This review article discusses the host response in apical periodontitis with the main focus on cytokines, produced under this pathological condition and contributing to the degradation of periradicular tissues. The pace of research in this field has greatly accelerated in the last decade. Here we provide an analysis of studies published in this area during this period.
Literature was selected through a search of PubMed electronic database. The keywords used for search were pathogenesis of apical periodontitis cytokines, periapical granuloma cytokines, inflammatory infiltrate apical periodontitis. The search was restricted to English language articles, published from 1999 to December 2010. Additionally, a manual search in the cytokine production, cytokine functions and periapical tissue destruction in the journals and books was performed.
In total, 97 literature sources were obtained and reviewed. The topics covered in this article include cellular composition of an inflammatory infiltrate in the periapical lesions, mechanisms of the formation of the innate and specific immune response. Studies which investigated cytokine secretion and functions were identified and cellular and molecular interactions in the course of apical periodontitis described.
The abundance and interactions of various inflammatory and anti-inflammatory molecules can influence and alter the state and progression of the disease. Therefore, periapical inflammatory response offers a model, suited for the study of many facets of pathogenesis, biocompatibility of different materials to periapical tissues and development of novel treatment methods, based on the regulation of cytokines expression
Apical periodontitis (AP) is an inflammation and destruction of periradicular tissues. It occurs as a sequence of various insults to the dental pulp, including infection, physical and iatrogenic trauma, following endodontic treatment, the damaging effects of root canal filling materials.
In response, the host mounts an array of defenses, consisting of several classes of cells, intercellular messengers, antibodies and effector molecules. The microbial factors and host defense forces encounter, clash with, and destroy much of the periapical tissue, resulting in a formation of various kinds of AP lesions, which most commonly take the form of reactive granulomas and cysts, with the concomitant resorption of bone surrounding the roots of affected teeth.
The pathogenesis of AP has been well reviewed [
The purpose of the present review is to overlook the factors, which interfere with the pathogenesis of AP and subsequent bone loss, evaluating findings, published in the vast literature on this subject.
Literature was selected through a search of PubMed electronic database. The keywords used for search were pathogenesis of apical periodontitis cytokines, periapical granuloma cytokines, inflammatory infiltrate apical periodontitis. The search was restricted to English language articles, published from 1999 to December 2010. Additionally, a manual search in the cytokine production, cytokine functions and bone resorption in the journals and books was performed. The included publications covered cellular composition, immunoregulatory mechanisms, the role of cytokines in human radicular cysts and periapical granulomas, as well as subsequent bone destruction and extracellular matrix degradation.
Histologically, a dense infiltration of immunocompetent cells is seen in
periradicular lesions. The analysis of these cells showed considerable heterogeneity
in their number, morphology and phenotypic properties. Therefore, various attempts
have been made to obtain evidence by means of immunofluoresence [
Microorganisms from the infected root canals, predominantly gramnegative anaerobs,
produce sufficient amount of lipopolysaccharide (LPS), also known as endotoxin,
which egress in high concentrations into the periapical area. LPS activates the
complement system via the alternate pathway leading to the generation of chemotactic
peptides [
The PMN approach to the site of infection because of the chemotaxis, induced
initially by the tissue injury, LPS, complement factor C5a [
Detectable levels of IL-8/CXCL8 were found in approximately 95% of periapical
exudates collected from root canals during routine endodontic treatment of human
periapical lesions, suggesting a pivotal role for IL-8 in neutrophil migration in
acute phases of apical disease [
IL-8 chemotactically attracts and activates PMNs, making them more available and more
competent to engage and kill the bacteria [
The GCP-2 (CXCL6) is a CXC chemokine. Similar to IL-8, it possesses potent
chemotactic and proangiogenic properties [
IL-1 production locally elevates cellular adhesion molecule (CAM-1) expression by
endothelial cells in the capillaries, thus enhancing the local attachment of PMNs
and monocytes and enhancing their migration into the area [
There are two distinct isoforms of IL-1: interleukin-1 alpha (IL-1α) and
interleukin-1 beta (IL-1β). IL-1β is a predominant form found in human
periapical lesions [
IL-1 together with IL-6 and TNF have been shown to induce an acute-phase response-
fever, an elevation in the erythrocyte sedimentation rate and major shifts in the
types of serum proteins synthesized by hepatocytes [
The IL-6 is an integral mediator of the acute phase response to injury and infection.
