T-dependent immunity in aged humans. I. Evaluation of T-cell subpopulations before and after short term administration of a thymic extract

T lymphocytes, T lymphocyte subpopulations and delayed hypersensitivity reactions to recall antigens were studied in 20 aged subjects clinically unaffected by disorders of the immune system. T-cells evaluated by the sheep rosette test and absolute T-cell number were within normal range. T-cell subsets were studied by the high affinity rosetting test and by the sheep rosette test after pre-incubation with theophylline; such methods have been shown by others to discriminate between T-cells with helper or suppressor activity. We did not observe an imbalance of such subpopulations. Skin reactivity to recall antigens (Candida and SK-SD) was depressed in several cases. Since a decline in serum levels of thymic hormones has been reported in the ageing and it is suggested that this could account for the T-dependent immunity defect, we treated these patients with a bovine thymic extract (thymostimulin, TP-1) 1 mg/kg/day for three consecutive days. Immunological tests were performed again after this therapy but no modifications were observed. We may conclude that the aged subjects studied in this paper present normal relative and absolute number of T-cells and T subpopulations, whereas the function of T-cells, as assessed by skin reactivity to recall antigens is often depressed. Short term therapy with TP-1 at the above-mentioned dosage was ineffective in modifying the parameters analysed.