Abstract The receptor tyrosine kinase-like Orphan Receptor 1 (ROR1) is highly expressed in hematologic and solid tumors and further upregulated during relapses following targeted or immunotherapies. In non-Hodgkin B-cell malignancies, ROR1 upregulation during disease progression contributes to relapses after CD19-CAR T-cell therapy, highlighting ROR1 as a compelling therapeutic target in mantle cell lymphoma (MCL). In this work, we demonstrate that ROR1 is markedly elevated in MCL and ROR1+ tumor cells become progressively enriched as the disease advances, particularly in specimens resistant to CD19-CAR T therapy. These observations reveal a clear association between heightened ROR1 level and relapse following CD19-CAR T-cell therapy, identifying ROR1 as an actionable vulnerability for therapeutic intervention in refractory/relapsed MCL, positioning ROR1-targeted bispecifics as a promising and clinically actionable approach to overcoming CAR T-cell resistance. To leverage this opportunity, we developed ROR1-directed bispecific modalities (ROR1-bsAb) for patients with CAR T-refractory disease and confirmed their functional activities. Engagement of healthy-donor T cells with the ROR1-bsAb markedly enhanced their effector function and induced robust, dose-dependent tumoricidal activity against ROR1high MCL cells, including primary MCL cells from CD19-CAR T-resistant patients, highlighting its therapeutic potential in overcoming CAR T-cell therapy relapses. Critically, while CD19-CAR T cells alone exhibited limited target specific tumoricidal activity, engagement with the ROR1-bsAb enabled CD19-CAR T cells to mediate robust and dose-dependent killing of ROR1high CAR T-resistant specimens. Given the emergence of CD19-independent CAR T cell-resistance, incorporating an alternative antigen-targeted strategy may overcome immune escape. Indeed, adding a Fc-less ROR1-bispecific significantly enhanced the antitumor activity of CD19-CAR T cells against both CD19-diminished Z-138-R and CAR T-resistant primary MCL. Consistently, Transwell antibody delivery from ROR1-bispecific-secreting T cells markedly increased cytokine production and reinstated the cytotoxicity against CAR T therapy-resistant MCL. We next validated the therapeutic potential of ROR1-bsAb in a cell line-derived xenograft model of MCL. Consistently, co-administration of either one of the ROR1-bispecific modalities and T cells elicited robust anti-lymphoma efficacy, as evidenced by pronounced suppression of tumor growth and prolonged survival. These findings highlight ROR1-targeted bispecific therapeutics as a clinically translatable strategy with strong potential to address the clinical unmet need. Taken together, this ROR1-directed bispecific platform, whether used either as a standalone modality or in combination with CAR T-cell therapy, offers considerable promise for overcoming ROR1-upregulation and antigen-loss-mediated resistance, ultimately improve outcomes for patients who relapse after CD19-CAR T-cell therapy. Citation Format: Yixin Yao, Fangfang Yan, Xiaolin Li, Yijing Li, Yue Fei, Joseph McIntosh, Wei Wang, Heng-Huan Lee, Yang Liu, Michael Wang. ROR1 upregulation after CD19-CAR T-cell relapse in mantle cell lymphoma reveals a targetable vulnerability for ROR1-directed bispecific modalities abstract. In: Proceedings of the Fifth AACR International Meeting on Advances in Malignant Lymphoma: From Discovery to Clinical Impact; 2026 Jun 24-27; Philadelphia, PA. Philadelphia (PA): AACR; Blood Cancer Discov 2026;7 (3Suppl): Abstract nr A004.
Yao et al. (Wed,) studied this question.