Evaluation Of Passive Immunity To Schistosomiasis In Mice Transfused With Hyper-Immune Baboon Serum

ABSTRACT

Schistosomiasis is a tropical disease caused by worms in the genus Schistosoma. It remains one of the most prevalent parasitic infections and has significant economic and public health consequences. Despite having an effective chemotherapeutic drug, praziquantel, there is still need for a vaccine since the immediate manifestations in most people are negligible or indeterminate. Several antigens have been tested as vaccine candidates with largely disappointing results. Both rodents and primates exposed to cercariae larvae optimally attenuated with gamma radiation show a highly significant reduction in challenge worm burden. Vaccination of baboons with RA (Radiation attenuated) vaccine gave 54% protection after three exposures and 86% protection after five exposures. Protection due to RA vaccine is both humoral and cellular mediated with the IgG antibody levels thought to mediate the effector response declining over a relatively short period after the last vaccination. This study therefore aimed at evaluating the role of humoral immunity against migrating and or the pre-liver lung stage schistosomulae by passively immunizing mice with either 500 µl of hyper-immune serum or infection serum from baboons. In the first study, mice were immunized on day 7 post challenge and boosted on the 14th day. In the second study, mice were immunized on the 3 rd day post challenge with no boost. A 24 hour egg count was carried out on the 5th, 6th , and 7th week post challenge using Kato Katz thick smear technique after which perfusion was done seven weeks post challenge. Blood was collected at week 0, 2, and 4 in experiment two from the tail and through heart puncture during perfusion in both experiments for IgG ELISA assays. Sections of preserved livers were processed for histopathology and for eggs recovery. Immunization against the migrating schistosomes on the 3rd day with the hyper-immune serum gave a 25.8 % protection but there was no protection obtained when pre-liver lung stage schistosomulae was targeted. There was no significant difference between the 24 hour faecal egg number, number of worms, liver eggs and in number and size of granulomas on both experiment. There was thus no significant protection that was conferred in mice against pre-liver-lung stage schistosomulae and or the migrating schistosome larvae by hyper-immune serum from RA vaccinated baboons probably due to the inability of the baboon serum to cooperate with the mouse effector system necessary in killing the schistosomulae. These results suggest that protective mechanism in baboons against schistosome worms could be different from those in mice and that though antibodies could be involved in protection, other factors such as cytokines could also play a major role. There is thus the need to further evaluate the mechanisms of protection in mice and experiment on homologous passive transfer of hyper-immune serum (baboon to baboon) .Other stages of the schistosome lifecycle should also be targeted during passive transfer experiments.