Osteoimmunology - Unleashing the concepts

Abstract

Osteoimmunology is an emerging field of research dedicated to the relationship between the immune processes and the bone metabolism of various inflammatory bone diseases. The regulatory mechanisms governing the osteoclast and osteoblast are critical for understanding the health and disease of the skeletal system. These interactions are either by cell to cell contact or by the secretion of immune regulatory mediators like cytokines and chemokines by immune cells that are governed by the RANKL (TRANCE)-RANK- OPG axis. TRANCE-RANK signaling has served as a cornerstone of osteoimmunology research. There is increased recognition of the importance of the inflammatory and immune responses in the pathogenesis of periodontal disease. Thus, this field has provided a framework for studying the mechanisms underlying periodontal destruction. As bone homeostasis is mainly regulated by both the immune and endocrine systems, there emerged osteoimmunoendocrinology where adipokines take the lead. This review focuses on the underlying concepts of osteoimmunology, its relation to Periodontics.

INTRODUCTION

The immune system is an organization of cells and molecules with specialized roles in defending against pathogenic bacteria (extra and intracellular), viruses (intracellular), fungi, protozoa other parasites and also to protect us from neoplastic cells. The responses to invading microbes are categorized as innate and adaptive, according to their antigen recognition receptors and ability to establish immunological memory.

When a wild pathogen enters the body for the first time, it immediately encounters cells of the innate immune system that are constantly patrolling for invaders. These sentries include macrophages, neutrophils and dendritic cells, which engulf and destroy pathogens as well as infected body cells. The guard cells then break down the material they have ingested and display samples of the intruder's components-known as antigens so that members of the adaptive immune system, T and B cells, can become familiar with the pathogen's appearance. At the same time, the antigen-presenting cells release signaling chemicals called cytokines that induce inflammation and alert T and B cells to the emergency.

Once a population of T and B cells adapted to the specific pathogen matures, the B cells release antibody molecules, and killer T cells seek out and destroy cells that have already been colonized by the invader. It takes a few days for interactions with antigen presenting cells to create these tailored T and B cells, but a subset of them remain in the body as “memory” cells-some times for decades-ready to squelch any attempted reinfection by the same organism.[1,2]

There is a close relationship between the immune and bone systems. Research into the bone destruction associated with inflammatory diseases such as periodontal disease, Paget's disease, osteoporosis, rheumatoid arthritis, osteoarthritis, multiple myeloma and metastatic bone tumors has highlighted the importance of the inter play of the immune and skeletal systems.[3]

The crosstalk between these systems has lead to the emergence of an interdisciplinary field called osteoimmunology. Although osteoimmunology started with the study of the immune regulation of osteoclasts, its scope has been extended to encompass a wide range of molecular and cellular interactions, including those between osteoblasts and osteoclasts, lymphocytes and osteoclasts, and osteoblasts and hematopoietic cells. The immune system is spawned in the bone marrow reservoir and it is now recognized that important niches also exist there for memory lymphocytes. At the same time, various factors produced during immune responses are capable of profoundly effecting regulation of bone. Therefore, the two systems should be understood to be integrated and operating in the context of the osteoimmune system, a heuristic concept that provides not only a framework for obtaining new insights by basic research, but also a scientific basis for the discovery of novel treatments for diseases related to both systems.

The term osteoimmunology has been coined to encourage an interdisciplinary approach to understand the cross talk between bone and the immune system by Arron and Choi Y to describe the interaction of cells from the immune and skeletal systems. In fact, one year before, (RANKL) was found in T lymphocytes and described as a regulator of dendritic cell and osteoclast function, having an important role in promoting osteoclastogenesis.[4]

Penninger and colleagues contributed to the development of osteoimmunology by generating a series of knockout mice and establishing the role of the RANKL/RANK system in the pathogenesis of bone related diseases.[5]

On the other hand, adipose tissue is also involved in regulation of bone cells mainly by the production of adipokines; which not only act on osteoblasts and osteoclasts but also on immune cells. Therefore, these cross-talk mechanisms between skeletal, immune, and adipose systems create several points of contact that can be used as potential therapeutic targets for controlling bone remodeling.[6]

