Abstract Purpose of the study: To gain insight to the proportion of solid tumors that express IDO1, CD73 and PD-L1 in lung cancer, colon adenocarcinoma, renal cell carcinoma (RCC) and melanoma, and the correlation between expression of these immune checkpoints and immune cell infiltration within the tumor. Experimental procedures: Using a battery of properly validated antibodies, we have performed an in situ analysis of IDO1, CD73 and PD-L1 expression by immunohistochemistry in a panel of colon cancer (n=68), lung cancer (n=36), RCC (n=67) and melanoma (n=48) tissue microarray samples. Tumor infiltrating T cells, CD3+ and CD8+, were also detected by IHC and counted in serial sections for each sample. We search for a possible a correlation between the expression of these immunosuppressor proteins and the number of infiltrating CD3+ and CD8+ T cells as well as their expression in the different tumor components: tumor cells, stroma and endothelium. Results: Our IHC analysis allowed us to classify the analyzed samples into two main groups: those with expression of IDO1, CD73 and PD-L1, which were also the ones with a high presence of immune cells within the tumors, “hot tumors”, and those that showed to be negative for all three proteins, and that correlated with lower infiltration of T cells, “cold tumors”. We also analyzed the location of the proteins within the tumors, and found an effect between endothelial expression of Ido1 and reduced infiltration in lung and RCC tumors. Conclusions: The response to combined immune checkpoint and IDO1 therapy could depend on the proper stratification of patients; therefore, it is of significant interest to determine the proportion of solid tumors that express IDO1, CD73 and PD-L1, and the correlation of the expression of these three immune check points to immune cell infiltration within the tumor. Thus, proper patient stratification based on the expression of these immune checkpoints may improve the efficacy of IDO1 inhibitor based immune therapies. Citation Format: Ana Cerezo, Gloria Martinez-del Hoyo, Sandra Peregrina, Tamara Mondejar, Eduardo Caleiras, Patricia Gonzalez, Eva Lospitao, Sonia Hernandez-Tiedra, Laura Diezma, Estela Casas, Frank Dorsey, Karim Benhadji, Raymond Gilmour, Sandaruwan Geeganage, Susana Velasco. Investigating the expression of indolamine deoxygenase I (IDO1) and other immune markers in tissue microarrays [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 513.