Dipeptidyl peptidase 2 apoptosis assay determines the B-cell activation stage and predicts prognosis in chronic lymphocytic leukemia

Objective: Dipeptidyl peptidase 2 (DPP2/DPP7) is a regulator of quiescence as inhibition of DPP2 results in apoptosis of resting, but not activated lymphocytes. The purpose of the present study was to investigate the prognostic value of DPP2 inhibition and the role of DPP2 in cell cycle in chronic lymphocytic leukemia (CLL). Materials and Methods: We screened 152 peripheral blood samples from patients with CLL in an apoptosis assay with AX8819, a DPP2-specific inhibitor. The apoptotic response was correlated with B-cell receptor signaling and cell cycle and molecular prognostic factors. Results: We categorized CLL into two prognostic subgroups. Inhibition of DPP2 induced apoptosis in 60% of CLL, while 40% were resistant to apoptosis. Resistance to apoptosis correlated with unmutated IgV_H and increased ZAP-70 expression and was associated with unfavorable clinical outcomes. Sensitive CLL B cells expressed high p27, low c-Myc protein levels and decreased Syk phosphorylation, indicative of a resting phenotype. DPP2 inhibition in those cells resulted in apoptosis accompanied by enhanced phosphorylation of Syk, degradation of p27 and p130, and upregulation of c-Myc, indicative of activation and inappropriate cell cycle entry. Resistant CLL demonstrated baseline low p27 and high c-Myc protein levels and increased pSyk, indicative of an activated phenotype. Inhibition of heat shock protein 90 in this subset of CLL partially reversed apoptosis resistance. Conclusions: The DPP2 apoptosis assay provides a reliable prognostic factor in CLL. CLL B cells sensitive to DPP2 inhibition are in true G₀, while resistant CLL B-cells are partially activated. DPP2 inhibition alone or with concomitant inhibition of heat shock protein 90 warrants investigation as a therapeutic modality in CLL.