BONE AND IMMUNE CELLS INVOLVED

The bony skeleton; enables the storage of calcium and the harboring of hematopoietic stem cells from which immune cells are derived. This multifunctional organ is characterized by calcified hard tissue composed of type I collagen and highly organized deposits of calcium phosphate (hydroxyapatite).[7] Although it seems to be metabolically inert, bone is restructured at such a high speed that approximately 10% of the total bone content is replaced per year in adult vertebrates. This process, called bone remodeling, is dependent on the dynamic balance of bone formation and resorption, which are mediated by osteoblasts and osteoclasts, respectively. A delicate regulation of this process is a prerequisite for normal bone homeostasis and an imbalance is often linked to metabolic bone diseases in humans, such as osteoporosis and inflammatory bone loss.[8]

Osteoblasts are cells of mesenchymal origin that secrete bone-matrix proteins and promote mineralization.[7,8] The proliferation and differentiation of osteoblasts are under the control of a number of soluble factors and transcription factors such as Runx2 (runt-related transcription factor 2) and OSTERIX (also known as SP7) .[9] Differentiated osteoblasts embedded in the bone matrix are called osteocytes, and might have a specific but as-yet unclear role in mechanotransduction by Wnt signaling. Osteoblasts expresses RANKL, a TNF related activation induced cytokine (TRANCE), a type II membrane protein. Binding of RANKL to RANK, its receptor on osteoclasts is inhibited by its decoy receptor OPG.[10]

During chronic inflammatory conditions, the balance between bone formation and resorption is skewed towards osteoclast mediated bone resorption. Moreover, in inflamed tissues, the osteoclasts are located between inflamed tissues and bone and, as in physiological conditions; the major player in bone resorption is the RANKL (TRANCE)/RANK (TRANCE-R)/OPG axis.[11]

Characterization of the functions of RANKL and its receptors (RANK and OPG) have contributed significantly to the emergence of osteoimmunology, specifically with respect to examination of the interplay between active immunity and maintenance of bone homeostasis. Apart from osteoblasts, cells that produce RANKL are monocytes, neutrophils, dendritic cells, B and T lymphocytes which produce a variety of proinflammatory cytokines by potentiating the effects of RANKL-RANK signaling, thus contributing the bone damage.[12]

The proinflammatory cytokines such as TNF-α, IL-1, IL-2, IL-3, IL-6, IL-7, IL-11, IL-15, and 1L-17 potentiate the bone loss either by inducing RANKL expression by osteoblasts and increasing osteoclast generation. The cytokines such as IL-4, IL-5, IL-10, IL-12, IL-13, IL-18, IFN- α, IFN-β, and IFN-γ are inhibitors of osteoclastogenesis by blocking RANKL signaling either directly or indirectly.[13,14]

Osteoclasts are cells of hematopoietic origin that decalcify and degrade the bone matrix by acid decalcification and proteolytic degradation, respectively, and thus causing bone resorption. They are large, multinucleated cells formed by the fusion of precursor cells of the monocyte -macrophage lineage[15] [Figure 1].[16] Osteoclastogenic signals are mediated by RANKL and its co-stimulatory signals, in addition to M-CSF. M-CSF and the transcription factor PU.1 are crucial for the proliferation and survival of osteoclast precursor cells. M-CSF transmits its signal to the cell through the specific receptor c-fms, which is a member of the receptor tyrosine kinase superfamily [Figure 2].[3] M-CSF is also crucial for the proliferation and survival of precursor cells of macrophages, mainly by activating ERK through GRB2 and AKT through P I3K. M-CSF also stimulates the expression of RANK in monocyte-macrophage precursor cells, thereby rendering them able to efficiently respond to RANKL. PU.1, a member of the ETS family of transcription factors, regulates the development of macrophages and osteoclasts by controlling the expression of c-fms.

Figure 1.

A simplified view of osteoclast differentiation

Figure 2.

