Recombinant TadZ from the type IVc pilus system induces protective immunity against virulent Aeromonas hydrophila in channel catfish (Ictalurus punctatus)

Aeromonas hydrophila is a Gram-negative pathogen responsible for motile Aeromonas septicemia (MAS) in fish, particularly in catfish aquaculture. The emergence of virulent A. hydrophila (vAh) has intensified the need to identify promising immunogenic targets for disease control. In this study, we aimed to evaluate the immunogenic potential and in vivo relevance of recombinant TadZ, a conserved cytoplasmic pilus assembly protein essential for the tight adherence (tad) pilus system in virulent A. hydrophila . We generated a schematic model of the Tad pilus machinery and mapped operon organization across diverse bacteria, revealing its conserved architecture. Fish were immunized intraperitoneally with the 44 kDa TadZ protein purified by Ni-NTA chromatography and emulsified with adjuvant, and they were subsequently challenged with an LD 80 dose of vAh. After 7 days post-challenge, TadZ immunization significantly reduced mortality compared to the non-immunized (NI) group, with mortality decreasing from 79.6% in the NI group to 26.1% in the TadZ-immunized group, corresponding to a relative percent survival (RPS) of 67.2%. Bacterial load analysis demonstrated a significant reduction in the liver of TadZ-vaccinated fish. Serum antibody analysis revealed significantly elevated IgM titers against both recombinant TadZ and heat-killed wild-type antigens after immunization and challenge, indicating robust humoral immune activation. These findings suggest that TadZ immunization induces antigen-specific antibody responses that may contribute to protection against vAh infection. Together, these findings identify TadZ as an immunogenic component of the Tad pilus system with in vivo relevance and support its further evaluation as a promising antigen for future vaccine development in catfish.