PROTECTIVE SPECIFIC IMMUNITY INDUCED BY CYCLOPHOSPHAMIDE PLUS TUMOR-NECROSIS-FACTOR-ALPHA COMBINATION TREATMENT OF EL4-LYMPHOMA-BEARING C57BL 6 MICE/

A combination treatment protocol initiated 12 days after tumor injecti on, when the tumor was large, by administering cyclophosphamide (CY, 1 50 or 250 mg/kg) intraperitoneally followed by intravenous tumor necro sis factor alpha (TNF alpha, 1000 units/injection) on days 13, 16, 18, 21, and 23, resulted in about 60% longterm survival (i.e., survival f or at least 60 days) in the syngeneic C57BL/6 mouse/EL4 lymphoma model system. The establishment of a specific antitumor immune memory and i ts possible therapeutic relevance was verified by reinjecting 60-day s urvivors with EL4 cells; all 60-day survivors that had received the co mbination treatments rejected the implants and survived for a further 60 days. Thymic cellularity was reduced during treatment and its recov ery appeared to correlate with long-term survival and immunity. Thymoc ytes from mice treated with the combination were found to express sign ificant levels of specific anti-EL4 cytolytic activity following a 4-d ay stimulation culture with X-irradiated EL4 cells and low concentrati ons of interleukin-2. This response could not be generated with thymoc ytes from naive animals. In each case the effect seen with the combina tion of a moderate CY dose (150 mg/kg) with TNF alpha was better than that seen with either dose of CY alone and equal to or better than tha t seen with the higher dose of CY combined with TNF alpha. These resul ts indicate that treatment with a single moderate dose of CY in combin ation with TNF alpha is effective against a large, established tumor i n this murine model.