Human XIRP1 is a new podosome protein targeting cytosolic bacteria as part of the IFN-γ defense program

Interferon-gamma (IFN-γ) is a powerful transactivating signal eliciting hundreds of IFN-stimulated genes (ISGs) in humans to help combat infection. Most ISGs remain uncharacterized, and here we searched for actin-binding candidates that could potentially target intracellular pathogens to block their spread or promote immune cell migration into infected tissues. Dual RNA-Seq and in silico mining across 1,933 data sets discovered >225 actin-related genes; the most highly expressed was XIRP1 (xin actin binding repeat containing 1 protein), a new ISG with no reported immune function. We found XIRP1 induction required IFN-γ plus IL-1β or exposure to pathogenic Listeria, Shigella, or Salmonella in immune and non-immune cells. Within IFN-γ-activated human macrophages, the XIRP1 protein localized to actin-rich podosomes where it formed a dome-shaped cap facing the cytosol; genetic XIRP1 ablation led to significant actin loss from these structures. Within infected cells, XIRP1 was recruited onto cytosolic Listeria monocytogenes in an ActA-dependent manner. Live imaging found many listeriae were fully encapsulated by XIRP1 whereas incomplete XIRP1 coating allowed pathogen escape from the initial coat structure. Together, our results identify XIRP1 as a new podosome-associated ISG that targets cytosolic bacteria as part of the IFN-γ-induced defense program in humans.