We have investigated the specific contribution of protease-activated receptor-2 (PAR 2) to host defense during Porphyromonas gingivalis infection. Culture supernatants from P. gingivalis strains 33277 and W50 provoked Ca 2+ mobilization in cells transfected with PAR 2 (PAR 2-KNRK) and desensitized the subsequent responses to PAR 2-selective agonist. In addition, culture supernatants of P. gingivalis E8 (RgpA/RgpB double knockout) did not cause calcium response in PAR 2-KNRK cells, evidencing the involvement of the arginine-specific cysteine proteases RgpA and RgpB in PAR 2 activation by P. gingivalis . Injection of P. gingivalis into mouse subcutaneous chambers provoked an increased proteolytic activity, which was inhibited by serine protease inhibitors. Fluids collected from chambers of P. gingivalis -injected mice were able to activate PAR 2 and this activation was inhibited by serine protease inhibitors. P. gingivalis inoculation into subcutaneous chambers of wild-type mice induced an inflammatory response that was inhibited by a serine protease inhibitor and was significantly reduced in PAR 2-deficient mice. Finally, mice orally challenged with P. gingivalis developed alveolar bone loss, which was significantly reduced in PAR 2-deficient mice at 42 and 60 days after P. gingivalis infection. We conclude that PAR 2 is activated on P. gingivalis infection, in which it plays an important role in the host inflammatory response.