Reduced bone mineral density in male Severe Acute Respiratory Syndrome (SARS) patients in Hong Kong

Jaw-related complications in COVID-19 patients; a systematic review

OBJECTIVE

The aim of this review was to highlight jaw-related complications in COVID-19 manifestations, their etiology, and prevention methods.

METHODS

A systematic review of literature was conducted. MEDLINE/PubMed, and Google Scholar were searched for the following keywords: "COVID-19" "Oral manifestations", "Musculoskeletal patients", "Mandible", "Jaw", "Osteonecrosis", "MRONJ", and "dry socket".

RESULTS

Only nine articles were included in this review. Jaw-related disorders associated with COVID-19 were dry socket, osteonecrosis, and orofacial pain related to temporomandibular joint disorders (TMD) and giant cell arteritis (GCA).

CONCLUSION

COVID-19 potentially predisposes to osteonecrosis due to thrombotic inflammatory phenomena caused by the disease itself or its therapeutic modalities. All jaw osteonecrosis cases reported so far in relation to COVID-19 affected the upper jaw. Orofacial pain in COVID-19 patients was related to TMD and GCA. Clinical evidence-based studies are required to investigate the actual prevalence and possible correlation between COVID-19 and jaw-related disorders.

Risks of incident major osteoporotic fractures following SARS-CoV-2 infection among older individuals: a population-based cohort study in Hong Kong

Population-based epidemiological studies on post-acute phase coronavirus 2019 (COVID-19)-related fractures in older adults are lacking. This study aims to examine the risk of incident major osteoporotic fractures following SARS-CoV-2 infection among individuals aged ≥50, compared to individuals without COVID-19. It was a retrospective, propensity-score matched, population-based cohort study of COVID-19 patients and non-COVID individuals identified from the electronic database of the Hong Kong Hospital Authority from January 2020 to March 2022. The primary outcome was a composite of major osteoporotic fractures (hip, clinical vertebral, and upper limb). COVID-19 patients were 1:1 matched to controls using propensity-score according to age, sex, vaccination status, medical comorbidities and baseline medications. Hazard ratios (HRs) with 95% confidence intervals (CIs) were calculated using Cox proportional hazards regression models. A total of 429 459 COVID-19 patients were included, 1:1 matched to non-COVID individuals. Upon median follow-up of 11 months, COVID-19 patients had higher risks of major osteoporotic fractures (5.08 vs 3.95 per 1000 persons; HR 1.22 95%CI [1.15-1.31]), hip fractures (2.71 vs 1.94; 1.33 [1.22-1.46]), clinical vertebral fractures (0.42 vs 0.31; 1.29 [1.03-1.62]), and falls (13.83 vs 10.36; 1.28 [1.23-1.33]). Subgroup analyses revealed no significant interaction. In acute (within 30 days) and post-acute phases (beyond 30 days) following severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, we consistently observed a significant increase in fractures and falls risks. Our study demonstrated increased risk of major osteoporotic fractures after SARS-CoV-2 infection in both acute and post-acute phases in older adults, partly due to increased fall risk. Clinicians should be aware of musculoskeletal health of COVID-19 survivors.

Bone marrow alterations in COVID-19 infection: The root of hematological problems

INTRODUCTION

The 2019 coronavirus disease (COVID-19) is a respiratory infection caused by the SARS-CoV-2 virus with a significant impact on the hematopoietic system and homeostasis. The effect of the virus on blood cells indicates the involvement of the bone marrow (BM) as the place of production and maturation of these cells by the virus and it reminds the necessity of investigating the effect of the virus on the bone marrow.

METHOD

To investigate the effects of COVID-19 infection in BM, we reviewed literature from the Google Scholar search engine and PubMed database up to 2022 using the terms "COVID-19; SARS-CoV-2; Bone marrow; Thrombocytopenia; Hemophagocytosis; Pancytopenia and Thrombocytopenia.

RESULTS

Infection with the SARS-CoV-2 virus is accompanied by alterations such as single-line cytopenia, pancytopenia, hemophagocytosis, and BM necrosis. The presence of factors such as cytokine release syndrome, the direct effect of the virus on cells through different receptors, and the side effects of current treatments such as corticosteroids are some of the important mechanisms in the occurrence of these alterations.

