Purpose: Mouse challenge experiments were conducted with murinized Lm strains to determine if VA Lm strains are capable of (i) eliciting a Lm-specific memory CD8+T cell response, and (ii) providing protection against a subsequent challenge with a fully-virulent strain.
Methods: Mice were orally administered doses of a VA Lm strain, a fully-virulent Lm strain, or L. innocua as a “vaccination,” or a PBS placebo (“unvaccinated”). Forty days post-inoculation animals were either (i) sacrificed and CD8+ T cells were isolated from spleens and examined for INF-g production and memory phenotyping by flow cytometry using anti-CD44 and -CD62L antibodies, or (ii) challenged with a fully-virulent strain at 107to probe protective immunity through exposure to VA strains as measured by bacterial counts from livers, spleens, and small intestines.
Results: Memory cell phenotype and CD8+ T cell INF-g frequency (~1.6%) were similar for VA and fully-virulent strains at 107. Mice vaccinated with the VA strain at 105 CFU showed reduced bacterial loads in internal organs, indicating reduced severity of infections in mice exposed to VA strains. Specifically, in unvaccinated animals 2.52 and 1.63 mean log CFU of the challenge strain was recovered in livers and small intestines, respectively, as compared to 1.94 and 1.16 mean log CFU in organs for vaccinated animals. Interestingly, mice vaccinated with the fully-virulent strain at 103CFU did not show a protective effect, as log CFU in organs were similar in vaccinated vs. unvaccinated groups.
Significance: Data support that VA Lm strains commonly in food elicit a memory CD8+ T cell population leading to long-lived protection. Data on VA Lm immune responses will be communicated to food safety policy makers, as they correlate to levels of predicted human exposure through consumption of contaminated foods.