Impaired nuclear responsiveness to cytoplasmic signals in lymphocytes from elderly humans with depressed proliferative responses.

Lymphocytes from many elderly individuals exhibit depressed proliferative responses to the plant phytohemagglutinin (PHA). We have previously reported that this proliferative defect is not due to a failure to generate a cytoplasmic activator of DNA replication (ADR). In the present study, we tested the DNA synthetic response of nuclei derived from aged cells to an exogenous source of ADR. We found that nuclei from aged lymphocytes exhibiting low PHA responses were impaired in their ability to synthesize DNA in responses to ADR, compared with nuclei from younger adult donors. In contrast, those aged cells maintaining intact PHA responses provided nuclei that were also unimpaired in their response to ADR. The relationship between PHA responsiveness of the intact cells and ADR responsiveness of nuclei derived from these was a linear one. These results suggest that the depressed cellular reactivity of aged lymphocytes to PHA (when seen) may be due to a failure of these nuclei to respond to cytoplasmic stimulatory signals induced by the mitogen.