Binding specificity of polymyxin B, BPI, LALF, and anti-deep core/lipid a monoclonal antibody to lipopolysaccharide partial structures

The deep core/lipid A (DCLA) region of gram-negative bacterial lipopolysacc haride (LPS) is common to most gram-negative pathogens and contains anionic phosphoryl groups plus numerous acyl chains as par? of the toxic lipid A m oiety. Several disparate agents that antagonize the effects of LPS exhibit extensive physicochemical similarities (hydrophobicity, cationic charge) wi thin their binding domains. It is presumed that binding to the DCLA region by each of these antagonists-cross-reactive anti-LPS monoclonal antibodies (mAbs), polymyxin B (PmB), plus bactericidal permeability-increasing protei n (BPI) and Limulus anti-LPS factor (LALF)-may be related to these properti es. Therefore, we hypothesized that in addition to secondary and tertiary p rotein conformation, electrostatic interactions involving the negatively ch arged phosphoryl groups, hydrophobic interactions involving the acyl chains of lipid A, or both might be important factors that promote LPS antagonism . Binding of PmB, BPI, LALF, or anti-DCLA mAb 1B6 to Salmonella minnesota m onophosphoryl lipid A (MPLA), diphosphoryl lipid A (DPLA), and Salmonella m innesota Re (which possess a common structural moiety, but vary considerabl y in structure and charge) was examined. Highly phosphorylated DNA and bovi ne serum albumin served as unrelated structural controls. BPI bound MPLA, w hich is hydrophobic and minimally charged, while mAb 1B6 bound anionic DNA; neither PmB nor LALF were reactive with MPLA or DNA. We surmised that hydr ophobic interactions play a role in BPI binding to LPS, and although electr ostatic interactions appear to be important for binding of mAb 1B6 to DCLA, they may not contribute to as great an extent for PmB, BPI, or LALF. Thus our data support the contention that the contribution of these specific phy sicochemical factors varies among endotoxin antagonists.