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IL-6 is induced often together with the proinflammatory cytokines TNFalpha and IL-1 in many alarm conditions, and circulating IL-6 plays an important role in the induction of acute phase reactions. However, whether this endogenous IL-6 plays any additional pro- or antiinflammatory roles in local or systemic responses remains unclear. In this study, the role of IL-6 in acute inflammatory responses was investigated in animal models of endotoxic lung or endotoxemia by using IL-6+/+ and IL-6-/- mice. Aerosol exposure of endotoxin induced increased IL-6 and proinflammatory cytokines TNFalpha and MIP-2 and a neutrophilic response in the lung of IL-6+/+ mice. However, the levels of TNFalpha and MIP-2 and neutrophilia were significantly higher in the lung of IL-6-/- mice. The rate of neutrophil apoptosis in these mice was similar to that in IL-6+/+ mice. A low constitutive level of antiinflammatory cytokine IL-10 was not …