Porphyromonas gingivalis gingipains cause defective macrophage migration towards apoptotic cells and inhibit phagocytosis of primary apoptotic neutrophils

Abstract

Resolution of inflammation involves suppression of inflammatory cell infiltration, effective clearance of apoptotic cells and induction of an anti-inflammatory response thereby re-establishing tissue homeostasis. Defects in this process may lead to chronic inflammatory diseases and tissue destruction. Periodontitis is a chronic oral inflammatory disease characterised by an aberrant host response to a pathogenic plaque biofilm resulting in local tissue damage and frustrated healing that can result in tooth loss. Cysteine proteases (Gingipains) from the key periodontal pathogen Porphyromonas gingivalis have been implicated in periodontal disease pathogenesis by inhibiting inflammation resolution and are linked with systemic chronic inflammatory disease e.g. rheumatoid arthritis. Given the importance of successful clearance of AC by the innate immune system in the restoration of tissue homeostasis and maintenance of healthy tissues, this project sought to characterise the impact of proteolytic enzymes from P.gingivalis on the innate immune response. The impact on AC clearance and innate immune responses was assessed.This project addressed the specific hypothesis that gingipains promote disease by acting to inhibit multiple stages of the AC clearance process. The effect of gingipains on the ability of macrophages to find AC, interact with and remove AC and respond appropriately were assessed. Gingipain treatment of macrophages was shown to reduce the ability of macrophages to migrate towards AC. In addition, expression of CD14, a key receptor known to tether AC, was also reduced. Together these gingipain-induced innate immune changes reduced effective clearance of AC at two separate points in the clearance process (migration and tethering) thus reducing the AC binding to phagocytes. Data presented here also suggest that gingipains reduced anti-inflammatory mediators (TGF-β and IL-10) in macrophages phagocytosing apoptotic neutrophils. Furthermore, whilst apoptotic neutrophils turn off the inflammation induced by lipopolysaccharide from P.gingivalis, they fail to turn off the novel gingipain induced inflammatory cytokine production. These data suggest that gingipains induce a strong and dominant pro-inflammatory response that may be resistant to inhibition by AC. Taken together, the work presented here reveals several key stages of the resolution phase of inflammation are inhibited. The failure of this process will result in immune cell necrosis and release of inflammatory mediators into the gingival crevicular fluid to cause catastrophic consequences of inflammation in the periodontal region. For the first time, this work suggests that gingipains may be potent contributors to chronic periodontal inflammation via their impact on multiple stages of the AC clearance process and thus represent a potential therapeutic target.