Bone and the Immune System (Osteoimmunology)

C H A P T E R

17 Bone and the Immune System (Osteoimmunology) Julia F. Charles1, Mary C. Nakamura2, Mary Beth Humphrey3 1Department

of Orthopaedics, Brigham and Women’s Hospital and Harvard Medical School, Boston, MA, United States; of Medicine, University of California at San Francisco, and San Francisco Veterans Administration Health Care System, San Francisco, CA, United States; 3Department of Medicine, University of Oklahoma Health Sciences Center and the Oklahoma City Veterans Administration, Oklahoma City, OK, United States

2Department

Osteoimmunology is defined as the study of interactions between the immune and skeletal systems. The relationship between these systems has been clear for some time given that osteoclasts are derived from immune cells (Chapter 3), and that much of the immune system develops within the bone marrow. However, interest in the field was catalyzed by the near simultaneous discovery of the role of the receptor activator of nuclear factor kappa-B (RANK)/ receptor activator of nuclear factor kappa-B ligand (RANKL); system in osteoclastogenesis by both immunologists and bone biologists. Since then, numerous studies have defined molecular interactions between immune and skeletal cells that have demonstrated bidirectional influences between cells from the two systems that regulate development and function of bone and immune responses. The immune system has the ability to react to diverse stimuli and stress to enable responses to infection, trauma, cancer, and environmental stressors to maintain and/or restore homeostasis. Traditionally, the immune system is divided into two major arms: (1) the innate immune system which generates more nonspecific and immediate responses to infection or injury and (2) the adaptive immune system which enables a highly specific cellular response against foreign pathogens that can be educated by the innate system. Both arms of the immune system can influence bone turnover in normal and pathological states. The major distinctions between innate and adaptive immunity are in the specificity of the response to foreign pathogens, the speed and duration of the response, and the types of effector cells involved (Table 17.1).

Basic and Applied Bone Biology, Second Edition https://doi.org/10.1016/B978-0-12-813259-3.00017-8

INNATE IMMUNITY Innate immunity is a rapid response system that can be mobilized against foreign pathogens or “danger” to the organism. Innate immune cells and mechanisms are utilized by all plants and animals. The innate immune system not only involves specific immune cells such as neutrophils, natural killer cells, basophils, eosinophils, dendritic cells, monocytes, and macrophages but also utilizes tissue barriers such as skin and mucous membranes and secreted proteins such as complement. Compared with adaptive immune cells, innate immune cells respond in a more nonspecific manner to pathogens and act as sensors of microenvironmental change. Innate immune recognition has been demonstrated to occur via pattern recognition receptors that detect conserved molecular patterns found in pathogens but not in normal cells. The detection of pathogen-associated molecular patterns (PAMPs) by innate cells can identify the presence of invading pathogens and alert the adaptive immune system to mobilize a more specific immune response. Pattern recognition receptors such as those in the TLR (toll-like receptor) family are genetically encoded and recognize PAMPs based on specific molecular patterns seen in pathogens, such as lipopolysaccharide, flagellin, and peptidoglycans (Fig. 17.1, Table 17.2). In response to these local signs of "danger", innate immune cells function to produce cytokines and chemokines, commence phagocytosis, and can also initiate and stimulate a more specific adaptive immune response.

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TABLE 17.1  Distinctions Between Innate and Adaptive Immunity Innate Immunity

Adaptive Immunity

Speed of response

Rapid—minutes to hours

Delayed—hours, days, weeks

Specificity of response

Nonspecific response Molecular pattern based (LPS, LTA, mannans, glycans)

Antigen-specific response Against details of molecular structure (proteins, peptides, carbohydrates)

Nonclonal response

Clonal response

Organisms

All plants and animals

Jawed vertebrates

Memory

Generally no memory response

Retains immunological memory that can be recalled

Receptors

Germline encoded

Gene segments encoded

No rearrangement

Undergo gene rearrangement to create diversity

Each cell expresses multiple different pattern recognition receptors

Each cell expresses single TCR or BCR

Neutrophils, basophils,

T cells B cells

Cells

Eosinophils Natural killer (NK) cells, NK T cells Monocytes, macrophages Dendritic cells Osteoclasts BCR, B-cell receptor; LPS, lipopolysaccharide; LTA, lipoteichoic acid; TCR, T-cell receptor.

Innate immunity also plays an important role in the response to tissue injury due to trauma, toxins, ischemia, or inflammation. Damaged cells release damage-associated molecular patterns (DAMPs) that also stimulate pattern recognition receptors similar to PAMPs (Table 17.3). DAMPs include intracellular components, such as histones, high mobility group box 1 (HMGB1), ATP, and uric acid that are released by dying cells. This enables the innate immune system to be activated to “clean up” cellular debris, following tissue injury from trauma, ischemia, or inflammation in the absence of specific pathogens. This function is important to maintain tissue homeostasis and plays a role in wound healing and tissue repair. Resolution of the innate immune response is also of importance to prevent ongoing tissue destruction. The innate immune system is important to bone in this way because of its need to remodel throughout life. Osteoclasts are hematopoietic cells of the innate immune system, derived from the same precursor cells as many other innate immune cells in the myeloid cell lineage. Similar to other innate immune cells, they are functionally activated by cytokines (such as RANKL, tumor necrosis factor alpha (TNFα), and interleukin (IL)-1β to initiate bone resorption and mobilize calcium. Like other innate immune cells, they respond to local microenvironmental stimuli and are regulated by innate immune receptors. Classic stimuli through TLR receptors regulate osteoclast differentiation, although the effect depends on the stage of osteoclastogenesis. TLR4 activation inhibits the differentiation of early osteoclast precursors but can

increase survival of mature osteoclasts. While osteoblasts and osteocytes are derived from cells in the mesenchymal lineage, they are also regulated by innate immune stimuli and also by other immune cells. An example is that activation of the pattern recognition receptors TLRs on osteoblasts or stromal cells stimulates these cells to produce the osteoclastogenic cytokines, such as RANKL and TNFα.

