Acceleration of human lymphocyte activation after preliminary culture

Human peripheral blood mononuclear cells which are precultured in tissue culture medium containing 5% fetal calf serum (FCS) have markedly augmented reactivity to a suboptimal concentration of phytohemagglutinin (PHA). The augmentation of response is due in part to an acceleration of lymphocyte activation. Precultured cells also respond in an accelerated manner to allogeneic cells. The augmentation is apparent after 24 hr of preliminary culture, and reaches a maximum at 4 to 7 days. Kinetic studies of the culturally augmented PHA response indicate that an increased number of reactive lymphocytes rather than an increased rate of proliferation accounts for the effect. Adding fresh cells to the precultured cells does not suppress their augmented response. When human serum is substituted for FCS in the preliminary culture, the augmentation of response does not occur. Serum-mixing experiments indicate a suppressive effect of human serum. These studies may be relevant for understanding immunologic mechanisms related to transplantation and autoimmunity.