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The disruption of signal transducer and activator of transcription 4 (STAT4) signal can inhibit the inflammation and protect organs from injury during severe bacterial infection.However, the mechanism of STAT4 signal in lung injury remains poor understood.Here we report that STAT4deficiency decreased the lethality and protein leakage in STAT4-/-mice and protected lipopolysaccharid (LPS)-induced lung injury with ameliorated edema, inflammatory infiltration and hemorrhage.The expression of CD11b+Gr-1+ myeloid derived suppressor cells (MDSCs) markedly increased in the circulation of STAT4-/-mice after LPS stimuli, accompanying with increased macrophages infiltration in inflamed lung tissue.In addition, the levels of pro-inflammatory cytokines including tumor necrosis factor (TNF)-α, interleukin (IL)-1β and IL-6 decreased while anti-inflammatory cytokine (IL-10) increased in the bronchoalveolar lavage fluid of STAT4-/-mice.Thus, these results indicate that the accumulation of MDSCs and macrophages play a critical role in LPS-induced lung injury.Targeting MDSCs and macrophages polarization through a STAT4 dependent signaling pathway might help to reduce the inflammation and damage of lung tissue.