ADAPTIVE IMMUNITY The adaptive immune system is known for the high level of specificity of its response, as well its ability to undergo education. Adaptive immune responses are the basis for protection against infection conferred by vaccination. The acquisition of adaptive immune memory is the ability to remember a specific response to a specific pathogen and allow a faster and more robust response if that same pathogen is encountered a second time. The adaptive immune system involves two major types of responses: (1) humoral immunity or antibody-mediated responses and (2) cellular immunity which involves both cell-mediated cytotoxicity and cellular activation to release cytokines, chemokines, and initiate phagocytosis. Antibodies, which are immunoglobulins, are proteins produced by immune cells in response to foreign proteins or antigens. Antigens are the foreign proteins that the body recognizes and develops an immune response against. Antibodies bind to foreign antigens in a highly specific manner to facilitate removal of the antigens

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Adaptive Immunity

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FIGURE 17.1  Pattern recognition receptors (PRRs). The innate immune response utilizes PRRs to sense changes in the microenvironment and activate cellular responses. These PRRs can recognize either pathogen‐specific molecules (PAMPs or pathogen‐associated molecular patterns) or endogenous host‐derived signals released during cellular damage (DAMPs or damage‐associated molecular patterns). Each pathogen is recognized by its specific molecular signature or PAMP by specific PRRs (see Tables 17.2 and 17.3). DAMPs are danger signals released during inflammatory stress such as burns, trauma, and infection that can activate the same PRRs and lead to cytokine and chemokine secretion by immune cells, mobilization of innate immune cells, and activation of acquired immune cells as well. PRRs are also present on osteoclasts and osteoblasts and can be directly activated by PAMPs and/or DAMPs.

TABLE 17.2 Types of Pattern Recognition Receptor Families TLR

Toll-like receptors

CLR

C-type lectin receptors

RIG-I

Retinoic acid-inducible gene-1-like receptors

NLR

Nucleotide-binding domain, leucine-rich repeat-containing protein receptors

STING

Stimulator of interferon genes and associated cytosolic DNA sensors

AIM

Absent in melanoma-like receptors

from the body. Antibodies are produced by plasma cells which are differentiated from B lymphocytes. Antibodies are secreted in the blood and other bodily fluids and serve a number of immune functions which together are termed humoral immunity. Immunoglobulin functions

include the following: (1) Neutralization of pathogens: blocking important surface molecules on pathogens and/or preventing pathogen function or infectability, (2) Opsonization: antibodies can coat pathogens, and then the antibodies bind to receptors on innate immune cells that lead to phagocytosis of pathogens, and (3) Complement activation: antibody binding to antigen triggers the complement system which can directly kill some pathogens and enhance opsonization (Fig. 17.2). The major cells involved in adaptive immunity are the lymphocytes: T cells and B cells. B cells develop in the bone marrow. Each B cell contains immunoglobulin gene segments that undergo random gene rearrangement during development which enables the creation of a large number of B cells with different antigenic specificities. The process of recombination is a highly regulated process that is controlled in a lineage, temporal and allele

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specific manner. Each B cell expresses a unique immunoglobulin B-cell receptor that binds a unique antigen, and the binding of receptor and antigen leads to B-cell activation. During B-cell development, B cells undergo selection to remove cells that are reactive with self-antigens. TABLE 17.3 Pathogen- and Danger-Associated Molecules Recognized by Pattern Recognition Receptors Pathogen-Associated Molecular Patterns

Damage-Associated Molecular Patterns

LPS (lipopolysaccharide)

HSPs (heat shock proteins)

LTA (lipoteichoic acid)

Fibrinogen

Peptidoglycan

Hyaluronan

Flagella

Biglycan

DNA

HMGB1

RNA

S100 proteins

Formyl peptides

Beta defensins Cathelicidin RNA DNA Histones

T cells also arise in the bone marrow but migrate to the thymus where they undergo maturation and similar random gene rearrangement which leads to T cells with T-cell receptors (TCRs) with numerous antigenic specificities. TCR rearrangement is regulated by recombination similar to immunoglobulin gene rearrangement. Each individual T cell expresses only one TCR sequence. T cells also undergo selection to remove self-reactive T cells. The process of eliminating self-reactivity is also known as the development of tolerance. Each TCR is activated by binding to antigen presented in the context of an MHC (major histocompatibility complex) molecule on an antigen-presenting cell. MHC molecules in humans are also known as HLAs (human leukocyte antigens). These molecules are the cell surface molecules that define “self” and need to be matched during organ transplantation. T cells differentiate into CD8+ cytotoxic T cells or CD4+ helper T cells under specific stimuli by cytokines and regulatory transcription factors. Cytotoxic T cells can specifically recognize foreign antigens in virally infected cells or cancer cells and initiate lysis of the abnormal cells. CD4+ cells further differentiate under the influence of specific transcription factors into polarized T cells that secrete specific cytokine profiles

FIGURE 17.2  Antibody effector functions. Antibodies play a number of roles in the humoral immune response. Antibodies can inhibit and clear infection by (A) Neutralization: antibodies can block pathogen binding and entry into cells, (B) Opsonization: antibody binding to pathogen can facilitate binding to immune cells and enhance phagocytosis, and (C) Complement activation: antibodies can fix complement and activate cellular lysis or enhance phagocytosis. V.  THE INTERACTION OF BONE WITH OTHER ORGAN SYSTEMS

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B Cells and Bone Remodeling

with specific effector functions (Fig. 17.3). During an adaptive immune response, specific T-cell specificities are clonally expanded in response to stimulation by a specific antigen. Some of these T cell subsets are defined by either their function such as Tregs or T regulatory cells which are T cells that can downregulate the immune response or Th17 cells which are T cells that produce the cytokine IL-17.

B CELLS AND BONE REMODELING As discussed above, B cells are the antibody-producing cells of the immune system and are essential for humoral immunity. B cells are derived from bone marrow progenitors and are continuously generated throughout life. B cells differentiate from hematopoietic stem cell (HSC) progenitors in a specialized structure called the bone marrow niche, and the interaction with stromal and other cells of bone is essential for the initial stages of B-cell development (Chapter 2). B cells can, in turn, influence cells of the bone both through production of particular cytokines (Chapter 4) and in some cases by production of particular antibodies that activate Th1

Tbet Th2 Gata3 Th9 Stat6 Treg Naive

FoxP3 Th17 Roryt Tfh Bcl6

osteoclasts. The initial stages of B-cell development, from HSC through a common lymphoid progenitor to immature B cells, occur in the bone marrow. Immature B cells must then leave the marrow and be activated by antigen-presenting cells to differentiate into mature B cells and the terminally differentiated antibody secreting B cells known as plasma cells. These mature B cells can then return to the bone marrow. Thus, both immature and mature B cells in the bone marrow may affect the function of osteoclasts and osteoblasts. B lymphocytes are a major source of OPG (osteoprotegerin), a decoy receptor for RANKL that blocks osteoclastogenesis. Mature B cells and plasma cells are particularly efficient producers of OPG (Fig. 17.4). Activation of B cells through the costimulatory Cluster of differentiation 40 (CD40)-CD40L pathway further increases B-cell production of OPG. B cells also produce TGFβ, another osteoclast inhibiting cytokine. B cells are essential for maintaining normal bone turnover, as evidenced by the osteoporotic phenotype of μMT/μMT mice, which lack mature B cells. These mice have evidence of enhanced osteoclast activity, suggesting that the primary effect of B cells on bone remodeling under normal conditions is inhibition of osteoclastogenesis.