Generation of osteoclast precursor by M-CSF

M-CSF induces Bcl-2, which has a pivotal role in cell survival. MITF, activated by transcription factors such as c-Fos, and NF-kB have been shown to be essential for osteoclast differentiation; and factors such as cSrc, Vav3, β₃ -integrin, chloride-channel family member ClC7, vacuolar ATPase and cathepsin K are crucial for osteoclast function.[17]

T cells represent the major regulatory players in bone destruction. Conventional T cells comprise two subtypes: Cytotoxic T cells and helper T (T _H ) cells which are distinguished by their restriction molecules for antigen presentation.[18] When T_H cells recognize antigen in association with supportive signals (costimulatory signals), they undergo a transition from naοve to effector T_H cells. Effector T_H cells can be divided into several functional phenotypes dependent on their cytokine profile, such as T_H1, T_H2, T_H17, T_reg cells.[19] Effector T_H cells regulate adaptive humoral immunity by interacting with B cells and influence the maturation and activation of innate immune cells, such as macrophages, dendritic cells, and granulocytes. Recently, studies have demonstrated that T_H cells also have effects on osteoclast commitment.

T_H cell subset involved in the production of IL-17 (T_H1 cells) is considered to be the typical osteoclastogenic. IL-17 induces the synthesis of MMPs inducing the bone and connective tissue degradation. These effects are balanced by T_reg cells which inhibit bone destruction through suppression of osteoclast formation by a cell contact dependent manner that might be mediated by the expression of CTLA-4, which binds to B7-1 and B7-2 in the preosteoclasts.[20,21] Thus directly inhibiting osteoclast differentiation.[22] T regulatory cells also express cytokines, like IL-4, IL-10, and TGF-β which not only have anti-inflammatory properties, but also suppress osteoclastogenesis.

IMMUNE MEDIATORS IN OSTEOIMMUNOLOGY

These include a) Transcription factors b) Immune receptors and costimulation.

• (A)Transcription factors include NF-kB, AP-1, NFAT, STAT 1, Schnurri -3 (SHN 3), a DNA binding protein.
• • (a)
    NF-kB is a family of dimeric transcription factors including REL (cREL), RELA (p65), RELB, NF-kB1 (p50) and NF-kB2 (p52). NF-kB is required for NFATc1 induction. Signals induced by the pro-inflammatory cytokines probably converge at the activation of NF-kB which therefore has an integral role in osteoimmunological interactions.[23]
  • (b)
    AP-1 component cFos is essential for the RANKL- mediated induction of NFATc1.AP 1 transcription factor is a dimeric complex composed of the FOS-family proteins (c-Fos, FODB, Fos-related antigen (FRA1 and FRA2), JUN-family proteins (cJUN, JUNB and JUND) and activating transcription factor (ATF) -family proteins.[24]
  • (c)
    The NFAT transcription-factor family was originally identified in T cells. It comprises five known members: NFATc1 (NFAT2), NFATc2 (NFAT1), NFATc3 (NFAT4), NFATc4 (NFAT3), and NFAT5.[25,26] The activation of NFATc1-NFATc4 is mediated by the phosphatase calcineurin through control of the phophorylation status of NFAT. In osteoclasts and T cells, the activity of this enzyme is regulated by calcium-calmodulin signaling. Calcineurin inhibitors, such as, FK 506 and cyclosporine A strongly inhibit osteoclastogenesis. It is notable that NFATc1 has an exclusive role in osteoclasts, but that NFATc1 and NFATc2 have a redundant role in T cells.[27] The importance of NFAT in the regulation of bone homeostasis is also supported by the recent findings that NFAT and calcineurin regulate osteoblast differentiation and bone formation. It has been shown that NFAT regulates bone formation through interaction with OSTERIX, but further studies are needed to clarify what activates calcineurin and NFAT in osteoblasts.[28]
  • (d)
    Inhibitor of DNA binding 2 (ID2) is a helix -loop-helix protein, a negative regulator of helix-loop-helix transcription factors. It functions downstream of RANKL, but depending on the context, it can be both a positive or negative regulator of RANKL signaling. This indicates a unique osteoimmunological function for this molecule.[29]
  • (e)
    STAT1-has a crucial role in the immune system as a mediator of signal transduction and a regulator of gene transcription in both I and II IFN systems. STAT 1 was found to have a unique, non-canonical function as a cytoplasmic attenuator of Runx2.[30] Therefore, the loss of STAT1 results in excessive Runx2 activation and osteoblast differentiation, thereby tipping the balance in favor of bone formation over bone resorption.[31]
  • (f)
    Schnurri-3 (SHN3), a zinc- finger protein, identified as a DNA-binding protein involved in the regulation of bone formation through proteolytic degradation of Runx2.[32]

  Therefore, accumulating evidence indicates that a great number of transcriptional regulators are shared by the immune and the bone systems, and they control not only osteoclasts but also osteoblasts.

