Susceptibility of Porphyromonas gingivalis and P. asaccharolytica to the non-oxidative killing mechanisms of human neutrophils

A pleiotropic role of sialidase in the pathogenicity of Porphyromonas gingivalis

As one of the keystone pathogens of periodontitis, the oral bacterium Porphyromonas gingivalis produces an array of virulence factors, including a recently identified sialidase (PG0352). Our previous report involving loss-of-function studies indicated that PG0352 plays an important role in the pathophysiology of P. gingivalis. However, this report had not been corroborated by gain-of-function studies or substantiated in different P. gingivalis strains. To fill these gaps, herein we first confirm the role of PG0352 in cell surface structures (e.g., capsule) and serum resistance using P. gingivalis W83 strain through genetic complementation and then recapitulate these studies using P. gingivalis ATCC33277 strain. We further investigate the role of PG0352 and its counterpart (PGN1608) in ATCC33277 in cell growth, biofilm formation, neutrophil killing, cell invasion, and P. gingivalis-induced inflammation. Our results indicate that PG0352 and PGN1608 are implicated in P. gingivalis cell surface structures, hydrophobicity, biofilm formation, resistance to complement and neutrophil killing, and host immune responses. Possible molecular mechanisms involved are also discussed. In summary, this report underscores the importance of sialidases in the pathophysiology of P. gingivalis and opens an avenue to elucidate their underlying molecular mechanisms.

Periodontal Pathogens' strategies disarm neutrophils to promote dysregulated inflammation

Periodontitis is an irreversible, chronic inflammatory disease where inflammophilic pathogenic microbial communities accumulate in the gingival crevice. Neutrophils are a major component of the innate host response against bacterial challenge, and under homeostatic conditions, their microbicidal functions typically protect the host against periodontitis. However, a number of periodontal pathogens developed survival strategies to evade neutrophil microbicidal functions while promoting inflammation, which provides a source of nutrients for bacterial growth. Research on periodontal pathogens has largely focused on a few established species: Tannerella forsythia, Treponema denticola, Fusobacterium nucleatum, Aggregatibacter actinomycetemcomitans, and Porphyromonas gingivalis. However, advances in culture-independent techniques have facilitated the identification of new bacterial species in periodontal lesions, such as the two Gram-positive anaerobes, Filifactor alocis and Peptoanaerobacter stomatis, whose characterization of pathogenic potential has not been fully described. Additionally, there is not a full understanding of the pathogenic mechanisms used against neutrophils by organisms that are abundant in periodontal lesions. This presents a substantial barrier to the development of new approaches to prevent or ameliorate the disease. In this review, we first summarize the neutrophil functions affected by the established periodontal pathogens listed above, denoting unknown areas that still merit a closer look. Then, we review the literature on neutrophil functions and the emerging periodontal pathogens, F. alocis and P. stomatis, comparing the effects of the emerging microbes to that of established pathogens, and speculate on the contribution of these putative pathogens to the progression of periodontal disease.

Human neutrophils and oral microbiota: a constant tug-of-war between a harmonious and a discordant coexistence

Neutrophils are a major component of the innate host response, and the outcome of the interaction between the oral microbiota and neutrophils is a key determinant of oral health status. The composition of the oral microbiome is very complex and different in health and disease. Neutrophils are constantly recruited to the oral cavity, and their protective role is highlighted in cases where their number or functional responses are impeded, resulting in different forms of periodontal disease. Periodontitis, one of the more severe and irreversible forms of periodontal disease, is a microbial-induced chronic inflammatory disease that affects the gingival tissues supporting the tooth. This chronic inflammatory disease is the result of a shift of the oral bacterial symbiotic community to a dysbiotic more complex community. Chronic inflammatory infectious diseases such as periodontitis can occur because the pathogens are able to evade or disable the innate immune system. In this review, we discuss how human neutrophils interact with both the symbiotic and the dysbiotic oral community; an understanding of which is essential to increase our knowledge of the periodontal disease process.

Major neutrophil functions subverted by Porphyromonas gingivalis

Polymorphonuclear leukocytes (neutrophils) constitute an integrated component of the innate host defense in the gingival sulcus/periodontal pocket. However, the keystone periodontal pathogen Porphyromonas gingivalis has in the course of evolution developed a number of capacities to subvert this defense to its own advantage. The present review describes the major mechanisms that P. gingivalis uses to subvert neutrophil homeostasis, such as impaired recruitment and chemotaxis, resistance to granule-derived antimicrobial agents and to the oxidative burst, inhibition of phagocytic killing while promoting a nutritionally favorable inflammatory response, and delay of neutrophil apoptosis. Studies in animal models have shown that at least some of these mechanisms promote the dysbiotic transformation of the periodontal polymicrobial community, thereby leading to inflammation and bone loss. It is apparent that neutrophil–P. gingivalis interactions and subversion of innate immunity are key contributing factors to the pathogenesis of periodontal disease.

