SPP1 + Macrophage‐ POSTN + Fibroblast‐Endothelial Triad Dictates Immunotherapy Response in Bladder Cancer

ABSTRACT Although immunotherapy has shown promise in improving outcomes for bladder cancer (BCa) patients, treatment responses remain highly variable. A comparative examination of the tumor microenvironment (TME) between responders and non‐responders may reveal key resistance mechanisms and identify potential therapeutic targets. We integrated spatial transcriptomics, single‐cell RNA sequencing, and multiplexed immunofluorescence to characterize spatial structures within the TME that influence response to anti‐PD‐1 therapy in BCa patients. In non‐responders, we observed an accumulation of stem‐like malignant epithelial cells with high MYBL2 expression near the tumor boundary. Furthermore, we identified a spatial triad structure—composed of SPP1 + tumor‐associated macrophages (TAMs), POSTN + cancer‐associated fibroblasts (CAFs), and endothelial cells—located at the tumor periphery. This structure was associated with T‐cell exclusion and reduced efficacy of immune checkpoint blockade. In a preclinical model, inhibiting SPP1 enhanced the response to anti‐PD‐1 therapy, resulting in reduced CAF infiltration and increased recruitment of cytotoxic T cells. Our study reveals a triad cellular structure mediated by SPP1 + TAMs, POSTN + CAFs, and endothelial cells that contribute to immunotherapy resistance in BCa. Targeting this structure, particularly through SPP1 blockade, represents a promising strategy to augment the efficacy of immune checkpoint inhibitors.