Protection of mice against visceral leishmaniasis by immunization with promastigote antigen incorporated in liposomes.

Leishmanial antigens (LAg) were used as a vaccine against Leishmania donovani, the causative agent of visceral leishmaniasis. BALB/c mice, immunized intraperitoneally with 20 micrograms of the antigen in phosphate-buffered saline (PBS) or entrapped in liposomes, were infected intravenously with 2 x 10(7) L. donovani promastigotes. Mice immunized with PBS and empty liposomes showed similar levels of parasite burdens in the liver and spleen. Injection of the antigen alone or entrapped in liposomes, followed with infection, induced significant levels of protection against the disease. After 2 and 4 mo of infection, mice immunized with free antigen induced 7.4% and 50.7% reduction in the liver parasite burden, respectively, compared to control (PBS) mice. With antigen encapsulated in liposome, the liver parasite burden was further reduced by 30.4% and 73% at 2 and 4 mo by infection, respectively. Splenic parasite burden was very low at 2 mo of infection. At 4 mo, the parasite level was reduced by 54.2% with free antigen and 69.3% with antigen entrapped in liposomes. Whereas the protection induced by the free antigen is mainly cell mediated, stimulation of an antibody response together with a strong delayed-type hypersensitivity may be responsible for the better protection with liposomal antigen.