The tryptophan catabolic enzyme indoleamine 2,3-dioxygenase-1 (IDO1) has attracted enormous attention in driving cancer immunosuppression, neovascularization and metastasis. IDO1 suppresses local CD8+ T effector cells and natural killer cells and induces CD4+ T regulatory cells (iTreg) and myeloid-derived suppressor cells (MDSC). The structurally distinct enzyme tryptophan dioxygenase (TDO) also has been implicated recently in immune escape and metastatic progression. Lastly, emerging evidence suggests that the IDO1-related enzyme IDO2 may support IDO1-mediated iTreg and contribute to B-cell inflammed states in certain cancers. IDO1 and TDO are upregulated widely in neoplastic cells but also variably in stromal, endothelial and innate immune cells of the tumor microenviroment and in tumor-draining lymph nodes. Pharmacological and genetic proofs in preclinical models of cancer have validated IDO1 as a cancer therapeutic target. IDO1 inhibitors have limited activity on their own but greatly enhance ‘immunogenic’ chemotherapy or immune checkpoint drugs. IDO/TDO function is rooted in inflammatory programming, thereby influencing tumor neovascularization, MDSC generation and metastasis beyond effects on adaptive immune tolerance. Discovery and development of small molecule enzyme inhibitors of IDO1 derived from the hydroxylamidine and phenylimidazole chemotypes have advanced furthest to date in Phase II/III human trials (epacadastat and GDC-0919/navoximod, respectively). Second generation combined IDO/TDO inhibitors may broaden impact in cancer treatment, for example, in addressing IDO1 bypass (inherent resistance) or acquired resistance to IDO1 inhibitors. This review surveys knowledge about IDO1 function and how IDO1 inhibitors reprogram inflammation to heighten therapeutic responses in cancer.

The transcriptional repressor Bcl6 controls development of the follicular helper T cell (TFH) lineage, however the precise mechanisms by which Bcl6 regulates this process are unclear. A model has been proposed whereby Bcl6 represses the differentiation of T cells into alternative effector lineages, thus favoring TFH differentiation. Analysis of T cell differentiation using Bcl6-deficient mice has been complicated by the strong pro-inflammatory phenotype of Bcl6-deficient myeloid cells. Here, we report data from a novel mouse model where Bcl6 is conditionally deleted in T cells (Bcl6fl/flCreCD4 mice). After immunization, PD-1high TFH cells in Bcl6fl/flCreCD4 mice are decreased over 90% compared to control mice, and antigen-specific IgG is sharply reduced. Residual PD-1high CXCR5+ TFH cells in Bcl6fl/flCreCD4 mice show a significantly higher rate of apoptosis than PD-1high CXCR5+ TFH cells in control mice. Immunization of Bcl6fl/flCreCD4 mice did not reveal enhanced differentiation into TH1, TH2 or TH17 lineages, although IL-10 expression by CD4 T cells was markedly elevated. Thus, T cell extrinsic factors appear to promote the increased TH1, TH2 and TH17 responses in germ-line Bcl6-deficient mice. Furthermore, IL-10 may be a key target gene for Bcl6 in CD4 T cells, which enables Bcl6 to promote the TFH cell phenotype. Finally, our data reveal a novel mechanism for the role of Bcl6 in promoting TFH cell survival.

We discuss how small molecule inhibitors of the tryptophan catabolic enzyme indoleamine 2,3-dioxygenase (IDOi) represent a vanguard of new immunometabolic adjuvants to safely enhance the efficacy of cancer immunotherapy, radiotherapy or 'immunogenic' chemotherapy by leveraging responses to tumor neoantigens. IDO activation in cancer supports inflammatory processes that IDOi can re-program to help clear tumors by blunting tumor neovascularization and restoring immunosurveillance. Studies of regulatory and effector pathways illuminate IDO as an inflammatory modifier. Recent work suggests that coordinate targeting of the Trp catabolic enzymes TDO and IDO2 may also safely broaden efficacy. Understanding IDOi as adjuvants to turn immunologically 'cold' tumors 'hot' can seed new concepts in how to improve the efficacy of cancer therapy while limiting its collateral damage.

The immune tolerogenic effects of IDO1 (indoleamine 2,3-dioxygenase 1) have been well documented and genetic studies in mice have clearly established the significance of IDO1 in tumor promotion. Dichotomously, the primary inducer of IDO1, the inflammatory cytokine IFNγ (interferon-γ), is a key mediator of immune-based tumor suppression. One means by which IFNγ can exert an anti-cancer effect is by decreasing tumor neovascularization. We speculated that IDO1 might contribute to cancer promotion by countering this anti-neovascular effect of IFNγ, possibly through IDO1-potentiated elevation of the pro-tumorigenic inflammatory cytokine IL6 (interleukin-6). In this study, we investigated how genetic loss of IDO1 affects neovascularization in mouse models of oxygen-induced retinopathy and lung metastasis. Neovascularization in both models was significantly reduced in mice lacking IDO1, was similarly reduced with loss of IL6, and was restored in both cases by concomitant loss of IFNγ. Likewise, the lack of IDO1 or IL6 resulted in reduced metastatic tumor burden and increased survival, which the concomitant loss of IFNγ abrogated. This insight into IDO1's involvement in pro-tumorigenic inflammatory neovascularization may have important ramifications for IDO1 inhibitor development, not only in cancer where clinical trials are currently ongoing, but in other disease indications associated with neovascularization as well.