A proposal for purification of salivary polymorphonuclear leukocytes by combination of nylon mesh filtration and density-gradient method: a validation by superoxide- and cyclic AMP-generating responses

Neutrophils and neutrophil extracellular traps in oral health and disease

Neutrophils perform essential functions in antimicrobial defense and tissue maintenance at mucosal barriers. However, a dysregulated neutrophil response and, in particular, the excessive release of neutrophil extracellular traps (NETs) are implicated in the pathology of various diseases. In this review, we provide an overview of the basic concepts related to neutrophil functions, including NET formation, and discuss the mechanisms associated with NET activation and function in the context of the prevalent oral disease periodontitis.

Neutrophils are gatekeepers of mucosal immunity

Mucosal tissues are constantly exposed to the outside environment. They receive signals from the commensal microbiome and tissue-specific triggers including alimentary and airborne elements and are tasked to maintain balance in the absence of inflammation and infection. Here, we present neutrophils as sentinel cells in mucosal immunity. We discuss the roles of neutrophils in mucosal homeostasis and overview clinical susceptibilities in patients with neutrophil defects. Finally, we present concepts related to specification of neutrophil responses within specific mucosal tissue microenvironments.

Oral Polymorphonuclear Neutrophil Contributes to Oral Health

PURPOSE OF REVIEW

Oral health is maintained in a dynamic equilibrium between the host immunity and the oral microbiome. Oral polymorphonuclear neutrophils (oPMNs) are important innate immune cells in the oral cavity.

RECENT FINDINGS

The oPMNs play a co-controlling part in the maintenance of oral equilibrium. In human saliva, the oPMNs integrity is preserved, and their function remains unaffected. In general, oPMNs are in a higher state of baseline activation compared to peripheral PMNs. However, in periodontitis, the oPMNs' activation state can result in excessive release of damaging molecules in the extracellular environment.

SUMMARY

The presence of oPMNs may unwittingly negatively impact the integrity of the oral tissues. While most of the oPMN functions occur intracellularly, release of their potent active mediators into the extracellular environment may jeopardize oral homeostasis and its integrity. The dual nature of oPMNs, both beneficial and detrimental, remains a challenging and understudied topic.

Oral polymorphonuclear neutrophil characteristics in relation to oral health: a cross-sectional, observational clinical study

Polymorphonuclear neutrophils (PMNs) have a major role in the innate immune system. However, little is known about PMN contribution in relation to oral health. The objective of this study was to investigate the numbers and functional characteristics of oral PMNs (oPMNs) compared with circulatory PMNs (cPMNs). Oral rinse and venous blood samples were obtained from 268 systemically and orally healthy volunteers in a cross-sectional observational study. PMN counts, cell cycle analysis and cellular activation state were investigated. Also, reactive oxygen species (ROS) production was analyzed, with and without bacterial stimulation (Fusobacterium nucleatum). In males, 1.2 × 10⁶±1.0 × 10⁶ oPMNs were collected, and showed a tendency to correlate with the levels of gingival bleeding (r=0.215, P=0.008). Comparable oPMNs counts were found among females (1.0 × 10⁶±0.7 × 10⁶). More late-stage apoptotic/necrotic cells were found among the oPMNs (53.1%) compared with the cPMNs (8.5% P<0.001). Without additional stimulation, oPMNs were more activated than cPMNs, as indicated by higher expression of CD11b, CD63 and CD66b, and higher constitutive ROS levels (P<0.001). Notably, in response to bacterial stimulation, oPMNs released comparable ROS levels as cPMNs (P=0.042). In conclusion, this study provides data on viable oPMNs showing high levels of activation in orally and systemically healthy individuals, free of apparent caries lesions and periodontal disease. These data suggests that although the oPMNs are in a more mature stage of their life cycle compared with the cPMNs, oPMNs are still responsive to stimulation, which indicates their functional potential and possible contribution to a healthy oral ecosystem.

