Induction of colony-stimulating factor expression following Staphylococcus or Salmonella interaction with mouse or human osteoblasts

Intracellular Staphylococcus aureus in osteoblasts and osteocytes and its impact on bone homeostasis during osteomyelitis

Osteomyelitis is a severe infection of bone tissue that can lead to bone loss and even osteonecrosis. This condition is mostly caused by Gram-positive bacteria, with Staphylococcus aureus being the most common etiological agent. Among the pathophysiological mechanisms involved in osteomyelitis, the ability of S. aureus to be internalized by osteoblasts or osteocytes and to survive within these cells, is particularly noteworthy. Infected osteoblasts and osteocytes not only serve as reservoirs in chronic cases of osteomyelitis but also play an active role in the osteoimmunology process, notably by producing mediators that promote the bone resorption activity of osteoclasts, thereby disrupting bone homeostasis. The present review explores both historical and recent literature on the internalization of S. aureus by osteoblasts and osteocytes, its intracellular behavior following internalization, and its mechanisms for inducing cell death. Additionally, it examines how S. aureus affects bone formation activity and promotes the production of inflammatory and pro-osteoclastic mediators. This review aims to highlight the limitations of current findings and outline key questions for future investigations.

RIG-I and cGAS mediate antimicrobial and inflammatory responses of primary osteoblasts and osteoclasts to Staphylococcus aureus

Staphylococcus aureus is the primary causative agent of osteomyelitis, and it is now apparent that osteoblasts and osteoclasts play a significant role in the pathogenesis of such infections. Their responses can either be protective or exacerbate inflammatory bone loss and are mediated by the recognition of microbial motifs by various pattern recognition receptors. We have recently reported that osteoblasts can respond to S. aureus challenge with the production of the type I interferon, interferon-beta, which can reduce the number of viable bacteria harbored within infected cells. In the present study, we demonstrate that S. aureus viability and internalization are necessary for maximal inflammatory cytokine and type I interferon responses of primary bone cells to this pathogen. Importantly, we show that primary murine and human bone cells constitutively express the cytosolic nucleic acid sensors, retinoic acid inducible gene I (RIG-I) and cyclic GMP-AMP synthase (cGAS), and demonstrate that such expression is markedly upregulated following S. aureus infection. The functional status of RIG-I and cGAS in osteoblasts and osteoclasts was confirmed by showing that specific ligands for each can also elevate their expression and induce cytokine responses. We have verified the specificity of such responses using siRNA knockdown or pharmacological inhibition and used these approaches to demonstrate that both sensors play a pivotal role in bone cell responses to infection with clinically relevant strains of S. aureus. Finally, we have begun to establish the biological significance of RIG-I- and cGAS-mediated bone cell responses with the demonstration that their attenuation increases S. aureus burden in infected cells, suggesting a potentially protective role for these sensors in osteomyelitis.IMPORTANCEStaphylococcal osteomyelitis is a severe infection that is often recalcitrant to current treatment strategies. We and others have demonstrated that resident bone cells are not merely passive victims but can respond to bacteria with the production of an array of immune mediators, including type I interferons, that could serve to limit such infections. Here, we demonstrate the functional expression of two cytosolic nucleic acid sensors, retinoic acid inducible gene I and cyclic GMP-AMP synthase, in primary murine and human osteoblasts and murine osteoclasts. We show that these pattern recognition receptors mediate potentially protective bone cell type I interferon responses to Staphylococcus aureus infection.

A review of bacterial and osteoclast differentiation in bone infection

Bone infections are characterized by bacterial invasion of the bone microenvironment and subsequent bone structure deterioration. This holds significance because osteoclasts, which are the only cells responsible for bone resorption, are abnormally stimulated during bone infections. Multiple communication factors secreted by bone stromal cells regulate the membrane of osteoclast progenitor cells, thereby maintaining bone homeostasis through the expression of many types of receptors. During infection, the immunoinflammatory response triggered by bacterial invasion and multiple virulence factors of bacterial origin can disrupt osteoclast homeostasis. Therefore, clarifying the pathways through which bacteria affect osteoclasts can offer a theoretical basis for preventing and treating bone infections. This review summarizes studies investigating bone destruction caused by different bacterial infections. In conclusion, bacteria can affect osteoclast metabolic activity through multiple pathways, including direct contact, release of virulence factors, induction of immunoinflammatory responses, influence on bone stromal cell metabolism, and intracellular infections.

Dual-functional composite scaffolds for inhibiting infection and promoting bone regeneration

The treatment of infected bone defects is an intractable problem in orthopedics. It comprises two critical parts, namely that of infection control and bone defect repair. According to these two core tasks during treatment, the ideal approach of simultaneously controlling infection and repairing bone defects is promising treatment strategy. Several engineered biomaterials and drug delivery systems with dual functions of anti-bacterial action and ostogenesis-promotion have been developed and demonstrated excellent therapeutic effects. Compared with the conventional treatment method, the dual-functional composite scaffold can provide one-stage treatment avoiding multiple surgeries, thereby remarkably simplifying the treatment process and reducing the treatment time, overcoming the disadvantages of conventional bone transplantation. In this review, the impaired bone repair ability and its specific mechanisms in the microenvironment of pathogen infection and excessive inflammation were analyzed, providing a theoretical basis for the treatment of infectious bone defects. Furthermore, we discussed the composite dual-functional scaffold composed of a combination of antibacterial and osteogenic material. Finally, a series of advanced drug delivery systems with antibacterial and bone-promoting capabilities were summarized and discussed. This review provides a comprehensive understanding for the microenvironment of infectious bone defects and leading-edge design strategies for the antibacterial and bone-promoting dual-function scaffold, thus providing clinically significant treatment methods for infectious bone defects.

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Antibacterial and bone-promoting dual-function scaffolds are ideal strategies for treatment of infectious bone defects.

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The effect of infection on bone repair was summarized in detail from four important aspects.

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A variety of dual-function scaffolds based on antibacterial and osteogenic materials were discussed.

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Dual-function drug delivery systems promoting repair of infectious bone defects by locally releasing functional agents.

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Leading-edge design strategies, challenges and prospects for dual-functional biomaterials were provided.

The osteoblast secretome in Staphylococcus aureus osteomyelitis

Osteomyelitis (OM) is an infectious disease of the bone predominantly caused by the opportunistic bacterium Staphylococcus aureus (S. aureus). Typically established upon hematogenous spread of the pathogen to the musculoskeletal system or contamination of the bone after fracture or surgery, osteomyelitis has a complex pathogenesis with a critical involvement of both osteal and immune components. Colonization of the bone by S. aureus is traditionally proposed to induce functional inhibition and/or apoptosis of osteoblasts, alteration of the RANKL/OPG ratio in the bone microenvironment and activation of osteoclasts; all together, these events locally subvert tissue homeostasis causing pathological bone loss. However, this paradigm has been challenged in recent years, in fact osteoblasts are emerging as active players in the induction and orientation of the immune reaction that mounts in the bone during an infection. The interaction with immune cells has been mostly ascribed to osteoblast-derived soluble mediators that add on and synergize with those contributed by professional immune cells. In this respect, several preclinical and clinical observations indicate that osteomyelitis is accompanied by alterations in the local and (sometimes) systemic levels of both pro-inflammatory (e.g., IL-6, IL-1α, TNF-α, IL-1β) and anti-inflammatory (e.g., TGF-β1) cytokines. Here we revisit the role of osteoblasts in bacterial OM, with a focus on their secretome and its crosstalk with cellular and molecular components of the bone microenvironment and immune system.

Vitamin K alleviates bone calcium loss caused by Salmonella Enteritidis through carboxylation of osteocalcin

Background

The present study aimed at evaluating the effect of vitamin K (VK) supplementation on bone health of laying hens challenged by Salmonella Enteritidis.

