Effect of serum concentration on rosette formation byP falciparum PA1-infected RBC.

PRBC cultured in CM-NHS and rosetting at high levels (more than 60%) were stripped of serum components using combination heparin treatment to disrupt the rosettes and Percoll density gradient centrifugation to separate the PRBC from uninfected cells and medium components. The PRBC were mixed with freshly washed, uninfected erythrocytes in a serum-free medium (CM-Albumax I) in the absence or presence of increasing concentrations of serum, and rosetting was assessed by fluorescence microscopy (×1000 objective in incident ultraviolet light). The relative percentage rosetting is defined as ×100 the rate of rosetting in a culture in which the CM-Albumax I medium is supplemented with the indicated amount of serum/the rate of rosetting in a culture in which the CM-Albumax I medium is supplemented with 10% (vol/vol) NHS. Results are presented as the mean of 3 separate experiments, each using a different batch of human nonimmune AB serum ± SD.