Single-cell transcriptomics identifies key immune-suppressive cells and their driver genes in the bladder cancer microenvironment with prognostic implications.

The immunosuppressive tumor microenvironment (TME) serves as a central driver of bladder cancer (BCa) progression and prognosis. While its significance is widely acknowledged, the key cellular subsets that mediate this immunosuppressive state and their core regulatory genes remain incompletely understood. Key immunosuppressive cellular subsets and their signature genes were systematically identified using single-cell RNA sequencing (scRNA-seq) data from BCa samples. A prognostic risk model was then constructed via univariate and multivariate Cox regression analyses, based on bulk RNA-seq datasets and the identified signature genes. The role of SUSD2 was further validated in vitro using qRT-PCR, Western blot, immunofluorescence, proliferation, and invasion assays. Compared with adjacent normal tissues, BCa tissues showed significant enrichment of stromal cells (e.g., epithelial cells, fibroblasts). Among these stromal populations, the proportion of myofibroblast-like cancer-associated fibroblasts (myCAFs) was significantly increased in BCa tissues, and high myCAF infiltration was closely associated with poor patient prognosis. Pseudotime trajectory analysis confirmed that fibroblast differentiation in BCa shifts toward a terminal state (State 3), which is predominantly composed of myCAFs. A prognostic model established using myCAF-related signature genes (TMEM74B, ABCC9, FCMR, ALG9, SUSD2, and ETV7) exhibited stable predictive performance in both training and validation cohorts, with SUSD2 identified as a risk-related gene. In vitro experiments revealed that SUSD2 knockdown inhibited myCAF activation and extracellular matrix secretion, thereby attenuating its promotional effects on BCa cell proliferation and invasion. The TGF-β receptor inhibitor SB-431542could reverse the facilitative effects of SUSD2 overexpression on tumor cell proliferation and migration. Our findings identify myCAFs as a core regulatory cellular subset and SUSD2 as a key molecule within the immunosuppressive TME of BCa. Additionally, SUSD2 may trigger the activation of the TGF-β/Smad signaling cascade to induce myCAF activation, thereby accelerating BCa progression. These results provide novel potential targets and a theoretical basis for prognosis assessment and TME-targeted therapy in BCa.