IFNγ

Inhibits osteoclastogenesis

IL-4

Inhibits osteoclastogenesis

IL-9

Increases osteoclastogenesis by increasing IL-17

IL-10, TGFβ, CTLA-4

Inhibits osteoclastogenesis

IL-17, IL-21, TNFα, RANKL

Increases osteoclastogenesis by increasing IL-17

IL-6, IL-21, TGFβ, TNFα

Increases osteoclastogenesis by increasing IL-17

IFNγ, TNFα

Depends on ratio of pro- and antiosteoclastogenic cytokines

NKT Gata3

FIGURE 17.3  T helper cells modulate osteoclastogenesis. When naïve CD4+ T cells are stimulated with cytokines in the microenvironment, they upregulate the expression of specific transcription factors that drive T helper (Th) differentiation. Each type of Th cell produces signature cytokines that modulate immune responses and osteoclastogenesis. Th17 cells have the largest impact on bone remodeling. Figure adapted from Srivastava, Dar, Mishra; 2018. V.  THE INTERACTION OF BONE WITH OTHER ORGAN SYSTEMS

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FIGURE 17.4  B cells use a variety of mechanisms to influence bone remodeling in a context dependent manner. Under homeostatic conditions (left panel), osteoclasts in the bone remodeling unit differentiate from myeloid precursors under the influence of RANKL. Although adjacent mature B cells or plasma cells produce some RANKL, the primary source of RANKL is osteoblast lineage cells. In contrast, B cells are the primary source of OPG, which binds RANKL and inhibits its function, thus reducing osteoclast differentiation. TGFβ produced by B cells can also inhibit osteoclasts. B-cell production of anti-osteoclastic cytokines is important for normal bone remodeling as B cell–deficient mice have low bone mass. B cells also produce WNT1, which promotes new bone formation to fill in the area resorbed by osteoclasts. In the setting of inflammation (right panel), activation of B cells stimulates their production of RANKL, while inflammatory cytokines including TNFα, IL-17, and IL-6 stimulate RANKL production by osteoblasts. The resultant increase in RANKL drives osteoclast formation and bone loss.

B cells also produce RANKL, although the contribution of B cell–derived RANKL to normal bone remodeling does not appear to be significant, as mice lacking RANKL only in B cells have no bone phenotype. However, on activation, B-cell production of RANKL increases significantly, which could contribute to bone loss in inflammatory conditions (Fig. 17.4). B cell–derived RANKL appears to be particularly important for the loss of alveolar bone in periodontitis, a disease driven by B cells, and may contribute to bone loss in estrogen deficiency, which is discussed in more detail in a later section. Whether B-cell RANKL contributes to bone loss in T cell–driven inflammatory diseases such as rheumatoid arthritis (RA) is not unknown, but there is emerging evidence that pathogenic autoantibodies produced by plasma cells can activate osteoclasts and bone resorption (see section on autoimmune diseases below). Other cytokines produced by B cells, particularly TNFα and IL-6, may promote osteoclast differentiation and bone resorption. B cells are also a source of the Wnt pathway ligand WNT1, which plays a critical role in bone homeostasis as evidenced by association of WNT1 mutations with osteoporosis and the identification of causal mutations in WNT1 in some families with osteogenesis imperfecta. WNT1 is also produced by the central nervous system and by granulocytes, among other tissues, and the relative contribution of B cell–produced WNT1 is not known. Thus, B cells can modulate skeletal homeostasis through several pathways, and the predominant effect of B cells on bone is likely context dependent (Fig. 17.4). While the primary link between B cells and bone is production of anti-osteoclastogenic cytokines OPG and TGFβ, in an inflammatory context B-cell production of the pro-osteoclastogenic cytokines RANKL, TNFα, and

IL-6 may become important. Lastly, B cells may also promote osteoblasts through production of WNT1.

T CELLS AND BONE REMODELING T cells account for approximately 5% of total bone marrow and play important roles in homeostatic and pathologic bone remodeling. T-cell progenitors arise in the bone marrow but migrate to, proliferate, and develop in the thymus. Within the thymus, T cells become CD4+ or CD8+ taking on distinct functions. CD4+ T cells primarily serve as T helpers (Th) that provide defense against bacterial, viral, and parasitic infections as well as provide antitumor and antiautoimmunity functions. CD4+ T cells also help B cells to produce antibodies. CD8+ T cells, also called cytotoxic T cells, provide protection from foreign organisms and are responsible for tumor cell surveillance and elimination. Both CD4+ and CD8+ T cells participate in bone remodeling by direct and indirect stimulation or inhibition of bone cells. Depending on the microenvironment around them, CD4+ T cells can differentiate into multiple subtypes including Th1, Th2, Th9, Th17, Treg, and follicular helper T (TFH) cells (Fig. 17.3). These subtypes of CD4+ T cells have differing effects on bone remodeling by producing cytokines that stimulate or inhibit osteoclasts and osteoblasts (Fig. 17.3). The Th17 subset of T cells has the most significant ability to stimulate pathogenic bone diseases. Th17 cells robustly stimulate osteoclast differentiation and bone resorption by secreting high levels of RANKL, IL-6, IL-17, IL-1, and TNFα. IL-17 further increases osteoclastogenesis by stimulating resident bone cells, osteocytes and osteoblasts, to upregulate their production of RANKL. Th17 cells are increased in many disease states

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Infections and Bone Cells

affecting bone health including RA, psoriatic arthritis (PsA), and postmenopausal osteoporosis. Other CD4+ T cells that promote osteoclastogenesis and bone resorption include Th9 CD4+ T cells, a newly described subset that secretes IL-9 and promotes autoimmune diseases including RA and psoriasis. IL-9 indirectly promotes osteoclastogenesis by driving Th17 T-cell differentiation. TFH cells, that function to help B cells to produce antibodies, secrete IL-6 that stimulates osteoclast activation and bone resorption. Other T-cell subsets inhibit bone resorption. Th1 CD4+ T cells, despite being associated with significant inflammation, inhibit osteoclastogenesis by secreting interferon gamma (IFNγ), which disrupts RANK receptor signaling within the osteoclast. Th2 CD4+ T cells, important in preventing allergic diseases, secrete immunosuppressive cytokines IL-4, IL-10, and IL-33 that act as potent inhibitors of osteoclastogenesis. Antiinflammatory Treg CD4+ T cells, identified by the presence of FoxP3 transcription factor, express cytotoxic T-lymphocyte-associated protein4 (CTLA-4) that directly binds to its receptor CD80/CD86 on osteoclast precursors and inhibits osteoclastogenesis. Treg cells also secrete Il-10, IL-4, and TGFβ1 to suppress osteoclast activation. Recently, CD8+ FoxP3+ Treg cells have been identified and found to directly suppress osteoclast activation by preventing osteoclast actin ring formation. Interestingly, osteoclasts themselves can induce CD8+ FoxP3+ Treg cells thus establishing a negative feedback loop. Indeed, pulsed stimulation of osteoclastogenesis with low dose RANKL following ovariectomy strongly induces CD8+ FoxP3+ Treg cells that in turn promote bone formation. Typical cytotoxic CD8+ Treg cells are potent inhibitors of osteoclastogenesis by secreting OPG and stimulation of osteoblast activity by production of Wnt10b, thus promoting an anabolic affect. Thus, depending on the ratios of T-cell subtypes, immune activation may promote bone resorption or bone formation.

