IL-10 is a potent, endogenous anti-inflammatory cytokine known to decrease cytokine and keratinocyte-derived chemokine (KC) expression. Traditionally, in vivo effects of IL-10 were extrapolated from studies employing systemic antibody neutralization. As a result, divergent data regarding the protective and/or harmful roles of IL-10 have been reported. In this study, we used a lung-specific, tetracycline-inducible IL-10 overexpression-transgenic (IL-10 OE) mouse to study the effects of IL-10 overexpression on Pseudomonas aeruginosa-induced lung inflammation and corresponding survival in mice. Overexpression of IL-10 in the lung significantly increased mortality. During the early phase after infection (6-hours after infection), neutrophil recruitment as well as cytokine (TNF-alpha) and chemokine (KC) expression were significantly decreased in the IL-10 OE mice, which resulted in attenuated bacterial clearance. In contrast, overzealous production of KC and TNF-alpha intensified neutrophil infiltration and increased vascular leakage in IL-10 OE mice at the later stage of infection (24 hours after infection). Neutrophil depletion showed impaired bacterial clearance in both control and IL-10 OE mice, and further enhanced mouse mortality, whereas exogenous administration of KC reversed this finding. Our data indicate that early neutrophil recruitment is important for combating bacterial infection, and that the inhibition of neutrophil recruitment by IL-10 results in insufficient bacteria clearance in the lung, leading to excessive development of inflammation and increased mortality.

Induction of NO may be a key mediator in driving the cytokine response to endotoxin toward an increased type-2 (interleukin-10) response and a diminished type-1 (interleukin-12) response.

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