Spontaneous and induced herpesvirus genome expression in Marek's disease tumor cell lines

We incubated 31 newly established Marek's disease tumor cell lines at 41 degrees C for 48 h after subculturing and then examined them to determine the spontaneous rates of expression of viral internal antigen(s), viral membrane antigen(s), and virus isolation. All but two of the lines were isolated from tumors induced by clone-purified Marek's disease virus strain JM-10, GA-5, RB-1B, and BC-1A in nine different genetic strains of chickens with defined histocompatibility antigens. The line-to-line variations in the rates of spontaneous expression for the antigens or virus rescue were great, but the levels of expression were very low in most cases. The median rates of expression for viral internal antigen, viral membrane antigen, and virus isolation were 32, 8, and 2 positive cells per 10(5) cells, respectively (ranges, 0 to 20,280, 0 to 22,990, and 0 to 220 positive cells per 10(5) cells, respectively). The ratio of viral internal antigen expression to virus isolation was extremely variable and often high, whereas the ratio of viral internal antigen to viral membrane antigen expression was more consistent and generally low. The virus strain which induced the cell line influenced the level of virus genome expression, but the cell genotype did not. Cell lines transformed by JM-10 virus, which exhibited low oncogenicity, had significantly (p less than 0.01) higher rates of expression than cell lines transformed by CA-5 and RB-1B viruses, which exhibited high oncogenicity. Treatment with iododeoxyuridine or incubation at 37 degrees C induced increased rates of expression in most lines but not in all lines. The degree of enhanced expression was inversely proportional to the rate of spontaneous expression.