Osteoimmunology in periodontal health and diseases

Periodontal arena harbors a very complex biofilm which consists of numerous bacterial species 1, 2. As this biofilm matures, there is an increased accumulation of facultative anaerobic, Gram-negative microorganisms 3, which result in early vascular changes in the periodontium, with exudation and migration of phagocytic cells, including neutrophils and monocytes/ macrophages, into the junctional epithelium and gingival sulcus, causing initial gingival inflammation. The most important and most prevalent anaerobic Gram-negative bacteria in the subgingival arena are Aggregatibacter actinomycetemcomitans, Porphyromonas gingivalis, Prevotella intermedia, and Tannerella forsythensis. These bacteria play a crucial role in periodontal connective tissue destruction, and alveolar bone resorption by means of an immunopathogenic mechanism, by participating in the periodontal pocket formation and subsequently, development of periodontitis. Once periodontitis has been established, an inflammatory infiltrate is formed consisting of different kinds of cells, such as macrophages and lymphocytes that will produce different cytokine subtypes, which are responsible for the disease progression. Gingival sulcus is the site for initiation of host-microbial interactions. The inflammatory cells, including neutrophils, macrophages, monocytes, etc., produce many chemical mediators that lead to various immunopathological processes. Cytokines induced by the host response play a critical role in periodontal tissue breakdown 4-7. The periodontal bone loss is the result of the extension of inflammation in the periodontal tissues.

It must be noted here that periodontal breakdown may not be a continuous process. Periodontitis may represent a series of brief insults, or ‘bursts’, which accumulate and appear to be chronic over time with extended periods of remission. However, the duration of time of the ‘burst’ is unknown. Alternatively, there may be relative constant stimulation over time, but it is not known how long the chronic destructive period lasts in the chronic model. In spite of evidence for both models 8-10, the nature of periodontal disease progression remains uncertain.

In the following discussion, we shall concentrate on the immunological aspect of the alveolar bone-related changes in both physiological and pathological conditions.

Mechanism of physiological bone resorption 11

To understand the bone resorption in periodontal diseases, first, let us understand physiological bone resorption. The osteoclasts typically contain multiple copies of the Golgi complex, organized as rings surrounding each nucleus, and many readily identifiable transport vesicles moving toward the ruffled border membrane. The enzymes are secreted via the ruffled border, into the extracellular bone-resorbing compartment, where they reach a sufficiently high extracellular concentration that allows extracellular degradation of the bone matrix, within the sealed compartment. The transport and targeting of these secreted enzymes at the apical pole of the osteoclast involves mannose-6-phosphate receptors. Furthermore, the cell secretes several metalloproteases, including collagenase and gelatinase which are responsible for the breakdown of the matrix. It occurs in the following steps:

  • After getting a signal, the osteoclast precursor becomes activated and becomes a multi-nucleated cell, osteoclast.
  • Osteoclast now attaches to the bone matrix. The attachment of the cell to the matrix is performed via integrin receptors (mostly αvβ3, αvβ5, and α2β1), which bind to specific sequences in matrix proteins, and activate a specific signaling cascade requiring several signaling molecules to ensure adhesion and cell motility (c-Src, Pyk2, Cbl, Gelsolin).
  • There are deep foldings of the plasma membrane in the area facing the bone matrix, called as the ruffled border, which is formed as a result of the directional insertion of the vesicles required for active secretion of protons and lysosomal enzymes toward the bone surface, as well as directional endocytic activity from the bone resorbing compartment.
  • The area of the interface between the cell and the bone surface is surrounded by a ring of contractile proteins (sealing zone) that serves to attach the cell to the bone surface while ensuring its continued ability to migrate.
  • The basolateral plasma membrane of the osteoclast is highly and specifically enriched in Na+/K+ ATPase (sodium-potassium pump), HCO3/Cl exchangers, and Na+/H+ exchangers, as well as several ion channels.
  • This membrane also expresses RANK, the receptor for RANK ligand (discussed later) and the M-CSF receptor, all of which are responsible for osteoclast differentiation, as well as the calcitonin receptor, which is capable of rapidly inactivating the osteoclast.
  • The osteoclast acidifies the extracellular compartment 12 by secreting protons across the ruffled border membrane, a process that involves the transporting activity of vacuolar proton pumps 13.
  • The protons are provided to the pumps by carbonic anhydrase, an enzyme that is highly concentrated in the cytosol of the osteoclast.
  • ATP and CO2 are provided by the mitochondria, which are characteristically found in very high numbers in the osteoclast.
  • Apical chloride channels (ClC-7) are also found in the ruffled border membrane14, where they serve to prevent the hyperpolarization created by the massive extrusion of positively charged protons by the V-ATPase.
  • First, the hydroxyapatite crystals are mobilized by digestion of their link to collagen (the noncollagenous proteins) and dissolved by the acidic environment. Then, the residual collagen fibers are digested by the specific action of cathepsin K at low pH, and possibly by the activation of latent collagenase.
  • During active resorption, the degraded bone matrix is processed extracellularly, after which some products are endocytosed into the osteoclast and degraded within secondary lysosomes, and others are transported through the cell by transcytosis and secreted through the basal membrane.
  • The degraded products from the bone matrix may also be released from the bone matrix into the local microenvironment during periods of relapse of the sealing zone.

The normal mechanism of bone resorption by an osteoclast

Normal mechanism of bone resorption by an osteoclast

Mechanism of bone destruction in periodontal diseases

The microorganisms that are present in a biofilm have been shown to be the primary etiological factors for periodontal bone destruction 15. But, now it is clear that the host immune responses to these microorganisms are responsible for the destruction of periodontal bone tissues by the production of RANKL from activated lymphocytes that react to such periodontal pathogens 16, 17.

