Construction of LPS mutants.

Lipopolysaccharide (LPS) is a major component of the meningococcal outer membrane. It consists of a hexa-acylated glucosamine disaccharide substituted at both ends with diphosphoethanolamine, to which an oligosaccharide chain of up to 10 sugar residues is attached (1,2). It lacks a long repeating O-antigen side chain, as is typically found in many Enterobacteriaceae, and is therefore also sometimes referred to as lipooligosaccharide or LOS. The oligosaccharide part shows structural variation among strains, which forms the basis for division into the different immunotypes L1 to L12 (3). In addition, individual strains can vary their LPS structure through high-frequency phase variation of several genes encoding glycosyltransferases (4). This can affect virulence-related properties such as invasion of host cells and serum resistance (5). In the context of vaccine development, meningococcal LPS is relevant in several ways. First, the cell surface-exposed oligosaccharide part may contain epitopes recognized by bactericidal or otherwise protective antibodies; however, the presence of host-identical structures such as the terminal lacto-N-neotetraose means that the possibility of inducing autoimmune pathology should also be considered (6). Second, the membrane-anchoring lipid A part has strong endotoxin activity, by inducing the synthesis of proinflammatory cytokines in a variety of host cells (7). This plays a major role in the pathological manifestations of meningococcal sepsis, and is also responsible for most of the reactogenicity found with outer membrane vesicle (OMV)-based vaccines.