Aging is associated with a numerical and functional decline in plasmacytoid dendritic cells, whereas myeloid dendritic cells are relatively unaltered in human peripheral blood.

Dendritic cells (DC) are potent antigen-presenting cells that initiate and regulate T-cell responses. In this study, the numbers and functional cytokine secretions of plasmacytoid and myeloid DC (pDC and mDC, respectively) in peripheral blood from young and elderly subjects were compared. Overall, pDC numbers in peripheral blood were lower in healthy elderly compared with healthy young subjects (p = 0.016). In response to influenza virus stimulation, isolated pDC from healthy elderly subjects secreted less interferon (IFN)-alpha compared with those from healthy young subjects. The decline in IFN-alpha secretion was associated with a reduced proportion of pDC that expressed Toll-like receptor-7 or Toll-like receptor-9. In contrast, there was little difference in the numbers and cytokine secretion function between healthy young and healthy elderly subjects (p = 0.82). However, in peripheral blood from frail elderly subjects, the numbers of mDC were severely depleted as compared with either healthy young or elderly subjects (p = 0.014 and 0.007, respectively). Thus, aging was associated with the numerical and functional decline in pDC, but not mDC, in healthy young versus elderly subject group comparisons, while declining health in the elderly can profoundly impact mDC negatively. Because of the importance of pDC for antiviral responses, the age-related changes in pDC likely contribute to the impaired immune response to viral infections in elderly persons, especially when combined with the mDC dysfunction occurring in those with compromised health.