Cell stimulation by haemopoietic lymphokines

Haemopoietic cells appear to respond to two distinct classes of growth factors, cell-bound molecules and lymphokines. Stimulation by lymphokines can be modelled by IL-3-dependent cell lines, and evidence is presented that such stimulation may represent a novel system of cell signalling unrelated to those where stimulation is a progression from a resting cell state into DNA synthesis and mitosis. The concept of a "rolling" cycle is introduced, and discussed in relationship to recent results suggesting that phosphorylation of a specific 33 kDa protein may be part of the control mechanism. Based on responses to different IL-3s, it is suggested that one function of lymphokines is to modulate responses to others by mechanisms other than regulation of receptor expression.