Manipulation of Neutrophils by Porphyromonas gingivalis in the Development of Periodontitis

Abstract

The pathogenesis of the chronic periodontal disease is associated with a skewed host inflammatory response to periodontal pathogens, such as Porphyromonas gingivalis, that accounts for the majority of periodontal tissue damage. Neutrophils are the most abundant leukocytes in periodontal pockets and depending on the stage of the disease, also plentiful PMNs are present in the inflamed gingival tissue and the gingival crevice. They are the most efficient phagocytes and eliminate pathogens by a variety of means, which are either oxygen-dependent or -independent. However, these secretory lethal weapons do not strictly discriminate between pathogens and host tissue. Current studies describe conflicting findings about neutrophil involvement in periodontal disease. On one hand literature indicate that hyper-reactive neutrophils are the main immune cell type responsible for this observed tissue damage and disease progression. Deregulation of neutrophil survival and functions, such as chemotaxis, migration, secretion of antimicrobial peptides or enzymes, and production of reactive oxygen species, contribute to observed tissue injury and the clinical signs of periodontal disease. On the other hand neutrophils deficiencies in patients and mice also result in periodontal phenotype. Therefore, P. gingivalis represents a periodontal pathogen that manipulates the immune responses of PMNs, employing several virulence factors, such as gingipains, serine proteases, lipid phosphatases, or fimbriae. This review will sum up studies devoted to understanding different strategies utilized by P. gingivalis to manipulate PMNs survival and functions in order to inhibit killing by a granular content, prolong inflammation, and gain access to nutrient resources.

Keywords: Porphyromonas gingivalis; inflammation; neutrophils; periodontitis; virulence factors.

Figure 1

Immune responses to pathogens. During an infection with pathogens, for example E. coli, lipopolisaccharide enhances the secretion of chemotactic IL-8 and stimulates the upregulation of E- or P-selectins expression on gingival endothelial cells (GECs). Selectins facilitate neutrophils adhesion during transmigration as they interact with PSGL-1 expressed on PMNs. Moreover, the presence of microbes and their particles activates the complement cascade. C3a and C5a are anaphylatoxins with a strong chemotactic and pro-inflammatory potential. IgG and IgM antibodies or C3b recognize bacterial antigens and opsonize invading pathogens thus facilitating bacterial phagocytosis. LPS activates the TLR4 signaling pathway in recruited neutrophils, eliciting strong inflammatory responses designed to inactivate the pathogen. Inflammatory responses include the production of reactive oxygen species, secretion of pro-inflammatory cytokines and antimicrobial enzymes or peptides, such as cathepsin G, elastase, cathelicidins or defensins. After a successful bacterial clearance, neutrophils undergo apoptosis, an essential process triggering the resolution of inflammation.

Figure 2

Manipulation strategies of neutrophils and the immune system by P. gingivalis at the initial phase of infection. During the initial phase of an infection with the keystone pathogen P. gingivalis (Pg) secretes serine phosphatase (SerB), inhibiting IL-8 production. At the same time, a tetra-acylated lipid A variant of P. gingivalis LPS suppresses the expression of L- and P-selectins on gingival epithelial cells. These manipulation strategies hinder neutrophil recruitment, giving periodontal pathogens enough time for colonization of periodontal pockets.

Figure 3

Manipulation strategies of neutrophils and the immune system by P. gingivalis at the later phase of infection. During a later phase of the infection P. gingivalis releases a penta-acylated LPS variant leading to the increased expression of L- and P-selectins on GECs and enhanced production of IL-8. This strongly stimulates neutrophil chemotaxis and transmigration to the site of infection. Moreover, P.g.-derived LPS and fimbriae strongly stimulate neutrophil pro-inflammatory and anti-bacterial responses, such as the secretion of reactive oxygen species and pro-inflammatory cytokines, and the production of anti-microbial peptides and enzymes. An elevated secretion of these anti-bacterial molecules results in gingival tissue destruction, while many virulence factors secreted by P. gingivalis protect this periodontopathogen from the consequences of hyper-inflammation. The keystone pathogen is protected from oxidative stress, as it expresses ruberythrin (Rbr) protein. Additionally, the expression of Lys-specific gingipains degrades immunoglobulins at the hinge region, and, coupled with the activation of the TLR2 signaling pathway (by LPS), abolishes bacterial opsonization and phagocytosis. Also, gingipains manipulate anti-bacterial responses by deregulating the complement cascade and IL-8-mediated neutrophil chemotaxis. In particular, depending on the concentration and the position of gingipains within the biofilm, these enzymes can exert opposing effects. C3, C5, and IL-8 are degraded at high gingipain concentration or by gingipains associated with bacterial cells or vesicles thus inhibiting pro-inflammatory responses and protecting bacteria from elimination. In contrast, low levels of soluble Arg-specific gingipains activate C5 and C3 by limited proteolysis that results in the generation of C5a and C3a anaphylatoxins. Furthermore, under such circumstances, gingipains can selectively cleave IL-8 generating a truncated, hyperactive IL-869aa variant (written in red) acting in concert with C5a and C3a in order to excessively activate neutrophil pro-inflammatory responses. Additionally, the secreted penta-acylated LPS variant diminishes neutrophil apoptosis which delays the resolution of inflammation Together, these events strongly contribute to the inflammatory tissue destruction observed in periodontitis and to excessive bleeding, providing P.g. and bystander bacteria with access to nutritional resources.