Appropriate conditions for the plaque forming cell (PFC) assay in rats and effects of cyclophosphamide on PFC response.

The appropriate conditions for the plaque forming cell (PFC) assay using rat splenocytes were determined and effects of cyclophosphamide on PFC response were investigated using these conditions. The number of PFCs produced by immunization with a suspension of sheep red blood cells (SRBCs) was higher with i.v. injection than with i.p. injection. Subcutaneous injection of the suspension did not produce PFCs. The highest PFC response was observed when the number of PFCs was determined 4 days after i.v. immunization with 0.5 ml of a 1% SRBC suspension. Cyclophosphamide (3, 10, 30, 100 and 300 mg/kg, p.o.) dose-dependently decreased PFC response under the above-mentioned optimal conditions, and decreased PFC responses were noted even at the very low dose of 3 mg/kg: a dose at which a decrease in the number of PFCs has not been reported in studies using mice. From these results, the appropriate conditions for the PFC assay in rats are considered to be i.v. immunization with 0.5 ml of a 1% SRBC suspension and determination of the number of PFCs 4 days after immunization. Furthermore, it is considered that the PFC assay using rats might be more sensitive to immunosuppressive agents than that using mice.