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Journal of Clinical Case Reports and Medical Research Review Article
Article Id: JCCRMR-20-04
The Role of Inflammatory Cytokines and Interleukin-6
Expression /Gene Polymorphism in Chronic Periodontitis:
Review of the Literature
Nikolaos Andreas Chrysanthakopoulos* *Dental Surgeon, Resident in Maxillofacial and Oral Surgery, 401 General Military Hospital of Athens, Athens, Greece
*Oncologist, Specialized in Clinical Oncology, Cytology and Histopathology, Department of Pathological Anatomy, Medical
School, University of Athens, Athens, Greece
Abstract
Periodontitis is a chronic inflammatory condition of the periodontal
tissues involving interactions between bacterial products,
inflammatory mediators, and various cell populations. It is accepted
that periodontitis is initiated by the dental plaque formation on the
teeth surfaces. Dental plaque ingredients such as antigens,
lipopolysaccharides, and other factors, initiate an inflammatory and
immune response, leading to the activation of host cells which results
in the release of inflammatory mediators such as cytokines,
chemokines, arachidonic acid metabolites and proteolytic enzymes
which contribute to tissue destruction and bone resorption. Recent
research regarding the pathogenesis of periodontal diseases has
clarified the involvement of various cytokines in the biological
activities that are implicated in its pathogenesis. Cytokines play
crucial roles in the maintenance of tissue homeostasis, whereas
excessive or continuous cytokines production in inflammatory
periodontal tissues is responsible for the progress of periodontitis and
periodontal tissue destruction. Inflammatory cytokines-such as IL-6,
IL-13, and IL-8 are present in the damaged periodontal tissues, and
their production is involved in chronic leukocyte recruitment, tissue
and bone destruction. It is possible that cytokine production and
expression may result to diagnose an individual's periodontal disease
status or susceptibility to the disease. Moreover, despite the
contradictory suggestions, it has been proposed that T-cell subsets Thl
and Th2 with different cytokine profiles play crucial roles in the
immunopathogenesis of periodontal disease. Chronic periodontitis is
suggested to be related to gene variations whereas; genetic variations
which are observed in cytokines may affect periodontitis
susceptibility. The aim of the current review was to present the role of
inflammatory cytokines and Interleukin-6 expression and its gene
polymorphism in chronic periodontitis pathogenesis.
*Corresponding author:
Dr. Nikolaos Andreas
Chrysanthakopoulos, University
of Athens, 35, Zaimi Street, PC
26 223, Patra, Greece, Tel./Fax:
0030-2610-225288; E-mail:
Received: 12 June 2020
Accepted: 04 July 2020
Published: 08 July 2020
Citation:
Chrysanthakopoulos NA. The
Role of Inflammatory Cytokines
and Interleukin-6 Expression
/Gene Polymorphism in Chronic
Periodontitis: Review of the
Literature. J Clin Case Rep Med
Res 2020;1:04
Copyright:
Chrysanthakopoulos NA. This
article is an open access article
distributed under the terms and
conditions of the Creative
Commons Attribution (CC-BY)
license
Journal of Clinical Case Reports and Medical Research Review Article
Article Id: JCCRMR-20-04
Keywords: Interleukin-6, single nucleotide polymorphism, chromic periodontitis, genotype
Introduction
Periodontal Disease (PD) affects nearly 50.0% of adults in the United States, whereas 8.5% of those have
been diagnosed with severe periodontitis. In addition, PD is regarded as one of the most common diseases
worldwide, showing a prevalence of 10%-15% (1). Risk of PD initiation and progression is associated
with smoking history, diabetes mellitus, age, poor oral hygiene, furcation involvement, residual pocket
depths, frequency of supportive periodontal care and genetics (2-6).
Periodontal pathogens that have been shown to be responsible for the PD initiation and progression, are
mainly Porphyromonas gingivalis, Treponema denticola, and Tannerella forsythia, and are considered
the most important pathogens in periodontitis (7,8). Periopathogen products initiate an inflammatory
reaction that leads to local tissue destruction including connective tissue and bone support loss, which
may eventually result in tooth loss (9). The toxic products of the subgingival periopathogens disrupt the
immune surveillance and enhance the toxic environment of the periopathogens leading to a disruptive
tissue homeostasis which concerns connective tissue, periodontal ligament, cementum and alveolar bone
(10). The inflammation progression induces the production of insistent inflammatory signals by the cells
of periodontal tissues. Such signals, which involve the proinflammatory cytokines, IL-1,-2, and Tumor
Necrosis Factor-alpha (TNF-α) induce the production of secondary inflammatory mediators which in turn
amplify the inflammatory signalling pathways, that are able to lead to an increased production of
proteases and osteoclastic signals that result in connective tissue and bone destruction (11,12).
