The immune modulation of B-cell responses by Porphyromonas gingivalis and interleukin-10

Background: Polyclonal B-cell activation induced by periodontopathic bacteria has been cited as being important for elevated numbers of B cells, but the role of bacteria in the pathogenesis of periodontal disease remains unknown. In this study, we used an in vitro model to investigate the activation of immune cells by the periodontopathic bacterium Porphyromonas gingivalis in healthy subjects. Methods: Peripheral blood mononuclear cells (PBMC) or purified subsets of lymphocytes were stimulated with sonicated extracts of P. gingivalis for 24 hours. Cells were harvested and monitored for expression of CD69 by flow cytometry. Cytokine production (IL-10, IL-12, and IL-15) in gingivalis-stimulated PBMC cultures was measured by ELISA. To identify IL-10 producer cells, a cell depletion experiment was used and confirmed by the ability of the purified cell population to produce IL-10. To evaluate the effect of P. gingivalis and IL-10, the proliferative response of purified B cells was assessed by [H-3] thymidine uptake. Results: PBMC cultured with P. gingivalis led to a large number of activated B and natural killer (NK) cells as monitored by CD69 expression. When positively sorted cells were used, the bacterium itself could directly activate only B cells but not NK cells, alpha beta, and gamma delta T cells. Measurement of B-cell regulatory cytokine production in P. gingivalis-stimulated PBMC cultures revealed a large amount of IL-10 but no detectable IL-12 or IL-15; the major producing cells were monocytes, not B cells or ap T cells. When IL-10 was added to B cells in the presence of bacteria, significantly increased B-cell proliferative responses were observed. Conclusions: These results suggest that P. gingivalis, both directly and indirectly via macrophage IL-10, may play an important role in polyclonal B-cell activation associated with periodontal disease.