<scp>TIM</scp>‐3 marks measurable residual leukemic stem cells responsible for relapse after allogeneic stem cell transplantation

In this study, we investigated the measurable residual leukemic stem cell (MR-LSC) population after allogeneic stem cell transplantation (allo-SCT) for high-risk acute myeloid leukemia (AML), utilizing T-cell immunoglobulin mucin-3 (TIM-3) expression as a functional marker of AML leukemic stem cells (LSCs). Analysis of the CD34⁺CD38^- fraction of bone marrow cells immediately after achievement of engraftment revealed the presence of both TIM-3⁺LSCs and TIM-3^- donor hematopoietic stem cells (HSCs) at varying ratios. Genetic analysis confirmed that TIM-3⁺ cells harbored patient-specific mutations identical to those found in AML clones, whereas TIM-3^- cells did not, indicating that TIM-3⁺CD34⁺CD38^- cells represent residual AML LSCs. In 92 allo-SCT occasions involving 83 AML patients, we enumerated the frequencies of TIM-3⁺LSCs immediately after achieving hematologic complete remission with complete donor cell chimerism. Notably, only 22.2% of patients who achieved a TIM-3⁺MR-LSC^low status (<60%) experienced relapse, with a median event-free survival (EFS) of 1581 days (median follow-up duration was 2177 days among event-free survivors). Conversely, 87.5% of patients with TIM-3⁺MR-LSC^int/high (≥60%) relapsed, with a median EFS of 140.5 days. Furthermore, MR-LSC status emerged as a significant independent risk factor for relapse (hazard ratio, 8.56; p < 0.0001), surpassing the impact of patient disease status prior to allo-SCT, including failure to achieve complete remission (hazard ratio, 1.98; p = 0.048). These findings suggest that evaluating TIM-3⁺ MR-LSCs immediately after engraftment, which reflects the competitive reconstitution of residual TIM-3⁺ LSCs and donor HSCs, may be valuable for predicting outcomes in AML patients undergoing allo-SCT.