POSTN+ cancer-associated fibroblasts forming fortress-like barrier to mediate immunotherapy resistance in colorectal cancer.

Immune checkpoint blockade (ICB) therapy has demonstrated promising efficacy in patients with mismatch repair-deficient (dMMR) or microsatellite instability-high (MSI-H) colorectal cancer (CRC). However, a substantial proportion of these patients remain resistant to treatment. This study aims to investigate the mechanisms underlying ICB resistance in dMMR/MSI-H CRCs. We analyzed single-cell transcriptomic data from 34 CRC patients treated with ICB and integrated ten spatial transcriptomic datasets to compare the tumor microenvironment (TME) features between responders and non-responders. Our analysis identified a distinct subset of tumor-associated fibroblasts (CAFs), marked by elevated POSTN expression (POSTN + CAFs), which was significantly enriched in non-responders. These cells encapsulate tumor cell aggregates and contribute to extensive extracellular matrix (ECM) remodeling, forming a dense, fortress-like stromal architecture at tumor periphery. Notably, cytotoxic T cells including CXCL13+CD8+ T cells were enriched in the ECM surrounding POSTN + CAF regions but were largely excluded from the adjacent tumor core, suggesting that POSTN + CAFs function as a physical and biochemical barrier to T cell infiltration. Differentiation trajectory analyses revealed that POSTN + CAFs originate from precursor CAFs (PI16+ CAFs and SFRP2+ CAFs), potentially driven by malignant cell-derived cues. Specifically, we identified GDF15 as key tumor-cell ligands that may interact with precursor CAFs to promote POSTN + CAF differentiation. Our findings uncover a stromal mechanism of ICB resistance in dMMR/MSI-H CRC mediated by POSTN + CAFs and highlight novel therapeutic strategies to enhance immunotherapy efficacy.