Phospholipid transmethylation in human mononuclear cells is not influenced by mitogens

The importance of accelerated methyl transfer from methionine to membrane phospholipid (PL) as an early event in lymphocyte activation is vigorously debated. We have examined pokeweed mitogen (PWM)- and phytohemagglutinin (PHA)-stimulated human peripheral blood lymphocytes for this early activation event. Without mitogen, methyl groups rapidly entered into the PL of unfractionated human lymphocytes by 10 min, and then adopted a rate which stayed constant for at least 2 hr. Whether or not mitogen was added after a 60-min preincubation, with [3H]methyl methionine, after a further 20 min, Sephadex G-10 adherent monocytes were incorporating methyl groups into PL 4 times faster than B-cells and 9 times faster than T-cells. At 10(7) cells/ml, neither PWM nor PHA changed total PL labeling kinetics, or produced a significant change of methyl groups from phosphatidyl-N-mono- and dimethylethanolamine to phosphatidylcholine or lysophosphatidylcholine in B-cells, T-cells or monocytes, whether these populations were cultured separately or together. Even for the slowly incorporating human T-cells, the background rate of methyl transfer per cell was much greater than that previously reported for mouse or pig mononuclear cells. Accelerated methylation of phospholipid appears not to be a favorable early event by which to study human lymphocyte activation, and we agree with Moore et al. [J. biol. Chem. 257, 8183-8189 (1982)] that it may not be a universal activation event.