Escape of Small Numbers of Allogeneic Lymphoma Cells From Immune Surveillance 2

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Abstract
Cells of 4 radiation-induced lymphomas in C57BL/10, B10.129(5M), and B10.A mice were transplanted into congenic-resistant hosts across the major histocompatibility barrier. Relatively small grafts (10-105 cells) consistently escaped immune surveillance and grew progressively, whereas larger grafts (106-107 cells) were rejected. The mechanism of this dilution escape was investigated with the B10.A lymphoma (H-2a) transplanted into H-2K-lr incompatible B10.A(5R) mice (H-2f). The recipients were subjected to splenectomy, adoptive or active immunization before and after tumor grafting, and various schedules of antitumor and immunosuppressive treatments. The tumor cells were used after either serial syngeneic or allogeneic passages under conditions of dilution escape. The results did not support the hypotheses that active enhancing responses by the host, antigen simplification of tumor cells, and reduced immunosensitivity of lymphoma grafts influenced dilution escape. The hypothesis of creeping tumor growth to a population size overwhelming the host's immune defenses (i.e., the “sneaking through” mechanism) was inadequate to account for all experimental findings. Evidence was obtained that factors depressing antilymphoma allograft reactivity were associated with tumor cells. Hence dilution escape may be the result of a nonspecific inhibition of antilymphoma allograft reactions before the initially small tumor cell population reaches the size to be immunogenic.

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