The cytokine interferon-gamma (IFN gamma) is a central coordinator of innate and adaptive immunity, but its highly pleiotropic actions have diminished its prospects for use as an immunotherapeutic agent. Here, we took a structure-based approach to decoupling IFN. pleiotropy. We engineered an affinity-enhanced variant of the ligand-binding chain of the IFN gamma receptor IFN gamma R1, which enabled us to determine the crystal structure of the complete hexameric (2:2:2) IFN gamma-IFN gamma R1-IFN gamma R2 signalling complex at 3.25 A resolution. The structure reveals the mechanism underlying deficits in IFN. responsiveness in mycobacterial disease syndrome resulting from a T168N mutation in IFN gamma R2, which impairs assembly of the full signalling complex. The topology of the hexameric complex offers a blueprint for engineering IFN gamma variants to tune IFN gamma receptor signalling output. Unexpectedly, we found that several partial IFN gamma agonists exhibited biased gene-expression profiles. These biased agonists retained the ability to induce upregulation of major histocompatibility complex class I antigen expression, but exhibited impaired induction of programmed death-ligand 1 expression in a wide range of human cancer cell lines, offering a route to decoupling immunostimulatory and immunosuppressive functions of IFN gamma for therapeutic applications.