Generation and function of Porphyromonas gingivalis lipid A heterogeneity.

摘要

Porphyromonas gingivalis is a Gram negative, asaccharolytic, anaerobic bacterium that is closely associated with the development of adult periodontitis. Its known virulence factors include multiple proteases, major and minor fimbria and lipopolysaccharide (LPS). P. gingivalis LPS is known to have an unusual lipid A structure characterized by low acylation; low phosphorylation; long chain, branched, odd numbered carbon fatty acids and by substitution with ethanolamine. P. gingivalis produces a complex, heterogeneous mixture of lipid A species that appears to change in composition with environmental conditions. Consistent with what is known about the relationship between lipid A structure and recognition by host innate immune cells (and the subsequent response), P. gingivalis LPS exhibits a low biological activity and an ability to antagonize the activity of other LPS. Variations in the composition may thus provide a means to alter interactions with the host and create an advantageous environment for the organism. In this work, we examine the role of specific lipid A synthesis genes in the generation of lipid A heterogeneity in P. gingivalis with regard to fatty acid chain length and degree of phosphorylation, how these specific alterations affect the recognition of P. gingivalis LPS by cells of the innate immune system and characterize how the differences in recognition are the result of differential interaction with receptors of the LPS recognition pathway.; By expressing the P. gingivalis homologs of the lipid A acyltransferases htrB, lpxA and lpxD in E. coli acyltransferase mutants, it was found that the observed heterogeneity of P. gingivalis lipid A could be accounted for in part by the acyl chain specificity of these enzymes but that other factors such as substrate availability must play a role as well. Using the chimeric LPS derived from these strains, it was observed that changes in the number of fatty acids or the length of fatty acids altered the ability of LPS to stimulate inflammatory mediator production from cells of the innate immune system. It was further determined that specific features of lipid A structure are recognized at different points in the LBP/CD14/TLR4/MD2 recognition pathway. P. gingivalis LPS also may act as an antagonist to other LPS. The LPS structural requirements for this antagonism were examined and it was found that isolated lipid A could account for the observed antagonism when considered on a molar basis relative to LPS. The LPS of a related organism Tannerella forsythia was characterized and compared to P. gingivalis LPS with regard to chemical structure and biological activity.