Monocyte Priming by Lipopolysaccharides from Periodontal Pathogens

Monocytes defend the body against many infectious organisms. Lipopolysaccharide (LPS), a component of the outer membrane of gram-negative bacteria, has been shown to enhance the bactericidal activity of monocytes, by a process known as “priming”. Objective: We investigated differences in the ability of LPS preparations from four periodontal pathogens to prime monocytes for enhanced release of superoxide anion, a bactericidal oxygen radical. Methods: LPS was prepared under identical conditions from four periodontal pathogens: Fusobacterium nucleatum, Actinobacillus actinomycetemcomintans (Aa), Prevotella denticola, and Porphyromonas gingivalis. Monocytes were isolated from human blood buffy coats using histopaque gradients. Monocytes were incubated with various concentrations of LPS for 24 hours, then phorbol myristate acetate (PMA) was added to trigger release of superoxide, in the presence of cytochrome c to measure release of superoxide spectrophotometrically. Results: LPS preparations from P. gingivalis and P. denticola were found to be less active in priming monocytes than the LPS isolated from Aa and F. nucleatum. Priming by LPS (10 ng/ml) from Aa and F. nucleatum caused PMA-triggered release of 50 and 65 nmol of superoxide/million monocytes, respectively. The same amount of LPS from P. gingivalis and P. denticola primed less well, with PMA-triggered release of 13 and 18 nmol of superoxide, respectively. LPS from E. coli (1 ng/ml) released 103 nmol/million monocytes. F. nucleatum LPS (1 ng/ml) gave 21 nmol superoxide, but it took 100 ng/ml of P. gingivalis LPS to prime for the same amount of superoxide. Conclusion: The LPS from periodontal pathogens appear to differ in their ability to prime monocytes. Some microbes that cause severe periodontal disease, such as P. gingivalis and P. denticola, are poor primers of monocytes, compared with other pathogenic organisms. We conclude that the ability of LPS from some pathogens to avoid priming monocytes may be a virulence factor.