Subjection of EL-4-1eukemia cells to hydrostatic pressure of 1200-1500atm for 15min increased their weak basal immunogenicity to a potent practical level. Injection of such pressure-treated and irradiated EL-4 cells into syngeneic naive C57B1/6 mice significantly delayed tumor development and increased survival after subsequent challenge with untreated EL-4 cells. Application of pressure of 1500 atm for a longer period of time (e.g., 120 rain) resulted in cell death and a smaller increase in tumor immunogenicity which could be partially accounted for by passive shedding of membrane material. Unlike previously studied tumor cells, incorporation of cholesteryl hemisuccinate (CHS) into the plasma membrane of EL-4 cells increased their apparent tumor immunogenicity only slightly. In addition, isolated EL-4 plasma membranes, untreated, CHS-treated or pressure-treated, as well as the material shed thereof by hydrostatic pressure, were all of weak immunogenicity.
Modulation in the projection of surface antigens upon pressure treatment could account for the observed increase in tumor immunogenicity and was monitored via the Thy 1.2 antigen. Fluorescence cell sorting anylsis indicated that upon application of 1500 atm for different periods of time the projection of Thy 1.2 progressively and irreversibly increased to a maximal level of about 140% at 15 rain. At longer pressurizations the availability of Thy 1.2 to antibody binding decreased sharply to levels below that of the untreated cells. It is suggested that pressure promotion of tumor immunogenicity is induced by changes in projection and surface distribution of the relevant antigens.