Induction of Hepatic Inflammatory Response by Plasmodium berghei Sporozoites Protects BALB/c Mice against Challenge with Plasmodium yoelii Sporozoites

BALB/c mice are about 2,000 times less susceptible to sporozoites of Plasmodium berghei than to Plasmodium yoelii. Associated with this is the innate cellular response mounted after injection with P. berghei. Host inflammatory cells do not normally attack P. yoelii during their development as exoerythrocytic forms (EEFs) in the liver. We used P. berghei sporozoites to induce host inflammation that might act against developing P. yoelii EEFs. Mice injected with P. berghei sporozoites followed 1 hr later with P. yoelii had a 58% reduction in P. yoelii EEFs. To establish whether this was due to events that occurred before vs. after invasion of hepatocytes by P. yoelii sporozoites, mice received P. yoelii sporozoites that were allowed to invade for 1 hr before subsequent injection with P. berghei; these mice showed minimal reduction in P. yoelii EEFs. Thus, most of the deleterious effects of P. berghei sporozoites appear to have been directed against P. yoelii sporozoites prior to their invasion of hepatocytes. Plasmodium yoelii that had already invaded were relatively unaffected. Further timing experiments showed that this effect was induced only by viable P. berghei sporozoites, which may thus induce rapid changes in sinusoid physiology leading to host resistance against P. yoelii sporozoites.