MD-2 opsonization enhances cytokine and nitrite production. pMφ from wild-type (left column), TLR4 (middle column), and MD-2 mice (right column) were left untreated (□) or treated with LPS (■), live Yp (), or MD-2–coated Yp (). RANTES, TNF-α, and nitrite concentrations were determined from the same supernatants by ELISA. The arrowheads indicate the restoration of the “wild-type” phenotype in MD-2^−/− cells, when MD-2 is provided on the surface of bacteria. *In the absence of MD-2 treatment, MD-2–deficient pMφ resemble the TLR4 knockout phenotype. Results are the average of triplicate readings plus SD, and the experiment is representative of 3.