Differential inductions of TNF‐α and IGTP, IIGP by structurally diverse classic and non‐classic lipopolysaccharides

Summary The innate immune system recognizes microbes by characteristic molecules like the Gram‐negative lipopolysaccharide (LPS). Lipid A (the LPS bioactive moiety) signals through toll‐like receptors (TLRs) to induce pro‐inflammatory molecules and small GTPases of the p47 family involved in intracellular pathogen control. We tested TNF‐α and p47‐GTPase induction in macrophages using classical LPSs [lipid As with glucosamine backbones, ester‐ and amide‐linked C14:0(3‐OH) and C12 to C16 in acyloxyacyl groups] of wild type and mutant Escherichia coli and Yersinia species and non‐classical LPSs [lipid As with diaminoglucose, ester‐linked 3‐OH‐fatty acids and C28:0(27‐OH) and C23:0(29‐OH) in acyloxyacyl groups] of plant endosymbionts ( Rhizobium ), intracellular pathogens ( Brucella and Legionella ) and phylogenetically related opportunistic bacteria ( Ochrobactrum ). Classical but not non‐classical LPSs efficiently induced TNF‐α, IIGP and IGTP p47‐GTPase expression. Remarkably, the acyloxyacyl groups in classical LPSs necessary to efficiently induce TNF‐α were not necessary to induce p47‐GTPases, suggesting that different aspects of lipid A are involved in this differential induction. This was confirmed by using PPDM2, a non‐endotoxic lipid A‐structurally related synthetic glycolipid. Despite their different bioactivity, all types of LPSs signalled through TLR‐4 and not through TLR‐2. However, whereas TNF‐α induction was myeloid differentiation factor 88 (MyD88)‐dependent, that of p47‐GTPases occurred via a MyD88‐independent pathway. These observations show that different aspects of the LPS pathogen‐associated molecular pattern may be triggering different signalling pathways linked to the same TLR. They also reinforce the hypothesis that non‐classical lipid As act as virulence factors by favouring the escape from the innate immune system.