Effect of immunization or cyclophosphamide treatment on growth of EL-4 leukemia cells in syngenic C57BL/6 mice.

Prolonged survival of C57BL/6 (B6) mice bearing syngenic EL-4 leukemia cells resulted from immunization with irradiated EL-4 cells on the day of inoculation of live leukemia cells. No prolongation of survival was observed if the irradiated cells were injected 6 or 13 days after live cell inoculation. Protection also was observed in EL-4-bearing mice that were treated with cyclophosphamide (CY) rather than by immunization, however, the protective effects were observed only when CY treatment was instituted 6 days after inoculation of live leukemia cells. No protection was seen when CY was administered on the day of, or 13 days after inoculation of live leukemia cells. In fact, administration of CY on the day live EL-4 cells were inoculated appeared to enhance the lethal effects of the tumor. In mice that underwent combined treatment, i.e., immunization and CY, prolonged survival was seen only in the group that received combined treatment 6 days after inoculation of live leukemia cells. No protection was seen in mice receiving combined treatment on the day of or 13 days after inoculation of live leukemia cells. The role that suppressor T cells might play in the observed results is discussed.