• (B)Immunoreceptors and costimulation

  The immune receptors are RANK, c-fms, OSCAR, DC-STAMP, ephrin, and semaphorin receptors, costimulatory molecules like B7-H3 (CD276).[33] The immunoglobulin-like receptors identified in osteoclast-precursor cells include OSCAR, TREM2, SIRPb1, and PIRA.[34]

  • (a)
    RANK, the signaling receptor for RANKL, mediates osteoclastogenesis. RANK signal transduction is mediated by adapter proteins called TRAFs. The key signals sent through RANK in osteoclast precursors are mediated by the adapter molecule TRAF 6. Downstream of TRAF 6, TRANCE signaling in osteoclasts activates P I3K, c-Src, JNK1, ERK, p38 MAPK, and subsequently a series of transcription factors including NF-κB, c-Fos, Fra-1, and NFATc1.
  • (b)
    c-fms is a receptor for M-CSF. It is also known as colony stimulating factor receptor. M CSF transmits its signal to the cell through the specific receptor c-fms which is a member of the receptor tyrosine kinase superfamily. M-CSF is crucial for the proliferation and survival of precursor cells of osteoclasts as well as macrophages.
  • (c)
    DC-STAMP, an “osteoimmunology” molecule, was originally identified in dendritic cells (DCs). DC-STAMP is highly expressed in osteoclasts and DCs, and play an essential role in regulating cell-cell fusion and bone resorbing efficiency in osteoclasts or phagocytosis and antigen presentation activity in DCs, respectively.[35]
  • (d)
    OSCAR is an FcRg-associated receptor that is expressed by myeloid cells and is involved in antigen presentation and activation of human dendritic cells. OSCAR expression is detected specifically in preosteoclasts or mature osteoclasts. It is a member of leukocyte receptor complex (LRC) protein family and is an important bone-specific regulator of osteoclast differentiation.[36]
  • (e)
    Ephrin receptor B4 and ephrin B2 were newly identified as mediators of osteoblast-osteoclast interactions.[6]
  • (f)
    Semaphorin 6D and its receptor plexin-A1 might have an important role in the activation of the DAP12-mediated ITAM signal through the interactions among plexin-A1, DAP12, and TREM2.[3]

Costimulatory molecules are of two families, TNF-TNF family and immune regulators. Costimulation was originally introduced in T cell biology to describe the requirement of naive T cells to receive a second signal besides T cell receptor (TCR) ligation for their activation and commitment to functional effector T cells. In T cells, the requirement for costimulation helps to maintain peripheral tolerance, since the expression of costimulatory ligands and activation of naive T cells is tightly regulated.[37]

B7-H3 is an Ig superfamily member, a new costimulatory molecule of B7 family that is expressed on the surface of antigen presenting cells. It stimulates CD4+ and CD8+T cells to increase the activity of cytotoxic T lymphocyte. B7-H3 is regarded as a positive regulation molecule. It increases the secretion of IFN-γ and can upregulate IL-8 and TNF-α. Recently, B7-H3 was found to be expressed on developing osteoblasts, with its expression increasing during cell maturation.[38]

THE CROSS TALK BETWEEN OSTEOBLASTS AND OSTEOCLASTS

Two key factors produced by osteoblasts play a crucial role in regulating bone resorption by osteoclasts. (a) RANKL/TRANCE/OPGL/ODF (b) OPG.

• (a)RANKL - a cytokine/secreted protein belonging to the TNF superfamily of proteins, a key differentiation factor of osteoclasts and originally discovered in activated T cells. RANKL is synthesized as a transmembrane protein by osteoblasts. In bone, expression of RANKL by osteoblasts allows the maturation and activation of osteoclasts by binding to its receptor RANK on preosteoclasts surface. Under physiological conditions, osteoclast formation is mediated by RANKL expressed in bone marrow mesenchymal cells. The expression of RANKL is upregulated in these cells by osteoclastogenic factors such as vitamin D3, prostaglandin E2, parathyroid hormone, IL-1, IL-6, IL-11, IL-17 and TNF-α.[39,40]

  RANKL is of two types, a membrane associated cytokine and soluble cytokine. In immune system, RANKL seems to be multifunctional and its role is dependent on the RANKL-expressing cell type.

