IL-27 showed no direct cytotoxicity toward B-ALL cells, supporting an indirect, niche-mediated mechanism.<h4>Interpretation</h4>IL-27 constrains B-cell haematopoiesis and B-ALL progression through coordinated progenitor inhibition and BM niche remodelling, revealing a cytokine-driven strategy with the potential to enhance leukaemia therapy.<h4>Funding</h4>This work was supported by the National Key R&D Program of China (Grant No. 2021YFA1100800 to A.-B.L). Here, IL27 is linked to precursor B-cell acute lymphoblastic leukemia.