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Periodontology 2000. 2020;83:26–39.
wileyonlinelibrary.com/journal/prd
DOI : 10.1111 /prd.1229 7
REVIEW ARTICLE
The role of inflammation and genetics in periodontal disease
Bruno G. Loos1 | Thomas E. Van Dyke2
1Department of Periodontology, Academic Centre for Dentistry Amsterdam (ACTA), University of Amsterdam and Vrije Universiteit Amsterdam, Amsterdam,
The Netherlands
2Center for Clinical and Translational Research, Forsyth Institute, Cambridge, Massachusetts, USA
Correspondence
Bruno G . Loos, De partm ent of Perio dontolog y, Academic Centre for Dentist ry Amsterdam (AC TA), Gustav Mahlerlaan 300 4, 1081 LA Amsterd am, The
Netherlands.
Email: b.g.loos@acta.nl
1 | INTRODUCTION
Patients with periodontitis show inflammatory destruction of the
supporting tissues around the teeth. Loss of connective tissue and
collagen in the gingiva is characteristic, along with loss of periodon-
tal ligament and resorption of alveolar bone. Thus the tooth roots
become exposed to the oral environment, and the root and root
cementum are colonized with a bacterial biofilm, which can calcify
to form dental calculus. The chronicity and mostly slow progression
of this disease results in tooth mobility, loss of chewing function,
esthetic disturbances and, ultimately, if left untreated, tooth exfo-
liation. Moreover, periodontal inflammation has systemic ef fects;
it can induce low grade systemic inflammation, which has negative
effects on other organs.
Traditionally, the most common forms of periodontitis have
been separated into 2 types: aggressive periodontitis and chronic
periodontitis.1 However, it has recently been acknowledged that
the scientific basis for this classification is weak and based on a vari-
able clinical presentation.2-5 In particular, in light of the complexity
of the causative factors for periodontitis that will be discussed in
this paper, the distinction between various clinical presentations
has been removed.5,6 The shortcomings of clinical diagnoses in-
cluded substantial overlap and lack of clear pathobiology-based
distinctions between the stipulated categories, diagnostic impreci-
sion, and treatment implementation difficulties. Although the 1999
classification1 provided a workable framework that has been used
extensively in both clinical practice and scientific investigations in
periodontology during the past 17 years, the shortcomings were
deemed too great for further utility,5,6 and a new classification was
introduced.7
Epidemiological research has shown that severe periodontitis
occurs in about 7%-14% of the population in western Europe and
North America, depending on the definitions used for severe peri-
odontitis, and depending on the specific study population evalu-
ated.8 -10 In populations and countries with low availability of dental
care with limited dent al health awareness, and when limited preven-
tive measures are available, the prevalence of severe periodontitis
may be 10%-15%.9 It may well be that the genetic susceptibility
factors are more pronounced in certain racial/ethnic populations;
the prevalence of severe periodontal disease in central and east sub-
Saharan Africa, and within some racial/ethnic groups in the USA,
was found to be up to 20%.8,9
2 | CHRONIC INFLAMMATORY DISEASES
Humans can suffer from a range of chronic immune-mediated inflam-
matory conditions, collectively called chronic inflammatory diseases.
These include autoimmune diseases, allergies, immune deficiencies,
and perhaps some psychiatric disorders such as depression.11 More
than 10% of people in all populations may suffer from 1 or more
chronic inflammatory diseases. For many chronic inflammatory dis-
eases, the question remains as to which factors contribute to their
onset and progression, while, on the other hand, which mechanisms
keep the immune system tolerant for self as well as for the daily chal-
lenges of trillions of antigens from bacterial, fungal, and viral micro-
organisms. In essence, uncontrolled and unresolved inflammation is
the basis of chronic inflammatory diseases. Chronic inflammatory
diseases share common causal factors, including genetic factors (ie,
pleiotropy) and immune pathways.11-16 This knowledge has helped
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LOOS and Van dYKE
in our general understanding of individual chronic inflammatory dis-
eases, including periodontal disease.1 7,18
Chronic inflammatory diseases are complex conditions because
they often involve multiple causal component s that play a role si-
multaneously, and they interact with each other, often in an unpre-
dictable way. In other words, the disease is the result of complex
interactions between genetics and environment, such as microbial
communities (biofilms) and the host response, which is hard to ex plain
by a few individual factors.14,15 ,19,20 Thus it should be noted that each
of the separate causal components vary in relative importance, which
varies in individual cases, yielding a tremendous number of combina-
tions of causal factors that impact upon how each case may develop
and progress.14,15, 19,21 Never theless, clinically, many cases of a given
chronic inflammatory disease share causal components, which of
course is helpful for determining diagnosis, prognosis, and treatment.
Complex chronic inflammatory diseases are recognized as non-
linear systems.20, 22-24 Nonlinearity in complex systems means that
cause and effect are disproportional so that various causal path-
ways may account for mechanically dif ferent effects, which var y
in individuals. For example, in one individual a causal pathway may
contribute little to the disease, while in another individual the same
pathway has a major effec t. Disease progression rate fluc tuates, or
rather can move (‘cycle’) from one state (homeostasis, ie, a stable
state with high resilience) to another (disease activity, ie, relapse)
and back again, in often unpredictable ways.20, 23 For many chronic
inflammatory diseases, it has been established that the disease can
relapse (occurrence of bursts of activity) after a period of remission
or quiescence.20, 24 Many clinicians have noted heterogeneity in the
clinical course of patients with chronic inflammatory disease, at-
testing to the nonlinearity of the condition. For patient s suffering
from rheumatoid arthritis, there are large variations reported on the
frequency and time periods of disease remission,25 and this nonlin-
earity can be related back to stability or changes in the patients’ im-
mune fitness.11, 22 Such periods of stability and disease progression
appear also to be characteristic of periodontitis.
