Targeted Activation of B Cell Autoimmunity Checkpoints in Acute Lymphoblastic Leukemia
Authors: Chen Zhengshan , Geng Huimin , Lowell Clifford A. , Weiss Arthur , Hunger Stephen P. , Melnick Ari , Muschen Markus 
Published: October 2019

Abstract
Background: Unlike other cell types, B cells are selected for an intermediate level of signaling strength. Critical survival and proliferation signals emanate from the B cell receptor (BCR): If B-cells fail to express a functional BCR, signaling output is too weak, resulting in "death by neglect". If the BCR binds to ubiquitous self-antigen, BCR signals are exceedingly strong. Both attenuation below minimum (non-functional BCR; death by neglect) and hyperactivation above maximum (autoreactive BCR) thresholds of signaling strength trigger negative selection and cell death.
Rationale: Unlike any other types of cancer, we recently discovered that pre-B acute lymphoblastic leukemia (ALL) cells are bound by the same rules that also govern normal B cell selection. The oncogenic BCR-ABL1 tyrosine kinase mimics active pre-BCR signaling in Ph+ acute lymphoblastic leukemia which defines the ALL subgroup with the worst clinical outcome. Current therapy approaches are largely focused on the development of more potent tyrosine kinase inhibitors (TKI) to suppress oncogenic signaling. However resistance to TKI is developed invariably. Here, we test the hypothesis that targeting hyperactivation above a maximum threshold will selectively kill Ph+ ALL cells through a mechanism that is functionally equivalent to removal of self-reactive B cells.
Results: ALL typically originates from pre-B cells that critically depend on survival signals emanating from a functional pre-BCR. Despite their pre-B cell origin, Ph+ ALL cells lack expression of pre-BCR signaling chains Iga and Igb, indicating lack of a functional pre-BCR. Reconstitution of Iga and Igb was indeed sufficient to induce cell death in BCR-ABL1 ALL. TKI-treatment, while designed to kill leukemia cells, seemingly paradoxically rescued Ph+ ALL cells in this experimental setting. These findings suggest that Ph+ ALL are uniquely sensitive to even incremental increases of pre-BCR signaling. Consistent with this concept, patient-derived Ph+ ALL cells express high levels of inhibitory surface receptors PECAM1, CD300A and LAIR1 that recruit and activate phosphatases SHP1 and SHIP1, which terminate pre-BCR signaling. Importantly, high expression levels of these surface receptors were strong predictors of poor outcome of patients with ALL in two clinical trials, including both pediatric and adult ALL patients. Genetic studies revealed that Pecam1, Cd300a and Lair1 were critical to calibrate pre-BCR signaling strength through recruitment of the inhibitory phosphatases SHP1 and SHIP1. Genetic deletion of Lair1, Ptpn6 or Inpp5d in BCR-ABL1 transformed ALL caused cell death in vitro and in vivo through hyperactivation of pre-BCR signaling. 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 an autoreactive BCR on ALL cells. Despite oncogenic transformation, this basic mechanism of negative selection was still functional in Ph+ ALL cells. Using chimeric PECAM1, CD300A and LAIR1 receptor decoys and a novel small molecule inhibitor of SHIP1, we demonstrated that pharmacological hyperactivation of pre-BCR signaling and engagement of negative B cell selection represents a promising new strategy to overcome drug-resistance in human Ph+ ALL.
Conclusion: These results indicated that inhibitory receptors and downstream phosphatases are critical regulators of pre-BCR signaling strength in Ph+ ALL, and identified targeting hyperactivation of pre-BCR signaling as a potential novel class of therapeutic strategy.

Disclosures
No relevant conflicts of interest to declare.