Abstract 5889: An immunoproteasome activator that expands the MHC-class I immunopeptidome, unmasks neoantigens and enhances antimyeloma T-cell activity

Abstract A hallmark of cancer is the ability to evade immunosurveillance mechanisms and thwart the efficacy of immunotherapeutic agents. The proteasome functions as an essential component of immunosurveillance by generating peptides from intracellular proteins that are then presented to T-cells. Antigens generated by the proteasome promote the infiltration of immune cells into tumors and improve tumor regression in response to immunotherapy. Cancer cells can evade elimination by the immune system by deregulating the antigen presentation machinery to downregulate the expression of proteins recognized by immune cells as antigens, creating an immunosuppressive microenvironment. In contrast to constitutive proteasomes, immunoproteasomes are a highly specialized proteasome variant that are highly expressed in antigen-presenting cells. Immunoproteasomes contain distinct catalytic subunits with active site substrate specificities distinct from constitutive proteasomes. Immunoproteasome degrade intracellular proteins to generate antigenic peptides that are the presented by MHC-class I (MHC-I) molecules on tumor cells for recognition by cytotoxic T-cells. We hypothesized that immunoproteasome activation could concomitantly increase the relative abundance and diversity of MHC-I antigens presented on multiple myeloma (MM) cells. High-throughput screening identified a novel small molecule (Compound A) that selectively increased immunoproteasome catalytic activity. Global proteomic integral stability assays determined that Compound A binds the proteasome structural subunit PSMA1 and that treatment with Compound A increased association of the proteasome activator PA28α/β (PSME1/2) activator with immunoproteasomes. The effect of Compound A on immunoproteasomes was abolished after the ablation of PSMA1, PSME1, or PSME2 or by the treatment of cells with suicide inhibitors that target immunoproteasome-specific catalytic sites. Treatment of MM cells with Compound A doubled the total number of MHC-I-bound peptides and increased the level of specific MHC-bound peptides up to 200-fold. Interestingly, a number of tumor-associated antigens and neoantigens were dramatically unmasked following treatment of MM cells with the immunoproteasome activator. Importantly, the treatment of patient CD138+ cells with Compound A promoted the antimyeloma activity of allogenic and autologous patient-derived CD8+ T-cells. Taken together, our results demonstrate the paradigm-shifting translational impact of immunoproteasome activators to expand the myeloma immunopeptidome and have revealed novel, actionable antigenic targets to personalize T-cell-directed immunotherapy. Citation Format: Priyanka S. Rana, James J. Ignatz-Hoover, James J. Driscoll. An immunoproteasome activator that expands the MHC-class I immunopeptidome, unmasks neoantigens and enhances antimyeloma T-cell activity abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 5889.