IL-6 IS AN ANTIINFLAMMATORY CYTOKINE REQUIRED FOR CONTROLLING LOCAL OR SYSTEMIC ACUTE INFLAMMATORY RESPONSES

IL-6 is induced often together with the proinflammatory cytokines TNF alpha and IL-1 in many alarm conditions, and circulating IL-6 plays an important role in the induction of acute phase reactions. However, wh ether this endogenous IL-6 plays any additional pro-or antiinflammator y 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 p roinflammatory cytokines TNF alpha and MIP-2 and a neutrophilic respon se in the lung of IL-6+/+ mice. However, the levels of TNF alpha and M IP-2 and neutrophilia were significantly higher in the lung of IL-6-/- mice. The rate of neutrophil apoptosis in these mice was similar to t hat in IL-6+/+ mice. A low constitutive level of antiinflammatory cyto kine IL-10 was not enhanced by endotoxin and remained similar in the l ung in both IL-6+/+ and IL-6-/- mice. Systemically, intraperitoneal de livery of endotoxin resulted in much more pronounced circulating level s of TNF alpha, MIP-2, GM-CSF, and IFN gamma in IL-6-/- mice than in I L-6+/+ mice, and administration of recombinant IL-6 to IL-6-/- mice ab olished these differences. In contrast, circulating IL-10 levels were induced to a similar degree in both IL-6+/+ and IL-6-/- mice. Thus, ou r studies reveal that endogenous IL-6 plays a crucial antiinflammatory role in both local and systemic acute inflammatory responses by contr olling the level of proinflammatory, but not antiinflammatory, cytokin es, and that these antiinflammatory activities by IL-6 cannot be compe nsated for by IL-10 or other IL-6 family members.