Impaired functions of peripheral blood monocyte subpopulations in aged humans

Abstract

Aging is associated with increased susceptibility to microbial infections, and monocytes play an important role in microbial defense. In this study, we have identified and compared four subpopulations of monocytes (CD14(++(high))CD16(-), CD14(+(low))CD16(-), CD14(++(high))CD16(+), and CD14(+(low))CD16(+)) in the peripheral blood of young and aged subjects with regard to their numbers, cytokine production, TLR expression, and phosphorylation of ERK1/2 in response to pam3Cys a TLR-1/2 ligand. Proportions and numbers of CD14(++(high))CD16(+) and CD14(+(low))CD16(+) monocytes were significantly increased, whereas proportions of CD14(+(low))CD16(-) monocytes were decreased in aged subjects as compared to young subjects. In aged subjects, IL-6 production by all four subsets of monocytes was significantly decreased, whereas TNF-α production was decreased in monocyte subsets, except the CD14(+(low))CD16(-) subset. A significantly reduced expression of TLR1 was observed in CD14(++(high))CD16(+) and CD14(+(low))CD16(+) monocyte subsets in aged subjects. Furthermore, following pam3Cys stimulation, ERK1/2 phosphorylation was significantly lower in CD14(+(low))CD16(+), CD14(++(high))CD16(+), and CD14(+(low))CD16(-) subsets of monocytes from aged subjects. This is the first study of four subpopulations of monocytes in aging, which demonstrates that their functions are differentially impaired with regard to the production of cytokines, expression of TLR, and signaling via the ERK-MAPK pathway. Finally, changes in the number of monocyte subsets, and impairment of TLR1 expression, TNF-α production, and EK1/2 phosphorylation was more consistent in CD16(+) monocyte subsets regardless of expression of CD14(high) or CD14(+low), therefore highlighting the significance of further subdivision of monocytes into four subpopulations.

Fig. 1

Monocyte subsets in young and aged humans. a Representative cytofluorograph of four subsets of monocytes based upon expression of CD14 and CD16 antigens from a young subjects. b Cumulative data for the proportions and c numbers of four subsets of monocytes from 17 young and 17 aged subjects. Data are expressed as mean ± SD. *P < 0.05, **P < 0.01

Fig. 2

Cytokine production by four subsets of monocytes in young and aged subjects. a Representative cytoflourograph of IL-6 and TNF-α production by four subsets of monocytes from young subjects. b Cumulative data for IL-6 and c for TNF-α from young and aged subjects

Fig. 3

Expression of TLR1 on four subsets of monocytes in young and aged subjects. Each subset of monocytes was gated, and proportions of each subset with TLR1 expression were expressed as percent of a particular subset of monocyte. a Representative cytofluorograph from a young subject, b cumulative data for the proportions of TLR-1+ subsets, and c the fluoresecence intensity of TLR1 (mean fluorescence channel number (MFC#)) of TLR-1 in each subset of monocytes from young and aged subjects

Fig. 4

Phosphorylation of ERK1/2 in monocyte subsets from young and aged subjects. a Representative Western blot of ERK1/2 phosphorylation and densitometric analysis following Pam3CSK4 stimulation of total monocytes from young and aged subjects. b Cumulative data of phosphor ERK1/2 in four subsets of monocytes by flow cytometry