Importance in timing of cyclophosphamide on the enhancement of interleukin-2-induced cytolysis

Summary We investigated the in vivo effects of cyclophosphamide (CY) on interleukin-2(IL-2)-induced cytolytic function and spleen cell immunophenotype. Pretreatment of A/J mice with CY (25 mg/kg or 75 mg/kg) i.p. on days –10 and –15 followed by IL-2 (50 000 U i.p. on days 0 to +3) resulted in increased lysis of YAC-1 target cells compared to the group receiving IL-2 without previous CY therapy. In contrast, when CY was given on day -5, the cytotoxicity against YAC-1 was not enhanced. Phenotypic analysis of splenocytes obtained from mice treated with CY on day –10 or –15 revealed a relative decrease in L3T4- and Lyt2-positive T cells. In vivo depletion of natural killer (NK) cells by anti-asialoG_M1, prior to IL-2 therapy, abrogated the enhancing effect of CY on cytolysis while in vivo elimination of T cells by anti-L3T4 and anti-Lyt2 monoclonal antibodies did not, indicating that in the absence of T cell antigenic challenge, the increased cytolytic function after CY administration is probably mediated through NK cells. These findings provide evidence that CY may be used more effectively in IL-2-based immunotherapy protocols, if consideration is given to timing of CY and IL-2 administration.Supported in part by the Children's Cancer Research Fund, the Concern II Foundation, RO1-CA-21 737, NO1-AI-85 002 and a contract to the University of Minnesota from OncoTherapeutics Inc. Dr. E. Katsanis is supported by a fellowship from the Medical Research Council of Canada. Dr. P. M. Anderson is supported by a Clinical Oncology Career Development Award from the American Cancer Society. This is paper no. 550 of the Immunobiology Research Center