Serologic Analyses of an Immunosuppressive Cell-Free Factor from Mastocytoma Cells and Neutralization by Antiserum

Mastocytoma cells, a rapidly growing tumor derived from mast cells of DBA/2 mice initially injected with methylcholanthrene, were used as a source of cellular and cell-free extracts for immunizing rabbits. Antisera prepared in rabbits against intact tumor cells, grown either in vivo or in vitro, as well as against cell-free tumor extracts, ascitic fluid, and culture fluids were examined for their ability to neutralize the immunosuppressive activity of cell-free ascitic fluid from mastocytoma-bearing mice and the effects of antisera on the in vivo susceptibility of mice were also tested. The antisera prepared against tumor cells, cell-free extracts or culture fluids neutralized the immunosuppressive activity of cell-free ascitic fluid from tumor-bearing mice. On the other hand, none of the sera passively protected mice upon challenge with living tumor cells. Furthermore, some mice treated with sera from rabbits immunized with intact tumor cells succumbed more rapidly to the tumor upon challenge, but this seemed due to a serum sickness-like reaction. Direct immunization of mice with either cell-free extracts or culture fluids did not protect the animals from direct challenge with mastocytoma cells. Thus, the presence of similar antigens in intact tumor cells, cell-free extracts and culture fluids, which stimulated antibodies readily detected by in vitro serologic assays, did not necessarily result in antibody activity which directly or indirectly protected mice in vivo.