg activation via IDO following high-dose CpG treatment. Consequently, CD4(+)CD25(+) T cells uniformly expressed IL-17 shortly after TLR9 ligation. These data support the hypothesis that constitutive interactions from activated T cells or Tregs and IDO-competent DCs via concomitant CTLA4→B7 and PD-1→PD-ligand signals maintain the default potential to regulate T cell responsiveness via IDO. Acute disruption of these nonredundant interactions abrogated regulation via IDO, providing novel perspectives on the proinflammatory effects of costimulatory blockade therapies. Moreover, interactions between IDO-competent DCs and activated T cells in lymphoid tissues may attenuate proinflammatory responses to adjuvants such as TLR ligands.