The effects of human saliva on the hemolytic activity of complement

Gene Regulation, Two Component Regulatory Systems, and Adaptive Responses in Treponema Denticola

The oral microbiome consists of a remarkably diverse group of 500-700 bacterial species. The microbial etiology of periodontal disease is similarly complex. Of the ~400 bacterial species identified in subgingival plaque, at least 50 belong to the genus Treponema. As periodontal disease develops and progresses, T. denticola transitions from a low to high abundance species in the subgingival crevice. Changes in the overall composition of the bacterial population trigger significant changes in the local physical, immunological and physiochemical conditions. For T. denticola to thrive in periodontal pockets, it must be nimble and adapt to rapidly changing environmental conditions. The purpose of this chapter is to review the current understanding of the molecular basis of these essential adaptive responses, with a focus on the role of two component regulatory systems with global regulatory potential.

Analysis of the complement sensitivity of oral treponemes and the potential influence of FH binding, FH cleavage and dentilisin activity on the pathogenesis of periodontal disease

Treponema denticola, a periopathogen, evades complement-mediated killing by binding the negative complement regulatory protein factor H (FH) to its surface via the FhbB protein. Paradoxically, bound FH is cleaved by T. denticola's dentilisin protease, a process hypothesized to trigger localized dysregulation of complement activation in periodontal pockets. The ability of other oral treponemes to evade complement-mediated killing and bind and cleave FH has not been assessed. In this report, we demonstrate that representative isolates of Treponema socranskii, Treponema medium, Treponema pectinovorum and Treponema maltophilum are also serum resistant, whereas Treponema vincentii and Treponema amylovorum are serum sensitive. Although T. denticola's ability to evade complement-mediated killing is strictly dependent on FH binding, other serum-resistant treponemal species lack FhbB and do not bind FH, indicating an FH-independent mechanism of complement evasion. To assess the influence of FhbB sequence variation on FH binding and cleavage by T. denticola, fhbB sequences were determined for 30 isolates. Three distinct phyletic types were identified. All T. denticola strains bound FH and were serum resistant, but differences in binding kinetics, dentilisin activity and FH cleavage ability were observed. Based on these analyses, we hypothesize that the composition of the T. denticola population is a determining factor that influences the progression and severity of periodontal disease.

Sequence divergence in the Treponema denticola FhbB protein and its impact on factor H binding

Treponema denticola is an anaerobic spirochete whose abundance in the subgingival crevice correlates with the development and severity of periodontal disease. The ability of T. denticola to survive and thrive in the hostile environment of the periodontal pocket is due, at least in part, to its ability to bind factor H (FH), a negative regulator of the alternative complement pathway. The FH binding protein of T. denticola has been identified as FhbB and its atomic structure has been determined. The interaction of FH with T. denticola is unique in that FH bound to the cell surface is cleaved by the T. denticola protease, dentilisin. It has been postulated that FH cleavage by T. denticola leads to immune dysregulation in periodontal pockets. In this study, we conduct a comparative assessment of the sequence, properties, structure and ligand binding kinetics of the FhbB proteins of strains 33521 and 35405. The biological outcome of the interaction of these strains with FH could differ significantly as 33521 lacks dentilisin activity. The data presented here offer insight into our understanding of the interactions of T. denticola with the host and its potential to influence disease progression.

Structure of factor H-binding protein B (FhbB) of the periopathogen, Treponema denticola: insights into progression of periodontal disease

Periodontitis is the most common disease of microbial etiology in humans. Periopathogen survival is dependent upon evasion of complement-mediated destruction. Treponema denticola, an important contributor to periodontitis, evades killing by the alternative complement cascade by binding factor H (FH) to its surface. Bound FH is rapidly cleaved by the T. denticola protease, dentilisin. In this report, the structure of the T. denticola FH-binding protein, FhbB, was solved to 1.7 Å resolution. FhbB possesses a unique fold that imparts high thermostability. The kinetics of the FH/FhbB interaction were assessed using surface plasmon resonance. A K(D) value in the micromolar range (low affinity) was demonstrated, and rapid off kinetics were observed. Site-directed mutagenesis and sucrose octasulfate competition assays collectively indicate that the negatively charged face of FhbB binds within FH complement control protein module 7. This study provides significant new insight into the molecular basis of FH/FhbB interaction and advances our understanding of the role that T. denticola plays in the development and progression of periodontal disease.

