Submitted November 30, 2007
Accepted January 16, 2008
Phagocytosis and intracellular killing of MD-2 opsonized Gram-negative bacteria depend on TLR4 signaling
Vishal Jain, Annett Halle, Kristen A Halmen, Egil Lien, Marie Charrel-Dennis, Sanjay Ram, Douglas T Golenbock, and Alberto Visintin*
Division of Infectious Diseases and Immunology, University of Massachusetts Medical School, Worcester, MA, United States
* Corresponding author; email: alberto.visintin{at}umassmed.edu.
Both Toll-like receptor 4 (TLR4) and MD-2 deficient mice succumb to otherwise non-fatal Gram negative bacteria inocula, demonstrating the pivotal role played by these proteins in anti-bacterial defense in mammals. MD-2 is a soluble endogenous ligand for TLR4 and a receptor for lipopolysaccharide (LPS). LPS-bound MD-2 transmits an activating signal onto TLR4. In this report we show that both recombinant and endogenous soluble MD-2 bind tightly to the surface of live Gram-negative bacteria. As a consequence, MD-2 enhances cellular activation, bacterial internalization and intracellular killing, all in a TLR4 dependent manner. The enhanced internalization of MD-2 coated bacteria was not observed in macrophages expressing Lpsd, a signaling incompetent mutant form of TLR4, suggesting that the enhanced phagocytosis observed is dependent on signal transduction. The data confirms the notion that soluble MD-2 is a genuine opsonin which enhances pro-inflammatory opsonophagocytosis by bridging live Gram-negative bacteria to the LPS transducing complex. The presented results extend our understanding of the role of the TLR4/MD-2 signaling axis in bacterial recognition by phagocytes.
