Age-associated changes in mitogen-induced protein phosphorylation in murine T lymphocytes

Since T lymphocyte proliferation declines with age, and since activation of a still only partially defined set of protein kinases is thought to play a critical role in T cell activation, we have carried out a systematic survey of age-related changes in protein phosphorylation in T lymphocytes. We used two-dimensional electrophoretic analysis to examine lysates prepared from T cells after 10 min of exposure to mitogens [anti-CD3, concanavalin A (Con A) and phorbol 12-myristate 13-acetate (PMA) plus ionomycin] and nonmitogenic activators (PMA or ionomycin used separately). Our results show a progressive, life-long decline in the levels of anti-CD3-induced phosphorylation of all 16 phosphoproteins that respond vigorously in T cells of young mice. Mice 10-18 months of age showed levels of response that were clearly below those induced in young mice, while responses of mice 22-24 months of age were even more severely diminished. Responses to Con A, PMA, ionomycin and a combination of PMA plus ionomycin were equally blunted in old mice, except for 2 (of 12) phosphoproteins that continued to respond to PMA even in the old animals. None of the phosphoproteins which were ionomycin responsive in young mice continued to respond in old mice, although 1 (of 3) ionomycin-inhibitable phosphoproteins remained inhibitable in old mice. We also found three phosphoproteins which became phosphorylated in response to anti-CD3, PMA and Con A (but not ionomycin) only in old mice, and a pair of phosphoproteins which were unresponsive to all mitogens but showed a higher "baseline" phosphorylation in T cells from old mice. Comparing phosphoprotein patterns between CD8- and CD8-CD45RB- T cells from young mice allowed us to identify nine phosphoproteins that were strongly responsive to anti-CD3 only when CD45RB+ (i.e. virgin) cells were present. Although the immune system of old mice consists largely of memory T cells, the change in phosphoprotein phosphorylation patterns cannot simply be explained by the accumulation of this cell type. We conclude that aging leads to a global impairment of several distinct protein kinase pathways in both virgin and memory T cell sets.