Induction of partial protection against Leishmania donovani by promastigote antigens in negatively charged liposomes

Negatively charged liposomes, proposed as potential vaccine adjuvants, have been extensively studied in association with various antigens. In the pres ent study, we investigated the adjuvanicity of negatively charged liposomes to enhance the protective immunity of membrane antigens of Leishmania dono vani promastigotes (LAg). In comparison to the control mice immunized with phosphate-buffered saline and empty liposomes, immunization with free LAg l ed to significant levels of protection against infection with virulent prom astigotes. Encapsulation of LAg in liposomes also induced effective protect ion. However, the level of protection by LAg-liposome was not significantly different from that induced by free LAg. Investigation of the Immune respo nses showed, in contrast to free LAg, that immunization with LAg-liposome e licited strong antibody responses. IgG isotype analysis revealed the presen ce of all 4 isotypes. However, the titer of IgG1 was significantly higher t han IgG2a, IgG2b, and IgG3. Following infection, stimulation of IgG and IgG isotypes did not differ in the different immunization groups. Delayed-type hypersensitivity (DTH) analysis after immunization showed significant indu ction by LAg and LAg-liposomes, in comparison to controls. With infection, again, the level of DTH in all the groups became almost comparable. Stimula tion of insufficient cellular response, as reflected by DTH and potentiatio n of IgG1 over IgG2a, IgG2b, and IgG3 suggest a dominance of Th2 response w ith this liposome-antigen formulation, resulting in weak protection against visceral Leishmaniasis.