Chronic Periodontitis (CP) is a chronic inflammatory pathological condition caused by the interaction
between microorganisms that produce supragingival and subgingival biofilms and the host inflammatory
response (13), and is characterized by the inflammatory gingiva and the destruction of tooth-supporting
structures (14). CP etiopathogenesis involves many factors such as environmental and genetic factors (15-
18). The development of CP is promoted by the interaction between periodontal pathogens and the host
immune response (13,19). Host immune re-sponse of the periodontal tissues is caused by bacterial
infection, whereas, the inflammatory response of the periodontal tissues is influenced by environmental
and genetic factors (15,20). Concerning the genetic factors, cytokine genes may play an important role in
the immune pathology of PD and their analysis of polymorphisms may be possibly associated with it (21).
Those gene polymorphisms may cause an alteration in the encoded cytokine, or its expression, possibly
resulting in alterations in innate and adaptive immunity, and may hence be a definitive factor of PD
outcome determination (22,23).
One of the most investigated cytokines genes whose polymorphisms are associated with PD development
is IL-6. That cytokine shows various cellular functions, including differentiation and activation of
macrophages and T-lymphocytes and development and final differentiation of B-lymphocytes. IL-6 levels
regulation plays a crucial anti-inflammatory role in both systemic and local inflammatory reactions (24).
Previous reports have shown associations between IL-6 gene polymorphisms and periodontitis in different
ethnicities and it has been proposed that IL-6 -174G/C polymorphism is associated with periopathogens
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identification, variations in inflammation severity, attachment loss and tissue destruction in PD (25,26).
Individual changes IL-6 blood levels may modify the predisposition to many inflammatory diseases, such
as PD, as it is a proinflammatory mediator that activates host cells and may lead to extracellular matrix
destruction. Such IL-6 changes are regulated by common nucleotide variations in its encoding gene,
which is located on chromosome 7p21 (27).
Pathogenesis of Periodontitis
Periodontitis has been considered to be a complex interaction between the bacterial invasion and the host
response, as already mentioned, which alters connective tissue and bone metabolism and causes
periodontal damage, as clinical attachment loss (28). PD consists of two main pathological conditions,
gingivitis and periodontitis and is a progressive condition, which leads to supporting tissue destruction
and in case of periodontitis leads to alveolar bone loss. Severe periodontitis and edentulism influence 743
and 158 million individuals worldwide, respectively (29,30). Gingivitis, is characterized by inflammation
of the gingival tissue, while in susceptible individuals, gingivitis may progress to periodontitis (23).
Although periopathogens are responsible for the PD initiation, the host immune response is implicated in
its pathogenesis as it seems to play a crucial role in the destruction of connective tissue and bone. The
mentioned immune and inflammatory reaction differs among individuals due to individuals genetic
factors as their susceptibility, environmental factors and possible polymorphisms in cytokines and
chemokines that are implicated in those reactions. Bacterial antigens and toxic agents, such as
lipopolysaccharides (LPS), peptidoglycans, lipoteichoic acids, proteases and toxins released by bacteria,
are recognized by toll like receptors (TLRs) on the surface of host cells, induce an immune and
inflammatory reaction, in which innate and adaptive immune systems are involved (31,32). The host
response to the bacterial invasion includes the stimulation of various inflammatory cell types such as
polymorphonuclear leukocytes (PMN), monocytes, macrophages, B- and T-lymphocytes (28,33). The
inflammatory reaction activates mast cells which in turn release vasoactive amines, TNF-α, and various
inflammatory mediators that are able to increase vascular permeability and the expression of adhesion
molecules such as inter-cellular adhesion molecule-1 (ICAM-1) and P-selectin on endothelial cell
surfaces. PMNs are recruited into the damaged tissue, and release lysosomal enzymes, such as
glycosidases, proteases, sulfatases which contribute to connective tissue breakdown (34). Despite the fact
that macro-phages and lymphocytes contribute to the destruction of a rate of 60-70% of the collagen in
