CD34⁺CD38^-CD58^- leukemia-Propagating Cells at Diagnosis Could Identify Patients at High-Risk for Relapse in Philadelphia Chromosome-Positive Acute Lymphoblastic Leukemia after Allogeneic Hematopoietic Stem Cell Transplantation

Background: Relapse of Philadelphia-chromosome-positive acute lymphoblastic leukemia (Ph⁺ALL) may result from the persistence of leukemia stem cells sometimes termed leukemia-propagating cells (LPCs). We recently found that Ph⁺ALL LPCs are enriched in the CD34⁺CD38^-CD58^- fraction using anti-CD122-conditioned NOD/SCID xenograft assay by intra-bone marrow injection, which translating to adverse clinical outcomes (Kong Y, et al. Leukemia 2014. accepted). Despite the widespread use of abelson tyrosine kinase inhibitors (TKIs) in Ph⁺ALL, allogeneic hematopoietic stem cell transplantation (allo-HSCT) remains the best curative option. However, whether the prognostic significance of the identified LPCs phenotype to identify patients at high risk for relapse could retain in Ph⁺ALL after allo-HSCT, if any, is unknown.

Aims: To investigate the prognostic significance of the candidate CD34⁺CD38^-CD58^- LPCs in Ph⁺ALL subjects underwent allo-HSCT.

Methods: A total of 80 consecutive adults (18-60 years) with Ph⁺ALL underwent allo-HSCT were eligible for the study at Peking University Institute of Hematology from January 1, 2009 to December 31, 2013. Imatinib was routinely administered in subjects pre- and post-HSCT as previously reported. A multi-parameter flow cytometry analysis of CD58-FITC/CD10-PE/CD19-APC-Cy7/CD34-PerCP/CD45-Vioblue/ CD38-APC on gated leukemia blasts of bone marrow was performed at diagnosis. Furthermore, minimal residual disease (MRD) was monitored by BCR/ABL transcripts in bone marrow samples at diagnosis, directly before transplantation, as well as serially at 1, 2, 3, 6, 9, 12,24,36,60 months post-HSCT and at relapse using real-time quantitative polymerase chain reaction. Cumulative incidences of relapse (CIR) and non-relapse mortality were calculated using the Kalbfleisch and Prentice method. Leukemia-free survival (LFS) and overall survival (OS) were estimated using the Kaplan-Meier method and compared using the log-rank test. Factors at a level of P<0.1 were included as variables in the multivariate Cox regression model. The study was approved by the Ethics Committee of Peking University People’s Hospital.

Results: On the basis of blasts phenotypes at diagnosis, subjects were stratified into CD34⁺CD38^-CD58^- group (N=15) and other phenotype group (N=65). The demographic and clinical characteristics showed no significant difference between the two phenotype groups. Median follow-up was 25.5 mo (range, 6-65 mo) for all subjects and 33 mo (range, 6-65 mo) for survivors. During the MRD monitoring, significantly higher levels of BCR/ABL transcripts were detected in subjects in CD34⁺CD38^-CD58^- group than persons in other phenotype group especially at 3 mo post-HSCT [0.12(0-152.4)% vs. 0(0-100)%, P=0.001]. Additionally, CD34⁺CD38^-CD58^- LPCs phenotype directly correlated with higher 3-year CIR (63.2% [58.2-68.1%] vs. 5.3% [5.1-5.5%]; P<0.0001), worse LFS (30.2% [8.1-56.6%] vs. 78.7% [64.5-87.7%]; P=0.001) and OS (37.7% [12.6-63.2%] vs. 82.3% [68.5-90.4%]; P=0.0004). Multivariate analyses indicated that CD34⁺CD38^-CD58^- LPCs phenotype at diagnosis and BCR-ABL reduction at 3 mo post-HSCT were independent risk factors for relapse, LFS and OS in adults with Ph⁺ALL underwent allo-HSCT.

Summary/Conclusion: Our data suggest that a candidate CD34⁺CD38^-CD58^- LPCs phenotype at diagnosis allows rapid identification of high-risk patients for relapse even after allo-HSCT. Risk-stratification post-HSCT therapy incorporating analysis of CD34⁺CD38^-CD58^- LPCs phenotype at diagnosis promises to benefit the adults with Ph⁺ALL in the future.

Acknowledgment: Supported by the National Natural Science Foundation of China (grant nos. 81370638&81230013), the Beijing Municipal Science and Technology Program (grant no. Z141100000214011), and Peking University People’s Hospital Research and Development Funds (grant no. RDB2012-23).