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Keywords:

  • Bone;
  • PTH

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Lymphocytes Relevant to Bone
  5. T Cells Mediate the Effects of Estrogen Deficiency in Bone
  6. Role of T Cells in the Catabolic and Anabolic Effects of PTH in Bone
  7. Role of T Cells in the Expansion of HSCs Induced by PTH
  8. Evolutionary and Teleologic Considerations
  9. Conclusions
  10. Disclosure
  11. References

Osteoimmunology is a field of research dedicated to the study of the interactions between the immune system, the hemopoietic system and bone. Among the cells of the immune system that regulate bone cells and the hemopoietic function are T lymphocytes. These cells secrete inflammatory cytokines that promote bone resorption, as well as Wnt ligands that stimulate bone formation. In addition, T cells regulate bone homeostasis by cross talking with BM stromal cells and osteoblastic cells via CD40 ligand (CD40L) and other costimulatory molecules. This article describes the immune cells relevant to bone and the hemopoietic function, reviews the role of lymphocytes as mediators of the effects of PTH and estrogen in bone and the hemopoietic system and discusses the implication of osteoimmunology for transplant medicine.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Lymphocytes Relevant to Bone
  5. T Cells Mediate the Effects of Estrogen Deficiency in Bone
  6. Role of T Cells in the Catabolic and Anabolic Effects of PTH in Bone
  7. Role of T Cells in the Expansion of HSCs Induced by PTH
  8. Evolutionary and Teleologic Considerations
  9. Conclusions
  10. Disclosure
  11. References

Posttransplant osteoporosis is a well-recognized complication of solid organ and bone marrow (BM) transplantation [1]. The conditions results, in part, from the negative effects of glucocorticoids and calcineurin inhibitors on bone. Hypogonadism, the secondary and tertiary hyperparathyroidism of patients undergoing kidney transplant and alterations of vitamin D metabolism often present in patients with end stage liver disease, represents additional pathogenetic factors. However, changes in the immune system and the chronic inflammatory state that follow transplantation play a relevant contributory role. The specific mechanisms by which changes in the immune system induce bone loss in transplant patients remain largely unknown.

The association between inflammatory and autoimmune diseases and osteoporosis has long been recognized [2, 3], leading to the assumption the cells of the immune system may participate in the control of bone formation and bone resorption. The molecular links between the immune system and bone have emerged clearly only with the discovery of RANKL and its receptor RANK. These molecules were first identified as factors expressed on T cells and dendritic cells (DCs), respectively. RANKL and RANKL were shown to augment the ability of DCs to stimulate naive T cell proliferation and enhance DC survival. They were later identified as the key osteoclastogenic molecules. The discovery that T cell produce RANKL and that T cell produced RANKL contribute to the bone loss observed adjuvant arthritis [2] has later provided the first incontrovertible evidence about the capacity of T cells to cause bone loss. It is now clear that a host of immune factors including costimulatory receptors, lymphocyte derived cytokines such as IL-17, TNF, IFNγ and RANKL and T and B lymphocytes play a fundamental role in the regulation of bone cell development and bone turnover, and in the pathogenesis of bone diseases.

Lymphocytes Relevant to Bone

  1. Top of page
  2. Abstract
  3. Introduction
  4. Lymphocytes Relevant to Bone
  5. T Cells Mediate the Effects of Estrogen Deficiency in Bone
  6. Role of T Cells in the Catabolic and Anabolic Effects of PTH in Bone
  7. Role of T Cells in the Expansion of HSCs Induced by PTH
  8. Evolutionary and Teleologic Considerations
  9. Conclusions
  10. Disclosure
  11. References

The effects of B cells and T cells in bone and the consequences of the lack of B cells and T cells in bone are summarized in Tables 1 and 2.

Table 1. Effects of T and B cells in bone
Cell lineageActivity/factor secretedEffect of OVXEffect of cPTHEffect of iPTH
  1. ? = unknown.

