Cutting Edge: ROR1/CD19 Receptor Complex Promotes Growth of Mantle Cell Lymphoma Cells Independently of the B Cell Receptor–BTK Signaling Pathway

: Inhibitors of Bruton tyrosine kinase (BTK), a kinase downstream of BCR, display remarkable activity in a subset of mantle cell lymphoma (MCL) patients, but the drug resistance remains a considerable challenge. In this study, we demonstrate that aberrant expression of ROR1 (receptor tyrosine kinase-like orphan receptor 1), seen in a large subset of MCL, results in BCR/BTK-independent signaling and growth of MCL cells. ROR1 forms a functional complex with CD19 to persistently activate the key cell signaling pathways PI3K-AKT and MEK-ERK in the BCR/BTK-independent manner. This study demonstrates that ROR1/CD19 complex effectively substitutes for BCR-BTK signaling to promote activation and growth of MCL cells. Therefore, ROR1 expression and activation may represent a novel mechanism of resistance to inhibition of BCR/BTK signaling in MCL. Our results provide a rationale to screen MCL patients for ROR1 expression and to consider new therapies targeting ROR1 and/or CD19 or their downstream signaling pathways for MCL-expressing ROR1.