Work

447 Molecular and immunological characterization of anti-PD-L1 response in GEMM-derived melanoma cell lines

Year: 2026

Type: conference-abstract

Abstract: Background Tumor cell lines from genetically engineered mouse models are widely used to study cancer immunity, but many are weakly immunogenic and lack a traceable tumor antigen. We engineered the Braf/PTEN melanoma model to express the model antigen EGFPOVA (BPO) to enable tracking of tumor antigen and to interrogate tumor–immune coevolution, checkpoint sensitivity, and the determinants of anti-programmed death-ligand 1 (PD-L1) response.Methods Tumors were enzymatically digested and cultured to derive melanoma cell lines, and EGFPOVA expression was assessed. Two EGFPOVA-positive lines (891.BPO and 2742.BPO) were purified to 100% EGFPOVA and serially passaged in vivo, with tumor take monitored across passages. PD-L1 expression was analyzed directly ex vivo in early (P0) versus late (P4) passage tumors, and anti-PD-L1 therapy was tested in vivo. Five EGFPOVA-negative BPO lines were evaluated for anti-PD-L1 responsiveness, and whole-exome sequencing was used to compare mutational burden between responder and non-responder lines.Results From 328 digested tumors, approximately ~16% (55/328) yielded melanoma cell lines with continuous in vitro growth and a condensed morphology. EGFPOVA expression varied widely, consistent with dynamic immunoediting in primary tumors. When purified EGFPOVA-positive lines (891.BPO and 2742.BPO) were engrafted into mice, most tumors were spontaneously rejected; however, serial passage of a rare escape variant, while at the same time maintaining 100% EGFPOVA expression, led to progressively higher tumor take, indicating stepwise immune escape. The late-passage tumor (P4) expressed higher PD-L1 than the early passage (P0) ex vivo, suggesting increased IFNγ responsiveness and adaptive immune resistance. Accordingly, this tumor cell line responded to anti-PD-L1 therapy in vivo. Among the five EGFPOVA-negative lines, anti-PD-L1 responses were heterogeneous: 605.BPO showed a strong response; 1876.BPO, 736.BPO, and 1845.BPO showed intermediate responses; and 3142.BPO showed no response. Whole-exome sequencing revealed fewer mutations in responder 605.BPO than in non-responder 3142.BPO, arguing that mutational burden alone is insufficient to predict checkpoint sensitivity.Conclusions These data highlight roles for neoantigen quality and the evolving tumor microenvironment in shaping anti-PD-L1 outcomes and establish BPO-derived melanoma cell lines as a versatile platform to study immune editing, escape, resistance, and immunotherapy response.

Source: Journal for ImmunoTherapy of Cancer

DOI: 10.1136/jitc-2026-sitc2026.0447