We have investigated three human fetal thymuses by various induction procedures in a search for retrovirus. Thymus cells were cultured for 7 days together with mitomycin C-treated B cells and examined for reverse transcriptase activity. The following results were obtained. 1) RNA-directed DNA polymerase (RDDP) activity was detected in the culture medium on day 3 and persisted until day 7. This enzyme, after partial purification on phos-phocellulose column, showed template-primer specificity of a typical reverse transcriptase. 2) In unstimulated thymus cells, no reverse transcriptase activity was detected. 3) Stimulation with lymphocyte mitogens such as phytohaemagglutinin (PHA) and concanavalin A (Con A) exerted no effect on inducing the reverse transcriptase activity. 4) No reverse transcriptase activity was obtained in the culture medium of B cells pretreated with mitomycin C. Taken together these results indicate that in human fetal thymus cells reverse transcriptase activity is induced by human B cell stimulation, pointing to an intriguing possibility of protovirus activation.
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