HSCs Expand HSCsExpand HSCsExpand HSCs
  Expand B lineage  
B cellsOPGIncrease OPG production??
 TNF, RANKL, IFNγ   
CD4+Increase bone resorption and/or bone formationIncrease number and activation stateIncrease TNF production
 TNF, RANKL, IFNγ,   
CD8+Increase bone resorption and/or bone formationIncrease number and activation stateIncrease TNF productionIncrease Wnt10b production
 TNF, RANKL, IFNγ   
Th17Increase bone resorptionIncrease number and activation stateExpand Th17 pool?
 IL-10, IL-35, TGFβ   
TregsBlock OC formationDecrease number and activation state??
 Decrease bone resorption   
Table 2. Effects of immune cells and immune cells receptors deletion on bone
Mouse strainBaseline phenotypeEffect of ovxEffect of cPTHEffect of iPTH
  1. ? = unknown.

WT miceNormalBone lossBone lossBone growth
T cell−/− mice (nude mice)Low bone massNo bone lossNo bone lossBlunted/no bone anabolism
 Increased bone resorption   
Αβ null mice (TCRβ−/− mice)Low bone massNo bone lossNo bone lossBlunted bone anabolism
 Increased bone resorption   
μMT/μMT mice (Mature B cell−/−)Low bone massBone loss??
 Increased bone resorption   
CD40L−/− miceLow bone massNo bone lossNo bone lossBlunted bone anabolism
 Increased bone resorption   
 Decreased osteoblast number and life span   
Wnt10b−/− miceVery low bone mass??No bone anabolism
 Decreased bone formation   
IL-17R−/− miceLow bone massControversial??
 Increased bone resorption   
Treg TG miceHigh bone massNo bone loss??
 Decreased bone resorption   

Activated T cells promote bone loss in inflammatory diseases such as rheumatoid arthritis [2] and periodontitis [3]. Activated T cells also play a critical role in bone metastasis [4] and postmenopausal osteoporosis [5]. Conversely, resting T cells may contribute to dampen bone resorption in vivo [6]. Activated T cells regulate bone homeostasis via direct interactions with BM stromal cells (SCs) and osteoblasts (OBs) [7] and by releasing osteoclastogenic cytokines and Wnt ligands. More recently has become clear that T cell contribute to the regulation of the hemopoietic stem cell (HSC) niche [8], thus revealing a novel regulatory loop.

Another novel link between T cells and bone has been provided by studies in germ free mice.

The gastrointestinal tract is colonized for life with 100 trillion indigenous bacteria, creating a diverse ecosystem known as the microbiota, whose contributions to human health are profound [9]. Mice kept in germ-free conditions have a blunted T cell activity and a significantly higher bone density than controls [10].

The most osteoclastogenic subsets of T cells are Th17 cells [11] as these populations produces high levels of IL-17, RANKL and TNF and low levels of IFNγ [11, 12]. The development of Th17 is promoted by TGF-β, IL-1 and IL-6. The Th17 cells signature cytokine IL-17 and IL-23, a cytokine that activates Th17, contribute to the bone pathology in various models of inflammatory arthritis. This occurs in part by upregulating the RANKL/OPG ratio. Blocking IL-17A or IL-23 in collagen-induced arthritis reduces disease, and IL-17 deficient mice are resistant to arthritis. In addition, IL-17 induces production of TNF and IL-1β in macrophages, and synergizes with IL-1β and TNF in many cell backgrounds to increase the synthesis of pro-inflammatory effectors. IL-1β, IL-6 and TNF are all associated with increased osteoclastogenesis following ovx. Thus, IL-17A and related pro-inflammatory cytokines have potent bone-destructive capacities in RA and other bone-destructive settings.

The most bone-sparing population of T cells are regulatory T cells (Tregs), a population of CD4+ T cells defined by the expression of the transcription factor FoxP3. It is now clear that Tregs regulate bone turnover in health and disease [13-15]. For example, studies have shown that Tregs suppress bone resorption in ovariectomy (ovx) induced bone loss and in models of rheumatoid arthritis by secreting anti-osteoclastogenic factors [13-15]. It is indeed remarkable that the cytokines produced by Tregs to repress effector T cells also possess a strong anti-osteoclastogenic activity.