Oral neutrophils display a site-specific phenotype characterized by expression of T-cell receptors

BACKGROUND

Neutrophils, key cells of the innate immune system, were previously thought to be terminally differentiated cells, incapable of altering their gene expression after differentiation and maturation in the bone marrow. Only recently has it been shown that neutrophils perform rapid and complex changes in gene expression during inflammatory responses. Previous work by the authors has demonstrated differences in reactive oxygen species production between oral and peripheral blood neutrophils isolated from patients with chronic periodontitis, suggesting that oral neutrophils present with a unique oral phenotype. Understanding differences in the neutrophil transcriptome after transit from circulation into the site of inflammation will give new insights into how these innate immune cells function during inflammation.

METHODS

Venous blood and oral rinse samples were obtained from five healthy participants. Blood neutrophils were isolated using a standard gradient method. Oral neutrophils were isolated through nylon mesh filters of different pore sizes (40 to 10 μm). RNA was purified from isolated neutrophils, and gene expression microarray analysis was completed. Results were confirmed by quantitative reverse transcription-polymerase chain reaction and immunofluorescence microscopy.

RESULTS

Oral neutrophil isolation, which is critical when analyzing gene expression with samples clear of epithelial cell contamination, was optimized. It was also demonstrated that oral neutrophils present with a significant increase in T-cell receptor expression compared with circulating neutrophils, suggesting a role for oral neutrophils in crosstalk between the innate and adaptive immune system in the mouth.

CONCLUSION

To the best of the authors' knowledge, it is demonstrated for the first time that, compared with circulating neutrophils, oral neutrophils present a site-specific gene expression profile in healthy individuals.

Refractory periodontitis population characterized by a hyperactive oral neutrophil phenotype

BACKGROUND

Neutrophils, in addition to being the primary protective component of the innate immune system, also contribute to periodontal destruction through production of reactive oxygen species (ROS), which cause damage to connective tissues and extracellular matrix after neutrophil activation. We have previously shown that hyperactive neutrophils are present in peripheral blood samples of patients diagnosed with refractory periodontitis. To test the hypothesis that oral neutrophil hyperactivity is related to periodontal disease severity, we used a flow cytometric approach to isolate and analyze oral neutrophil ROS (oROS) production in a refractory periodontal disease patient population.

METHODS

Oral rinse samples and venous blood were obtained from 13 patients diagnosed with refractory periodontitis. After isolation of neutrophils from both samples, dihydrorhodamine 123 was used as a fluorescent probe for phorbol 12-myristate 13-acetate-mediated ROS production as assessed through flow cytometry. For each patient, oROS production levels were expressed as a percentage of their baseline to maximal peripheral blood neutrophil ROS production range.

RESULTS

Two distinct groups of refractory patients were identified based on levels of phorbol 12-myristate 13-acetate-stimulated oROS production. The patient group with high oROS production had significantly more clinical attachment loss (AL) compared to the patient group with low oROS production.

CONCLUSIONS

Our findings demonstrate that a group of refractory patients with increased clinical AL present a hyperactive oral neutrophil phenotype characterized by increased potential for ROS production. Identification of this exaggerated oral neutrophil phenotype could allow clinicians to identify which patients are more susceptible to rapid disease progression.

Comparison of superoxide detection abilities of newly developed spin traps in the living cells

This study compared the superoxide detection abilities of four spin traps, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO), 5-(diphenylphosphinoyl)-5-methyl-1pyrroline N-oxide (DPPMPO) and 5-(2,2-dimethyl-1,3-propoxy cyclophosphoryl)-5-methyl-1-pyrroline N-oxide (CYPMPO) in living cells. Electron spin resonance (ESR) signals of the superoxide adducts were observed when spin traps were added to a suspension of human oral polymorphonuclear leukocytes (OPMNs) stimulated by phorbol 12-myristate 13-acetate. The ESR signal of the CYPMPO-superoxide adduct (CYPMPO-OOH) increased for 24 min after the initiation of the reaction, whereas the signals from DMPO-OOH and DPPMPO-OOH peaked at 6 and 10 min, respectively. The maximum concentrations of DMPO-OOH, DPPMPO-OOH and CYPMPO-OOH in OPMNs were 1.9, 6.0 and 10.7 microM, respectively. Furthermore, CYPMPO could more efficiently trap superoxide in blood PMNs compared with DEPMPO. From these results, it was concluded that CYPMPO performs better than DMPO, DPPMPO and DEPMPO for superoxide measurements in living cell systems because it has lower cytotoxicity and its superoxide adduct has a longer lifetime.

