Engineered invasive Lactococcus lactis as an oral vaccine delivery platform is associated with compartmentalized immune modulation and enhances control of Leishmania infantum infection

Visceral leishmaniasis caused by Leishmania infantum remains a major challenge for vaccine development due to the intracellular localization of the parasite and the need for effective antigen delivery and cellular immune responses. Genetically engineered lactic acid bacteria have emerged as promising platforms for mucosal vaccination because of their genetic tractability and capacity to interact with the host immune system. Here, we evaluated an engineered invasive Lactococcus lactis strain expressing fibronectin-binding protein A (FnBPA) and carrying the chimeric TryPan antigen-encoding plasmid as an oral vaccine delivery platform, and compared its immunological and parasitological outcomes with subcutaneous immunization using recombinant TryPan formulated with monophosphoryl lipid A (MPLA). Oral administration of invasive L. lactis was associated with changes in intestinal chemokine expression, Th1/Th17-associated cytokines, and inflammatory cell markers, together with modulation of systemic immune parameters. Following L. infantum challenge, oral administration of L. lactis carrying the TryPan construct resulted in an approximately 99% reduction in hepatic parasite burden compared with the corresponding infected control. Notably, administration of L. lactis alone also reduced hepatic parasite burden, indicating a contribution of the bacterial vector to the observed parasitological outcome, while the greater reduction observed with TryPan delivery suggests an additional effect associated with the vaccine construct. Subcutaneous immunization with recombinant TryPan and MPLA induced broader systemic cytokine responses and reduced hepatic parasite burden by approximately 90%. Multivariate analysis identified distinct patterns of association among mucosal, systemic, and hepatic parameters according to the vaccination strategy. Together, these findings support the potential of invasive L. lactis as an oral vaccine delivery platform and highlight the contribution of both bacterial-vector-associated immunomodulation and antigen delivery to the observed control of hepatic L. infantum burden.