Immunization with idiotypic immunoglobulin protects against development of B lymphocytic leukemia, but emerging tumor cells can evade antibody attack by modulation.

Immunization of strain 2 guinea pigs with idiotypic IgM obtained from serum of animals in the terminal phase of the L2C leukemia produced high levels of circulating anti-idiotypic antibody that was cytotoxic for the tumor cells in the presence of syngeneic C. Such animals showed good protection against injected tumor cells, but after a long delay, leukemia did develop. As tumor cells appeared in the blood, anti-idiotype was consumed, but the cells displayed little or no surface Ig and could not be killed in vitro by anti-idiotype and C. However, as serum antibody levels fell, the cells displayed increasing amounts of surface IgM lambda of the same idiotype and regained their susceptibility to the antibody and C in vitro. It appears, therefore, that the ability of tumor cells to modulate their target idiotype can present a considerable problem for antibody-mediated attack.