Abstract PR05: Blockade of AHR activation by IDO/TDO-derived kynurenine restricts cancer immune suppression

Immune checkpoint blockade (ICB) has been shown to convey significant clinical activity across a spectrum of malignancies, yet there is now recognition that multiple mechanisms of resistance can impair response. The catabolism of tryptophan into metabolites known as kynurenines (Kyn) by enzymes such as indoleamine 2,3-dioxygenase (IDO) or tryptophan 2,3-dioxygenase (TDO) plays a major suppressive role. Recently it was shown that Kyn acts as an endogenous agonist of the Aryl hydrocarbon receptor (AhR). In order to gain a better understanding of this pathway, we sought to characterize the mechanisms of immunosuppression associated with AhR and evaluate its potential as therapeutic target. Gene-expression analysis of IDO-overexpressing melanomas (B16-IDO vs. B16-WT) demonstrated reduced expression levels of Type 1 inflammatory genes, including IFNy, TNF, GzmB, and CD40. In addition, B16-IDO presents higher infiltration of tumor-associated macrophages TAMs, which upregulate the AHR as well as classic AhR-regulated genes (Cyp1a1 and Cyp1b1) and are differentially skewed towards an immunosuppressive M2 phenotype. Tumor-antigen specific CD8+T cells show reduced expression of activation markers (GzmB and CD44) and proliferation rate when primed by Kyn-treated antigen-presenting cells. In addition, TAMs from B16-IDO tumors suppressed activation of CD8+T cells in vitro and their depletion delayed tumor growth. When B16-IDO cells are implanted in mice depleted of Foxp3 expressing cells, TAMs do not accumulate. Treatment of B16-IDO tumors with an AhR-specific antagonist (CH-223191) upregulates MHC II in APCs, activation markers in CD8s, and reduced frequency of T-regs in B16-IDO tumors. AhR inhibition slows tumor growth and prolongs survival of tumors with active IDO/TDO/Kyn pathway (B16-IDO and B16-TDO), and this is enhanced when PD-1 blockade is used in combination. In summary, our findings demonstrate that targeting the Kyn pathway through AhR inhibition could overcome key suppressive mechanisms and sensitize tumors to ICB. This abstract is also being presented as Poster A57. Citation Format: Luis F. Campesato, Sadna Budhu, Mathieu Gigoux, Jeremy Tchaicha, Stephane Pourpe, Cailian Liu, Dmitriy Zamarin, Mark G. Manfredi, Karen McGovern, Jedd D. Wolchok, Taha Merghoub. Blockade of AHR activation by IDO/TDO-derived kynurenine restricts cancer immune suppression [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr PR05.