The immune function of any individual can be equated with
immune fitness; the way the host deals with the challenges and
perturbations encountered during life, including normal inflamma-
tion-resolving mechanisms.11,14, 26 There is a wide variet y of determi-
nants (ie, causal factors) that regulate the immune system, and these
factors are both intrinsic and acquired. The intrinsic causal factors
are the inherited risk factors, ie, genetic susceptibility. Common to
chronic inflammator y diseases is the fact that they are associated
with 100s of disease-associated genetic variant s (single nucleotide
polymorphisms).1 3,15,27 Chronic inflammatory diseases are under-
stood to be polygenic, and the various chronic inflammator y dis-
eases often share particular single nucleotide polymorphisms that
are considered to play a role in immune fitness. This is the concept
of pleiotropy and demonstrates the existence of common patho-
genic pathways for different chronic inflammatory diseases based
on shared single nucleotide polymorphisms.1 2,16,27
In addition to variations in genomic sequences that have been
associated with chronic inflammatory diseases, there are epigenetic
modifications of DNA that are in par t acquired during life, but can
also be inh erited and thus ca n be intrinsic. 28-30 It has been est ablished
that aging, microbial exposure, dietary factors, systemic conditions
(eg, obesity, diabetes, osteoporosis, and depression), environmental
factors (eg, pollution), as well as modifiable risk and lifestyle factors,
such as smoking, stress, and alcohol consumption, have the capacity
to induce epigenetic changes.29 Interestingly, microRNA sequences
have also been implicated in DNA methylation abnormalities.31 The
major epigenetic modifications are DNA methylation, as well as his-
tone modification involving acet yl, methyl, and phosphate groups,
leading to molecular alterations of the normal molecular structure
of DNA, and to remodeling of chromatin. For example, acetylation
of histones alters accessibility of chromatin and allows DNA binding
proteins (transcription factors) to interact with exposed sites to tran-
scribe the gene, or vice versa, DNA accessibility for func tional tran-
scription may be inhibited. Thus epigenetic mechanisms determine
gene expression levels; epigenetic changes, especially in promoters,
enhancer sequences, and DNA sequences for long noncoding RNA,
can contribute to altered on/off mechanisms of gene transcription, or
cause genes to behave in an aberrant way. In chronic inflammatory
diseases, epigenetic mechanisms can, like single nucleotide polymor-
phisms, contribute to gene expression and func tion, altered immune
function, and the magnitude of the inflammatory responses.29,30,32,33
Interestingly, for epigenetic patterns, the DNA modifications can
be shared by various chronic inflammatory diseases, especially via
microinhibitory RNA sequences that may play an important role in
intracellular regulation and feedback loops.33,34
Further, the microbiomes that are in and on all the various eco-
logic niches of the body are of importance for determining normal
immune function and may induce an aberrant host response in var-
ious chronic inflammatory diseases. For example, the mucous sur-
faces of the complete gastrointestinal tract, starting with the mouth,
are lined with biofilm, normally without eliciting any pathological
immune reaction.35 A series of studies36 -40 support an important
role for local (peripheral) Treg cells in the maintenance of mucosal
or oral tolerance that avoids damaging immune reactions to microor-
ganisms colonizing the mucosal sur faces of the human body. In fact,
earlier research indicated that Treg cells were induced by microbial-
produced metabolites, in particular short-chain fatty acids, thus sug-
gesting that the intestinal biofilm induced its own tolerance.3 6, 39,4 0
More recent research showed that there is a reciprocal and func-
tional metabolite-driven loop, where the gut bacteria control the
local Treg cells numbers via their metabolites, and the local Treg cells
alter the gut bacteria composition and determine which bacteria are
tolerated.3 7,3 8 Interestingly, the importance of Treg cells for immune
tolerance and biofilm control has recently been recognized for peri-
odontitis.41 This mucosal or oral tolerance is critically important in
controlling destructive immune reactions to the commensal flora of
the gastrointestinal tract.
Lifest yle factors constitute an important group of risk factors
that can have a significant impact on immune function and immune
fitness. For many chronic inflammatory diseases, lifestyle factors
such as smoking, poor diet, and nutrition are considered important
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LOOS and Van dYKE
extrinsic and shared causal factors.19,42-44 Smoking is one of the
major sources of toxic chemical exposure to humans (there are at
least 4,500 components in cigarette smoke) and can induce epi-
genetic alterations, which in turn leads to heightened inflamma-
tion.45,46 Also, in general, oxidative stress is increased, with an
excess of free oxygen radicals leading to cell damage. Apoptosis,
programmed cell death, is pathologically both decreased and in-
creased in smokers in different cellular compartments. Smoking
impacts the immune-inflammatory system and has consequences
for inflammatory cellular activation. The nuclear factor-kappa B
pathway, which when activated results in the production of proin-
flammatory mediators, has autocrine, paracrine, and endocrine
consequences. In smokers, low grade systemic inflammation is
often present and macrophage elastase (matrix metalloprotein-
ase-12) is elevated. The immune system in general shows signs of
dysfunction, with an increased tendency to produce autoantibod-
ies, and reduced polymorphonuclear neutrophil chemotaxis and
phagocytic capabilities.43 Also, smoking has an impact on the mi-
crobiome of the gastrointestinal tract; smokers with inflammatory
bowel disease have a dysbiotic intestinal microbiome.45 However,
the mechanisms by which smoking may induce a dysbiotic gut
microbiota are not yet clear. There are indications that there is
a vicious cycle, in that the inflammatory process of inflammatory
bowel diseases may induce the dysbiosis, but the dysbiosis itself
perpetuates and worsens inflammatory reactions in Crohn's dis-
ease and ulcerative colitis. Interestingly, there have been some
reports that indicate that smoking may be protective for ulcer-
ative colitis; however, there seems to be little data to explain these
observations.45
Similar to smoking, the central aspect of poor diet and nutrition
is the induction or increase in low grade systemic inflammation. The
