Abstract PR006: C5ORF30/MACIR is a regulator of HLA-DR expression and immune evasion in high-risk DLBCL

Abstract

Understanding the molecular mechanisms governing relapse of diffuse large B-cell lymphoma (DLBCL) after R-CHOP immunochemotherapy is central to the development of novel therapeutic strategies. A recursive Bayesian screen across 13 gene expression datasets (N=3,775 patients) identified C5ORF30 (MACIR) as a strong and consistent predictor of poor survival, significant in 11 of 13 independent cohorts. MACIR is a relatively unknown cytoplasmic protein, described previously only in rheumatoid arthritis. We therefore evaluated its role and significance in DLBCL. We knockout-validated a MACIR antibody for immunohistochemistry (IHC) and evaluated its expression in reactive lymphoid tissues and DLBCL (N=250; 2 cohorts). In normal lymphoid tissues, MACIR protein was restricted to CD38-positive, CD20-negative plasmablast-like cells, pointing to a role in plasmacytic differentiation. In DLBCL, MACIR expression was noted in CD20-positive malignant B cells, especially in cases of ABC cell-of-origin. Importantly, MACIR protein expression remained independently prognostic of IPI and cell-of-origin on multivariate analysis. MACIR perturbation had minimal effects on proliferation, apoptosis, or sensitivity to CHOP chemotherapy in vitro, suggesting a non-cell-intrinsic role. In an immunocompetent syngeneic mouse model, Macir overexpression led to reduced T-cell infiltration, demonstrating a cell-extrinsic effect on the tumor immune microenvironment (TIME). We therefore examined MACIR’s relationship with the TIME through high-plex spatial profiling in DLBCL (N=152) with 58 immuno-oncology markers. While overall immune cell proportions and cellular neighborhoods were largely preserved, MACIR-High malignant B cells showed a significant reduction in HLA-DR expression. Loss of MHC class II is a hallmark of plasmablastic lymphoma (PBL), a highly aggressive lymphoma with unique immune evasion phenotypes and poor overall survival. In a cohort of PBL cases (N=8), we noted MACIR protein expression in CD138-positive PBL tumor cells with concordant absence of HLA-DR. MACIR knockdown in vitro resulted in a significant increase in HLA-DR surface expression by flow cytometry, validating this inverse relationship. Mass spectrometry pulldown identified UNC119B, a cytosolic adaptor protein involved in secretory and vesicular trafficking, as a reproducible MACIR interactor, providing a plausible mechanistic basis for MACIR-mediated regulation of HLA-DR and MHC class II surface expression. Together, our data identifies MACIR as a novel regulator of HLA-DR surface expression and plasmacytic differentiation, that delineates a subset of high-risk DLBCL with immune-evasive properties shared with PBL, and possibly representing an intermediate state between ABC DLBCL and PBL. IHC evaluation of MACIR represents a clinically applicable approach to identify high-risk DLBCL, which may require alternate approaches to overcome these HLA-DR linked immune evasion phenotypes.