Methods

A total of 80 32-week-old double negative salmonella-free brown-egg laying hens were randomly assigned to 4 treatments with 20 replicates each (1 bird per replicate) according to a 2 × 2 factorial design with 2 dietary VK supplementation levels [0 mg/kg (VK0) vs 2 mg/kg VK (VK2) and 2 challenge treatments [Salmonella Enteritidis (SE) vs physiological saline solution (PS)]. During the last 3 days of week 43 of age, birds of both VK treatments were either orally challenged with 1.0 mL suspension of 10⁹ cfu/mL S. Enteritidis daily or received the same volume of PS.

Results

The laying rate, daily egg mass, tibia strength, CT, cOC and cOC/(cOC + ucOC) of VK2 treatment increased (P < 0.05) in contrast to VK0, however, the medullary area and ucOC of VK2 treatment decreased (P < 0.05) in contrast to VK0. Mortality, medullary area, serum Ca content of SE treatments increased (P < 0.05) in contrast to PS treatments. In both SE treatments, the decrease (P < 0.05) in birds’ tibia strength was associated with higher (P < 0.05) Ca levels in serum. There is an interaction (P < 0.05) between SE challenge and VK levels with regard to tibia strength and serum Ca levels. At week 42, serum CT was positively correlated with cOC (R = 0.99, P = 0.009); at week 44, tibia strength was positively correlated with BMD (R = 0.95, P = 0.045), but negatively correlated with medullary area (R = − 0.98, P = 0.018).

Conclusions

VK (2 mg/kg) supplementation to diets of laying hens can enhance bone strength under challenge situations with Salmonella Enteritidis. Medullary area has proven to be a sensitive biomarker for bone calcium loss caused by SE infection.

Printing the Pathway Forward in Bone Metastatic Cancer Research: Applications of 3D Engineered Models and Bioprinted Scaffolds to Recapitulate the Bone-Tumor Niche

Breast cancer commonly metastasizes to bone, resulting in osteolytic lesions and poor patient quality of life. The bone extracellular matrix (ECM) plays a critical role in cancer cell metastasis by means of the physical and biochemical cues it provides to support cellular crosstalk. Current two-dimensional in-vitro models lack the spatial and biochemical complexities of the native ECM and do not fully recapitulate crosstalk that occurs between the tumor and endogenous stromal cells. Engineered models such as bone-on-a-chip, extramedullary bone, and bioreactors are presently used to model cellular crosstalk and bone-tumor cell interactions, but fall short of providing a bone-biomimetic microenvironment. Three-dimensional bioprinting allows for the deposition of biocompatible materials and living cells in complex architectures, as well as provides a means to better replicate biological tissue niches in-vitro. In cancer research specifically, 3D constructs have been instrumental in seminal work modeling cancer cell dissemination to bone and bone-tumor cell crosstalk in the skeleton. Furthermore, the use of biocompatible materials, such as hydroxyapatite, allows for printing of bone-like microenvironments with the ability to be implanted and studied in in-vivo animal models. Moreover, the use of bioprinted models could drive the development of novel cancer therapies and drug delivery vehicles.

A comprehensive review of bacterial osteomyelitis with emphasis on Staphylococcus aureus

Osteomyelitis, a significant infection of bone tissue, gives rise to two main groups of infection: acute and chronic. These groups are further categorized in terms of the duration of infection. Usually, children and adults are more susceptible to acute and chronic infections, respectively. The aforementioned groups of osteomyelitis share almost 80% of the corresponding bacterial pathogens. Among all bacteria, Staphylococcus aureus (S. aureus) is a significant pathogen and is associated with a high range of osteomyelitis symptoms. S. aureus has many strategies for interacting with host cells including Small Colony Variant (SCV), biofilm formation, and toxin secretion. In addition, it induces an inflammatory response and causes host cell death by apoptosis and necrosis. However, any possible step to take in this respect is dependent on the conditions and host responses. In the absence of any immune responses and antibiotics, bacteria actively duplicate themselves; however, in the presence of phagocytic cell and harassing conditions, they turn into a SCV, remaining sustainable for a long time. SCV is characterized by notable advantages such as (a) intracellular life that mediates a dam against immune cells and (b) low ATP production that mediates resistance against antibiotics.

From Crosstalk between Immune and Bone Cells to Bone Erosion in Infection

Bone infection and inflammation leads to the infiltration of immune cells at the site of infection, where they modulate the differentiation and function of osteoclasts and osteoblasts by the secretion of various cytokines and signal mediators. In recent years, there has been a tremendous effort to understand the cells involved in these interactions and the complex pathways of signal transduction and their ultimate effect on bone metabolism. These crosstalk mechanisms between the bone and immune system finally emerged, forming a new field of research called osteoimmunology. Diseases falling into the category of osteoimmunology, such as osteoporosis, periodontitis, and bone infections are considered to have a significant implication in mortality and morbidity of patients, along with affecting their quality of life. There is a much-needed research focus in this new field, as the reported data on the immunomodulation of immune cells and their signaling pathways seems to have promising therapeutic benefits for patients.

Cancer Metastases to Bone: Concepts, Mechanisms, and Interactions with Bone Osteoblasts

The skeleton is a unique structure capable of providing support for the body. Bone resorption and deposition are controlled in a tightly regulated balance between osteoblasts and osteoclasts with no net bone gain or loss. However, under conditions of disease, the balance between bone resorption and deposition is upset. Osteoblasts play an important role in bone homeostasis by depositing new bone osteoid into resorption pits. It is becoming increasingly evident that osteoblasts additionally play key roles in cancer cell dissemination to bone and subsequent metastasis. Our laboratory has evidence that when osteoblasts come into contact with disseminated breast cancer cells, the osteoblasts produce factors that initially reduce breast cancer cell proliferation, yet promote cancer cell survival in bone. Other laboratories have demonstrated that osteoblasts both directly and indirectly contribute to dormant cancer cell reactivation in bone. Moreover, we have demonstrated that osteoblasts undergo an inflammatory stress response in late stages of breast cancer, and produce inflammatory cytokines that are maintenance and survival factors for breast cancer cells and osteoclasts. Advances in understanding interactions between osteoblasts, osteoclasts, and bone metastatic cancer cells will aid in controlling and ultimately preventing cancer cell metastasis to bone.

Biomimetic 3D in vitro model of biofilm triggered osteomyelitis for investigating hematopoiesis during bone marrow infections

In this work, we define the requirements for a human cell-based osteomyelitis model which overcomes the limitations of state of the art animal models. Osteomyelitis is a severe and difficult to treat infection of the bone that develops rapidly, making it difficult to study in humans. We have developed a 3D in vitro model of the bone marrow, comprising a macroporous material, human hematopoietic stem and progenitor cells (HSPCs) and mesenchymal stromal cells (MSCs). Inclusion of biofilms grown on an implant into the model system allowed us to study the effects of postoperative osteomyelitis-inducing bacteria on the bone marrow. The bacteria influenced the myeloid differentiation of HSPCs as well as MSC cytokine expression and the MSC ability to support HSPC maintenance. In conclusion, we provide a new 3D in vitro model which meets all the requirements for investigating the impact of osteomyelitis.

STATEMENT OF SIGNIFICANCE

Implant-associated osteomyelitis is a persistent bacterial infection of the bone which occurs in many implant patients and can result in functional impairments or even entire loss of the extremity. Nevertheless, surprisingly little is known on the triangle interaction between implant material, bacterial biofilm and affected bone tissue. Closing this gap of knowledge would be crucial for the fundamental understanding of the disease and the development of novel treatment strategies. For this purpose, we developed the first biomaterial-based system that is able to mimic implant-associated osteomyelitis outside of the body, thus, opening the avenue to study this fatal disease in the laboratory.

