Human cytomegalovirus-specific CD8+ T-cell expansions contain long-lived cells that retain functional capacity in both young and elderly subjects

P>The immune response to human cytomegalovirus (HCMV) infection is characterized by the accumulation of HCMV-specific CD8+ T cells, particularly in the elderly; such expansions may impair immune responses to other pathogens. We investigated mechanisms underlying HCMV-specific expansions in 12 young and 21 old healthy subjects (although not all analyses were performed on all subjects). Phenotypically, HCMV-pentamer+ CD8+ T cells were characterized by marked V beta restriction, advanced differentiation (being predominantly CD27- CD28-), and variable CD45RO/RA expression. Although more common and larger in older subjects, expansions had similar phenotypic characteristics in the young. In one old subject, repeated studies demonstrated stability in size and V beta distribution of pentamer+ populations over 6 years. We tested whether HCMV-specific CD8+ T-cell expansions arose from accelerated proliferation or extended lifespan by in vivo labelling with deuterated glucose and ex vivo Ki-67 expression. Uptake of deuterated glucose was lower in pentamer+ cells than in pentamer- CD8+ CD45RO+ or CD8+ CD45RA+ cells in three old subjects, consistent with reduced proliferation and extended lifespan. Similarly Ki-67 labelling showed no evidence for increased proliferation in HCMV-specific CD8+ expansions in older subjects, although pentamer- CD45RA+ cells from young donors expressed very little Ki-67. We investigated Bcl-2 and CD95 as possible anti-apoptotic mediators, but neither was associated with pentamer-positivity. To investigate whether expansion represents a compensatory response to impaired functionality, we performed two tests of functionality, peptide-stimulated proliferation and CD107 expression; both were intact in pentamer+ cells. Our data suggest that HCMV-specific CD8+ expansions in older subjects accumulate by extended lifespan, rather than accelerated proliferation.