Immunoprotection in mice susceptible to waning memory against the pre-erythrocytic stages of malaria after validated immunisation with irradiated sporozoites of Plasmodium berghei.

The induction of immunity by irradiated sporozoites has been a bench-mark of immunological protection against the malaria parasite. Herein we confirm that different mouse strains exhibit different susceptibilities to sporozoite-induced infection of Plasmodium berghei. We note, however, that after hepatic schizogony, early parasite growth in the blood demonstrates no strain preference between C57BL/6 and BALB/c mice. Sporozoite-susceptible C57BL/6 mice, although initially protected by irradiated sporozoite immunisation against a challenge of 10(3) live sporozoites, progressively lose this protection; a challenge with fewer sporozoites 2 months later elicits a blood infection. BALB/c mice treated in parallel remain protected. Analysis of the kinetics of blood parasitaemia (a measure of hepatic schizont burden) with waning protection shows clearly that immunocompetence remains, as indicated by a reduction in the effective exo-erythrocytic schizont load. This immunocompetence can be shown to be absolutely protective, given an appropriately low dose of viable infective sporozoites. We discuss the testable proposition that this elicitation of protective memory is a consequence either of 'unsaturated' threshold levels of recirculating immunoeffector CD8+ cells or of CD4 cell activation by nonviable sporozoites.