Age-dependent systemic antibody responses and immunisation-associated changes in mice orally and nasally immunised with Lactococcus lactis expressing a malaria parasite protein.

Gram positive food-grade bacteria such as lactococci have significant advantages over attenuated pathogens as vaccine delivery vehicles because of their inherently greater safety. Plasmodium falciparum merozoite surface antigen 2 (MSA2) was expressed in recombinant Lactococcus lactis both intracellularly and covalently anchored to the peptidoglycan of the cell wall (MSA2cP). Balb/c mice of different ages were immunised with the MSA2cP expressing L. lactis in a combined oral and nasal immunisation procedure. Serum IgG antibody responses to MSA2 were higher in young adult Balb/c mice compared to old mice and neonates. The elicited serum IgG antibodies reacted with native MSA2 on the surface of P. falciparum merozoites in an immunofluorescence assay. The serum IgG antibody isotypes in young adult mice were mainly IgG1, IgG2a and IgG2b, while IgG3 tended to be higher in old mice. IgA antibodies to MSA2 were also produced in young mice. Enlarged mesenteric lymph nodes, and more prominent lymphoid tissue in the lamina propria of the ileum and lymphoid follicles in the spleen, were observed in mice fed L. lactis. These findings are relevant for developing L. lactis as a vector to deliver vaccines in human populations.