In the search for a
leishmaniasis vaccine, extensive studies of
cutaneous leishmaniasis have been carried out. Investigations in this regard with the visceral form are limited. As an initial step in the identification of the protective molecules, leishmanial
antigens extracted from the membranes of Leishmania donovani promastigotes, alone or in association with
liposomes, were evaluated for their immunogenicity and ability to elicit a protective immune response against challenge
infection. Intraperitoneal immunization of hamsters and BALB/c mice with the leishmanial
antigens conferred protection against
infection with the virulent promastigotes. Encapsulation in positively charged
liposomes significantly enhanced the protective efficacy of these
antigens. The splenic parasite burden of hamsters was reduced by 97% after 6 months of
infection. BALB/c mice exhibited 87 and 81.3% protection in the liver and spleen, respectively, after 4 months of
infection. These protected animals elicited profound delayed-type
hypersensitivity and increased levels of Leishmania-specific
immunoglobulin G (
IgG)
antibodies. Protection in mice also coincided with elevated levels of
IgM and
IgA antibodies, which decreased with
disease progression in the control-infected animals. Although both
IgG1 and
IgG2a antibodies were present in the sera of infected mice,
IgG1 appeared to be the predominant isotype, suggesting a preferential induction of the Th2 type of immune response over that of Th1. Effective stimulation of all the
IgG isotypes, particularly
IgG2a, after immunization with
liposome encapsulated
antigens seems to be responsible for the significant levels of resistance against the disease. Taken together, these data indicate a potential for the liposomal
antigens as a
vaccine which could trigger both humoral and cell-mediated immune responses.