Identification of SKI-II as a host-protective immunomodulator against Staphylococcus aureus infection

Antibiotic resistance threatens the effectiveness of conventional antimicrobial agents, underscoring the need for host-directed therapies that enhance immune defense. Here, we present a cross-species discovery pipeline that couples a Caenorhabditis elegans-Staphylococcus aureus liquid-based infection screen with mammalian mechanistic validation to identify small-molecule immunomodulators. Using six C. elegans innate immunity reporter strains, we identified a known small molecule, SKI-II, as a previously unrecognized host-protective compound that activates the C. elegans SKN-1/Nrf2 oxidative-stress pathway. In mouse RAW264.7 macrophages, SKI-II binds to the ATPase pocket of VCP (Valosin-Containing Protein), activating the PERK-dependent (Protein kinase R [PKR]-like Endoplasmic Reticulum Kinase) Nrf2 signaling regulation axis that reduces the levels of pathogenic ROS (reactive oxygen species). SKI-II treatment also promotes macrophage M1 polarization and a mitochondrial metabolic shift. This work identifies VCP as a druggable node for host-directed immunomodulation and highlights SKI-II as a prototype small molecule that boosts host tolerance to infection, thereby validating our C. elegans-based screening platform for discovering immunomodulators active in mammalian systems.IMPORTANCEEnhancing host immunity is a promising strategy to combat S. aureus infection, particularly multidrug-resistant strains. In this study, we applied a C. elegans liquid-based infection screening model to identify the immunomodulatory compound SKI-II. We demonstrate that SKI-II protects against S. aureus infection in both nematodes and mouse macrophages by regulating host oxidative stress pathways rather than directly targeting the pathogen. We reveal a regulatory circuit in which VCP functions as a central node controlling cellular ROS responses and activating the downstream PERK-dependent Nrf2 antioxidant signaling pathway. These findings advance our understanding of host cellular responses to bacterial infection and highlight VCP as a druggable target for host-directed therapeutic strategies against infectious diseases.