SEPARATION OF A CYSTEINE-RICH SURFACE ANTIGEN-EXPRESSING VARIANT FROMA CLONED GIARDIA ISOLATE BY FLUORESCENCE-ACTIVATED CELL SORTING

Clonally derived trophozoites (clone O2-4A1, from a sheep) of the morp hologically defined group of Giardia duodenalis change their cysteine- rich surface proteins (CRISPs) spontaneously during in vitro culture. This phenomenon constitutes a serious obstacle for studies that rely o n a pure population of cells bearing a particular variant CRISP. We de scribe herein the successful separation and quantitation of a trophozo ite subpopulation expressing a 90-kDa major surface antigen (CRISP-90) from a heterologous population using fluorescence-activated cell sort ing (FACS) on the basis of fluorescein-tagged antibodies directed spec ifically against O2-4A1 CRISP-90. During subsequent in vitro culture, the purified cell population exhibited a progressive decline in the pr oportion of cells labeled by CRISP-90 specific antibodies. After 46 ge nerations, only one-third of the total trophozoite population reacted with the anti-CRISP-90 antibodies.