Researchers reported that a bacterial metabolite detected in patients colonized with carbapenem-resistant Klebsiella pneumoniae can weaken CD19 CAR-T cell activity in vitro. In a Cancer Immunology, Immunotherapy paper, Tongji Hospital investigators describe how 1,5-pentanediamine disrupts CAR-T function, pointing to microbiome-derived factors as a potential obstacle to engineered T-cell efficacy.