Specificity and function of monoclonal antibodies reactive with discrete structural elements of bacterial lipopolysaccharide

Abstract

We examined the binding and functional activities of monoclonal antibodies (mAbs) reactive with different structural elements of Escherichia coli and Salmonella minnesota LPS. O-side chain-reactive mAbs were highly specific for homologous, smooth LPS, bound avidly to intact bacteria, mediated complement-dependent bactericidal and/or opsonic activity, and protected against live, homologous IP challenges in mice. Core- and lipid A-specific mAbs, on the other hand, were more cross-reactive, although this cross-reactivity was severely restricted by the relative inaccessability of epitopes in the core/lipid A region. This was reflected in the general inability of these mAbs to react with isolated smooth LPS or wild type bacteria, or to mediate bactericidal or opsonic functions. No LPS-reactive mAbs, regardless of molecular specificity, was able to block LPS- or lipid A-induced TNF produciton by RAW 264.7 macrophages, thus raising doubts concerning the putative endotoxin-neutralizing properties of mAbs reactive with the core/lipid A complex. Bacterial lipopolysaccharides (LPS) exhibit a complex identity. They represent an essential structural element of the outer membrane of all Gram-negative bacteria (7); they are toxins (5); they mediate a variety of immunomodulatory activities; and they are important bacterial surface antigens (2). In general, LPS macromolecules consist of three genetically, biochemically, and antigenetically distinct regions or domains: the O-side chain, core oligosaccharide, and lipid A moiety (15). Of these three regions, the O-side chain is the most phylogenetically diverse. It also represents the most antigenetically exposed element on isolated or cell-associated, native LPS. The core and lipid A structures, in contrast, are relatively conserved among different bacteria and are less accessible to antibody attack by virtue of overlying sugars contained in the O-side chain or outer core (8). In this study, we investigated selected functional activities of monoclonal antibodies (mAbs) specific for different epitopes within the three major structural domains of Escherichia coli and Salmonella minnesota LPS. The possible endotoxin-neutralizing and antibacterial properties of these mAbs were our particular focus.