Immunotherapy of established murine tumors with large multivalent immunogen and cyclophosphamide

Plasma membrane vesicles isolated from tumor cells can be incorporated onto 5-microns diameter microspheres and antigen in this form, termed large multivalent immunogen (LMI), augments generation of tumor-specific cytotoxic T lymphocyte (CTL) responses in vivo. Treatment of mice with LMI at the time of challenge with tumor significantly reduced growth of several tumors in their syngeneic hosts. Our report describes the effects of LMI on established progressing tumors, including P815 solid tumor and two fibrosarcomas in a lung-metastasis model. Treatment of mice bearing established tumors (7 to 12 days) with LMI alone did not significantly reduce tumor growth or extend host survival, but highly synergistic effects of combined treatment with cyclophosphamide (Cy) and LMI were found. Cy alone reduced the size of P815 solid tumors, but within a few days, the tumors began to grow progressively, and survival was only marginally extended. However, Cy followed 2 to 3 days later by a single injection of LMI resulted in prolonged reduction of tumor growth and significant extension of survival; in some experiments, tumors became undetectable in the majority of treated mice, and the mice survived indefinitely. Essentially the same results were obtained in experiments examining survival of mice bearing established MCA-203 fibrosarcoma. LMIs were uniquely effective in acting synergistically with Cy; antigen in the form of irradiated tumor cells or plasma membrane in adjuvant were ineffective, and free plasma-membrane antigen (not on microspheres) had only marginal effects. There has been considerable interest in the possibility of using tumor antigen to enhance tumor-specific immune responses, and clinical trials using this approach are showing some promise. The results described here suggest that altering the form of antigen by purifying plasma membranes and incorporating them onto microspheres might significantly improve the efficacy of tumor immunotherapy with antigen.