The lysine gingipain adhesin domains from Porphyromonas gingivalis interact with erythrocytes and albumin: Structures correlate to function

The crystal structure of the K1 domain, an adhesin module of the lysine gingipain (Kgp) expressed on the cell surface by the periodontopathic anaerobic bacterium, Porphyromonas gingivalis W83, is compared to the previously determined structures of homologues K2 and K3, all three being representative members of the cleaved adhesin domain family. In the structure of K1, the conformation of the most extensive surface loop is unexpectedly perturbed, perhaps by crystal packing, and is displaced from a previously reported arginine-anchored position observed in K2 and K3. This displacement allows the loop to become free to interact with other proteins; the alternate flipped-out loop conformation is a novel mechanism for interacting with target host proteins, other bacteria, or other gingipain protein domains. Further, the K1 adhesin module, like others, is found to be haemolytic in vitro, and so, functions in erythrocyte recognition thereby contributing to the haemolytic function of Kgp. K1 was also observed to selectively bind to haem-albumin with high affinity, suggesting this domain may be involved in gingipain-mediated haem acquisition from haem-albumin. Therefore, it is most likely that all cleaved adhesin domains of Kgp contribute to the pathogenicity of P. gingivalis in more complex ways than simply mediating bacterial adherence.

How has neutrophil research improved our understanding of periodontal pathogenesis?

BACKGROUND

Neutrophils are the predominant cells responsible for host defence against bacterial infection. Loss of neutrophil defence, due either to deficient number or function, strongly predisposes to bacterial infections such as periodontitis. Yet, the neutrophil oxidative and proteolytic arsenal has also been implicated in perpetrating periodontal tissue damage in periodontitis.

AIM

In this review, we focus on recent developments that shed light on these two aspects of neutrophil function in periodontitis.

METHODS

Primary search: using PubMed search for "neutophil", "periodontal", and "periodontitis". Secondary search: using references from the articles found in the first stage.

RESULTS

Early histological studies showed that infiltrating neutrophils form a wall of cells abutting the junctional epithelium in periodontal inflammatory lesions. The chronic standoff between these neutrophils and the bacterial community suggests that bacterial evasion of neutrophil clearance is a major characteristic of periodontitis. Indeed, not all functional neutrophil deficiencies increase the risk of periodontitis, an observation that points the way towards identification of particular anti-bacterial pathways essential for protection against periodontal pathogens. The net result in the majority of periodontitis patients who exhibit normal neutrophil number and function, is that neutrophils accumulate in the periodontal tissue where they are available to participate in tissue destruction. Diminished neutrophil clearance further contributes to the persistence of activated neutrophils in the periodontal tissue.

CONCLUSIONS

Data on the role of neutrophils in the pathogenesis of periodontitis are mixed. Neutrophils are a critical arm of the defence against periodontitis, but bacterial evasion of the neutrophil microbicidal machinery coupled with delayed neutrophil apoptosis may transform the neutrophil from defender to perpetrator. At this stage of knowledge, attempts to induce host modulation through neutrophil suppression or activation are premature.

Proteases of Porphyromonas gingivalis: what don't they do?

The gram-negative anaerobic bacterium Porphyromonas gingivalis has been strongly associated with the causation of human periodontal diseases. One distinguishing property of these organisms that has been implicated in periodontal destruction is the expression of potent protease activity. Recent biochemical and genetic approaches have clearly demonstrated that at least five distinct proteases are elaborated by these organisms. The utilization of monospecific mutants defective in individual proteases has demonstrated that protease activity is important in virulence but also has suggested the complexity of the functions of the enzymes in the physiology of these microorganisms. This review summarizes current progress in assessing the role of these enzymes in periodontal inflammation and discusses some unresolved issues relevant to the significance of P. gingivalis proteases in virulence.

Porphyromonas gingivalis proteinases as virulence factors in the development of periodontitis

Porphyromonas gingivalis contains exceedingly high concentrations of cysteine proteinases with trypsin-like activity which have been implicated as virulence factors in adult-onset periodontitis. These enzymes, referred to as gingipains, cleave protein and peptide substrates after arginine (gingipain R) and lysine residues (gingipain K), and it has been found that neither is easily inhibited by host proteinase inhibitors. Examination of the properties of each proteinase clearly indicates a role(s) for both in the dysregulation of a number of normally tightly controlled pathways. The effects of such uncontrolled proteolysis are the development of edema (kallikrein/kinin pathway activation by gingipain R), neutrophil infiltration (complement pathway activation by gingipain R), and bleeding (degradation of fibrinogen by gingipain K). Since three of the major hallmarks of periodontitis involve increased crevicular flow, neutrophil accumulation at infected sites and bleeding on probing, it seems likely that both P. gingivalis-derived proteinases are important virulence factors in the development of periodontal disease.

