Nematode Serine Protease Inhibitor SPI-I8 Negatively Regulates Host NF-κB Signalling by Hijacking MKRN1-mediated Polyubiquitination of RACK1

Parasitic roundworms are remarkable for their ability to manipulate host immune systems and ameliorate inflammatory diseases. Although much is known about the nature of nematode effectors in immune modulation, little is known about the action mode of these molecules. Here, we report that a serine protease inhibitor SPI-I8 in the extracellular vesicles of blood-feeding nematodes like Ancylostoma ceylanicum, Haemonchus contortus and Nippostrongylus brasiliensis, effectively halts excessive inflammatory responses in vitro and in vivo. We demonstrate that H. contortus SPI-I8 promotes the role of a negative regulator of RACK1 and enhances the effects of RACK1 on tumor necrosis factor (TNF)-α-IκB kinases (IKKs)-nuclear factor kappa beta (NF-κB) axis in mammalian cells, by hijacking E3 ubiquitin protein ligase MKRN1-mediated polyubiquitination of RACK1. Administration of recombinant N. brasiliensis SPI-I8 effectively protects mice from dextran sulfate sodium (DSS)-induced colitis and lipopolysaccharide (LPS)-induced sepsis. Considering the structural and functional conservation of SPI-I8s among Strongylida nematodes and the conservation of interactive mediators (i.e., MKRN1 and RACK1) among mammals, our findings provide insights into the host-parasite interface where parasitic roundworms secret molecules to suppress host inflammatory responses. Harnessing these findings should underpin the exploitation of nematode's immunomodulators to relief excessive inflammation associated diseases in animals and humans.