Divergent Effects of Cyclophosphamide Administration on Mononuclear Killer Cells: Quantitative Depletion of Cell Numbers Versus Qualitative Suppression of Functional Capabilities

Abstract

The effects of various regimens of cyclophosphamide administration on guinea pig peripheral blood leukocytes were studied. Cyclophosphamide-induced immunosuppression was assessed by the effect of drug administration on the proportions and absolute numbers of leukocyte populations, and by the effect on functional capabilities of unfractionated and adherent cell-depleted mononuclear cell suspensions as measured by the PHA-induced cellular cytotoxicity and antibody-dependent cellular cytotoxicity assays against chicken erythrocyte targets. Intraperitoneal administration of five daily doses of cyclophosphamide (5 mg/kg) caused a modest absolute leukopenia but no change in cytotoxic effector function of the mononuclear cells remaining in the circulation. As the dosage of cyclophosphamide was increased to 20 mg/kg/day to produce a pronounced leukopenia, a profound neutropenia (< 300 polymorphonuclear leukocytes/mm³) together with a marked decrease in mononuclear cell effector function was noted. A single i.p. injection of cyclophosphamide (100 mg/kg), which produced identical degrees of leukopenia of each leukocyte class as did daily administration of cyclophosphamide (20 mg/kg/day), caused no change in mononuclear cell effector function when compared to saline controls. Complement receptor-bearing and Fc-receptor bearing mononuclear cells were decreased to the same degree by both regimens of cyclophosphamide administration. Removal of adherent cells from mononuclear cell suspensions by column purification resulted in a marked decrease in cytotoxic effector function at low effector to target ratios. At higher effector to target ratios there was no difference in cytotoxic effector function between unfractionated and column-purified cells. In contrast, the functional defect in mononuclear cell suspensions from animals that received five daily doses of cyclophosphamide (20 mg/kg) could not be compensated for at higher effector to target ratios, indicating that this functional defect was not an artifact of relative depletion of monocytes by cyclophosphamide, but was due to an actual suppression of the effector functional capabilities of the killer cells. This study indicates that, dependent on the particular regimen of drug administration, the quantitative depletion of mononuclear cell populations by cyclophosphamide administration can be clearly distinguished from the qualitative effect on certain functional capabilities of surviving cells.