Tobacco-induced suppression of the vascular response to dental plaque.

Abstract

Cigarette smoking presents oral health professionals with a clinical and research conundrum: reduced periodontal vascular responsiveness to the oral biofilm accompanied by increased susceptibility to destructive periodontal diseases. This presents a significant problem, hampering diagnosis and complicating treatment planning. The aim of this review is to summarize contemporary hypotheses that help explain mechanistically the phenomenon of a suppressed bleeding response to dysbiotic plaque in the periodontia of smokers. The influence of smoke exposure on angiogenesis, innate cell function, the production of inflammatory mediators including cytokines and proteases, tobacco-bacterial interactions and potential genetic predisposition, are discussed.

Periodontal diseases and cigarette smoking

Cigarette use is a, or perhaps the, major risk factor for destructive forms of periodontal disease^1,2. Tobacco use has been associated with increased disease progression and severity, as well as with refractory disease^3,4. Risk appears dose-related^1,5,6 while environmental smoke exposure may also be associated with increased risk of destructive forms of periodontitis^7–9. At the population level, smoking alone may account for most cases of periodontitis in adults in developed nations^10–13. Indeed, smoking rates and sales each predict periodontitis prevalence^10,14. In 2000, US NHANES data ascribed almost 80% of the attributable risk of destructive forms of periodontitis in cigarette users to smoking¹³. The 2016 NHANES data suggest that the prevalence of periodontal diseases in smokers and non-smokers is 62% and 32%, respectively 15,16, while severe periodontitis prevalence is 26% and 7%, respectively¹⁷. In Sweden, the population attributable fraction for smoking was 80% in 1970 and is presently close to 60%¹¹. Similar tobacco-periodontitis relationships have now been reported at the population level in New Zealand¹². In many developing nations, smoking rates are rising dramatically¹⁸, suggesting that an increase in the periodontitis burden in such countries should be expected. In Pakistan, for example, 55% of those with chronic periodontitis are smokers, while 82% of smokers have this form of disease¹⁹. Thus, epidemiological data from across the globe clearly indicates an intimate association between tobacco smoking and periodontal diseases.

Interpretation of the clinical signs of inflammation of the periodontal tissues is an important aid in the diagnosis and prognosis of plaque-associated periodontal diseases²⁰. Indeed, key warning signs are spontaneous gingival bleeding or bleeding that occurs in response to tooth brushing or periodontal probing in the dental office. However, tobacco-related suppression of the periodontal vascular response to plaque in cigarette smokers is long established. Pindborg noted, in the 1940s, that many individuals with an aggressive form of periodontitis were smokers and suggested that nicotine-mediated “vascular spasms” may be a contributing factor^21,22. McMurray et al later suggested a potential relationship between gingival bleeding and smoking status²³. It is now accepted that tobacco smoking presents oral health professionals with a clinical conundrum: decreased gingival bleeding but increased disease susceptibility. This apparent paradox also represents a challenge to the research community as, although tobacco use may account for the majority cases of chronic periodontitis¹³, the underlying mechanisms of tobacco-induced vascular suppression and disease promotion remain unclear.

While smoking has effects on multiple biological systems that may predispose to chronic periodontitis, including amplification of cholinergic anti-inflammatory signaling^24–26; a reduced antibody response^27–31; suppressed immune cell function^32–35; promotion of osteoclast/osteoblast imbalance^36,37; a depleted antioxidant defense 38,39; and compromised tissue remodeling^40–42, this review will focus specifically on the vascular response itself.

Suppressed gingival bleeding in tobacco smokers

Multiple studies have firmly established that cigarette smoking exerts a profound, potentially dose-dependent suppressive effect on the bleeding response to plaque in humans, as shown in Figure 1A and B and as evidenced in Table 1. As an example, Ditriech et al reported that, in those smoking more than 10 cigarettes/day, the odds ratio (O.R.) of bleeding at healthy periodontal sites was 0.6 (95% CI: 0.4–0.7) compared to non-smokers, and diseased sites were considerably more likely to bleed in non-smokers (O.R. 5.7; 95% CI, 4.3–7.6)⁴³.

