Resistance of macrophages to the suppressive effect of interleukin-10 following thermal injury

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Abstract
The activation of a macrophage (Mφ)-dependent proinflammatory cascade following thermal injury plays an important role in the development of immunosuppression and increased susceptibility to subsequent sepsis in burn patients. In contrast, although interleukin (IL)-10, an anti-inflammatory cytokine that can downregulate Mφ activity, has also been implicated in postburn immune dysfunction, its role in the regulation of Mφ function postburn remains unclear. To study this, C57BL/6 female mice were subjected to a 25% total body surface area third-degree scald burn, and splenic Mφs were isolated 7 days later. Lipopolysaccharide (LPS)-stimulated IL-10, IL-6, tumor necrosis factor (TNF)-α, and nitric oxide (NO) production were significantly increased in the burn group compared with shams. Blockade of endogenous IL-10 activity enhanced IL-6 and TNF-α release, but not NO release, in both groups. The addition of exogenous IL-10 to the Mφ cultures dose dependently suppressed production of these inflammatory mediators in both groups. The timing of IL-10 addition to the cultures in relation to LPS stimulation, however, was critical. The suppressive effect of exogenous IL-10 was attenuated in both groups when the cells were exposed to IL-10 at 4–6 h after LPS stimulation; however, Mφs from injured mice were significantly better able to maintain inflammatory mediator-productive capacity. The resistance of Mφs from injured mice to IL-10-mediated suppression correlated with decreased IL-10 receptor (IL-10R) expression and increased CD11b expression. These findings suggest that Mφs, following thermal injury, display resistance to suppression by IL-10 due in part to downregulation of IL-10R expression.