  • (i)
    T cells expressing RANKL facilitate the survival of interacting dendritic cells[41] and accelerate immune reactions in certain autoimmune conditions.[42]
  • (ii)
    Keratinocytes express RANKL in response to ultraviolet stimulation of the skin activating local resident dendritic cells and triggering the expansion of regulatory T (Treg) cells which suppresses immune reactions.[43]
  • (iii)
    In fetal lymphoid tissue inducer cells in the development of secondary lymphoid organs.[44]
  • (iv)
    In thymocytes, for the induction of autoimmune regulator (AIRE), a factor required for the elimination of autoreactive thymocytes and the establishment of central tolerance.[45]
• (b)OPG- a factor that exerts a protective effect on bone. OPG is a member of the TNF receptor superfamily and has a very important role in the skeletal system, acting as a decoy receptor for RANK-RANKL binding. It binds to RANKL with high specificity and thus, prevents osteoclasts differentiation and activation and osteoclast apoptosis. OPG inhibits the ability of RANKL to induce osteoclastogenesis[11] Figure 3.[11]

Figure 3.

Cells, ligands, receptors and decoys

RANKL is not only been reported to be involved in physiological osteoclastogenesis, but also in pathological bone loss. Therefore, the balance between RANKL and OPG determines bone resorption. The key factor for osteoclast proliferation and differentiation is signaling through RANK -RANKL.

RANK, present on the preosteoclast and osteoclast surfaces by itself does not have intrinsic enzymatic activity and needs to recruit adaptor proteins such as TRAF, especially TRAF 6.[3] Binding of this protein to RANK induces trimerization of TRAF 6 leading to the activation of NF-KB and MAPK inducing JNk and p38.[46] Therefore, TRAF 6 acts downstream of RANK inducing in preosteoclasts, the expression of target gene activator AP-1 and NFATc1leading to preosteoclasts fusion and to osteoclast differentiation and in osteoclastic bone resorption by inducing membrane ruffling action and actin ring formation through the activation of c-Src signaling cascade.

Interestingly, IL-1 is a stimulator of TRAF 6 expression on the osteoclast and therefore activates NF-kB and MAPKs and synergises with RANKL, thereby potentiating the RANK-RANKL signaling cascade, where as IFN-γ is known to down regulate TRAF 6 by proteossomal degradation aborting osteoclast formation [Figure 4].[47]

Figure 4.

Role of activated t cells in osteoclast formation and function

In addition to the signaling pathways mentioned above, TRANCE stimulation also triggers reactive oxygen species (ROS) production. ROS, such as superoxide anions, hydroxyl radicals, and H₂O₂, have been associated with many cellular responses including metabolic bone diseases.

Recent reports suggest that ROS act as a key second messenger during osteoclastogenesis, such that TRANCE stimulation induces the production of ROS in OC precursors via the ROS-inducing factor NADPH oxidase (Nox) 1. ROS may potentiate mitogen-activated protein kinase (MAPK) activation by inactivating protein tyrosine phosphatase activity in a manner similar to mechanisms recently described in B cells.[3]

Osteoclast proliferation also depends on the presence of M-CSF produced by other cells and by osteoblasts, that upon binding to its receptor, c-fms, at the surface of pre-osteoblast cells, activates and intracellular cascade that ultimately leads to proliferation and survival of osteoclasts.[48] In this way, the osteoblasts produce the key factors RANKL and M-CSF that promote osteoclasts proliferation and differentiation.

Although RANKL and M-CSF are essential factors and key players in osteoclast differentiation, it was hypothesized that costimulatory molecules such as immunoglobulin-like receptors identified in osteoclast-precursor cells are the third essential factors required for osteoclastogenesis. Among these molecules, OSCAR and (PIR) -A signal FcRγ.[49] OSCAR, PIR-A, TREM-2 and SIRP, which signal through DAP 12, activate an intracellular calcium signaling cascade, thus, promoting dephosphorylation of calcineurin, there by activating the auto amplification of NFATc1 which consequently promotes osteoclast differentiation. Therefore, FcRγ and DAP 12 are adaptor molecules that associate with immunoglobulin like receptors helping their expression and transducing signals through ITAM.[50]

RANKL also interacts with ITAM by inducing its phospho-rylation, thus increasing the expression of immunoglobulin-like receptors and enhancing ITAM signal. ITAM mediated signals cooperate with RANK to stimulate calcium signaling through ITAM phosphorylation and resulting in activation of SYK and PLCg. Further modulation of osteoclastogenesis is provided by Toll-like receptors.TLR expression was detected on bone cells and direct signaling through TLR activates a signaling cascade mediated by TRAF 6 leading to the activation of transcription factors such as NF-kB and AP-1 and also synthesizes and release proinflammatory cytokines.

