A LIPID-A-ASSOCIATED PROTEIN OF PORPHYROMONAS-GINGIVALIS, DERIVED FROM THE HEMAGGLUTINATING DOMAIN OF THE RI PROTEASE GENE FAMILY, IS A POTENT STIMULATOR OF INTERLEUKIN-6 SYNTHESIS

There is evidence that the lipid A-associated proteins (LAPs) of enter ic bacteria can induce the synthesis of interleukin 1 (IL-1) and there fore may be important virulence factors. Porphyromonas gingivalis is n ow recognized as a major pathogen in the chronic inflammatory periodon tal diseases and it has previously been reported that a crude LAP frac tion from this organism could induce IL-1 and interleukin 6 (IL-6) syn thesis. In the present study the chemical and biological properties of the LAPs of this bacterium have been further characterized. Analysis by SDS-PAGE has shown that the LAPs comprise nine proteins of molecula r masses 81, 68, 48, 47, 28, 25, 20, 17 and 16 kDa. These LAPs, at con centrations as low as 100 ng ml(-1), were shown to stimulate human gin gival fibroblasts, human peripheral blood mononuclear cells and whole human blood to produce the pro-inflammatory cytokine IL-6. The cytokin e-inducing activity of the LAPs was reduced after heat-inactivation an d trypsinization, suggesting that the activity was not due to contamin ating LPS. To establish which proteins in this mixture were the active cytokine inducers, the LAPs were separated by electrophoresis on poly acrylamide gels. The majority of the activity was associated with the 17 kDa LAP. N-terminal sequence analysis demonstrated that this protei n was homologous to an internal region of a conserved adhesin domain c ontained within a family of P. gingivalis extracellular proteins inclu ding the RI protease, Lys-gingipain, porphypain and haemagglutinin A. In addition to a role in adherence, the adhesin domain(s) of these pro teins may also have cytokine-inducing properties. Furthermore, it has also been shown that the previously observed degradation of cytokines by P. gingivalis may be attributable to the catalytic domain of the RI protease. Thus, different domains within the same molecule appear to have opposing actions on pro-inflammatory cytokine levels and the bala nce between these two activities may influence the cytokine status of the periodontium in patients with the common chronic inflammatory cond itions known as the periodontal diseases.