Identification of murine protective epitopes on the Porphyromonas gingivalis fimbrillin molecule

Protective and Destructive Immunity in the Periodontium: Part 2—T-cell-mediated Immunity in the Periodontium

Based on the results of recent research in the field and Part 1 of this article (in this issue), the present paper will discuss the protective and destructive aspects of the T-cell-mediated adaptive immunity associated with the bacterial virulent factors or antigenic determinants during periodontal pathogenesis. Attention will be focused on: (i) osteoimmunology and periodontal disease; (ii) some molecular techniques developed and applied to identify critical microbial virulence factors or antigens associated with host immunity (with Actinobacillus actinomycetemcomitans and Porphyromonas gingivalis as the model species); and (iii) summarizing the identified virulence factors/antigens associated with periodontal immunity. Thus, further understanding of the molecular mechanisms of the host’s T-cell-mediated immune responses and the critical microbial antigens related to disease pathogenesis will facilitate the development of novel therapeutics or protocols for future periodontal treatments. Abbreviations used in the paper are as follows: A. actinomycetemcomitans ( Aa), Actinobacillus actinomycetemcomitans; Ab, antibody; DC, dendritic cells; mAb, monoclonal antibody; pAb, polyclonal antibody; OC, osteoclast; PAMP, pathogen-associated molecular patterns; P. gingivalis ( Pg), Porphyromonas gingivalis; RANK, receptor activator of NF-κB; RANKL, receptor activator of NF-κB ligand; OPG, osteoprotegerin; TCR, T-cell-receptors; TLR, Toll-like receptors.

A targeted fimA DNA vaccine prevents alveolar bone loss in mice after intra-nasal administration

AIM

To construct a dendritic cell (DC)-targeted DNA vaccine against FimA of Porphyromonas gingivalis and evaluate the immunogenicity and protection in mice.

MATERIALS AND METHODS

A targeted DNA plasmid pCTLA4-FimA, which encodes the signal peptide and extracellular regions of mouse cytotoxic T lymphocyte-associated antigen 4 (CTLA4), the hinge and Fc regions of human Igγ1 and FimA of P. gingivalis, was constructed. Mice were immunized with pCTLA4-FimA, the non-targeted DNA plasmid pFimA, which contains only fimA gene, or pCI vector intra-nasally. Serum and saliva antibody responses were detected by enzyme-linked immunosorbent assay. The protection against P. gingivalis-induced periodontitis was evaluated by measuring alveolar bone loss in mice.

RESULTS

Mice immunized with pCTLA4-FimA showed elevated levels of specific serum IgG and salivary IgA antibody responses compared with mice immunized with pFimA (p<0.01). Both pFimA and pCTLA4-FimA immunized groups showed significantly lower alveolar bone loss, with the magnitude protection greater in the latter (p<0.01), compared with the pCI immunized group.

CONCLUSIONS

The DC-targeted DNA construct pCTLA4-FimA enhanced both systemic and mucosal immunity following intra-nasal immunization. A DNA-based immunization strategy may be an effective way to attenuate periodontitis induced by P. gingivalis.

Destructive periodontitis lesions are determined by the nature of the lymphocytic response

It is now 35 years since Brandtzaeg and Kraus (1965) published their seminal work entitled "Autoimmunity and periodontal disease". Initially, this work led to the concept that destructive periodontitis was a localized hypersensitivity reaction involving immune complex formation within the tissues. In 1970, Ivanyi and Lehner highlighted a possible role for cell-mediated immunity, which stimulated a flurry of activity centered on the role of lymphokines such as osteoclast-activating factor (OAF), macrophage-activating factor (MAF), macrophage migration inhibition factor (MIF), and myriad others. In the late 1970s and early 1980s, attention focused on the role of polymorphonuclear neutrophils, and it was thought that periodontal destruction occurred as a series of acute exacerbations. As well, at this stage doubt was being cast on the concept that there was a neutrophil chemotactic defect in periodontitis patients. Once it was realized that neutrophils were primarily protective and that severe periodontal destruction occurred in the absence of these cells, attention swung back to the role of lymphocytes and in particular the regulatory role of T-cells. By this time in the early 1990s, while the roles of interleukin (IL)-1, prostaglandin (PG) E(2), and metalloproteinases as the destructive mediators in periodontal disease were largely understood, the control and regulation of these cytokines remained controversial. With the widespread acceptance of the Th1/Th2 paradigm, the regulatory role of T-cells became the main focus of attention. Two apparently conflicting theories have emerged. One is based on direct observations of human lesions, while the other is based on animal model experiments and the inability to demonstrate IL-4 mRNA in gingival extracts. As part of the "Controversy" series, this review is intended to stimulate debate and hence may appear in some places provocative. In this context, this review will present the case that destructive periodontitis is due to the nature of the lymphocytic infiltrate and is not due to periodic acute exacerbations, nor is it due to the so-called virulence factors of putative periodontal pathogens.

