Aging is associated with immune dysfunction that is measurable in blood monocytes in humans. Measurements include monocyte concentration, subpopulation proportions and cell-surface receptor expression. Despite widespread use of mouse models in aging research, age-related phenotypical differences in mouse monocytes have not been determined. Purpose. To determine differences in monocyte concentration, supopulation proportions and cell-surface expression of receptors involved in cytokine production, antigen presentation and T-cell activation between young and old mice. Methods. Blood (80 μL) was drawn from the saphenous vein of old (age 80 weeks) and young (age 20 weeks) CD-1 mice. 3-color flow cytometry was used to assess CD115+ monocytes, classic (CD115+/Ly6C+) and non-classic (CD115+/Ly6C-) monocyte subpopulations and their cell-surface expression of toll-like receptor 4 (TLR4), major-histocompatibility complex class II (MHC II), CD80 and CD86. Independent T-tests were used to identify significant (P ≤ 0.05) group differences. Results. Monocyte concentration was 163% greater in old mice (P<0.05) and the proportion of classic monocytes was 70% greater in old mice compared to young (P<0.05). TLR4 expression on classic monocytes was 43% lower in old mice. MHC II expression on non-classic monocytes was 34% lower in old mice (P<0.05). Similarly, CD80 and CD86 expression on non-classic monocytes was significantly lower in old mice compared to young (P<0.05). Conclusion. Increased monocyte concentration and proportion of classic monocytes, the subpopulation preferentially recruited into inflamed tissue, likely indicates immune dysfunction and low-grade inflammation that is routinely detected in aging humans. Lower TLR4 expression in classic monocytes may indicate immune suppression and reduced pro-inflammatory response to infection. Reduced expression of receptors involved in antigen-presentation and T-cell activation on non-classic monocytes, the subpopulation that is maintained in circulation, may further indicate immune suppression in old mice. These findings denote that, like in humans, age-related changes in monocyte cell-surface receptor expression can be detected in mice. Further analysis is necessary to determine the functional consequences of these alterations.
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