Figure 1: The clinical conundrum in smoking-related periodontitis.

A. Typical bleeding response in a non-smoker (left) and smoker (middle and right) with generalized aggressive periodontitis.

B. 1. Cigarette smoking is associated with reduced edema, gingival index and bleeding response in humans but increased periodontal destruction. Photo kindly provided by Dr. Richard Palmer, King’s College London, England. 2. This may be due to a chronically suppressed angiogenic response accompanied by suppressed endothelial activation rather than an acute vasoactive phenomenon. Figure and legend reproduced, with permission, from^159#. 3. Tobacco also induces a dysbiotic microbiota, with phenotypically tobacco-altered P. gingivalis interacting with the immune response in a profoundly different manner than unexposed bacteria. Figure and legend reproduced, with permission, from^111##. 4. The inflammatory and bleeding response to plaque, however, rebound within weeks of smoking cessation. Figure extrapolated, with permission, from 72. The mean (s.d.) proportion of sites exhibiting bleeding on probing and plaque at baseline (white bars) and at 4–6 weeks after quitting smoking (black bars). 5. These anti-inflammatory events are accompanied by an increased local (periodontal) and systemic proteolytic burden and dysregulated innate cell function.

^# Typical histological sections of gingival tissue from a smoker (upper) and a non-smoker (lower) with chronic periodontitis. Tissue sections were labelled with monoclonal antibodies to ICAM-1 and secondarily labelled. The percentage of ICAM-1-positive vessels of the gingival microvasculature was determined following immunostaining of von Willebrand factor, an endothelial marker, in adjacent sections. ICAM-1 expression microvascular development was compromised in smokers.

^## Pro-inflammatory TNF release by primary human monocytes (0.5 × 10⁶) stimulated for 20 h with control P. gingivalis (10⁷); CSE-treated P. gingivalis; or P. gingivalis cells that were first grown in CSE-treated medium for two passages then reconditioned in untreated medium for 2 passages (10⁷ P. gingivalis cells). Error bars represent the mean (s.d.) of 3 experiments.

Table 1:

Comparison of direct vascular indices in the periodontal tissues and GCF of smokers and non-smokers.

Vascular Index	Vascular relevance	Smokers vs Non-smokers
GCF flow/volume	Serum-derived trans-periodontal tissue exudate	124 CP n.s.d. 125 ↓ LCP 126 ↓ CPs 127 ↓ H 128 ↓ H 129 ↓ P 129 ↓ H
Gingival / sulcular bleeding	Spontaneous or provoked (probing; brushing) vascular response to plaque	78 ↓ P 130 ↓ P 131 ↓ P 132 ↓ P 133 ↓ P 86 ↓ H 134 ↓ H 125 ↓ LCP 79 ↓ CP 43 ↓ H, P 135 ↓ SPT 136 n.s.d. CP 137 n.s.d. CP 138 n.s.d. CP ↓ CS ↓ AP
Microvascular density	Density of small blood vessels, reflecting angiogenesis	44 ↓ P 141 n.s.d. P
Gingival index	Measure of the clinical severity of gingival inflammation	125 ↓ LCP 47 n.s.d P 141 n.s.d. P 126 ↓ CP
Neutrophil numbers	Bacterial killing, promotion of inflammation, reflection of vascular activation	141 n.s.d. CP 86 ↓, ↑ H
sICAM-1	Marker of vascular activation	68 ↓ P
VE-cadherin	Endothelial cell junction promotion	125 n.s.d. LCP
VEGF	Potent promoter of angiogenesis	125 n.s.d. LCP

↓, significantly reduced in smokers; ↑, significantly enhanced in smokers compared to non-smokers; n.s.d., no significant difference.

