Baram, N. | bioRxiv Science

bioRxiv2022

Post-phagocytosis activation of NLRP3 inflammasome by two novel T6SS effectors

The type VI secretion system (T6SS) is used by bacteria to deliver toxic effectors directly into target cells. Most T6SSs mediate antibacterial activities, whereas the potential anti-eukaryotic role of T6SS remains understudied. Here, we found a Vibrio T6SS that delivers two novel effectors into mammalian host immune cells. We showed that these effectors induce a pyroptotic cell death in a phagocytosis-dependent manner; we identified the NLRP3 inflammasome as being the underlying mechanism leading to the T6SS-induced pyroptosis. Moreover, we identified a compensatory T6SS-induced pathway that is activated upon inhibition of the canonical pyroptosis pathway. Genetic analyses revealed possible horizontal spread of this T6SS and its anti-eukaryotic effectors into emerging pathogens in the marine environment. Our findings reveal novel T6SS effectors that activate the host inflammasome and possibly contribute to virulence and to the emergence of bacterial pathogens.

immunology↗

bioRxiv2019

Vibrio pore-forming leukocidin activates pyroptotic cell death via the NLRP3 inflammasome

Cell death mechanisms are central to combat infectious microbes and to drive pathological inflammation. One such mechanism, the inflammasome, controls infection through either activation of caspase-1 and the subsequent secretion of the mature pro-inflammatory cytokine, interleukin 1{beta} (IL-1{beta}), or by stopping the dissemination of intracellular pathogens by inducing pyroptotic cell death in infected cells. Hemolysins, which are pore-forming toxins (PFTs), target the host cell plasma membrane by producing pores with different diameters. These pores alter the permeability of the target membrane, often leading to cell death. We previously discovered a functional and potent pore-forming, leukocidin domain-containing hemolysin produced by the Gram-negative marine bacterium Vibrio proteolyticus (V. proteolyticus), termed VPRH. Although leukocidin domains are found in other known PFTs, VPRH constitutes a distinct, understudied class within the leukocidin superfamily. Since PTFs of other pathogens were shown to induce cell death by activating the inflammasome pathway, we hypothesized that VPRH-induced cell death is mediated by direct activation of the inflammasome in mammalian immune host cells. Indeed, we found that VPRH induced a two-step cell death in primary macrophages. The first, a rapid step, was mediated by activating the NLRP3 inflammasome, leading to caspase-1 activation and GSDMD cleavage that resulted in IL-1{beta} secretion and pyroptotic cell death. The second step was independent of the inflammasome; however, its mechanism remains unknown. This study sets the foundation for better understanding the immunological consequences of inflammasome activation by a new leukocidin class of toxins.

immunology↗