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David H Munn

Publications and source records attributed to David H Munn.

At least 19 recordsLinked to original sources
PubMed2006

A high-affinity, tryptophan-selective amino acid transport system in human macrophages.

Tryptophan catabolism via the enzyme indoleamine 2,3-dioxygenase (IDO) allows human monocyte-derived macrophages (MDM) and other APC to suppress T cell proliferation. IDO helps protect murine fetuses from rejection by the maternal immune system and can promote tolerance and immunosuppression. For tryptophan to be catabolized by IDO, it must first enter the APC via transmembrane transport. It has been shown that MDM in vitro readily deplete tryptophan present in the extracellular medium to nanomolar levels via IDO activity; yet, no currently known amino acid transport system displays high affinity and specificity sufficiently to permit efficient uptake of tryptophan at these low concentrations. Here, we provide biochemical characterization of a novel transport system with nanomolar affinity and high selectivity for tryptophan. Tryptophan transport in MDM was predominantly sodium-independent and occurred via two distinct systems: one consistent with the known system L transporter and a second system with 100-fold higher affinity for tryptophan (Km<300 nM). Competition studies showed that the high-affinity system did not correspond to any known transporter activity and displayed a marked selectivity for tryptophan over other amino acids and tryptophan analogs. This new system was expressed at low levels in fresh monocytes but underwent selective induction during MDM differentiation. In contrast, resting human T cells expressed only the conventional system L. We speculate that the high-affinity, tryptophan-specific transport system allows MDM to take up tryptophan efficiently under conditions of low substrate concentration, such as may occur during interaction between T cells and IDO-expressing APC.

Amino Acid Transport System L↗
PubMed2006

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.

Animals↗
PubMed2006

The tumor-draining lymph node as an immune-privileged site.

Lymph nodes that lie immediately downstream of tumors [tumor-draining lymph nodes (TDLNs)] undergo profound alterations due to the presence of the upstream tumor. The antigen-presenting cell population in TDLNs becomes modified such that tumor-derived antigens are cross-presented by host cells in a tolerizing fashion. In addition, the number and suppressor activity of regulatory T cells (Tregs) are increased in the TDLN. Emerging evidence suggests that some of these Tregs may be generated de novo against specific tumor-derived antigens, and thus they arise as a direct consequence of antigen presentation in the TDLN. Others may represent Tregs against self-antigens, which undergo preferential activation in the tolerogenic milieu of the TDLN. The TDLN thus becomes an anatomic context in which presentation of new antigens not only fails to elicit a protective immune response but also actively creates systemic tolerance. In this regard, the TDLN displays features analogous to classical immune privilege. Accumulating evidence thus suggests that the TDLNs, although small in size, may exert a profound tolerizing influence on the rest of the immune system. These mechanisms will need to be interrupted in order for clinical anti-tumor immunotherapy to be successful.

Animals↗
PubMed2005

Cutting edge: CpG oligonucleotides induce splenic CD19+ dendritic cells to acquire potent indoleamine 2,3-dioxygenase-dependent T cell regulatory functions via IFN Type 1 signaling.

CpG oligodeoxynucleotides (CpG-ODNs) stimulate innate and adaptive immunity by binding to TLR9 molecules. Paradoxically, expression of the immunoregulatory enzyme indoleamine 2,3-dioxygenase (IDO) is induced following i.v. CpG-ODN administration to mice. CpG-ODNs induced selective IDO expression by a minor population of splenic CD19+ dendritic cells (DCs) that did not express the plasmacytoid DC marker 120G8. Following CpG-ODN treatment, CD19+ DCs acquired potent IDO-dependent T cell suppressive functions. Signaling through IFN type I receptors was essential for IDO up-regulation, and CpG-ODNs induced selective activation of STAT-1 in CD19+ DCs. Thus, CpG-ODNs delivered systemically at relatively high doses elicited potent T cell regulatory responses by acting on a discrete, minor population of splenic DCs. The ability of CpG-ODNs to induce both stimulatory and regulatory responses offers novel opportunities for using them as immunomodulatory reagents but may complicate therapeutic use of CpG-ODNs to stimulate antitumor immunity in cancer patients.