Intravital microscopy studies have disclosed that Tregs are not randomly distributed in the BM, but rather reside in close proximity with endosteal bone surfaces and osteoclasts [16]. Osteoclasts selectively recruit and activate CD8 T cells [17, 18]. The osteoclast-recruited CD8 T cells express CD25 and FoxP3, and therefore are defined as osteoclast-induced regulatory CD8 T cells [19]. It is presently unknown if CD8+ Tregs home preferentially to bone as CD4+ Tregs. Together, these studies suggest the existence of a novel regulatory loop, whereby osteoclasts induce Tregs and Tregs blunt osteoclastic bone resorption.

B cells have recently been directly implicated in the regulation of bone resorption as they represent a major source of osteoprotegerin (OPG), a soluble decoy receptor for RANKL. The major sources of OPG were initially thought to be osteoblasts and its stromal precursors [20]. However, an analysis of the bone phenotype of B cell knockout (KO) (μMT/μMT) mice has show that B cells and plasma cells are the dominant producers of OPG in the bone microenvironment in vivo [6]. Among cells of the B lineage, BM plasma cells are those that secrete the highest concentrations of OPG. Plasma cell production of OPG is in fact five- to sixfold higher than mature B cells. However, since plasma cells are few relative to the number of mature B cells and B cell precursors, it is estimated that plasma cells contribute to ∼20% of total B-lineage OPG production [6].

Interestingly, it has previously been reported that B cell OPG production by human tonsil derived B cells could be significantly upregulated by the activation of CD40 signaling by an activating antibody [21]. CD40 is a costimulatory molecule constitutively expressed by professional antigen presenting cells (APC) such as macrophages, dendritic cells and B cells, and partners with a receptor that is transiently upregulated on the surface of activated T cells. Mouse splenic B cells likewise produced upregulated concentrations of OPG in response to a recombinant soluble ligand to CD40 (sCD40L). In line with these data both CD40 and CD40L KO mice displayed an osteoporotic phenotype and a significant deficiency in BM OPG concentrations. This deficiency in total OPG further correlated with a B cell specific deficiency in OPG production [22]. Thus the emerging data suggest that the B lineage, rather than the osteoblast lineage, is likely the major source of OPG in the bone microenvironment and that T cell signaling to B cells through the costimulatory molecules CD40L and CD40 play an important role in regulating basal osteoclast formation and in regulating bone homeostasis.

Recently, it has been shown that mice lacking RANKL in B lymphocytes were partially protected from the bone loss caused by ovx due to a failure of ovx to increase osteoclast numbers and bone resorption [23]. By contrast, deletion of RANKL from B cells had no impact on bone mass in estrogen-replete mice [23]. Since mice lacking mature B cells are not protected against ovx-induced none loss [22], the data suggest that RANKL might be produced mostly by B cell precursors.

These findings may provide in part a novel explanation for the propensity for osteopenia and osteoporosis development in numerous pathological conditions in which altered immune function or immunodeficiency in B cells and/or T cells results. Such conditions include solid organ and bone marrow transplantation and patients treated with immunosuppressive agents.

T Cells Mediate the Effects of Estrogen Deficiency in Bone

  1. Top of page
  2. Abstract
  3. Introduction
  4. Lymphocytes Relevant to Bone
  5. T Cells Mediate the Effects of Estrogen Deficiency in Bone
  6. Role of T Cells in the Catabolic and Anabolic Effects of PTH in Bone
  7. Role of T Cells in the Expansion of HSCs Induced by PTH
  8. Evolutionary and Teleologic Considerations
  9. Conclusions
  10. Disclosure
  11. References

Menopause is followed by a period of rapid bone loss that is central to the pathogenesis of postmenopausal osteoporosis. The effects of menopause are modeled by ovx, a procedure that induces rapid bone loss by increasing osteoclast formation [24] and lifespan [25, 26]. This initial phase of bone loss is followed by a slower but more prolonged loss of mainly cortical bone due to incomplete refilling of the resorption cavities due to insufficient osteoblast activity and lifespan [27]. The net bone loss caused by ovx is limited by an increase in bone formation resulting from stimulated osteoblastogenesis [28]. This compensation is fueled by an expansion of the pool of BM SCs, increased commitment of such pluripotent precursors toward the osteoblastic lineage [28] and enhanced proliferation of early osteoblast precursors [29]. An increase in osteoblast apoptosis [30, 31] together with an increased secretion of cytokines which suppress bone formation such as IL-7 and TNF prevents bone formation from increasing as much as bone resorption.

