Dysregulated haemolysin promotes bacterial outer membrane vesicles-induced pyroptotic-like cell death in zebrafish.

Inflammasomes are important innate immune components in mammals. However, the bacterial factors modulating inflammasome activation in fish, and the mechanisms by which they alter fish immune defences, remain to be investigated. In this work, a mutant of the fish pathogen Edwardsiella piscicida (E. piscicida), called 0909I, was shown to overexpress haemolysin, which could induce a robust pyroptotic-like cell death dependent on caspase-5-like activity during infection in fish nonphagocyte cells. E. piscicida haemolysin was found to mainly associate with bacterial outer membrane vesicles (OMVs), which were internalised into the fish cells via a dynamin-dependent endocytosis and induced pyroptotic-like cell death. Importantly, bacterial immersion infection of both larvae and adult zebrafish suggested that dysregulated expression of haemolysin alerts the innate immune system and induces intestinal inflammation to restrict bacterial colonisation in vivo. Taken together, these results suggest a critical role of zebrafish innate immunity in monitoring invaded pathogens via detecting the bacterial haemolysin-associated OMVs and initiating pyroptotic-like cell death. These new additions to the understanding of haemolysin-mediated pathogenesis in vivo provide evidence for the existence of noncanonical inflammasome signalling in lower vertebrates.