Adjuvants, Immunologic↗
PubMed2005

A minor population of splenic dendritic cells expressing CD19 mediates IDO-dependent T cell suppression via type I IFN signaling following B7 ligation.

By ligating CD80/CD86 (B7) molecules, the synthetic immunomodulatory reagent CTLA4-Ig (soluble synthetic CTLA4 fusion protein) induces expression of the enzyme indoleamine 2,3-dioxygenase (IDO) in some dendritic cells (DCs), which acquire potent T cell regulatory functions as a consequence. Here we show that this response occurred exclusively in a population of splenic DCs co-expressing the marker CD19. B7 ligation induced activation of the transcription factor signal transducer and activator of transcription (STAT1) in sorted CD19+, but not CD19(NEG), DCs. STAT1 activation occurred even when DCs lacked receptors for type II IFN (IFNgamma); however, STAT1 activation and IDO up-regulation were not observed when DCs lacked receptors for type I IFN (IFNalphabeta). Thus, IFNalpha, but not IFNgamma, signaling was essential for STAT1 activation and IDO up-regulation in CD19+ DCs following B7 ligation. Consistent with these findings, B7 ligation also induced sorted CD19+, but not CD19(NEG), DCs to express IFNalpha. Moreover, recombinant IFNalpha induced CD19+, but not CD19(NEG), DCs to mediate IDO-dependent T cell suppression, showing that IFNalpha signaling could substitute for upstream signals from B7. These data reveal that a minor population of splenic DCs expressing the CD19 marker is uniquely responsive to B7 ligation, and that IFNalpha-mediated STAT1 activation is an essential intermediary signaling pathway that promotes IDO induction in these DCs. Thus, CD19+ DCs may be a target for regulatory T cells expressing surface CTLA4, and may suppress T cell responses via induction of IDO.

Animals↗
PubMed2005

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.

Animals↗
PubMed2005

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.

Adoptive Transfer↗
PubMed2004

Specific subsets of murine dendritic cells acquire potent T cell regulatory functions following CTLA4-mediated induction of indoleamine 2,3 dioxygenase.

Murine dendritic cells (DCs) expressing indoleamine 2,3 dioxygenase (IDO) catabolize tryptophan and can suppress T cell responses elicited in vivo. Here, we identify specific subsets of splenic (CD11c+) dendritic cells competent to mediate IDO-dependent T cell suppression following CTLA4-mediated ligation of B7 molecules. IDO-competent DC subsets acquired potent and dominant T cell suppressive properties as a consequence of IDO up-regulation, as they blocked the ability of T cells to respond to other stimulatory DCs in the same cultures. Soluble CTLA4 (CTLA4-Ig) and cloned CTLA4+ regulatory T cells (Tr1D1) up-regulated IDO selectively in DC subsets co-expressing B220 or CD8alpha. The ability of Tr1D1 T cells to suppress CD8+ T cell responses was completely dependent on their ability to induce tryptophan catabolism in DCs. Selective IDO up-regulation in DCs did not inhibit T cell activation, but prevented T cell clonal expansion due to rapid death of activated T cells. T cell responses were restored by genetic or pharmacologic inhibition of IDO enzyme activity, or by adding excess tryptophan. DCs from interferon gamma (IFNgamma)-receptor-deficient mice were effective in promoting IDO-dependent T cell suppression following CTLA4-Ig exposure in vivo, indicating that IFNgamma signaling was not necessary for IDO up-regulation in this model. These findings suggest that IDO-competent DCs provide a regulatory bridge, mediated by CTLA4-B7 engagement, between certain regulatory T cell subsets and naive responder T cells.

