The role of serum amyloid a (SAA) in sepsis and LPS neutralization

Background: IJpoproteins play a major role in LPS neutralization. During sepsis acute phase proteins are produced by the liver. One of these proteins, serum amyloid A (SAA) a 12 KD apoprotein, binds to high density lipoprotein, and thus may influence LPS neutralization in sepsis. This study tries to elucidate the role of SAA in LPS neutralization. Methods and Results: LJpoproteins were isolated from (non-)acutc phase plasma by density-gradient ultracentrinigation. SDS-PAGE and Western blot with anti-SAA were used to detect SAA in the lipoprotein fractions. SAA was detected hi the lipoprotein fraction of plasma from patients with sepsis, mainly in the HDL fraction. SAA was not detected in normal human plasma or the protein traction of the acute phase plasma. Recombinant-SAA was used to reconstitute acute phase HDL in vitro. Dequenching of fluorescent LPS, binding to LPS-coated beads and neutrophil priming in a chemolulincscence assay were used to measure LPS-SAA Interactions. Reconstituted acute phase HDL was able to tranfer monomeric fluorescent LPS out of micelles (3 fold increase), similar to normal HDL. Unbound SAA was able to neutralize LPS in a dose dependent manner in neutrophil priming. SAA and SAA-HDL were also able to bind to LPS-coated beads (10 fold increase at lpg/ml) Conclusions: In sepsis patients, SAA is mainly bound to HDL. SAA doesn't influence the LPS binding capacity of HDL.Unbound SAA has LPS-blnding am) neutralizing properties.