the gingival connective tissue, no alveolar bone resorption is observed at that phase (35). The gingival
tissues damage at this phase is still reversible and it is possible to be repaired and restored after removal
of the etiological factors. However, in some cases the inflammatory reaction fails to resolve and
macrophages form preosteoclasts which, after maturing into osteoclasts are responsible for the
irreversible alveolar bone loss, due to innate susceptibility and/or environmental factors (36). Another
implication of an unsolved inflammatory reaction is that the bacterial antigens interact with antigen
presenting cells (APC) such as dendritic cells, macro-phages and B-lymphocytes. The interaction between
naive CD4 (+) T helper cells (T0) with APC, leads to the differentiation of those into various subsets of
cells including Th1, Th2, Th17 and regulatory T cells (Tregs), and this differentiation depends on the
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cytokines that they produce, such as IL-1, IL-2, IL-4, IL-5, IL-6, IL-10, IL-13, IL-17, IL-22, IL-23, TNF-
α, TGF-β, IFN-γ, etc. Th1 cells are responsible for initiation of the cell-mediated immune response and
release IFN-γ, TGF-β, IL-2, and TNF-α in the presence of IL-12, which is released by dendritic cells,
macrophages and PMNs cells. Th2 cells are responsible for the humoral immune response and release the
cytokines IL-4, IL-5, IL-6, IL-10, IL-13 and TGF-β in the presence of IL-4. Th17 subset of cells produce
IL-17, -23, -22, -6 and TNF-α in the presence of TGF-β, IL-1β and -6, whereas Tregs appear in the
presence of TGF-β and release the immunosuppressive cytokines IL-10 and TGF-β. IL-17 is implicated in
the pro-duction of various inflammatory mediators such as TNF-α, PGE2, IL-6 and -1β, mediating bone
resorption by osteoclasts activation (34,36). Tregs and Th17 cells are present in periodontal tissue with an
increased expression of Foxp3 and IL-17, suggesting their crucial role in the immunoregulation of
periodontitis (33,34,37). It has also been shown a plasticity between Th17 and Treg cell subsets which are
present in the same tissues, and especially in periodontitis lesions (37). However, the role of the
association between the Treg/Th17andTh1/Th2, and their cross-talk in the pathogenesis of periodontitis it
still remains unclear.
The inflammatory reaction involves in addition to recruited inflammatory cells, blood vessels, and their
endothelial cells and smooth muscle cells that are come in contact with the recruited inflammatory cells.
Gingival fibroblasts, resident cells of the connective tissue, produce inflammatory mediators, such as
cytokines, chemokines, proteolytic enzymes and PGs which are involved in the inflammatory response
and contribute to disease persistence (38-44). Periodontal ligament fibroblasts, which are located between
the tooth and the alveolar bone, are also implicated in the inflammatory reaction and produce
inflammatory mediators such as PGs, proteolytic enzymes and factors which affect bone resorption.
During the inflammatory process, proinflammatory mediators are released and in turn affect various cell
types and promote the inflammatory pathway (45-47).
Cytokines and Chemokines
Various pro-inflammatory cytokines such as IL-1, -6, -12, -17, -18, -21, TNF-α and IFN-γ and
chemokines such as IL-8, Monocyte Chemoattractant Protein-1 (MCP-1), Macrophage Inflammatory
Protein-1α (MIP1α), and chemokine (C-C motif) ligand 5 (CCL5) have been identified in the Gingival
Crevicular Fluid (GCF), and in gingival tissue from patients with periodontitis and are implicated in its
pathogenesis (48).
TNF-α is released from mast cells in response to bacterial antigens and has a crucial role at an early
phase of the inflammatory reaction. Periodontal tissue cells that also produce TNF-α and IL-1 are PMN
cells, monocytes/macrophages, epithelial and endothelial cells, fibroblasts, and osteoblasts (48). Those
inflammatory cytokines are increased in GCF and gingival tissue with periodontitis (49-51). Their role in
periodontitis has been proven as after periodontal treatment their levels were reduced (52, 53).
Experimental studies have shown that soluble receptors of IL-1 and TNF-α inhibit the progress of
periodontitis in a primate model (54,55). During the inflammatory reaction those cytokines are implicated
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in the induction of other inflammatory mediators, such as IL-6, -8, Matrix Metalloproteinases (MMPs)
and PGE2, whereas are also involved in bone resorption (56-60).