AP, aggressive periodontitis; CP, chronic periodontitis; CS, cross-sectional; GCF, gingival crevicular fluid; H, periodontally healthy; sICAM-1, soluble intracellular adhesion molecule-1; LCP, localized chronic periodontitis; P, periodontitis; SPT, supportive periodontal therapy subjects; VE-vadherin, vascular endothelial cadherin; VEGF, vascular endothelial growth factor.

While healthy and diseased periodontal tissues in smokers exhibit greater junctional and marginal epithelial thickness than do non-smokers^44,45, the relevance of this phenomenon to gingival bleeding, if any, remains unclear. Smoking contributes to a lowered oxygen tension in the periodontal pocket^46,47. The lack of oxygen sufficiency in gingival pockets could influence gingival bleeding directly or indirectly. For example, it is possible that vascular responsiveness, which occurs primarily beneath the junctional epithelium adjacent to plaque accumulation, may be less efficient in anoxic conditions and upon exposure to the composite insult of chemicals found in cigarette smoke. It is also clear that smoking leads to altered dental plaque composition, with the accumulating oral microbiome the key driver of gingival inflammation and bleeding. We shall address tobacco-bacterial interactions in more detail later.

There has been some debate as to whether cigarette consumption reduces gingival bleeding by suppression of angiogenesis or due to the influence of vasoactive smoke components or metabolites on the existing gingival microvasculature. The evidence remains somewhat ambiguous, particularly for acute vascular events, and it is possible that both phenomena occur concomitantly.

As we and others have reviewed previously^20,48,49, the reduced bleeding response in smokers was once attributed primarily to nicotine-mediated vasoactivity. In support of this theory, intraartererial nicotine infusion in rabbits has been shown to induce gingival ischemia⁵⁰. Also in rabbits, the same group later determined the influence of epinephrine and nicotine on gingival blood flow, using thermal diffusion. Initial, transitory dilatory effects were observed with the reverse, considered a reflection of vasoconstriction, noted within ten minutes⁵¹. Oral or systemic nicotine administration to dogs has been associated with increased blood flow to the anterior gingiva, relative to untreated controls, as assessed using radiolabeled microspheres 52. However, central, intracerebroventricular injection of nicotine reduced gingival blood flow in rats in a dose-related manner⁵³.

In humans, nicotine can influence variant vascular beds differentially^48,54–58. Grudianov and Kemulariia have reported that microvascular blood flow in humans with periodontitis, as measured by laser doppler flowmetery (LDF), is reduced immediately upon smoking a cigarette and that the recovery period is longer, the more severe the periodontal disease⁵⁹. However, Mavropoulos et al showed a minor increase in blood flow to the gingiva upon smoking in 13 periodontally healthy, casual tobacco users (1 self-reported cigarette per week up to 5 per day), as determined by LDF. Importantly, this gingival hyperemic response was minor compared to the major vasoconstriction observed in the peripheral (thumb) vasculature and was overcome by arterial perfusion pressure⁵⁸. The same group later reported, in patients with periodontitis, that resting gingival blood flow did not differ between smokers and non-smokers, again as determined by LDF⁶⁰. The LDF study of Meekin et al also concluded that smoking did not impair gingival blood flow in humans, although an acute cutaneous forehead vasoactive phenomenon was apparent⁵⁵. More recently, Molnar et al have shown that the gingiva of smokers are less responsive to a heat provocation test, which assesses vascular reactivity, than non-smokers, as measured by induction of gingival crevicular fluid flow⁶¹. Interestingly, the authors reported that, independent of periodontal disease classification, the volume of GCF produced is significantly lower in smokers compared to non-smokers. The combined evidence that gingival vasoconstriction is promoted by tobacco smoke is, at most, limited in comparison with other areas of the body in humans^48,55. The use of LDF to monitor gingival blood flow in humans has been reviewed recently by Kouadio et al⁶².

