Irradiated sporozoites prime mice to produce high antibody titres upon viable Plasmodium berghei sporozoite challenge, which act upon liver-stage development

C57BL6 mice were protected against Plasmodium berghei sporozoite challenge by immunization with live 12 krad dose-irradiated sporozoites, but not by 2 0 krad dose-irradiated sporozoites. Immunization with 12 krad irradiated sp orozoites generated low levels of antibody reactive to liver-stage parasite s (titres of 1/100). Inoculation of as few as 100 live P. berghei sporozoit es induced complete host protection accompanied by a very quick and high bo ost of antibody titres up to 1/4000. This sporozoite challenge-drive antibo dy boost was absent in mice immunized by 20 krad sporozoites and in non-pro tected, and non-immunized mice. Antibody was mainly liver-stage (LS) specif ic and due to an increase of IgG(2a) and IgG(2b). The in vitro effect of pr e- and post-challenge sera upon either sporozoite invasion or LS developmen t was assessed in Hep-G2 cultures. Both were found to have a strong effect upon LS development even at 1/2500 dilution, and conversely a low effect up on invasion. These results suggest that sporozoites irradiated at doses tha t induce protection are able to prime T-cells which, upon challenge by non- irradiated sporozoites, provide help to B-lymphocytes to trigger the produc tion of high titres of anti-LS antibodies that can inhibit LS development i n vitro.