IL-6 is a pro-inflammatory cytokine, released by lymphocytes, monocytes, epithelial cells, and
fibroblasts in response to bacterial LPS, IL-1 and TNF-α and is implicated in the formation of osteoclasts
in vitro (57,61). Periodontitis patients’ GCF and gingival tissue are characterized by increased levels of
IL-6, compared with healthy locations (62, 63). Similarly, after nonsurgical periodontal treatment IL-6
levels have been found to be decreased (64). Chemokines are cytokines implicated in inducing
chemotaxis in responsive cells. IL-8 is a chemokine released by various cells, such as epithelial and
endothelial cells, monocytes, lymphocytes, and fibroblasts, in response to IL-1, TNF-α and LPS and
recruits neutrophils and other leucocytes to the inflammation site (56, 65). In aggressive periodontitis
gingival tissues show increased levels of IL-8 expression, which especially are identified to areas with
high concentrations of PMN cells (66). Similarly, elevated levels of IL-8 have been found in GCF in
periodontitis sites compared with healthy ones, whereas its levels were decreased after periodontal
treatment (67).
The chemokine MCP-1 is produced by epithelial cells, fibroblasts, and endothelial cells, in response to
bacterial endotoxins such as LPS or inflammatory mediators (57, 68), whereas in periodontitis MCP-1
and MIP-1α attract neutrophils and other leucocytes to the inflammation site (56,65). In periodontitis
pathogenesis is also involved the chemokine (C-C motif) ligand 5(CCL5) as they have been found
elevated levels in gingival biopsies and/or GCF, whereas after periodontal treatment were identified
decreased levels of CCL5 and other chemokines (49,53,69-73).
IL-1a, -1b, -12 (p40), -17, -18, -21, IFN-γ and TNF-α cytokines have been found to be increased in GCF
in periodontitis patients(49,52,53, 62,69-71,73-79). Similarly, chemokines such as MCP-1, MIP1a, and
CCL5 were also increased in periodontitis patients (53,70,74).
Interleukin-6 Expression and Gene Polymorphism
Bacterial invasion and environmental factors are implicated in initiation and modulation of PD
progression, however, evidence supports that genes have an important role in the predisposition to and
progression of periodontitis (80,81).
Cytokines and oral pathogens play central roles in the inflammatory process associated with the etiology
of periodontitis (82-84). Polymorphisms in the regulatory regions of the cytokine genes may change
cytokine expression. The cytokine gene polymorphism and expression could be contribute to the
identification of genetic factors that play a significant role in the periodontitis etiology and define the
susceptibility of individuals with respect to PD and clinical outcomes (85).
Based on previous reports it seems that IL-6 is implicated in the periodontitis pathogenesis (23,86). The
development of periodontitis is characterized by biological events that could be media-ted by the binding
of IL-6 and its receptor (IL-6R), including immunocyte activation, angiogene-sis, hematopoiesis, and
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osteoclast differentiation (87). IL-6 levels are elevated in periodontitis patients (88,89) and decreased after
successful periodontal therapy (90,91), whereas in those patients IL-6 has been identified in serum, GCF,
and salivary, condition that proposes a modified production of IL-6 (92,93). IL-6 release has been
associated with the persistence of oral inflammation and tissue destruction caused by proteases,
osteoclasts, and methylation alterations (94-97), whereas increased levels of IL-6 expression is positively
correlated with attachment loss (98). IL-6 is also a potent stimulator of osteoclast differentiation and bone
resorption (99,100), whereas also regulates the expressionof cytokines such as IL-1, -10, and TNF-α
(101,102).
As already mentioned genetic factors may predispose to periodontitis (80,81). Thus, several studies have
been carried out to research whether IL-6 gene polymorphisms predispose to periodontitis. However, the
results regarding the possible associations between these polymorphisms and clinical forms of
periodontitis are conflicting.
Clinical studies indicated that IL-6 plays a crucial role in the inflammatory response to Gram-negative
bacteria (103) by affecting the constitution of the subgingival microbiota and increasing the susceptibility
to colonization with periopathogens bacteria, and in general is induced inresponse to several
inflammatory stimuli (104-107).
The simple nucleotide polymorphism (SNP) of the IL-6 promoter gene may affect the production and
expression of that cytokine, and consequently, this change in serum levels may result in a relevant
biological response (108).
Previous studies have reported an association of the IL-6-174G/C polymorphism and periodonti-tis and
suggested that IL-6 may be considered an important marker for its pathogenesis (4,109-112). Nibali et al.