MYELOID CELLS AND BONE REMODELING Myeloid cells also have significant effects on both bone resorption and bone formation. Osteoclasts are myeloid cells derived from immature myeloid progenitors differentiated under the stimulation of macrophage colony-stimulating factor (MCSF) and RANKL (Chapter 3). Interestingly, both osteoclasts and osteoclast precursors have also been found to regulate T cells, by suppressing T-cell expansion when they are in close physical proximity. Osteoclast precursor cells have been found to be phenotypically the same as a cell subset termed monocytic myeloid suppressor cells (MDSCs), defined by their ability to suppress T-cell activation. Studies have shown that MDSC expanded in a tumor environment can

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differentiate to osteoclasts and lead to lytic lesions and bone destruction in animal models of multiple myeloma and metastatic breast cancer. Thus, the dual function of T-cell suppression and osteoclast progenitor by these cells facilitates the expansion of tumor cells in the bone. Most likely, this is due to an unfortunate subversion of the homeostatic innate immune function to dampen inflammation in the setting of bone remodeling and repair. The same myeloid progenitors that can differentiate into osteoclasts can also differentiate to other innate immune cells such as dendritic cells and macrophages. Dendritic cells are also known as professional antigen-presenting cells that play an essential role in the stimulation and activation of antigen-specific T cells. Immature dendritic cells can easily transdifferentiate into osteoclasts and may contribute to pathologic bone loss because dendritic cell to osteoclast transdifferentiation can be promoted by rheumatoid synovial fluid or multiple myeloma cells. Dendritic cells express the RANK receptor which can interact with RANKL expressed on T cells, though the precise immune function of this interaction is not well understood. Dendritic cells are not critical for normal bone homeostasis, however, because dendritic cell-deficient mice do not have a bone phenotype. Tissue-resident macrophages around the periosteum and at sites of bone remodeling have recently been described and termed “osteomacs.” The macrophages are present in bone marrow stromal cell/osteoblast cultures and have been shown to promote osteoblast differentiation and mineralization both in vitro and in vivo. Maintenance of the osteoblast bone modeling surface is diminished if osteomacs are depleted, and studies in bone injury and fracture models suggest that bone repair is facilitated by the presence of osteomacs. Interestingly, osteomacs have also been demonstrated to play a role in PTH (parathyroid hormone)-induced anabolism, thus these cells may play a role in both normal and pathologic bone turnovers. Deletion of these osteal macrophages in mouse models leads to decreased bone mass and bone formation, while expansion of osteal macrophages is seen on the periosteal and endosteal surface of cortical bone in mice during PTHinduced bone formation. Deletion of early lineage macrophages in mice blunts the anabolic effect of PTH suggesting the osteomacs are necessary to support osteoblast differentiation during PTH-induced bone formation.

INFECTIONS AND BONE CELLS A primary function of the immune system is the eradication of infection. While infection of the bone is not an everyday occurrence, acute and chronic bone and joint infections can be extremely severe given that they can be difficult to treat and may be highly destructive. Osteomyelitis is an infection of the bone and surrounding tissues that is hard to eradicate even with

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FIGURE 17.5  Acute osteomyelitis. (A) Acute osteomyelitis in the absence of trauma, generally, begins via hematogenous seeding of bone by bacteria such as Staphylococcus aureus in the bloodstream. (B) Terminal branches of metaphyseal arteries from loops at the growth plate where they connect with afferent venous sinusoids and blood flow is considered sluggish in this zone. It is in this area that initial infection generally advances and develops in the metaphysis which then leads to vascular compromise in the area of infection. (C) This leads to avascular necrosis of surrounding bone and extension of the infection into the subperiosteal space where an abscess can form. The localized vascular compromise and cellular necrosis facilitates development of chronic infection. Only some arterial branches cross the growth plate; the infection primarily spreads via sinus tracts, and along the Haversian canals.

appropriate antibiotic treatment. Bone cells directly interact with pathogens to limit spread of the infection but can also provide a microenvironment where the infection can persist. In adults, healthy bone tissue is fairly resistant to infection. Osteomyelitis is generally a complication of trauma or surgery and is most often due to infection by commensal skin organisms such as Staphylococcus aureus due to contiguous spread from a contaminated site or open wound. Staphylococcal organisms express high affinity receptors for collagen, fibrinogen, fibronectin, and laminin which facilitate their adhesion to bone tissue, helping sequester the bacteria from the immune system, leading to the establishment of chronic infection. The incidence of fracture-associated bone infection varies from 1.8% to 27% depending on the bone involved and the grade/type of fracture, with severe, high energy, lower extremity, open, and complex fractures having the highest occurrence of infection. Necrotic bone can also be more easily infected, and osteomyelitis is more common in individuals with vascular or neurologic insufficiency due to poor blood

supply, diabetes, neuropathy causing loss of protective sensation, or compromised immune systems. S. aureus is the cause of 80%–90% of acute osteomyelitis cases, while Staphylococcus epidermidis more commonly infect medical devices and orthopedic hardware. Additional pathogens are seen in diabetic or immunocompromised patients. Infection by bacteria in the bone associated with external trauma induces an acute inflammatory reaction affecting the periosteum. Spreading of the infection leads to bone cell death and necrosis. In children, osteomyelitis is usually due to hematogenous spread from a distant site of infection to infect the metaphysis. Salmonella enterica remains an important cause of osteomyelitis in young patients with sickle cell anemia. Osteomyelitis beginning by hematogenous spread in children or adults often seeds the metaphysis at the site where terminal branches of metaphyseal arteries form loops and join with afferent venous sinusoids. Establishment of the infection leads to local vascular compromise and cellular necrosis which further facilitates spread of the infection, and abscesses can form in the periosteal space (Fig. 17.5).

