Elderly persons have been exposed to a myriad of pathogens over their lifespan. This life-
long immunological history leads, in some cases, to the generation of expanded populations
of memory CD8 T cells that have reached the end stage of replicative senescence. In cell
culture, CD8 T cells that are subjected to repeated rounds of antigen-driven proliferation
eventually show irreversible cell cycle arrest, permanent and complete loss of CD28 gene
expression, apoptosis resistance, reduced gene transcription of the major stress protein in …