Abstract
The blastogenic response of human lymphocytes in vitro to hyperoptimal concentrations of concanavalin A (Con A) has been studied by means of volume spectroscopy (measuring cellular and nuclear volume), flow cytofluorometry (measuring cellular DNA content) and incorporation of [^3^H]thymidine ([^3^H]dThd). The optimal Con A dose with respect to [^3^H]dThd incorporation was about 30 u, g/ml. In cultures given hyperoptimal doses, e.g. 100 ΞΌg/ml, [^3^H]dThd incorporation was strongly inhibited, whereas the number of cells entering Sβphase and significantly increasing their cellular and nuclear volume was considerably larger than with 30 ΞΌg/ml. With 200 ΞΌg/ml Con A, which induced negligible [^3^H]dThd incorporation, the percentage of responding cells was even larger. Hence, doses of Con A, which were hyperoptimal with regard to [^3^H]dThd incorporation, induced blastogenic response, including DNA synthesis, in a larger percentage of the cells than did the optimal dose. However, in cultures with hyperoptimal Con A doses, the progression of the cell cycle stagnated mainly during Sβ and G2βphase and few cells completed mitosis. Thus, the blocking effect of hyperoptimal doses was not confined to any particular point of the cell cycle. The reduced [^3^H]dThd incorporation, seen with hyperoptimal doses, is attributed partly to a failure of this assay under such conditions.