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Endocrine Activation of Immune-Mediated Bone Loss

TABLE 17.4  Factors Contributing to Osteomyelitis Infection/ Persistence Bacterial

Host/Immune

Adherence of organism to extracellular matrix

Vascularization

Biofilm formation

Host immune response (presence of immune deficiency)

Invasion of host cells

Phagocytosis capacity

Sequestration in canaliculi and osteocyte lacunae

Presence of prosthesis

Following a bacterial bone infection, there is a local increase of neutrophils and macrophages which lead to cytokine release and bacterial phagocytosis (Fig. 17.5). Thus, the innate immune system mobilizes in an attempt to control bacterial invasion. Osteoblasts have also been described to produce antimicrobial peptides, such as beta defensin-3, that may function to limit infection of the bone and to phagocytize bacteria. In children, the periosteum is loosely attached to the cortex which allows formation of subperiosteal abscesses along the surface that can compromise the vascular supply to the bone. The inflammatory reaction and local cytokine production simulates osteoclastogenesis and bone resorption, ingrowth of fibrous tissue, and deposition of new bone in the periphery. Despite initiation of a T- and B-cell response, osteomyelitis can become a chronic infection, in part due to poor vascularization that inhibits both antibiotic and cellular responses, and in part because of formation of bacterial biofilms in bone which protect the organisms from phagocytosis (Table 17.4). In animal models, S. Aureus is observed within canaliculi of live cortical bone, leading to sequestration of proliferating bacteria within osteocyte lacunae. A recent study suggested that in humans with osteomyelitis, measurement of the host antibody response against 14 known S. aureus antigens is a predictor of ongoing infection (sepsis) that may prove to have prognostic value which suggests that the specificity of humoral immunity generated may also be of importance in bacterial clearance. Bacteria should be opsonized by antibodies to lead to activation of neutrophils and bacterial clearance; however, in the presence of biofilms, this response is dampened. Bacteria also induce inflammatory cytokines that promote osteoclastogenesis and limit osteoblast function (See Fig. 6.23). This can lead to localized lytic lesions at the site of infection, causing significant pain and disability. Other infections of the bone include septic arthritis and infections of prosthetic joints. The incidence of joint infection following arthroplasty (joint replacement) ranges from 0.3% to 3.0%. Synovial joints are difficult to clear infection from and generally require debridement and irrigation in addition to prolonged antibiotics. Prosthetic joint infection generally requires removal of the infected

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hardware followed by prolonged antibiotics before the joint can be replaced again. Prosthetic joint infection can occur early (<3 months) due to direct perioperative infection. Delayed infections (3 months–2 years) may be due to perioperative inoculation of a less virulent bacterium. Late infections (>2 years) are due to infection via a hematogenous source. Preclinical studies show prosthetic joints quickly become coated with host adhesins (fibronectin, fibrin, and fibrinogen) on implantation, which facilitates bacterial adherence, sequestration, and biofilm formation. The formation of a biofilm helps evade normal host immune defenses and leads to a persistent chronic infection that can continue to stimulate bone degradation. The infection can remain localized with low ongoing levels of tissue necrosis or can become systemic leading to development of sepsis. Sepsis is a clinical syndrome caused by systemic infections leading to a severe host inflammatory response. The syndrome is associated with high mortality that is exacerbated by an associated lymphopenia and immunodeficiency. An acute interaction exists between systemic infection leading to sepsis and bone cells. Studies in septic mice have shown that the development of lymphopenia during sepsis is dependent on loss of osteoblasts during systemic infection. Osteoblasts provide a source of IL-7 which is needed to support common lymphoid progenitors. Sepsis leads to rapid ablation of osteoblast cells, and in the absence of osteoblast produced IL-7, the number of common lymphoid progenitors is reduced and lymphopenia results. Thus, there is a reciprocal interaction between the immune and bone cells in which acute inflammation due to a pathogen induces a defect in bone cells that results in lymphopenia-associated immunodeficiency. Thus, infectious organisms have direct effects on osteoclast and osteoblast cell differentiation, function, and survival and indirect effects through stimulation of immune cells, growth factors, and cytokines. Bone infection involves both innate and adaptive immunity, and the development of persistent infection is likely due to the combined effects of local changes in vasculature, cell death, and immunodeficiency that develop as bacteria, immune cells, and bone cells reciprocally interact. Given the significant morbidity and mortality that is caused by severe and chronic infections, further delineation of these bacterial and cellular interactions is needed to better target and enhance the eradication of these complex infections.

ENDOCRINE ACTIVATION OF IMMUNEMEDIATED BONE LOSS The basic effects of endocrine hormones, including estrogen and PTH, are described elsewhere (see Chapter 15), but these hormones have specific immunological effects that contribute to bone loss or anabolism.

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Role of Immune Activation in Estrogen Deficiency With age, women naturally loose ovarian function leading to menopause. Younger women may undergo early menopause due to surgical removal of ovaries, ovarian failure secondary to toxic medications, or hormone deprivation therapy related to anticancer therapeutics. Menopause is associated with significant rapid bone loss and leads to the excessive activation of basic multicellular units (BMU) comprised of osteoclasts and osteoblasts (see Chapters 10 and 21). However, with menopause an imbalance occurs within the BMU leading to increased resorption resulting from increased osteoclast survival and a decrease in bone formation due to increased osteoblast apoptosis. Menopause also induces immune activation leading to increased production of TNFα and IL-7 that directly inhibit osteoblastogenesis while stimulating osteoclastogenesis (Fig. 17.6). TNFα also works synergistically with RANKL to stimulate osteoclast activity, thereby leading to an imbalance of bone resorption and bone formation. The importance of TNFα in postmenopausal bone loss is demonstrated by the protection of bone loss by treatment with TNFα inhibitor after ovariectomy and by failure to develop ovariectomy-induced bone loss in TNFα- or TNF receptor p55-deficient mice. Activated T cells are a significant source of the increased TNFα Estrogen deficiency ↓TGFβ ↑IL-7

B cell B cell

CD4

CD4

IFNγ

Th17

Th17 Th17

B cell

Mac TNFα

TNFα, IL-6, Il-1β

Stromal Cells Osteoblasts IL-6, IL-1β, MCSF, RANKL

RANKL, IL-17, TNFα, IL-6, IL-1β RANKL ROS

FIGURE 17.6  Estrogen deficiency induces activation of the innate and adaptive immune system. In the absence of estrogen, TGFβ is decreased and IL-7 is increased leading to expansion of CD4+ T cells and B cells and activation of macrophages. T-cell production of IFNγ further amplifies macrophage and dendritic cell antigen presentation resulting in production of Th17 cells producing copious osteoclastogenic cytokines driving excessive osteoclastogenesis. Osteoclastogenesis is further amplified by the production of RANKL by activated B cells and by IL-6, Il-1β, and TNFα production from macrophages and stromal cells. IL-7 also inhibits osteoblastogenesis and prevents coupled bone formation leading to significant bone loss.

