Host defenses in murine malaria: characteristics of protracted states of immunity to Plasmodium berghei.

Vaccination of 8-week-old B6D2 (C57BL/6 x DBA/2) mice with Formalin-killed asexual erythrocytic stages of Plasmodium berghei provided a capacity to survive challenge with lethal P. berghei for more than 542 days. Although long-lived, this immunity did not provide a capacity to immediately neutralize parasites in a challenge; significant levels of erythrocytic infection occurred transiently after each challenge. The quality of long-lived immunity was not enhanced by an injection of live parasites 2 weeks after the last injection of killed antigen. The results suggest that it will be difficult to design a vaccination regimen which will provide long-lived protection from overt erythrocytic-stage infection.