Several B-lymphocyte mitogens have been previously characterized as efficient inducers of endogenous C-type viruses in mouse spleen cell cultures. We now report that foetal calf serum is also capable of inducing C-type virus release in such cultures. While virus induction by B-cell mitogens was found to be serum independent, the combined effects of serum and mitogens were found to be additive and, with some serum batches, synergistic. The kinetics of induction of virus release by serum was very similar to the established pattern using mitogens. The effect of serum was concentration-dependent. The serum lipoprotein fraction prepared by density ultracentrifugation contained virus-inducing activity. By co-cultivation with mink CCL64 cells, stable lines of mouse xenotropic C-type virus could be recovered from cultures which contained serum, serum lipoprotein fraction or mitogens, but not from control cultures. Preliminary evidence indicates that human sera contained a similar virus-inducing activity in the lipoprotein fraction.


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