Naturally Derived Outer Membrane Vesicles confer Immunity to Salmonella typhimurium in a Murine Model

Background: Typhoid fever causes massive morbidity and mortality in developing countries, inflicting 21.7 million illnesses and 217,000 deaths yearly worldwide. The causative agent is Salmonella typhi, a flagellated Gram-negative bacterium that is spread through fecal-oral transmission. While current vaccines against the organism, such as the live attenuated strain Ty21a, protect travelers from disease, there is continued need for better protection of vulnerable populations in endemic countries. Outer membrane vesicles (OMVs) are an attractive vaccine platform because they are multi-antigenic, immunostimulatory, stable, and non-infectious.

Methods: OMVs were purified from Salmonella typhimurium (St) strain SL1433, characterized by LC-MS analysis, and evaluated for protective efficacy in a murine model of typhoid fever. Mice were immunized twice with 10 μg of OMVs subcutaneously (subQ), 21 days apart, and challenged 28 days later with 1 x 10⁵ CFU of St strain SL1433 by oral lavage. Control groups of mice were immunized subQ with saline only (sham) or with 1x 10⁷ CFU of heat killed St. Another group of mice was immunized orally with 1x 10⁵ CFU of live-attenuated St strain SL3261. Mice were monitored and weighed every 24 hours for 12 days.

Results: St OMVs measured 20-100 um in diameter and were shown to be free of contaminants by transmission electron microscopy. OMVs contained a number of SPI-1 proteins, such as SipA, and immunoreactive proteins, such as OmpF. OMV-immunized mice demonstrated an increase in survival (60%) compared to sham-immunized mice (10%) at day 12 post-exposure. There was no significant difference in survival of mice immunized with live attenuated St (80%) compared to those immunized with OMVs. Furthermore, mice immunized with OMVs, heat killed St, or live attenuated St demonstrated significantly less weight loss than sham-immunized mice.

Conclusion: St OMVs may represent a safe and effective vaccine platform for typhoid fever and other Salmonella infections.