Flow Cytometry Based Identification of CD26+ Leukemic Stem Cells in the Peripheral Blood Has a Potential Role in the Rapid Diagnosis of Chronic Myeloid Leukemia

Background: Demonstration of t(9;22)(BCR-ABL1) fusion is gold standard for the diagnosis of chronic myeloid leukemia (CML). We performed a flowcytometric assay to identify CD26+ CML leukemic stem cells (LSCs) for its value as a standalone diagnostic investigation for the diagnosis of CML and its utility for detection of residual disease in CML patients on therapy.

Methods: Patients of CML/ CML on follow-up were included and peripheral (PB) and/or bone marrow (BM) samples were utilized for flowcytometric analysis. PB and/or BM of patients with diseases other than CML were used as controls. Under 'lyse-wash-stain-wash' sequence, the sample was incubated with a pre-titrated custom-made antibody cocktail in a 'test' tube containing CD45, CD34, CD38 and CD26 mo-abs. Acquisition was carried out on BD FACS Canto II and analysis was done with Diva Software. Clinical data including demographic details, complete blood count and BM findings were also noted.

Results: A total of 104 samples (63 PB and 41 BM) from 64 patients [confirmed & treatment naïve CML (n=30), CML on follow-up (n=15), non-CML (n=19)] were tested. The median (range) time for reporting of PB/BM examination, molecular genetic studies and flow cytometry for CD26+ CML LSCs was 5 (3-11 days), 4 (3-6 days) and 1 (0-1 day) respectively. CD26+ LSCs were identified in all patients with a confirmed diagnosis of CML (Median=0.07%, range 0.002%-26.79%), and also in 8/15 patients of the follow-up group, who also reported persisting levels of BCR-ABL1. None of the patients in the non-CML group and follow-up CML patients with negative RT-PCR results showed the presence of CD26+ LSCs. Also, there was a strong correlation between CD26+ CML LSCs in the PB and BM (r=0.917).

Conclusion: Flow cytometric assessment for CD26+ LSCs is quick with reporting time of even less than an hour. Flow cytometric identification of CD26+ LSCs in the peripheral blood can be a cheap, rapid, robust and potential diagnostic tool for the diagnosis of CML compared to available testing methods. It is independent of BCR-ABL1 transcript type and its role in residual disease monitoring needs further investigation.