produced during menopause. Athymic nude mice lacking T cells are protected from ovariectomy-induced bone loss. Additionally, transfer of wild-type T cells, but not TNFα-deficient T cells, into nude mice restores ovariectomy-induced bone loss. However, compared to standard treatments for postmenopausal osteoporosis, TNFα inhibitors have a wide range of serious side effects, are very expensive, and require frequent monitoring; therefore, these drugs are not used to prevent postmenopausal osteoporosis in women. TNFα also stimulates the release of IL-7 from T cells that drives postmenopausal bone loss (Fig. 17.6). IL-7 uncouples the BMU by suppressing osteoblastic differentiation and stimulating osteoclast activity. In response to exogenous IL-7, wild-type mice develop osteoporosis, whereas mice lacking IL-7 receptor have increased bone acquisition. Ovariectomy significantly induces IL-7 expression in bone marrow, and treatment with IL-7 neutralizing antibodies in vivo can prevent estrogendeficient bone loss with decreases in markers of bone resorption and increases in osteocalcin, indicating that IL-7 suppresses bone formation. IL-7 induces T-cell proliferation by promoting T-cell activation to weak antigens, such as self-antigens. IL-7 also leads to upregulation of IFNγ that promotes antigen presentation. IL-7 stimulates RANKL production by T cells, thus increasing osteoclastogenesis and bone resorption. In vivo, exogenous IL-7 fails to increase bone resorption in mice lacking T cells indicating the requirement for T-cell activation and RANKL production in bone loss. In addition to T-cell proliferation, IL-7 also induces B-cell proliferation, a population that is greatly induced by estrogen deficiency. B-cell production of RANKL is also increased in estrogen deficiency in both mice and humans, and mice lacking RANKL only in B cells were partially protected from ovariectomy-induced bone loss. However, mice lacking all mature B cells still lose bone after ovariectomy and thus the relative importance of this B-cell proliferation in menopausal bone loss is unclear. Recent studies suggest that estrogen deficiency leads to significant changes in the gut microbiome (see Chapter 19) leading to expansion of Th17 cells producing TNFα, RANKL, and IL-17 in the small intestines promoting bone loss. Mice raised in germ-free environments fail to exhibit this immune activation and bone loss after ovariectomy. Dysbiosis, a microbial imbalance in the gut that occurs with menopause, leads to increased gut wall permeability and increased antigen presentation by dendritic cells and macrophages within the gut. This culminates in an increased activation of T cells and production of TNFα, RANKL, IL-1, IL-17, and Il-6 that drives bone loss. Use of Lactobacillus acidophilus, a probiotic, prevents ovariectomy-induced bone loss by increasing Tregs and inhibiting Th17 cells in mice. Microbial products such as short-chain fatty acids induce increases in insulin-like

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Endocrine Activation of Immune-Mediated Bone Loss

growth factor-1 (IGF-1) that promote bone formation. Additional studies are needed to determine specific bacterial species or microbial communities that provide optimal bone and immune system health.

Role of T Cells in Parathyroid Hormone Responses in Bone PTH is a critical regulator of calcium metabolism and tightly regulates serum and urinary calcium levels in the normal ranges (Chapter 13). PTH binds to the PTH/ PTH-related peptide (PTHrP) receptor (PTH-1R) found on osteocytes, osteoblasts, and lining cells to mediate bone remodeling. PTH acts as a Wnt signaling agonist to increase β-catenin within these cells leading to proliferation and differentiation while also preventing cellular apoptosis. This expansion of osteoblasts leads to an anabolic effect in bone. PTH also stimulates RANKL expression in osteocytes and osteoblasts inducing osteoclastogenesis and bone resorption. Thus, PTH stimulates both bone formation and resorption. High dose PTH, like that seen in primary hyperparathyroidism or tumor production of PTH/PTHrP, stimulates osteoclast activation over osteoblast differentiation leading to bone loss (see Chapters 20 and 22). In contrast, intermittent, daily injections of PTH stimulate osteoblast differentiation over osteoclast recruitment inducing bone accumulation (see Chapters 15 and 21). The exact mechanisms

allowing these differences in responses to the same stimulation are poorly understood, yet studies have revealed a requirement for T cells for the catabolic action and anabolic actions of PTH. Like bone marrow stromal cells, CD4+ and CD8+ T cells express the PTH-1R and respond to PTH (Fig. 17.7). Continuous infusion of PTH, modeling hyperparathyroidism, induces the expression of TNFα in CD4+ and CD8+ T cells that is necessary for the continuous PTHinduced bone loss, as evidenced by the failure of continuous PTH to induce bone loss in mice with TNFα deficiency only in their T cells. Conditional silencing of PTH-R1 in T cells also blunts continuous PTH-induced bone loss. T cell–derived TNFα directly induces osteoclastogenesis and indirectly does so by upregulating TGFβ and IL-6, driving Th17 cells that produce IL-17 and TNFα leading to increased RANKL production from osteoblasts and osteocytes. Humans with hyperparathyroidism have increased IL-17 expression in peripheral mononuclear cells that is normalized after parathyroidectomy. Neutralizing IL-17 antibodies or silencing IL-17A receptor prevents continuous PTH-induced bone loss in mice. Thus, T cells actively participate in continuous PTH-induced catabolic bone loss. Intermittent PTH is an approved treatment for severe osteoporosis- and glucocorticoid-induced osteoporosis in men and women (see Chapter 21). Unlike continuous PTH, intermittent PTH induces bone anabolism

iPTH

cPTH

PTH

PTH-R1

CD8

↑ TGFβ, IL-6

Treg

CD8

Treg

CD8

Treg

OCY

Th17

X

↑ TGFβ, IGF-1

↑ Wnt10b

CD40L

LRP4,5,6 OB

OB

↑ Proliferaon

Bone anabolism

OB

↑ IL-17

RANKL, TNFα, IL-6, IL-1β

CD8 CD4

CD40

Th17

↓Sclerosn

MSC

Th17

CD4

↑ TNFα

↓OPG ↑ RANKL, MCSF

OB

↑ Survival

Bone catabolism

FIGURE 17.7  Parathyroid hormone (PTH) induces T-cell activation promoting bone anabolism and catabolism. (Left) Intermittent PTH strongly induces bone anabolism in a CD8+ T cell and Treg-dependent manner. PTH stimulation of CD8+ T cells induces secretion of Wnt10b that engages with LRP 4, 5, 6 receptors to induce mesenchymal stem cells (MSC) and osteoblasts (OBs) to induce OB differentiation, proliferation, and protection from apoptosis leading to bone anabolism. PTH also suppresses osteocyte production of Wnt inhibitor sclerostin allowing for more osteoblastogenesis. OBs stimulated with PTH produce TGFβ and IGF-1 promoting proliferation of Treg cells. Tregs promote anabolism through a yet unknown mechanism. (Right) Continuous PTH induces CD40 ligand (CD40L) on CD4+ T cells, engaging with CD40 receptor expressed on stromal cells, leading to decreased osteoprotegerin (OPG) and increased RANKL and MCSF promoting osteoclastogenesis. Continuous PTH also induces TGFβ and IL-6 promoting Th17 cells that directly promote osteoclastogenesis by producing RANKL, TNFα, IL-1β, and IL-6. Th17 cells secrete IL-17 that further stimulates stromal cells to produce RANKL. Excessive osteoclastogenesis leads to bone catabolism.