Abatacept↗
PubMed2004

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

Human monocyte-derived dendritic cells (DCs) are capable of expressing the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase (IDO), which allows them to suppress Ag-driven proliferation of T cells in vitro. In DCs that express IDO, the activity of the enzyme is tightly regulated, with the protein being constitutively expressed, but functional activity requiring an additional set of triggering signals supplied during Ag presentation. We now show that triggering of functional IDO obligately requires ligation of B7-1/B7-2 molecules on the DCs by CTLA4/CD28 expressed on T cells. When this interaction was disrupted, IDO remained in the inactive state, and the DCs were unable to inhibit T cell proliferation. Inhibition could be fully restored by direct Ab-mediated cross-linking of B7-1/B7-2. Although both CD4(+) and CD8(+) T cells were susceptible to inhibition once IDO was induced, the ability to trigger functionally active IDO was strictly confined to the CD4(+) subset. Thus, the ability of CD4(+) T cells to induce IDO activity in DCs allowed the CD4(+) population to dominantly inhibit proliferation of the CD8(+) population via the bridge of a conditioned DC. We hypothesize that IDO activation via engagement of B7-1/B7-2 molecules on DCs, specifically, engagement by CTLA4 expressed on regulatory CD4(+) T cells, may function as a physiologic regulator of T cell responses in vivo.

Abatacept↗
PubMed2004

Indoleamine 2,3-dioxygenase expression is restricted to fetal trophoblast giant cells during murine gestation and is maternal genome specific.

Pharmacologic inhibition of indoleamine 2,3 dioxygenase (IDO) activity during murine pregnancy results in rejection of allogeneic fetuses by the maternal immune system. Here, we show that IDO expression is restricted to perinuclear regions of primary trophoblast giant cells (TGCs) of fetal origin at mid-gestation (E10.5). After placentation (E14), no IDO expression was detected at the maternal-fetal interface. Matings involving IDO-deficient females revealed that paternally inherited IDO alleles were inactive in primary TGCs, presumably due to paternal genome-specific gene inactivation. Allogeneic matings in which both parents were genetically IDO-deficient produced litters of normal sizes at normal rates compared to IDO-sufficient parental mice, implying that compensatory or redundant immunosuppressive mechanisms protected allogeneic fetuses during gestation in IDO-deficient mice. Consistent with this notion, treatment with IDO inhibitor did not affect allogeneic pregnancy rates when both parents were IDO-deficient, confirming that IDO was the relevant pharmacologic target of the IDO inhibitor in matings involving IDO-sufficient mice. Hence, IDO is a key immunosuppressive mechanism in normal murine pregnancies, and it is regulated entirely through maternally inherited fetal genes.

Animals↗
PubMed2004

Regulation of macrophage foam cell formation by alphaVbeta3 integrin: potential role in human atherosclerosis.

The accumulation of macrophage foam cells in atherosclerotic lesions is associated with both initiation and progression of this disease. Scavenger receptors CD36 and SRA are the primary receptors responsible for conversion of macrophages into foam cells. Integrin alphaVbeta3 plays a role in the differentiation of several cell types, but its involvement in the transition of macrophages into foam cells and the potential role of this receptor in atherosclerosis have not been examined. Using an in vitro model of single surface receptor activation by binding with an immobilized monoclonal antibody specific to alphaVbeta3 integrin we show that ligation of alphaVbeta3 integrin prevents differentiation of blood monocytes and macrophages into the foam cell phenotype via coordinate down-regulation of CD36 and SRA. This effect of alphaVbeta3 integrin ligation can be reproduced by contact with endothelial cells, whereas the inhibition of alphaVbeta3 receptor ligation restores the uptake of oxidized low-density lipoprotein. Moreover, we found that alphaVbeta3 integrin is readily detected in situ on macrophages in early and advanced atherosclerotic lesions and that in vitro exposure to oxidized low-density lipoprotein up-regulates alphaVbeta3 integrin expression. We hypothesize that alphaVbeta3 integrin regulates macrophage functional maturation into foam cells in a persistent manner, and therefore, by targeting alphaVbeta3 receptor it could potentially be possible to regulate progression of atherosclerosis in humans.