Chlorogenic acid in coffee can prevent the formation of dinitrogen trioxide by scavenging nitrogen dioxide generated in the human oral cavity

Coffee contains antioxidants like chlorogenic acid and its isomers. In this report, effects of coffee on the nitrite-induced N2O3 formation were studied using whole saliva and bacterial fraction prepared from the saliva. The formation of N2O3 was measured by fluorescence increase due to the transformation of 4,5-diaminofluorescein to triazolfluorescein. Coffee inhibited the nitrite-induced fluorescence increase, and 50% inhibition was observed at several microg of coffee/mL in bacterial fraction of saliva as well as whole saliva. During the inhibition of the fluorescence increase, concentration of chlorogenic acid and its isomers decreased. It is discussed that the reduction of NO2 by chlorogenic acid and its isomers contributed to the coffee-dependent inhibition of the fluorescence increase as N2O3 is formed from NO and NO2. When coffee was added to whole saliva, chlorogenic acid and its isomers bound to cells in the saliva. The rate of the fluorescence increase in bacterial fraction, which was prepared at defined periods after the ingestion of coffee, was increased to the rate before the ingestion of coffee with a half-time of about 1 h. This result suggests that chlorogenic acid and its isomers remained in the oral cavity for a few hours after ingestion of coffee. The significance of coffee drinking and rinsing of the mouth with coffee for the health of the oral cavity is proposed.

Quercetin-dependent scavenging of reactive nitrogen species derived from nitric oxide and nitrite in the human oral cavity: interaction of quercetin with salivary redox components

In the human oral cavity, nitrite is reduced to nitric oxide (NO) by certain bacteria. The NO formed reacts with O2 to generate NO2 and then with NO2 producing N2O3. In this study, N2O3 produced by the reaction between NO and NO2 was detected by fluorescence increase due to the transformation of 4,5-diaminofluorescein to fluorescent triazolfluorescein. Nitrite-induced fluorescence increase in the bacterial fraction of saliva was completely inhibited by 1muM quercetin and the complete inhibition continued until almost all quercetin had been oxidized. Nitrite-induced fluorescence increase was also observed in the saliva which contained salivary redox components. Quercetin effectively inhibited the fluorescence increase. During the inhibition of the fluorescence increases by quercetin, the flavonol was oxidized. NO2 seemed to participate in the oxidation. The main oxidation product was 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofurane. Thiocyanate inhibited the fluorescence increase in bacterial fraction and duration of the complete inhibition by quercetin was prolonged by SCN-. The inhibition and the prolongation are discussed to be due to SCN--dependent inhibition of oxidation of nitrite to NO2 by salivary peroxidase. Quercetin cooperated with ascorbate to inhibit the fluorescence increase. From the results obtained in this study, it is deduced (1) that quercetin can protect human oral cavity from damages induced by reactive nitrogen species and (2) that the protective function of quercetin may be significant when antioxidant capacity of saliva is decreased by periodontal diseases.

Quercetin-dependent inhibition of nitration induced by peroxidase/H2O2/nitrite systems in human saliva and characterization of an oxidation product of quercetin formed during the inhibition

Local pH in the oral cavity can decrease to below 7 at the site where acid-producing bacteria are proliferating. Effects of pH on nitration of 4-hydroxyphenylacetic acid were studied using dialyzed human saliva. Dialyzed saliva nitrated 4-hydroxyphenylacetic acid to 4-hydroxy-3-nitrophenylacetic acid in the presence of nitrite and H(2)O(2). The rate of the nitration was dependent on pH, and the maximal rate was observed between pH 5.5 and 7.2. The optimum pH seemed to reflect rates of formation of nitrogen dioxide and 4-hydroxyphenylacetic acid radicals. Quercetin inhibited the nitration. The quercetin-dependent inhibition might be due to scavenging of nitrogen dioxide and 4-hydroxyphenylacetic acid radicals, which were formed by salivary peroxidase-dependent oxidation of nitrite and 4-hydroxyphenylacetic acid, respectively, and competition with nitrite and 4-hydroxyphenylacetic acid for peroxidase in saliva. An oxidation product of quercetin was formed during inhibition of the nitration by quercetin. The oxidation product was identified as 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone. This component could also be oxidized by salivary peroxidase and nitrogen dioxide radicals. The oxidation products were 2,4,6-trihydroxyphenylglyoxylic and 3,4-dihydroxybenzoic acids. On the basis of the results, the significance of quercetin for inhibition of nitrogen dioxide formation and for scavenging of nitrogen dioxide radicals in the oral cavity is discussed.

