Synthesis and biological evaluation of lipid A derived from commensal Bacteroides

The inflammation-inducing properties of lipopolysaccharides (LPS) of gram-negative bacteria reside in its lipid A moiety. Bacillus fragilis, which is a commensal gram-negative bacterium, biosynthesizes lipid A that is structurally distinct from that of E. coli and other enteric bacteria. It is composed of a beta1,6-linked glucosamine (GlcN) disaccharide that is only phosphorylated at the anomeric center. The major species of B. fragilis has five fatty acids and the amine of the distal GlcN carries the unusual (R)-3-(13-methyltetradecanoyloxy)-1.5-methylhexadecanoic acid. A recent study indicates that LPS of B. fragilis has anti-viral activity by selective induction of Interferon (IFN)- and was protective in mouse models of vesicular stomatitis virus (VSV) and influenza A. Heterogeneity in the structure of LPS and lipid A and possible contamination with other inflammatory components makes it difficult to unambiguously define immune-modulatory properties of LPS or lipid A. Therefore, we developed a synthetic approach for preparation of the unusual lipid A derived from B. fragilis, which includes a synthetic approach for (R)-3-(13-methyltetradecanoyloxy)-1.5-methylhexadecanoic acid by Wittig olefination to install the terminal isopropyl moiety. The proinflammatory and antiviral responses of synthetic B. fragilis lipid A was investigated in several cell lines and primary human monocytes by examining the production of the interleukin (IL)-6 and IFN-beta. It was found that B. fragilis does not induce the production of IL-6 and IFN-beta but can partially antagonize the production of pro-inflammatory cytokines induced by E. coli LPS and lipid A.