Abstract 2075: Signaling thresholds and negative B cell selection in acute lymphoblastic leukemia

Proceedings: AACR 106th Annual Meeting 2015; April 18-22, 2015; Philadelphia, PA Introduction: Unlike other cell types, B cells are selected for an intermediate level of signaling strength. Critical survival and proliferation signals emanate from the (pre-) B cell receptor (BCR): Both attenuation below minimum (e.g. non-functional pre-BCR) and hyperactivation above maximum (e.g. autoreactive pre-BCR) thresholds of signaling strength trigger negative selection and cell death. The oncogenic BCR-ABL1 tyrosine kinase mimics active pre-BCR signaling in Ph+ acute lymphoblastic leukemia (ALL) 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, similar to removal of self-reactive B cells. Results: The Ph+ ALL cells don not express ITAM (immunoreceptor tyrosine-based activation motif) receptor Igα or Igβ on the cell surface, indicating defects for a functional pre-BCR. Reconstitution of ITAM receptor was sufficient to induce cell death through increasing pre-BCR signaling strength indicated by phosphorylation of SYK, SRC, BTK and PLCγ2. TKI-treatment, while designed to kill leukemia cells, seemingly paradoxically rescued Ph+ ALL cells in this experimental setting. Surprisingly, patient-derived Ph+ ALL cells express the ITIM (immunoreceptor tyrosine-based inhibitory motif) receptors PECAM1, CD300A and LAIR1 at high levels compared to normal pre-B cells. Importantly, high expression levels of ITIM-receptors are predictive of poor outcome 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 Ptpn6 (Shp1) and Inpp5d (Ship1). Genetic deletion of Lair1, Ptpn6 or Inpp5d in BCR-ABL1 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 a self-reactive BCR on ALL cells. Using chimeric PECAM1, CD300A and LAIR1 receptor decoys and a novel small molecule inhibitor of INPP5D, 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. Note: This abstract was not presented at the meeting. Citation Format: Zhengshan Chen, Seyedmehdi Shojaee, Maike Buchner, Huimin Geng, Jae Woong Lee, Lars Klemm, Eugene Park, Ying Xim Tan, Anne Satterthwaite, Elisabeth Paietta, Stephen P. Hunger, Mignon L. Loh, Jae U. Jung, John E. Coligan, Silvia Bolland, Tak W. Mak, Andre Limnander, Hassan Jumaa, Michael Reth, Arthur Weiss, Clifford A. Lowell, Markus Muschen. Signaling thresholds and negative B cell selection in acute lymphoblastic leukemia. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2075. doi:10.1158/1538-7445.AM2015-2075