result of the high intake of animal fats (saturated fatty acids), red
meat, salt, refined carbohydrates, fried foods, and low intake of fruit,
vegetables, fiber, vitamin C, and other antioxidants, and shortage of
vitamin D, results in increased inflammation. This can be through
direct actions on the immune system or through epigenetic modi-
fications that lead to nuclear factor-kappa B or activator protein-1
activation.19 Conversely, exercise, and low-calorie diets (fruit, veg-
etables, and fish), perhaps probiotics and prebiotics, can act on the
nuclear receptors and enzymes that upregulate oxidative metabo-
lism and reduce the production of proinflammatory molecules. For
example, the enzymatic conversion of polyunsaturated fatty acids
from fish is essential for the generation of specialized proresolving
mediators such as lipoxins, resolvins, protectins, and maresins.47, 48
Also, the effects of nutrition on the commensal gut microbiota
are becoming increasingly understood as important.42,4 4,49 For ex-
ample, good diet may help to maintain eubiosis, while poor dietary
habits tend to induce a gut dysbiosis. The interaction of the gut mi-
crobiome and systemic health has now been established.50 A dysbi-
otic intestinal microbiome (with a reduced diversity) through poor
diet leads to inflammation of the mucosal lining, and in turn, inflam-
mation (“leaky gut ”) and dysregulated mucosal immunity (altered
Treg cells function). Together, this will induce or further propagate
a poor quality gut microbiome, with reduced production of useful
metabolites.19
Other aspects within the cluster of lifestyle factors are psy-
chological stress and other types of psychosocial elements.11
Psychosocial aspec ts, including cognitive behavior (ie, coping with
stress), have been identified as being part of the shared, causal
factors deterministic to immune fitness. Over the years there has
been consistent evidence for psychosocial fac tors having the capa-
bility to suppress immune functions at various levels.51-54 In fact,
it has been established that there is a bidirec tional loop between
the immune and nervous systems.52,55,56 Basically, neurons produce
various immune mediators and cy tokines that have effects on im-
mune cells, and vice versa; immune cells produce neuropeptides
and neurotransmitters, as well as express receptors for neurological
mediators. Interestingly, susceptibility for depression is increased in
situations of chronic inflammatory diseases.52,55,56 Between the var-
ious chronic diseases (morbidities), bi- or triple comorbidity is not un-
common.57 This is not surprising as the various chronic inflammatory
diseases share genetic and epigenetic risk factors (pleiotropy is out-
lined above), immune pathways, and have shared lifest yle and psy-
chosocial risk factors.11 For example, heart failure is associated with
various comorbidities,57 and there is evidence of an association of
psoriasis with rheumatoid arthritis, depression, inflammator y bowel
disease, and cardiovascular disease.58 Also, a met a-analysis showed
a 48% increased risk of incident cardiovascular disease in patients
with rheumatoid arthritis.59 Recently, the importance of the risk for
cardiovascular comorbidity in patients with rheumatoid arthritis has
been further stressed.60 In fact, one chronic inflammatory disease
can negatively affect the immune fitness for another disorder; for
example, this has been suggested for rheumatoid arthritis. The sys-
temic inflammation in rheumatoid ar thritis, as evidenced by elevated
levels of C-reactive protein and the consistent elevated erythrocyte
sedimentation rate, may contribute to a heightened inflammatory
burden, which is atherogenic.21, 61 Interestingly, a dysbiotic subgin-
gival microbiome in periodontitis may affec t the pathobiology and
immune reactions of several autoimmune diseases such as rheuma-
toid arthritis, Sjogren's syndrome, systemic lupus erythematosus,
and Crohn's disease.62 For rheumatoid arthritis, it is speculated that
an altered oral microbiome with the outgrowth of Porphyromonas
gingivalis and its enz ymatic machinery for peptide citrullination may
contribute to the generation of autoimmune anti-citrullinated pep-
tide antibodies. Taken together, comorbidities including periodon-
titis and/or dysbiotic microbiomes can have a major impact across
different medical conditions.
3 | PERIODONTITIS IS A COMPLEX
CHRONIC INFLAMMATORY DISEASE
Periodontitis is considered a chronic inflammatory disease, and can
be defined as a multicausal, complex, chronic inflammatory disor-
der.17,63-65 Periodontitis is a chronic inflammatory disease exhibit-
ing immune dysregulation (aberrant immune function) at its base,
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LOOS and Van dYKE
involving multiple causal components, which interplay simultane-
ously (as outlined above for other chronic inflammatory diseases).11
In Figure 1 we have provided a schematic drawing illustrating that
the blueprint of the host response could determine immune fitness,
which can either have normal tolerance and homeostasis with the
dental biofilm, or an aberrant host response leading to an imbalance
with the dental biofilm resulting in inflammation-driven destruc-
tion of periodontal tissues, ie, periodontitis. In 20 03, experimen-
tal work in rabbits overexpressing 15-lipoxygenase types I and II
demonstrated that inflammation is the driving force behind neutro-
phil-mediated tissue degradation and alveolar bone loss in P. gingi‐
valis-induced periodontitis.66 Controlling excess inflammation with
lipoxins showed dramatically reduced to no periodontal bone loss
despite the ligature and P. gingivalis challenge to the periodontal tis-
sues and bone.
In analog y to other chronic inflammatory diseases, we would ex-
pect that periodontitis behaves in a nonlinear fashion.17 Although
we have limited evidence for periodontitis, we propose that the
causes and effects may be disproportional to each other and that
the disease progression rate fluc tuates, or rather, can move from a
homeostatic state or resolving state to an active st ate and back.67-
69 The nonlinearity in periodontitis is seen in daily practice, where
the disease often reveals its heterogeneity in the clinical course be-
tween any 2 patients. In periodontitis, it has long been established
that the disease can occur in bursts of activity followed by periods of
quiescence or stability,70-72 although the bout s of disease progres-
sion may occur in micromillimeters, which are not easily measured
clinically. Also, the periodontitis disease state and disturbance of the
homeostasis may change throughout life, where aging, epigenetic
modifications, and comorbidities alter immune function.7 3-76 These
factors are further outlined below.
