Immunogenicity of subcellular fractions and molecular species of MuLV-induced tumors. I. screening of immunogenic components by isopycnic ultracentrifugation and polyacrylamide electrophoresis of a tumor homogenate

The isolation of immunogenic determinants from subcellular fractions of tumor cells by a biochemical screening method is described in this paper. Separated subcellular fractions and molecular species of tumor cells were screened for their ability to stimulate antitumor immune responses in syngeneic hosts. Homogenates of the in vivo-carried tumor YAC of A/J mice and the corresponding in vitro-cultivated tumor YAC-1 were fractionated on a sucrose gradient. The immunogenic capacity of the fractions was screened by injecting each fraction into A/J mice and testing the ability of the splenocytes of these mice to generate anti-YAC cytotoxic responses. The strongest cytotoxic responses were generated by the heavy subcellular fractions of YAC and YAC-1, which contained the highest concentration of protein, and the light fraction of YAC-1, which contained high 5′-nucleotidase activity. These immunogenic fractions and homogenates of intact tumor cells were further analyzed by sodium dodecyl sulfate-polyacrylamde gel electrophoresis (SDS-PAGE). The immunogenicity of the molecular species separated by SDS-PAGE was tested by injecting the gel slices into different groups of mice. It was found that certain molecular species stimulated specific antitumor cytotoxic responses, whereas others failed to stimulate such responses. Mice injected with some of these immunogenic molecular species were able to reject viable tumors. Other immunogenic molecular species, although stimulating cellular cytotoxic responses, failed to generate rejection capacity in the syngeneic hosts. To the best of our knowledge this is the first attempt to isolate immunogenic determinants from tumor cells by a biochemical screening method. Furthermore, by this method evidence was obtained that immunogenic determinants can even be isolated from a non-immunogenic tumor.