Cyclic nucleotides in mitogen-induced lymphocyte proliferation

Laboratory of Immunopharmacology, Sloan-Kettering Institute for Cancer Research, New York, N. Y. 10021 Fundamental to understanding the nature of the induction of proliferation is the elucidation of the earliest events following mitogen action and an analysis of their cause and effects. In recent years a concept has emerged that agents which induce lymphocyte proliferation do so through an action restricted to the cell surface. Greaves and Bauminger 1 showed that mitogenic plant lectins like concanavalin A (Con A) and pbytohemagglutinin (PHA) bound to 'Sepharose' beads can induce tile proliferation of T lymphocytes without entering the cell. In addition PHA must apparently be present in optimal mito- genic concentration lot only as little as 3 h in order to trigger maximal proliferation 2. Thus, the duration and intensity of the cell surface stimulus appears to deter- mine a short-term 'switch-on' or 'trigger signal' process. The initial phase of lymphocyte activation following mitogen binding is characterized by nuclear activation with new RNA and protein synthesis, by blastogenesis, i.e. cellular and nuclear enlargement, and by the appearance of receptors for T-cell growth factor (TCGF or interleukin 2 (IL2). A second phase involves mitogen-dependent production of lympho- cyte activating factor (interleukin I (ILl)) by acces- sory cells, resulting in the production of IL2 by T lym- phocytes which spurs the mitogen-primed lympho- cytes to enter DNA synthesis and to undergo clonal replication as long as the IL2 is present. The process is thought then to involve a double signal - the first supplied by the mitogen for initial cellular activation which moves the cell from a restricted 'G~' or 'G~j' phase of the cell cycle into a late 'G~' phase and the second, supplied by IL2, which moves the cell into S phase and mitosis (Fig. 1). The surface characteristics of PHA and Con A action have prompted a number of investigators to examine their role in the early events of cell activation, viewing them as hormones which exert their effects through receptors on the cell surface*. Cyclic nucleotides Parker and co-workers 4 initially observed that PHA and Con A induced early increases in the concentra- tion of lymphocyte cyclic AMP and considered this effect to be related to mitogen action. Earlier work showed that agents which increase cyclic AMP in lymphocytes inhibit the induction of proliferation by phytomitogens. This action is limited to the early phase of the triggering process. In studying the effects of PHA and Con A on the concentration of both cyclic AMP and cyclic GMP, we found that at optimal mito- genic concentration, both mitogens increased cyclic GMP 5 but that neither mitogen influenced the con- centration of cyclic AMP. The ability of plant lectins to increase the cellular concentration of cyclic AMP in Iymphocytes has remained controversial. The disputes appear to rest on differences in incubation conditions, mitogen con- centrations, and nucleotide assay methods. While most investigators have observed that optimal mito- genie concentrations of PHA and Con A do not increase the concentration of cyclic AMP in the lymphocyte a number have found that at high doses cyclic AMP concentrations are raised while the mito- genic effect is lost (high dose inhibition). It is at these high concentrations that one clearly observes the agglutinating activity and the induction of patch and cap formation by these agents. Edelman and co- workers 6 showed that mitogenic succinylated subunits of Con A do not produce patch and cap formation and also do not show high dose inhibition characteristic of *These studies were reviewed in 1978 and the reader is referred to Ref. 3 for more complete referencing of the pre-1978 studies. © Elsevier Biomedical Pless 1982 01674919/82/0000 0000/$1.00