The nature of immunity to Snyder-Theilen fibrosarcoma virus induced tumors in cats.

Snyder-Theilen fibrosarcoma virus (ST-FeSV) induced tumors evoked a vigorous immune response in adolescent cats. The response was characterized histologically by a lymphoid and histiocytic cell infiltrate beginning around the 9th day post inoculation. Hyperemia edema, hemorrhage, and necrosis of the tumors occurred shortly thereafter. Gross regression of the tumors commenced around the 15th day. Viable fibrosarcoma cells could be recovered as almost pure cultures from tumors biopsied on the 9th day. Biopsies taken between days 9 and 15 contained progressively fewer tumor cells and increasing numbers of lymphoid cells, histiocytes, giant cells, and normal fibroblasts. Tumor cells in such mixed cultures did not replicate as fast as normal and died out within 7 to 14 days. Viable tumor cells were not recovered from biopsies taken after day 15. Fibrosarcoma regression was associated with the appearance of tumor cell specific cytotoxic lymphocytes and antibodies in the blood. Cell mediated immunity, as determined by a chromium release assay, consisted of both antibody dependent and independent mechanisms. Fluorescent and complement dependent cytolytic antibodies were detected in the blood at the same time as cytotoxic lymphocytes, but persisted after regression. In a preliminary experiment, serum from tumor regressor cats was injected into susceptible kittens, and the kittens were then challenged with ST-FeSV transformed fibroblasts or whole FeSV. Immune serum did not prevent the appearance of initial growth of tumors, but did slow their subsequent growth and increased the rate of regression. Immune serum had a much more dramatic inhibitory effect on the accompanying retrovirus infection.