Prior exposure of dendritic cells (DCs) and monocytes/macrophages to LPS causes unresponsiveness to subsequent LPS stimulation, a phenomenon called endotoxin tolerance (ET). ET impairs antigen presentation of these cells to T cells by down-regulating expression of MHC class II and co-stimulatory molecules such as CD86 and CD40. Some epidemiological studies have shown that endotoxin acts as a protective factor for allergic diseases. Accordingly, LPS has beneficial effects on the onset of airway allergic inflammation in model animals by T(h)1 skewing or induction of regulatory T cells. However, results derived from asthma model animals are controversial, probably due to the difficulty of handling LPS. We previously generated a monoclonal agonistic antibody against Toll-like receptor (TLR) 4, named UT12, which mimics the biological activities of LPS, exhibiting more potent and sustained ET than does LPS. In this study, we took advantage of UT12 to generate prolonged ET to explore the possibility that ET is involved in the inhibitory effects of the TLR4 signals on asthma model mice. Induction of ET by UT12 inhibited the capacity of DCs to expand ovalbumin (OVA)-specific T(h)2 and T(h)17 cells, without inducing T(h)1 cell or regulatory T-cell populations or producing inhibitory cytokines. Accordingly, administration of UT12 before the OVA sensitization significantly suppressed airway allergic inflammation by OVA inhalation. Taken together, these results demonstrate that ET induced by activating TLR4 signals attenuates airway allergic inflammation through direct suppression of the T-cell stimulatory effect of DCs in asthma model mice.