Effect of cyclophosphamide on Toxoplasma gondii infection: reversal of the effect by passive immunization

Treatment of mice with cyclophosphamide (CY) at a dose of 250 mg/kg body weight 24 hr prior to infection with an avirulent strain of Toxoplasma gondii delays the appearance of antibody by about one week and results in 70% mortality. To discount other effects of CY besides inhibition of antibody synthesis, CY-treated infected mice were passively immunized with a pooled specific serum collected from chronically infected syngeneic animals. Passive immunization reversed the effect of CY treatment if the titre of antibody in recipients reached 1 : 512 or more, as measured by the indirect immunofluorescence technique (IFT). It is therefore suggested that antibody plays an important role in establishing an infection-immunity (premunition) in this system.