Comparison of approaches for augmenting the serologic response to the individually specific methylcholanthrene-induced sarcoma-Meth A: pretreatment with cyclophosphamide is most effective.

We have previously reported the production of antisera against a highly restricted antigen expressed on the BALB/c sarcoma Meth A. Because induction of the antibody required many vaccinations with irradiated and unirradiated Meth A cells over a prolonged period, we have investigated methods of improving the efficiency of producing Meth A antibody in BALB/c mice. Three general approaches were used: immunizing with irradiated Meth A cells mixed with adjuvants, immunizing with irradiated Meth A cells modified by treatment with chemicals, or administration of tumor cell vaccines in conjunction with low dose cyclophosphamide. These vaccine preparations were injected subcutaneously into groups of BALB/c mice five to six times at 2-wk intervals. Sera from all mice were screened for reactivity to Meth A by complement-dependent cytotoxicity, protein A, and mixed hemadsorption assays. Twenty-three vaccine preparations containing adjuvants or modified cells were tested in individual groups of mice. Mice in some groups receiving adjuvants and in all groups receiving modified cells produced antibody, but only after four to six vaccinations. In contrast, nine of the 14 mice immunized with irradiated Meth A cells (unmodified and without adjuvant) in conjunction with 10 or 100 mg/kg body weight of cyclophosphamide, and all nine of the mice receiving 25 mg/kg cyclophosphamide made Meth A antibodies after only one or two vaccinations. The titer of antibodies produced after one treatment with cyclophosphamide and irradiated Meth A cells was at least as high as that achieved after five or six vaccinations in our other trials. The specificity of these antibodies for the Meth A antigen was established by absorption analysis. We conclude that treatment with cyclophosphamide before vaccination is highly effective in augmenting the humoral immune response to the Meth A antigen.