T cells play a key role in the mechanism of ovx induced bone loss. In support of this hypothesis are the failure of ovx to induce trabecular and cortical bone loss in T cell deficient mice [32-36], mice treated with anti T cells antibodies [7], or Abatacept [37] (a drug which induces T cell anergy and apoptosis) and mice lacking the T cell costimulatory molecule CD40L [7]. By contrast, Lee et al. [38] showed that nude mice are protected against the loss of cortical, but not trabecular bone induced by ovx. In the same study other strains of T cell, and T and B cell deficient mice were found to lose either trabecular or cortical bone after ovx [38]. The discrepancy between reports from us and other attesting to a role of T cells [7, 32, 33, 35, 37, 39, 40] and that of Lee at al. [38] is likely explained by differences in the experimental design, the lack of B cells in some models, and compensatory mechanisms such as an increase in natural killer (NK) cells producing the osteoclastogenic factor IL-17 [41].

Both CD4+ and CD8+ cells have been found to play a role in ovx induced bone loss. The differentiation of the Th17 subset of CD4+ cells is inhibited by estrogen via a direct effect mediated by ERα [42]. The specific contribution of Th17 in ovx induced bone loss remains controversial as one study reported that IL-17R null mice are more susceptible to ovx induced bone loss than controls [43], while another group found mice lacking IL-17R and mice treated with anti IL-17 antibody to be protected against ovx induced bone loss [44]. Because IL-17 signaling has been documented to produce bone loss only in the context of severe inflammation, the fact that it is an essential component of ovx induced bone loss is apparently surprising. However, it should be underscored that postmenopausal osteoporosis is considered to be a pro-inflammatory disease with many parallels to RA [24].

More abundant information is available about Tregs. Firstly, estrogen increases the relative number of Tregs [45]. Moreover, transgenic mice overexpressing Tregs develop high bone mass as they age due to inhibition of bone resorption, and are protected against ovx induced bone loss [15]. Importantly, transfer of Tregs into T cell deficient mice increases bone mass, indicating that Tregs directly affect bone homeostasis without the need to engage other T cell lineages [15].

Two mechanisms have been described to explain how T cells contribute to ovx induced bone loss. The first involve an increase in T cell activation leading to increased production of TNF by BM T cells. The second is a regulatory cross talk between T cells and SCs resulting in enhanced production of osteoclastogenic cytokines by SCs (Figure 1).

image

Figure 1. Schematic representation of the role of T cells in the mechanism by which ovx promotes osteoclastogenesis, osteoblastogenesis and hemopoiesis. Estrogen deficiency promotes T cell activation by increasing the interaction of antigen (Ag) loaded MHC molecules with on bone marrow macrophages (BMM) and dendritic cells (DC) with the T cell receptor (TCR). The Ags are likely to be non-self peptides derived from the intestinal macrobiota. T cell activation also requires at least two costimulatory signals provided by the binding of BMM and DC expressed CD40 and CD80 to the T cell surface molecules CD40L and CD28, respectively. A critical upstream event is the increased production of reactive oxygen species (ROS) which activate DCs by increasing their expression of CD80. The expansion of T cells in the BM is partially driven by an ovx-induced increase in the thymic output of naïve T cells. Activated T cells secrete TNF that stimulates osteoclasts formation primarily by potentiating the response to RANKL. In addition, T cell expressed CD40L and DLK1/FA-1 increase the osteoclastogenic activity of SC by blunting their secretion of OPG and augmenting their production of RANKL, M-CSF and other pro-inflammatory factors. Reproduced with permission from Pacifici [5].

