The efficacy evaluation of mutant strain ΔpilV-AH as a live attenuated vaccine in Leiocassis longirostris.

Aquaculture and human health are seriously threatened by the opportunistic pathogen Aeromonas hydrophila (A. hydrophila), creating a desperate need for effective and environmentally friendly solutions. Based on initial lab research, an A. hydrophila pilV deletion mutant strain (ΔpilV-AH) with significantly lower virulence and stable inheritance was created. This mutant strain was examined in Leiocassis longirostris (L. longirostris) as a live attenuated vaccine (LAV) candidate. Experiments on safety evaluation demonstrated that L. longirostris could safely be exposed to ΔpilV-AH administered at a dose of 1 × 106 CFU/mL. Vaccination elicited a rapid innate response, reflected by significant early increases in humoral enzyme activities (lysozyme, alkaline phosphatase, acid phosphatase) and superoxide dismutase antioxidant activity within 12 to 48 h. Transcription candidate was evaluated analyses of gill, spleen and head-kidney during the first 144 h post-vaccination revealed early upregulation of pattern recognition receptor (toll-like receptor 4, TLR4) and gene implicated in antigen processing and presentation (major histocompatibility complex class I, MHC-I). Head-kidney transcriptomics at day 21 corroborated activation of antigen-presentation pathways and mitogen-activated protein kinase (MAPK) signaling cascades. Mechanistically, these responses are consistent with local phagocytic uptake and recognition of vaccine antigen, intracellular signaling amplification, and subsequent antigen transport to central immune organs where presentation to lymphocytes promotes B-cell activation and differentiation into antibody-secreting cells. Functionally, the relative percent survival (RPS) of L. longirostris in the ΔpilV-AH group was up to 70.83%, which is significantly higher than that in the inactivated vaccine group (45.83%, P < 0.05). In conclusion, these findings demonstrate that ΔpilV-AH is capable of eliciting protective immunity in L. longirostris and provides a promising candidate for LAV against A. hydrophila.