The role of cytotoxic T-lymphocytes in the prevention and immune surveillance of tumors - lessons from normal and immunodeficient mice

The idea of immunological surveillance against cancer has existed for nearl y 100 years but as no conclusive evidence has yet been published the import ance of the cellular immune defense in the detection and removal of incipie nt or existing tumors is still a hotly debated subject. However, in order t o select a relevant immunotherapeutic strategy in the treatment of cancer, a fundamental understanding of the basic immunologic conditions under which a tumor develops and exists is a prerequisite. Therefore, a murine model w as set up that we hoped would enable us to confirm or reject the theory of immunological surveillance. A large panel of methylcholanthrene induced tum ors was established in T-cell immunodeficient nude mice and congenic normal mice to study the influence of the immune system on developing tumors. As nude mice developed tumors fastest and with the highest incidence, we concl uded that in this model the immune system constituted a 'tumor-suppressive factor' delaying and sometimes abrogating tumor growth, i.e. performing imm une surveillance. Immunogenicity of the tumors was assessed by transplantat ion back to normal histocompatible mice. Tumors originating from the immuno deficient nude mice turned out to be far more immunogenic than tumors from normal mice, resulting in a high rejection rate. CD8+ cytotoxic T cells wer e found to be indispensable for this rejection, leading to the conclusion t hat the cytotoxic T cells perform immune selection in normal mice, eliminat ing immunogenic tumor cell variants in the incipient tumor. In this review, we discuss the difficulties facing immunotherapy when conclusions are draw n from the presented observations and hypotheses.