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Increased T cell production of TNF in response to ovx is due to an increased number of TNF producing T cells [33] and enhanced production of TNF per cell [7, 46]. Ovx also increases the population of premature senescent CD4+CD28− T cells [46], a lineage that produces high levels of TNF. Studies have also shown that menopause increases T cell activation and T cell production of TNF and RANKL in humans [47]. The mechanism by which estrogen deficiency expands the pool of TNF producing T cells involves reactivation of thymic function and induction of T cell activation in the BM. T cell activation is driven by enhanced Ag presentation by macrophages and dendritic cells (DCs) [48, 49].

The most upstream effects of ovx in the BM are to stimulate the production of reactive oxygen species (ROS) and to impair the generation of antioxidants [27, 37, 50, 51]. In response to ovx, ROS are produced by most BM cells including T cells [52]. ROS play an important role in postmenopausal bone loss by generating a more oxidized bone microenvironment [53, 54]. Multiple enzymatic pathways regulate the intracellular redox state through modulation of ROS levels [55]. Ovx blunts the BM levels of GSH, a critical ROS scavenger, and reduces expression of APE1/Ref-1 and Prx-1 proteins which collectively limit the production of intracellular ROS [56].

ROS have important direct effects on osteoblasts and osteoclasts that have been addressed elsewhere [57-59]. However, additional pivotal effects of ROS include expanding the pool of mature DCs that express the costimulatory molecule CD80, and increasing DC mediated Ag presentation [37]. Antioxidants potently inhibit DC differentiation and ability to activate T cells [60, 61] in part by suppressing expression of MHC class II and costimulatory molecules in response to antigen [62]. N-Acetyl-cystein (NAC), which acts as an intracellular scavenger of ROS by restoring intracellular concentrations of glutathione, can block DC maturation [63] and DC-mediated T cell activation [64].

In vivo support for a role of ROS is provided by experiments demonstrating that administration of antioxidants prevents ovx induced bone loss [37, 50, 57], while depletion of glutathione by buthionine sulfoximine (BSO), which inhibits glutathione synthesis, enhances bone loss [50]. Bone loss caused by BSO has significant similarities to bone loss induced by estrogen deficiency, as both processes are TNF-dependent [65].

It is now clear that direct interactions between SCs and T cells mediated by the CD40L/CD40 system are important for SC function and ovx induced bone loss. CD40L is a key surface ligand expressed on T cells [66]. CD40L binds to CD40 [67] and several integrins [68, 69]. CD40 is expressed on antigen presenting cells, hemopoietic progenitors and SCs and their osteoblastic progeny [70, 71]. CD40L has been linked to postnatal skeletal maturation because T cells, through the CD40L/CD40 system, promote production of the anti-osteoclastogenic factor OPG by B cells [6]. Consequently, CD40L deficient mice attain a reduced peak bone volume due to exaggerated bone resorption [6]. Low bone density has also been found in children affected by X-linked hyper-IgM syndrome, a condition in which CD40L production is impaired due to a mutation in the CD40L gene [72]. However, mice lacking T cell expressed CD40L are protected against parathyroid hormone (PTH) induced bone loss [71] and ovx induced bone loss [7], demonstrating that CD40L exerts anti resorptive activities in unstimulated conditions, but promotes bone resorption under conditions of bone stress.

Studies with CD40L null mice and with WT mice treated with anti CD40L Ab have revealed that silencing of CD40L completely prevents ovx induced bone loss [7]. A dual mechanism was shown to be involved. Firstly, silencing of CD40L blocks the activation of T cells and the resulting production of TNF. Secondly, T cell expressed CD40L and direct cell-to-cell contact between T cells and SCs is required for ovx to increase the proliferation and the differentiation of SCs and their capacity to support osteoclast formation through enhanced production of M-CSF and RANKL, and diminished secretion of OPG. Thus, a critical additional mechanism by which T cells disregulate bone homeostasis in ovx mice is through a CD40L mediated cross talk between T cells and SCs that results in enhanced osteoclastogenesis and, to a lesser degree, enhanced osteoblastogenesis.

