PD-1 signalling defines and protects leukaemic stem cells from T cell receptor-induced cell death in T cell acute lymphoblastic leukaemia.

T cell acute lymphoblastic leukaemia (T-ALL) is an aggressive malignancy with poor prognosis, but a decisive marker and effective treatment for leukaemia stem cells (LSCs) remain unclear. Here, using lineage tracing, limiting dilution assays and in vivo live imaging approaches, we identify rare inhibitory receptor programmed cell death 1 (PD-1)-expressing cells that reside at the apex of leukaemia hierarchy for initiation and relapse in T-ALL. Ablation of PD-1-expressing cells, deletion of PD-1 in T-ALL cells or blockade of PD-1 or PD-1 ligand 1 significantly eradicated LSCs and suppressed disease progression. Combination therapy using PD-1 blockade and chemotherapy substantially extended the survival of mice engrafted with mouse or human T-ALL cells. Mechanistically, PD-1+ LSCs had high NOTCH1-MYC activity for disease initiation. Furthermore, PD-1 signalling maintained quiescence and protected LSCs against T cell receptor-signal-induced apoptosis. Overall, our data highlight the hierarchy of leukaemia by identifying PD-1+ LSCs and provide a therapeutic approach for the elimination of LSCs through PD-1 blockade in T-ALL.

Overcoming Wnt-ß-catenin dependent anticancer therapy resistance in leukaemia stem cells.

Leukaemia stem cells (LSCs) underlie cancer therapy resistance but targeting these cells remains difficult. The Wnt-ß-catenin and PI3K-Akt pathways cooperate to promote tumorigenesis and resistance to therapy. In a mouse model in which both pathways are activated in stem and progenitor cells, LSCs expanded under chemotherapy-induced stress. Since Akt can activate ß-catenin, inhibiting this interaction might target therapy-resistant LSCs. High-throughput screening identified doxorubicin (DXR) as an inhibitor of the Akt-ß-catenin interaction at low doses. Here we repurposed DXR as a targeted inhibitor rather than a broadly cytotoxic chemotherapy. Targeted DXR reduced Akt-activated ß-catenin levels in chemoresistant LSCs and reduced LSC tumorigenic activity. Mechanistically, ß-catenin binds multiple immune-checkpoint gene loci, and targeted DXR treatment inhibited expression of multiple immune checkpoints specifically in LSCs, including PD-L1, TIM3 and CD24. Overall, LSCs exhibit distinct properties of immune resistance that are reduced by inhibiting Akt-activated ß-catenin. These findings suggest a strategy for overcoming cancer therapy resistance and immune escape.