Background and hypothesis: B cells critically depend on continuous survival and proliferation signals from a functional B cell receptor (BCR). In >50% of B cell malignancies, the tumor clone is driven by an oncogenic BCR-mimic. Oncogenic mimics of BCR-signaling include BCR-ABL1 (Ph+ ALL) and other ABL1-fusion genes (Ph-like ALL), viral oncoproteins (e.g. EBV; KSHV), BRAF- (hairy cell leukemia) and NF-kB-pathway lesions (Hodgkin's lymphoma, primary mediastinal B cell lymphoma). B cell tumors driven by oncogenic BCR-signaling represent a highly heterogeneous group of various cellular origins, genetic lesions and divergent clinical outcomes. Despite this heterogeneity, our correlative analyses showed that these B cell malignancies are uniformly characterized by high expression levels of CD25.
Features of CD25 function in B cell malignancies: While CD25 mediates IL2 signaling on T cells and is highly expressed on regulatory T cells (Tregs), we recently discovered that CD25 is a critical feedback regulator of B cell receptor (BCR) and oncogenic BCR-mimics in human B cell malignancies. Indeed, we found that CD25 is a biomarker of tumor clones driven by oncogenic BCR-mimics and predicts sensitivity to small molecule inhibitors of the BCR signaling pathway (e.g. Ibrutinib, entospletinib). Studying Cd25-/- models for B cell tumors, our genetic experiments demonstrated that CD25 is critical for the initiation of B cell leukemia in transplant recipient mice. Pulldown and mass-spectometry proteomic analyses identified strong binding and phosphorylation of the CD25 cytoplasmic tail by PKCd. Surface expression is rapidly induced by activity of PKCd and NF-kB downstream of the BCR. CD25 then recruits an inhibitory complex to the surface to reduce kinase signaling downstream of the BCR or its oncogenic mimics. Analysis of three clinical cohorts revealed that high expression levels of CD25 are strongly predictive of poor clinical outcome in various B cell malignancies. While CD25 expression is associated with drug-resistance, inhibition of CD25 sensitized multiple B cell malignancies to conventional drug-treatment.
CD25 is required for B cell tumor-initiation: Our genetic studies showed that BCR signaling rapidly induces CD25 cell surface translocation by PKCd-dependent phosphorylation of the CD25 cytoplasmic tail (S268). In addition, BCR signaling induces CD25 expression at the transcriptional level through activation of NF-kB. In response to oncogenic BCR signaling, CD25 shuttles inhibitory phosphatases (e.g. SHIP1) from the cytoplasm to the cell membrane. CD25-mediated membrane recruitment of inhibitory phosphatases calibrates signal output and enables robustness of oncogenic signaling. Consistent with a role as dynamic feedback regulator, CD25 surface expression levels on individual cells oscillate in parallel with BTK- and PKCd-dependent Ca2+ oscillations. Genetic experiments demonstrated that CD25 is critical for the initiation of B cell leukemia and lymphoma in transplant recipients. Genetic ablation of CD25 subverted the ability of B cell lymphoma cells to balance oncogenic signal strength, resulting in p53 checkpoint activation, cell cycle arrest and cell death. Analysis of three clinical cohorts revealed that CD25+ B cell tumors are associated to relapse and poor clinical outcome. While CD25 expression induced drug-resistance, CD25-inhibition sensitized to conventional drug-treatment.
Conclusions: Based on these and other findings, we propose CD25 as a biomarker of BCR-dependent B cell tumors that are highly sensitive to established BCR signaling antagonists (e.g. Ibrutinib, entospletinib) and therapeutic target in refractory B cell tumors.
Melnick:Janssen: Research Funding.
Asterisk with author names denotes non-ASH members.