CONCLUSION

To our knowledge, this review is the first study to comprehensively investigate BM alterations caused by SAR-CoV-2 virus infection. The available findings show that the significant impact of this viral infection on blood cells and the clinical consequences resulting from them are deeper than previously thought and it may be rooted in the changes that the virus causes in the BM of patients.

Acute coronavirus infection triggers a TNF-dependent osteoporotic phenotype in mice

AIMS

Millions of people died during the COVID-19 pandemic, but the vast majority of infected individuals survived. Now, some consequences of the disease, known as long COVID, are been revealed. Although the respiratory system is the target of Sars-CoV-2, COVID-19 can influence other parts of the body, including bone. The aim of this work was to investigate the impact of acute coronavirus infection in bone metabolism.

MAIN METHODS

We evaluated RANKL/OPG levels in serum samples of patients with and without acute COVID-19. In vitro, the effects of coronavirus in osteoclasts and osteoblasts were investigated. In vivo, we evaluated the bone phenotype in a BSL2 mouse model of SARS-like disease induced by murine coronavirus (MHV-3).

KEY FINDINGS

Patients with acute COVID-19 presented decreased OPG and increased RANKL/OPG ratio in the serum versus healthy individuals. In vitro, MHV-3 infected macrophages and osteoclasts, increasing their differentiation and TNF release. Oppositely, osteoblasts were not infected. In vivo, MHV-3 lung infection triggered bone resorption in the femur of mice, increasing the number of osteoclasts at 3dpi and decreasing at 5dpi. Indeed, apoptotic-caspase-3⁺ cells have been detected in the femur after infection as well as viral RNA. RANKL/OPG ratio and TNF levels also increased in the femur after infection. Accordingly, the bone phenotype of TNFRp55^-/- mice infected with MHV-3 showed no signs of bone resorption or increase in the number of osteoclasts.

SIGNIFICANCE

Coronavirus induces an osteoporotic phenotype in mice dependent on TNF and on macrophage/osteoclast infection.

The Impacts of COVID-19 on Musculoskeletal Health

PURPOSE OF REVIEW

Although COVID-19 was originally characterized as a respiratory disease, recent findings have shown lingering side effects in those who have recovered, and much is still unknown about the long-term consequences of the illness. Thus, the potential of unearthing multi-system dysfunction is high, with current data revealing significant impacts on musculoskeletal health.

RECENT FINDINGS

Multiple animal models of COVID-19 infection have revealed significant post-infection bone loss at several different skeletal sites. While how this loss occurred is unknown, this current review discusses the primary bone loss studies, and examines the possible mechanisms of action including: direct infection of bone marrow macrophages or hematopoietic progenitors, a proinflammatory response as a result of the COVID-19 induced cytokine storm, and/or a result of hypoxia and oxidative stress. This review will further examine how therapeutics used to treat COVID-19 affect the skeletal system. Finally, this review will examine the possible consequence that delayed care and limited healthcare accessibility has on musculoskeletal-related patient outcomes. It is important to investigate the potential impact COVID-19 infection has on musculoskeletal health.

Long-term implications of COVID-19 on bone health: pathophysiology and therapeutics

BACKGROUND

SARS-CoV-2 is a highly infectious respiratory virus associated with coronavirus disease (COVID-19). Discoveries in the field revealed that inflammatory conditions exert a negative impact on bone metabolism; however, only limited studies reported the consequences of SARS-CoV-2 infection on skeletal homeostasis. Inflammatory immune cells (T helper-Th17 cells and macrophages) and their signature cytokines such as interleukin (IL)-6, IL-17, and tumor necrosis factor-alpha (TNF-α) are the major contributors to the cytokine storm observed in COVID-19 disease. Our group along with others has proven that an enhanced population of both inflammatory innate (Dendritic cells-DCs, macrophages, etc.) and adaptive (Th1, Th17, etc.) immune cells, along with their signature cytokines (IL-17, TNF-α, IFN-γ, IL-6, etc.), are associated with various inflammatory bone loss conditions. Moreover, several pieces of evidence suggest that SARS-CoV-2 infects various organs of the body via angiotensin-converting enzyme 2 (ACE2) receptors including bone cells (osteoblasts-OBs and osteoclasts-OCs). This evidence thus clearly highlights both the direct and indirect impact of SARS-CoV-2 on the physiological bone remodeling process. Moreover, data from the previous SARS-CoV outbreak in 2002-2004 revealed the long-term negative impact (decreased bone mineral density-BMDs) of these infections on bone health.

