Checkpoint blockade and the stem-like T cell trade-off.

Stem-like T cells are central to the efficacy of programmed cell death protein 1 (PD1) blockade, sustaining long-term immune responses by serving as a renewable reservoir for anti-tumor effector CD8+ T cells. However, the mechanisms governing their maintenance and regulation in cancer remain incompletely understood. Addressing this gap, Hor et al. combined high-dimensional 3D-imaging with immunological profiling to define the niche of stem-like T cells within tumor-draining lymph nodes in murine cancer models. Their study identifies a critical role for conventional type 1 dendritic cells (cDC1s) and the PD1 pathway in preserving high-affinity tumor-specific stem-like T cells. cDC1s deliver sustained T-cell receptor (TCR) stimulation together with PD-L1/2 co-inhibitory signals that support stemness, proliferation, and survival. Strikingly, disruption of PD1 signaling transiently enhances effector T cell expansion but promotes differentiation and apoptosis of stem-like T cells, ultimately depleting this essential pool. These findings reveal a potential long-term vulnerability of immune checkpoint blockade, particularly when tumors are not eradicated during the initial treatment response.