Abstract B041: AR suppresses MHC Class I expression and T-cell response in prostate cancer
Authors: Chesner Lisa , Graff Julie , Polesso Fanny , Smith Alexis , Lundberg Arian , Sjoestroem Martin , Xia Zheng , Linder Simon , Bergman Andries , Ashworth Alan , Quigley David , Zwart Wilbert , Gilbert Luke , Moran Amy , Feng Felix 
Published: June 2023

Abstract
Immunotherapy is a treatment option that has had limited success with prostate cancer patients. The major histocompatibility complex (MHC) Class I plays a pivotal role in the adaptive immune response by presenting neo-antigens on the surface of cancer cells to CD8+ T-cells. Prostate cancer cells have markedly lower expression of MHC Class I genes compared to immune-responsive cancers. Loss of MHC Class I is also associated with more aggressive disease and immune evasion in prostate cancer. However, the molecular mechanisms that control MHC Class I downregulation in prostate cancer are still unknown. We hypothesize that increasing MHC Class I expression in prostate cancer cells will increase antigen presentation and improve the efficacy of immunotherapy agents. To investigate the mechanism of MHC Class I regulation in prostate cancer cells, we conducted a whole-genome CRISPRi flow cytometry screen. In this screen, C42B cells containing a non-catalytic Cas9 (dCas9) were infected with a lentivirus that contained a sgRNA library of 100,000 guides targeting 20,000 genes. Infected cells were selected and stained with a pan-Class I MHC antibody, sorted, and the highest and lowest 25-30% of MHC Class I expressing cells were collected for analysis. As expected, knockdown of MHC Class I genes such as B2M, HLA-A, and TAP1 decreased MHC Class I expression. Surprisingly, knockdown of AR and AR co-factors GRHL2 and FOXA1 resulted in a dramatic increase of MHC Class I surface expression. Additional AR inhibition experiments using enzalutamide, an AR degrader, or charcoal-stripped serum also showed increased MHC Class I expression over time. These in vitro results were substantiated with RNA expression analyses of patient biopsy samples taken before or after neoadjuvant enzalutamide treatment, which showed significantly increased MHC Class I expression post-treatment. Results from a small library CRISPR screen, showed that elimination of androgen response elements upstream of MHC Class I genes increased MHC expression, providing a mechanism by which AR regulates MHC Class I. Importantly, AR knockdown was shown to increase antigen-specific T-cell response in co-culture and transgenic mouse models. These observations were further validated in a mCRPC phase II clinical trial. Bulk RNA expression analyses comparing responders and non-responders to anti-PD1 therapy in enzalutamide treated patients, showed decreased expression of AR and increased expression of MHC Class I in patients who responded to immunotherapy. Overall, our data show that AR suppresses MHC Class I expression in prostate cancer, and that androgen-targeted therapies can increase antigen presentation and improve T-cell response. By understanding how AR regulates MHC Class I expression, we can identify new combination treatments utilizing AR signaling inhibition and immunotherapy that lead to a improved anti-cancer response.
Citation Format: Lisa Chesner, Julie Graff, Fanny Polesso, Alexis Smith, Arian Lundberg, Martin Sjoestroem, Zheng Xia, Simon Linder, Andries Bergman, Alan Ashworth, David Quigley, Wilbert Zwart, Luke Gilbert, Amy Moran, Felix Feng. AR suppresses MHC Class I expression and T-cell response in prostate cancer [abstract]. In: Proceedings of the AACR Special Conference: Advances in Prostate Cancer Research; 2023 Mar 15-18; Denver, Colorado. Philadelphia (PA): AACR; Cancer Res 2023;83(11 Suppl):Abstract nr B041.