Development of an in vitro assay using abscisic acid to study Toxoplasma gondii infectivity

The common parasite Toxoplasma gondii can infect all warm-blooded animals, including humans. Although most infections in humans remain asymptomatic, clinical toxoplasmosis can develop into a fatal disease. Infections are usually contracted by oral ingestion of tissue cysts or oocysts contained in cat feces. Currently, the mouse bioassay is applied as a final experiment to evaluate meat infectivity. This study aims to establish an alternative cell culture and quantitative polymerase chain reaction (qPCR)-based in vitro infectivity assay for tissue cysts. The phytohormone abscisic acid (ABA) is applied to increase parasite multiplication. A human foreskin fibroblast (HFF) host cell culture was infected with bradyzoites from mouse tissue. Treatment groups included uninfected controls, infected untreated controls, and infected ABA treated groups. The applied ABA concentrations used ranged from 0.2 ng/μl to 20 ng/μl, and ABA incubation times ranged from 2 h to 18 h before ABA removal. At 48 h after infection, T. gondii deoxyribonucleic acid (DNA) in the cell cultures was quantified by qPCR. The results indicate that parasite DNA copy numbers are markedly increased when using ABA at 2 ng/μl for 4-6 h or at 20 ng/μl for 2 h incubation. Our results indicate that this newly established in vitro assay is suitable to determine T. gondii cyst infectivity.