Dynamic expression of CD9 protein in T-cell acute lymphoblastic leukemia.

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematologic malignancy affecting both children and adults. Given its persistently poor prognosis, there is a critical need to identify additional factors involved in T-ALL oncogenesis and progression. CD9, a membrane protein of the tetraspanin family implicated in diverse cellular processes, has been associated with prognosis in several cancers, yet its role in T-ALL remains poorly understood. In this study, using a mouse model first, we found that CD9 overexpression is associated with leukemic T cells that have migrated outside the thymus into peripheral tissues. Then, analysis of a human T-ALL cohort shows that CD9 expression is heterogeneous, tends to increase at relapse and is enriched in the TAL1⁺ molecular subtype. We further demonstrate that CD9⁺ cells display enhanced migratory capacity compared with CD9⁻ counterparts, and that CD9 levels affect extracellular vesicle biogenesis. Altogether, our findings support a role for CD9 in T-ALL leukemogenesis and highlight its potential involvement in relapse.