C57BL6 mice were protected against Plasmodium berghei
sporozoite challenge by immunization with live 12 krad dose-irradiated sporozoites, but not by 20 krad dose-irradiated sporozoites.
Immunization with 12 krad irradiated sporozoites
generated low levels of antibody reactive to liver-stage parasites (titres
of 1/100). Inoculation of as few as 100 live P. berghei
sporozoites induced complete host protection accompanied by a very quick
and high boost of antibody titres up to 1/4000.
This sporozoite challenge-drive antibody boost was absent in mice immunized
by 20 krad sporozoites and in non-protected, and non-immunized mice. Antibody was mainly liver-stage (LS)
specific and due to an increase of IgG2a and
IgG2b. The in vitro effect of pre- and post-challenge
sera upon either sporozoite invasion or LS development 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 upon invasion. These results suggest that sporozoites irradiated
at doses that induce protection are able to
prime T-cells which, upon challenge by non-irradiated sporozoites, provide
help to B-lymphocytes to trigger the
production of high titres of anti-LS antibodies that can inhibit LS development
in vitro.