Most in vitro T cell proliferation experiments are performed by using serum-supplemented medium, yet the actual contributions of serum components to cell cycle progression remain ill-defined, thus complicating attempts to fully define requirements for cell division. By utilizing a functional separation between T cell receptor-triggered "competence" and IL 2-promoted "progression" to independently assess serum requirements during each cell cycle stage, it was shown that serum serves an essential, active role only during the early events of the competence phase (G0-G1 transition) of T cell activation. Serum is required for optimal IL 2 production and the cell surface expression of IL 2 receptors after the stimulation of the T3/Ti antigen receptor complex. In contrast, serum does not function actively during IL 2-mediated progression through the G1 phase of the cycle. Serum proteins serve only a passive role at this stage, preventing the adsorption of IL 2. This same effect can be provided by any number of proteins including IL 2 itself, or even a high cell concentration. Supplementation of serum-free T cell cultures solely with IL 2 and transferrin is sufficient for maximal T cell proliferation, although the time of the peak response is delayed owing to a suboptimal rate of IL 2 receptor expression. Accordingly, the realization that serum is only necessary for the earliest stage of T cell activation will now enable studies designed to identify the critical individual serum components and to define their mechanism of action.