Staphylococcal Osteomyelitis: Disease Progression, Treatment Challenges, and Future Directions

Osteomyelitis is an inflammatory bone disease that is caused by an infecting microorganism and leads to progressive bone destruction and loss. The most common causative species are the usually commensal staphylococci, with Staphylococcus aureus and Staphylococcus epidermidis responsible for the majority of cases. Staphylococcal infections are becoming an increasing global concern, partially due to the resistance mechanisms developed by staphylococci to evade the host immune system and antibiotic treatment. In addition to the ability of staphylococci to withstand treatment, surgical intervention in an effort to remove necrotic and infected bone further exacerbates patient impairment. Despite the advances in current health care, osteomyelitis is now a major clinical challenge, with recurrent and persistent infections occurring in approximately 40% of patients. This review aims to provide information about staphylococcus-induced bone infection, covering the clinical presentation and diagnosis of osteomyelitis, pathophysiology and complications of osteomyelitis, and future avenues that are being explored to treat osteomyelitis.

Bone responses in health and infectious diseases: A focus on osteoblasts

Historically, bone was thought to be immunologically inactive with the sole function of supporting locomotion and ensuring stromaness functions as a major lymphoid organ. However, a myriad of pathogens (bacteria such as staphylococcus as well as viruses including alphaviruses, HIV or HCV) can invade the bone. These pathogens can cause apoptosis, autophagy and necrosis of osteoblasts and lead to lymphopenia and immune paralysis. There are now several detailed studies on how osteoblasts contribute to innate immune and inflammatory responses; indeed, osteoblasts in concert with resident macrophages can engage an armory of defense mechanisms capable of detecting and controlling pathogen evasion mechanisms. Osteoblasts can express the so-called pattern recognition receptors such as TOLL-like receptors involved in the detection for example of lipids and unique sugars (polysaccharides and polyriboses) expressed by bacteria or viruses (e.g. LPS and RNA respectively). Activated osteoblasts can produce interferon type I, cytokines, chemokines and interferon-stimulated proteins through autocrine and paracrine mechanisms to control for viral replication and to promote phagocytosis or lysis of bacteria for example by defensins. Uncontrolled and sustained innate immune activation of infected osteoblasts will also lead to an imbalance in the production of osteoclastogenic factors such as RANKL and osteoprotegerin involved in bone repair.

Staphylococcus aureus vs. Osteoblast: Relationship and Consequences in Osteomyelitis

Bone cells, namely osteoblasts and osteoclasts work in concert and are responsible for bone extracellular matrix formation and resorption. This homeostasis is, in part, altered during infections by Staphylococcus aureus through the induction of various responses from the osteoblasts. This includes the over-production of chemokines, cytokines and growth factors, thus suggesting a role for these cells in both innate and adaptive immunity. S. aureus decreases the activity and viability of osteoblasts, by induction of apoptosis-dependent and independent mechanisms. The tight relationship between osteoclasts and osteoblasts is also modulated by S. aureus infection. The present review provides a survey of the relevant literature discussing the important aspects of S. aureus and osteoblast interaction as well as the ability for antimicrobial peptides to kill intra-osteoblastic S. aureus, hence emphasizing the necessity for new anti-infectious therapeutics.

Repercussions of NSAIDS drugs on bone tissue: the osteoblast

Non-steroidal anti-inflammatory drugs (NSAIDs) can act by modulating the behavior of osteoblasts, including their proliferation, differentiation, adhesion, and migration, but not all NSAIDs have these effects. Our objective was to update the information on this issue in a review of the literature in order to offer guidance on the prescription of the appropriate NSAID(s) to patients requiring bone tissue repair. To review current knowledge of this issue by searching for all relevant publications since 2001 in the MEDLINE, EMBASE and Cochrane Library databases, we used the following descriptors: bone tissue, osteoblast, NSAIDs, Anti-inflammatory drugs. Published studies show that most NSAIDs have an adverse effect on osteoblast growth by cell cycle arrest and apoptosis induction. The effect on differentiation varies according to the drug, dose, and treatment time. Osteoblast adhesion is increased and migration decreased by some NSAIDs, such as indomethacin and diclofenac. The antigenic profile or phagocytic function can also be modulated by NSAIDs. In general, NSAIDs have an adverse effect on bone tissue and given the routine administration of NSAIDs to individuals requiring bone repair, in which the osteoblast has an essential role, this effect on bone should be borne in mind.

Apoptosis-associated uncoupling of bone formation and resorption in osteomyelitis

The mechanisms underlying the destruction of bone tissue in osteomyelitis are only now being elucidated. While some of the tissue damage associated with osteomyelitis likely results from the direct actions of bacteria and infiltrating leukocytes, perhaps exacerbated by bacterial manipulation of leukocyte survival pathways, infection-induced bone loss predominantly results from an uncoupling of the activities of osteoblasts and osteoclasts. Bacteria or their products can directly increase osteoclast formation and activity, and the inflammatory milieu at sites of infection can further promote bone resorption. In addition, osteoclast activity is critically regulated by osteoblasts that can respond to bacterial pathogens and foster both inflammation and osteoclastogenesis. Importantly, bone loss during osteomyelitis is also brought about by a decline in new bone deposition due to decreased bone matrix synthesis and by increased rates of osteoblast apoptosis. Extracellular bacterial components may be sufficient to reduce osteoblast viability, but the causative agents of osteomyelitis are also capable of inducing continuous apoptosis of these cells by activating intrinsic and extrinsic cell death pathways to further uncouple bone formation and resorption. Interestingly, bacterial internalization appears to be required for maximal osteoblast apoptosis, and cytosolic inflammasome activation may act in concert with autocrine/paracrine death receptor-ligand signaling to induce cell death. The manipulation of apoptotic pathways in infected bone cells could be an attractive new means to limit inflammatory damage in osteomyelitis. However, the mechanism that is the most important in bacterium-induced bone loss has not yet been identified. Furthermore, it remains to be determined whether the host would be best served by preventing osteoblast cell death or by promoting apoptosis in infected cells.

A PDMS/paper/glass hybrid microfluidic biochip integrated with aptamer-functionalized graphene oxide nano-biosensors for one-step multiplexed pathogen detection

Infectious pathogens often cause serious public health concerns throughout the world. There is an increasing demand for simple, rapid and sensitive approaches for multiplexed pathogen detection. In this paper we have developed a polydimethylsiloxane (PDMS)/paper/glass hybrid microfluidic system integrated with aptamer-functionalized graphene oxide (GO) nano-biosensors for simple, one-step, multiplexed pathogen detection. The paper substrate used in this hybrid microfluidic system facilitated the integration of aptamer biosensors on the microfluidic biochip, and avoided complicated surface treatment and aptamer probe immobilization in a PDMS or glass-only microfluidic system. Lactobacillus acidophilus was used as a bacterium model to develop the microfluidic platform with a detection limit of 11.0 cfu mL(-1). We have also successfully extended this method to the simultaneous detection of two infectious pathogens - Staphylococcus aureus and Salmonella enterica. This method is simple and fast. The one-step 'turn on' pathogen assay in a ready-to-use microfluidic device only takes ~10 min to complete on the biochip. Furthermore, this microfluidic device has great potential in rapid detection of a wide variety of different other bacterial and viral pathogens.

Differential interactions of Streptococcus gordonii and Staphylococcus aureus with cultured osteoblasts

The impedance of normal osteoblast function by microorganisms is at least in part responsible for the failure of dental or orthopedic implants. Staphylococcus aureus is a major pathogen of bone, and exhibits high levels of adhesion and invasion of osteoblasts. In this article we show that the commensal oral bacterium Streptococcus gordonii also adheres to and is internalized by osteoblasts. Entry of S. gordonii cells had typical features of phagocytosis, similar to S. aureus, with membrane protrusions characterizing initial uptake, and closure of the osteoblast membrane leading to engulfment. The sensitivities of S. gordonii internalization to inhibitors cytochalasin D, colchicine and monensin indicated uptake through endocytosis, with requirement for actin accumulation. Internalization levels of S. gordonii were enhanced by expression of S. aureus fibronectin-binding protein A (FnBPA) on the S. gordonii cell surface. Lysosomal-associated membrane protein-1 phagosomal membrane marker accumulated with intracellular S. aureus and S. gordonii FnBPA, indicating trafficking of bacteria into the late endosomal/lysosomal compartment. Streptococcus gordonii cells did not survive intracellularly for more than 12 h, unless expressing FnBPA, whereas S. aureus showed extended survival times (>48 h). Both S. aureus and S. gordonii DL-1 elicited a rapid interleukin-8 response by osteoblasts, whereas S. gordonii FnBPA was slower. Only S. aureus elicited an interleukin-6 response. Hence, S. gordonii invades osteoblasts by a mechanism similar to that exhibited by S. aureus, and elicits a proinflammatory response that may promote bone resorption.

