Abstract:
A unique characteristic of advanced periodontal lesion is the accumulation of a large number of B-cells and plasma cells. Polyclonal B-cell activation induced by periodontopathic bacteria has been cited as being important for these elevated B-cell responses, however, the exact mechanisms underlying this event remain unknown. Our preliminary study of gingival mononuclear cell populations in periodontitis tissues revealed the majority of activated B-cells (CD69+ CD19+). In order to investigate the mechanisms of how immune cells, in particular B-cells, become activated by microorganisms in dental plaque, we examined the activation parameters of peripheral blood mononuclear cells (PBMC) and purified cell populations after stimulation with periodontopathic bacteria, Porphyromonas gingivalis, in healthy periodontal subjects. PBMC cultures derived from these healthy donors incubated with sonicated extracts of P.gingivalis led to a dose dependent activation of different lymphocyte subpopulations as monitored by a flow cytometric analysis of CD69 expression, a very early activation antigen. Large increase in number of CD69+ cells was consistently observed in B, NK and gammatriangle T-cells, and to a lesser degree in alphabeta T-cells. When flow cytometric sorted cells were used, it was found that only B-cells but not alphabeta, gammatriangle T-cells or NK cells were directly activated by the bacterial extracts, thus being suggestive of non specific B-cell activation by P.gingivalis. Furthermore, production of B-cell regulatory cytokines e.g., L-10, L-12 and L-15 was assessed by ELISA in P.gingivalis-stimulated PBMC cultures. Large amount of L-10 was detected in culture supematants but none of L-12 and L-15. By using cell depletion experiments, the major source of L-10 in P.gingivalis-stimulated PBMC was monocytes not B-cell or alphabeta T-cells. In addition, stimulation of sorted monocytes by these microorganisms induced high level of L-10 production. L-10 is initially described as a cytokine synthesis inhibition factor and also a potent growth and differentiation factor for B-cells. Our results support recent reports of high level of L-10 in periodontitis lesion which may be linked to the pathogenesis of periodontal disease. Upon exposure of B-cell with P.gingivalis and the cytokine L-10, proliferative response of B-cells was significantly increased. Therefore, L-10 may prove to be the critical cytokine involved in polyclonal B-cell activation associated with periodontal disease.