The major sources of IL-6 production are monocytes and macrophages, type 2 helper T
lymphocytes (Th2), activated B cells and PMN cells. Epithelial cells, vascular
endothelial cells and fibroblasts have also been shown to release IL-6 [
The IL-17 is secreted by type 17 helper T lymphocytes (Th17). It is able to
reactivate the inflammatory process including the induction of inflammation
characterized by the presence of neutrophils. There is also strong evidence that
IL-17 might induce the production of receptor activator for nuclear factor kappa B
ligand (RANKL), activating osteoclasts, with consequent bone resorption [
Colony stimulating factors (CSF) are a group of cytokines that regulate the
proliferation and differentiation of hematopoietic cells. They functionally activate
neutrophil leukocytes. GM-CSF is secreted by a large variety of cells, the possible
principle sources being macrophages, endothelial cells, activated T cells and PMN
[
Specific dentin proteins are capable of stimulating neutrophil migration via the
induction of KC/CXCL1 and MIP-2/CXCL2 release [
The CGRP is a 37-amino-acid peptide which is widely distributed throughout the
central and peripheral nervous systems and is found in particularly high levels in
sensory nerves. CGRP has potent vasodilator activity and is frequently co-localized
with SP [
The SP stimulates the release of histamine from the mast cells, which in turn results
in increased bradykinin production, while bradykinin, histamine and prostaglandin E2
(PGE2) all stimulate increase in pulpal vascular permeability. SP also increases
phagocytosis and oxidative metabolism [
The LTB4 is formed when arachidonic acid is oxidized via the lipoxygenase pathway. It
causes PMNs adhesion to the endothelial walls and attracts macrophages into the
area. Besides PMNs are not only attracted by LTB4, but also release them and
prostaglandins. So though neutrophil leukocytes are essentially protective cells,
they also cause severe damage to the host tissues [
The PMNs produce a wide range of cytokines: IL-1, TNF-α, IL-6, IL-8, macrophage
inflammatory proteins MIP-1α and MIP-1β [
The PMNs are the short lived cells. Their massive death is a major cause for tissue
breakdown in acute phases of apical periodontitis [
Frequency of macrophages within the inflammatory cellular infiltrate has been
reported to scatter between wide ranges of 4% to over 50%. This difference
considered to be an outcome of using the different methodology. Introduction of the
immunohistological methods is considered to be the breakthrough in identifying
macrophages and their subsets [
Infiltration of macrophages into sites of inflammation is relatively slow compared to
that of neutrophils, but they are capable to engulf almost any foreign agent and
their infiltration lasts for a longer time. They were shown to express CC chemokine
receptors: CCR1, CCR2 and CCR5 [
The MCP-3/CCL7 is one of the most pluripotent chemokines, but it acts predominantly
on monocyte-macrophage lineage [
The MIP-1α/CCL3 and MIP-1β/CCL4 belong to the CC chemokine family. Type 1
helper T lymphocytes (Th1) can attract and activate macrophages by producing
cytokines MIP-1α and MIP-1β [
The TGF-β family members are critical regulators of cell growth,
differentiation, repair and inflammation. TGF-β is one of the cytokines
involved in the repair process in periradicular lesions and belongs to the group of
anti-inflammatory cytokines [
The IFN-γ is a cytokine, produced by Th1 cells. The secretion of IFN-γ is
primarily triggered by IL-12 and down-regulated by IL-10. Both IL-12 and IL-10 are
produced by dendritic cells and activated macrophages [
The TNFα is produced both by macrophages and Th1 lymphocytes [
When present at the site of inflammation, macrophages have central roles in the
regulation of connective tissue destruction and repair; innate, nonspecific immunity
and the onset, regulation and outcome of antigen-specific, acquired, immunity [
Cytokines interaction in the course of apical periodontitis. Black arrow = stimulation; red arrow = suppresion.
