Aberrant expression of Ia-like antigens on tumor cells of regressing but not of progressing Rous sarcomas.

Unlike many other tumors, experimentally induced Rous sarcomas in chickens have a definite tendency to regress spontaneously. Regression is under genetic control, but it is immunologically mediated although the target antigens that stimulate an immune response are unknown. Tumor cells in regressing sarcomas were found to express class II major histocompatibility complex (Ia-like) antigens while tumor cells from progressing sarcomas were negative. This suggests that the induction of Ia-like antigen expression has a role in the initiation or perpetuation of regression, similar to that postulated for class II antigen expression in a variety of autoimmune diseases.