Background
Low tumor immunogenicity is associated with cancer immune escape, immunotherapy resistance, and poor patient survival.1–3Major histocompatibility complex (MHC) antigen presentation pathways (APPs) play a crucial role in promoting tumor T-cell recognition and initiating antitumoral immune responses. Epigenetic silencing of MHC APPs is a common mechanism leading to tumor progression and immune evasion in gastrointestinal malignancies.3 4 The polycomb repressive complex 2 (PRC2) has been identified as one of the main epigenetic effectors suppressing MHC expression through the production of histone H3K27me3, a gene repression mark in breast, lung and other malignancies.4–6 In the current investigation, we elucidated the role of the PRC2 complex and MHC regulation in the context of colon neoplastic and preneoplastic tumors, with a view to potential therapeutic targeting.
Methods
Mouse and human colon cancer cell lines, along with normal colonic epithelial cells, were treated with PRC2 inhibitors, and MHC class I/II expression was determined at transcriptional and protein levels. Changes in APP expression in mouse colon cancer cells were corroborated by T-cell co-culture assays. Additionally, human familial adenomatous polyposis (FAP) organoids and polyposis in rat colon (Pirc) colon tumor primary cell lines were also treated with PRC2 inhibitors to assess MHC regulation and target modulation in precancer models. Moreover, the Pirc model was used to evaluate antitumor efficacy of PRC2 inhibition in vivo. Subsequent immunoblotting and flow cytometry was performed to assess MHC upregulation/target modulation and immunomodulatory changes in Pirc rat colon tumors, respectively.
Results
Decreased histone H3K27me3 levels via PRC2 inhibition increased the expression of MHC class I and class II APP components in pre-cancer and cancer models, based on immunoblotting and RT-qPCR assays. Normal colonic epithelial cells were resistant to the treatments, indicating PRC2 inhibition cancer cell-specific effects. In mouse colon cancer cells, cell fractionation and enzyme-linked immunosorbent assays corroborated that PRC2 inhibition increased cell surface occupancy of MHC-I complexes and CD8+ T-cell activation. Additionally, PRC2 inhibition resulted in decreased colon tumor formation in the Pirc, with an increased CD8+/CD3+ ratio and marked reduction of T-cell exhaustion marks.
Conclusions
These findings have implications for the potential clinical application of epigenetic agents,6–8 aimed towards increasing tumor immunogenicity. Such agents might be used to enhance the efficacy of current immune or chemotherapeutic strategies, providing an avenue for novel drug combinations acting in therapeutic and preventive scenarios.
Acknowledgements
Work supported by NCI grants CA122959 and CA257559, grant RF NS119872 from NINDS/NIA, and the John S. Dunn Foundation.
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Ethics Approval
Ethics Approval: Colon polyp biopsies used in organoid experiments were collected from an FAP patient at The University of Texas MD Anderson Cancer Center under an approved IRB protocol (#PA12-0327).