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in humans and mice but not in mice lacking T cells. In humans and rodents, intermittent PTH-induced bone anabolism is robust in trabecular bone and leads to increased cortical porosity that is associated with increased mechanical strength. Studies have revealed that stromal cell expansion induced by intermittent PTH requires CD8+ T cells producing the Wnt ligand, Wnt10b (Fig. 17.7). Mice lacking Wnt10b fail to develop bone anabolism to intermittent PTH and T cells from Wnt10b−/− mice adoptively transferred into T cell-­ deficient mice fail to rescue intermittent PTH-induced bone anabolism, while wild-type T cells restore the effect of intermittent PTH. Interestingly, humans treated with intermittent PTH (PTH 1–34, teriparatide) also develop elevated Wnt10b in their blood T cells, while those with hyperparathyroidism do not. Intermittent PTH also induces increases in Treg cells in humans at 3 and 6 months and mice at 1–4 weeks. This Treg cell expansion is required for intermittent PTH-induced bone anabolism as depletion of Treg cells prevents anabolism. However, this effect is not direct via PTH stimulation of Treg proliferation but indirect, unknown mechanisms.

AUTOIMMUNE DISEASES CAN DRIVE LOCAL OR SYSTEMIC BONE LOSS Several autoimmune diseases, including RA, systemic lupus erythematosus (SLE), ankylosing spondylitis, and PsA, are associated with abnormal bone remodeling, joint destruction, and/or increased fracture risk. SLE patients have significantly lower bone mineral density (BMD) than healthy age- and sex-matched controls and also have a higher prevalence of fractures with increasing age. However, the mechanisms driving low bone mass and increased fracture in SLE patients are poorly understood in part because it is difficult to parse out the disease from medication effects (glucocorticoids [GCs] or cyclophosphamide), high prevalence of vitamin D deficiency, poor functional status due to fatigue, pain or weakness, or comorbid conditions such as kidney disease. On the other hand, mechanisms leading to pathological states of bone erosions in RA are becoming increasingly understood and have directed new therapies targeting specific immune cells, immune cell cross talk, or cytokines.

Rheumatoid Arthritis RA is one of the most common autoimmune diseases and leads to significant disability due to erosive destruction of joints, particularly those of the hands, wrist, elbows, hips, knees, ankles, and feet (Fig. 17.8). Erosions of the joints occur from the development of

chronic inflammation and synovial hyperplasia leading to the production of pannus, a tissue formed from proliferation of synovial cells and influx of immune cells. Pannus consists of synovial fibroblasts, T and B cells, plasma cells, dendritic cells, neutrophils, monocytes, and macrophages. These cells within the pannus are activated and secrete pro-inflammatory cytokines (TNFα, IL-1, and IL-6) and RANKL, whereas OPG is downregulated, collectively inducing robust osteoclastogenesis (Fig. 17.9). These same pro-inflammatory cytokines negatively impact osteoblast differentiation and their ability to produce mineralized matrix. The activated synovial fibroblasts also produce metalloproteinases leading to cartilage catabolism and juxta-articular bone erosions at the synovial cartilage interface. Once the erosions occur, they are difficult to fully repair due to abnormalities in the Wnt signaling preventing osteoblast differentiation. Pro-inflammatory cytokines like TNFα induce the upregulation of the potent WNT inhibitors, Dickkopf-related protein 1 (Dkk1), secreted frizzled-related protein 1(sFRP1), and sclerostin, in synovial tissue that suppress bone formation. All together, the RA-induced inflammation leads to uncoupling of bone resorption from bone formation locally. Treatment of RA with immunosuppressive diseasemodifying antirheumatic drugs promotes stabilization of bone erosions especially if remission can be achieved. In addition to local bone erosions, RA is associated with significant increases in fractures compared to ageand gender-matched controls. Fracture risk increases with length of time because diagnosis and with increases in RA disease activity. RA-associated fracture risk occurs in the absence of GC use but is significantly worsened with GC use. During active RA, there are increases in systemic pro-inflammatory cytokines, TNFα, IL-1, IL-17, and IL-6, leading to upregulation of RANKL and generalized osteoclast activation and bone resorption without compensatory bone formation due to inhibitory effect of these cytokines on osteoblasts. Treatment of RA with drugs targeting these cytokines (anti- TNFα, anti-IL-6, and anti-RANKL) not only prevents bone erosions but improves systemic BMD. Immunologically, RA results from a loss of immune tolerance to self-antigens, in particular to citrullinated versions of extracellular matrix proteins including vimentin, fibronectin, keratin, fibrinogen, collagen, and α-enolase. Citrullination is a posttranslational protein modification and may more easily induce an autoimmune response because citrullinated proteins may not be present during the development of immune tolerance. The citrullinated peptides bind particularly well to MHC molecules containing the HLA-shared epitope. The shared epitope is a specific amino acid motif commonly encoded by some alleles of the HLA-D-related (DR) locus, especially

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Autoimmune Diseases Can Drive Local or Systemic Bone Loss

(A)

(B)

(C)

FIGURE 17.8  Rheumatoid arthritis (RA) leads to bone loss and destruction of articular joints. Schematic of the small joints of the wrist and hand commonly affected by RA (A). (B and C) are radiographs of the wrist from an RA patient. (B) Minimal bone loss and erosions (arrows) are present early in disease but many more have developed after 4 years of disease (C).

Genec and environmental triggers Citrullinaon of pepdes Normal Joint

Immune cell acvaon

Rheumatoid Joint

APC Synovial hyperplasia or pannus Synovial membrane

Bone Erosions

Th17 cell RANKL TNFa IL-17 IL-6

T cell

APCA & RF Abs

B cell

Synovial Plasma Macrophage fibroblast Cell RANKL IL-6 MMPs DKK1

TNFa IL-1 IL-6 IL-8

T and B cells, plasma cells, macrophages, synovial fibroblasts

FIGURE 17.9  Rheumatoid arthritis (RA) is associated with synovial membrane pannus formation. Schematic of a normal knee joint with small synovial membrane (left) compared with a joint affected by RA with synovial hyperplasia and pannus formation (right). At the junction of pannus with bone, local bone erosions form. Pannus is the result of immune activation resulting from a genetic predisposition and environmental triggers including citrullination of extracellular matrix proteins. These antigenic peptides lead to activation of the adaptive immune system with expansion of Th17 and B cells, anti-citrullinated peptide antibodies (ACPA) and rheumatoid factor (RF) antibody production, and innate immune activation of macrophages and synovial fibroblasts. These activated cells produce cytokines and metalloproteinases (MMPs) that stimulate bone resorption, as well as Wnt inhibitors like Dkk1 that suppress bone formation.