On the other hand, osteoblasts expressed TLRs-TLR 4, TLR 5, and TLR 9, and exposure of these cells to PAMP induces the secretion of proinflammatory cytokines. Therefore, TLR contribute to the modulation of osteoclasts together by modulating the function of both osteoblasts and osteoclasts.[6]

Finally, bone also interacts with B cell biology as RANKL is known to influence B lymphocyte development, increasing pro-B cell proliferation. As discussed, the immune system has several cross talk points with skeletal system among which T and B lymphocyte interactions with bone cells, cytokine and chemokine dependent bone resorption, TLR signaling and costimulating molecules. Together these factors modulate bone remodeling.

Osteoimmunological interactions between osteoblasts and osteoclasts: The immune and skeletal systems share cytokines, receptors, signaling molecules and transcription factors, all of which cooperatively regulate osteoclasts and osteoblasts as well as their interactions. Osteoblasts regulate osteoclastogenesis through RANKL-RANK-OPG interactions, M-CSF-c-fms interactions and immunoglobulin-like receptors associated with ITAM harboring adaptor molecules, such as DAP12 and FcRg. Semaphorin 6D and its receptor plexin A1, and ephrin receptor B4 and ephrin B2 were newly identified as mediators of osteoblast-osteoclast interactions. Transmembrane receptor EphB4, in osteoblasts and its ligand ephrinB2 in osteoclasts signaling leads to the inhibition of osteoclast differentiation by blockage of c-fos and NFATc1 transcriptional cascade and stimulate osteoblast differentiation by inducing osteogenic regulatory genes. There are extensive signaling pathways in osteoclasts. RANK and Ig-like receptors stimulate downstream signaling cascades such as TRAF 6, NF-kB, MAPKs, AP1, calcineurin and NFATc1, which are influenced by number of immunoregulatory molecules including CD 40 ligand, IL-1, IFN-β, IFN-γ, TNF and lipopolysaccharide. DC-STAMP and ATP6V0D2 (a gene , an essential component of the osteoclast specific protein pump that mediates extracellular acidification in bone resorption) are necessary for the fusion of osteoclast precursor cells. P I3K-AKT and GRB2-ERK pathways are important for the proliferation and survival of the osteoclast lineage where as Vav3, cSrc, and cCbl are included in the molecules required for cytoskeletal reorganization and bone resorbing osteoclasts. [Figure 5].[3] Osteoclast activity is dependent on acidifying proton pump ATP61 and chloride channel CIC7 as well as matrix-degrading enzyme such as cathepsin K and MMP-9 [Figure 6].[34]

Figure 5.

Osteoimmunological interactions between osteoblast and osteoclast

Figure 6.

The immune interactions

OSTEOIMMUNOLOGY AND PERIODONTITIS

Periodontitis is caused by a host response to the presence of gram negative bacteria (Actinobacillus actinomycetem-comitans, Porphyromonas gingivalis) or their products that invade connective tissue and alveolar bone. Since the presence of bacteria is required, but not sufficient to trigger periodontitis, the recognition of microbial components as ‘danger signals’ by host cells and subsequent production of inflammatory mediators is an essential step in pathogenesis of periodontitis.

The critical aspect of periodontitis is uncoupling; where the bacteria induced bone loss is not followed by an equivalent amount of new bone formation. There is a failure to form an adequate amount of new bone following resorption resulting in net bone loss. It is possible that under conditions where inflammation is in close proximity to and along the bone, it will affect osteoclast numbers or function and interfere with coupling process[51] [Figure 7].[51]

Figure 7.

