BCR signaling activity stratifies patents with MCL with divergent clinical outcome

Mantle cell lymphoma (MCL) is a rare subtype of Non-Hodgkin lymphoma characterized by the expansion of mature B-cells originating from the mantle zone of the lymph nodes. MCL presents a high clinical variability, with some patients experiencing an indolent disease while others characterized by an aggressive clinical course. Recent evidence supports that B-cell receptor (BCR) signaling is crucial for MCL initiation and progression and is a target for therapeutic intervention. Although BCR inhibitors [i.e., Bruton's tyrosine kinase inhibitor (BTKi)] contribute to better outcomes, drug resistance inevitably emerges for reasons still poorly understood. Therefore, there is a critical need to define parameters that identify high-risk patients for aggressive disease and therapy resistance. We measured the activation status of nine BCR signaling kinases (SYK, LCK, BTK, PLCγ2, p38, ERK1/2, AKT, NF-B p65, STAT5) in 30 peripheral blood mononuclear cells (PBMC) from MCL patients, in the basal and anti-IgM modulated condition, using phospho-specific flow cytometry. To measure phosphorylation statuses as well as responses to external stimulation of the signaling proteins, flow cytometry data were normalized with respect to the controls and subjected to unsupervised hierarchical clustering analysis (HCA). Progression free survival (PFS) and overall survival (OS) curves were estimated using the Kaplan-Meier method and compared using the log-rank test. BCR modulation with anti-IgM induced the activation of BCR signaling that was heterogeneous among patients’ samples, with identification of two clusters showing differential responses to BCR stimulation (Figure 1A). The cluster comprising samples with higher BCR signaling response (HR) was associated with shorter survival than samples grouped in the lower BCR signaling response (LR) cluster (P=0.042 and P=0.041 for PFS and OS, respectively; Figure 1B). While basal BCR activation did not provide prognostic information, higher constitutive activation of AKT was predictive of inferior response to the BTKi ibrutinib (Figure 1C). Time-to-event analyses showed that the MCL international prognostic index (MIPI) high-risk category together with high STAT5 response were significant predictors of shorter PFS and OS. In addition, MIPI high-risk category combined with high SYK response predicted shorter OS. In conclusion, we identified BCR signaling activation profiles that were associated with poorer clinical outcome and resistance to ibrutinib. This study highlighted the prognostic and predictive significance of BCR activity in MCL and advanced our understanding of signaling heterogeneity underlying clinical behavior of MCL. Future challenges are to use BCR signaling data integrated with genetic signature to predict patient’s clinical behavior and drug response for a more personalized treatment approach. We thank Fondazione Italiana Linfomi (PGR Ed. 2019) for funding support.