Interaction of Staphylococcus aureus with osteoblasts (Review)

Orthopedic infection is refractory to cure. Staphylococcus aureus (S. aureus) is the main causative pathogen responsible for orthopedic infection. S. aureus is capable of not only colonizing bone matrix, but also invading osteoblasts, which may play a significant role in the persistence and recurrence of osteomyelitis. Internalization requires the involvement of cytoskeletal elements, including actin microfilaments, microtubules and clathrin-coated pits. Microfilaments are most significant in the invasion process. S. aureus is capable of remaining alive in osteoblasts for a long period of time. Decreased sensitivity to antibiotics capable of penetrating host cells increases the difficulties of eradicating S. aureus. Osteoblasts, invaded by S. aureus, play a significant role in the initiation and maintenance of inflammatory immune responses. These osteoblasts recruit leukocytes and phagocytes to the site of inflammation via the expression of cytokines. Apoptosis is observed in osteoblasts invaded by S. aureus. Recruitment of osteoclasts and other immunocytes plays a crucial role in the resorption and destruction of bone.

Cytotoxic effects of Kingella kingae outer membrane vesicles on human cells

Kingella kingae is an emerging pathogen causing osteoarticular infections in pediatric patients. Electron microscopy of K. kingae clinical isolates revealed the heterogeneously-sized membranous structures blebbing from the outer membrane that were classified as outer membrane vesicles (OMVs). OMVs purified from the secreted fraction of a septic arthritis K. kingae isolate were characterized. Among several major proteins, K. kingae OMVs contained virulence factors RtxA toxin and PilC2 pilus adhesin. RtxA was also found secreted as a soluble protein in the extracellular environment indicating that the bacterium may utilize different mechanisms for the toxin delivery. OMVs were shown to be hemolytic and possess some leukotoxic activity while high leukotoxicity was detected in the non-hemolytic OMV-free component of the secreted fraction. OMVs were internalized by human osteoblasts and synovial cells. Upon interaction with OMVs, the cells produced increased levels of human granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin 6 (IL-6) suggesting that these cytokines might be involved in the signaling response of infected joint and bone tissues during natural K. kingae infection. This study is the first report of OMV production by K. kingae and demonstrates that OMVs are a complex virulence factor of the organism causing cytolytic and inflammatory effects on host cells.

Causative agents of osteomyelitis induce death domain-containing TNF-related apoptosis-inducing ligand receptor expression on osteoblasts

Bacteria and their products are potent inducers of bone destruction. While inflammatory damage during conditions such as osteomyelitis is associated with increased formation and activity of bone-resorbing osteoclasts, it is likely that bone loss also results from the elimination of the cells responsible for matrix deposition. Consistent with this notion, we have previously demonstrated that bone-forming osteoblasts undergo apoptosis following bacterial challenge and that this cell death is due, at least in part, to the actions of TNF-related apoptosis-inducing ligand (TRAIL). In the present study, we demonstrate that primary osteoblasts constitutively express death domain containing TRAIL receptors. Importantly, we show that cell surface expression of the death-inducing receptors DR4 and DR5 on murine and human osteoblasts is restricted to cells infected with the principle causative agents of osteomyelitis, Staphylococcus aureus and Salmonella. In addition, we show that the robust constitutive production by osteoblasts of the decoy TRAIL receptor, OPG, is inhibited following bacterial infection. Finally, we report that while exogenous administration of TRAIL fails to activate apoptosis signaling pathways in uninfected osteoblasts, acute bacterial exposure sensitizes these cells to this ligand. Based upon these findings we suggest a model in which bacterially challenged osteoblasts express TRAIL while concomitantly decreasing the production of the decoy receptor OPG and upregulating cell surface death receptor expression. Such an increase in TRAIL bioavailability and induced sensitivity of infected osteoblasts to this ligand would result in apoptotic cell death of this bone-forming population, providing an additional mechanism underlying inflammatory bone loss during diseases such as osteomyelitis.

Granulocyte-macrophage colony-stimulating factor- and tumor necrosis factor alpha-mediated matrix metalloproteinase production by human osteoblasts and monocytes after infection with Brucella abortus

Osteoarticular complications are common in human brucellosis, but the pathogenic mechanisms involved are largely unknown. Since matrix metalloproteinases (MMPs) are involved in joint and bone damage in inflammatory and infectious diseases, we investigated the production of MMPs by human osteoblasts and monocytes, either upon Brucella abortus infection or upon reciprocal stimulation with factors produced by each infected cell type. B. abortus infection of the normal human osteoblastic cell line hFOB 1.19 triggered a significant release of MMP-2, which was mediated in part by granulocyte-macrophage colony-stimulating factor (GM-CSF) acting on these same cells. Supernatants from infected osteoblasts exhibited increased levels of monocyte chemoattractant protein 1 and induced the migration of human monocytes (THP-1 cell line). Infection with B. abortus induced a high MMP-9 secretion in monocytes, which was also induced by heat-killed B. abortus and by the Omp19 lipoprotein from B. abortus. These effects were mediated by Toll-like receptor 2 and by the action of tumor necrosis factor alpha (TNF-α) produced by these same cells. Supernatants from B. abortus-infected monocytes induced MMP-2 secretion in uninfected osteoblasts, and this effect was mediated by TNF-α. Similarly, supernatants from infected osteoblasts induced MMP-9 secretion in uninfected monocytes. This effect was mediated by GM-CSF, which induced TNF-α production by monocytes, which in turn induced MMP-9 in these cells. These results suggest that MMPs could be potentially involved in the tissue damage observed in osteoarticular brucellosis.

Differential roles for NOD2 in osteoblast inflammatory immune responses to bacterial pathogens of bone tissue

Osteoblasts produce an array of immune molecules following bacterial challenge that can contribute to inflammation and the recruitment of leukocytes to sites of infection during bone diseases such as osteomyelitis. However, the mechanisms by which osteoblasts perceive and respond to facultative intracellular pathogens such as Salmonella species and Staphylococcus aureus have not been determined. Recently, our laboratory has described the expression in osteoblasts of members of the nucleotide-binding domain leucine-rich repeat region containing family of proteins that include nucleotide-binding oligomerization domain-2 (NOD2), a molecule that functions as an intracellular receptor for bacterial peptidoglycans. In the present study, we demonstrate that NOD2 expression is required for select inflammatory mediator production by osteoblasts following infection with the invasive pathogen Salmonella. In contrast, we have found that the inflammatory immune responses of osteoblasts to the passively internalized bacterial species Staphylococcus aureus, heat-killed pathogenic Salmonella, a non-invasive Salmonella strain and specific Toll-like receptor ligands are not reduced in the absence of NOD2 expression but are, in fact, elevated. Based upon these findings, we suggest that NOD2 serves differential roles in osteoblasts, promoting inflammatory responses to invasive bacteria while tempering cell responses to extracellular and/or passively internalized bacterial species.