METHODOLOGY

We used the keywords "immunopathogenesis of SARS-CoV-2," "SARS-CoV-2 and bone cells," "factors influencing bone health and COVID-19," "GUT microbiota," and "COVID-19 and Bone health" to integrate the topics for making this review article by searching the following electronic databases: PubMed, Google Scholar, and Scopus.

CONCLUSION

Current evidence and reports indicate the direct relation between SARS-CoV-2 infection and bone health and thus warrant future research in this field. It would be imperative to assess the post-COVID-19 fracture risk of SARS-CoV-2-infected individuals by simultaneously monitoring them for bone metabolism/biochemical markers. Importantly, several emerging research suggest that dysbiosis of the gut microbiota-GM (established role in inflammatory bone loss conditions) is further involved in the severity of COVID-19 disease. In the present review, we thus also highlight the importance of dietary interventions including probiotics (modulating dysbiotic GM) as an adjunct therapeutic alternative in the treatment and management of long-term consequences of COVID-19 on bone health.

COVID-19 Pandemic and Osteoporosis in Elderly Patients

Coronavirus disease 2019 (COVID-19), which is caused by an infection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is rapidly becoming a worldwide epidemic and poses a significant threat to human life and health. SARS-CoV-2 can cause damage to organs throughout the body through ACE2 receptors. It may have direct and indirect effects on osteoclasts, and osteoblasts and lead to osteoporosis. Vitamin D (VitD) is a key hormone for bone health and has immunomodulatory actions of relevance in the context of the COVID-19 pandemic. Vitamin D deficiency has a significant positive association with both infection and the mortality rate of COVID-19. Elderly patients infected by COVID-19 were more likely to develop acute respiratory distress syndrome (ARDS), which was primarily caused by an inflammation storm. The production of proinflammatory cytokines increases with COVID-19 infection and immobilization may result in bone loss and bone resorption in seriously ill patients, especially aging patients. It is well known that glucocorticoids are beneficial in the treatment of acute respiratory distress syndrome (ARDS) because they reduce inflammation and improve the functioning of the lung and extrapulmonary organs. Glucocorticoid therapy is widely used to treat patients with COVID-19 in most parts of the world. During COVID-19 clinical treatment, glucocorticoids may accelerate bone loss in elderly people, making them more susceptible to the development of osteoporosis. Therefore, it is worthwhile to draw the attention of clinicians and researchers to the linkages and interactions between COVID-19, glucocorticoids, and osteoporosis (especially in elderly patients).

Sports Imaging of COVID-19: A Multi-Organ System Review of Indications and Imaging Findings

BACKGROUND

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was first identified in December 2019 in Wuhan, China. Although coronavirus disease-19 (COVID-19) affects every population group, the sports community and athletes require special consideration of the effects on cardiovascular, musculoskeletal, neurologic, and respiratory systems. A comprehensive understanding of imaging indications, findings, and features of COVID-19 supports appropriate imaging utilization and effective patient management and treatment.

PURPOSE

To review the spectrum of sports imaging in COVID-19 infection, organ system manifestations, vaccine effects, and complications in recreational and competitive athletes.

STUDY DESIGN

Narrative review.

LEVEL OF EVIDENCE

Levels 4 and 5.

METHODS

Based on a PubMed database search, studies describing the imaging findings of COVID-19 infection, organ system manifestations, vaccine effects, and complications in recreational and competitive athletes were included.

RESULTS

On March 11, 2020, World Health Organization officially declared COVID-19 a global pandemic. As of May 9, 2022, more than 515 million confirmed cases of COVID-19 were reported globally. While the multisystem effects of COVID-19 are incompletely understood, the role of imaging in diagnosing, monitoring, and prognosticating active disease, long-term effects, and complications is evolving. In the respiratory system, imaging plays an important role in diagnosing, characterizing, and monitoring pulmonary COVID-19 infections, barotrauma, and COVID-19-associated chronic pulmonary opacities and fibrotic-like lung changes. Ultrasonography, computed tomography, and magnetic resonance imaging aid in the timely diagnosis of ischemic, embolic, and thrombotic peripheral and central cardiovascular events, including deep venous thrombosis, pulmonary embolism, myocarditis, and stroke. COVID-19-associated musculoskeletal and peripheral nervous system manifestations include rhabdomyolysis and myonecrosis, plexus and peripheral neuropathies, Guillain-Barré syndrome, and shoulder injury related to vaccine administration.

