IMMUNE PRESSURE SELECTS FOR PLASMODIUM-FALCIPARUM PARASITES PRESENTING DISTINCT RED-BLOOD-CELL SURFACE-ANTIGENS AND INDUCING STRAIN-SPECIFIC PROTECTION IN SAIMIRI-SCIUREUS MONKEYS

The passive transfer of specific antibodies to a naive splenectomized Saimiri sciureus monkey infected with the Pale Alto FUP/SP strain of P lasmodium falciparum resulted in the emergence of parasites resistant to the transferred antibodies. Molecular typing indicated that the ori ginal and resistant parasites were isogenic. Saimiri monkeys primed wi th original parasites were fully susceptible to a challenge by the res istant ones, and vice versa. This absence of crossprotection indicates that strain-specific determinants would be the major targets of prote ctive immunity developed in these monkeys. Phenotypic analysis showed that the surface of the infected red blood cells differed in both line s. Original parasites formed rosettes, autoagglutinated, presented cha racteristic knobs at the surface of the infected red blood cell, and d id not agglutinate in the presence of a pool of human immune sera. In contrast, the resistant parasites did not form rosettes, did not spont aneously autoagglutinate, presented abnormal flattened knobs, and form ed large aggregates in the presence of a pool of human immune sera. Th e presence of strain-specific determinants at the surface of the resis tant parasites was confirmed by surface immunofluorescence and aggluti nation using homologous Saimiri serum. Neither the original nor the re sistant parasites cytoadhered to an amelanotic melanoma cell line, sug gesting that cytoadherence and agglutination can be dissociated. These results indicate that parasites that differ by the antigens exposed a t the surface of the red blood cell induce strain-specific immunity. F urthermore they show that resetting and nonrosetting parasites differ in their antigenic properties and do not crossprotect.