DP123 | CORRELATION BETWEEN CIRCULATING CD26+ LEUKEMIC STEM CELLS AND BCR::ABL1+ EXTRACELLULAR VESICLES IN CHRONIC MYELOID LEUKEMIA PATIENTS: A PRELIMINARY STUDY

Chronic myeloid leukemia (CML) is a blood disease marked by BCR::ABL1 fusion gene, which drives uncontrolled leukemic cell growth. Although tyrosine kinase inhibitors (TKIs) effectively control the disease, leukemic stem cells (LSCs) persist, contributing to relapse. A subset of LSCs, identifiable by the CD26 membrane marker, is found at similar levels in bone marrow (BM) and peripheral blood (PB), where they result Ki67+, indicating active proliferation. Beyond cellular markers, small extracellular vesicles (EVs) have emerged as a promising tool for disease monitoring. EVs carry BCR::ABL1 transcript, offering a minimally invasive method to detect residual disease and serving as a stable biomarker for monitoring disease burden and predicting relapse. We assessed vesicular BCR::ABL1 transcript alongside circulating CD26+ cells in a preliminary CML cohort of 44 CML patients who had achieved at least MMR on TKIs: 10/44 (22%) in MMR and 34/44 (78%) in DMR. Circulating CD26+ cells were measured via flow cytometry on CD45+/CD34+/CD38− cells using a four-color staining protocol. The absolute CD26+ count/μL was calculated as % of with blood cells. EVs were isolated from plasma using a commercial kit, and vesicular BCR::ABL1 was quantified by digital PCR (dPCR). BCR::ABL1 was also assessed by dPCR in PB cells. Spearman correlation was used to assess relationships between variables. We found an inverse correlation between CD26+ cell count and vesicular BCR::ABL1 transcript (r=–0.3919, p=0.0085). No correlation was found between CD26+ cells and BCR::ABL1 levels measured as IS% or MR. Similarly, BCR::ABL1 in PB cells by dPCR showed no correlation with CD26+ levels. CD26+ cells are present in comparable numbers in PB and BM. These proliferating Ki67+ LSCs significantly decline in response to TKI treatment. Our findings reveal for the first time that as CD26+ cells decrease, vesicular BCR::ABL1 transcript increases. This may suggest that a distinct CD26− leukemic population emerges and takes over by releasing BCR::ABL1-loaded vesicles, or that the decline of CD26+ cells activate a mechanism that enhances vesicular release. These findings require validation in a larger cohort, and further characterization is essential to determine the origin of the BCR::ABL1-loaded vesicles. Ultimately, a comprehensive understanding of the prognostic implications of this data must be elucidated, as it could offer valuable insights into disease progression and the biological basis of CML.