**10. Therapeutic approaches**

for NETosis and degranulation. Therefore, it can be assumed that implementation of such survival strategies coexists with the elimination of bacteria by neutrophils. Finally, inflamma‐ tion and tissue destruction mediated by neutrophils evoke frequent gingival bleeding, which these bacteria may use as an additional source of nutrients, such as iron and vitamin K.

*Porphyromonas gingivalis* are anaerobic, Gram‐negative, nonmotile, asaccharolytic rods that usually exhibit coccal or short rod morphologies. It is part of the black‐pigmented Bacteriodes group [32]. *P. gingivalis*, even in low colonization levels, can induce the shift from symbiotic microbiota to dysbiotic microbiota followed by inflammatory bone loss. This bacteria uses different mechanisms to destabilize neutrophil homeostasis, inhibition of phagocytic kill‐ ing, resistance to granule‐derived antimicrobial agents and to the oxidative burst, impaired recruitment and chemotaxis, promote inflammatory response, and delay of neutrophil apop‐ tosis. *P. gingivalis* has a number of virulence factors related to the subversion of the innate immune system. This ability is what often characterizes a successful pathogen, as it tends to disable the overall host response while simultaneously enhancing the pathogenicity of a poly‐ microbial community. *P. gingivalis* are resistant to oxidative killing [67] and recruit hyperac‐ tive neutrophils with an enhanced response, which is characterized by the release of reactive oxygen intermediates, several cationic peptides, and enzymes such as matrix metalloprotein‐ ases (MMPs). All this responses increased tissue damage [48]. *P. gingivalis* also can manipulate

*Porphyromonas gingivalis* gingipains are able to trigger the expression of proinflammatory sur‐ face receptor TREM‐1 on neutrophils, and several periodontopathogenic species can induce

*Treponema denticola* is an anaerobic, Gram‐negative, motile, spirochete that can be poorly detected in the gingival plaque of healthy individuals. However, it is present in very high numbers in the subgingival periodontal pocket and is associated with the dysbiotic microbiota biofilm for‐ mation in periodontal lesions. *T. denticola* limits neutrophil chemotaxis, and inhibits junctional epithelial cells to secrete IL‐8. Additionally, this pathogen is able to degrade IL‐8 that is already present at the infection site, which disables the neutrophil chemotactic gradient. *T. denticola* major outer sheath protein (Msp) is one of its most important virulence factors in contribut‐ ing to the disease progression. This membrane protein modulates neutrophil signaling path‐ ways involved in cytoskeletal dynamics that are relevant in chemotaxis and phagocytosis [70]. Msp controls neutrophil cytoskeletal functions like migration, adhesion, and cell shape. It also causes extracellular matrix degradation by stimulating the release of activated MMPs from

both complement and TLR signaling to induce bacterial persistence.

IL‐8 gene expression in gingival epithelial cells and fibroblasts [68, 69].

**7.** *Porphyromonas gingivalis*

80 Role of Neutrophils in Disease Pathogenesis

**8.** *Treponema denticola*

neutrophils.

As it was discussed along this chapter, it is clear that both, lack of neutrophils and excess of neu‐ trophils in the periodontium, can lead to periodontal disease. Because both situations involve IL‐17‐mediated inflammation and bone loss, it is conceivable that IL‐17 or IL‐17R inhibitors may be promising targets for treatment of human periodontitis. By blocking IL‐17 actions, neutrophil recruitment to the periodontium would be reduced and in consequence, the inflam‐ mation state would also be reduced. This should prevent tissue damage and loss of bone.

In chronic periodontitis, periodontal bacteria activate neutrophil subversion pathways that allow bacteria to escape neutrophil killing. For example, *P. aeruginosa* biofilms produce bac‐ terial surfactants that induce rapid neutrophil death [59], and *Aggregatibacter actinomycetemcomitans* and *S. aureus* produce bacterial toxins that induce neutrophil lysis and degranulation [61–63]. These bacterial products could be neutralized with antibodies or novel pharmaceuti‐ cal drugs to prevent their negative effects on neutrophils. Also, *P. gingivalis* can manipulate complement to induce bacterial persistence. Thus, complement components are also good therapeutic targets. In fact, in preclinical models of periodontitis the use of complement inhib‐ itors has led to a reduction of the inflammatory state [73, 74]. In addition, several bacteria including *P. gingivalis* can induce cells such as fibroblasts to produce IL‐8 and recruit more neutrophils to the inflamed periodontal tissues [68, 69]. Blocking IL‐8 is another interesting therapeutic strategy for reducing periodontitis. Several anti‐IL‐8 blocking antibodies are avail‐ able. Their potential benefit in periodontal disease should be evaluated in the near future.

Del‐1 is another promising candidate molecule to be used therapeutically to prevent neutro‐ phil recruitment and bone loss associated with periodontal inflammation [17]. Since Del‐1 blocks LFA‐1 binding to its ligand ICAM‐1 and prevents neutrophil transmigration [50], it could be administered to inflamed tissues, to reduce neutrophil recruitment, and to reduce inflammation. In fact, this is exactly what was found in a model of periodontitis with nonhu‐ man primates [75]. Local administration of Del‐1 also prevented inflammatory bone loss [75]. Preclinical studies for the use of Del‐1 are now underway.

All these potential therapeutic approaches promise a relief from periodontitis and perhaps other inflammatory disorders in the future.
