Inhibition of phytohaemagglutinin-induced transformation of lymphocytes by human spleen extracts.

The presence in human spleen extracts of a substance capable of inhibiting the phytohaemagglutinin (PHA)-induced transformation of lymphocytes into lymphoblasts was demonstrated by analysing the nuclear size of cultured human lymphocytes. In this way the artefacts produced by the methods based on the uptake of 3H-thymidine were avoided. The inhibitor did not compete with PHA for the same membrane receptor-site, since adding the extract 24 h after the PHA-stimulation still produced a significant inhibition. The lack of cytotoxicity of the extract was demonstrated by the fact that cultures that lasted for six days, instead of three days, resulted in a pattern similar to that of control cultures. An attempt to partially purify the inhibitor by means of ammonium sulphate fractionation showed that the highest activity precipitated between 0.5 and 0.6 saturation although, due to the characteristics of the assay, it was difficult to assess the actual degree of purification.