Presence of natural killer cells in specific-pathogen-free chickens.

Spleen cells of normal specific-pathogen-free N, P, and 15 X 7 chickens were cytotoxic in vitro for target cells of Marek's disease lymphoma line MSB-1. The natural killer (NK) cell activity, best expressed in chickens over 7 weeks old, varied among several genetic lines of chickens tested. The NK cells were thermolabile, and incubation of the cells at 37 degrees C for 30--60 minutes resulted in substantial loss of cytotoxicity. The specificity of NK effector cells was directed against common antigen(s) on tumor cell lines of diverse origin but not on normal adult or embryonic cells.

Cell-mediated cytotoxic response to cells bearing Marek's disease tumor-associated surface antigen in chickens infected with Marek's disease virus.

In a microcytotoxicity test in which 51Cr-labeled cells of a Marek's disease (MD) lymphoblastoid line (MSB-1 line) were used, cell-mediated cytotoxicity of spleen cell suspensions prepared from chickens inoculated with MD virus was demonstrated. This cytotoxic response, presumably directed against MD tumor-associated surface antigen, was detectable briefly after virus infection and paralleled the appearance of early lymphoproliferative lesions characteristic of MD.

Development of cell-mediated immunity to Marek's disease tumor cells in chickens inoculated with Marek's disease vaccines.

Chickens inoculated with herpesvirus of turkeys or with apathogenic or attenuated vaccine strains of Marek's disease virus (MDV) developed a T-cell-mediated immune response to Marek's disease (MD) tumor cells. This immune response was detected in a 4-hour 51Cr-release assay in which effector cells obtained from spleens of vaccinated chickens were reacted with 51Cr-labeled target cells of an MD lymphoblastoid cell line (MSB-1). The cytotoxic effector cells generated by the vaccine viruses had characteristics similar to those noted previously for anti-MSB-1 effector cells generated by MDV. The immune response was specific to MSB-1 cells, because another target cell line (TLT) antigenically unrelated to MSB-1 cells was not lysed by the effector cells nor did the unrelated target cells inhibit the cytotoxicity of effector cells against MSB-1 target in a cold-target inhibition assay. Because MSB-1 cells contain MD tumor-associated surface antigen, we postulated that the immune response detected in the vaccinated chickens may be directed against this antigen and that the antitumor antigen immunity may play a role in the mechanism of vaccine protection against lymphoma development by pathogenic MDV.

Effect of in vitro adaptation of Marek's disease virus on pock induction on the chorioallantoic membrane of embryonated chicken eggs.

Cell-associated preparations of several isolates of Marek's disease virus produced more pocks on the chorioallantoic membrane of embryonated chicken eggs than plaques in duck embryo fibroblasts, thus indicating that lesion response in eggs was more sensitive than cytopathic response in duck embryo fibroblasts for assaying low-passage Marek's disease virus. Adaptation of the virus to cell cultures by serial passages, however, substantially reduced its pock response so that the titer ratio (plaque-forming units in duck embryo fibroblasts/pock-forming units in eggs) of cell culture-adapted Marek's disease virus was 1 or higher. The decreased pock response could not be attributed to selection of preexisting virus variant(s) with low affinity for chorioallantoic membrane because cloned Marek's disease virus had a good pock response at low cell culture passage levels, but this response decreased as the virus was attenuated by serial cell culture passage.