Surface antigens of immunoprotective leukaemia x fibroblast hybrid cells which have lost malignant properties in histocompatible mice differ from the malignant parental cells.

Somatic hybrids of ASL-1 leukaemia cells and LM(TK-) fibroblast cells, established mouse cell lines, are rejected by immunocompetent histocompatible mice; however, they grow progressively in nude mice and proliferate indefinitely in vitro. Mice rejecting such hybrid cells exhibit immunity toward ASL-1 leukaemia cells, used as one of the parents in forming the hybrid. In histocompatible recipients, ASL-1 cells are highly malignant, but LM(TK-) cells are rejected. Several distinguishing characteristics in the properties of the surface antigens of the three cell types are described. ASL-1 cells and hybrid cells but not LM(TK-) cells form an antigenically cross-reactive leukaemia-associated antigen; however, it is not detected on the surface membranes of ASL-1 cells taken directly from leukaemic mice. The antigen becomes apparent after short-term culture of the cells. Serum from leukaemic animals, unlike specific antibodies, has no effect upon the expression of the leukaemia-associated antigen of either hybrid or ASL-1 cells. Hybrid cells form a 'second' antigen, foreign to F1 mice, which can be distinguished from the leukaemia-associated antigen of ASL-1 cells. It is not detected on either parental cell. The leukaemia-associated antigen of ASL-1 cells is more readily digested by each of three proteases used than the analogous antigen of hybrid cells. The heightened immunogenic properties of hybrid cells is reflected by the observation that spleen cells from mice injected with hybrid cells undergo extensive proliferation in short-term in vitro culture, with or without an added specific stimulus.