Human salivary peroxidase-catalyzed oxidation of nitrite and nitration of salivary components 4-hydroxyphenylacetic acid and proteins

Human saliva contains high activities of peroxidase and high concentrations of nitrite (about 0.2 mM in average). If H2O2 is provided by bacteria and leukocytes in the oral cavity, peroxidase-dependent formation of reactive nitrogen species, which can nitrate phenolics like 4-hydroxyphenylacetic acid (HPA) and tyrosine residues in salivary proteins, is possible. H2O2-dependent oxidation of nitrite and H2O2-dependent nitration of HPA were observed in dialyzed saliva and by partially purified salivary peroxidase (SPX). The nitration was inhibited by a physiological electron donor to salivary peroxidase, SCN-. When concentrations of H2O2 and nitrite were increased, nitration of HPA was also observed in control (non-dialyzed) saliva. In addition, H2O2-dependent nitration of tyrosine residues in salivary proteins was observed in dialyzed saliva as an increase in absorbance around 420 nm at pH 7.2. Kinetic studies of the increase in absorbance indicated that sulfhydryl groups in salivary proteins as well as glutathione, ascorbate, urate and SCN- could inhibit the nitration. Since the nitration of proteins can lead to impairment of their functions, it is discussed how the oral cavity is protected from the damages caused by reactive nitrogen species under normal conditions and also discussed that reactive nitrogen species generated by the H2O2/nitrite/peroxidase system can participate in the host defence mechanism in the oral cavity.

The loss of susceptibility to apoptosis in exudated tissue neutrophils is associated with their nuclear factor-kappa B activation

Tissue neutrophils, human salivary neutrophils donated from healthy subjects and synovial fluid neutrophils collected from patients with rheumatoid arthritis were compared with circulating blood neutrophils. Concomitant treatment of circulating blood neutrophils with tumor necrosis factor-alpha (TNF-alpha) and cycloheximide induced neutrophil apoptosis, whereas the same treatment failed to induce significant apoptosis in salivary and synovial fluid neutrophils. Caspase-3 activation by TNF-alpha was observed in these tissue neutrophils, although its activity was significantly weaker than that in circulating blood neutrophils. In circulating blood neutrophils, TNF-alpha induced activation of nuclear factor-kappa B (NF-kappa B), whereas, in tissue neutrophils, NF-kappa B had been already activated without any stimulation, and no further activation was induced by the treatment with TNF-alpha. Furthermore, while pretreatment of neutrophils with an NF-kappa B inhibitor produced typical apoptotic changes in circulating blood neutrophils, this inhibitor did not produce any morphological apoptotic changes induced by TNF-alpha in tissue neutrophils. These results indicate that neutrophils undergo marked functional changes such as altered sensitivity to apoptosis-inducing stimuli in association with their exudation from blood into tissue, and that NF-kappa B activation is involved in the acquisition of resistance to TNF-alpha-induced apoptosis.

Differential uptake of grepafloxacin by human circulating blood neutrophils and those exudated into tissues

The uptake of the antimicrobial quinolone agent, grepafloxacin, both by human circulating blood neutrophils and by those exudated into tissues, was evaluated in vitro by comparing the intracellular drug concentrations. In circulating blood neutrophils, the uptake of grepafloxacin was rapid and saturable at 37 degrees C. The uptake of grepafloxacin into circulating blood neutrophils was reduced by lowering the environmental temperature or by the presence of metabolic inhibitors, suggesting the involvement of an active transport mechanism. Furthermore, the uptake of grepafloxacin by tissue (salivary) neutrophils was also partially temperature-dependent and was significantly greater than that by circulating blood neutrophils, i.e. exudation of neutrophils into tissue results in a markedly enhanced transport mechanism for grepafloxacin. This phenomenon may be related to the higher defense activity against infection seen in exudated tissue neutrophils.

