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774 EARLY EVENTS IN HUMAN LYMPHOCYTE ACTIVATION MEASURED BY HYBRIDIZATION TO PURIFIED GENES

A specific hybridization assay for ribosomal RNA (rRNA) sequences reveals a dramatic increase in rRNA synthesis 2-6 hours after stimulation of human peripheral lymphocytes with phytohemagglutinin (PHA). Preliminary experiments demonstrated a serum requirement for long term survival and 3H-uridine incorporation by peripheral lymphocytes incubated in RPMI media at 37 . Purified proteins such as bovine serum albumin or lactalbumin could not substitute for this requirement and human serum gave lower background RNA synthesis than fetal calf serum. Lymphocyte rRNA synthesis, as measured by hybridization to cloned rDNA segments, closely3 parallels total RNA synthesis as measured by the incorporation of 3H-uridine. An early linear phase (2-14 hours) of PHA-stimulated rRNA synthesis was followed by a biphasic increase in synthesis at later times (14-32 hours). A more delayed and less complex response was observed in purified T lymphocytes, indicating heterogeneity among lymphocyte populations for PHA stimulation of rRNA synthesis. Synthesis of rRNA or total RNA by T cells was enhanced 25 fold 30 hours after PHA stimulation. This response affords a model system for gene activation which defines factors regulating rRNA synthesis in particular and cellular differentiation in general.