MAPK-activated protein kinase 2 (MK2) plays a critical role in the development of inflammation. However, the modulatory mechanisms in macrophage activation and acute lung injury (ALI) have not been completely defined. Here, we reported that MK2-deficient mice (MK2(-/-)) protected against sepsisinduced ALI. In response to lipopolysaccharide (LPS) challenge. MK2(-/-) mice and myeloid cell-specific MK2 conditional knockout mice (MK2(Lyz2-KO)) exhibited attenuated inflammatory response, especially producing fewer amounts of tumor necrosis factor-alpha (TNF-alpha), interleukin (IL)-6, and macrophage inflammatory protein 2 (MIP-2). LPS treatment in vitro resulted in reduced cytokine expression in MK2(-/-) bone marrow-derived macrophages (BMDMs). Furthermore. we found that LPS-induced micmRNA lethal-7e (let-7e) expression was significantly increased in MK2(-/-) macrophages. Transfection of let-7e antagomirs into MK2(-/-) BMDM rescued LPS-induced expression of TNF-alpha, IL-6, and MIP-2. In contrast. transfection of let-7e mimics into MK2'BMDM decreased cytokine expression. Meanwhile, I,PS-induced phosphorylation of cAMP response element-binding (CREB) protein, a substrate of MK2, was downregulated in MK2(-/-) BMDMs. Lin28, an inhibitory molecule of let-7, was significantly reduced in MK2(-/-) macrophages. Our results suggested that MK2 boosts LPS-induced macrophage activation and ALI via increasing activation of CREB and consequently, the expression of Lin28 and downregulation of let-7e.