The effects of cyclophosphamide on in vitro correlates of tumor immunity

The effects of cyclophosphamide (CY) on tumor immunity against isografts of rat sarcomas induced by polyoma virus were studied using in vitro techniques. Groups of sarcoma‐bearing animals received CY (250 mg/kg intraperitoneally 11 days after isografting) CY (150 mg/kg IP 8 days after isografting), or IP injections of 0.9% NaCl. In control rats tumor growth was progressive. All CY‐treated animals showed transient tumor regression of at least 50% of their pretreatment tumor volume. Despite a drastic depression in numbers of blood leukocytes as well as lymph‐node cells following CY treatment, animals treated with 150 mg/kg CY were shown to have blood lymphocyte and lymph‐node cell cytotoxicity in vitro against plated sarcoma target cells comparable to untreated sarcoma‐bearing animals. Sera obtained from sarcoma‐bearing rats prior to CY treatment specifically blocked lymphocyte cytotoxicity against sarcoma target cells. After CY treatment serum blocking activity could not be demonstrated during the period of tumor regression, but reappeared in parallel with tumor regrowth. Antibodies cytotoxic to sarcoma target cells when homologous complement was added could not be demonstrated in sera obtained from animals before CY treatment, but were present after treatment during tumor remission. Following CY treatment, sera obtained from treated animals, when mixed with blocking sera from control animals, could counteract or unblock the blocking activity of tumor‐bearer sera. Unblocking capacity was present only in sera obtained during CY‐induced tumor remission. Serum IgG concentration was significantly and temporarily decreased after CY treatment