Effects of aging on the immunopathologic response to sepsis

Abstract

Objective: Aging is associated with increased inflammation following sepsis. The purpose of this study was to determine whether this represents a fundamental age-based difference in the host response or is secondary to the increased mortality seen in aged hosts.

Design: Prospective, randomized controlled study.

Setting: Animal laboratory in a university medical center.

Subjects: Young (6-12 weeks) and aged (20-24 months) FVB/N mice.

Interventions: Mice were subjected to 2 x 25 or 1 x 30 cecal ligation and puncture (CLP).

Measurements and main results: Survival was similar in young mice subjected to 2 x 25 CLP and aged mice subjected to 1 x 30 CLP (p = 0.15). Young mice subjected to 1 x 30 CLP had improved survival compared with the other groups (p < 0.05). When injury was held constant but mortality was greater, both systemic and peritoneal levels of tumor necrosis factor-alpha, interleukin (IL)-6, IL-10, and monocyte chemotactic protein-1 were elevated 24 hours after CLP in aged animals compared with young animals (p < 0.05). When mortality was similar but injury severity was different, there were no significant differences in systemic cytokines between aged mice and young mice. In contrast, peritoneal levels of tumor necrosis factor-alpha, IL-6, and IL-10 were higher in aged mice subjected to 1 x 30 CLP than young mice subjected to 2 x 25 CLP despite their similar mortalities (p < 0.05). There were no significant differences in either bacteremia or peritoneal cultures when animals of different ages sustained similar injuries or had different injuries with similar mortalities.

Conclusions: Aged mice are more likely to die of sepsis than young mice when subjected to an equivalent insult, and this is associated with increases in both systemic and local inflammation. There is an exaggerated local but not systemic inflammatory response in aged mice compared with young mice when mortality is similar. This suggests that systemic processes that culminate in death may be age independent, but the local inflammatory response may be greater with aging.

Figure 1

Survival in young and aged septic mice. Survival was similar between young mice subjected to 2×25 CLP and aged mice subjected to 1×30 CLP. In contrast, young mice subjected to 1×30 CLP had improved survival compared to both of the other two groups of animals (* signifies p<0.05 for both). All animals were treated with fluid resuscitation following CLP and received two days of antibiotics post-operatively.

Figure 2

Plasma cytokine levels 24 hours after CLP. Cytokine levels were statistically higher in aged mice subjected to the same insult (1×30) than young mice. However, when mortality was similar (aged 1×30 vs. young 2×25), cytokine levels were not statistically different. Data shown are mean ± standard error of the mean.

Figure 3

Peritoneal cytokine levels 24 hours after CLP. Cytokine levels were statistically higher in aged mice subjected to the same insult (1×30) than young mice for all except IL-10. When mortality was similar (aged 1×30 vs. young 2×25), all cytokine levels except MCP-1 were statistically higher in aged mice than in young mice. Data shown are mean ± standard error of the mean.

Figure 4

Blood and peritoneal bacterial cultures 24 hours after CLP. Systemic (A) and peritoneal (B) cultures showed similar amounts of bacteria between young and aged mice subjected to the same insult (1×30) and between young and aged mice with similar mortalities despite the differences in the size of the fecal inoculum (2×25 vs. 1×30). Data were log transformed and are shown are mean ± standard error of the mean.

Figure 5

Splenic apoptosis 24 hours after CLP. Apoptosis is statistically higher in aged mice subjected to the same insult (1×30) than young mice (A). However, when mortality is similar (aged 1×30 vs. young 2×25), cytokine levels are not statistically different. Data shown are mean ± standard error of the mean. Representative sections stained for active caspase 3 (positive cells are brown) are shown for young 1×30 (B), aged 1×30 (C) and young 2×25 (D), magnification 400x.