Interestingly, we understand now that the aberrant host re-
sponse leading to periodontal inflammation can induce a microbial
dysbiosis in the submarginal and subgingival regions. The normal
submarginal and subgingival biofilm is diverse and may contain over
1,000 different species. With modern DNA techniques, the resident
microbiome has been studied and in health an abundance of gram-
positive species can be found. Also, in health, traces of the well-
known periodontal pathogens are present.77,7 8 In this way, they are
termed symbionts, natural members of the submarginal and subgin-
gival tooth biofilm. Normally their expansion is outcompeted by spe-
cies that are not dependent on proteinaceous and blood metabolite
materials such as hemin (an iron source), which are growth require-
ments for anaerobic gram-negative species. However, when the
host responds to the normal resident biofilm as it accumulates, then
this in turn triggers inflammation, altering the microbial ecology and
dysbiosis. The pathologic and dysfunctional immune response can
actually induce an “ecological catastrophe”, a self-feeding cycle of
escalating dysbiosis.65,79-82 Specifically, the inflammatory reactions
result in increased gingival crevicular fluid containing collagen break-
down products and the full range of host immune factors, includ-
ing immunoglobulins, complement, serum proteins, cytokines, and
chemokines, as well as increased amounts of immune cell remnants
(after apoptosis of, for example, polymorphonuclear leukocytes),
FIGURE 1 Panel (A) shows how
immune fitness of the host determines the
host response to the dental biofilm, which
can either be symbiosis and homeostasis,
or an aberrant host response leading to
an imbalance resulting in inflammation-
driven destruction of periodontal tissues,
ie, periodontitis. Panel (B) summarizes the
complexity of periodontitis
30
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LOOS and Van dYKE
cells from desquamated pocket epithelium and, importantly, abun-
dant collagen peptides from the degradation of gingival collagen
by matrix metalloproteineases. Also, because of increased capillary
permeability, serum exudates are found in inflamed pockets provid-
ing essential nutrients. Moreover, in inflammatory conditions, an
anoxic environment develops, an additional factor driving prolifer-
ation of anaerobic bacteria.77,78 Seminal experiments in rabbits have
proven the above reasoning; experimentally induced periodontitis
was associated with increased amounts of gram-negative, anaerobic,
and proteolytic bacteria, while treatment of the periodontal inflam-
matory lesions with the potent proresolving mediator resolvin E1
resulted in healing of the periodontal lesions and complete regres-
sion of these bacteria.79 In similar experimental settings reported by
Lee et al in 2016,83 when analyzing the microbiome of periodontitis
lesions af ter treatment with proresolving mediators it was shown
that resolution of inflammation reversed the periodontal dysbiosis.
Thus the altered ecological conditions transform some symbionts to
pathobionts, ie, commensals that under conditions of disrupted ho-
meostasis have the potential to cause disease.65,78,79 The outgrowth
of the pathobionts (the most well-known example is P. gingivalis) can
further induce and worsen the host inflammatory responses.77, 78
Indeed, a transcriptomic analysis of subgingival microbiomes in
periodontitis showed elevated expression of genes for proteolytic
enzymes and for iron acquisition, as well as for lipopolysaccharide
synthesis, highly suggestive that in periodontal inflammation many
asaccharolytic, anaerobic, and gram-negative bacteria exploit the
ecological changes for their nutritional and expansive needs.78,84
The bacterial biomass of human periodontitis-associated biofilms
increases with increasing periodontal inflammation. Thus the selec-
tive outgrowth of these inflammatory pathobionts can perpetuate
periodontal inflammation resulting in a vicious cycle for disease
progression, where the dysbiosis and inflammation reinforce each
other.63-65,77-79,83,85-89 In Figure 2, we have schematically illustrated
the vicious cycle in periodontitis.
The paradigm, as outlined above, is also current for inflammatory
bowel diseases, where dysregulated immune responses can induce
intestinal dysbiosis that perpetuates the inflammation and disease,
with reduction in microbial diversity and transformation of sym-
bionts into pathobionts.90-92 The dysbiosis in the gut is associated
with changes in gut permeability (“leaky gut”), leading to invasion of
bacteria into the mucosal lining. We speculate that a similar phenom-
enon occurs in the gingiva; bacterial invasion and persistence have
been described as critical events in the pathogenesis of periodonti-
tis (“leaky gums”).93-95 In particular, the pathobionts P. gingivalis and
Aggregatibacter actinomycetemcomitans have been shown to possess
these characteristics.
4 | THE ABERRANT HOST RESPONSE IN
PERIODONTITIS
The literature in the periodontal field indicates that the aber-
rant immune response is in fact a hyperactive immune response;
this is the central component of the pathobiolog y in periodonti-
tis.63-65,78,85-89,96-100 Thus the excessive inflammator y reactions lead
to the dysbiotic changes discussed above, concomitant with peri-
odontal tissue and alveolar bone breakdown. Polymorphonuclear
neutrophils are the most likely cells to contribute substantially to
destruction of periodontal tissues, and their hyper-functionality has
been found in periodontitis, in particular in aggressive periodonti-
tis.64,85-89,101-103 Neutrophils release elevated levels of tissue-de-
structive enzymes and substances such as reactive oxygen species,
lysozyme, collagenases, and elastase. In addition, they produce pro-
inflammatory cytokines and chemokines that add to the continued
chronicity of the inflammation; these immune mediators will leak
into the circulation and add to the systemic proinflammatory actions
of periodontitis.10 0 For example, it was shown in ex vivo experiments
that plasma samples from periodontitis patients were effective in
enhancing superoxide production by neutrophils from healthy do-
nors104; in this way a t ypical feedback loop or vicious cycle is pre-
sent, where hyperactive neutrophils are also primed in an endocrine
manner. Hyperactive neutrophils can also contribute to bone de-
struction through the stimulation of osteoclastic activity, in particu-
lar when they are in close proximity to the alveolar bone; most likely,
neutrophils participate in the chemotactic recruitment of Th17 cells,
which mediate and direct the activity of osteoclasts.105
Periodontitis patients showing regular relapses and nonrespon-
siveness to therapy are often characterized as having refractory
periodontitis. Their neutrophils have been shown to have increased
potential to produce oxygen radicals. In addition, for these refrac-
tory patients, increased phagocytosis by neutrophils was observed.
This might be associated with higher intrinsic intracellular activity of
the nicotinamide adenine dinucleotide phosphate oxidase system.98
A genetic polymorphism was identified in the nicotinamide adenine
dinucleotide phosphate oxidase gene106; however, it is not known
whether this genetic mutation is coupled to the increased produc-
tion of the enzyme.
Interestingly, in periodontitis, increased influx of B-cells and
plasma cells is observed.107,108 The plasma cell-dominated lesions,
which presumably produce antibodies, can be seen as a way for the
FIGURE 2 The vicious c ycle of the “ecological cat astrophe”
driven by an aberrant host response in periodontitis
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LOOS and Van dYKE
immune system to attempt to neutralize the pathobionts that have
proliferated in the dental biofilm. However, we can also hypothe-
size that the polyclonal B-cell activation in the periodontal lesions
represents a frustrated cellular immune system that has not found
an effec tive way to reduce B cell/plasma cell activity and therefore
shows overreactive antibody production.
