Characterization of the protective immune response to Yersinia pseudotuberculosis infection in mice vaccinated with an LcrV-secreting strain of Lactococcus lactis.

Background: Pseudotuberculosis is an infection caused by the bacterial enteropathogen Yersinia pseudotuberculosis and is considered to be a significant problem in veterinary medicine. We previously found that intranasal administration of a recombinant Lactococcus lactis strain that secretes the low-calcium response V (LcrV) antigen from Y. pseudotuberculosis (Ll-LcrV) confers protection against a lethal Y. pseudotuberculosis infection. Here, we aimed at characterizing the immunological basis of this LcrV-elicited protective response and at determining the duration of vaccine-induced immunity.

Methods: Splenocytes from BALB/c mice intranasally immunized with Ll-LcrV or Ll as control were immunostained then analyzed by flow cytometry. Protection against a lethal intravenous injection of Y. pseudotuberculosis was also determined (i) in immunized BALB/c mice depleted or not of CD4, CD8 or CD25 cells and (ii) in naïve BALB/c mice receiving serum from immunized mice by counting the number of bacteria in liver and spleen. Lastly, survival rate of immunized BALB/c mice following a lethal intravenous injection of Y. pseudotuberculosis was followed up to 9-months.

Results: We found that T and B lymphocytes but not non-conventional lymphoid cells were affected by Ll-LcrV immunization. We also observed that depletion of CD4 and CD25 but not CD8 cells in immunized mice eradicated protection against a lethal systemic Y. pseudotuberculosis infection, suggesting that activated CD4 T lymphocytes are required for vaccine-induced protection. Adoptive transfer of LcrV-specific antibodies from Ll-LcrV-immunized animals significantly reduced the bacterial counts in the liver compared to non-vaccinated mice. Lastly, the protective immunity conferred by Ll-LcrV decreased slightly over time; nevertheless almost 60% of the mice survived a lethal bacterial challenge at 9months post-vaccination.

Conclusion: Mucosal vaccination of mice with Ll-LcrV induced cell- and antibody-mediated protective immunity against Y. pseudotuberculosis infection in the mouse and the protection is long-lasting.