Antigenic modifications associated with 'spontaneous' malignant alterations of mouse fibroblasts propagated in vitro

Two types of 'spontaneous' malignant alteration in vitro of ST/a mouse lung fibroblasts (ST-L) have been observed. In contrast to cells which retained their fibroblastic appearance (R- ST-L cells), cells showing morphological signs of transformation (R+ ST-L cells) developed strong isoimmunizing properties. Both types of cells expressed MuLV antigens which were found to be responsible for serum as well as cell-mediated immune reactions in vitro. The higher concentration of gp70 in R+ ST-L cells as compared to R- cells and possibly also the morphological differences in surface structure between the two cell types may account for the differences in immunogenicity. Preimmunization with R+ ST-L cells protected ST/a mice against secondary challenge with two ascites tumors (STABAL leukemia and Ehrlich). R- ST-L cells did not have a similar protective effect. However, the two ascites tumors only showed weak or no cross-reactions in vitro with sera and lymphoid cells from ST/a mice sensitized to R+ ST-L cells, and the in vitro reaction between the latter cells and sera or lymphoid cells from mice immunized against the two ascites tumors was moderate. This discrepancy between in vivo and in vitro observations is discussed.