Oral immunization with recombinant Streptococcus gordonii expressing porphyromonas gingivalis FimA domains

Porphyromonas gingivalis, a gram-negative anaerobe, is implicated in the etiology of adult periodontitis. P. gingivalis fimbriae are one of several critical surface virulence factors involved in both bacterial adherence and inflammation. P. gingivalis fimbrillin (FimA), the major subunit protein of fimbriae, is considered an important antigen for vaccine development against P. gingivalis-associated periodontitis. We have previously shown that biologically active domains of P. gingivalis fimbrillin can be expressed on the surface of the human commensal bacterium Streptococcus gordonii. In this study, we examined the effects of oral coimmunization of germfree rats with two S. gordonii recombinants expressing N (residues 55 to 145)- and C (residues 226 to 337)-terminal epitopes of P. gingivalis FimA to elicit FimA-specific immune responses. The effectiveness of immunization in protecting against alveolar bone loss following P. gingivalis infection was also evaluated. The results of this study show that the oral delivery of P. gingivalis FimA epitopes via S. gordonii vectors resulted in the induction of FimA-specific serum (immunoglobulin G [IgG] and IgA) and salivary (IgA) antibody responses and that the immune responses were protective against subsequent P. gingivalis-induced alveolar bone loss. These results support the potential usefulness of the S. gordonii vectors expressing P. gingivalis fimbrillin as a mucosal vaccine against adult periodontitis.

RgpA-Kgp peptide-based immunogens provide protection against Porphyromonas gingivalis challenge in a murine lesion model

Porphyromonas gingivalis, a gram-negative bacterium, has been linked to the onset and progression of periodontitis, a chronic inflammatory disease of the supporting tissues of the teeth. A major virulence factor of P. gingivalis is an extracellular complex of Arg- and Lys-specific proteinases and adhesins designated the RgpA-Kgp complex (formerly the PrtR-PrtK complex). In this study we show that the RgpA-Kgp complex, when used as an immunogen with incomplete Freund adjuvant (IFA), protects against challenge with invasive and noninvasive strains of P. gingivalis in the murine lesion model. We identified a variety of peptide vaccine candidates from the RgpA and Kgp polyprotein sequences that involved the putative active site histidine of both proteinases and five repeat motifs in the adhesin domains of both polyproteins implicated in aggregation and binding to host substrates, designated adhesin-binding motif (ABM) peptides. These peptides were synthesized using standard, solid-phase protocols for 9-fluorenylmethoxy carbonyl chemistry with S-acetylmercaptoacetic acid (SAMA) as the N-terminal residue. The SAMA-peptides were then conjugated to diphtheria toxoid and used with IFA to immunize BALB/c mice. Both active-site peptides and three of the five ABM peptides gave protection (P < 0.005) against challenge with P. gingivalis in the murine lesion model. The three ABM peptide sequences that conferred protection exist within a 100-residue span in the RgpA44 and Kgp39 adhesins of the RgpA-Kgp complex. Protective anti-RgpA-Kgp complex mouse antisera recognized the RgpA27, Kgp39, and RgpA44 adhesins in an immunoblot. Epitope mapping of the RgpA27 adhesin using the protective anti-RgpA-Kgp antisera identified a major protective epitope that mapped immediately N terminal to one of the protective ABM peptides in the 100-residue span in RgpA44 and Kgp39. This identified protective epitope contains clusters of basic residues spatially surrounded by hydrophobic amino acids, a finding which is characteristic of a heparin binding motif.

Distribution and molecular characterization of Porphyromonas gingivalis carrying a new type of fimA gene

Fimbriae of Porphyromonas gingivalis are filamentous appendages on the cell surface and are thought to be one of the virulence factors. The fimA gene encoding the subunit protein of fimbriae, fimbrillin (FimA), was classified into four typeable variants (types I to IV). We previously examined the distribution of P. gingivalis in terms of fimA genotypes in periodontitis patients using a fimA type-specific PCR assay. However, some patients harbored P. gingivalis with untypeable fimA. In this study, we have cloned a new type (type V) of fimA from dental plaque samples. P. gingivalis with type V fimA was isolated from dental plaque of a periodontitis patient, and the isolate was named HNA-99. The deduced amino acid sequences were compared with those of type I P. gingivalis ATCC 33277, type II strain HW24D1, type III strain 6/26, and type IV strain HG564, and the homologies were found to be 45, 44, 43, and 55%, respectively. Southern blot analysis showed that the clinical isolate HNA-99 possessed P. gingivalis-specific genes sod and kgp. However, in terms of serological specificities, type V FimA showed a difference from other types of FimA. In addition, type V P. gingivalis bacteria were detected in 16.4% (12 of 73) of the P. gingivalis-positive patients with periodontitis by PCR assay using specific primers. Thus, a new type of fimA gene is now established, and the fimA genotyping could be useful in determining the disease-associated genotypes of P. gingivalis involved in the development of adult periodontitis.

