Phagocytosis and intracellular killing of MD-2 opsonized gram-negative bacteria depend on TLR4 signaling.
Both Toll-like receptor 4 (TLR4)- and MD-2-deficient mice succumb to otherwise nonfatal gram-negative bacteria inocula, demonstrating the pivotal role played by these proteins in antibacterial 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 Lps(d), a signaling-incompetent mutant form of TLR4, suggesting that the enhanced phagocytosis observed is dependent on signal transduction. The data confirm the notion that soluble MD-2 is a genuine opsonin that enhances proinflammatory 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.
Jain
Vishal
V
Division of Infectious Diseases and Immunology, University of Massachusetts Medical School, Worcester 01605, USA.
Halle
Annett
A
Halmen
Kristen A
KA
Lien
Egil
E
Charrel-Dennis
Marie
M
Ram
Sanjay
S
Golenbock
Douglas T
DT
Visintin
Alberto
A
D000818:Animals; D006090:Gram-Negative Bacteria / Q000276:immunology*; D051218:Lymphocyte Antigen 96 / Q000276:immunology*; D051379:Mice; D009895:Opsonin Proteins / Q000378:metabolism; D010587:Phagocytosis; D015398:Signal Transduction / Q000276:immunology*; D051197:Toll-Like Receptor 4 / Q000502:physiology*
C411728:Ly96 protein, mouse; D051218:Lymphocyte Antigen 96; D009895:Opsonin Proteins; C493487:Tlr4 protein, mouse; D051197:Toll-Like Receptor 4