Effect of cyclophosphamide on tumor cell sensitivity to the action of immunological effectors.

A single sublethal dose of cyclophosphamide (CY) given to mice with Ehrlich ascites carcinoma (EAC) kills tumor cells and inhibits tumor growth for the first five days after its administration. From day 7 on, treated tumor cells reassume their growth, but their transplantability in normal recipients is greatly reduced for further three weeks more. On the other hand, the transplantability of untreated (control) and CY-treated cells in T cell-deficient mice is similar. Experiments with criss-cross immunization and challenge have shown that CY-treated cells are equally, or even less, immunogenic than un-treated cells. On the other hand, CY-pretreated cells are several times more sensitive to immunological attack of passively transferred immune spleen cells. These data show that CY does not increase the tumor cell immunogenicity but only increases its sensitivity to immunological attack.