APC-dependent impairment of T cell proliferation in aging: Role of CD28- and IL-12/IL-15-mediated signaling

The age-related impairment of phytohaemagglutinin (PHA)-triggered peripheral blood mononuclear cell (PBMC) proliferation was paralleled by an expansion of CD28^- T lymphocytes with a poor capacity to undergo lectin-induced blastogenesis. However, both CD28^- and CD28⁺ T cells isolated from aged individuals exhibited a significant reduction of proliferative response to PHA in comparison with young controls, this implies that the CD28-mediated signaling is not the only defective pathway in the elderly. Thus, PBMC or T cell subsets plus monocytes from aged donors were stimulated with PHA and assayed for the production of, or the response to cytokines known to regulate T cell functions. Results can be so summarized: (i) interleukin (IL)-2 as well as IL-10 release was unaffected by age; (ii) in both groups of subjects, IL-15 concentrations were similar to those spontaneously released by PBMC; (iii) surprisingly, IL-12 p70 and IL-12 p40 production by PBMC was markedly increased in the aged group; (iv) in spite of this finding and of the experimental outcome that IFN-γ synthesis was almost completely dependent on IL-12. PBMC from old individuals did not release higher amounts of IFN-γ in comparison with young controls; (v) moreover, only a slight increase in IFN-γ production was observed in PBMC cultures from the aged group as a result of IL-12 and/or IL-15 costimulation; (vi) at the same time, even though IL-12 as well as IL-15 were necessary for an efficient T cell proliferation, the addition of exceeding doses of cytokines proved to be ineffective in enhancing the proliferative outcome of PBMC or of both CD28⁺ and CD28^- T cells in the aged group. Taken together, the data outline the role of CD28 and IL-12/IL-15 signaling impairment in T cell proliferative deficiency during senescence.