Development of an effective adaptive immune response relies greatly upon appropriate
recognition of antigen by cells of the innate immune system and presenting it to
adaptive immune cells. The APCs have important pathogen recognition skills and
operate at the interface of innate and adaptive immunity. However, even if a
considerable proportion of macrophages in the lesion may express major
histocompatibility complex (MHC) class II molecules, suggesting that they may act as
APCs, recent researches showed that dendritic cells act as efficient APCs, compared
to macrophages [
Dendritic cells function as sentinels of the immune system by trafficking from the
vasculature to the tissues where, while immature, they capture antigens. The APCs
possess special receptors on their surface that recognize specific pathogen
associated molecular pattern (PAMP) and trigger appropriate intra-cellular events to
continue capture of antigen and further induce co-stimulatory molecules for T cells.
In humans, Toll-like receptors (TLRs) identify PAMPs and activate multiple steps in
the inflammatory reaction. Once activated, TLRs up regulate the genes encoding
inflammatory cytokines such as IL-8, TNFα, IL-6, IL-12 and IL-1β in
immunocompetent cells. Appropriate ligand recognition by TLRs stimulates
intracellular signal transduction pathways and induction of different genes that
function in host defense, including those for inflammatory cytokines, chemokines,
MHC and co-stimulatory molecules. In addition, TLR activation induces multiple
effector molecules, such as inducible nitric oxide synthase and antimicrobial
peptides, which can destroy microbial pathogens. Then, following inflammatory
stimuli, antigen loaded dendritic cells leave the tissues and move to the draining
lymphoid organs where, converted into mature dendritic cell, they prime naive T
cells [
The RANKL is an important regulator of the interactions between T cells and dendritic
cells during the antigen presentation process. RANKL is also expressed on the
surface of the dendritic cells and the interaction with its receptor can induce
cluster formation and activation of T cells, dendritic cell survival, regulate the
dendritic cell functions, and T cell–dendritic cell communication [
Continuous or severe infections at levels beyond the capacity of innate immunity are
mediated by adaptive immunity, which is much more specific toward exogenous
antigens. Adaptive immunity is also called specific immunity, and possesses the
ability to memorize and respond more vigorously to repeated exposures to the same
antigen. The major components of adaptive immunity are T and B lymphocytes [
T lymphocytes are classified into two categories according to their surface T cell
receptors (TCRs), which are either the alpha- beta (αβ) or gamma- delta
(γδ) type. Although the functions of TCR γδ- expressing cells
are still uncertain, they are reportedly related to nonspecific defense systems
against exogenous stimuli [
The understanding about the role of suppressor/ regulatory T cells changed a few
times through the time. Initially it was considered that these cells were CD8+ ,
later that they do not exist at all. Everything changed when the forkhead/winged
helix transcription factor (Foxp3), known as a key orchestrator of inhibitory
function, was discovered. Such T cells occur within both the CD4 and the CD8 T cell
lineages. Their suppressive functions are inhibition of antigen presenting cells and
production of inhibitory cytokines [
The CD8+ cells are also known to induce target cell death by means of granule- and
FAS- mediated pathways and have cytotoxic effects when secreting several cytokines,
such as TNFα and IFNγ, in the vicinity of target cells [
IP-10/CXCL10 is expressed by various inflammatory cells as well as endothelial cells.
It was also reported to attract Th1 cells [
The Th1 type response can be characterized by the production of IL-12, IFN-γ
[
The IL-12 is produced by activated APC, such as macrophages [
The IFN-γ, as noted before, is produced by Th1 cells. And it mainly serves to activate macrophages at the site of inflammation.