Spatial relationship between an inflammatory infiltrate and periodontal bone loss

One of the critical components of host responses to bacteria or their products is a family of receptors called the Toll like receptors (TLRs). The TLRs activate the immune response binding to various microbial components. After TLR, an intracellular signaling cascade leads to activation of transcription factors, such as NF-kB, AP-1, will stimulate pro-inflammatory cytokine synthesis and production, leading to indirect tissue damage through excess inflammation. NF-kB is a redox-sensitive transcription factor which sits in the cytoplasm of cells bound to its inhibitor I-kB. When that complex is phosphorylated, the NF-kB is released from its inhibitor and is small enough to cross the nuclear membrane, bind to DNA and then to stimulate transcription to proinflammatory cytokines and subsequent production of various cytokines and chemokines.[52] Recent studies describe a role for both TLR-2 and TLR-4 in the recognition of A. a – comitans, whose impact range from stimulating cytokine expression and inflammatory cell migration to osteoclastogenesis and alveolar bone loss. Besides TLRs, the nucleotide-binding oligomerization (NOD) receptors and the inflammasome system have been pointed out as potential accessory molecules that trigger the host response against periodontal pathogens.[51,53]

One of the major signaling pathways involved in osteoclastogenesis is TRANCE-RANK-OPG axis. RANKL/OPG ratio, in tissues seems to be a major indicator of potential bone resorption[37][Figure 8].[11] The RANKL/OPG axis greater than 1 predominates in chronic periodontitis lesion while a ratio of 0.5 or less is found in chronic gingivitis lesion.[54]

Figure 8.

RANKL-RANK-OPG Axis

The RANKL-RANK-OPG axis clearly is involved in the regulation of bone metabolism in periodontitis in which an increase in relative expression of RANKL or a decrease in OPG can tip the balance in favor of osteoclastogenesis and the resorption of alveolar bone that is the hallmark of periodontal disease.Interference with the RANKL-RANK-OPG axis may have a protective effect on bone loss.

Potential intervention strategies

Current treatment for periodontitis often relies on mechanical procedures and neglects the immune cells. Newer treatments for periodontal diseases must address the major immune cell contribution to periodontal bone resorption. Therefore, an important emphasis of new therapies should involve the development and evaluation of therapeutic strategies to treat immune cell-mediated periodontal disease. In recent years, biological agents and molecular inhibitors were developed for the treatment of inflammatory diseases accompanied by bone destruction which target molecules expressed in T cells and osteoclasts such as a specific antibody to RANKL, a fusion protein, comprising CTLA-4 and human immunoglobulin, certain drugs which are under development are inhibitors of IL 17, NFkB inhibitors and SYK inhibitors, in vitro suppression of NFATc1, Bisphosphonates, Cyclosporin A,TNF α antagonists, and drugs inhibiting Cathepsin K.

Although it is difficult to specifically target signaling molecules and transcription factors in therapy, it is now clear that multiple novel therapies may be implemented which more directly address the new periodontal disease pathogenesis concept.[55]

Osteoimmunoendocrinology

Apart from immune cells, adipocytes also play role on immune as well as skeletal systems [Figure 9].[16] Adipocytes present in adipose tissue releases adipokines such as leptin, adiponectin, visfatin, and resistin. Adipocytes also secrete IL-1 and TNF-α which are positive stimulus for the expression of leptin. Leptin, a major adipokine, is also released by immune cells and osteoblasts. It regulates the body weight through appetite suppression and increased energy expending. Induces peripheral blood mononuclear cells to secrete pro-inflammatory cytokines, such as IL-6, TNF-α and IFN-α and itself acts as a pro-inflammatory cytokine by activating monocytes, modulating phagocytosis of macrophages and stimulates neutrophil production of reactive oxygen species several cells involved in the immune response. Leptin plays a major role in bone homeostasis, reproduction, development, hematopoiesis, and angiogenesis interferes in the T-cell balance inducing T-cell activation toward a Th₁ type response.

Figure 9.

Bone environment

Leptin exerts two opposing mechanisms on bone metabolism:

1. Acts on bone receptors that promote the development of osteoprogenitor cells and stimulate osteoblasts to form new bone.

2. Acts through CNS decreasing osteoblast activity.

Leptin increases bone formation by enhancing human osteoblast proliferation, collagen synthesis and mineralization and positively favoring OPG/RANKL ratio through down regulation of RANKL. Leptin exerts antiapoptotic role by reducing the mRNA levels of Bax/Bcl-2, which facilitate the transition of mature osteoblasts to osteocytes.