Role of T Cells in the Catabolic and Anabolic Effects of PTH in Bone

  1. Top of page
  2. Abstract
  3. Introduction
  4. Lymphocytes Relevant to Bone
  5. T Cells Mediate the Effects of Estrogen Deficiency in Bone
  6. Role of T Cells in the Catabolic and Anabolic Effects of PTH in Bone
  7. Role of T Cells in the Expansion of HSCs Induced by PTH
  8. Evolutionary and Teleologic Considerations
  9. Conclusions
  10. Disclosure
  11. References

PTH plays a critical regulatory role in calcium metabolism. Secreted in response to small decrements in serum ionized calcium, this hormone defends against hypocalcemia, in part by stimulating bone resorption and thereby the release of calcium from the skeleton. In addition to its role in regulating the level of serum calcium, sustained overproduction, or in vivo continuous infusion of PTH (cPTH) causes bone loss by stimulating bone resorption, a phenomenon only in part mitigated by a concomitant stimulation of bone formation. Primary hyperparathyroidism is common cause of cortical and trabecular bone loss in aging women while secondary hyperparathyroidism is a common cause of bone disease in men and women with end-stage renal disease and vitamin D deficiency.

However, when injected daily in humans and animals at low dose, a regimen known as intermittent PTH (iPTH) treatment, the hormone stimulates trabecular and cortical bone formation, leading to marked increases in bone volume and strength. Attesting to potency, iPTH has been shown to decrease the risk of fractures in humans, and is an FDA approved treatment modality for postmenopausal women and men with osteoporosis [73, 74]. The PTH receptor PPR (also known as PTH1R) was first discovered in SCs, osteoblasts and osteocytes. Accordingly PTH induced bone loss has been ascribed mainly to a stimulatory effect of PTH on the production of RANKL by osteoblastic cells. However, PPR is also expressed in lymphoid cells, suggesting that lymphocytes may contribute to the effects of PTH in bone.

Unexpectedly, studies have shown that cPTH fails to induce osteoclast formation, bone resorption and cortical bone loss in mice lacking T cells or T cell expressed CD40L [71]. This is because direct activation of CD40 signaling in SCs and osteoblasts by T cell expressed CD40L increases SC proliferation, differentiation and life span. It also increases SCs production of RANKL and M-CSF in response to PTH [71]. Therefore, SCs from T cell deficient mice or mice lacking T cell expression of CD40L have a lower capacity to support osteoclast formation in vivo and in vitro [71]. Moreover, cPTH stimulates T cell production of TNF [75]. TNF further increases the SC production of RANKL, and upregulates the expression of CD40 in SCs, thus increasing their response to T cell expressed CD40L (Figure 2). Attesting to the relevance of T cell produced TNF, cPTH fails to induce bone loss and stimulate bone resorption in mice specifically lacking T cell TNF production [75]. Deletion of the PTH receptor PPR in T cells blunts the stimulation of bone resorption induced by cPTH without affecting bone formation, thus blocking cortical bone loss and converting the effects of cPTH in trabecular bone from catabolic to anabolic [75]. These findings demonstrate the critical relevance of direct PPR signaling in T cells.

image

Figure 2. Schematic representation of the role of T cells in the mechanism by which continuous PTH induces bone loss. PTH binding to PPR in T cells stimulates the production of TNF. This cytokine increases CD40 expression by SCs. Binding of CD40 by T cell expressed CD40L increases SC sensitivity to PTH resulting in enhanced SC production of RANKL and diminished secretion of OPG in response to PTH. T cell produced TNF further stimulates osteoclast formation through its direct effects on maturing osteoclast precursors. The red arrows represent the main modifications induced by activation of PPR signaling in T cells.

