A promising strategy to improve the stability and immunogenicity of killed but metabolically active vaccines: low-temperature preparation and coating of nanoparticles.

Bacteria are becoming an increasingly serious threat to human health. The emergence of super bacteria makes clinical treatment more difficult. Vaccines are one of the most effective means of preventing and treating bacterial infections. As a new class of vaccines, killed but metabolically active (KBMA) vaccines provide the immunogenicity of live vaccines and the safety of inactivated vaccines. Herein, a promising strategy is proposed to improve the stability and immunogenicity of KBMA vaccines. KBMA vaccines were produced at low temperature (4 °C), and the bacterial surface was engineered using mesoporous silica nanoparticle (MSN) coating. Compared to vaccines prepared at room temperature, the metabolic activity of KBMA vaccines prepared at 4 °C remarkably improved. Benefiting from the induction of MSNs, the stability of KBMA vaccines was increased and the preservation time was prolonged at 4 °C. Meanwhile, metabolomics analysis showed that the metabolite spectrum of live bacteria changed after photochemical treatment and MSN coating, which interfered with organic acid metabolism pathways, lipid metabolism and biosynthesis of secondary metabolites. Furthermore, the immune response in the mice treated with KBMA/MSN vaccines was similar to that in those treated with live vaccines and stronger than that in those treated with inactivated vaccines. In comparison with the control group, bacteria tissue burdens of KBMA/MSN group were significantly reduced. CD4 ⁺ T cells dominated immune responses for the protection of mice. Thus, the current work promotes the application of KBMA vaccines, providing an alternative choice for treating bacterial infections.