Protection of squirrel monkeys against virulent Plasmodium falciparum infections by use of attenuated parasites.

We previously showed that the Uganda Palo Alto line of Plasmodium falciparum propagated in Saimiri monkeys and the line maintained in culture in human erythrocytes for many years in our laboratory are genetically unrelated (T. Fandeur, S. Bonnefoy, and O. Mercereau-Puijalon, Mol. Biochem. Parasitol. 47:167, 1991). When injected into a splenectomized Saimiri monkey, the in vitro-derived Palo Alto population procured a long-lasting, low-grade parasitemia that was spontaneously resolved by the animal. This line was propagated by serial blood transfers in two other monkeys without enhancement of the virulence of the parasites. The genetic characteristics of parasite samples corresponding to the different passages of the line in monkeys were stable for the several markers examined (pPF11.1, MSA1, and MSA2), although microheterogeneity was detected in telomeric and subtelomeric regions of chromosomes. Interestingly, in vitro-derived Palo Alto parasites induced a strong, potent immunity that enabled the monkeys to completely block subsequent challenge with two different heterologous lethal P. falciparum lines. These attenuated parasites are thus genetically stable in monkeys and represent an attractive model for assessing the feasibility of a live attenuated malaria vaccine.