Signaling thresholds and negative B cell selection in acute lymphoblastic leukemia

B cells are selected for an intermediate level of B cell receptor (BCR) signaling strength: Attenuation below minimum (e.g. non-functional BCR)(1) or hyperactivation above maximum (e.g. self-reactive BCR)(2–3) thresholds of signaling strength causes negative selection. In ~25% of cases, acute lymphoblastic leukemia (ALL) cells carry the oncogenic BCR-ABL1 tyrosine kinase (Ph(+)), which mimics constitutively active pre-BCR signaling(4,5). Current therapy approaches are largely focused on the development of more potent tyrosine kinase inhibitors to suppress oncogenic signaling below a minimum threshold for survival(6). Here, we tested the hypothesis that targeted hyperactivation above a maximum threshold will engage a deletional checkpoint for removal of self-reactive B cells and selectively kill ALL cells. Testing various components of proximal pre-BCR signaling, we found that an incremental increase of Syk tyrosine kinase activity was required and sufficient to induce cell death. Hyperactive Syk was functionally equivalent to acute activation of a self-reactive BCR on ALL cells. Despite oncogenic transformation, this basic mechanism of negative selection was still functional in ALL cells. Unlike normal pre-B cells, patient-derived ALL cells express the inhibitory receptors PECAM1, CD300A and LAIR1 at high levels. Genetic studies revealed that Pecam1, Cd300a and Lair1 are critical to calibrate oncogenic signaling strength through recruitment of the inhibitory phosphatases Ptpn6(7) and Inpp5d(8). Using a novel small molecule inhibitor of INPP5D(9), we demonstrated that pharmacological hyperactivation of SYK and engagement of negative B cell selection represents a promising new strategy to overcome drug-resistance in human ALL.