Correlation between the presence of T-antigen and the reinitiation of host DNA synthesis in senescent human diploid fibroblasts after SV40 infection

Senescent human diploid fibroblasts, TIG-1, had labelling indices of about 0.5-3% when labelled with 3H]thymidine for 3 days in fresh medium containing 10% fetal bovine serum. When these cells were infected with SV40, the percentage of nuclei incorporating 3H]thymidine increased by about 10-fold. The frequency of T-antigen-positive cells and that of 3H]thymidine-incorporating cells were almost the same. About 80% of T-antigen-positive cells were also positive to incorporation of 3H]thymidine, and the same result was obtained in infected young cells. These results indicated that senescent human diploid cells which are brought to synthesize T-antigen always initiate DNA synthesis as young cells do. The characteristics of senescent cells as compared with younger cells was low incidence of T-antigen-positive cells after infection. The basis of low susceptibility of senescent cells to initiate DNA synthesis by SV40 infection thus seems to be concerned with an event after the adsorption of virus, but before the synthesis of a detectable amount of T-antigen.