Construction and immunologic evaluation of a Porphyromonas gingivalis subsequence peptide fused to hepatitis B virus core antigen

Several proteins of Porphyromonas gingivalis contain multiple copies of a 47 amino acid conserved repeated sequence. A fusion protein was constructed in which the P. gingivalis peptide was fused to the carboxy terminus of the hepatitis B core protein. This fusion protein was expressed in Escherichia coli, purified, and used to vaccinate mice that were later challenged with P. gingivalis W83 using the mouse abscess model. Although the mice were not protected against bacterial challenge, Western blot analysis showed that sera from the mice and from rabbits immunized with the fusion protein reacted with a number of vesicle proteins from P. gingivalis W83. These data suggested that this peptide is recognized by the host's immune system but that the antibodies are not protective.

Life below the gum line: pathogenic mechanisms of Porphyromonas gingivalis

Porphyromonas gingivalis, a gram-negative anaerobe, is a major etiological agent in the initiation and progression of severe forms of periodontal disease. An opportunistic pathogen, P. gingivalis can also exist in commensal harmony with the host, with disease episodes ensuing from a shift in the ecological balance within the complex periodontal microenvironment. Colonization of the subgingival region is facilitated by the ability to adhere to available substrates such as adsorbed salivary molecules, matrix proteins, epithelial cells, and bacteria that are already established as a biofilm on tooth and epithelial surfaces. Binding to all of these substrates may be mediated by various regions of P. gingivalis fimbrillin, the structural subunit of the major fimbriae. P. gingivalis is an asaccharolytic organism, with a requirement for hemin (as a source of iron) and peptides for growth. At least three hemagglutinins and five proteinases are produced to satisfy these requirements. The hemagglutinin and proteinase genes contain extensive regions of highly conserved sequences, with posttranslational processing of proteinase gene products contributing to the formation of multimeric surface protein-adhesin complexes. Many of the virulence properties of P. gingivalis appear to be consequent to its adaptations to obtain hemin and peptides. Thus, hemagglutinins participate in adherence interactions with host cells, while proteinases contribute to inactivation of the effector molecules of the immune response and to tissue destruction. In addition to direct assault on the periodontal tissues, P. gingivalis can modulate eucaryotic cell signal transduction pathways, directing its uptake by gingival epithelial cells. Within this privileged site, P. gingivalis can replicate and impinge upon components of the innate host defense. Although a variety of surface molecules stimulate production of cytokines and other participants in the immune response, P. gingivalis may also undertake a stealth role whereby pivotal immune mediators are selectively inactivated. In keeping with its strict metabolic requirements, regulation of gene expression in P. gingivalis can be controlled at the transcriptional level. Finally, although periodontal disease is localized to the tissues surrounding the tooth, evidence is accumulating that infection with P. gingivalis may predispose to more serious systemic conditions such as cardiovascular disease and to delivery of preterm infants.

Expression of functional Porphyromonas gingivalis fimbrillin polypeptide domains on the surface of Streptococcus gordonii

Genetically engineering bacteria to express surface proteins which can antagonize the colonization of other microorganisms is a promising strategy for altering bacterial environments. The fimbriae of Porphyromonas gingivalis play an important role in the pathogenesis of periodontal diseases. A structural subunit of the P. gingivalis fimbriae, fimbrillin, has been shown to be an important virulence factor, which likely promotes adherence of the bacterium to saliva-coated oral surfaces and induces host responses. Immunization of gnotobiotic rats with synthetic peptides based on the predicted amino acid sequence of fimbrillin has also been shown to elicit a specific immune response and protection against P. gingivalis-associated periodontal destruction. In this study we engineered the human oral commensal organism Streptococcus gordonii to surface express subdomains of the fimbrillin polypeptide fused to the anchor region of streptococcal M6 protein. The resulting recombinant S. gordonii strains expressing P. gingivalis fimbrillin bound saliva-coated hydroxyapatite in a concentration-dependent manner and inhibited binding of P. gingivalis to saliva-coated hydroxyapatite. Moreover, the recombinant S. gordonii strains were capable of eliciting a P. gingivalis fimbrillin-specific immune response in rabbits. These results show that functional and immunologically reactive P. gingivalis fimbrillin polypeptides can be expressed on the surface of S. gordonii. The recombinant fimbrillin-expressing S. gordonii strains may provide an effective vaccine or a vehicle for replacement therapy against P. gingivalis. These experiments demonstrated the feasibility of expressing biologically active agents (antigens or adhesin molecules) by genetically engineered streptococci. Such genetically engineered organisms can be utilized to modulate the microenvironment of the oral cavity.