A lack of bleeding response to plaque accumulation could reflect suppressed angiogenesis rather than vasoactivity. In a study of 8 young smokers and 8 non-smokers, in whom gingivitis was promoted by the cessation of oral hygiene measures, Bergstrom et al noted that, although dental plaque accumulated in both groups, the intensity of the vascular reaction, i.e., the number of gingival vessels determined sterophotometrically, was reduced by approximately 50% in the smokers⁶³. In a small number of smokers (n = 4), Mirbod et al reported that, compared to non-smokers, cigarette consumption was associated with a higher number of microvessels with low internal circumference in the gingiva and a lower number of high internal circumference microvessels⁶⁴. Scardina and Messina further observed that while, compared to non-smokers, the gingival microvasculature of smokers exhibited a larger number of capillaries, the capillaries were of smaller caliber, as determined by videomicroscopy⁶⁵. Lindeboom et al, though, found no difference in gingival microvascular density, as examined by spectroscopically, in a young and periodontally healthy set of smokers and non-smokers⁶⁶.

When looking specifically at inflamed areas of the periodontium in subjects with chronic periodontitis, Rezavandi et al noted an increased number of vessels, as determined by Von Willebrand Factor staining, in non-smokers compared to smokers⁶⁷, suggesting tobacco-induced angiogenic suppression. While the number of activated, ICAM-1 and E-selectin expressing small vessels was greater in sites with inflammation compared to non-inflamed sites in smokers and non-smokers alike, reduced ICAM-1 expression was noted in the non-inflamed tissues of the smokers. A reduction in ICAM-1 expression could reflect a lack of gene activity. However, the substantive increase in soluble ICAM-1 in smokers hints at vascular ICAM-1 shedding, a risk factor for vascular diseases^68,69. These data are supportive of dysregulated activation of the periodontal vascular endothelium in smokers.

Finally, there is evidence for potentially thermally-induced nerve damage in the oral cavities of smokers^70,71. While this has not been directly examined in the gingiva, it is possible that another consequence of smoking is compromise of local innervation networks that can influence the microvascular response not least through the release of vasoactive neuropeptides.

Importantly, within weeks of biochemically validated (expired air CO) cessation of cigarette smoking, bleeding on probing is increased. Recovery in bleeding occurred concomitantly with improved plaque control, presumably representing more rigorous oral hygiene prompted by participation in a dental research trial⁷². Similarly, Morozumi et al later reported that gingival bleeding and GCF volume increased shortly after validated smoking cessation⁷³.

Smoking clearly suppresses the bleeding response to plaque in human smokers. While the mechanistic data are somewhat inconsistent, we hypothesize that smoking leads to a reduced angiogenic response to plaque but appropriate periodontal inflammation, reflected in increased bleeding, is quick to recover upon quitting cigarette use.

Inflammatory mediator imbalance in tobacco smokers

Inflammation encompasses manifold processes that occur simultaneously. Some aspects of the inflammatory response will be upregulated by components of cigarette smoke, or their metabolites, e.g., production of some proteolytic enzymes. Other inflammation-related events will be suppressed, e.g. pro-inflammatory cytokine production and angiogenesis. Still other aspects of the host response to oral biofilms will be unaffected. While it is clear that the systemic levels of multiple inflammatory mediators and biomarkers, e.g. CRP, sICAM-1, MMP-9, myeloperoxidase and neutrophil elastase, are increased in cigarette smokers, compared to non-smokers (e.g.^68,74–76), the inflammatory burden has also been extensively examined in the local periodontal environment. Studies on gingival crevicular fluid (GCF) content have been particularly enlightening. GCF is a trans-periodontal, serum-derived exudate that contains multiple inflammation-related molecules garnered on its journey across the gingival tissues. As such, it is considered to reflect the disease process across the periodontium and has been extensively characterized in order to better understand periodontal disease progression. A comparison of key inflammatory indices and mediators in the GCF of smokers and non-smokers is presented in Tables 1–3. While certain bioactive molecules noted in Tables 1–3 will have direct effects of angiogenesis and vasoactivity, such as VEGF, IL-8 and prostaglandins, the overall balance and intensity of pro- versus anti-inflammatory mediators is critically important.