CONCLUSIONS

In athletes, COVID-19 infections and associated effects on cardiovascular, musculoskeletal, neurologic, and respiratory systems require special consideration. With the increasing understanding of the multisystem effects of COVID-19, the role of imaging in diagnosing, monitoring, and prognosticating active disease, long-term effects, and complications is evolving. A comprehensive understanding of imaging indications, COVID-19 imaging features, and organ system effects aids in appropriate imaging utilization and effective patient management and treatments.

Pathological changes in the osteoarticular system during COVID-19 drug therapy (review of literature)

The recommended drugs for the treatment of COVID-19 are, on the one hand, experimental in nature, but at the same time, they have many side effects that cause long-term complications in organs and systems, including osteoarticular. Based on the analysis of modern domestic and foreign literature, to determine the effect of not only the new coronavirus infection COVID-19, but also the drugs used to treat it, on the human osteoarticular system. In the course of the study, a scientific search was made for publications in the electronic databases PubMed, MedLine and e-Library for the period from January 2000 to October 2021 for the main keywords. Due to successive “waves” of the COVID-19 pandemic, the number of patients receiving non-specific therapy, including corticosteroids, will increase in the coming years. Preliminary data on COVID-19 and similar trends during the Sars-COV-1 epidemic of 2003 show that the pathogenesis of Sars- Cov-2 and its treatment with high doses of corticosteroids may increase the risk of osteonercosis in patients, which will inevitably lead to an increase in orthopedic diseases in patients not only the middle age group, but also young patients in the near future. Currently, studies are required aimed at risk stratification, studying the pathogenesis of damage to the musculoskeletal system after COVID-19 and the effectiveness of preventive and therapeutic measures in such patients.

Extra-Pulmonary Complications in SARS-CoV-2 Infection: A Comprehensive Multi Organ-System Review

Coronavirus disease (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, is typically presented with acute symptoms affecting upper and lower respiratory systems. As the current pandemic progresses, COVID-19 patients are experiencing a series of nonspecific or atypical extra-pulmonary complications such as systemic inflammation, hypercoagulability state, and dysregulation of the renin-angiotensin-aldosterone system (RAAS). These manifestations often delay testing, diagnosis, and the urge to seek effective treatment. Although the pathophysiology of these complications is not clearly understood, the incidence of COVID-19 increases with age and the presence of pre-existing conditions. This review article outlines the pathophysiology and clinical impact of SARS-CoV-2 infection on extra-pulmonary systems. Understanding the broad spectrum of atypical extra-pulmonary manifestations of COVID-19 should increase disease surveillance, restrict transmission, and most importantly prevent multiple organ-system complications.

Effects of COVID-19 on the Musculoskeletal System: Clinician's Guide

The global pandemic caused by SARS-CoV-2, or COVID-19, continues to impact all facets of daily life. Clinical manifestations of COVID-19 commonly include musculoskeletal symptoms such as myalgias, arthralgias, and neuropathies/myopathies. The inflammatory response and its impact on the respiratory system have been the focus of most studies. However, the literature is more limited regarding the inflammatory response and its implications for other organ systems, specifically the musculoskeletal system. Previous studies have described how systemic inflammation may play a role in bone and joint pathology. Furthermore, it is important to understand the effects current therapeutics used in the treatment of COVID-19 may have on the musculoskeletal system. In this study, we will review the current understanding of the effect COVID-19 has on the musculoskeletal system, provide an overview of musculoskeletal symptoms of patients infected with the virus, and address key issues for clinicians to address during the care of COVID-19 patients.

The Risk of Osseointegration in the Coronavirus Disease 19 Pandemic

ABSTRACT

Coronavirus disease 19 (COVID-19) is associated with respiratory complications but also with alterations on bone metabolism. Coronavirus disease 19, therefore, might be a risk factor for osseointegration. Recent studies suggest that severe acute respiratory syndrome coronavirus 2 is related with bone abnormalities mainly for act via renin-angiotensin system. This report aims to list the bone alterations caused by coronavirus disease 19 and the possible consequences on the peri-implant bone healing. The current data add to the accumulating knowledge that coronavirus disease 19 may negatively impact the osseointegration and it requires further research.

