Interactions of bacterial lipopolysaccharide and peptidoglycan with a 70 kDa and an 80 kDa protein on the cell surface of CD14+ and CD14- cells

Abstract

Bacterial cell wall components, lipopolysaccharide (LPS), lipoteichoic acid (LTA), and peptidoglycan (PGN) are known to stimulate cells of the immune, inflammatory and vascular systems contributing to septic shock. CD14 has been identified as the main LPS receptor, a process that is accelerated by the serum protein LPS-binding protein (LBP). CD14 has also been found to bind LTA and PGN from the cell wall of gram positive bacteria. Recently, toll-like receptor proteins TLR-2 and TLR-4 have been shown to be required for LPS and LTA-induced intracellular signalling. Although CD14 functions as either a glycosylphosphatidylinositol (GPI)-anchored molecule that does not transverse the cell membrane or as a soluble serum protein, the mechanisms by which the CD14-LPS/LTA complex interacts with the TLRs remains to be elucidated. We have looked directly for cell surface protein(s) that bind LPS or LTA in a CD14-dependent manner. Using biochemical approaches we have identified two proteins of molecular weight 70 kDa (LAP-1) and 80 kDa (LAP-2) that can be precipitated from both CD14(+) and CD14(-) cells with LPS- or LTA-specific antibodies. Binding of LPS and LTA to LAP-1 and -2 required serum. While soluble CD14 (sCD14) was sufficient to allow precipitation of these two proteins from CD14(-) cells, serum could not be replaced by purified sCD14 and/or LBP when mCD14-expressing cells were used.