The covalent conjugation of fatty acid to a tumor cell membrane preparation transformed it from an antigen that enhanced tumor growth to one that suppressed it. A crude cell membrane preparation was made by sequential hypertonic and hypotonic salt extraction of tumor cells from a fibrosarcoma induced in hamsters by simian virus 40. The membranes were chemically conjugated with dodecanoic anhydride in 0.5 M carbonate buffer (pH 9.0). Injection of unmodified membranes 10 days before transplantation of live tumor cells produced clear-cut enhancement of the tumor growth rate. In contrast, injection of lipid-conjugated membranes in a similar dose and protocol suppressed tumor growth. The lymphoid proliferative reactions to the tumor cells as demonstrated by the histology of both the tumor and regional lymph nodes were consistent with the hypothesis that unmodified membranes stimulated the production of antibody which participated in the enhancement of tumor growth, and that lipid-conjugated membranes stimulated the production of cell-mediated immunity which suppressed this growth.
