Interleukin-10 (IL-10) is a central anti-inflammatory cytokine that suppresses immune responses in myeloid cells but can also activate T cells, limiting its therapeutic use in chronic inflammatory diseases owing to narrow efficacy windows and dose-limiting toxicities. Here we show that myeloid and T cells differ in their dependence on JAK-family kinase pairing for IL-10 signalling and function. We took advantage of this differential kinase dependence to engineer a tetravalent nanobody-based myeloid-selective IL-10 receptor agonist (msIL-10R agonist) that fully recapitulates IL-10’s potent suppression of monocytes, macrophages and dendritic cells while avoiding stimulation of T cells. Cryo-electron microscopy structural analysis reveals a distinct receptor cross-linking geometry of the msIL-10R agonist compared with native IL-10 family cytokines. This tetravalent architecture renders signalling strictly TYK2-dependent and highly sensitive to IL-10Rα and IL-10Rβ receptor abundance, thereby conferring selective activity in myeloid cells. This msIL-10R agonist reduces inflammation in humanized mouse models of lipopolysaccharide challenge, delayed-type hypersensitivity and colitis. A humanized version exhibits IgG-like pharmacokinetics, sustained receptor occupancy and durable myeloid suppression up to 30 days without T cell activation or haematological toxicities in non-human primates. These findings establish a geometry-based strategy for achieving cell-type-selective cytokine signalling and identify the msIL-10R agonist as a therapeutic candidate for myeloid-driven inflammatory diseases, including inflammatory bowel diseases.
