Age-related tyrosine-specific protein phosphorylation defect in human T lymphocytes activated through CD3, CD4, CD8 or the IL-2 receptor

Although transmembrane signaling defect has been recognized as one of the major functional alterations involved in immune senescence, its biochemical nature as well as its precise molecular localization are still unknown. The available data indicate that an early step in the signaling cascade may be affected during the aging process. Because protein tyrosine kinases (PTK) are ubiquitously implicated in the initiation of physiological signals, they appear as prime candidates for age-related changes. The present investigation examined the effect of age on the activity of PTK associated with CD3, CD4, CD8 or the IL-2 receptor (IL-2R) in human T lymphocytes. By comparison with cells derived from young individuals, anti-CD3-activated T lymphocytes from elderly donors were more susceptible to herbimycin A, a PTK inhibitor known to prevent signal transduction by the T cell antigen receptor. This increased sensitivity of cells from senescent organisms to PTK inhibitors is most likely related to a lesser PTK activity since a significant decrease in the tyrosine phosphorylation of particular endogenous substrates was observed as a consequence of either CD3, CD4, CD8 or IL-2R activation. However, no age-related difference in tyrosine phosphorylation could be demonstrated when T cells were activated by pervanadate, a pharmacological activator of PTK. These results suggest that the intrinsic activity of the enzymes is preserved and that the age-associated defect in PTK activation occurs as a consequence of an upstream biochemical alteration. The defect in PTK activation could be the primary cause for the dysfunction of various components of the signaling cascade observed during the course of aging

Excessive apoptosis of mature T lymphocytes is a characteristic feature of human immune senescence

Recent evidence suggests that apoptotic deletion of activated mature lymphocytes is an essential physiological process implicated in both the regulation of the immune response and the control of the overall number of immunocompetent cells. Tightly interrelated signaling mechanisms convey either activation or death messages, achieving the necessary equilibrium between cell proliferation and cell deletion. During the course of aging, numerous alterations of these signaling pathways may shift the balance toward cell death. In the present investigation, the reduced DNA synthesis of anti-CD3 activated T lymphocytes isolated from elderly individuals is associated with an important and early cell deletion from the cultures. Visualization of DNA fragmentation in the remaining activated cells argues in favour of the apoptotic nature of the cell deletion. Quantification of histone-associated DNA fragments shows that the apoptotic process is greatly amplified in activated lymphocytes derived from senescent organisms. Further analysis reveals that IL-2 deprivation does not play a significant role in the age-related increase in apoptosis. Partial correction of this excessive apoptosis by products that bypass the early steps of the signaling cascade suggests that transmembrane signaling defects are involved in this process. Exploration of the antioxidant pathway reveals that the increased susceptibility of lymphocytes from senescent organisms to apoptosis is not explained by a decreased Bcl-2 expression and is not influenced by a modification of the intracellular concentration of glutathione (GSH)

Age-associated decline in cdk1 activity delays cell cycle progression of human T lymphocytes

Despite the repeatedly observed impaired proliferative response of T lymphocytes from aged donors, the precise molecular basis underlying such a defect is still poorly understood. The aim of this study was to determine whether cyclin-dependent kinase 1 (cdk1), a serine-threonine kinase required for entry into mitosis, is implicated in this age-associated dysregulation of the cell cycle. T lymphocytes derived from young and elderly donors were blocked in S phase by hydroxyurea after a 48-h activation by anti-CD3 Abs. Under these experimental conditions, only the cells that were already located beyond the S phase were able to complete the cell cycle, decreasing their DNA content from 4n to 2n chromosomes. Using this procedure, a delay in the accomplishment of mitosis could be observed in cells from elderly individuals, as evidenced by propidium iodide staining. In this age group, only a minimal cdk1 activity could be immunoprecipitated from cells sorted in G2/M after nocodazole block. The decrease in cdk1 activity observed in T lymphocytes from aged donors could be accounted for by at least three mechanisms: 1) a failure of these cells to express a sufficient amount of cdk1, 2) a reduced level of the associated cyclin B1, and 3) an incomplete dephosphorylation of the kinase on tyrosine. This low cdk1 activity is likely to postpone the progression through the G2/M transition and participates in the dysfunction of the cell cycle during the process of aging