Adjuvanticity and protective immunity elicited by Leishmania donovani antigens encapsulated in positively charged liposomes

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