EFFECT OF LIPOPOLYSACCHARIDE (LPS) CHAIN-LENGTH ON INTERACTIONS OF BACTERICIDAL PERMEABILITY-INCREASING PROTEIN AND ITS BIOACTIVE 23-KILODALTON NH2,-TERMINAL FRAGMENT WITH ISOLATED LPS AND INTACT PROTEUS-MIRABILIS AND ESCHERICHIA-COLI/

The target-specific cytotoxicity for gram-negative bacteria and the en dotoxin-neutralizing activity of the 55-kDa bactericidal/Permeability- increasing protein (BPI) and its bioactive 23-kDa NH2-terminal fragmen t depend on the strong attraction of BPI for the lipid A region of lip opolysaccharides (LPS). We have shown before that smooth gram-negative bacteria with long-chain LPS are more resistant to BPI (especially ho lo-BPI) than are rough strains. It has been suggested that the high BP I resistance of some gram-negative bacteria, such as Proteus mirabilis , might also reflect the structural diversity of lipid A. To explore t his possibility, we compared the antibacterial activity and binding of natural and recombinant holo-BPI and a recombinant NH2-terminal fragm ent (rBPI-23) to an isogenic rough (Re-LPS chemotype) and a smooth (S- LPS chemotype) strain of P. mirabilis and to LPS isolated from the two strains. Holo-BPI and rBPI-23 were both potently active against the R e strain of P. mirabilis (90% lethal dose, 20 nM). In contrast, the sm ooth strain was greater than or equal to 100 times more resistant to h olo-BPI but only 10 times more resistant to rBPI-23. rBPI-23 was also more potent against several Escherichia coli strains from clinical bac teremia isolates. Differences in the antibacterial potency of BPI towa rd the Re and S strains of P. mirabilis correlated with differences in the binding of holo-BPI and rBPI-23 to these bacteria. In contrast, t he binding of biosynthetically (in vitro transcribed and translated) S -35-labeled holo-BPI and NH2-terminal fragment to isolated Re- and S-L PS from P. mirabilis in solution was similar. Moreover, in the Limulus amebocyte lysate assay, holo-BPI and rBPI-23 potently neutralized bot h forms of LPS with equal effectiveness. Together, these results stron gly suggest that BPI recognizes Proteus lipid A and that the relative resistance of (Smooth) P. mirabilis to holo-BPI is due to the inhibito ry effect of long polysaccharide chains of tightly packed LPS in the e nvelope.