COVID-19 in age-related neurodegenerative diseases: is there a role for vitamin D3 as a possible therapeutic strategy?

The coronavirus disease (COVID-19), identified in Wuhan, China, on December 2019, was declared a pandemic by the World Health Organization, on March, 2020. Since then, efforts have been gathered to describe its clinical course and to determine preventive measures and treatment strategies. Adults older than 65 years of age are more susceptible to serious clinical symptoms and present higher mortality rates. Angiotensin-converting enzyme 2 (ACE2) is a major receptor for some coronavirus infection, including SARS-COV-2, but is also a crucial determinant in anti-inflammation processes during the renin-angiotensin system (RAS) functioning - converting angiotensin II to angiotensin 1-7. The decline in ACE2 expression that occurs with aging has been associated to the higher morbidity and mortality rates in older adults. These observations highlight the importance of investigating the association between COVID-19 and age-related neurodegenerative disorders, i.e., Parkinson's and Alzheimer's diseases. A possible option to reduce the risk of COVID-19 is vitamin D supplementation, due to its anti-inflammatory and immune-system-modulating effects. It has also been suggested that vitamin D supplementation plays a role in slowing progression of Parkinson and Alzheimer. The present study is a literature review of articles published on the theme COVID-19, Parkinson and Alzheimer's diseases, and the role played by vitamin D. PUBMED, MEDLINE, and EMBASE databases were consulted. Results confirm neurodegenerative and neuroinflammatory effects of COVID-19, aggravated in Parkinson's and Alzheimer's patients, and the important role of vitamin D as a possible therapeutic strategy. Nevertheless, randomized controlled trials and large population studies are still warranted.

Musculoskeletal Consequences of COVID-19

Coronavirus disease 2019 (COVID-19) is an emerging pandemic disease caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Although the majority of patients who become infected with SARS-CoV-2 are asymptomatic or have mild symptoms, some patients develop severe symptoms that can permanently detract from their quality of life. SARS-CoV-2 is closely related to SARS-CoV-1, which causes severe acute respiratory syndrome (SARS). Both viruses infect the respiratory system, and there are direct and indirect effects of this infection on multiple organ systems, including the musculoskeletal system. Epidemiological data from the SARS pandemic of 2002 to 2004 identified myalgias, muscle dysfunction, osteoporosis, and osteonecrosis as common sequelae in patients with moderate and severe forms of this disease. Early studies have indicated that there is also considerable musculoskeletal dysfunction in some patients with COVID-19, although long-term follow-up studies have not yet been conducted. The purpose of this article was to summarize the known musculoskeletal pathologies in patients with SARS or COVID-19 and to combine this with computational modeling and biochemical signaling studies to predict musculoskeletal cellular targets and long-term consequences of the SARS-CoV-2 infection.

Src siRNA prevents corticosteroid-associated osteoporosis in a rabbit model

In an established steroid-associated osteonecrosis (SAON) rabbit model we found recently that blockage Src by siRNA could improve reconstructive repair of osteonecrosis via enhancing osteogenesis and inhibiting bone resorption. The current study investigated if blocking Src was able to prevent steroid-associated osteoporosis (SAOP) in the same SAON animal model. Rabbits were treated with pulsed lipopolysaccharide (LPS) and corticosteroid methylprednisolone (MPS). At 2, 4, and 6weeks after induction, Src siRNA, control siRNA and saline were intramedullary injected into proximal femur, respectively. Two fluorescent dyes xylenol orange and calcein green were injected before sacrificing the animals for in vivo labeling of the newly formed bone. At week 6 after induction, proximal femora of rabbits were dissected for micro-CT and histological analysis. Results showed significant bone loss in the metaphysis of femoral head in the control rabbits after SAON induction. Src siRNA treatment was able to prevent steroid-associate bone loss in trabecular bone and increase cortical bone thickness at femoral neck. Histomorphometry showed that Src siRNA increased the osteoblastic bone formation and decreased the eroded bone surfaces suggesting decreased osteoclastic bone resorption. This was the first study to report bone loss after SAON induction in rabbit model that could be prevented by knocking down Src by siRNA.

