PROJECT SUMMARY B cells critically depend on continuous survival and proliferation signals from a functional B cell receptor (BCR). Likewise, in ~50% of B cell malignancies, the tumor clone is driven by an oncogenic BCR-mimic. Oncogenic mimics of BCR-dependent proliferation and survival signals include BCR-ABL1 (Ph+ ALL), viral oncoproteins (e.g. EBV), RAS- and NF-κB-pathway activating lesions (Hodgkin's lymphoma, PMBL, ABC-DLBCL, hairy cell leukemia, Waldenström's macroglobulinemia). In preliminary studies, we found that CD25 is selectively expressed on malignant B cell clones driven by oncogenic BCR-mimics. While CD25 functions as IL2 receptor α-chain on T cells, we recently discovered that CD25 is a critical feedback regulator of BCR signaling and oncogenic BCR- mimics in human B cell tumors. Genetic experiments demonstrated that CD25 is critical for the initiation of B cell leukemia and lymphoma in transplant recipients. Surface expression is rapidly induced by activity of BTK and PKCδ downstream of the BCR and induced by FOXM1 and NF-κB at the transcriptional level. CD25 then recruits an inhibitory complex to the cell membrane to reduce and recalibrate BCR signaling or oncogenic mimicry of BCR-signaling. Analysis of three clinical cohorts revealed that high expression levels of CD25 are associated with poor clinical outcome in various B cell malignancies. While CD25 expression is associated with drug-resistance, inhibition of CD25 or disabling of CD25-dependent feedback control sensitizes multiple B cell malignancies to conventional drug-treatment. Based on these and other findings, we propose three Aims to (1) elucidate mechanisms of CD25 regulation, (2) explore usefulness of pharmacological subversion of CD25-mediated feedback control and (3) targeted eradication of CD25+ cells by CART25 cells and antibody-drug conjugates (ADC) as therapeutic adjuvant.