Cyclophosphamide pretreatment induces eosinophilia to nonparasite antigens

Eosinophilia is usually regarded as a T cell-dependent hemopoietic response, although in most instances of immune responses by T cells there is no accompanying eosinophilia. BALB/c mice immunized with keyhole limpet hemocyanin (KLH) in complete Freund's adjuvant (CFA) were found not to develop eosinophilia, whereas pretreatment of such mice with 150 mg/kg cyclophosphamide (CY) induced marked eosinophilia in the blood (1000 to 3000/mm3) and bone marrow. CY as well as CY and KLH in CFA cause a leukocytosis and neutrophilia, but significant eosinophilia was seen only in mice receiving both CY and KLH-CFA. The eosinophilic response depended on the dose of CY and peaked 13 days after immunization, and was preceded by a bone marrow eosinophilia that peaked 8 days after immunization. No eosinophilia occurred in BALB/c nu/nu mice after CY and KLH-CFA but did occur in the nu/+ littermates. This model in mice of eosinophilia in response to CY followed by a nonparasite antigen should be useful in the analysis of T cell signals that might regulate eosinophilia.