A lipid A associated protein of Porphyromonas gingivalis, derived from the haemagglutinating 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 enteric bacteria can induce the synthesis of interleukin 1 (IL-1) and therefore may be important virulence factors. Porphyromonas gingivalis is now recognized as a major pathogen in the chronic inflammatory periodontal diseases and it has previously been reported that a crude LAP fraction from this organism could induce IL-1 and interleukin 6 (IL-6) synthesis. 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 molecular masses 81, 68, 48, 47, 28, 25, 20, 17 and 16 kDa. These LAPs, at concentrations as low as 100 ng ml(-1), were shown to stimulate human gingival fibroblasts, human peripheral blood mononuclear cells and whole human blood to produce the pro-inflammatory cytokine IL-6. The cytokine-inducing activity of the LAPs was reduced after heat-inactivation and trypsinization, suggesting that the activity was not due to contaminating LPS. To establish which proteins in this mixture were the active cytokine inducers, the LAPs were separated by electrophoresis on polyacrylamide gels. The majority of the activity was associated with the 17 kDa LAP. N-terminal sequence analysis demonstrated that this protein was homologous to an internal region of a conserved adhesin domain contained within a family of P. gingivalis extracellular proteins including the RI protease, Lys-gingipain, porphypain and haemagglutinin A. In addition to a role in adherence, the adhesin domain(s) of these proteins 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 balance between these two activities may influence the cytokine status of the periodontium in patients with the common chronic inflammatory conditions known as the periodontal diseases.