Active immunization with EPF suppresses the formation of immune ascites in BALB/c mice

The production of early pregnancy factor (EPF) is not confined to pregnancy--EPF has been detected as a product of tumour and transformed cells in vivo and in vitro. In this study, EPF (or an EPF-like substance) was detected also in the serum of BALB/c mice, 7 days after intraperitoneal (i.p.) administration of the mineral oil pristane. Furthermore, EPF was present in serum from mineral oil-induced plasmacytoma-susceptible mice throughout the latent period potentially leading to tumour development, peaking around the time neoplastic cells were identified in ascites. Since mineral oil-induced granuloma is essential to development of immune ascites, involvement of EPF in this process was investigated. Active immunization of BALB/c mice with EPF was shown to suppress the production of immune ascites, induced by multiple i.p. injections of antigen in complete Freund's adjuvant. Of the mice immunized with EPF (n = 19), only 47% produced ascites, compared with 94% of mice receiving saline or human chorionic gonadotrophin and 100% of mice receiving keyhole limpet haemocyanin. Further investigations revealed that ascites was only produced in mice that maintained free-circulating EPF. These mice displayed the classic mineral oil-induced granuloma covering the tissues of the peritoneum. In contrast, the serum of mice that did not produce ascites tested negative for EPF and the peritonea of these mice were devoid of the oil granuloma. These studies suggest that EPF is involved in the initiation and maintenance of the inflammatory response of the peritoneum to mineral oil.