Identification of the primary mechanism of complement evasion by the periodontal pathogen, Treponema denticola

Treponema denticola, a periodontal pathogen, binds the complement regulatory protein Factor H (FH). Factor H binding protein B (FhbB) is the sole FH binding protein produced by T. denticola. The interaction of FhbB with FH is unique in that FH is bound to the cell and then cleaved by the T. denticola protease, dentilisin. A ∼ 50-kDa product generated by dentilisin cleavage is retained at the cell surface. Until this study, a direct role for the FhbB-FH interaction in complement evasion and serum sensitivity had not been demonstrated. Here we assess the serum resistance of T. denticola strain 35405 (Td35405wt) and isogenic mutants deficient in dentilisin (Td35405-CCE) and FhbB production (Td35405ΔfhbB), respectively. Both dentilisin and FhbB have been postulated to be key virulence factors that mediate complement evasion. Consistent with conditions in the subgingival crevice, an environment with a significant concentration of complement, Td35405wt was resistant to serum concentrations as high as 25%. Deletion of fhbB (Td35405ΔfhbB), which resulted in the complete loss of FH binding ability, but not inactivation of dentilisin activity (Td35405-CCE), rendered T. denticola highly sensitive to 25% human serum with 80% of the cells being disrupted after 4 h of incubation. Heat treatment of the serum to inactivate complement confirmed that killing was mediated by complement. These results indicate that the FH-FhbB interaction is required for serum resistance whereas dentilisin is not. This report provides new insight into the novel complement evasion mechanisms of T. denticola.

Analysis of a unique interaction between the complement regulatory protein factor H and the periodontal pathogen Treponema denticola

Treponema denticola, a spirochete associated with periodontitis, is abundant at the leading edge of subgingival plaque, where it interacts with gingival epithelia. T. denticola produces a number of virulence factors, including dentilisin, a protease which is cytopathic to host cells, and FhbB, a unique T. denticola lipoprotein that binds complement regulatory proteins. Earlier analyses suggested that FhbB specifically bound to factor H (FH)-like protein 1 (FHL-1). However, by using dentilisin-deficient mutants of T. denticola, we found that T. denticola preferentially binds FH and not FHL-1, and that FH is then cleaved by dentilisin to yield an FH subfragment of approximately 50 kDa. FH bound to dentilisin-deficient mutants but was not cleaved and retained its ability to serve as a cofactor for factor I in the cleavage of C3b. To assess the molecular basis of the interaction of FhbB with FH, mutational analyses were conducted. Replacement of specific residues in widely separated domains of FhbB and disruption of a central alpha helix with coiled-coil formation probability attenuated or eliminated FH binding. The data presented here are the first to demonstrate the retention at the cell surface of a proteolytic cleavage product of FH. The precise role of this FH fragment in the host-pathogen interaction remains to be determined.

Functional analysis of IgA antibodies specific for a conserved epitope within the M protein of group A streptococci from Australian Aboriginal endemic communities

The mucosa is one of the initial sites of group A streptococcal (GAS) infection and salivary IgA (sIgA) is thought to be critical to immunity. However, the target epitopes of sIgA and the function of sIgA in GAS immunity, in particular the role of accessory cells and complement, is largely unknown. We studied the aquisition and the function of sIgA specific for a conserved region epitope, p145 (sequence: LRRDLDASREAKKQVEKALE) of the M protein. Peptide 145-specific sIgA is highly prevalent within an Aboriginal population living in an area endemic for GAS and acquisition of p145-specific sIgA increases with age, consistent with a role for such antibodies in immunity to GAS. Human sIgA and IgG specific for p145 were affinity purified and shown to opsonize M5 GAS in vitro. Opsonization could be specifically inhibited by the addition of free p145 to the antibodies during assay. Opsonization of GAS was totally dependent on the presence of both complement and polymorphonuclear leukocytes, and, moreover, affinity-purified p145-specific sIgA was shown to fix complement in the presence of M5 GAS. These data show that mucosal IgA to this conserved region peptide within the M protein has an important role in human immunity against GAS and may be useful in a broad-based cross-protective anti-streptococcal vaccine.