Notably, various ex vivo whole blood cell culture systems have
been employed to investigate immune reactivity to lipopolysaccha-
ride preparations in periodontitis.109,110 Most likely the cells respon-
sive to stimuli in these systems represent the cells of the monocyte/
macrophage lineage, which are also antigen-presenting cells.111,112
Collectively, these studies have shown higher reactivity in whole
blood cell cultures from patients with aggressive or chronic peri-
odontitis, as evidenced by elevated levels of key cytokines produced
in the cultures. This was also suggested when employing isolated
monocytes from peripheral blood samples.113 In some studies,
monocytic hyperresponsiveness was partly attenuated after peri-
odontal therapy,11 0,112 while others did not observe the reduction.114
The various studies cited here confirm possible intrinsic aberrant re-
activity with the ultimate result that heightened inflammatory reac-
tions feed the proteolytic species in the dental biofilm.
Although the majority of the literature indicates that neutro-
phils are hyperreactive in periodontitis, in particular in severe, early
onset forms, at the same time investigators have observed reduced
neutrophil functions.100 In particular, reduced chemotaxis and Ca2+
uptake seems to be a comorbidity associated with primed neutro-
phils.96,100 -102 Further, aberrations in phagocytosis, respiratory
burst, and intracellular killing have been reported.115 Rare mutations
leading to rare forms of periodontitis (Papillon-Lefèvre syndrome,
leukocyte adhesion defect-syndrome, and other congenital diseases
with rampant periodontitis) can be regarded as forms of periodon-
tal inflammation where specific neutrophil functions are altered or
absent.100 Another aspect of the immune system that can be less
functional in periodontitis is the decreased production of immuno-
globulins.101,116 This is especially seen in smokers, further explaining
how smoking reduces immune fitness.
How hypoac tivity of parts of the immune system, such as leuko-
cyte adhesion deficiency, actually results in excessive inflammation
with concomitant conversion of a normal microbiome to a dysbiotic
microbiota, is becoming clearer. Moustopoulous and co-workers
have demonstrated that an aberrant host response is overcompen-
sated by other parts of the immune system.117 For example, where
neutrophils in the leukocyte adhesion deficiency syndrome are un-
able to exit the circulation to phagocytize bacteria, the system is
compensated by Th17 cells and interleukin-17 production.
5 | EVIDENCE FOR FAILURE OF
RESOLUTION PATHWAYS
Inflamma tory respons es are protective b iological proc esses designed
to eliminate the harmful stimuli and promote return of the affected
tissue to its preinflammatory state and function (homeostasis).
There are a series of anti-inflammator y mediators, including tissue
inhibitors of metalloproteinases, which are important tissue-de-
stroying enzymes released by macrophages during the inflammator y
response. There is also an inhibitor set of mediators that results in
active resolution of inflammation.11 8 In the past, it was believed that
inflammatory resolution was a passive event that resulted from the
dilution of chemokine gradients over time, thus reducing the chemo-
taxis of le ukocytes to the site of injury, and pr oduction of anti-inf lam-
matory mediators. However, evidence from studies performed over
the last few decades has shown that it is a carefully orchestrated
active process.119 The identific ation of specific proresolving path-
ways and lipid mediators introduced a paradigm shift and opened a
new window to underst anding the resolution of inflammation.119 It is
now widely appreciated that resolution of inflammation represents a
sequence of overlapping events during which proinflammatory me-
diators induce the generation of specialized proresolving mediators
that stimulate their receptor targets.26 The specialized proresolving
mediators are lipoxins, resolvins, protectins, and maresins, which
originate from the enzymatic conversion of omega-3 polyunsatu-
rated fat ty acids in the human diet. In other words, the signals that
regulate the resolution of acute inflammatory responses are tightly
interrelated with the mediators which initiate these responses. Thus
the peak of an acute inflammatory response is considered the begin-
ning of resolution.120
Resolution of inflammation is a well-orchestrated active pro-
cess mediated by a variety of specialized proresolving mediators.
Resolvin E1 is biosynthesized from eicosapentaenoic acid and selec-
tively interacts with specific receptors to inhibit further leukocyte
infiltration and cytokine/chemokine generation, to induce the apop-
tosis of neutrophils and their removal by macrophages to restore
tissue homeostasis. In periodontitis, as well as in other inflamma-
tory conditions, inflammation fails to resolve and results in chronic
pathology. Recent studies have detected lower levels of specialized
proresolving mediators in the stimulated whole blood of localized
aggressive periodontitis patients suggesting a defect, which might
contribute to the failure of resolution.121 A growing body of in vitro
and in vivo evidence point s to the effects of resolvin E1 and other
specialized proresolving mediators on different cell types in regu-
lating the resolution of periodontal inflammation. To date, no study
has addressed the issue of the tissue expression levels of specialized
proresolving mediators in gingival biopsies retrieved from chronic
gingivitis patients, as opposed to periodontitis patients.119 With this
important piece of information lacking, it is as yet unknown whether
a robust engagement of the resolution pathways can efficiently pre-
vent the conversion of gingivitis to periodontitis.
To compensate for rapid metabolism or dilution of proresolv-
ing lipid mediators, specialized and stable proresolving mediator
analogs have been constructed as mimetics of endogenous resolu-
tion, offering new therapeutic approaches. In addition, incorpora-
tion of specialized proresolving mediators in microparticles and the
employment of nano-proresolving medicines have provided a new
possibility for local deliver y at the site of inflammation.122 Although
promising results have been obtained in animal studies, the efficacy
32
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LOOS and Van dYKE
of this experimental data has yet to be established in human clinical
trials. Human studies involving omega-3 fatty acid supplementation
as a source of polyunsaturated fatty acid and low-dose aspirin as
adjunct to periodontal treatment provide promising results and indi-
cate a synergistic effect of these agents in periodontal treatment.123
Up to now there have been no long-term randomized clinical trials
investigating the clinical benefits of omega-3 fatty acids vs other
broadly used pharmacological agents, such as antibiotics, as an ad-
junct to periodontal treatment in humans. Furthermore, large scale
in vivo and ex vivo studies evaluating the effec ts of treatment with
specialized proresolving mediators in individuals with periodontitis
may shed more light on the complex molecular mechanisms involved
in the resolution of periodontal inflammation. Further studies are
warranted to elucidate if resolvin E1, alone or in combination with
other specialized proresolving mediators, is effective for elimination
of periodontal inflammation and regeneration of lost tissues in hu-
mans, as was seen in various animal models.79, 83
6 | WHICH FACTORS CO‐CONTRIBUTE TO
DYSREGULATION OF IMMUNE FITNESS IN
PERIODONTITIS?
