Background: S100A8 is involved in the pathological processes of a variety of central nervous system(CNS) diseases related to inflammation including traumatic brain injury (TBI). However, the underlying mechanism for the induction of inflammation in the brain by S100A8 after TBI remains unclear, which was investigated in the present study.Methods: The weight-drop TBI model was used in this study. The mice were randomly assigned into 5 groups: the Sham, S100A8, S100A8 + TAK-242, TBI, and TBI + TAK-242 groups. In the S100A8 + TAK-242 and TBI + TAK-242 groups, mice were treated with TAK-242, an inhibitor of Toll-like receptor (TLR) 4, intraperitoneally at half an hour before TBI. In the S100A8 and S100A8 + TAK-242 groups, S100A8 recombinant protein was injected into the lateral ventricle of the brain. To explore the relationship between S100A8 and TLR4, Western Blot (WB), immunofluorescence, enzyme-linked immunosorbent assay (ELISA) and Nissl staining were employed. Neurological score and the brain water content were also assessed. Additionally, BV-2 microglial cells were stimulated with lipopolysaccharide (LPS) or S100A8 recombinant protein with/without TAK-242 in vitro. The expressions of the related proteins were subsequently detected with WB or ELISA.Results: The levels of S100A8 protein and pro-inflammatory cytokines were significantly increased after TBI. After intracerebroventricular administration of S100A8, the neurological scores of non-TBI animals were decreased remarkably with severe brain edema. Furthermore, the levels of TLR4, p-p65 and myeloid differentiation factor 88(MyD88) were all increased after S100A8 administration or TBI, which could be restored by TAK-242. Meanwhile, p-p65 and MyD88 were upregulated after S100A8 or LPS stimulation in vitro, which also could be suppressed by TAK-242.Conclusions: The present study demonstrated that TLR4-MyD88 pathway was activated by S100A8, which was essential to the development of inflammation in the brain after TBI.