MD-2 is a novel molecule essential for lipopolysaccharide (LPS) signaling through Toll-like receptor 4 (TLR4). TLR4, a mammalian homologue of Drosophila Toll, is involved in innate immune responses against pathogens. However, transfection of TLR4 alone does not confer LPS responsiveness, suggesting the need for an additional molecule. MD-2, which physically associates with TLR4 on the cell surface, is required for LPS signaling. MD-2 acts as a link between TLR4 and LPS signaling, enabling the receptor to respond to LPS. The identification of MD-2 as a critical component of the TLR4-LPS signaling complex has implications for understanding host defense mechanisms and pathophysiology. MD-2 is structurally similar to MD-1 and shares several cysteine residues. It is expressed in various human and mouse tissues and is physically associated with TLR4. Immunoprecipitation experiments confirmed the physical interaction between MD-2 and TLR4. MD-2 enhances TLR4-dependent activation of NF-κB, a key transcription factor in inflammatory responses. Stable transfectants expressing TLR4 and MD-2 showed increased LPS responsiveness, indicating that MD-2 is necessary for TLR4 to recognize and respond to LPS. The TLR4-MD-2 complex is capable of sensing bacterial endotoxins, suggesting a broader specificity compared to TLR2. These findings highlight the importance of MD-2 in LPS signaling and provide insights into the mechanisms of innate immunity. The study was supported by grants from the Ministry of Education, Science, and Culture of Japan and other foundations. The research was conducted by scientists from Saga Medical School, Japan.MD-2 is a novel molecule essential for lipopolysaccharide (LPS) signaling through Toll-like receptor 4 (TLR4). TLR4, a mammalian homologue of Drosophila Toll, is involved in innate immune responses against pathogens. However, transfection of TLR4 alone does not confer LPS responsiveness, suggesting the need for an additional molecule. MD-2, which physically associates with TLR4 on the cell surface, is required for LPS signaling. MD-2 acts as a link between TLR4 and LPS signaling, enabling the receptor to respond to LPS. The identification of MD-2 as a critical component of the TLR4-LPS signaling complex has implications for understanding host defense mechanisms and pathophysiology. MD-2 is structurally similar to MD-1 and shares several cysteine residues. It is expressed in various human and mouse tissues and is physically associated with TLR4. Immunoprecipitation experiments confirmed the physical interaction between MD-2 and TLR4. MD-2 enhances TLR4-dependent activation of NF-κB, a key transcription factor in inflammatory responses. Stable transfectants expressing TLR4 and MD-2 showed increased LPS responsiveness, indicating that MD-2 is necessary for TLR4 to recognize and respond to LPS. The TLR4-MD-2 complex is capable of sensing bacterial endotoxins, suggesting a broader specificity compared to TLR2. These findings highlight the importance of MD-2 in LPS signaling and provide insights into the mechanisms of innate immunity. The study was supported by grants from the Ministry of Education, Science, and Culture of Japan and other foundations. The research was conducted by scientists from Saga Medical School, Japan.