Adult↗
PubMed2004

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.

Animals↗
PubMed2004

Expression of indoleamine 2,3-dioxygenase by plasmacytoid dendritic cells in tumor-draining lymph nodes.

One mechanism contributing to immunologic unresponsiveness toward tumors may be presentation of tumor antigens by tolerogenic host APCs. We show that mouse tumor-draining LNs (TDLNs) contained a subset of plasmacytoid DCs (pDCs) that constitutively expressed immunosuppressive levels of the enzyme indoleamine 2,3-dioxygenase (IDO). Despite comprising only 0.5% of LN cells, these pDCs in vitro potently suppressed T cell responses to antigens presented by the pDCs themselves and also, in a dominant fashion, suppressed T cell responses to third-party antigens presented by nonsuppressive APCs. Adoptive transfer of DCs from TDLNs into naive hosts created profound local T cell anergy, specifically toward antigens expressed by the transferred DCs. Anergy was prevented by targeted disruption of the IDO gene in the DCs or by administration of the IDO inhibitor drug 1-methyl-D-tryptophan to recipient mice. Within the population of pDCs, the majority of the functional IDO-mediated suppressor activity segregated with a novel subset of pDCs coexpressing the B-lineage marker CD19. We hypothesize that IDO-mediated suppression by pDCs in TDLNs creates a local microenvironment that is potently suppressive of host antitumor T cell responses.

Adoptive Transfer↗
PubMed2003

Cutting edge: induced indoleamine 2,3 dioxygenase expression in dendritic cell subsets suppresses T cell clonal expansion.

In mice, immunoregulatory APCs express the dendritic cell (DC) marker CD11c, and one or more distinctive markers (CD8alpha, B220, DX5). In this study, we show that expression of the tryptophan-degrading enzyme indoleamine 2,3 dioxygenase (IDO) is selectively induced in specific splenic DC subsets when mice were exposed to the synthetic immunomodulatory reagent CTLA4-Ig. CTLA4-Ig did not induce IDO expression in macrophages or lymphoid cells. Induction of IDO completely blocked clonal expansion of T cells from TCR transgenic mice following adoptive transfer, whereas CTLA4-Ig treatment did not block T cell clonal expansion in IDO-deficient recipients. Thus, IDO expression is an inducible feature of specific subsets of DCs, and provides a potential mechanistic explanation for their T cell regulatory properties.

Abatacept↗
PubMed2003

Pattern of recruitment of immunoregulatory antigen-presenting cells in malignant melanoma.

The mechanism by which the immune system of a tumor-bearing host acquires tolerance toward tumor antigens is still elusive. Antigen-presenting cells (APCs) are critical regulators of the decision between immune response and tolerance. APCs that express the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase (IDO) have been found to inhibit T-cell responses both in vitro and in vivo. We hypothesized that malignant tumors exploit this mechanism by recruiting IDO-expressing APCs to the tumor-draining lymph nodes. To test this hypothesis, archival tissues and records of 26 cases of lymph node dissection for invasive cutaneous melanoma were obtained. IDO immunohistochemistry was performed on 14 cutaneous tumors and 328 regional lymph nodes. Abnormal accumulations of IDO-positive cells with a monocytoid or plasmacytoid morphology were identified in the perisinusoidal regions of draining lymph nodes in 45% of nodes studied. Recruitment of IDO-positive cells was seen in nodes with and without malignancy. We hypothesize that these IDO-positive APCs may contribute mechanistically to acquired tolerance to tumor antigens. Immunostaining of tumor-draining lymph nodes for abnormal accumulation of IDO-expressing cells might thus constitute an adverse prognostic factor and could contribute to the decision process and the appropriate care of patients with this deadly disease.

Adult↗