Inhibition of macrophage activation by the myxoma virus M141 protein (vCD200)

The M141 protein of myxoma virus (MYXV) is a viral CD200 homolog (also called vOX-2) that inhibits macrophage activation in infected rabbits. Here, we show that murine myeloid RAW 264.7 cells became activated when infected with MYXV in which the M141 gene was deleted (vMyx-M141KO) but not with the parental wild-type MYXV. Moreover, transcript and protein levels of tumor necrosis factor and granulocyte colony-stimulating factor were rapidly upregulated in an NF-kappaB-dependent fashion in the RAW 264.7 cells infected with vMyx-M141KO. M141 protein is present in the virion and counteracts this NF-kappaB activation pathway upon infection with the wild-type MYXV. Our data suggest that upregulation of these classic macrophage-related proinflammatory cytokine markers following infection of myeloid cells with the M141-knockout MYXV is mediated via the rapid activation of the cellular NF-kappaB pathway.

Proinflammatory response of human osteoblastic cell lines and osteoblast-monocyte interaction upon infection with Brucella spp

The ability of Brucella spp. to infect human osteoblasts and the cytokine response of these cells to infection were investigated in vitro. Brucella abortus, B. suis, B. melitensis, and B. canis were able to infect the SaOS-2 and MG-63 osteoblastic cell lines, and the first three species exhibited intracellular replication. B. abortus internalization was not significantly affected by pretreatment of cells with cytochalasin D but was inhibited up to 92% by colchicine. A virB10 mutant of B. abortus could infect but not replicate within osteoblasts, suggesting a role for the type IV secretion system in intracellular survival. Infected osteoblasts produced low levels of chemokines (interleukin-8 [IL-8] and macrophage chemoattractant protein 1 [MCP-1]) and did not produce proinflammatory cytokines (IL-1beta, IL-6, and tumor necrosis factor alpha [TNF-alpha]). However, osteoblasts stimulated with culture supernatants from Brucella-infected human monocytes (THP-1 cell line) produced chemokines at levels 12-fold (MCP-1) to 17-fold (IL-8) higher than those of infected osteoblasts and also produced IL-6. In the inverse experiment, culture supernatants from Brucella-infected osteoblasts induced the production of IL-8, IL-1beta, IL-6, and TNF-alpha by THP-1 cells. The induction of TNF-alpha and IL-1beta was largely due to granulocyte-macrophage colony-stimulating factor produced by infected osteoblasts, as demonstrated by inhibition with a specific neutralizing antibody. This study shows that Brucella can invade and replicate within human osteoblastic cell lines, which can directly and indirectly mount a proinflammatory response. Both phenomena may have a role in the chronic inflammation and bone and joint destruction observed in osteoarticular brucellosis.

Staphylococcus aureus induces expression of receptor activator of NF-kappaB ligand and prostaglandin E2 in infected murine osteoblasts

Osteomyelitis is an inflammatory disease of the bone that is characterized by the presence of necrotic bone tissue and increased osteoclast activity. Staphylococcus aureus is responsible for approximately 80% of all cases of human osteomyelitis. While the disease is especially difficult to treat, the pathogenesis of S. aureus-induced osteomyelitis is poorly understood. Elucidating the molecular mechanisms by which S. aureus induces osteomyelitis could lead to a better understanding of the disease and its progression and development of new treatments. Osteoblasts can produce several soluble factors that serve to modulate the activity or formation of osteoclasts. Receptor activator of NF-kappaB ligand (RANK-L) and prostaglandin E(2) (PGE(2)) are two such molecules which can promote osteoclastogenesis and stimulate bone resorption. In addition, previous studies in our laboratory have shown that osteoblasts produce inflammatory cytokines, such as interleukin 6, following infection with S. aureus, which could induce COX-2 and in turn PGE(2), further modulating osteoclast recruitment and differentiation. Therefore, we hypothesized that following infection with S. aureus, osteoblasts will express increased levels of RANK-L and PGE(2). The results presented in this study provide evidence for the first time that RANK-L mRNA and protein and PGE(2) expression are upregulated in S. aureus-infected primary osteoblasts. In addition, through the use of the specific COX-2 inhibitor NS 398, we show that when PGE(2) production is inhibited, RANK-L production is decreased. These data suggest a mechanism whereby osteoblasts regulate the production of RANK-L during infection.

Osteoblasts express NLRP3, a nucleotide-binding domain and leucine-rich repeat region containing receptor implicated in bacterially induced cell death

Bacterially induced osteoblast apoptosis may be a major contributor to bone loss during osteomyelitis. We provide evidence for the functional expression in osteoblasts of NLRP3, a member of the NLR family of cytosolic receptors that has been implicated in the initiation of programmed cell death.

INTRODUCTION

Osteoblasts undergo apoptosis after exposure to intracellular bacterial pathogens commonly associated with osteomyelitis. Death of this bone-forming cell type, in conjunction with increased numbers and activity of osteoclasts, may underlie the destruction of bone tissue at sites of bacterial infection. To date, the mechanisms responsible for bacterially induced apoptotic osteoblast cell death have not been resolved.

MATERIALS AND METHODS

We used flow cytometric techniques to determine whether intracellular invasion is needed for maximal apoptotic cell death in primary osteoblasts after challenge with Salmonella enterica. In addition, we used real-time PCR and immunoblot analyses to assess osteoblast expression of members of the nucleotide-binding domain leucine-rich repeat region-containing family of intracellular receptors (NLRs) that have been predicted to be involved in the induction of programmed cell death. Furthermore, we have used co-immunoprecipitation and siRNA techniques to confirm the functionality of such sensors in this cell type.

RESULTS

In this study, we showed that invasion of osteoblasts by Salmonella is necessary for maximal induction of apoptosis. We showed that murine and human osteoblasts express NLRP3 (previously known as CIAS1, cryopyrin, PYPAF1, or NALP3) but not NLRC4 (IPAF) and showed that the level of expression of this cytosolic receptor is modulated after bacterial challenge. We showed that osteoblasts express ASC, an adaptor molecule for NLRP3, and that these molecules associate after Salmonella infection. In addition, we showed that a reduction in the expression of NLRP3 attenuates Salmonella-induced reductions in the activity of an anti-apoptotic transcription factor in osteoblasts. Furthermore, we showed that NLRP3 expression is needed for caspase-1 activation and maximal induction of apoptosis in osteoblasts after infection with Salmonella.

CONCLUSIONS

The functional expression of NLRP3 in osteoblasts provides a potential mechanism underlying apoptotic cell death of this cell type after challenge with intracellular bacterial pathogens and may be a significant contributory factor to bone loss at sites of infection.

TRAIL expression is induced in both osteoblasts containing intracellular Staphylococcus aureus and uninfected osteoblasts in infected cultures

Staphylococcus aureus is the principal etiological agent of osteomyelitis (bone infection), which is characterized by a progressive inflammatory response resulting in extensive damage to bone tissue. Recent studies have demonstrated the ability of S. aureus to invade and persist inside osteoblasts (bone matrix-forming cells) and other eukaryotic cells. The presence of intracellular S. aureus in bone tissue may be relevant to the pathology of osteomyelitis, a disease often refractory to antibiotic treatment and subject to recurrence months and even years after apparently successful therapy. The present study examined the production of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) following S. aureus infection, and whether expression of the molecule was induced by those osteoblasts containing intracellular S. aureus. Results from this study suggest that osteoblasts containing intracellular S. aureus induce TRAIL expression in uninfected osteoblasts present in infected cultures.

