Live attenuated hfq-deleted Acinetobacter baumannii vaccine provides protection against systemic infection.

Acinetobacter baumannii is a multidrug-resistant (MDR) nosocomial pathogen with high mortality rates, necessitating alternative preventive strategies. The RNA chaperone Hfq plays a key role in bacterial virulence, and its deletion attenuates pathogenicity. In this study, we evaluated an hfq deletion mutant (Δhfq) of A. baumannii ATCC 17978 as a live-attenuated vaccine candidate. Mice vaccinated intraperitoneally with 2.3 × 107 CFU of Δhfq, followed by a booster on day 28, exhibited 90 % survival after an intraperitoneal lethal challenge with MDR A. baumannii AB5075, whereas all unvaccinated controls succumbed. Vaccinated mice showed a significant reduction in bacterial burden in key organs (p < 0.001), sustained IgG levels, and an enhanced cytokine response with early IFN-γ and TNF-α activation, followed by a controlled IL-10 response. Serum pentraxin-3 (PTX3) levels were significantly lower in vaccinated mice post-challenge, correlating with reduced systemic inflammation and sepsis risk. Histopathological analysis revealed intact organ architecture in vaccinated mice, while unvaccinated mice displayed severe pathology. Functional immunogenicity assays demonstrated that serum from vaccinated mice enhanced neutrophil-mediated bacterial killing, and passive transfer of heat-inactivated serum conferred complete protection, confirming antibody-mediated immunity independent of complement activation. These findings highlight Δhfq as a promising live-attenuated vaccine that induces strong humoral and cellular immunity, preventing bacterial dissemination and lethal infection. Further studies are needed to elucidate its protective mechanisms and clinical potential.