To determine whether endogenous IL-6 contributes to mediate hypoxic PH and lung inflammation, we studied IL-6-deficient (IL-6^-/-) and wild-type (IL-6^+/+) mice exposed to hypoxia for 2 weeks.

Right ventricular systolic pressure, right ventricle hypertrophy, and the number and media thickness of muscular pulmonary vessels were decreased in IL-6^-/- mice compared to wild-type controls after 2 weeks' hypoxia, although the pressure response to acute hypoxia was similar in IL-6^+/+ and IL-6^-/- mice. Hypoxia exposure of IL-6^+/+ mice led to marked increases in IL-6 mRNA and protein levels within the first week, with positive IL-6 immunostaining in the pulmonary vessel walls. Lung IL-6 receptor and gp 130 (the IL-6 signal transducer) mRNA levels increased after 1 and 2 weeks' hypoxia. In vitro studies of cultured human pulmonary-artery smooth-muscle-cells (PA-SMCs) and microvascular endothelial cells revealed prominent synthesis of IL-6 by PA-SMCs, with further stimulation by hypoxia. IL-6 also markedly stimulated PA-SMC migration without affecting proliferation. Hypoxic IL-6^-/- mice showed less inflammatory cell recruitment in the lungs, compared to hypoxic wild-type mice, as assessed by lung protein levels and immunostaining for the specific macrophage marker F4/80, with no difference in lung expression of adhesion molecules or cytokines.