Regulatory effects of interleukin-6 in immunoglobulin G immune-complex-induced lung injury

Interleukin-6 (IL-6) is a cytokine produced in response to a variety of inflammatory stimuli. Although IL-6 is often observed in increased amounts in acute respiratory distress syndrome, its role in the development of lung injury is unclear. The role of IL-6 was studied in the rat model of lung injury induced by the intra-alveolar deposition of IgG immune complexes. IL-6 induction, as determined by Northern blot analysis and bioactivity, was found as a function of time during the course of development of injury. Recombinant IL-6 instilled intratracheally at commencement of injury led to substantial reductions in lung vascular permeability, neutrophil accumulation, and levels of tumor necrosis factor (TNF)-alpha and macrophage inflammatory protein (MIP)-2 in bronchoalveolar lavage fluids. Conversely, blocking of intrinsic IL-6 by a neutralizing antibody resulted in increases in lung vascular permeability, neutrophil content, and TNF-alpha levels in bronchoalveolar lavage fluids. Rat alveolar macrophages stimulated in vitro with lipopolysaccharide in the presence of IL-6 showed a significant reduction in TNF-alpha expression. Together, these findings suggest that IL-6 acts as an intrinsic regulator of lung inflammatory injury after deposition of IgG immune complexes and that the protective effects of exogenously administered IL-6 may be in part linked to suppressed TNF-alpha production.