Involvement of alpha5beta1-integrin and TNF-alpha in Staphylococcus aureus alpha-toxin-induced death of epithelial cells

Staphylococcus aureus causes suppurative infections which are often associated with tissue destruction and cell death. In the present study, we investigated the molecular and cellular basis of S. aureus-induced apoptosis and death in a human lung epithelial cell line (A549). We found that staphylococcal alpha-toxin is an important mediator of cytotoxicity in these epithelial cells. Specifically, we found that downregulating alpha-toxin production eliminated the cytotoxicity of S. aureus, whereas the addition of alpha-toxin to the cell culture medium significantly increased cell death in a dose-dependent manner. Importantly, we found that alpha-toxin-mediated cell death may partially function through alpha5beta1-integrin, because both the beta1-integrin antibody and the ligand fibronectin inhibited the cytotoxicity of alpha-toxin. Furthermore, we found that the overexpression of the inflammatory cytokine interferon (TNF)-alpha is associated with alpha-toxin-induced cell death, because both the TNF-alpha release inhibitor and antibody effectively inhibited the cytotoxicity of alpha-toxin. In contrast, the cytotoxicity of alpha-toxin was enhanced by the inhibition of the MAPK p38 and NF-kappaB pathways. Taken together, our results suggest that the activation of the MAPK p38 and NF-kappaB pathways are stress responses for survival, rather than direct contributes to alpha-toxin-induced cell death, and that the interaction of alpha-toxin with alpha5beta1-integrin and overproduction of TNF-alpha may contribute to destruction of epithelial cells during S. aureus infection.

Heterotopic ossification following traumatic and combat-related amputations. Prevalence, risk factors, and preliminary results of excision

BACKGROUND

Although infrequently reported in amputees previously, heterotopic ossification has proven to be a common and problematic clinical entity in our recent experience in the treatment of traumatic and combat-related amputations related to Operation Enduring Freedom and Operation Iraqi Freedom. The purpose of the present study was to report the prevalence of and risk factors for heterotopic ossification following trauma-related amputation as well as the preliminary results of operative excision.

METHODS

We identified 330 patients with a total of 373 traumatic and combat-related amputations who had been managed at our centers between September 11, 2001 and November 30, 2005. We reviewed the medical records and radiographs of 187 patients with 213 amputations who had adequate radiographic follow-up. Additional analysis was performed for twenty-four patients with twenty-five limbs that required excision of symptomatic lesions. The mechanism and zone of injury, amputation level, timing of excision, use of prophylaxis against recurrence, and other confounding variables were examined. Outcomes were assessed by determining clinical and radiographic recurrence rates, perioperative complications, preoperative and follow-up pain medication requirements, and the ability to be fit with a functional prosthesis.

RESULTS

Heterotopic ossification was present in 134 (63%) of 213 residual limbs, with twenty-five lesions requiring excision. A final amputation level within the zone of injury was a risk factor for both the development and the grade of heterotopic ossification (p < 0.05). A blast mechanism was predictive of occurrence (p < 0.05) but did not correlate with grade. All patients who had been managed with excision were tolerating the prosthetic limb at an average of twelve months of follow-up. Twenty-three limbs demonstrated no evidence of recurrence, and two limbs had development of clinically asymptomatic, radiographically minimal recurrences. Six patients experienced wound-related complications that required reoperation, and two patients required subsequent minor revision surgery. There was a significant decrease in the use of pain medication following surgery (p < 0.05).

CONCLUSIONS

Heterotopic ossification following trauma-related amputation is more common than the literature would suggest, particularly following amputations that are performed within the initial zone of injury and those that are due to blast injuries. Many patients are asymptomatic or can be successfully managed with modification of the prosthesis. For patients with refractory symptoms, surgical excision is associated with low recurrence rates and decreased medication requirements, with acceptable complication rates.

Murine osteoblasts respond to LPS and IFN-gamma similarly to macrophages

Osteoblasts are bone-forming mesenchymal cells, while macrophages are cells of hematopoietic origin responsible for innate immunity. Lipopolysaccharide (LPS) can induce tolerance in macrophages, whereas interferon (IFN)-gamma can activate macrophages to produce cytokines, exert bactericidal effects, and present antigens. In this study, we examined such macrophagic phenotypes regulated by LPS and IFN-gamma in murine osteoblasts. In both primary calvarial osteoblasts and osteoblastic MC3T3-E1 cells, LPS pretreatment resulted in reduced production of IL-6 in response to a subsequent LPS stimulation or to Salmonella infection, indicating the existence of LPS-induced tolerance in osteoblasts. Furthermore, IFN-gamma treatment of MC3T3-E1 cells resulted in both enhanced IL-6 production in response to LPS and upregulation of major histocompatibility complex class II (MHC II). Following infection, Salmonella-containing vacuoles (SCVs) were formed in MC3T3-E1 cells, and IFN-gamma pretreatment enhanced bactericidal effects on intracellular Salmonella. Taken together, these observations indicate that osteoblasts can exhibit a subset of phenotypes reminiscent of macrophages in the course of bacterial infection.

Intracellular Staphylococcus aureus and antibiotic resistance: implications for treatment of staphylococcal osteomyelitis

Staphylococcus aureus is responsible for 80% of human osteomyelitis. It can invade and persist within osteoblasts. Antibiotic resistant strains of S. aureus make successful treatment of osteomyelitis difficult.

NULL HYPOTHESIS

antibiotic sensitivities of S. aureus do not change after exposure to the osteoblast intracellular environment. Human and mouse osteoblast cultures were infected and S. aureus cells were allowed to invade. Following times 0, 12, 24, and 48 h ( +/- the addition of erythromycin, clindamycin, and rifampin at times 0 or 12 h), the osteoblasts were lysed and intracellular bacteria enumerated. Transmission electron microscopy was performed on extracellular and intracellular S. aureus cells. In mouse osteoblasts, administration of bacteriostatic antibiotics at time 0 prevented the increase in intracellular S. aureus. If the antibiotics were delayed 12 h, this did not occur. When rifampin (bactericidal) was introduced at time 0 to human and mouse osteoblasts, there was a significant decrease in number of intracellular S. aureus within osteoblasts compared to control. If rifampin was delayed 12 h, this did not occur. Significant time-dependent S. aureus structural changes were observed after exposure to the osteoblast intracellular environment. These studies demonstrate that once S. aureus is established intracellularly for 12 h, the bacteria are less sensitive to antibiotics capable of eukaryotic cell penetration (statistically significant). These antibiotic sensitivity changes could be due in part to the observed structural changes. This leads to the rejection of our null hypotheses that the antibiotic sensitivities of S. aureus are unaltered by their location.

Osteoblasts produce monocyte chemoattractant protein-1 in a murine model of Staphylococcus aureus osteomyelitis and infected human bone tissue

Incidences of osteomyelitis caused by Staphylococcus aureus have increased dramatically in recent years, in part, due to the appearance of community-acquired antibiotic-resistant strains. Therefore, understanding the pathogenesis of this organism has become imperative. Recently, we have described the surprising ability of bone-forming osteoblasts to secrete a number of important immune mediators when exposed to S. aureus in vitro. In the present study, we provide the first evidence for the in vivo production of the pivotal inflammatory chemokine, monocyte chemoattractant protein-1 (MCP-1), by osteoblasts during S. aureus-associated bone infection. Quantitative real-time PCR was employed to determine levels of mRNA encoding MCP-1 in vivo using a mouse model that closely resembles the pathology of trauma-induced staphylococcal osteomyelitis. Expression of this inflammatory chemokine and osteoblast-specific markers was investigated by confocal laser scanning microscopy in bone tissue from organ cultures of neonatal mouse calvaria and from the in vivo mouse model. Furthermore, the clinical relevancy of these findings was investigated by performing similar studies on infected human bone tissue from patients with S. aureus-associated osteomyelitis. Here, we confirm that expression of mRNA encoding MCP-1 is elevated in bacterially infected murine bone tissue. Importantly, we show that these increases translate into marked elevations in the expression of MCP-1 protein that co-localizes with osteoblast markers in infected bone tissue. Such increases could not be attributed solely to mechanical damage as a similar response was observed in infected but otherwise undamaged organ cultures. Finally, we have demonstrated the in vivo production of MCP-1 by osteoblasts in bone specimens from patients with S. aureus-associated osteomyelitis. As such, these studies demonstrate that bacterial challenge of osteoblasts during bone diseases such as staphylococcal osteomyelitis induces cells to produce a key inflammatory chemokine that can direct appropriate host responses or may contribute to progressive inflammatory damage.