As explained above for other chronic inflammatory diseases, mul-
tiple factors can contribute to the blueprint of immune fitness at a
certain time/moment, and the dysregulation of immune fitness in
periodontitis occurs by the total of all risk fac tors and their interplay.
The onset of periodont al inflammation is triggered by the microbial
communities in the dental biofilm; without microorganisms there is
little or no gingival inflammation. Above, we have already concluded
that the inflammatory reactions in the periodontal tissues can derail
from normal and tolerant to destructive. At that point, the change
in ecosystem allows the outgrowth of pathobionts, which subse-
quently, in a vicious cycle, can worsen the inflammatory reactions
and allow the disease to become chronic.
We group the causal risk factors for periodontitis into the fol-
lowing main clusters: (a) the subgingival bacterial biofilm on both the
tooth root surface and on the epithelial lining; (b) genetic risk fac-
tors and epigenetic modifications; (c) lifest yle-related risk factors; (d)
systemic diseases; and (e) miscellaneous factors. The 5 main causal
clusters for periodontitis can be brought together into a pie chart
model (Figure 3) (based on17 and modified from124-126).
In Figure 3 we present a generic multi-causalit y model for peri-
odontitis, where each of the 5 causal components have an equal
contribution. However, we should emphasize, that for each individ-
ual periodontitis case, the relative contribution (ie, size of piece of
pie) of each of the 5 clusters of causal factors varies and needs to
be estimated based on all available clinical dat a. At this point clin-
ical judgement is required. Nevertheless, in general, older patients
with periodontitis are considered to have a major contribution from
expansion of pathobionts, possibly induced by increasing inflamma-
tion with age, unfavorable lifest yle factors, and of ten concomitant
with another chronic inflammator y disease. On the other hand,
periodontitis in younger patient s, for example, in those suffering
from early onset periodontitis, can be linked to a greater ex tent to
genetic factors.17
Below we summarize knowledge on the genetic risk factors for
periodontitis. Risk factors from the clusters “lifestyle” (such as smok-
ing, stress, and diet), “systemic diseases” (such as diabetes, rheuma-
toid arthritis, and other autoimmune diseases), and “miscellaneous”
(such as dentition and tooth-related factors, but also stochastic fac-
tors) are not reviewed here, but have been reviewed elsewhere.127
At the onset , we want to stress that the identification of the myriad
of risk factors playing a role in the whole pathobiology of periodon-
titis is hampered by the fact that it is so difficult to identify and study
one factor among the others simultaneously playing a role. Each
of the factors individually, including the genetic factors discussed
below, is not a “black and white” determinant; we stress again that
the mono-causality concept is obsolete and that the disease is com-
plex and nonlinear.
The genetic blueprint of an individual is very dependent on the
type and function of genomic variants in multiple genetic loci: (a)
multiple genes play a role (polygenic) (gene-gene interactions); (b)
not all disease cases have the same genetic risk variants; and (c) mul-
tiple genetic variations are present at the same time. All these ge-
netic variations potentially modify the host response, which is also
not at a constant level. We discussed earlier that the host response
FIGURE 3 A generic multi-causality model for periodontitis,
where 5 clusters of causal (risk) factors are playing a role
simultaneously, (epi)genetic factors (light blue), lifestyle factors
(orange), comorbidities (systemic diseases) (gray), microbial
communities, ie, dental biofilms (yellow) and other factors (tooth
and dention related and stochasticity) (dark blue). Notably, for each
individual periodontitis patient, the relative contribution of the 5
clusters of causal factors varies and needs to be estimated, and
as such, theoretically, for each patient, a unique pie chart can be
created. Adapted from17 based on124-126
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33
LOOS and Van dYKE
is trying to maintain homeostasis with its environment, and can
cycle between remission or relapse states where the genetic varia-
tions may cause modifications in the immune system, while immune
fitness is also affected by lifestyle factors and certain systemic
diseases. Furthermore, epigenetic changes of DNA and mutations
during a lifetime may fur ther modify an individual's susceptibility to
periodontitis.
On the basi s of epidemiologi cal studies i n twins and in fa mily stud-
ies with a high rate of early onset periodontitis, it can be concluded
that in younger patients the genetic contribution may be as large as
50% to the total sum of causal factors, while in older patients the
genetic contribution to the total of all causes is at most 25%.17, 12 8-1 31
Similar to other complex chronic diseases, it is important to realize
that a multitude of genetic variations (probably >100) are involved
and therefore the disease is called polygenic.132 Currently, for peri-
odontitis, the genetic factors associated with or actually contribut-
ing to the pathogenesis have only been identified to a limited extent
and are often poorly validated. Research into specific genetic poly-
morphisms is hampered by the fact that genetic variants, which are
associated with periodontitis in, for example, Caucasians, may not be
associated with disease in other ethnic populations such as Asians,
Brazilians, and Africans.133 Although a considerable overlap of ge-
netic variants (often single nucleotide polymorphisms) between the
different races and ethnicities is expected,134 it is important to real-
ize that there might also be population-specific risk gene variants.132
The risk genes or genomic noncoding regions (loci) for periodon-
titis, with the variants or single nucleotide polymorphisms within
them, have been identified either by the candidate gene approach or
by genome-wide association studies.18 Many candidate gene stud-
ies have been performed in periodontitis yielding varied and often
contradictory results. Because of the absence of sufficient statisti-
cal power, and also because of the absence of correction for multi-
ple testing, false positive results are a common problem.17,18,132,133
Recently, we have seen the sharing of data and patient samples be-
tween research groups to further explore and to discover new can-
didate genes, or to extend the results of genome-wide associated
studies.135,136 Other issues also exist. For example, earlier studies
often included just 1 or a few candidate single nucleotide polymor-
phisms within the genetic locus of interest, but today, confirming
whether a given single nucleotide polymorphism has an association
with periodontitis requires exploring the complete genetic locus in-
cluding upstream and downstream regulatory regions and/or neigh-
boring loci; and requires a robust mechanistic explanation on the
functional consequences of a given genetic variant. Furthermore,
the phenotype classification of periodontitis and control subjects
has not been consistent across the various studies. Also, many stud-
ies have not taken into account lifestyle factors that could play a role
in the periodontitis phenotype, or the presence of comorbidities.
