Abstract 1154: Investigating the functional role of GPNMB in glioblastoma and the tumor immune microenvironment and its targeted elimination using CAR-Ts
Authors: Savage Neil , Zemp Franz J. , Mikolajewicz Nick , Han Hong , Venugopal Chitra , Chokshi Chirayu , Tatari Nazanin , Kislinger Thomas , Moffat Jason , Mahoney Doug , Singh Sheila K. 
Published: April 2023

Abstract
Glycoprotein nonmetastatic melanoma protein B (GPNMB) is known to be active in the extracellular matrix of glioblastoma and has been identified as a promising immunotherapy target for both tumor cells and immunosuppressive macrophages.
Methods: Immunohistochemistry was performed on patient derived xenograft (PDX) brains and tissue samples of 16 patient matched primary/recurrent GBMs as well as 23 normal organ tissues. Whole cell proteomics was performed on 43 matched primary/recurrent GBM samples. Flow cytometry measured surface expression levels of GPNMB to confirm CAR-T accessibility. CRISPR/Cas9 was used to eliminate expression in GBM lines to measure proliferation and mouse survival times. GPNMB knockout clones were generated in GL261 and engrafted in immunocompetent mice to examine single cell transcriptomes using sciRNAseq at endpoint. A second-generation CAR-T was developed to target GPNMB-expressing populations, and efficacy was interrogated using standard in vitro assays and GBM PDX models.
Results: The absence of GPNMB throughout most normal tissues validates the rationale of administering CAR-Ts as a safe modality for patients. GPNMB detected in residual tumors of PDX models treated orthotopically with CD133 CAR-Ts revealed it as a targetable subpopulation of GBM cells and a rational co-target alongside CD133 in the heterogeneous tumor. Tissue microarrays and whole cell proteomics found GPNMB to be upregulated in recurrent GBMs compared to primary (p=0.0349 and p=0.0033 respectively) while being absent in normal tissues. Single cell sequencing data of patient GBMs revealed GPNMB was also highly expressed in tumor-associated macrophages. Eliminating GPNMB in GBM cell lines decreased proliferation (P&lt;0.001) and prolonged survival times in all mouse models (P&lt;0.01) indicating its functional relevance. GPNMB knockout clones displayed downregulation of hallmark signalling pathways of GBM such as PDGFR, TGF-beta, Integrins and Stats, as well as decreased innate/adaptive immune activation. CAR-T cytotoxicity and activation was observed in vitro and in vivo resulting in decreased tumor burden (P&lt;0.001) and increased survival times (P&lt;0.001). Ultimately a CD133+ population was observed in residual tumors of GPNMB CAR-T treated mice at endpoint and surface expression of CD133 and GPNMB revealed co-expression and distinct populations.
Conclusions: We show GPNMB influences tumor-intrinsic biology of GBM and is also active in macrophages in the recurrent GBM immune microenvironment. By targeting GPNMB along with CD133, combinatorial therapeutic regimens could target both the cancer stem cell hierarchy and its supportive niche. Administration of both CAR-T cell therapies to humanized mice engrafted with patient-derived GBMs will provide better cytotoxic coverage and potentially provide more durable therapeutic efficacy for GBM patients.
Citation Format: Neil Savage, Franz J. Zemp, Nick Mikolajewicz, Hong Han, Chitra Venugopal, Chirayu Chokshi, Nazanin Tatari, Thomas Kislinger, Jason Moffat, Doug Mahoney, Sheila K. Singh. Investigating the functional role of GPNMB in glioblastoma and the tumor immune microenvironment and its targeted elimination using CAR-Ts [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1154.