Subcutaneous Immunization with Recombinant Lactococcus lactis Expressing F1S1 Fusion Protein Induces Systemic and Mucosal Immune Responses in BALB/C Mice

Background:Lactic acid bacteria such as Lactococcus (L.) lactis are powerful tools that can function as live delivery vectors and heterologous protein expression hosts in development of novel vaccines. Pertussis toxin (PT) and filamentous hemagglutinin (FHA) are important virulence factors of Bordetella (B.) pertussis and constitute the major components of commercially available acellular pertussis (aP) vaccines. The purpose of the present study was to express F1S1 fusion protein, consisted of the N-terminal region of Si subunit from PT and FHA type 1 immunodominant domain by L. lactis and to evaluate its immunogenicity. Methods: The fusion gene composed of sequences encoding the F1s1 and the signal peptide of usp45 fragments (SECF1S1) was codon optimized for protein production in L. lactis and was synthesized and inserted in-frame inside pNZ8149 plasmid. The resulting pNZ8149-SECF1S1 construct was introduced by electroporation into L. lactis cells (LL-F1S1). BALB/c mice were subcutaneously immunized with LL-F1S1 or commercial DTaP vaccine. The immune responses were investigated. Results: The LL-F1S1-immunized mice produced significant levels of specific IFN-g compared to their respective controls and DTaP-immunized mice. The F1S1- specific IgG antibody response was lower in LL-F1S1-immunized mice while the IgG2a/IgG1 ratio was higher in this group compared to the DTaP-immunized mice. Moreover, anti-F1S1 IgA antibodies were only detected in the lung homogenates of the LL-F1S1immunized mice, suggesting the induction of a mucosal immune response. Conclusions: These results indicate the feasibility of expression of F1S1 fusion protein in L. lactis. This recombinant bacterium could induce mucosal and Th1-type systemic immune responses following subcutaneous administration.