Immune responses mediating survival of naive BALB/c mice experimentally infected with lethal rodent malaria parasite, Plasmodium yoelii nigeriensis.

The rodent malaria parasite, Plasmodium yoelii nigeriensis is known to cause fatal malaria infections in BALB/c mice. However, we found that nearly 5% of inbred BALB/c mice could overcome primary infections initiated with lethal inoculum of P. y. nigeriensis asexual blood-stages, without any experimental intervention. These 'survivor' mice developed peak parasitemia levels of about 5% and successfully resolved their infections in about two weeks time; infected blood collected during the descending phase of infection in these mice and subinoculated in naive recipients resulted in a normal lethal course of infection. Typically, the parasites in survivor mice looked 'sick' compared to those in the susceptible mice. In experiments to define temporal basis of this protection, we found that purified splenic B cells isolated from such a survivor mouse, plus T cells from an infected or naive mouse, could adoptively transfer this protection to an X-irradiated, naive mouse against a lethal parasite challenge. Purified T cells or B cells alone from the survivor mouse donor provided no protection to the X-irradiated, naive recipient. Passive transfer of sera collected from survivor mice animals a week after recovery from infection was also able to substantially alter the course of preestablished P. y. nigeriensis infection. These findings are discussed in the light of recent reports on the genetic control of blood parasitemia in mouse malaria models. In the generally lethal malaria infections such as those caused by P. y. nigeriensis in mice and by Plasmodium falciparum in naive children, it is not clear what constitutes a protective immune response in cases which survive primary infections without any experimental or therapeutic intervention. An understanding of these mechanisms and their regulation would help design better vaccination strategies.