The potential role of alpha 2-macroglobulin in the control of cysteine proteinases (gingipains) from Porphyromonas gingivalis

Porphyromonas gingivalis is closely associated with the development of some forms of periodontitis. The major cysteine proteinases released by this bacterium hydrolyze peptide bonds only after arginyl (gingipain R) or lysyl residues (gingipain K). No target protein inhibitors have been identified for either enzyme, leading us to investigate their inhibition by human plasma alpha 2-macroglobulin (alpha 2M). Both 50- and 95 kDa gingipain R were efficiently inhibited by alpha 2M, whereas the catalytic activity of gingipain K could not be eliminated. All 3 enzymes were, however, inhibited by a homologous macroglobulin from rat plasma, alpha 1-inhibitor-3 (alpha 1I3). alpha-Macroglobulins must be cleaved in the so-called "bait region" in order to inhibit proteinases by a mechanism involving physical entrapment of the enzyme. A comparison of the amino acid sequences of the 2 macroglobulins indicates that the lack of lysyl residues within the bait region of alpha 2M protects Lys-specific proteinases from being trapped. On this basis, other highly specific proteinases might also not be inhibited by alpha 2M, possibly explaining the inability of the inhibitor to control proteolytic activity in some bacterially induced inflammatory states, despite its abundance (2-5 mg/ml) in vascular fluids.

Functional properties of nonhuman primate antibody to Porphyromonas gingivalis

The nonhuman primate (NHP) serves as a useful model for examining the host-parasite interactions in Porphyromonas gingivalis-associated periodontal disease. This study determined the influence of NHP sera on (i) the direct killing of P. gingivalis, (ii) P. gingivalis-induced superoxide anion (O2-) release from human polymorphonuclear leukocytes (PMNs), and (iii) the ability of PMNs to bind and phagocytize P. gingivalis. Three types of NHP sera were utilized: (i) normal or baseline sera; (ii) sera obtained after ligature-induced periodontitis; and (iii) sera obtained following active immunization with formalinized P. gingivalis. All assays were performed with or without the addition of human complement. Significantly more (P < 0.01) direct killing of P. gingivalis occurred with immunized sera and complement than with any of the other treatments. The sera from ligature-induced periodontitis NHPs had significantly less (P < 0.03) killing capacity than the baseline sera, which contained natural antibody produced to P. gingivalis colonization. Sera from immunized NHPs were used to opsonize P. gingivalis and caused significantly greater (P < 0.01) levels of O2- release from PMNs. Finally, the sera from immunized NHPs significantly enhanced (P < 0.009) the uptake of P. gingivalis by PMNs, although binding of the bacteria to PMNs was similar among all three serum types. Active immunization of NHPs with P. gingivalis elicited a functional antibody that enhanced direct killing, positively influenced the activation of PMNs, and enhanced the ability of PMNs to phagocytize P. gingivalis. Moreover, antibody produced as a sequela of progressing periodontitis appeared to lack these functions. A wide variability in functional capacity of the sera from individual NHPs, which may contribute to an individual's susceptibility to P. gingivalis-induced disease, was noted. This variability suggested that results from functional tests of serum antibody may aid in predicting host susceptibility to disease and response to therapy.

Degradation of immunoglobulin G by periodontal bacteria

Several subgingival microorganisms were tested for their ability to utilize human immunoglobulin G (IgG) as a substrate for growth. This was done using a protein-free chemically defined medium, supplemented with IgG. Stimulation of growth was observed for Capnocytophaga ochracea, Porphyromonas asaccharolytica, Porphyromonas endodontalis, Porphyromonas gingivalis, Prevotella intermedia, Prevotella oralis, Lactobacillus catenaforme and Streptococcus intermedius. Immunoelectrophoresis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis and a protein assay demonstrated that P. intermedia and P. endodontalis completely degraded the protein chains of IgG. Partial breakdown of IgG was observed for P. asaccharolytica and C. ochracea, whereas P. oralis cleaved the IgG heavy chain, yielding Fc and Fab fragments. All these bacteria utilized IgG as a substrate for growth. Binding studies using an enzyme-linked immunosorbent assay, revealed complete loss of in vitro antigen-antibody binding capacity after incubation of specific IgG with P. endodontalis and partial loss of binding with P. intermedia, P. gingivalis, C. ochracea or Fusobacterium nucleatum. Degradation or inactivation of IgG by oral bacteria is thought to be important in the causation of polymicrobial infections.