Negatively charged
liposomes, proposed as potential
vaccine adjuvants, have been extensively studied in association with various
antigens. In the present study, we investigated the adjuvanicity of negatively charged
liposomes to enhance the protective immunity of membrane
antigens of Leishmania donovani promastigotes (LAg). In comparison to the control mice immunized with
phosphate-buffered saline and empty
liposomes, immunization with free LAg led to significant levels of protection against
infection with virulent promastigotes. Encapsulation of LAg in
liposomes also induced effective protection. However, the level of protection by LAg-
liposome was not significantly different from that induced by free LAg. Investigation of the immune responses showed, in contrast to free LAg, that immunization with LAg-
liposome elicited strong antibody responses.
IgG isotype analysis revealed the presence of all 4 isotypes. However, the titer of
IgG1 was significantly higher than
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 induction by LAg and LAg-
liposomes, in comparison to controls. With
infection, again, the level of DTH in all the groups became almost comparable. Stimulation of insufficient cellular response, as reflected by DTH and potentiation of
IgG1 over
IgG2a,
IgG2b, and
IgG3 suggest a dominance of Th2 response with this
liposome-
antigen formulation, resulting in weak protection against
visceral leishmaniasis.