Neutrophil activation by bacterial lipoprotein versus lipopolysaccharide: Differential requirements for serum and CD14

Neutrophil activation plays an important role in the inflammatory response to Gram-negative bacterial infections. LPS has been shown to be a major med iator of neutrophil activation which is accompanied by an early down-regula tion of L-selectin and up-regulation of CD11b/CD18, In this study, we inves tigated whether lipoprotein (LP), the most abundant protein in the outer me mbrane of bacteria from the family Enterobacteriaceae, can activate neutrop hils and whether this activation is mediated by mechanisms that differ from those used by LPS or Escherichia coli diphosphoryl lipid A (EcDPLA), Neutr ophil activation was assessed by measuring down-regulation of L-selectin an d up-regulation of CD11b/CD18, When comparing molar concentrations of LP vs EcDPLA, LP was more potent (four times) at activating neutrophils, In cont rast to LPS/EcDPLA, LP activation of neutrophils was serum independent, How ever, LP activation of neutrophils was enhanced by the addition of soluble CD14 and/or LPS-binding protein. In the presence of serum, LP activation of neutrophils was inhibited by different mAbs to CD14. This inhibition was s ignificantly reduced or absent when performed in the absence of serum, Diph osphoryl lipid A from Rhodobacter spheroides (RaDPLA) completely inhibited LPS/EcDPLA activation of neutrophils but only slightly inhibited LP activat ion of neutrophils, These results suggest that LP activation of human neutr ophils can be mediated by a mechanism that is different from LPS activation and that LP is a potentially important component in the development of dis eases caused by Gram-negative bacteria of the family Enterobacteriaceae.