Cell surface IL-1α trafficking is specifically inhibited by interferon-γ, and associates with the membrane via IL-1R2 and GPI anchors

IL-1 is a powerful cytokine that drives inflammation and modulates adaptive immunity. Both IL-1 alpha and IL-1 beta are translated as proforms that require cleavage for full cytokine activity and release, while IL-1 alpha is reported to occur as an alternative plasma membrane-associated form on many cell types. However, the existence of cell surface IL-1 alpha (csIL-1 alpha) is contested, how IL-1 alpha tethers to the membrane is unknown, and signaling pathways controlling trafficking are not specified. Using a robust and fully validated system, we show that macrophages present bona fide csIL-1 alpha after ligation of TLRs. Pro-IL-1 alpha tethers to the plasma membrane in part through IL-1R2 or via association with a glycosylphosphatidylinositol-anchored protein, and can be cleaved, activated, and released by proteases. csIL-1 alpha requires de novo protein synthesis and its trafficking to the plasma membrane is exquisitely sensitive to inhibition by IFN-gamma, independent of expression level. We also reveal how prior csIL-1 alpha detection could occur through inadvertent cell permeabilisation, and that senescent cells do not drive the senescent-associated secretory phenotype via csIL-1 alpha, but rather via soluble IL-1 alpha. We believe these data are important for determining the local or systemic context in which IL-1 alpha can contribute to disease and/or physiological processes.