Characterisation of bovine lipopolysaccharide binding protein and the in vivo acute phase response to Pasteurella haemolytica Type A

Abstract

Lipopolysaccharide binding protein (LBP) is a liver-derived acute phase protein which is implicated in modulating the host responses to lipopolysaccharide (LPS) from Gram-negative bacteria. LBP interacts with circulatory LPS to form complexes which bind to the CD14 receptor or cells of the monocytic lineage and neutrophils resulting in their activation. This causes the release of mediators and cytokines which are responsible for initiating the acute phase response. LBP-like activity has now been identified in bovine serum and in this study LBP has been purified from acute phase bovine serum using ion exchange chromatography. On sodium dodecyl sulphate polyacrylamide electrophoresis, bovine LBP demonstrated a single band with a molecular mass of 58 kDa. Bovine LBP enhanced the binding of LPS to human monocytes while enzymatic removal of the CD14 receptor abrogated this interaction. Furthermore, bovine LBP increased the sensitivity of monocytes to LPS by at least 100-fold. Depletion of LBP by means of antibodies to bovine LBP inhibited the serum mediated LPS binding to monocytes. Antibodies to rabbit LBP or recombinant human LBP did not cross-react with bovine LBP. Studies on the kinetics of LBP activity in calves during the acute phase response demonstrated a four-fold increase in the serum concentration 36 h after a single intratracheal inoculation of Pasteurella haemolytica A1. The findings of this study indicate that cattle possess a LPS detection mechanism comparable to that described in man and experimental animals in which LBP forms complexes in serum with circulatory LPS enhancing the signal to the immune system to mount a host response. The isolation of LBP will allow further investigations into LBP-mediated responses to LPS in cattle.