Currently, variants in at least 65 genes have been suggested as
being associated with periodontitis. However, the number of genetic
variants proposed to be associated with periodontitis is very depen-
dent on the applied criteria in the original discovery studies and in
systematic reviews. For systematic reviews, questions such as what
is considered a sufficient number of cases and controls that were in-
cluded in the original studies, what were the phenotypic definitions,
and whether the investigators have also looked into replication/val-
idation of single or pilot studies, are important. Results of genome-
wide association studies are often more reliable as they include 100s
or up to 1000s of periodontitis patients, apply strict significance
levels to avoid false positive results because of multiple testing,
and mostly contain a validation or replication cohort .18,133,135,137-139
The genes having variant alleles (minor alleles) or haplotypes asso-
ciated with both aggressive and chronic periodontitis, or only with
aggressive periodontitis, or only with chronic periodontitis, or just
with unspecified periodontitis, are summarized in various current
reviews.3,17,129,140,141
Interestingly, reports have found pleiotropy between peri-
odontitis and cardiovascular diseases142; the same genetic variants
have been observed as being associated with both cardiovascular
diseases and periodontitis.139,142-145 This is an intriguing finding be-
cause a common genetic background for coronary artery disease
and periodontitis could be interpreted as similarly aberrant host
responses during inflammatory processes, irrespective of where
they take place. Notably, it is now well accepted that atherosclerotic
plaques can be regarded as inflammatory lesions, and that athero-
sclerotic cardiovascular disease can be regarded as an inflammatory
disease.146 -150 Similar host immune reactions and pathobiology re-
sponses could be hypothesized to be directed to the bacteria and
bacterial antigens that are transmigrated from the periodontium,
intestines, and other mucosal surfaces into macrophages/foam cells
residing in atherosclerotic plaques.151
One of the first and best replicated genetic loci associated with
coronar y arter y disease is the CD KN2B‐AS1 locus.17,139,142,143 The
CDKN2 B‐AS1 locus is a regulatory region and does not contain a pro-
tein-encoding gene. It is a long noncoding antisense RNA also known
as ANRIL. Importantly, it appears to be from a highly pleiotropic ge-
netic region (chromosome 9, p21.3), as it is also associated with type
2 diabetes, ischemic stroke, and Alzheimer's disease. Since 2009, it
has been reported that certain genetic variations in C DK N2B‐A S1
are also consistently associated with periodontitis.17,142,143,145,1 52 Its
function and role has recently been further investigated and found
to be related to regulation of gene expression.153 Interestingly, a pilot
study identified that one of the genetic variants in the C DK N2B ‐A S1
locus is associated with the extent of elevated levels of C-reactive
protein in periodontitis, but det ails of the possible pathway have not
yet been established.154
Further, a conserved noncoding element within CAMTA1 up-
stream of VAMP3, first identified as a genetic susceptibility locus
for coronary artery disease, was also found to be associated with
periodontitis.144,155 Interestingly, previously, a genome-wide associ-
ation study suggested that the VAMP3 locus was associated with a
higher probability of subgingival overgrowth of periodontal patho-
gens.156 Another coronary artery disease risk locus PLG was also
found to be associated with periodontitis. There is now evidence for
PLG as a shared genetic risk factor of coronary artery disease and
periodontitis.144 Plasminogen is converted into plasmin, which can
34
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LOOS and Van dYKE
dissolve the fibrinogen fibers that entangle the blood cells in a blood
clot, in a process called fibrinolysis. The plasminogen-plasmin axis
has an important function in tissue degradation and control of the
blood coagulation system. Interestingly, bacteria (including P. gingi‐
valis) can convert plasminogen to plasmin, and this complex is highly
proteolytic and can possibly inactivate plasmin inhibitors, causing
uncontrolled plasmin activity. Yet another shared risk locus for cor-
onary artery diseases and periodontitis has been identified. Based
on a genome-wide association meta-analysis of individuals of north
European ancestr y, it is a haplotype block at the VAMP8 locus.14 3
Despite the strong significance of the shared genetic variants
in the VAMP3 and VAMP8 loci, it is not clear whether they are the
causative variants and what the functional consequences are. The
VAMP3 and VAMP8 variants encode for vesicle-associated mem-
brane proteins and are simultaneously expressed in various cell
types, including mast cells, adipocy tes, and in the secretory gran-
ules of platelets. These vesicle-associated membrane proteins may
play a role in membrane trafficking and the release of inflammatory
mediators from platelets during coagulation, pathogen recognition,
aggregation, and wound healing.143 Adipocytes may also be part of
biological pathways that connect glucose and fatty acid metabolism
steps with immune responses via the vesicle-associated membrane
proteins.143,144,155
Collectively, these shared genetic factors suggest mechanistic
links or immunologic commonalities between coronary artery dis-
ease, periodontitis, diabetes, metabolic syndrome, obesity, and in-
flammation. The impairment of the regulatory pathways by genetic
factors may be a common pathogenic denominator of at least coro-
nary artery disease and periodontitis.142,143,153,155 We hypothesize
that aberrant inflammatory reactivity, determined in part by genetic
variants in the loci CDKN2B‐AS1 (ANRIL), PLG, CAMTA1/VAMP3, and
VAMP8, could explain in part the epidemiological link between peri-
odontitis and cardiovascular diseases.157-1 61 Thus the shared genes
suggest that periodontitis per se is not actually causally related to
atherosclerosis, but rather both conditions are sequelae of similar
(the same?) aberrant inflammatory pathways, which contribute to
the pathogenesis of both periodontitis and cardiovascular disease.
