Background <p>Cancer-associated fibroblasts (CAFs) are key regulators in tumor microenvironment and tumor immunity, partly through MHC-II expression that modulates T-cell differentiation. However, the upstream cytokine signals controlling MHC-II expression in fibroblasts still remain poorly defined.</p> Methods <p>We examined MHC-II expression on fibroblasts under stimulation with interferon-γ (IFN-γ) and interleukin-1β (IL-1β) by using flow cytometry, transcriptomic analysis, and qRT-PCR. To dissect transcriptional regulation, we generated CIITA-overexpressing and CIITA-deficient fibroblast lines by lentiviral transduction and CRISPR/Cas9-mediated editing. Public scRNA-seq, ATAC-seq, and ChIP-seq datasets were further analyzed to validate molecular mechanisms.</p> Results <p>IFN-γ robustly up-regulated MHC-II expression on fibroblasts, while IL-1β selectively suppressed this induction without affecting PD-L1. Mechanistically, IL-1β attenuated IFN-γ–induced CIITA expression at the mRNA level but did not alter STAT1 abundance or phosphorylation. Functional assays confirmed that CIITA was indispensable for IFN-γ–driven MHC-II expression in fibroblasts. Integration of transcriptomic and epigenomic data demonstrated that CIITA directly bound MHC-II gene promoters and regulated chromatin accessibility.</p> Conclusions <p>Our study identifies an IFN-γ/STAT1/CIITA axis as the central regulator of MHC-II expression in fibroblasts and reveals IL-1β as a potent suppressor of this pathway. These findings highlight a novel cytokine-mediated regulatory mechanism underlying CAF-driven immunosuppression within the tumor microenvironment.</p>