The TNF-α is produced both by macrophages and Th1 lymphocytes [
Differentiation of native T cells to Th2 lymphocytes is driven by the activation of T
cells via TCR and IL-4 receptor, which leads to phosphorylation of signal transducer
and activator of transcription, interleukin-4 induced (STAT6). The pSTAT6 is
critical for the induction of the Th2 transcription factor GATA3, which in turn
transactivates Th2 specific cytokines, such as IL-4 [
IL-4 has been found in periapical granulomas and cysts: Ihan Hren and Ihan [
Differentiation of Th17 lymphocytes from the native T cells is driven by the
combination of the cytokines TGF-β and IL-6. In the absence of IL-6, Th17
differentiation can also be induced by TGF-β plus IL-21. IL-23, produced by
APC, reinforces Th17 differentiation programme and decreases the chance of
dedifferentiation and plasticity in Th17 cells. IL-1β was also described as
critical differentiation factor for Th17 cells. In addition, as Th1 and Th2 subsets
cross-regulate each other, they also appear to regulate the development of Th17
cells, because both IFN-γ and IL-4 inhibit the formation of Th17 [
The Th17 cells produce IL-17, which is able to reactivate the inflammatory process,
including the induction of inflammation characterized by the presence of neutrophils
[
There is evidence that the type of the immune response in the periapical granuloma
tissue is determined by their apically resident bacteria [
Minor population of CD4+ T cells, which coexpress the IL-2 receptor α-chain
(CD25) and are crucial for the control of auto reactive T cells and the regulation
of the immune response to infection, is named "T regulatory cells" (Treg).
They were found to express the Foxp3, which is uniquely present in this cell type
and is essential for Treg differentiation [
The TGF-β alone induces the Treg transcription factor Foxp3 and is also
essential for Treg development [
B lymphocytes are the lymphocytes, directly responsible for antibody production. On
receiving signals from antigens and the Th2 cells, some of the B cells transform
into large plasma cells [
Cytokines, involved in immune response formation in apical periodontitis
[
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IL-1 | MǾ, PMN, Oc, Epithelial cells | Chemotactically attracts and activates PMNs; Stimulates the production of prostaglandines, proteolytic enzymes and cytokines IL-6, IL-8; Stimulates bone resorption and inhibits bone formation |
IL-8 | MǾ, PMN, Th1 | Chemotactically attracts and activates PMNs; Stimulates osteoclast recruitment and activity |
IL-6 | MǾ, PMN, Th2, B lymphocytes, endothelial cells | Activates PMNs, T cells; Stimulates the differentiation of B lymphocytes into plasma cells; Induces bone resorption; Down-regulates the production of IL-1 |
TNFα | MǾ, Th1, PMN | Activates lymphocytes and MǾ's; Stimulates bone resorption |
GCP-2 | Endothelial cells | Chemotactically attracts PMNs |
IL-17 | Th17 | Activates IL-1, IL-6, TNFα, GCP-2 and IL-8 secretion; Stimulates bone resorption |
GM-CSF | MǾ, T lymphocytes, endothelial cells, PMN | Functionally activates MǾ and PMNs |
MCP-3 | Endothelial cells, lymphocytes, fibroblasts, plasma cells | Chemotactically attracts MǾ |
MIP-1 | Th1 | Chemotactically attracts and activates MǾ and Oc |
TGFβ | MǾ, lymphocytes, fibroblasts, Ob, Oc | Suppresses the proliferation and differentiation of T and B lymphocytes; Down regulates the production of IL-1, IL-6, TNFα and IFNγ; Blocks the production of nitric oxide by MǾ; Inhibits bone resorption; Inhibits Th17 formation and promotes Treg formation |
IFNγ | Th1 | Activates MǾ; Induces IL-1, NO and O2 production |
IL-12 | MǾ, dendritic cells | Stimulates the production of IL-1 and IFNγ; Stimulates Th1 differentiation; Suppresses Th2 differentiation |
IL-10 | MǾ, dendritic cells | Suppresses the production of IL-1 and IFNγ |
IL-4 | Th2 | Inhibits bone resorption; Inhibits Th17 formation, Suppresses the production of IL-1 |
MǾ = macrophages; PMN = polymorphonuclear leucocytes; Th = T helper cells; Ob = osteoblasts; Oc = osteoclasts; NO = nitric oxide.
The integrity of bone tissues depends on the maintenance of a delicate equilibrium
between bone resorption by osteoclasts and bone deposition by osteoblasts.
