Oral immunization using live Lactococcus lactis co-expressing LACK and IL-12 protects BALB/c mice against Leishmania major infection.

Leishmaniasis is a parasitic disease affecting over 12 million individuals worldwide. Current treatments are laborious, expensive, cause severe side effects, and emerging drug resistance has been reported. While vaccination is the most cost-effective means to control infectious diseases there is no human vaccine currently available against Leishmania infections. Lactococcus lactis is a non-pathogenic, non-colonizing Gram-positive lactic acid bacterium commonly used in the dairy industry. Recently, L. lactis was used for the expression and delivery of biologically active molecules, such as antigens and cytokines, in mice and humans. In this study, we report the generation of L. lactis(alr-) strains solely expressing the protective Leishmania antigen, LACK, in the cytoplasm, secreted or anchored to the bacterial cell wall or co-expressing mouse IL-12. We show that oral immunization using live L. lactis, secreting both LACK and IL-12 was the only regimen that partially protected BALB/c mice against subsequent Leishmania major challenge. This highlights the importance of temporal and physical proximity of the delivered antigen and adjuvant for optimal immune priming by oral immunization since co-administration of L. lactis strains independently expressing secLACK and secIL-12 did not induce protective immunity. Protected animals displayed a delay in footpad swelling, which correlated with a significant reduction of parasite burden. Immunization with the L. lactis strain secreting both LACK and IL-12 induced an antigen-specific mucosal immune response and a LACK-specific T(H)1 immune response in splenocytes and mesenteric lymph node cells. Further, protection in immunized animals correlated with a strong Leishmania-specific T(H)1 immune response post-challenge, detectable in splenocytes and lymph node cells draining the site of infection. This report demonstrates the use of L. lactis as an oral live vaccine against L. major infection in susceptible BALB/c mice. The vaccine strains generated in this study provide the basis for the development of an inexpensive and safe oral live vaccine against the human parasite Leishmania.