David H. Munn

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.

Indoleamine 2,3-dioxygenase and tumor-induced tolerance

Tumors arise from normal cells of the body through genetic mutation. Although such genetic mutation often leads to the expression of abnormal antigens, the immune system fails to respond effectively to these antigens; that is, it is tolerant of these antigens. This acquired state of tolerance must be overcome for cancer immunotherapy to succeed. Indoleamine 2,3-dioxygenase (IDO) is one molecular mechanism that contributes to tumor-induced tolerance. IDO helps create a tolerogenic milieu in the tumor and the tumor-draining lymph nodes, both by direct suppression of T cells and enhancement of local Treg-mediated immunosuppression. It can also function as an antagonist to other activators of antitumor immunity. Therefore, strategies to block IDO might enhance the effectiveness of tumor immunotherapy.

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.

Plasmacytoid dendritic cells from mouse tumor-draining lymph nodes directly activate mature Tregs via indoleamine 2,3-dioxygenase

A small population of plasmacytoid DCs (pDCs) in mouse tumor-draining LNs can express the immunoregulatory enzyme indoleamine 2,3-dioxygenase (IDO). We show that these IDO+ pDCs directly activate resting CD4+CD25+Foxp3+ Tregs for potent suppressor activity. In vivo, Tregs isolated from tumor-draining LNs were constitutively activated and suppressed antigen-specific T cells immediately ex vivo. In vitro, IDO+ pDCs from tumor-draining LNs rapidly activated resting Tregs from non-tumor-bearing hosts without the need for mitogen or exogenous anti-CD3 crosslinking. Treg activation by IDO+ pDCs was MHC restricted, required an intact amino acid-responsive GCN2 pathway in the Tregs, and was prevented by CTLA4 blockade. Tregs activated by IDO markedly upregulated programmed cell death 1 ligand 1 (PD-L1) and PD-L2 expression on target DCs, and the ability of Tregs to suppress target T cell proliferation was abrogated by antibodies against the programmed cell death 1/PD-L (PD-1/PD-L) pathway. In contrast, Tregs activated by anti-CD3 crosslinking did not cause upregulation of PD-Ls, and suppression by these cells was unaffected by blocking the PD-1/PD-L pathway. Tregs isolated from tumor-draining LNs in vivo showed potent PD-1/PD-L-mediated suppression, which was selectively lost when tumors were grown in IDO-deficient hosts. We hypothesize that IDO+ pDCs create a profoundly suppressive microenvironment within tumor-draining LNs via constitutive activation of Tregs.

Tryptophan catabolism and T-cell tolerance: immunosuppression by starvation?

Some macrophages inhibit microbial infections by producing indoleamine 2,3 dioxygenase (IDO), which catabolizes tryptophan. Here, Andrew Mellor and David Munn discuss evidence that cells that synthesize IDO protect the mammalian fetus from maternal T-cell attack and argue that this mechanism might have wider implications for the control of T-cell responses.

Activation of Gpr109a, receptor for niacin and the commensal metabolite butyrate, suppresses colonic inflammation and carcinogenesis

Commensal gut microflora and dietary fiber protect against colonic inflammation and colon cancer through unknown targets. Butyrate, a bacterial product from fermentation of dietary fiber in the colon, has been implicated in this process. GPR109A (encoded by Niacr1) is a receptor for butyrate in the colon. GPR109A is also a receptor for niacin, which is also produced by gut microbiota and suppresses intestinal inflammation. Here we showed that Gpr109a signaling promoted anti-inflammatory properties in colonic macrophages and dendritic cells and enabled them to induce differentiation of Treg cells and IL-10-producing T cells. Moreover, Gpr109a was essential for butyrate-mediated induction of IL-18 in colonic epithelium. Consequently, Niacr1(-/-) mice were susceptible to development of colonic inflammation and colon cancer. Niacin, a pharmacological Gpr109a agonist, suppressed colitis and colon cancer in a Gpr109a-dependent manner. Thus, Gpr10a has an essential role in mediating the beneficial effects of gut microbiota and dietary fiber in colon.

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 (CD8α, 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.

Inhibition of Indoleamine 2,3-Dioxygenase in Dendritic Cells by Stereoisomers of 1-Methyl-Tryptophan Correlates with Antitumor Responses

Indoleamine 2,3-dioxygenase (IDO) is an immunosuppressive enzyme that contributes to tolerance in a number of biological settings. In cancer, IDO activity may help promote acquired tolerance to tumor antigens. The IDO inhibitor 1-methyl-tryptophan is being developed for clinical trials. However, 1-methyl-tryptophan exists in two stereoisomers with potentially different biological properties, and it has been unclear which isomer might be preferable for initial development. In this study, we provide evidence that the D and L stereoisomers exhibit important cell type-specific variations in activity. The L isomer was the more potent inhibitor of IDO activity using the purified enzyme and in HeLa cell-based assays. However, the D isomer was significantly more effective in reversing the suppression of T cells created by IDO-expressing dendritic cells, using both human monocyte-derived dendritic cells and murine dendritic cells isolated directly from tumor-draining lymph nodes. In vivo, the d isomer was more efficacious as an anticancer agent in chemo-immunotherapy regimens using cyclophosphamide, paclitaxel, or gemcitabine, when tested in mouse models of transplantable melanoma and transplantable and autochthonous breast cancer. The D isomer of 1-methyl-tryptophan specifically targeted the IDO gene because the antitumor effect of D-1-methyl-tryptophan was completely lost in mice with a disruption of the IDO gene (IDO-knockout mice). Taken together, our findings support the suitability of D-1-methyl-tryptophan for human trials aiming to assess the utility of IDO inhibition to block host-mediated immunosuppression and enhance antitumor immunity in the setting of combined chemo-immunotherapy regimens.

ArticleActivation of Gpr109a, Receptor for Niacin and the Commensal Metabolite Butyrate, Suppresses Colonic Inflammation and Carcinogenesis

Commensal gut microflora and dietary fiber protect against colonic inflammation and colon cancer through unknown targets. Butyrate, a bacterial product from fermentation of dietary fiber in the colon, has been implicated in this process. GPR109A (encoded by Niacr1) is a receptor for butyrate in the colon. GPR109A is also a receptor for niacin, which is also produced by gut microbiota and suppresses intestinal inflammation. Here we showed that Gpr109a signaling promoted anti-inflammatory properties in colonic macrophages and dendritic cells and enabled them to induce differentiation of Treg cells and IL-10-producing T cells. Moreover, Gpr109a was essential for butyrate-mediated induction of IL-18 in colonic epithelium. Consequently, Niacr1(-/-) mice were susceptible to development of colonic inflammation and colon cancer. Niacin, a pharmacological Gpr109a agonist, suppressed colitis and colon cancer in a Gpr109a-dependent manner. Thus, Gpr10a has an essential role in mediating the beneficial effects of gut microbiota and dietary fiber in colon.

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.