Effect of thymosin on cellular immunity in old age

Cell-mediated immunity (CMI) in a population of 28 well-nourished, disease-free old-age individuals (65--103 years) was compared to that of healthy young adult controls (22--41 years) and the effects in vitro of thymosin fraction 5 (TF5) evaluated. The number of T cells (T cell rosettes, TCR) in the peripheral blood was the same as controls. However, there was a significant depression in lymphocyte response to phytohemagglutinin (PHA) (p Less Than 0.005). Limiting dilution of the PHA response revealed an inherent defect in the function of those cells that are capable of responding to PHA. The formation of TCR from old-age subjects was similar to controls when incubated with TF5, i.e. there was no enhancement. When TF5 was incubated with old-age and normal control lymphocytes in the mixed lymphocyte reaction (MLR), there were significant increases in the responses in both (71 +/- 39% and 64 +/- 52%, respectively; p Greater Than 0.2). There was also a mitogenic effect of TF5 on resting lymphocytes which was significantly greater in the control than in the old-age population (p Less Than 0.002). Our results suggest that in old age: (1) there is no loss in absolute numbers of T lymphocytes; (2) there is a defect in the ability of individual lymphocytes to function; and (3) the response of lymphocytes in vitro to TF5 in forming TCR and in the MLR is statistically similar to young adult controls, and suggests that the decline in CMI with age is not due to a loss of thymosin-responsive cells.