Table 1 summarizes direct vascular indices. There is a clear suppression in smokers, relative to non-smokers, of gingival bleeding, GCF production and gingival index (a clinically evaluated combined inflammation and bleeding scale, summarized in Scott and Singer²⁰; alterations to the microvasculature architecture; while reports on intravascular gingival blood flow provide varied results. Interestingly, the systemic neutrophilia and vascular activation (sICAM-1) seen systemically in smokers are not reflected in the GCF^68,77,78, indicating a limitation in vascularity and/or diapedesis. Table 2 addresses cytokines, chemokines and related molecules, with the weight of evidence suggesting a reduced GCF content of pro-inflammatory mediators, such as IL-1α, IL-1β IL-8, MCP-1 and MIP-1. Table 3 (Supplemental Material) considers GCF concentrations of proteases, anti-proteases and other inflammatory biomolecules involved in tissue remodeling. Overall, there appears to be a cigarette-induced protease-antiprotease imbalance, at least as determined by comparing MMP-8, MMP-9 (proteases) and α1-antitrypsin, α2-macroglobulin levels (protease inhibitors). Indeed, serum derived from smokers with chronic periodontitis has been shown to enhance MMP-1 and -9, but suppress TIMP-1 (a key MMP inhibitor), release in P. gingivalis-exposed epithelial cells compared to serum from non-smokers⁷⁹. Further, Bondy-Carey et al have shown that cigarette smoke influences cytokine and protease production in a profound and complex manner in a tripartite (epithelial cell-neutrophil-P. gingivalis) model of the gingival crevice³³. The enhanced proteolytic burden is likely to contribute to the connective tissue destruction that helps define chronic periodontitis.

Table 2:

Comparison of cytokines, chemokines and related inflammatory mediators in the GCF of smokers and non-smokers.

Mediator	Vascular relevance	Smokers vs Non-smokers
IL-1α	Pro-inflammatory (NF-κB activator); endothelial activation and vasoactive amine release	141 ↓ CP 142 ↓ SP 143 n.s.d. H 143 ↓ P
II-1β	Pro-inflammatory (NF-κB activator); endothelial activation and vasoactive amine release	144 n.s.d., ↑* CS 145 ↓ P 141 ↓ CP 133 n.s.d. P 146 n.s.d. CP
IL-1Ra	Anti-inflammatory (IL-1α and β inhibitor)	145 ↓ P 133 n.s.d. P
IL-6	Pro-inflammatory (STAT-3 pathway, promotion of acute phase response) and potential anti-inflammatory (PI3 kinase pathway) functions	147 n.s.d. CP n.s.d. 143 ↓ H, P 140 ↑ H, AP
CXCL8 (IL-8)	Leukocyte chemoattractant, promoter of angiogenesis and vasoactive amine release	143 ↓ H, P 140 ↓ H, AP
IL-10	Anti-inflammatory (NF-κB activator inhibitor)	146 n.s.d. CP 79 ↑ CP
IL-12	Innate-adaptive immune response bridging cytokine; antagonist of angiogenesis	148 n.s.d. CP 143 ↓ H, P
IL-16	Pro-inflammatory (monocyte chemoattractant)	148 n.s.d.
CCL2 (MCP-1)	Leukocyte chemoattractant	143 ↓ H, P
CCL4 (MIP-1)	Leukocyte chemoattractant	143 ↓ H, P ↓ P
TNF	Pro-inflammatory (NF-κB activator)	147 n.s.d. CP 149 ↑ P 149 ↑ CP 79 ↓ CP 150 n.s.d. CP
PGE2	Pro-inflammatory (COX pathway; direct vasodilator)	144 ↓ CS 151 n.s.d. CP
CCL5 (RANTES)	Leukocyte chemoattractant	143 ↓ H, P
TGF-β	Suppressor of leukocyte activation; extracelluar matrix stimulation	152 ↑ CP 153 n.s.d. SP
PAI-2	Anti-inflammatory (blocks plasmin-C3a axis)	154 H ↓

↓, significantly reduced in smokers compared to non-smokers; ↑, significantly enhanced in smokers compared to non-smokers; nsd, no significant difference.

