ENHANCEMENT OF TUMOR-ASSOCIATED ANTIGEN EXPRESSION DURING THE REGRESSION PHASE OF XENOGENIZED TUMOR-CELL GROWTH IN-VIVO

Rat fibrosarcoma cells infected with Friend leukemia virus (FV-KMT-17) grow for a short time and then regress spontaneously in syngeneic hos ts. This regression was caused by immunological mechanisms, because th e tumor cells were xenogenized. In this study, we have tried to find o ut whether tumor-associated antigen (TAA) expression in these xenogeni zed tumor cells can be modulated by xenogenization. FV-KMT-17 cells (1 x 10(7)), which were subcutaneously transplanted into ten mts, sponta neously regressed after temporary growth. All rats which rejected FV-K MT-17 cells showed strong resistance to rechallenge with KMT-17 (1 x 1 0(6)) cells. To reveal the chronological modulation of TAA and virus-a ssociated antigen (VAA), a single-cell suspension was obtained from th e subcutaneous tumors and expression of these antigens was chronologic ally measured. TAA, termed CE7 antigen, was examined by anti-CE7 monoc lonal antibody (MoAb) and VAA was examined by anti-FK1 MoAb which reco gnizes the FV env gene product (gp 70). Expression of VAA was not modu lated through either the progression or the regression phase, but expr ession of TAA was strongly enhanced in the regression phase. These res ults show that enhancement of TAA expression occurs during the regress ion phase of FV-KMT-17 growth in vivo and that TAA-expressing cells ma y stimulate anti-tumor immunity resulting in acquisition of resistance against parental KMT-17 cells.