Purification and in vitro selection of rosette-positive (R+) and rosette-negative (R-) phenotypes of knob-positive Plasmodium falciparum parasites.

We have developed methods for in vitro selection of Plasmodium falciparum parasites that bear knob protrusions (K+) and are either of the rosette-positive (K+R+) or rosette-negative (K+R-) phenotypes. Cryopreserved parasites from spleen-intact Aotus monkeys that were K+, C32 cell adherence-positive (C+), CD36 adherence-positive, and R- with Aotus erythrocytes were adapted to continuous growth in human erythrocytes, and selected initially for adherence to C32 melanoma cells. In the absence of independent selection for rosettes, K+R-C+ parasites were produced that adhered to both C32 cells and CD36. Without selection for the C+ phenotype, K+R-C- parasites eventually predominated in such cultures. The R+ parasites were selected using differences in sedimentation behavior of rosette-infected cells versus non-rosette-infected cells. Methods were devised for selection of the R+ or R- phenotypes and for the purification of R+ or R- infected cells of high parasitemia that were suitable for molecular studies. With the repeated selection for K+R+ parasites, we were able to maintain the K+R+ phenotype for several months in vitro. These methods will allow systematic study of the molecular basis of the K+R+ and K+R- phenotypes.