Monocyte Galectin-9 is associated with CD8⁺ T-cell dysfunction and early relapse after haploidentical hematopoietic stem cell transplantation

Acute myeloid leukemia (AML) is the most common acute leukemia in adults. Haploidentical hematopoietic stem cell transplantation (haplo-HSCT) stands as a leading curative approach for AML in China. However, relapse remains a major challenge for patient survival, often occurring within six months post-transplant and associated with a five-year mortality rate of up to 80%. Here, we characterized early immune reconstitution after haplo-HSCT and investigated immune features associated with early relapse (ER) in AML. We analyzed two clinical cohorts: patients with AML ( n = 44) and aplastic anemia (AA; n = 38), and AML patients with continuous complete remission (CCR, n = 55) versus ER ( n = 38). Single-cell RNA sequencing was performed on longitudinal peripheral blood and bone marrow samples from AML and AA patients ( n = 3 and 4), while single-cell RNA/TCR sequencing was performed on bone marrow samples from CCR and ER patients ( n = 3 each). Flow cytometry assessed monocyte Galectin-9 surface expression and CD8⁺ T-cell phenotypes. Galectin-9-associated monocyte-T-cell interactions and the effects of the TIM-3 inhibitor MG-T-19 were evaluated in vitro and in a murine AML model. AML and AA patients showed different patterns of early immune reconstitution after haplo-HSCT, including differences in monocyte and T-cell recovery. Within AML, ER patients exhibited increased T-cell representation but reduced basal TNF-α expression and altered exhaustion-associated CD8⁺ T-cell phenotypes before relapse. Monocytes represented a prominent Galectin-9-expressing compartment, and surface Galectin-9 on monocytes was elevated in ER compared with CCR patients as early as day 30 post-transplantation. Exogenous Galectin-9 induced dose-dependent functional and exhaustion-associated changes in CD8⁺ T cells, while Galectin-9-high IFN-conditioned monocytes impaired CD8⁺ T-cell responses in vitro. MG-T-19 partially reversed these alterations, increased CD8⁺ T-cell TNF-α production, and enhanced leukemia-cell apoptosis. In the MLL-AF9 murine AML model, MG-T-19 reduced leukemia burden and prolonged survival. Increased monocyte Galectin-9 during early immune reconstitution is associated with subsequent early relapse and CD8⁺ T-cell dysfunction after haplo-HSCT. Further Galectin-9-specific mechanistic studies and prospective clinical validation are warranted to establish its causal relevance and potential utility as an early relapse biomarker.