Induction of Nod1 and Nod2 intracellular pattern recognition receptors in murine osteoblasts following bacterial challenge

Osteoblasts produce an array of immune molecules following bacterial challenge that could recruit leukocytes to sites of infection and promote inflammation during bone diseases, such as osteomyelitis. Recent studies from our laboratory have shed light on the mechanisms by which this cell type can perceive and respond to bacteria by demonstrating the functional expression of members of the Toll-like family of cell surface pattern recognition receptors by osteoblasts. However, we have shown that bacterial components fail to elicit immune responses comparable with those seen following challenge with the intracellular pathogens salmonellae and Staphylococcus aureus. In the present study, we show that UV-killed bacteria and invasion-defective bacterial strains elicit significantly less inflammatory cytokine production than their viable wild-type counterparts. Importantly, we demonstrate that murine osteoblasts express the novel intracellular pattern recognition receptors Nod1 and Nod2. Levels of mRNA encoding Nod molecules and protein expression are significantly and differentially increased from low basal levels following exposure to these disparate bacterial pathogens. In addition, we have shown that osteoblasts express Rip2 kinase, a critical downstream effector molecule for Nod signaling. Furthermore, to begin to establish the functional nature of Nod expression, we show that a specific ligand for Nod proteins can significantly augment immune molecule production by osteoblasts exposed to either UV-inactivated bacteria or bacterial lipopolysaccharide. As such, the presence of Nod proteins in osteoblasts could represent an important mechanism by which this cell type responds to intracellular bacterial pathogens of bone.

Osteoblast responses to bacterial pathogens: a previously unappreciated role for bone-forming cells in host defense and disease progression

Although the primary roles of osteoblasts are to synthesize the components of bone matrix and to regulate the activity of bone resorbing osteoclasts, there is growing realization that osteoblasts have an additional function during bone diseases, such as osteomyelitis. Based on our recent studies, we propose a novel role for osteoblasts during bacterial infections of bone, namely, the initiation and maintenance of inflammatory immune responses. In this article, we describe how these nonleukocytic cells can perceive bacterial pathogens of bone to initiate the production of an array of immune regulatory molecules. This pattern of expression is one that could promote the recruitment of leukocytes to sites of bacterial challenge, initiate antigen-specific activation of infiltrating cells, and facilitate the development of cell-mediated host responses to intracellular pathogens of bone tissue, thereby identifying this cell type as a previously unappreciated component in host responses.

Synovial fibroblasts infected with Salmonella enterica serovar Typhimurium mediate osteoclast differentiation and activation

The mechanisms whereby arthritogenic organisms may induce cartilage and bone erosions in infection-triggered arthritis remain unknown. In this study, we asked whether an arthritogenic organism could contribute to osteoclast differentiation and activation through regulation of the receptor activator of NF-kappaB ligand (RANKL) in synovial fibroblasts. Rat synovial fibroblasts were infected in vitro with Salmonella enterica serovar Typhimurium and monitored over time. The expression of RANKL in resting and infected synovial fibroblasts was quantified by reverse transcription-PCR and Western blotting. Osteoclast progenitors, isolated from femurs of 8-week-old rats and cultured in the presence of macrophage colony-stimulating factor, were cocultured with either infected or noninfected synovial fibroblasts for 2 to 4 days. Differentiation and maturation of osteoclasts were determined by morphology and tartrate-resistant acid phosphatase (TRAP) staining and by a bone resorption bioassay. RANKL expression was undetectable in resting synovial fibroblasts but was dose-dependently upregulated in cells after Salmonella infection. Osteoprotegerin was constitutively expressed by synovial fibroblasts and was not upregulated by infection. Further, we observed the formation of multinucleated TRAP-positive cells and formation of bone resorption pits in cocultures of bone marrow-derived osteoclast precursors with synovial fibroblasts infected with Salmonella but not with heat-killed Salmonella or noninfected cells. Arthritogenic bacteria may alter bone structure via synovial fibroblast intermediaries, since infected synovial fibroblasts (i) upregulate RANKL expression and (ii) enhance osteoclast precursor maturation into multinucleated, TRAP-positive, bone-resorbing, osteoclast-like cells. These data provide a link between infection and osteoclastogenesis. A better understanding of infection-mediated osteoclast differentiation and activation may provide new therapeutic strategies for inflammatory joint disease.

Osteoblasts express the inflammatory cytokine interleukin-6 in a murine model of Staphylococcus aureus osteomyelitis and infected human bone tissue

Staphylococcus aureus is the single most common cause of osteomyelitis in humans. Incidences of osteomyelitis caused by S. aureus have increased dramatically in recent years, in part due to the appearance of community-acquired antibiotic resistant strains. Therefore, understanding the pathogenesis of this organism has become imperative. Recently, we have described the surprising ability of bone-forming osteoblasts to secrete a number of important immune mediators when exposed to S. aureus in vitro. In the present study, we provide the first evidence for the in vivo production of such molecules by osteoblasts during bacterial infection of bone. These studies demonstrate the expression of the key inflammatory cytokine interleukin-6 by osteoblasts in organ cultures of neonatal mouse calvaria, and in vivo using a mouse model that closely resembles the pathology of trauma-induced staphylococcal osteomyelitis, as determined by confocal microscopic analysis. Importantly, we have established the clinical relevancy of these findings in infected human bone tissue from patients with S. aureus-associated osteomyelitis. As such, these studies demonstrate that bacterial challenge of osteoblasts during bone diseases, such as osteomyelitis, induces cells to produce inflammatory molecules that can direct appropriate host responses or contribute to progressive inflammatory damage.

Bacterial infection induces expression of functional MHC class II molecules in murine and human osteoblasts

A growing body of evidence has shown that bacterially challenged osteoblasts can play a significant role in the initiation of inflammatory immune responses at sites of bone disease. We have recently demonstrated the surprising ability of osteoblasts exposed to bacteria to express CD40, a molecule that plays a critical costimulatory role in the activation of T lymphocytes. In the present study, we have extended our investigations into the ability of osteoblasts to interact with CD4+ T lymphocytes by determining the expression of antigen-presenting major histocompatibility complex (MHC) class II molecules in murine and human osteoblasts following exposure to two common pathogens of bone, Staphylococcus aureus and Salmonella. Cultured osteoblasts were found to respond rapidly to bacterial challenge by induction of mRNA encoding MHC class II molecules or its transcriptional regulator. Increased mRNA expression translated into expression of MHC class II proteins in murine and human osteoblasts as determined by Western blot analysis and by immunohistochemical and immunofluorescent microscopy. Furthermore, the increased surface expression of these molecules on osteoblasts exposed to bacteria was confirmed by FACS analysis. Finally, we show that bacterial challenge results in the elevated functional expression of MHC class II molecules on osteoblasts by demonstrating the enhanced ability of these cells to interact with T lymphocytes and to initiate antigen-specific T cell activation. Taken together, these data suggest a previously unappreciated role for osteoblasts in the initiation of T lymphocyte activation at sites of bacterial infection in bone tissue.

Signaling via Toll-like receptor 5 can initiate inflammatory mediator production by murine osteoblasts

Murine osteoblasts express Toll-like receptor 5 (TLR5), and this expression is upregulated following exposure to bacteria or to the TLR5 agonist, flagellin. Importantly, flagellin activates transcriptional regulators and elicits proinflammatory cytokine production, suggesting TLR5 functionality. TLR5 may represent an important mechanism underlying the recognition of bacterial pathogens by osteoblasts during bone infections.

Staphylococcus aureus - induced tumor necrosis factor - related apoptosis - inducing ligand expression mediates apoptosis and caspase-8 activation in infected osteoblasts

BACKGROUND

Staphylococcus aureus infection of normal osteoblasts induces expression of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL).