Free radical scavenging activity in the nonenzymatic fraction of human saliva: a simple DPPH assay showing the effect of physical exercise

Free-radicals and other reactive oxygen species (ROS) have been implicated as being major damaging species in pathology and they have been widely investigated. Using 1,1'-diphenyl-2-picrylhydrazyl (DPPH), we estimated total free radical scavenging activity in the low-molecular-weight nonenzymatic fraction (LMNEF) of human whole saliva. The activity of the whole saliva and serum were measured in terms of the rate of decrease in the absorbance at 517 nm in a 40% ethanol DPPH solution (pH 7.4) at room temperature. The DPPH activity of saliva and serum showed a significant linear relationship. The mean DPPH activities of saliva from 257 subjects aged 4-72 was found to be 0.389+/-.190 micromol/ml and bore no relation to age or sex. The activity in saliva of 86 subjects aged 4-11 was significantly different before and after exhaustive aerobic dance exercise for 1 hr. Physical exercise markedly decreased free radical scavenging activity in whole saliva of children. On the basis of the above results, we concluded that DPPH is useful for evaluating the total antioxidant capacity of LMNEF of human saliva.

Comparison of susceptibility to apoptosis induced by rhTNF-alpha and cycloheximide between human circulating and exudated neutrophils

To determine whether exudated neutrophils differ from circulating ones in their apoptosis, rhTNF-alpha plus cycloheximide-induced apoptosis in human salivary neutrophils was compared to that in human neutrophils in peripheral blood. Concomitant treatment of peripheral blood neutrophils with rhTNF-alpha and cycloheximide-induced apoptosis in blood neutrophils within 3 hr, as evaluated both by light microscopic changes characteristic to apoptosis and by DNA fragmentation, whereas the same treatment failed to induce any apoptosis in salivary neutrophils. These results indicate that the exudation of neutrophils from blood into tissue is associated with marked changes in their functions such as alteration in their sensitivity to apoptosis-inducing stimuli.

Migration of neutrophils from blood to tissue: alteration of modulatory effects of prostanoid on superoxide generation in rabbits and humans

Alteration of neutrophil function is associated with their migration from blood into tissue. We evaluated this alteration in both human and rabbit neutrophils, by comparing the inhibitory effects of prostanoids on formylmethionyl-leucyl-phenylalanine (fMLP)-stimulated superoxide generation in human circulating blood neutrophils with those in saliva, and also comparing rabbit circulating blood neutrophils with those exudated into peritoneal cavity. We showed that EP-receptor agonists (PGE1) EP2/EP3 agonist (misoprostol), EP2-receptor agonist (butaprost) and DP-receptor agonist (PGD2) inhibited fMLP-stimulated superoxide production from human blood neutrophils in a concentration-dependent manner. In contrast, these prostanoids produced a significantly smaller maximum inhibition of fMLP-stimulated superoxide production in salivary neutrophils compared to those in blood neutrophils. Similar differences were observed for rabbit blood and peritoneal neutrophils. The inhibitory effect of EP2 agonist (butaprost) on the fMLP-stimulated superoxide generation in human blood neutrophils was significantly higher than that of EP3 agonist (ONO-AP-324). The EP1 antagonist (SC-51322) and EP4 antagonist (AH23848B) employed in this study could not antagonize the inhibitory effect of PGE2. TP agonist (U-46619) failed to show any inhibitory effect in either blood or salivary neutrophils. These results indicated that EP2 and DP receptors are the primary receptors mediating the prostanoids inhibition of fMLP-stimulated superoxide generation from neutrophils. Furthermore, it can be concluded that neutrophils become less responsive to prostanoids in terms of fMLP-stimulated superoxide production in association with their migration from blood to tissue.