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Abstract: Korean mistletoe lectin (KML-C) is an adjuvant that activates systemic and mucosal immune cells to release cytokines including TNF-α, which induces immunity against viruses and cancer cells. Although the immunomodulatory activity of KML-C has been well established, the underlying mechanism of action of KML-C has yet to be explored. When mouse peritoneal macrophages were treated with KML-C, both transcription and translation of TLR4 were upregulated. KML-C-induced TLR4 downstream events were similar to those activated by LPS: the upregulation of interleukin-1 receptor-associated kinase-1 (IRAK1); resulting in macrophage activation and TNF-α production. When TLR4 was blocked using a TLR4-specific neutralizing antibody, TNF-α production from the macrophages was significantly inhibited. Moreover, TLR4-deficient mouse macrophages treated with KML-C also secreted greatly reduced level of TNF-α secretion. Finally, TLR4 molecules were co-precipitated with KML-C, to which agarose beads were conjugated, indicating that those molecules are associated. These data indicate that KML-C activates mouse macrophages to secrete TNF-α by interacting with the TLR4 molecule and activating its signaling pathways. [Copyright &y& Elsevier]