Intracellular mechanisms of lymphoid cell activation

Activation of lymphocytes for proliferation is associated with the appearance of an intracellular factor (ADR) that can induce DNA synthesis in isolated quiescent nuclei. ADR plays a role in the sequence of intracellular events leading to activation for IL-2-mediated proliferation. Because of the nature of the defining assay, the locus of ADR action appears to be near the terminal end of the transduction pathway. Interestingly, although lymphocytes from aged individuals respond poorly to proliferative stimuli, they appear to produce normal to above-normal levels of ADR. In contrast, their nuclei are only poorly responsive to stimulation by ADR. Preparations rich in ADR activity have proteolytic activity as well. In addition, aprotinin, as well as a variety of other protease inhibitors, suppresses ADR-induced DNA synthesis in a dose-dependent manner. ADR activity can be removed from active extracts by absorption with aprotinin-conjugated agarose beads, and can be removed from the beads by elution at pH 5.0. This latter suggests that ADR itself is a protease. However, its endogenous substrate is not yet known. We have also detected an inhibitor of ADR activity in the cytoplasm of resting lymphocytes. This is a heat-stable protein of approximately 60,000 Da. In addition to suppressing the interaction of ADR with quiescent nuclei, the inhibitor can suppress DNA synthetic activity of replicative nuclei isolated from mitogen-activated lymphocytes. Interestingly, these preparations had little or no activity on replicative nuclei derived from several neoplastic cell lines. The resistance of tumor cell nuclei to spontaneously occurring cytoplasmic inhibitory factors such as the one described here may provide one explanation for the loss of growth control in neoplastic cells.