Steroid-associated hip joint collapse in bipedal emus

In this study we established a bipedal animal model of steroid-associated hip joint collapse in emus for testing potential treatment protocols to be developed for prevention of steroid-associated joint collapse in preclinical settings. Five adult male emus were treated with a steroid-associated osteonecrosis (SAON) induction protocol using combination of pulsed lipopolysaccharide (LPS) and methylprednisolone (MPS). Additional three emus were used as normal control. Post-induction, emu gait was observed, magnetic resonance imaging (MRI) was performed, and blood was collected for routine examination, including testing blood coagulation and lipid metabolism. Emus were sacrificed at week 24 post-induction, bilateral femora were collected for micro-computed tomography (micro-CT) and histological analysis. Asymmetric limping gait and abnormal MRI signals were found in steroid-treated emus. SAON was found in all emus with a joint collapse incidence of 70%. The percentage of neutrophils (Neut %) and parameters on lipid metabolism significantly increased after induction. Micro-CT revealed structure deterioration of subchondral trabecular bone. Histomorphometry showed larger fat cell fraction and size, thinning of subchondral plate and cartilage layer, smaller osteoblast perimeter percentage and less blood vessels distributed at collapsed region in SAON group as compared with the normal controls. Scanning electron microscope (SEM) showed poor mineral matrix and more osteo-lacunae outline in the collapsed region in SAON group. The combination of pulsed LPS and MPS developed in the current study was safe and effective to induce SAON and deterioration of subchondral bone in bipedal emus with subsequent femoral head collapse, a typical clinical feature observed in patients under pulsed steroid treatment. In conclusion, bipedal emus could be used as an effective preclinical experimental model to evaluate potential treatment protocols to be developed for prevention of ON-induced hip joint collapse in patients.

Severe acute respiratory syndrome (SARS)

This article reviews the virology, history, pathology, epidemiology, clinical presentations, complications, radiology, laboratory testing, diagnosis, treatment, and prevention of severe respiratory distress syndrome, with reference to documented outbreaks of the disease.

Effectiveness of ribavirin and corticosteroids for severe acute respiratory syndrome

OBJECTIVE

Ribavirin and corticosteroids were used widely as front-line treatments for severe acute respiratory syndrome; however, previous evaluations were inconclusive. We assessed the effectiveness of ribavirin and corticosteroids as the initial treatment for severe acute respiratory syndrome using propensity score analysis.

METHODS

We analyzed data on 1755 patients in Hong Kong and 191 patients in Toronto with severe acute respiratory syndrome using a generalized propensity score approach.

RESULTS

The adjusted excess case fatality ratios of patients with severe acute respiratory syndrome receiving the combined therapy of ribavirin and corticosteroids within 2 days of admission, compared with those receiving neither treatment within 2 days of admission, were 3.8% (95% confidence interval, -1.5 to 9.2) in Hong Kong and 2.1% (95% confidence interval, -44.3 to 48.5) in Toronto.

CONCLUSIONS

Our results add strength to the hypothesis that the combination of ribavirin and corticosteroids has no therapeutic benefit when given early during severe acute respiratory syndrome infection. Further studies may investigate the effects of these treatments later in disease course.

Potential enhancement of osteoclastogenesis by severe acute respiratory syndrome coronavirus 3a/X1 protein

Severe acute respiratory syndrome coronavirus (SARS-CoV) causes a lung disease with high mortality. In addition, osteonecrosis and bone abnormalities with reduced bone density have been observed in patients following recovery from SARS, which were partly but not entirely explained by the short-term use of steroids. Here, we demonstrate that human monocytes, potential precursors of osteoclasts, partly express angiotensin converting enzyme 2 (ACE2), a cellular receptor of SARS-CoV, and that expression of an accessory protein of SARS-CoV, 3a/X1, in murine macrophage cell line RAW264.7 cells, enhanced NF-kappaB activity and differentiation into osteoclast-like cells in the presence of receptor activator of NF-kappaB ligand (RANKL). Furthermore, human epithelial A549 cells expressed ACE2, and expression of 3a/X1 in these cells up-regulated TNF-alpha, which is known to accelerate osteoclastogenesis. 3a/X1 also enhanced RANKL expression in mouse stromal ST2 cells. These findings indicate that SARS-CoV 3a/X1 might promote osteoclastogenesis by direct and indirect mechanisms.