Decoding PilQ: From pathogenicity to vaccine potential against fish Edwardsiellosis

Edwardsiella piscicida is an intracellular bacterium causing fish Edwardsiellosis. PilQ is the structural protein of type IV pili and crucial for bacterial virulence. This research aimed to investigate the pathogenic roles and immune protective effects of E. piscicida PilQ. PilQ contains a secretin-N domain and a secretin domain, classified as a member of the secretin protein family, and was located on the external membrane of E. piscicida. The PilQ mutation (TXD1ΔPilQ) significantly reduced bacterial survival under acidic and oxidative conditions, and the motility and biofilm formation of bacteria. Moreover, compared to wild E. piscicida, the PilQ mutation markedly decreased the adhesive and invasive capacity of E. piscicida to peripheral blood leukocytes (PBLs); meanwhile, the TXD1ΔPilQ could boost the reactive oxygen species (ROS) production of PBLs. On the other hand, antibody-mediated PilQ blocking yielded consistent phenotypes with PilQ gene deletion; moreover, the block of PilQ conspicuously suppressed the E. piscicida reproduction in PBLs. Additionally, we observe that recombinant PilQ (rPilQ) could bind to PBLs and activate the PBLs' ROS and acid phosphatase (ACP), and the vaccinated rPilQ could further activate head kidney macrophages (HKMs) vitality, heighten expression levels of immune genes, and boost the fish serum's antibacterial properties via enhancing the production of IgM, resulting in that rPilQ acted as a subunit vaccine against E. piscicida infection. The results provided the first evidence that PilQ is crucial to the E. piscicida virulence and acts as a potential candidate for developing subunit vaccines in teleost.