(25) suggested a positive association between IL-6 -174 G/G genotype and the presence of A.
actinomycetemcomitans and Capnocytophaga sputigena in subgingival plaque. Similarly, Shao et al.
(113) in a systematic review and meta-analysis demonstrated that IL-6 G/G genotype could not modify
the risk of CP, but increased the risk of aggressive periodontitis. An increased expression of IL-6 and IL-6
-174G/C polymorphism was found to be associated with PD severity in Brazilian individuals (98),
whereas in another study was found that IL-6 -174 G/G genotypes and G allele seems to be associated
with aggressive periodontitis (114) and the extend of it (115). Trindade et al. (26) reported associations
between cytokine gene polymorphisms and periodontitis in distinct populations. The effect of that
polymorphism on the susceptibility of CP has been studied by multiple researchers and has been found
positive associations (110,116-121). The IL-6 -572 C/G polymorphism has also been studied in the
context of susceptibility to periodontitis. However, compared with C allele carriers (genotypes GC and
CC), the GG genotype significantly increased the risk of CP, suggesting that the -572 G allele is
associated with the pathogenesis of CP under the compliant genetic model (122-125). It has also been
shown that the same polymorphism was associated with the pathogenesis of periodontitis, as it
predisposes to either CP or aggressive periodontitis (113).
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Polymorphisms in the regulatory regions of genes may alter the expression of cytokines revealing an
important role of genetic predictors of disease susceptibility and clinical measures (126). More
specifically, IL-6 -572 G/C polymorphisms have been associated with an increased protein expression
and inflammatory response (127).
Besides the IL-6 -174 G/C and -572 C/G polymorphisms, the -6331 T/C polymorphism in the promoter
region of the gene could also alter the transcriptional activity of IL-6. It is suggested that IL-6 -6331 T/C
polymorphism is implicated in the pathogenesis of periodontitis, but that association with risk of
periodontitis requires to be confirmed in other ethnics (128).
A study by Nibali et al. (129) supported the hypothesis of a link between IL-6 genetic factors and
aggressive periodontitis and highlights the importance of two IL-6 polymorphisms (-1363 and –1480) in
modulating disease phenotype and susceptibility. Another similar study (125) suggested the hypothesis
that IL-6 polymorphisms and haplotypes are moderately associated with periodontitis, possibly acting
through influencing tissue levels of IL-6.
On the contrary, some reports have shown no association between the polymorphism IL-6 -174 G/C and
PD. Fan et al. (130) observed a very low prevalence of the C allele and found no significant difference in
the genotypes distribution for the IL-6-174G/C polymorphism in the analysis of groups with PD and
coronary artery disease. Similarly, Holla et al. (122) found no significant differences between patients
with periodontitis and healthy controls regarding the genotype or allele frequencies between both groups
for IL-6 -174 G/C and-597 G/A polymorphisms. Another study conducted by Wohlfahrt et al. (131) also
revealed no association between IL-6 -174G/C polymorphism and other polymorphisms in CTLA-4,
DEFB1, ICAM-1, FasL, ICOS, CCR5, OPG, and OPN genes in individuals with PD. The same study
showed no significant association between this polymorphism and CP, finding that was in accordance
with the results of a previous analysis (132). Loo et al. (133) showed no association between genetic
polymorphisms in IL-6,-1b and IFN-γ genes and PD however, in the same study, associations with
polymorphisms in IL-1, TNF-α, IL-4 and -10 genes were described. Ianni et al. (134) found no
association, whereas in another report (130) was found that IL-6 -572C/G polymorphism did not correlate
with CP susceptibility, but might be a potential risk factor for Coronary Heart Disease in a Chinese popu-
lation. In addition, Holla et al.(122) suggested that the -572 G/C polymorphism of the IL-6 gene may be
one of the protective factors associated with lower susceptibility to chronic periodontitis.It was also
observed that the IL-6-373 A9T11 allele was associated with a reduced susceptibility to CP among
Japanese individuals and decreased IL-6 serum levels (124).
It is clear that the prevalence of IL-6 polymorphism varies in different samples and populations, and it
might also vary the association with susceptibility to periodontitis. That observation would lead to
discrepancies and contradictory results in genetic polymorphism studies carried out on periodontitis
patients of different ethnicities and various genetic backgrounds. Except from racial and genetic
differences, other differences could be attributed to possible confounders such as clinical diagnosis
criteria, environmental variables, biologic plausibility, and significant heterogeneity among the studies
reviewed, penetrance and logic of association studies (135).
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