Abstract Tumors evade immune surveillance using several mechanisms such as induction of regulatory T-cells (Tregs) and upregulation of indolemine-2,3-dioxygenase (IDO) enzyme. IDO is an intracellular enzyme that mediates its immunosuppressive effects by depleting tryptophan leading to accumulation of kynurenine and in turn reducing the proliferation of effector cells while activating Tregs. IDO is expressed by several suppressive immune cells including Tregs, suppressive antigen presenting cells (APCs), as well as by tumor cells. These immunosuppressive effects of IDO make it an attractive target for cancer immunotherapy, including a target for vaccine therapy. To test the anti-tumor immune effect of IDO vaccine, we evaluated the anti-tumor effect of an IDO-derived MHC I restricted peptide (IDO-pep) vaccine in the low IDO-expressing (TC-1) and high IDO-expressing B16 (melanoma) mouse tumor models. For this, mice were treated with three doses of IDO-peptides in combination with the respective tumor-specific vaccines (E7 for TC-1 and gp100 for B16) at an interval of seven days, followed by assessment of tumor growth and mice survival. We observed a significant delay in the tumor growth and prolonged survival in both tumor models when IDO vaccine was combined with tumor-specific antigen vaccine. We found that tumor-infiltration of antigen-specific CD8+ T-cells was significantly enhanced following the addition of IDO vaccine. Furthermore, while CD4+ T-cells increased in the tumor microenvironment (TME), there was a significant reduction in the numbers of Treg cells. We also found a significant reduction in the numbers of IDO-expressing myeloid-derived suppressor cells (MDSCs), dendritic cells as well as macrophages out of the total CD45+CD11b+ myeloid cells in the TME after the administration of IDO-pep in combination with tumor antigen-specific vaccine. In conclusion, our findings suggest that IDO-pep vaccine enhances the specific anti-tumor immune response by increasing the tumor antigen-specific effector T-cell infiltration in the TME through inhibition of immune suppressive cells. These results also suggest that MHC I restricted IDO peptide treatment is a promising strategy to improve cancer immunotherapy. Citation Format: Rahul Nandre, Vivek Verma, Pankaj Gaur, Pooja Vir, Hua Wang, Baolin Kang, Ayako Wakatsuki Pedersen, Mai-Britt Zocca, Mads Hald Andersen, Seema Gupta, Samir Khleif. IDO vaccine enhances antigen-specific anti-tumor effects by reducing IDO-expressing antigen presenting cells and MDSCs [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3763.