Osteoclasts originate from hematopoetic precursors of the monocyte-macrophage
lineage that reside within the bone marrow and, guided by chemokines, emigrate from
the peripheral circulation into bone. Chemokines, known to cause osteoclasts
chemotaxis and differentiation, are: MCP-1/CCL2, SDF-1α/CXCL12,
MIP-1α/CCL3, MIP-1γ/CCL9, RANTES/CCL5, IL-8/CXCL1, MCP-3/CCL7, CKβ8/
CCL23, MIG/CXCL9 and IP-10/CXCL10. The activation of osteoclasts is achieved only
with RANKL [
Marton et al. [
The MIP-1α/CCL3 induces adhesion of osteoclasts to primary osteoblasts, thereby
suggesting a function for this chemokine in regulation of the interaction between
those two cell types. The MIP-1γ/CCL9 plays an important role in the survival
of osteoclasts, and part of the RANKL effect on osteoclast survival is dependent on
its ability to induce MIP-1γ/CCL9 production [
Regulated on activation, normal T cell expressed and secreted (RANTES/CCL5) is a
member of the CC chemokine family. Yu et al. [
The β-chemokine (CKβ8/CCL23) is a member of the CC chemokine family.
Functionally, CCL23 has chemotactic activity for monocytes, dendritic cells,
lymphocytes, neutrophils, osteoclast precursor cells, and endothelial cells [
Osteoclast precursors have also been found to express CXCR3, which make them
responsive to the chemokine MIG/CXCL9 and results in their migration and the
adhesion of osteoclast precursors [
T cells are able to regulate osteoclastogenesis by the RANKL and IFN-γ
production [
Vernal et al. [
The natural decoy receptor for RANKL is osteoprotegerin (OPG), also known as
osteoclastogenesis inhibitory factor (OICF), and osteoclast formation is regulated
by the balance between OPG and RANKL [
It has become evident that osteoblasts have a global role in orchestrating the bone
remodeling process. Their function is not restricted solely to bone formation, but
it is now firmly established that they are responsible for initiating bone
resorption. Osteoblasts provide the essential and sufficient stimuli that control
the behavior of the osteoclasts, an event that occurs via cell-cell interaction.
There are molecular determinants responsible for that, namely M-CSF and RANKL, the
former secreted and the latter mainly cell-membrane bound. The M-CSF binds to c-Fms
on the surface of osteoclast precursors and so enhances their proliferation and
survival [
The periodontal ligament is a dense connective tissue, localized between the cementum
and alveolar bone, supporting the tooth. This ligament is composed mainly of
collagen fibers and elastic system fibers. The destruction of the periodontal
ligament is initiated by the degradation of ECM. Enzymes involved in ECM degradation
comprise both matrix metalloproteinases (MMPs) and serine proteases (including
neutrophil elastase and cathepsin G) [
The MMPs are a family of zinc- dependant endopeptidases collectively capable of
degrading all extracellular matrix components, including collagen and proteoglycans.
The MMPs have been suggested to play an important role in inflammatory conditions of
periodontal, pulpal and periapical tissues, as well as dentin mineralization [
The subfamily of collagenases includes: interstitial collagenase (MMP-1), collagenase
of neutrophils (MMP-8) and collagenase-3 (MMP-13). These enzymes disintegrate native
fibrillar interstitial collagens by cleaving the single peptide bond α-chains
[
The MMP-1 has been reported to degrade non-mineralized extracellular matrix and
stimulate osteoclastogenesis through generating collagen-degradation fragments on
bone surfaces. MMP-1 expressing cells are considered to be macrophages [
The MMP-8 (collagenase-2) degrades gelatin, type I, II, III, V and XI collagens
[
The MMP-13 has an exceptionally wide substrate specificity compared with other MMPs.
