The Effect of Phytohemagglutinin on Ribonucleic Acid Synthesis and Histone Acetylation in Equine Leukocytes

It is well known that peripheral lymphocytes rarely divide in tissue culture; indeed, they have usually been considered to be the end cell in the pathway of differentiation. However, in the presence of certain mitogenic agents, of which phytohemagglutinin (PHA) is the best known example, a striking transformation occurs; the cells increase their over-all metabolic activity, enlarge, and divide. This response of the small lymphocyte, widely studied since its discovery in 1960 (1, 2), is now known to involve an increased synthesis of ribonucleic acids (3-5). It has also been observed that histone metabolism is altered very early in the process of gene activation for RNA synthesis (5). The acetylation of arginine-rich histones, in particular, is increased very soon after exposure of the cells to the mitogenic agent. Relatively little attention has been paid to the effect of PHA on other cell types. Recently Andrew and Gabourel (6) described cytotoxic effects of PHA on rapidly growing cells in vitro and in vivo. On the other hand, Ioachin (7), found increased mitotic indexes in cell lines in tissue culture under the effect of PHA; Gamble also observed a stimulatory response in all cell types in mouse spleen in vivo (8). Ameba growth was also stimulated by PHA (9). In polymorphonuclear leukocytes, only vacuolization has been found after some hours in tissue culture in the presence of PHA (10, 11), and Razari found fluorescence in the polymorphonuclear cell cytoplasm as soon as 15 min after exposure to fluorescent PHA (12). In a previous publication (5), the correlation between RNA synthesis and histone acetylation was studied in lymphocytes stimulated by PHA. We also observed that PHA inhibited both processes in equine granulocytes. A detailed account of these effects in equine lymphocytes and granulocytes (with special reference to RNA synthesis and histone acetylation) will be reported here.