AP, aggressive periodontitis; COX, cyclooxygenase; CP, chronic periodontitis; CS, cross-sectional; GCF, gingival crevicular fluid; H, periodontally healthy; IL-1Ra, IL-1 receptor antagonist; P, periodontitis; PAI-2, plasminogen activator inhibitor-2; PGE2, prostaglandin E₂; SP, severe periodontitis; SPT, supportive periodontal therapy subjects; STAT-3, signal transducer and activator of transcription-3; TNF, tumor necrosis factor (formerly TNF-α).

^*n.s.d. for former and current smokers with a pack year history of less than 20 years.

The response of innate cells to plaque in tobacco smokers

Innate cells activated in the host response to plaque release large amounts of factors that drive inflammation, angiogenesis and tissue remodeling. The primary innate cells found in the gingival crevice are neutrophils, where they attempt to form a barrier between the junctional epithelium and the accumulating microbiome⁸⁰.

A detrimental effect of cigarette smoking on oral neutrophil function has been suspected for at least 40 years^81,82. Tobacco-related cytoskeletal, chemotactic, oxidative burst, phagocytic, bactericidal and other functional defects, as well as inefficient neutrophil differentiation and compromised viability, have all been noted^48,83–87.

While smoking induces a chronic systemic neutrophilia⁸⁸, the crevicular neutrophil count in smokers and non-smokers in disease-matched subjects appear similar⁷⁸, which, again, suggests compromised vascular-neutrophil interactions controlling diapedesis into the gingival tissues and crevice.

Multiple research avenues suggest that neutrophil interactions with oral microbes are compromised by smoking, although how this may influence gingival bleeding is not entirely clear. For example, nicotine has been shown to diminish elements of the oxidative burst and inhibit the ability of human neutrophils to kill multiple bacteria, including Actinomyces naeslundii, Aggregatibacter actinomycetemcomitans, Fusobacterium nucleatum, Porphyromonas gingivalis and Staphylococcus aureus^34,35,85.

The function of other myeloid cell types may be similarly compromised by cigarette smoke. For example, Yanagita et al reported that nicotine exposure during differentiation suppressed subsequent production of multiple cytokines (TNF, IL-10, IL-12 p40 and p70, RANTES/CCL5) by P. gingivalis-derived LPS-stimulated monocyte-derived dendritic cells, while MCP-1/CCL2 and CCL-22 (macrophage-derived chemokine, MDC) release was enhanced⁸⁹. Ryder et al have observed a profound influence of tobacco smoke on the monocytic transcriptome⁹⁰. A reduced IL-12 (p70) response to oral bacteria (Prevotella intermedia and Fusobacterium nucleatum) in PBMCs isolated from smokers with generalized aggressive periodontitis, as compared to innate cells isolated from diseased non-smokers, has also been noted⁹¹.

Interestingly, in vitro exposure of endothelial cells to high levels of nicotine (10 mM) resulted in reduced production of the leukocyte chemoattractant, IL-8, and a suppression of endothelial migration in response to stimulation with P. gingivalis-derived lipopolysaccharide (LPS)⁹². Such evidence is supportive of the anti-angiogenesis hypothesis.

Thus, a profound influence of cigarette consumption on innate cell function is apparent. What is less clear is how cigarette-innate cell interactions may be mechanistically linked to a reduced vascular response to dental plaque. Further research in this area seems warranted.