Smokeless tobacco extracts activate complement in vitro: a potential pathogenic mechanism for initiating inflammation of the oral mucosa

The use of smokeless tobacco has been linked to an increased incidence of inflammation of the buccal and gingival mucosa. However, the mechanisms by which smokeless tobacco initiates inflammation are not well understood. The complement cascade is a ubiquitous source of proinflammatory molecules and can be activated rapidly by a wide variety of agents. Therefore, the effect of smokeless tobacco on complement was investigated as a potential pathogenic mechanism for triggering inflammation of the oral mucosa. Aqueous extracts of loose leaf chewing tobacco (1S1), dry snuff (1S2), and moist snuff (1S3), added to normal human serum, depleted complement hemolytic activity in a dose-dependent manner. Experiments utilizing sera deficient in one specific complement component indicated that the smokeless tobacco-induced depletion of hemolytic activity was due largely to consumption of C3. Furthermore, assays designed to test the activity of the alternative pathway of complement clearly showed that all three extracts depleted the hemolytic activity of this pathway. Finally, all three smokeless tobacco extracts activated the alternative pathway since significantly elevated levels of the cleavage fragments iC3b and Bb were detected in extract-treated serum. High quantities of the classical pathway cleavage fragment C4d also were detected in serum treated with moist snuff (1S3). The results clearly demonstrate that smokeless tobacco extracts activate the alternative pathway and also suggest some measure of classical pathway activation. Activation of complement by smokeless tobacco may be a mechanism for initiating inflammation of the oral mucosa.

Smooth muscle and epithelial cells express specific binding sites for the C1q component of complement

In injury and inflammation, interactions of complement C1q with C1q receptors may provide attachment sites for cell localization and tissue regeneration. Cultured smooth muscle cells (58%), epithelial cells (26%), and endothelial cells (25%) attach to C1q-coated surfaces, while only 6% of cultured B cells (Raji) attach. Endothelial and Raji cells express C1q receptors, but C1q receptors (C1qR) on smooth muscle cells and epithelial cells have not previously been demonstrated. Evidence is provided that smooth muscle cells express an average of 1.5 x 10(6) C1qR/cell (K alpha = 10(8) M-1) and that epithelial cells express an average of 0.7 x 10(6) C1qR/cell (K alpha = 1.4 x 10(8) M-1). Binding properties of C1qR, and immunoreactivity to anti-C1qR antibodies, are characterized. The antibodies specifically recognize a 67-kDa component of smooth muscle cell lysates and inhibit cell attachment to C1q substrates. We conclude that distribution of C1qR may be ubiquitous; binding properties, size, and antigenicity of various C1qR may be related, but adhesive function may be tissue specific.

The interaction of salivary secretions with the human complement system--a model for the study of host defense systems on inflamed mucosal surfaces

When complement first contacts salivary secretions, as when gingival crevicular fluid first meets saliva at the gingival margin, complement function is enhanced. The immediate potentiation of the complement system at equal volume ratios of serum to saliva is due to several factors, including the lower ionic strength of saliva when compared with serum and the presence of certain salivary glyproteins such as the nonimmunoglobulin agglutinins that appear to simultaneously activate C1 and affect (sequester) certain complement control proteins, such as Factor H. This initial potentiation of the complement cascade by saliva may aid in defending the area immediately above the gingival crevice from oral microbiota that are being coated with a combination of serous exudate components and salivary components. As serum becomes much more diluted with saliva (i.e., crevicular fluid moves away from the supragingival area), the acidic proline-rich salivary proteins (APRP) begin to disrupt the unbound C1q-C1r2-C1s2 macromolecular complexes. Thus, the APRP along with other C1 fixing substances in saliva appear to restrict complement function, but only when the ratios of saliva to serum exceed 250:1. Since certain salivary glycoproteins bind to viruses, the potentiation of the complement system by saliva may also play a role in neutralizing certain viral infections on mucosal surfaces where tissue transudates containing complement begin to contact mucosal secretions such as saliva. Again, the ratio of serous fluid to mucosal secretion appears to be an important factor. This article also discusses some of our preliminary data and speculations concerning the binding of the self-associating high-molecular-weight nonimmunoglobulin salivary agglutinins (NIA) with the envelope of the human immunodeficiency virus (HIV) and the possible cooperative role of C1q and fibronectin in aiding neutralization of HIV infectivity.

The first component of complement as a constituent of human salivary sediment

The sediment from human saliva is complement-reactive. Evidence presented shows that C1 (first component of complement) is a constituent of sediment from healthy human donors. Sediment (Sed) inactivated functionally pure C4 (fourth component of complement), and this action on C4 was inhibited by EDTA, phenylmethylsulphonylfluoride (PMSF, a serine-esterase inhibitor) and C1-inhibitor (C1-In). When Sed was incubated with 0.15 ionic strength buffer and separated by centrifugation, C1 haemolytic activity was found in the supernatant. By incubating Sed with EAC4 cells (sheep erythrocytes sensitized with rabbit antibody to which C4 has been fixed), transfer was shown of C1 from the Sed to the cells, resulting in the formation of EAC14; this transfer was inhibited by IgG directed against a subunit of C1 (anti-C1s).

