Some early events in the primary mitogenic stimulation of lymphocytes differ from later interleukin stimulation and other quiescence to growth activation systems.

The requirement for activity of the enzyme ADP-ribosyl transferase (ADPRT) and changes in single-strand DNA breaks were assessed during the initial stimulation of quiescent murine splenic lymphocytes with mitogen alone, the stimulation of activated blasts with IL-2-containing medium and, for comparison, the serum stimulation of quiescent fibroblasts and the induction of haemoglobin synthesis in an erythromyeloid cell line K562. Inhibitors of ADPRT, at concentrations previously found to have no effect on the proliferation of lymphoblastoid cell lines, blocked the stimulation of spleen cells by Con A or LPS; non-inhibitory analogues had much less effect. No early increase in ADPRT activity after mitogenic stimulation was detectable. The rejoining of single-strand breaks was observed after stimulation of splenic lymphocytes with Con A, but not consistently with LPS. Conversely, ADPRT inhibitors had only little effect on the IL-2-induced stimulation of Con A blasts, and no effect on the stimulation of fibroblasts or K562. Neither were any changes in strand breaks associated with these systems. These findings implicate ADPRT activity and the rejoining of strand breaks in the early mitogenic response as being distinct from later IL-2 activation and changes from quiescence to growth in other cell types.