The identification and characterization of RANKL, its receptor RANK, and soluble decoy receptor osteoprotegerin (OPG) have significantly contributed to the understanding of the skeletal remodeling mechanism involving differentiation of osteoclasts (osteoclastogenesis) and their activation 18-20.

Bone Remodeling and influence of cytokines on bone remodeling

Bone remodeling is a cyclic process of bone matrix osteosynthesis/degradation, rigorously controlled by the RANKL/RANK/OPG system together with other factors of bone metabolism. A number of hormones and cytokines modulate osteoclastogenesis by enhancing osteoclast differentiation, activation, lifespan, and function. These include parathyroid hormone (PTH), calcitriol, PTH-related protein, prostaglandin E2 and thyroxine 21, 22.

Several pro-inflammatory cytokines have been identified as key molecules contributing to the destruction of periodontal tissue, including interleukin-1 (IL-1), tumor necrosis factor-alpha (TNF-α), interferon-gamma (IFN-γ), interleukin-6 (IL-6) 23. Pro-inflammatory cytokines IL-1 and TNF-α have been shown to be very important factors in …………………………..123123

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RANKL:

RANKL is an important factor associated with bone metabolism. It is also known as receptor activator of NfκB ligand or OPGL (osteoprotegerin ligand) or ODF (osteoclast differentiation factor) or TRANCE (TNF-related activation-induced cytokine). It was cloned and identified through different strategies by four independent groups of researchers: Yasuda 31, Lacey 32, Anderson 33 and Wong 34. RANKL belongs to the TNF family and is the only cytokine which has a role in the development and activation of osteoclasts 35-37. RANKL is a trimeric molecule that can be membrane associated or soluble molecule released through enzymatic cleavage by membrane metalloproteinases. The proteolytic cleavage of RANKL is carried out by matrix metalloproteinases (MMP3 or MMP7) 38 or ADAM (a disintegrin and metalloprotease domain) 39.

Sources:

  • Osteoblasts,
  • Bone marrow stromal cells,
  • Chondrocytes,
  • Activated T-lymphocytes.

Biological effects of RANKL:

RANKL mediates osteoclastogenesis i.e. it is involved in bone resorption. Activation of mature osteoclasts exclusively depends on RANKL, which in turn are the main effector cells of bone resorption. Gene knockout mice deficient in RANKL or RANK, respectively, not only present osteopetrotic phenotypes but also lack peripheral lymph nodes 40, 41. RANKL also interacts with osteoprotegrin which is an antagonist to osteoclast action. Thus, RANKL has a dual antagonistic type action on osteoclastogenesis, depending upon the type of receptor it interacts with i.e. RANK or OPG, although both receptors belong to the same TNF receptor family.

Regulation of osteoclast differentiation and function by RANK-RANKL and OPG

rank copy copy

RANK:

RANK (receptor activator of NfκB) is also known as TRANCE-R. It is a heterotrimer, expressed in a transmembrane way on the surface of osteoclast progenitor cells, mature osteoclasts, chondrocytes, dendritic cells, trophoblasts and epithelial cells of mammary glands. When hematopoietic stem cells differentiate from the colony forming unit for granulocytes and macrophages (CFU-GM) to the colony forming unit for macrophages (CFU-M), the macrophage colony-stimulating factor (M-CSF) induces the expression of RANK on CFU-M. Subsequently, RANK remains expressed on osteoclast lineage cells throughout their lifespan until terminally differentiated into multinucleated cells 42. The interaction of the RANK receptor with its ligand RANKL represents the essential stage in initiating osteoclastogenesis and osteoclast activation 43, 44. The signaling mechanism through RANK receptor after binding to RANKL consists of transducing activating signals into the osteoclasts through adapter proteins.

Osteoprotegerin (OPG):

It’s bone protective action justifies the name of osteoprotegerin. OPG is secreted by many cell types in addition to osteoblasts, including those in the heart, kidney, liver, and spleen. It is also called as OCIF (osteoclasts inhibitor factor). Osteoprotegerin functions as a soluble decoy receptor, which binds to RANKL, through competition with the RANK receptor. Osteoprotegerin has an anti-osteoclastogenic effect through its role as the antagonistic endogenous receptor which, after binding to RANKL, inhibits osteoclast maturation and activation via RANKL and blocks bone resorption 45, 46. OPG expression is regulated in osteoblasts not only by a variety of cytokines but also by some hormones and growth factors 47.

RANKL/OPG ratio in periodontitis

Research has been done to find out and examine the expression pattern of RANKL and OPG in periodontal bone destruction in periodontitis 48-50The ratio of RANKL/OPG is critically involved in regulating and directing osteoclastogenic and/or osteoblastogenic development 51.

Studies have been done on periodontitis patients, where tissue homogenate isolated from diseased periodontal lesion with bone resorption showed significantly higher levels of RANKL protein than gingival tissue taken from healthy subjects, whereas OPG protein detected in diseased tissue was lower than that in healthy gingival tissue 52-53.

In another study, gingival crevicular fluid (GCF) samples from patients with gingivitis demonstrated an elevated…………………

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Conclusion

Recent studies have highlighted the role of RANK/RANKL/OPG and pro-inflammatory cytokines in the periodontal bone resorption. Host modulation therapies are also being investigated in this direction to minimize the periodontal connective tissue loss. We need to further investigate and analyze the host-microbial interactions and modulation of host response to minimize the periodontal destruction.

References:

References available in the hard copy of the website

Periobasics: A textbook of periodontics and implantology

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