Aged mice have increased inflammatory monocyte concentration and altered expression of cell-surface functional receptors.

The expression of monocyte cell-surface receptors represents one index of immune dysfunction, which is common with aging. Although mouse models of aging are prevalent, monocyte subset assessment is rare. Our purpose was to compare cell receptor expression on classic (CD115+/Gr-1 high) and non-classic (CD115+/Gr-1 low) monocytes from 80- or 20-week-old CD-1 mice. Three-colour flow cytometry was used to determine the concentration of monocyte subsets and their respective cell-surface expression of TLR2, TLR4, CD80, CD86, MHC II and CD54. These receptors were selected because they have been previously associated with altered monocyte function. Data were analysed with independent t-tests; significance was set at P less than 0.05. Old mice had a greater concentration of both classic (258%, P=0.003) and non-classic (70%, P=0.026) monocytes. The classic : non-classic monocyte ratio doubled in old as compared with that in young mice (P=0.006), indicating a pro-inflammatory shift. TLR4 ( 27%, P=0.001) and CD80 ( 37%, P=0.004) were decreased on classic monocytes from old as compared with those from young mice. TLR2 ( 24%, P=0.002) and MHCII ( 21%, P=0.026) were altered on non-classic monocytes from old as compared with those from young mice. The increased classic : non-classic monocyte ratio combined with changes in the cell-surface receptor expression on both monocyte subsets is indicative of immune dysfunction, which may increase age-associated disease risk.