| CXCL9 (C‑X‑C motif chemokine ligand 9), also known as monokine induced by gamma interferon (MIG), is a small secreted CXC chemokine induced predominantly by IFN‑γ that binds the G‑protein–coupled receptor CXCR3 to direct the migration, activation and differentiation of T lymphocytes, NK cells and other immune effectors in type 1‑polarized inflammatory responses. The protein is synthesized as a basic 125‑residue chemokine with the characteristic CXC motif near the N‑terminus and an extended, positively charged C‑terminal tail that contributes to direct antimicrobial activity and to interactions with heparan sulphate proteoglycans on endothelial and stromal surfaces, creating immobilized gradients that guide chemotaxis. Mechanistically, CXCL9 is part of the IFN‑γ–inducible CXCR3 ligand triad (CXCL9, CXCL10, CXCL11) and activates CXCR3 on Th1 cells, CD8⁺ memory T cells, NK cells, NKT cells, activated B cells and monocytes, triggering Gαi‑dependent signaling, calcium release and downstream pathways that promote integrin activation, endothelial adhesion, diapedesis and tissue infiltration, thereby reinforcing Th1 responses and antagonizing IL‑4/Th2‑associated chemokines such as eotaxin to limit Th2 cell recruitment. CXCL9 has anti‑angiogenic and tumor‑modulatory properties: it inhibits endothelial cell proliferation through CXCR3 expressed at the S/G2‑M phase of the endothelial cell cycle and suppresses colony formation from hematopoietic progenitors, and both CXCL9 and CXCL10 have been shown to inhibit neovascularization and non‑small cell lung carcinoma growth and metastasis in vivo, linking IFN‑γ–CXCL9 axes to IL‑12‑mediated anti‑tumor angiogenesis. Tumor models with enforced CXCL9 expression exhibit slower primary growth, reduced lung metastases and prolonged survival, with NK cells crucial for metastasis limitation and T‑cell responses more important for local control, supporting a role for CXCL9 in shaping anti‑tumor immunity and tumour microenvironment composition. At the same time, context‑dependent autocrine CXCL9/CXCR3 signaling in cancer cells can enhance proliferation, angiogenesis and metastasis, and clinical studies show that CXCL9 levels may correlate with favourable prognosis in some cancers (e.g. ER‑negative breast and uterine endometrioid carcinoma) while contributing to immune‑driven pathology or severe humoral responses in others, such as chronic respiratory disease and severe COVID‑19, reflecting dual tumour‑suppressive and tumour‑promoting potential depending on cell type and receptor expression patterns. In CNS and viral infections, CXCL9 produced by astrocytes and microglia participates in T‑cell‑mediated immunity, and knockout studies indicate that CXCL9 is required, together with CXCL10, for efficient recruitment of NK and virus‑specific cytotoxic T cells and for clearance of neurotropic and hepatotropic viruses, with CXCL10 more important in acute phases and CXCL9 in subacute or chronic phases. |
