Impaired macrophage maturation after exposure to Porphyromonas gingivalis lipopolysaccharide

Macrophages are a critical component of the innate immune response to pathogenic bacteria. Ligation of Toll-like receptors on the macrophage by bacterial molecules such as lipopolysaccharide (LPS) leads to macrophage activation. The cytokine environment has a profound effect on the phenotype of the activated macrophage. Interferon gamma primed macrophages exposed to LPS mature into classically activated macrophages that produce microbicidal compounds. Interleukin 4 priming directs a macrophage down an alternate activation pathway which is characterised by arginase production. LPS from P. gingivalis has been shown to induce variable immune responses in mammalian cells compared to other gram negative pathogens. Objectives: To investigate the effect of P. gingivalis LPS on the maturation and function of macrophages. Methods: A mouse macrophage cell line was primed with interferon gamma or interleukin-4 then exposed to Escherichia coli LPS, a synthetic lipoprotein or P. gingivalis LPS. Macrophage maturation was then monitored by analysis of cell surface markers, production of arginase and nitric oxide, and the ability to phagocytose of P. gingivalis. Results: Macrophage primed with IFN-gamma and activated with E. coli LPS or lipoprotein resulted in a significant upregulation of maturation molecules and the production of nitric oxide. However exposure to P. gingivalis LPS did not, even at concentrations 100 fold higher. The IL-4 primed macrophages produced high levels of arginase, independent of the amount or type of LPS added. Classically activated macrophages stimulated with P. gingivalis LPS were also impaired in their ability to phagocytose bacteria. Conclusions: we have demonstrated that cytokine primed macrophages were unable to differentiate into a classically activated macrophage when activated with P. gingivalis LPS. However macrophages were able to mature into alternatively activated macrophages regardless of the TLR ligand used. These results suggest that the impaired ability of macrophages to kill P. gingivalis may contribute to chronic periodontitis.