Tobacco-microbe interactions

It has now been firmly established that smoking profoundly influences the composition of the subgingival biofilm^93–102; that the microbiome of smokers is more diverse, pathogen rich and commensal poor^{94,95,97–100}; that it is more difficult to eradicate key pathogens from smokers, compared to non-smokers^93,103; and that smokers are more susceptible to the re-establishment of a pathogenic subgingival biofilm than are non-smokers⁹³. The elegant study by Joshi et al, showed that smokers exhibit early colonization by, and more abundant infection with, pathogens relative to non-smokers as established in an experimental gingivitis setting⁹⁴.

While the relevance of an altered microbiome to a reduced bleeding response in smokers is not transparent, certain clues are available. Indeed, the cigarette-altered oral microbiota may have less pro-inflammatory potential than plaque in non-smokers. For example, key drivers of gingival inflammation are the lipopolysaccharide (LPS) molecules produced by Gram-negative bacteria. Variant LPS structures have differential capacities to engage the Toll-like receptors (TLRs) that drive inflammatory mediator release from epithelial and immune cells. Certainly, the overall LPS profile in smokers, as measured in saliva, is composed of lower levels of LPS with the optimal TLR-activating structure than that of non-smokers¹⁰⁴.

The best studied periodontal pathogen is P. gingivalis. Smokers are more likely to be infected with, and to harbor higher numbers of, Porphyromonas gingivalis, relative to non-smokers^105–109. In other words, the periodontium of smokers is a preferred niche for P. gingivalis. Indeed, P. gingivalis is resistant to very high doses of cigarette smoke and tobacco constituents^110–113. Yet, tobacco-pathogen interactions generally, and P. gingivalis specifically, remain poorly understood.

Recent work has established that smoke exposure exerts a profound influence on P. gingivalis physiology resulting in dramatically altered pathogen-innate immune interactions. For example, the whole blood leukocyte response to P. gingivalis, other bacteria or bacterial products, as monitored by cytokine release (IL-1β), has been shown to be suppressed in smokers compared to non-smokers¹¹⁴. In keeping with de Heens et al, P. gingivalis cells exposed to cigarette smoke extract (CSE) have been shown induce a lower pro-inflammatory response (TNF, IL-6, IL-12 p40) from monocytes and peripheral blood mononuclear cells than do unexposed bacteria¹¹¹. Further, CSE leads to differential regulation of a large number of P. gingivalis genes¹¹¹, the products of several having the potential to regulate the inflammatory response in the periodontium. For example, the production of bacterial capsule, highly inflammatory in most strains, is down-regulated in CSE-exposed P. gingivalis cells¹¹⁵. At the same time, FimA, the primary component of the major fimbriae, is upregulated. FimA protein can induce TLR hyposensitivity in immune cells as monitored by pro-inflammatory cytokine production, yet efficiently induce anti-inflammatory IL-10 release^111,115. CSE exposure can also promote mono- and dual species P. gingivalis biofilms¹¹⁰. This phenomenon is likely to promote P. gingivalis persistence. Persistence coupled with lower inflammatory potential may, therefore, contribute to a chronic, low-grade infection and, subsequently, the reduced bleeding response noted in smokers relative to non-smokers.

Animal models

We discussed the influence of tobacco or nicotine on gingival blood flow earlier in, rabbits and rodents. There are, perhaps surprisingly, only a few other studies that have employed animals to model tobacco-induced periodontal diseases directly. For example, in 1977, Kraal et al observed that a solution of tobacco smoke applied to the gingiva of dogs could suppress subsequent crevicular innate cell migration in neutrophils isolated from both healthy and inflamed sites¹¹⁶. In nicotine-exposed rats, both capillary length and height were noted to be reduced, compared to untreated controls, in the maxillary gingiva¹¹⁷. Breivik et al, 2009 reported that, in ligature-induced periodontitis in rats, nicotine administration enhanced alveolar bone loss concomitant with a reduced pro- and anti-acute inflammatory cytokine response to LPS (TNF, TGF-β, IL-10)¹¹⁸. It is unclear why anti-inflammatory cytokines, which were not influenced by pre-treatment of the non-selective nicotinic receptor antagonist, mecamylamine, should be suppressed in this model. Animal studies, therefore, reflect the reduced inflammatory burden noted in diseased adults. Further, the obvious in vivo model, where animals are exposed to direct or indirect cigarette smoke in an exposure chamber is lacking but likely to be particularly informative.

