The phosphate exporter XPR1 regulates a gasdermin D-independent mature IL-1β secretion pathway in LPS-stimulated human monocytic cells

The inflammatory cytokine interleukin (IL)-1{beta} is a leaderless protein that is not secreted via the classical endoplasmic reticulum-Golgi pathway but instead is secreted during pyroptosis, a form of caspase-dependent inflammatory cell death mediated by gasdermin D (GSDMD) cleavage and pore formation at the plasma membrane. However, human monocytes can secrete IL-1{beta} in the absence of cell death, and the contribution of GSDMD in this secretory pathway is not established. Here, we identify two mechanisms of mature IL-1{beta} secretion by living human monocytic cells: a rapid, GSDMD-dependent pathway and a slower, GSDMD-independent pathway. Using a CRISPR-Cas9 loss-of-function screen, we identified XPR1 (Xenotropic and Polytropic retrovirus Receptor 1) as a key regulator of the GSDMD-independent pathway. XPR1 is the only phosphate exporter identified in metazoans and has no previously described function in cytokine secretion. XPR1 invalidation in GSDMD-/- monocytic cells impaired IL-1{beta} secretion. We further show that this regulatory function requires cell-surface expression of XPR1 and is linked to its phosphate export activity. Our results reveal a previously undescribed mechanism of IL-1{beta} secretion by living human monocytic cells, independent of GSDMD and unexpectedly linked to phosphate homeostasis. Deciphering this pathway could lead to new therapeutic modalities for IL-1{beta}-driven inflammatory diseases.