In addition to fibrillar type I, II and III collagens, MMP-13 degrades type IV, IX,
X and XIV collagens, gelatin, tenacin-C, fibronectin and proteoglycan core proteins
[
Type IV collagen/gelatin is the main component of basement membrane and the
degradation of this structural protein as well as denatured gelatins, laminin,
elastin, fibronectin and basement membrane zone- associated collagen is favored by 2
MMPs: gelatinase A (MMP-2) and gelatinase B (MMP-9) [
The MMP-2 is known to degrade gelatin, fibronectin, elastin, laminin and collagen I,
III, IV, V, VII, X, XI [
The MMP-9 degrades gelatin, elastin, type IV, V, VII, X, XI and XIV collagens. It is
mainly secreted by neutrophils, although macrophages, T cells, mast cells and
odontoblasts can also express this enzyme [
Several studies have shown that MMP-2 and -9 participate in the pathogenesis of pulp
and periapical inflammation. Shin et al. [
Belmar et al. [
Serine proteases known to degrade ECM in apical periodontitis are neutrophil elastase
(NE) and cathepsin G [
Cathepsin G degrades type III collagen and proteoglycan. It is a nonspecific serine
protease which cleaves individual amino acids from protein molecules. Cathepsin G is
mainly stored in primary granules of neutrophils and is also detectable in monocytes
and mast cells. Tsuji et al. [
Enzymes involved in degradation of the extracellular matrix [
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Interstitial collagenase |
Macrophages | Degrades non-mineralized extracellular matrix Stimulates osteoclastogenesis |
Collagenase of neutrophils |
Polymorphonuclear leukocytes, macrophages | Degrades gelatin, type I, II, III, IV and XI collagens | ||
Collagenase-3 |
Fibroblasts, epithelial cells, plasma cells | Degrades type I, II, III, IV, IX, X and XIV collagens, gelatin, tenacin-C, fibronectin and proteoglycan core proteins | ||
|
Gelatinase A (MMP-2) | Epithelial cells, fibroblasts | Degrades gelatin, fibronectin, elastin, laminin, collagen I, III, IV, V, VII, X, XI | |
Gelatinase B (MMP-9) | Polymorphonuclear leukocytes, macrophages, T cells, mast cells, odontoblasts | Degrades gelatin, elastin, type IV, V, VII, X, XI and XI collagens | ||
|
Neutrophil elastase |
Polymorphonuclear leukocytes | Degrades elastin, collagen, fibrinogen, hemoglobin, proteoglycans | |
Cathepsin G | Polymorphonuclear leukocytes, monocytes, mast cells | Degrades type III collagen and proteoglycan |
Reactive oxygen species (ROS) are highly reactive and may modify and inactivate proteins, lipids, DNA, and RNA and induce cellular dysfunctions. O2-, H2O2 and NO (nitric oxide) play an important role in the host defense, as well as in inflammation-induced tissue lesions.Superoxide anion is a highly reactive oxygen radical involved in cell and tissue damage in a variety of disorders, including inflammatory diseases. While production of superoxide by neutrophils and other phagocytic cells is essential for the killing of microorganisms, it causes tissue damage at the site of inflammation. An altered balance between oxygen radical production by phagocytic cells in periapical lesions and its elimination was suggested to contribute to periapical damage and bone loss in chronic apical periodontitis.
Superoxide anion has also been shown to be produced by osteoclasts and involved in
bone resorption. Furthermore, superoxide anion may react with a precursor in plasma
to generate a factor that is chemotactic for neutrophils. In addition to production
by host cells, bacteria can also produce superoxide anion. Production of superoxide
by a clinical isolate of a
Mynczykowski et al. [
The arrangement of one atom of nitrogen and one of oxygen in the molecule of nitric
oxide leaves an unpaired electron, which makes the molecule a highly reactive free
radical. NO is synthesized by a complex family of enzymes, called NO synthases
(NOS). There are three NOS: NOS1, NOS2 and NOS3. NOS1 and NOS3 are constitutive, and
NOS2 is an inducible, calcium-independent isoform, also called iNOS. Unlike NOS1 and
NOS3, induction of NOS2 results in continuous production of NO [
It is suggested that NO plays a pivotal role in regulating inflammatory reaction in
apical lesions with the association of cytokines. However, more detailed mechanisms
of how NO is related to periapical lesions are unknown. Hama et al. [
Apical periodontitis is one of the most common endodontic diseases, which endodontists and general practitioners face daily in their practice. It's a very complex pathology with the multiple factors involved. The abundance and interactions of various inflammatory and anti-inflammatory molecules can influence and alter the state and progression of the disease. Therefore, periapical inflammatory response offers a model, suited for the study of many facets of pathogenesis, biocompatibility of different materials to periapical tissues and development of novel treatment methods, based on the regulation of cytokines expression.
The authors declare that they have no conflict of interests.