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Background The interplay between the immune system and tumors can drive the evolution of immune evasion, a process known as cancer immunoediting. While immunotherapies are designed to enhance anti-tumor immune responses, they can also accelerate immunoediting and contribute to immune escape. Previously, we reported that immunoediting induces tumor antigen shifting, which serves as an immune escape mechanism in brain tumors that evade adoptive cellular therapy (ACT). 1 Here, we identify an additional immune escape mechanism involving immunoediting-driven dendritic cell (DC) dysfunction in ACT-escaped brain tumors.Methods KR158B glioblastoma cells were implanted, and ACT treatment were given including 9 Gy whole body irradiation and one dose of hematopoietic stem cell and tumor reactive T cell transfer and 3 does of DC vaccines. Primary (no treatment control) and ACT escaped tumors were harvested at endpoint for T cell immune phenotyping, and DC sorting. DCs from primary and ACT escaped tumors were assessed for T cell activation function and subjected for RNA sequencing. Gene set enrichment analysis (GSEA) was performed on transcriptomic data to identify enriched pathways. The impact of hypoxia and factors secreted from tumor-T cell interactions on DC tolerance was evaluated using RT-qPCR. In vivo correlations between hypoxia and DC tolerance were analyzed using GeoMx spatial transcriptomics.Results ACT-escaped tumors retained adoptively transferred cytotoxic, non-exhausted T cells that failed to recognize the antigen-shifted tumor cells. DCs from both untreated primary and ACT-escaped tumors showed impaired T cell activation and reduced expression of antigen-presentation genes. With note, DCs from ACT-escaped tumors exhibited higher expression of tolerance-associated genes compared to those from primary tumors. GSEA showed that hypoxia pathway enriched in ACT escaped tumor DCs. In vitro hypoxia treatment could induce the expression of DC tolerance genes in a HIF1α-dependent manner and impair T cell activation. Spatial transcriptomics further confirmed a strong correlation between HIF1α and ARG1 in tumor-bearing brains. Additionally, a strong correlation of HIF1α and CD45 in ACT treated brain tumor suggests that increased immune infiltration in ACT-treated gliomas exacerbated hypoxia, further contributing to DC dysfunction. Conditioned medium from tumor-T cell co-cultures also activated the hypoxia pathway and upregulated DC tolerance genes, indicating that factors secreted during tumor-T cell interactions can further promote DC tolerance via hypoxia pathway activation.Conclusions ACT enhances hypoxia pathway activation to induce DC tolerance, facilitating tumor immune evasion.Reference Wildes TJ, Dyson KA, Francis C, et al. Immune escape after adoptive t-cell therapy for malignant gliomas. Clin Cancer Res. 2020;26(21):5689-5700.Ethics Approval The investigators adhered to the ‘Guide for the Care and Use of Laboratory Animals’ as proposed by the committee on care of Laboratory Animal Resources Commission on Life Sciences, National Research Council. The facilities at the University of Florida Animal Care Services are fully accredited by the American Association for Accreditation of Laboratory Animal Care, and all studies were approved by the University of Florida Institutional Animal Care and Use Committee.