Evidence for an Immunological Reaction of the Host Directed Against Its Own Actively Growing Primary Tumor2

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Abstract
Cells isolated from primary benzo[a]pyrene-induced fibrosarcomas in rats of a pure line were tested for their ability to grow as autografts when injected back into the autochthonous host. The autograft did not take if the primary tumor had been removed but grew occasionally in animals in which the major part of the tumor was left. In every instance the sarcoma cells grew when injected into syngeneic recipients. In these recipients the growth of the sarcoma cells was prevented or retarded when they were mixed in vitro, prior to injection, with spleen cells from animals that had been immunized against the tumor. Autochthonous spleen cells taken from the animal with the tumor behaved in this test like those from immunized animals as long as the spleen was taken 3 weeks after removal of the tumor. When the spleen and tumor were removed at the same time, however, autochthonous spleen cells did not behave like spleen cells from immunized animals and did not interfere with the growth of the tumor. The results of both the autograft and spleen cell experiments suggest that rats react actively against their own growing primary tumors, but that the tumor exhausts the supply of lymphocytes responsible for this reaction. After the tumor is removed, the concentration of antitumor lymphocytes in the spleen builds up and the animals can reject an autograft.