Tumors Immunological Escape Of

Karl Erik Hellström and Ingegerd Hellström, Pacific Northwest Research Foundation, 720 Broadway, Seattle, WA, USA

Many experimentally induced rodent tumors express tumor-specific transplantation antigens (TSTAs). These are detectable by the rejection of tumor cells transplanted to immunized syngeneic hosts. Many spontaneous mouse tumors express antigens that can, under certain circumstances, be recognized by the immune system. Human tumors can also induce an immune response which can be detected by assays for serum antibodies, T helper cell activity and/or the generation of cytotoxic T lymphocytes (CTI.s), with most of the targets being tumor-associated differentiation antigens.

Even those tumors that have strong TSTAs appear in immunocompetent hosts in which they grow progressively. A question that needs to be answered is, therefore, why are they not immediately eliminated by immunological mechanisms? Furthermore, immunological mechanisms which can kill tumor cells in vitro and reject transplants of small numbers of tumor cells in vivo commonly fail to destroy neoplasms which are more than a few millimeters in diameter. Even the transplantation immunity that can be induced against TSTA-positive tumors can be overcome by challenge with a 100- to 10000-fold larger number of tumor cells than that needed for outgrowth in nonimmune animals. The next question is, therefore, what mechanisms make this possible? We shall discuss several such 'escape' mechanisms here.

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