Immunogenicity of an Oil-in-Water Emulsion Containing Hafnia Alvei-Derived Lipopolysaccharide, with TLR4 and Dectin-2 Agonist Activity In Vitro

Background: Lipopolysaccharide (LPS) functions as a Toll-like receptor 4 (TLR4) agonist that triggers innate immunity; however, structural variations between pathogenic and commensal bacteria distinctly influence its immunostimulatory profile. This study evaluated the immunostimulatory activity of LPS derived from the commensal bacterium Hafnia alvei and explored its potential as an exploratory vaccine adjuvant. Methods: Cytokine induction was evaluated in immune cells across diverse host species, and receptor activation was assessed via reporter assays. To investigate in vivo immunogenicity and preliminary tolerability, H. alvei LPS was formulated into a prototype oil-in-water (O/W) emulsion utilizing ovalbumin (OVA) as a model antigen. Results: LPS from H. alvei strain BA2000346 exhibited immunostimulatory activity comparable to that of Escherichia coli, while inducing greater TNF-α expression than pathogenic Salmonella and Pseudomonas strains. Distinct from E. coli LPS, it demonstrated the capacity to activate both TLR4 and the mannose-recognizing Dectin-2 receptor in reporter systems. This cytokine induction was consistent across various strains and host species. Furthermore, the prototype O/W emulsion formulation enhanced antigen-specific humoral and cellular immune responses while demonstrating preliminary tolerability based on body-weight monitoring and visual clinical observation. Conclusions: H. alvei-derived LPS exhibits TLR4 and Dectin-2 agonist activity in vitro. When synergized with an O/W emulsion delivery system, it provides a preliminary indication of cross-species stimulatory potential and supports further investigation as a hypothesis-generating platform for future vaccine adjuvant development.