Pathogen-Mimicking Nanovaccine Induces Potent Humoral and Cellular Immunity against <i>Brucella</i>

Brucellosis is a widespread zoonotic disease caused by Brucella, which is a facultative intracellular pathogen. Brucellosis poses a significant challenge to vaccine development due to the ability of Brucella to evade innate immunity, primarily through its atypical and low-toxicity lipopolysaccharide (LPS). To improve the suboptimal immunogenicity of subunit vaccines, a pathogen-mimicking nanovaccine was engineered to deliver the antigen and monophosphoryl lipid A (MPLA, a detoxified LPS analogue) in the present study. A fusion protein (SO) of the outer membrane protein 19 (Omp19) and Cu/Zn superoxide dismutase (SOD) acted as the antigen. SO was conjugated with an octaarginine peptide to absorb anionic MPLA (a TLR4 agonist), followed by self-assembly into uniform nanoparticles by electrostatic and hydrophobic interactions. The resultant vaccine (SOMs) enhanced the uptake by APCs, promoted the maturation of dendritic cells in vitro, and effectively activated the TLR4 signaling pathway. Immunization with SOMs induced a robust and balanced immune response in BALB/c mice, as reflected by substantially elevated antigen-specific IgG1 and IgG2a antibody titers and potent cellular immunity. The vaccine protected BALB/c mice against Brucella melitensis strain M5, along with no apparent systemic toxicity. This work validates a pathogen-mimicking vaccine strategy that counteracts the immune stealth of Brucella by replenishing critical TLR4 signaling. This strategy is promising for developing effective vaccines against intracellular pathogens.