RESULTS

Normal osteoblasts were incubated in the presence of purified bacterial products over a range of concentrations. Results demonstrate that purified surface structures and a selected superantigen present in the extracellular environment are not capable of inducing TRAIL expression by osteoblasts. Osteoblasts were co-cultured with S. aureus at various multiplicities of infection utilizing cell culture chamber inserts. Results of those experiments suggest that direct contact between bacteria and osteoblasts is necessary for optimal TRAIL induction. Finally, S. aureus infection of osteoblasts in the presence of anti-TRAIL antibody demonstrates that TRAIL mediates caspase-8 activation and apoptosis of infected cells.

CONCLUSIONS

Collectively, these findings suggest a mechanism whereby S. aureus mediates bone destruction via induction of osteoblast apoptosis.

Functional CD40 expression induced following bacterial infection of mouse and human osteoblasts

Bacterially induced bone infections often result in significant local inflammatory responses which are coupled with loss of bone. However, the mechanisms necessary for the protective host response, or those responsible for pathogen-induced bone loss, are not clear. Recent evidence demonstrates that bacterially infected osteoblasts secrete chemokines and cytokines, suggesting that these cells may have an unappreciated role in supporting localized inflammation. In this study, mouse and human osteoblasts were investigated for their ability to express functional CD40 upon exposure to two important pathogens of bone, Staphylococcus aureus and Salmonella enterica serovar Dublin. Bacterial infection of cultured mouse or human osteoblasts resulted in increased CD40 mRNA and CD40 protein expression induced by either pathogen. Importantly, CD40 expression by osteoblasts was functional, as assessed by ligation of this molecule with recombinant, soluble CD154. CD40 activity was assessed by induction of interleukin-6 and granulocyte-macrophage colony-stimulating factor in osteoblasts following ligation. Cocultures of activated CD4(+) T lymphocytes and osteoblasts could interact via CD40 and CD154, since an antibody against CD40 could block macrophage inflammatory protein-1alpha secretion. Taken together, these studies conclusively demonstrate that infected osteoblasts can upregulate expression of functional CD40 molecules which mediate cytokine secretion. This surprising result further supports the notion that bone-forming osteoblasts can directly interact with CD154-expressing cells (i.e., T lymphocytes) and can contribute to the host response during bone infection.

Infection by Streptococcus pyogenes induces the receptor activator of NF-kappaB ligand expression in mouse osteoblastic cells

Group A Streptococcus pyogenes is known to induce nongonococcal septic arthritis in addition to pharyngitis, scarlet fever, and poststreptococcal sequelae. However, little is known about the interaction between S. pyogenes and bone cells. We report here that S. pyogenes strain JRS4 (M6) attached to and invaded mouse primary osteoblasts. Reverse transcription-PCR demonstrated that S. pyogenes infection of osteoblasts stimulated expression of mRNA for the receptor activator of NF-kappaB ligand (RANKL). Western blot analysis followed by ligand precipitation with the receptor activator of NF-kappaB receptor showed that there was an increase in RANKL protein in infected osteoblasts. Production of interleukin-6 was also stimulated, but no production of interleukin-1beta or tumor necrosis factor alpha was observed. Stimulation of RANKL production was not observed in osteoblasts stimulated with heat-inactivated S. pyogenes, suggesting that an active interaction of S. pyogenes with osteoblasts is essential for this phenomenon. A Western blot analysis performed with antibodies specific for phosphorylated signal transduction proteins demonstrated that S. pyogenes infection induces phosphorylation of p38 mitogen-activated protein kinase. A specific inhibitor of this kinase, SB203580, inhibited RANKL production by infected osteoblasts. These results suggest that infection of osteoblasts by S. pyogenes stimulates RANKL production and may trigger bone destruction in infected bone tissue.

Bacterial infection of osteoblasts induces interleukin-1beta and interleukin-18 transcription but not protein synthesis

A growing body of evidence has shown that bacterially challenged bone-forming osteoblasts are a significant source of an array of cytokines and chemokines that can support immune responses during bone disease. In the present study, Staphylococcus aureus and Salmonella, two common pathogens of bone, were investigated for their ability to induce production of two related inflammatory cytokines, interleukin-1beta (IL-1beta) and IL18, in osteoblasts. Cultured mouse osteoblasts were found to respond rapidly to either bacterial challenge by upregulation in the levels of mRNA encoding both IL-1beta and IL-18. Surprisingly, this mRNA expression did not translate into intracellular accumulation of IL-1beta or IL-18 precursor proteins or secretion of mature cytokines, despite the presence of detectable caspase-1 activity in these cells. These studies demonstrate that although osteoblasts can secrete a number of key proinflammatory mediators in response to bacterial pathogens, IL-1beta and IL-18 are not among this number. We suggest that osteoblasts are an unlikely source of these cytokines during the progression of bacterial infection of bone.

Bacterium-induced CXCL10 secretion by osteoblasts can be mediated in part through toll-like receptor 4

Two common pathogens known to cause bone infection, Salmonella and Staphylococcus aureus, were investigated to determine their abilities to induce chemokine expression in cultured mouse and human osteoblasts. While these cells are responsible for bone formation, we were surprised to find that they could respond to bacterial infection by upregulating expression of the chemokine CXCL10 (IP-10). However, there were significant differences in the abilities of the gram-negative bacterium Salmonella and the gram-positive bacterium S. aureus to induce expression of CXCL10. Reverse transcription-PCR and enzyme-linked immunosorbent assay analyses showed high levels of Salmonella-induced CXCL10 mRNA and protein expression, respectively, whereas the osteoblast response to S. aureus was significantly less. Consistent with these findings, Salmonella-derived lipopolysaccharide (LPS), but not S. aureus-derived peptidoglycan, could induce expression of CXCL10. An antibody against toll-like receptor 4 (TLR4) could block the LPS-induced CXCL10 production, demonstrating the functional expression of TLR4 by osteoblasts. Despite the inducible nature of TLR2 mRNA expression by bacterium-infected osteoblasts, peptidoglycan failed to stimulate CXCL10 secretion. Immunofluorescent staining of bacterium-infected calvaria (i.e., skull bone) demonstrated the presence of CXCL10 in osteoblasts. The fact that osteoblasts did not express CXCR3 mRNA, whereas T lymphocytes can express high levels of this receptor, suggests that osteoblast-derived CXCL10 may recruit T lymphocytes to the sites of bone infections.

Cytokines in host defense against Salmonella

Cytokines are key communication molecules between host cells in the defense against the enteric pathogen, Salmonella. Infection with Salmonella induces expression of multiple chemokines and proinflammatory cytokines in cultured intestinal epithelial cells and macrophages. In animal models, protective roles have been shown for IL-1alpha, TNFalpha, IFN-gamma, IL-12, IL-18 and IL-15, whereas IL-4 and IL-10 inhibit host defenses against Salmonella.

Monocyte chemoattractant protein-1 expression by osteoblasts following infection with Staphylococcus aureus or Salmonella

Two common pathogens of bone, Staphylococcus aureus and Salmonella, were investigated for their ability to induce chemokine expression in bone-forming osteoblasts. Cultured mouse or human osteoblasts could rapidly respond to bacterial infection by upregulating the mRNA encoding the chemokine, monocyte chemoattractant protein-1 (MCP-1). This rapid induction occurred on infection with either the gram-positive pathogen, S. aureus, or the gram-negative pathogen, Salmonella. Increased mRNA expression translated into MCP-1 secretion by cultured mouse or human osteoblasts in response to viable bacteria, whereas UV-killed bacteria were less effective in stimulating chemokine secretion. There was a dose-response relationship observed between the amount of input bacteria and increases in MCP-1 secretion. Immunohistochemical staining of infected osteoblasts indicated that the majority of cells could express MCP-1, with some osteoblasts having a higher intensity of staining than others. Organ cultures of mouse calvaria (skullcap) bone showed increases in MCP-1 immunostaining following bacterial infection. The immunoreactive MCP-1 in infected calvaria localized to areas containing active osteoblasts. Taken together, these studies demonstrate a conserved osteoblast-derived MCP-1 response to two very different pathogens of bone.