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T cells are required not only for cPTH to induce bone loss but also for iPTH to exert its bone anabolic effect. In fact, T cell null mice and mice with a silent PTH receptor in T cells display a blunted increase in bone formation and trabecular bone volume in response to iPTH [76, 77]. Furthermore, adoptive transfer of T cells into T cell deficient mice restores a normal bone anabolic response to iPTH. Mechanistic studies have disclose that in the absence of T cells iPTH is unable to increase the commitment of SCs to the osteoblastic lineage, induce OB proliferation and differentiation, and mitigate OB apoptosis. All of these action are due to the capacity of iPTH to stimulate BM CD8+ T cells production of Wnt10b [76], a Wnt protein which activates Wnt signaling in SCs and OBs, thus increasing OB proliferation, differentiation and life-span. The key role of T cell produced Wnt10b was revealed by the hampered effect of iPTH on bone volume in global Wnt10b null mice [77] and T cell null mice reconstituted with T cells from Wnt10b−/− mice [76].

Together the data indicate that CD8+ T cells potentiates the anabolic activity of iPTH by providing Wnt10b, which is a critical Wnt ligand required for activating Wnt signaling in osteoblastic cells Therefore in the absence of CD8+ cells, stimulation of osteoblastic cells by PTH is not sufficient to elicit maximal Wnt activation due to the lack of a critical Wnt ligand (Figure 3). The residual bone anabolic activity of PTH observed in T cell deficient mice is presumably due suppressed production of the Wnt inhibitor sclerostin [78-80].

image

Figure 3. Schematic representation of the role of T cells in the mechanism by which intermittent PTH treatment stimulates bone formation. PTH stimulates T cells to secrete Wnt10b, a Wnt ligand required to activate Wnt signaling in SCs and OBs. In the presence of T cell produced Wnt10b, stimulation of osteoblastic cells by PTH result in the activation of the Wnt signaling pathway. This event leads to increased commitment of mesenchymal stem cells to the osteoblastic lineage, increased osteoblast proliferation and differentiation, and decreased osteoblast apoptosis. Reproduced with permission from Bedi [77].

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Role of T Cells in the Expansion of HSCs Induced by PTH

  1. Top of page
  2. Abstract
  3. Introduction
  4. Lymphocytes Relevant to Bone
  5. T Cells Mediate the Effects of Estrogen Deficiency in Bone
  6. Role of T Cells in the Catabolic and Anabolic Effects of PTH in Bone
  7. Role of T Cells in the Expansion of HSCs Induced by PTH
  8. Evolutionary and Teleologic Considerations
  9. Conclusions
  10. Disclosure
  11. References

An interesting and useful effect of PTH is that of increasing the HSC pool through regulatory actions on the HSC niche. The HSC niche comprises a variety of cells including SCs and OBs [81]. Early studies had linked this activity of PTH to its capacity to increase the osteoblastic expression of the Notch ligand, Jagged1, leading to the activation of Notch signaling in HSCs in vivo [82]. Comparisons of T cell replete mice and T cell deficient mice revealed that iPTH specifically expands HSCs but does so only in mice with T cells [8]. Studies with mice lacking the expression of PPR in T cells disclosed that iPTH targeting of T cells is required for iPTH to expand HSCs [8]. HSCs comprise at least two major populations. The first is the most primitive long-term reconstituting subset of HSCs (LT-HSCs). The second is the short-term reconstituting subset of HSCs (ST-HSCs), a population that arise from LT-HSCs and possess limited self-renewal activity. iPTH specifically expands ST-HSCs without compromising LT-HSCs (Figure 4) [83].

image

Figure 4. Schematic representation of the role of T cells in the mechanism by which intermittent PTH treatment stimulates HSPCs expansion. Direct PPR signaling in T cells stimulates T cells to secrete Wnt10b, a Wnt ligand required to activate Wnt signaling. In the presence of T cell produced Wnt10b, PTH activates Wnt signaling in stromal cells and HSPCs. Wnt signaling activation also upregulates the expression of the Notch ligand Jagged 1 in SCs. These events result in HSPCs expansion.

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Activation of Wnt signaling in both HSCs and SCs is known to be required for the expansion of HSCs. This consideration led to the hypothesis that increased production of Wnt10b by iPTH-stimulated T cells might lead to Wnt activation and HSCs expansion. Indeed, studies revealed that iPTH activates Wnt signaling in SCs and HSCs by inducing the release of Wnt10b by T cells. Demonstrating the relevance of Wnt10b, iPTH failed to expand HSCs in mice with T cell specific deletion of Wnt10b [8]. In addition, iPTH failed to promote HSCs engraftment after BM transplantation in Wnt10b null mice.

