The immunologic and stromal functions of nitric oxide in fibrotic tumor microenvironments: A mini-review

• NO promotes or suppresses immunity depending on tumor context. • PDAC: NO expands MDSCs and elevates PD-L1, impairing T-cell activity. • Desmoplastic Melanoma: Chronic NO may induce PD-L1 via JNK signaling. • iNOS inhibition reduces PD-L1 and reprograms TNBC immune profiles. • eNOS activity may protect against early breast cancer metastasis. Tumors characterized by a prominent desmoplastic stroma –including pancreatic ductal adenocarcinoma, the desmoplastic melanoma subtype, and a subset of triple-negative breast cancer feature a dense, collagen-rich stroma that impairs drug penetration, skews myeloid cellular function, and either excludes, permits, or exhausts effective lymphocyte function. Nitric oxide sits at the center of this microenvironmental interchange. To delineate nitric oxide’s dual function, this study surveyed mechanistic and translational studies on NO signaling in fibrotic tumor microenvironments indexed in PubMed and Web of Science through 2025. In pancreatic ductal adenocarcinoma, chronic inducible nitric oxide synthase activity within cancer-associated fibroblasts, tumor cells, and myeloid-derived suppressor cells stabilizes HIF-1α, drives PD-L1 expression, and reinforces a self-perpetuating loop of T-cell dysfunction. In desmoplastic melanoma, transient bursts of nitric oxide from M1-polarized macrophages augment antigen presentation and correlate with ∼70% objective response rates to PD-1 blockade. However, sustained nitric oxide flux may converge on JNK and PI3K/Akt dependent PD-L1 upregulation, fostering adaptive resistance. In triple negative breast cancer, Roughly 34% develop a fibrotic stroma where inducible nitric oxide synthase overexpression predicts poor survival. This poor survival is reflected in the highly fibrotic, immune-excluded milieu of TNBC with high TGF-β activity and HIF-1α activity. Like PDAC, sustained nitric oxide flux further stabilizes HIF-1α, amplifying hypoxia responsive gene programs and reinforcing stromal fibrosis. Collectively, these findings reveal a concentration, isoform, and context-specific spectrum of nitric oxide activity: pathologic high output inducible nitric oxide synthase-derived flux promotes immunosuppression and metastasis, whereas basal or controlled nitric oxide levels supports vascular integrity and antitumor immunity. Therapeutically, a multifaceted approach combining inducible nitric oxide inhibition, calibrated nitric oxide donors, myeloid-derived suppressor cell inhibition, and tumor associated macrophage repolarization with immune checkpoint inhibitors offers a precision framework to dismantle fibrotic stromal barriers and convert immune-cold desmoplastic cancers into responsive disease.