Differences in both Toll-like receptor 4 and MD-2 account for the host-specific recognition of Bordetella pertussis lipid A (P1240)

Abstract

Abstract The lipid A portion of lipopolysaccharide (LPS) activates innate immunity through the Toll-like receptor 4 (TLR4)/MD-2 complex on host cells. Variation in lipid A has significant consequences for TLR4 activation and thus may be a means by which pathogenic bacteria modulate host immunity. We have previously seen that strain BP338 of Bordetella pertussis modifies its lipid A by the addition of glucosamine moieties which promote TLR4 activation in human macrophages. In the absence of glucosamine modification, TLR4 activation is attenuated. This effect is host-specific: glucosamine modification does not affect mouse TLR4 activation. We hypothesized that the species-specific effect is due to differences in TLR4, and used inter-species chimeric receptors to test this hypothesis. We found that the middle 330 amino acids of TLR4 were sufficient to generate species-specific responses to B. pertussis lipid A, dependent on co-expression of the matching species of MD-2. Based on the published crystal structure of TLR4/MD-2 in complex with E. coli LPS, we narrowed our search to the interface where TLR4/MD-2 complexes dimerize for activation, and used site-directed mutagenesis to identify specific amino acid residues in TLR4 and MD-2 that promote activation in response to B. pertussis lipid A. Taken together, we have demonstrated that charged amino acids in TLR4 and MD-2 contribute to host-specific immune responses to lipid A variants from B. pertussis.