HLA-DRB1*01 and HLA-DRB1*04, which are significantly associated with the risk of developing RA. Anti-citrullinated peptide antibodies (ACPAs) may precede the development of clinical RA by a decade. ACPAs also appear to directly activate osteoclasts and induce bone resorption and erosions by binding to citrullinated vimentin present on preosteoclasts and osteoclasts. Although vimentin is expressed on

macrophages and osteoclasts, citrullination of vimentin only occurs when peptidylarginine deaminase 2 is upregulated in osteoclasts in response to high calcium conditions, such as those found during active bone remodeling. ACPA stimulation of synovial fibroblasts, antigen-presenting cells, and macrophages induces RANKL, TNFα, and 1L-8 expression further driving osteoclastogenesis and immune activation.

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ACPAs also have been associated with lower BMD in early untreated RA patients and in healthy subjects without clinical RA suggesting that ACPA is associated with dysregulated bone metabolism and bone loss prior to clinical disease. These findings implicate ACPA in directly promoting dysregulated bone remodeling and the presence of early erosions in newly diagnosed patients. Indeed, early RA patients with ACPA developed more radiological damage than those without ACPA, and a recent study demonstrated that ACPA titer inversely correlated with systemic bone density. Reductions in BMD and increased bone erosions in ACPA-positive patients may result from increased circulating RANKL, driving osteoclastic bone resorption, present in these patients. T cells play a prominent role in the pathophysiology of RA. Dendritic cells within the pannus present the citrullinated autoantigens to T cells and inducing several subsets including Th1, Th2, Th17, and TFH cells. Several models of RA are widely used including collagen-induced arthritis (CIA), K/BxN, and SKG mouse models. All three models require CD4+ T cells for full induction of disease. It is now recognized that Th17 cells are highly pathogenic in RA. Interestingly, some Th17 cells can be derived from Tregs and possess the highest pathogenic potential. Th17 cells produce copious RANKL and IL-17 that induces osteoblasts and stromal cells to produce additional RANKL and macrophages and synovial fibroblasts to produce TNFα and IL-6. In the CIA mouse model of RA, mice deficient in IL-17A fail to develop arthritis and neutralizing IL-17 antibodies prevents disease. CD4+ T cells from SKG mice induce RA-like disease when adoptively transferred to Rag2−/− mice lacking T and B cells; but IL-17-deficient SKG CD4+ T cells do not induce disease. These data indicate the critical role of Th17 and IL-17 in RA pathology. However, initial studies using anti-IL-17 therapies have proven disappointing for RA but efficacious for psoriasis and PsA. Other T-cell subsets contribute to regulating RA activity and associated bone loss. Treg cells dampen the immune response by producing IL-10, IL-4, and cytotoxic T-lymphocyte-associated protein4 (CTLA-4) and may inhibit osteoclast formation via production of OPG and CTLA-4. Th1 cells previously thought to be pathogenic in RA are now known to inhibit osteoclastogenesis by producing IFNγ. Likewise, Th2 cells secrete IL-4 and exert inhibitory effects on osteoclasts. Thus, the impact of T cells on bone loss in RA, and likely other inflammatory diseases associated with bone loss, depends on the specific T-cell subset. Evidence exists to support a role for each arm of the immune system in contributing to bone erosion and bone loss in RA. The adaptive immune system, in particular CD4+ IL-17 producing Th17 cells, is essential for disease

pathogenesis. IL-17 itself drives RANKL production and thus promotes osteoclast-mediated bone resorption. In contrast, other T-cell subsets produce cytokines that inhibit osteoclast formation and may be protective against bone loss. B cells make OPG, but whether this is important for protecting against inflammatory bone loss is not known. However, B cells clearly contribute to bone loss via production of ACPA. These antibodies both directly activate osteoclasts and stimulate innate immune cells to increase pro-osteoclastogenic inflammatory cytokines including TNFα, IL-1, and IL-6. These cytokines contribute to bone loss both by promoting osteoclastogenesis and by inhibiting osteoblastic bone formation.

STUDY QUESTIONS   

1. D  escribe the features that distinguish innate and adaptive immunity and delineate the major immune cell types involved in each. 2. Describe how T-cell subsets can promote or inhibit bone resorption. 3. Describe several bacterial and host factors that facilitate establishment of chronic bone infection. 4. Which cells of the immune system promote osteoclast formation through production of inflammatory cytokines such as TNFα, IL-1, and IL-6? 5. How do B cells inhibit osteoclast formation during bone remodeling? How do B cells promote osteoclast formation in RA?   

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Suggested Readings

10. Kono H, Rock KL. How dying cells alert the immune system to danger. Nat. Rev. Immunol. 2008;8:279–289. 11. Okamoto K, Nakashima T, Shinohara M, et al. Osteoimmunology: the conceptual framework unifying the immune and skeletal systems. Physiol. Rev. 2017;97:1295–1349. 12. Onal M, Xiong J, Chen X, et al. Receptor activator of nuclear factor kappaB ligand (RANKL) protein expression by B lymphocytes contributes to ovariectomy-induced bone loss. J. Biol. Chem. 2012;287:29851–29860. 13. Pacifici R. T cells, osteoblasts, and osteocytes: interacting lineages key for the bone anabolic and catabolic activities of parathyroid hormone. Ann. N. Y. Acad. Sci. 2016;1364:11–24. 14. Panaroni C, Fulzele K, Saini V, Chubb R, Pajevic PD, Wu JY. PTH signaling in osteoprogenitors is essential for B-lymphocyte differentiation and mobilization. J. Bone Miner. Res. 2015;30:2273–2286.

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15. Panaroni C, Wu JY. Interactions between B lymphocytes and the osteoblast lineage in bone marrow. Calcif. Tissue Int. 2013;93:261–268. 16. Sinder BP, Pettit AR, McCauley LK. Macrophages: their emerging roles in bone. J. Bone Miner. Res. 2015;30:2140–2149. 17. Wakkach A, Mansour A, Dacquin R, et al. Bone marrow microenvironment controls the in vivo differentiation of murine dendritic cells into osteoclasts. Blood. 2008;112:5074–5083. 18. Walsh MC, Takegahara N, Kim H, Choi Y. Updating osteoimmunology: regulation of bone cells by innate and adaptive immunity. Nat. Rev. Rheumatol. 2018;14:146–156. 19. Wu Y, Humphrey MB, Nakamura MC. Osteoclasts - the innate immune cells of the bone. Autoimmunity. 2008;41:183–194.

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