Venetoclax dose escalation rapidly activates a BAFF/BCL-2 survival axis in chronic lymphocytic leukemia.

Venetoclax, a first-in-class BH3 mimetic drug that targets B-cell lymphoma-2 (BCL-2), has improved the outcomes of patients with chronic lymphocytic leukemia (CLL). Early measurements of the depth of the venetoclax treatment response, assessed by minimal residual disease, are strong predictors of long-term clinical outcomes. However, there are limited data on the early changes induced by venetoclax treatment that might inform strategies to improve responses. To address this gap, we conducted longitudinal mass cytometric profiling of blood cells from patients with CLL during the first 5 weeks of venetoclax monotherapy. At baseline, we resolved CLL heterogeneity at the single-cell level to define multiple subpopulations in all patients based on proliferative, metabolic, and cell survival proteins. Venetoclax induced a significant reduction in all CLL subpopulations and caused rapid upregulation of the prosurvival BCL-2, BCL-extra large, and mantle cell lymphoma-1 proteins in surviving cells, which had reduced sensitivity to the drug. In mouse models, the venetoclax-induced elevation of survival proteins in B cells and CLL-like cells that persisted was recapitulated, and genetic models demonstrated that extensive apoptosis and access to the B-cell cytokine, B-cell activating factor (BAFF), were essential. Accordingly, in patients with CLL who were treated with venetoclax or the anti-CD20 antibody obinutuzumab there was marked elevation in BAFF and an increase in prosurvival proteins in leukemic cells that persisted. Overall, these data highlight the rapid adaptation of CLL cells to targeted therapies through homeostatic factors and support cotargeting of cytokine signals to achieve deeper and more durable long-term responses.