Genetic predisposition to tobacco-related periodontal diseases

With the profound negative influence of cigarette use on periodontal health well-established, it is unfortunate that some studies examining potential genetic links to disease susceptibility do not consider this key factor. Limited evidence, though, does suggest gene-tobacco interactions that may predispose to destructive periodontal diseases.

For example, Yoshihara et al¹¹⁹ reported that smoking enhances the progression of periodontal tissue destruction in elderly people with a FcγRIIIb (CD16b)-neutrophil antigen (NA) 2 polymorphism which determines neutrophil responses to IgG antibody (O.R., 3.0; 95% CI, 1.1–8.3). However, the bleeding response was not reported. CCL2, the leukocyte chemoattractant, is reduced in the GCF of smokers with periodontitis (Table 1). In a study of generalized aggressive periodontitis, the authors reported an increased disease risk for males who smoked in combination with either a particular CCL2 polymorphism (G+ genotype; O.R. 4.9) or VV polymorphism in the CCL2 receptor, CCR2, (O.R., 7.2; 95% CI, 1.3–41.1)¹²⁰. Again, the influence of such gene-tobacco interactions on specific components of the inflammatory response was not the study focus. In a study of chronic and aggressive periodontitis in the context of smoking and a variant vitamin D receptor (VDR) sequence, both the presence and the progression of disease was associated with the VDR-Taq-I TT polymorphism¹²¹. Bleeding on probing was measured, but not reported. Recently, others have reported an association between smoking, TNF-308 GA/AA genotypes and peri-implantitis¹²².

Thus, while increasing data are suggestive of genetic predisposition to destructive periodontal diseases in smokers, the influence of such tobacco-gene interactions on the vascular response to plaque remain unclear.

Conclusions

Tobacco use is a, or the, leading risk factor for chronic periodontitis. Cigarette smoking leads to a suppression of overt gingival inflammation, manifested as reduced angiogenesis and a compromised bleeding response to plaque (Figure 1), but simultaneously promotes periodontal tissue destruction relative to non-smokers. The bleeding response recovers rapidly following smoking cessation. The mechanisms underlying vascular suppression in cigarette users are yet to be fully elucidated. While there may be a genetic element to susceptibility, tobacco use profoundly influences the biological mediators that control the angiogenic and bleeding responses to bacterial challenges (Tables 1–3); promotes oral bacterial dysbiosis; compromises innate cell function; and promotes a protease-antiprotease imbalance in the gingival tissues. It may be possible that specific preventive and therapeutic approaches to the control of chronic periodontitis, and other destructive plaque-induced oral disease, may need to be tailored for smokers.

Review provisos

It is important to note that biochemical confirmation of smoking status and the level of smoke exposure, such as biofluid cotinine analysis or expired-air CO monitoring, are clearly preferable to self-report. However, the studies cited herein contain a variety of classification systems for smoking status. Additionally, while many contemporary studies delineate periodontal diseases using the 1999 Consensus Classification of Periodontal Diseases¹²³, those cited herein use a wide variety of definitions for “periodontitis”. It is not practical to note such nuances herein. The manuscript by Grudianov and Kemulariia⁵⁹ is in Russian with interpretation reliant on the English abstract. As a final proviso, because the profound negative influence of cigarette use on oral health is accepted, the exclusion of smokers from research studies is a not uncommon strategy. Therefore, smokers may be under-represented in the knowledge base.