Feline cytotoxic immune mechanisms against virus-associated leukemia and fibrosarcoma

Humoral and cellular cytotoxic immune mechanisms of cats were compared against feline leukemia virus (FeLV)- and feline sarcoma virus (FeSV)-transformed cells. The groups of animals studied were nonexposed control cats; FeLV-infected immune or viremic tumor-bearing cats; FeSV-inoculated tumor progressor or regressor cats, and cats immunized with FeSV-transformed autochthonous fibroblasts (ATF). Sera containing complement-dependent antibodies (CDA), which lysed FeLV-producer lymphoma lines, had no cytotoxic effects when tested against FeLV-producer FeSV-transformed fibroblasts. Sera with lytic CDA activity were also tested for antibody-dependent cellular cytotoxic (ADCC) effects with peripheral blood lymphocytes (PBL) from nonimmune cats. No ADCC activity was detected against either lymphoid or fibroblast target lines. To demonstrate that cat PBL contained ADCC effector cells, antibody-coated murine target cells were employed and positive results obtained. Natural killer (NK) assays were performed using PBL from normal and tumor-bearing cats. Cytotoxic effects were only detectable to FeLV-producer lymphomas, and comparable levels of NK activity were found in normal and lymphoid tumor-bearing animals. In cats immunized with ATF, a population of effector cells was found in peripheral blood which had functional characteristics of cytotoxic T lymphocytes (CTL). The killing of ATF by CTL-like cells was not inhibited by FeLV/FeSV immune sera or by sera from autochthonous immune cats. The comparative importance of humoral and cellular cytotoxic mechanisms against FeLV- and FeSV-induced tumors is discussed.