Investigating the Effect of Monophosphoryl Lipid A as a Co-adjuvant on Humoral Immune Response against Staphylococcus Aureus Infection in a Mouse Model

Staphylococcus aureus is one of the leading causes of nosocomial infections, therefore, it is necessary to develop a suitable vaccine candidate to prevent infections caused by this bacterium. Autolysin protein as a virulence factor plays an important role in bacterial division. Monophosphoryl lipid A (MPLA) as a Toll-like receptor 4 agonist is currently used as an adjuvant. In this study, autolysin protein along with MPLA was investigated as a vaccine candidate.

Recombinant autolysin was expressed with IPTG and purified by Ni-NTA affinity chromatography. To increase the immunogenicity of vaccine candidates, Monophosphoryl lipid A (biologic and synthetic) was formulated in combination with Alum in four groups of Balb/c mice. Animals were injected subcutaneously three times at intervals of two weeks. Total IgG and IgG1, IgG2a isotype antibodies were measured in sera by indirect ELISA technique. Then, experimental mice were challenged with a sub-lethal dose of Staphylococcus Strain (5×108 CFU) and following that, the number of bacteria from internal organs was assessed. Also, the survival rate was monitored daily for 30 days.

Total IgG, IgG1, and IgG2a isotype antibody levels were significantly improved in vaccinated groups in comparison to the control group. Bacterial burden in the internal organ (Liver, spleen, and kidney), and animal mortality of the vaccinated group especially r-Autolysin+Alum+MPLA Synthetic and r-Autolysin+Alum+MPLA biologic were decreased in comparison to the control group.

We concluded that synthetic MPLA is a reliable candidate for immune response improvement against Staphylococcal infection.