Glycosaminoglycans enhance complement hemolytic efficiency: theoretical considerations for GAG-complement-saliva interactions

When human serum is diluted and pre-incubated at 37 degrees C in low ionic strength buffer (LIS, u = 0.07; made iso-osmotic with dextrose), a spontaneous activation of complement (C) is observed as determined by C4 and C3 electrophoretic conversion. In this paper it is postulated that most species of glycosaminoglycans (GAG) restricted non-specific fluid phase complement consumption induced by LIS, an effect which conserved complement and thereby enhanced the subsequent residual serum C mediated hemolytic activity. The capacity of glycosaminoglycans, to have modulated the hemolytic activity at low ionic strength, depended on the charge of the GAG species tested. In general, the GAG regulatory effects may have been due to GAG mediated restriction of spontaneous non-specific fluid phase C1 autoactivation, and/or restriction of activated C1 activity. Such effects would result in the subsequent reduction of the spontaneous fluid phase C4 and C3 consumption. Although the precise mechanisms responsible for the effects were not identified, it is speculated that the potentiation of C1 inhibitor function and direct effects on C1 might be involved. Overall, the relative specific activities of the glycosaminoglycans, on a weight basis, in mediating the fluid phase C regulatory effect were heparin greater than dermatan sulfate greater than chondroitin-6-sulfate greater than chondroitin-4-sulfate greater than hyaluronic acid and keratan sulfate. When much higher concns of heparin (greater than or equal 0.2 micrograms/ml) were used, complement mediated lysis of EA was inhibited, probably due to the direct inhibition of C1, even C1 which may have bound to the sensitized erythrocytes (EA). Results similar to that of heparin were obtained using greater than 1 mg/ml of dermatan sulfate or dextran sulfate. In contrast, pre-incubation of human serum in LIS with high concns (up to 10 mg/ml) of hyaluronic acid or chondroitin-4-sulfate, which are much less charged, continued to result only in the restriction of hemolytically non-specific (fluid phase) C consumption, resulting in a higher residual complement hemolytic activity. A theory is developed that the binding of polyionic GAG to C1 and to C1 INH may provide a charged local environment which simulates a relatively higher ionic strength. Chemical degradation of hyaluronic acid or chondroitin-4 or -6 sulfate resulted in lowering of this C modulating effect, indicative of the importance of the structural integrity of these charged glycosaminoglycans.(ABSTRACT TRUNCATED AT 400 WORDS)

Hyaluronic acid-complement interactions--II. Role of divalent cations and gelatin

Native hyaluronic acid (HA) is reported to be a weak anticomplementary agent. However, the normal buffer systems used for complement tests incorporate gelatin, Ca2+ and Mg2+, which may bind to HA, influence its conformation and interfere with its anticomplementary reactions with complement components such as Cl. In this study, metal ions (Ca2+ and Mg2+), gelatin and fibronectin appeared to react with native HA preparations and block their anticomplementary effects on Cl. In previous studies, we obtained evidence for a relationship between reversible heat-induced HA conformational changes and a subsequent reversible increase in anticomplementary activity. The anticomplementary activity of heat-treated HA preparations was also reduced by gelatin.

Hyaluronic acid-complement interactions--I. Reversible heat-induced anticomplementary activity

The in vitro interaction of hyaluronic acid (HA) with complement (C) classical-pathway activity has been investigated. It was found that native HA, even at a high concn (greater than 3 mg/ml), has a relatively weak anticomplementary activity. However, we report here that native HA can be reversibly altered by heat treatment such that C-inhibitory properties are manifested. We have determined in this study that a potent C-inhibitory activity can be obtained if HA solutions are thermally treated (100 degrees C), and stabilized by prompt freezing with prompt thawing just prior to the interaction with human serum complement. Several investigators have proposed that the intermolecular-associated strands of HA undergo a reversible decoupling upon thermal treatment and this decoupled state of HA can be semi-stabilized by quickly cooling the sample. This heat-treated HA strongly inhibits C1 as well as classical-pathway-mediated C3 conversion. However, if heat-treated HA samples are not stabilized but, rather, slowly cooled after heating or if heated HA samples are snapfrozen and then slowly thawed, the anticomplementary activity is gradually lost. Interestingly, the activity for this same sample can be regenerated by retreatment of the same sample with heat followed by low-temp stabilization, indicating the reversibility of the physical state of HA responsible for the anticomplementary effect. Since no detectable molecular degradation of thermally-treated HA was found, it was assumed that a heat-induced physical transition of HA (decoupled state) was responsible for the C-inhibitory effect.