Autologous bone marrow transplant carries significant risks of infection, bleeding and death. To determine whether the stimulatory effects of iPTH on HSCs might be relevant in BM transplantation studies were conducted to determine whether iPTH increases survival of lethally irradiated mice subjected to BM transplantation. These investigations revealed that iPTH treatment of either donor or recipient mice increased survival by approximately threefold, but only in the presence of T cells. Global or T cell specific deletion of Wnt10b also resulted in the failure of iPTH to improve survival after BM transplantation. Thus, T cells and Wnt10b are required for iPTH to expand ST-HSCs and increase survival after BM transplantation.

Evolutionary and Teleologic Considerations

  1. Top of page
  2. Abstract
  3. Introduction
  4. Lymphocytes Relevant to Bone
  5. T Cells Mediate the Effects of Estrogen Deficiency in Bone
  6. Role of T Cells in the Catabolic and Anabolic Effects of PTH in Bone
  7. Role of T Cells in the Expansion of HSCs Induced by PTH
  8. Evolutionary and Teleologic Considerations
  9. Conclusions
  10. Disclosure
  11. References

It is certainly interesting to entertain the question of why the immune system is involved in the mechanism of action of PTH. Clues may emerge by considering calcium requirements for T cell activation. Common consequences of severe infection and inflammation are anorexia or the inability to obtain food due to the disability caused by the disease state [83, 84]. Both results in decreased calcium intake and a tendency to hypocalcemia. The resulting secondary hyperparathyroidism, which is characterized by a continuous production of PTH, stimulates bone resorption to restore appropriate calcium levels. The capacity of cPTH to induce activated T cells to stimulate bone resorption represents a means to release calcium and phosphorus at the site of infection and inflammation, an adaptation which fulfill the requirement to provide locally the calcium and phosphorous required for T cell and other immune cells activation in conditions of starvation and fasting [85, 86]. Conversely, feeding is followed by a temporary suppression of PTH secretion, thus converting the kinetics of PTH secretion from continuous to intermittent. Under conditions of normal feeding and adequate calcium intake, stimulation of bone formation and the building up of calcium storage in the skeleton represents an appropriate and useful response. Therefore, the capacity of iPTH treatment to induce T cell production of Wnt10b and bone anabolism might represent a recapitulation of a system designed to store calcium in the skeleton during health and normal feeding.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Lymphocytes Relevant to Bone
  5. T Cells Mediate the Effects of Estrogen Deficiency in Bone
  6. Role of T Cells in the Catabolic and Anabolic Effects of PTH in Bone
  7. Role of T Cells in the Expansion of HSCs Induced by PTH
  8. Evolutionary and Teleologic Considerations
  9. Conclusions
  10. Disclosure
  11. References

Remarkable progress has been made in understanding how T and B cells participate in the regulation of bone remodeling in health and disease. It is now clear that T cells and B cells play an unexpected role in the function of the major calciotrophic hormones estrogen and PTH, and therefore in common and clinically relevant forms of bone loss such postmenopausal osteoporosis and hyperparathyroidism. Since secondary hyperparathyroidism is a common feature of end stage renal disease and the post kidney transplant period, the connection between T cells and PTH is relevant for transplant medicine as well. Much remains to be discovered on the role of T cells in modulating the effects of estrogen and PTH in humans. Additional investigations are also required to determine how the aging process affects the interaction between the immune system and bone and the relevance of these interactions for senile osteoporosis.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Lymphocytes Relevant to Bone
  5. T Cells Mediate the Effects of Estrogen Deficiency in Bone
  6. Role of T Cells in the Catabolic and Anabolic Effects of PTH in Bone
  7. Role of T Cells in the Expansion of HSCs Induced by PTH
  8. Evolutionary and Teleologic Considerations
  9. Conclusions
  10. Disclosure
  11. References