Acquired folate deficiency in phytohaemagglutinin-stimulated human lymphocytes

3H-thymidine (3H-TdR) incorporation, deoxyuridine (dU)-suppressed values, modal cell volume and the relative DNA content of individual cells have been determined in phytohaemagglutinin (PHA)-stimulated normal human lymphocytes cultured for varying periods in folate-free medium and media supplemented with various concentrations of folic acid. The results indicate that when the intracellular folate content falls below 400-600 pg/10(6) cells, lymphocytes exhibit increased 3H-TdR incorporation into DNA, raised dU-suppressed values, an increase in cell volume and abnormal DNA synthesis. This critical concentration of folate is just present in resting lymphocytes, but falls on stimulation by PHA, and the effects of folate deficiency become increasingly evident beyond a culture period of 24 h unless the folic acid concentration in the medium is at least 0.32 microgram/ml. Thus human lymphocytes cultured in folate-free medium provide a model for folate-deficient DNA synthesis, but their usefulness for the performance of dU suppression tests in the diagnosis of clinical folate deficiency is likely to be limited.