The dominant regulator of adipocytes is PPAR-γ which is also an essential regulator of insulin metabolism too. Unsaturated fattyacids, eicosanoids, metabolites of linoleic acid activates PPAR-γ, which in turn down regulates cytokines, chemokine secretion, expression of costimulatory molecules, thus exerting an anti-inflammatory activity. Therefore PPAR-γ not only has a direct effect on osteoclast and osteoblast differentiation but also have indirect actions upon bone remodeling through regulation of cytokine and chemokine secretion.[56]

Thus, understanding the basis of osteoimmunology and application of these principles targeting the molecular mechanisms though at a preliminary phase, aspires immune modulation.

CONCLUSION

The pathogenesis of periodontal diseases involves both the inflammatory as well as immune mechanisms. The amplification and propagation of the inflammatory response through gingival tissue is critical to the pathogenesis of periodontitis. However, it is the spread of the response to areas adjacent to alveolar bone that drives the cellular machinery involved in bone loss. Characterization of the functions of TRANCE-RANK-OPG axis has contributed significantly to the emergence of osteoimmunology, specifically with respect to examination of the interplay between active immunity and maintenance of bone homeostasis. Though modulating the immune system is a delicate work, targeting the alterations of this axis may form the basis for rational drug therapy in treating the periodontal pathosis effectively. This review has highlighted on the critical aspects in the field of osteoimmunology, the osteblast-osteoclast cross talk and the new and innovative therapeutic interventions in treating periodontal disease.

ABBREVIATIONS

-AP-1-Activator Protein, ATF-Activating Transcription Factor, B7-1 and B7-2-Homologous costimulatory ligands found on antigen presenting cells, Bax/Bcl-2-Bcl-2-associated X protein/B cell lymphoma 2, c-fms (cellular-full mitochondrial sequence )/ MCSFR-Macrophage colony stimulating factor receptor, c-Fos is a cellular proto-oncogene belonging to the immediate early gene family of transcription factors, c-Src-proto-oncogenic tyrosine kinase, src-‘sarcoma’, cCbl -Casitas B-lineage lymphoma, CTLA-4- Cytotoxic T lymphocyte Asssociated, DAP 12 -DNAX-activation protein 12, DNA X-gene that codes for the T and γ subunits of the DNA clamp loader of prokaryotes, DC-STAMP- Dendritic cell-specific trancemembrane protein, ERK-Extracellular-signal-regulated kinase, FRA-1 and 2-Fos related antigen 1 and 2, Fos- FBJ murine osteosarcoma viral oncogene. The human oncogene c-fos is homologous to the Finkel-Biskis-Jinkins (FBJ) murine osteosarcoma virus oncogene. GM-CSF-Granulocyte -macrophage colony stimulating factor, GRB2-Growth Factor Receptor Bound Protein 2, HSC-Hematopoietic stem cells, ITAM-Immunoreceptor tyrosine-based activation motif, JNk-Jun N-terminal kinases, MITF-Microphthalmia-associated transcription factor, MAPK-Mitogen activated kinases, M-CSF-Macrophage-Colony Stimulating Factor, NF-kB-Nuclear factor kappa-light-chain-enhancer of activated B cells, NFATc1-Nuclear factor of activated T cells and cytoplasmic calcineurin dependent 1, NODP-Nucleotide-binding oligomerization, OSCAR-Osteoclast associated receptor, OPG-Osteoprotegerin, OPGL-Osteoprotegerin ligand, ODF-Osteoclast differentiation factor, PU.1- Purine rich, protein in humans coded for SPI 1 gene, PAMP-Pathogen associated molecular patterns, P I3K-Phosphoinositide-3-kinase, (PIR) -A- Paired immunoglobulin receptor, PPAR-γ -Peroxisome proliferative activated receptor γ, PLCγ-Phospholipase Cγ, RANKL-Receptor activator of nuclear factor kappa -light-chain-enhancer of activated B cell ligand, RANK-Receptor activator of nuclear factor kappa-light -chain-enhancer of activated B cell, SIRP- signal-regulatory protein, SOCS-Suppressors of cytokine signaling proteins, SYK- spleen tyrosine kinase, TRANCE- (R)-TNF related activation induced cytokine-receptor, TRAF-TNF receptor associated factor, TREM-2- Trigger receptors expressed on myeloid cells.