CD9 antibody enhances the anti-leukemia efficacy of γδT cells in adult B cell acute lymphoblastic leukemia

B cell acute lymphoblastic leukemia (B-ALL) is a hematological malignancy with a heterogeneous prognosis. Using single-cell RNA sequencing (scRNA-seq), we profiled 10 samples from three relapsed patients, three newly diagnosed patients, and four healthy volunteers. We identified a preB_<i>CD9</i> cluster with predominant S-phase distribution (53.5%), enhanced communication with leukemic clusters, and reduced interaction with immune cells, which were the most pronounced changes observed during the progression of adult B-ALL. <i>CD9</i> was overexpressed in relapsed patients, validated as a poor prognostic marker in the GSE34941 dataset. Relapsed cases showed immunosuppressive tumor microenvironments (TMEs) with diminished γδT cells, while the preB_<i>CD9</i> cluster expressed γδT cell-targeting antigen genes. <i>CD9</i> overexpression increased these antigen genes, and anti-CD9 antibodies enhanced γδT cell cytotoxicity against <i>CD9</i>-high leukemia cells <i>in vitro</i> and <i>in vivo</i>. Thus, <i>CD9</i> identifies an aggressive B-ALL subset and may provide therapeutic benefit in combination with γδT cell immunotherapy and anti-CD9 antibodies.