One other important aspect needs to be noted. The human ge-
nome, and genetic and epigenetic variants or mutations, can also
contribute to microbial colonization and infection patterns (the
gene-environmental axis).162 From several genetically modified
mouse models, we have observed that when apparent key genes
are silenced or knocked out, a dysfunctional immune system is
present, and with the ensuing inflammation and periodontal tissue
destruction, dental biofilm can proliferate and increase markedly in
biomass.163 It is important to note that the mouse gut microbiome
can be influenced by genetic modifications.164 Spor and colleagues
concluded that the overall architecture of host genetics impacts the
diversit y of the microbiome of the gut.165 The concept of infectoge-
nomics in the periodontal field was introduced in 2009 and several
papers have found genetic variants to be associated with the level
of some bacterial species.156,16 6-169 The latest review on this topic
lists all gene-bacterial interactions extensively; however, the results
need to interpreted with care, as already explained above, because
false positive associations can be present as a result of a lack of sta-
tistical power.167 Future studies should show statistical adjustment,
to minimize for the false positives that are common in genetic as well
as microbiome studies.
Finally, an additional important feature playing a role in modify-
ing the genetic blueprint of host responses to the dental biofilm is
epigenetic modification of coding and noncoding DNA.74,1 70-174 This
emerging field will likely yield new information in relation to the sus-
ceptibility to periodontitis and subsequent persisting inflammatory
reactions in periodontitis.171 Several studies suggest that smoking,
inflammatory processes in the periodont al tissues, and the micro-
biome compositions adjacent to the sulcular/pocket epithelial cells,
may induce aberrant epigenetic modifications of genomic DNA with
functional consequences.170,171,175,176 For example, based on gingi-
val biopsies from chronic periodontitis patients, hyper-methylation
and hypo-methylation of the promoter regions of the genes encod-
ing tumor necrosis factor-alpha and interferon-gamma, respectively,
were inversely associated with their expression.177,178 Generally,
hyper-methylation in gene-promoter regions seems to correlate with
gene silencing, while hypo-methylation is associated with increased
gene expression.173 Similarly, in a pilot study in 15 patients with ag-
gressive periodontitis and 10 controls, the methylation patterns for
22 inflammatory candidate genes in gingival biopsies were quanti-
fied,179 and reduced methylation was found for the genes CCL25 and
IL17C in periodontitis compared with control gingival biopsies. This
hypo-methylation pat tern was suggested as being associated with
an increase of the expression of these genes having a pro-inflam-
matory effect. In earlier studies, Barros and Offenbacher obser ved
that the periodontal tissues are epigenetically modified in particu-
lar at the biofilm-gingiva interface around the teeth.171 The authors
suggest that epigenetics may be partly responsible for the cycling
of chronic periodontal lesions from hyper-inflammation to hypo-in-
flammation and resolution, and alterations in the epigenome can re-
veal the molecular basis for certain risk exposures.171 Interestingly,
variance or major differences in subgingival microbiome from one
individual to the next may result in differential epigenetic changes
on various genes in the exposed tissues.171 The clinically important
suggestion is that epigenetic modification of periodontal tissues
may explain why periodontal disease preferentially persists at one
specific periodontal site relative to another and why these local tis-
sues will not fully repair or regenerate, thus persisting as a long-term
clinical management problem.171 The interesting clinical insight from
these studies is that, potentially, excisional periodontal surgery can
eliminate the epigenetically modified tissues yielding “normal” peri-
odontal tissues after healing, presumably able to maintain homeo-
stasis with the dental biofilm.
Rather than studying the local epigenome, Shaddox et al174
studied peripheral white blood cells for epigenetic signatures in
children and adolescents of African-American ethnicity. They found
that there were significant differences in the DNA methylation
status in the localized aggressive periodontitis patients compared
with healthy controls; the authors reported differences for both
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LOOS and Van dYKE
hyper- and hypo-methylation patterns in genes that are part of the
toll-like receptor signaling pathways.174 These patterns correlated
with lipopolysaccharide-stimulated inflammatory cytokines, sug-
gesting the functionality of the methylation patterns and possibly
explaining systemic susceptibility for periodontitis174 and the previ-
ously observed hyperresponsive phenotype in localized aggressive
periodontitis.109
7 | CONCLUSIONS
Periodontitis is a complex chronic inflammatory disease with non-
linear progression that is caused by various factors each playing a
role simultaneously and interacting with each other. The various
factors determine the immune fitness of a subject. The host ex-
ists in a symbiotic relationship with the oral microbiome to main-
tain homeostasis. Loss of homeostasis results from loss of the
host balance and an aberrant host response. This aberrant host
response can manifest as a hyper- or hyporesponsiveness and/or
lack of sufficient resolution of inflammator y reactions. The conse-
quent chronic inflammation elicit s changes in the ecolog y of the
subgingival environment providing favorable conditions for the
overgrowth of pathobionts that further propagate periodontal in-
flammation. The factors that determine immune fitness include:
(a) genetic factors and epigenetic factors; (b) lifestyle factors;
(c) comorbidities; (d) local or dental factors and factors that act
randomly; and (e) pathobionts in a dysbiotic subgingival biofilm.
Variants in at least 65 genes to date have been suggested as being
associated with periodontitis based on genome-wide association
studies and candidate gene case control studies. Interestingly,
reports have found pleiotropy between periodontitis and car-
diovascular diseases. To date, 4 genetic loci are shared between
coronar y arter y disease and periodontitis. The shared genes sug-
gest that periodontitis is not causally related to atherosclerotic
diseases, but rather both conditions are sequelae of similar (the
same?) aberrant inflammatory pathways. In addition to variations
in genomic sequences, epigenetic modifications of DNA can af-
fect the genetic blueprint of the host responses. Further studies
are required to verify and expand our knowledge base before final
cause and effect conclusions c an include specific genetic markers.
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How to cite this a rticle: Loos BG, Van Dyke TE. The role of
inflammation and genetics in periodontal disease. Periodontol
2000. 2020;83:26–39. https ://doi.org /10.1111 /prd.12297
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