The serologic response to Meth A sarcoma vaccines after cyclophosphamide treatment is additionally increased by...

We have shown previously that the serologic response of BALB/c mice to immunization with BALB/c sarcoma Meth A cells can be more effectively augmented by pretreatment with cyclophosphamide (Cy) than by the use of adjuvants. The serologic response was directed against a highly restricted cell surface antigen, closely related to or identical with the unique transplantation antigen characteristic for this tumor. In this paper, we report the results of our attempts to obtain additional augmentation by using Cy and adjuvants together. For these studies, the optimal Cy dose, interval between Cy and vaccine administration, and vaccine cell number were determined. Mice were injected with Cy 25 mg/kg i.p., and 3 days later, with viable irradiated (10,000 rad) Meth A cells subcutaneously, under conditions in which only few mice produced antibody. Sera were tested for antibody with reactivity against Meth A by complement dependent cytotoxicity (CDCX), which predominantly detects IgM, and by the protein A (PA) and anti-IgG assays, which detect IgG. Of the various adjuvants tested, only monophosphoryl lipid A (MPLA) and CP20,961 resulted in significantly increased titers of reactivity in both the CDCX and PA assays over that obtained when using Cy alone. Although the mean titers observed with CDCX ranged between 1/160 and 1/320, no titer above 1/40 was observed with the PA assay. The specificity of the CDCX reactivity detected by the assay for the Meth A antigen was ascertained by absorption analysis of selected sera by using a panel of BALB/c spleen and tumor cell lines grown in vitro or in vivo. PA titers were too low to permit absorption analysis, and the titers obtained in the anti-IgG assay were lower still. Attempts to augment the anti-Meth A IgG response or to convert the IgM response to IgG were unsuccessful. The combined approach described here (i.e., vaccination with irradiated syngeneic tumor cells plus MPLA in Cy-pretreated mice) was also shown to be effective in augmenting the serologic response against two additional murine leukemia virus-negative sarcomas that are known to be less immunogenic, CMS4 and CMS5. It appears, therefore, that this combined approach may be applicable to stimulating serologic responses against a variety of tumor cell surface antigens.