key: cord-0977798-9zd88mba
authors: Singh, Sunit K.
title: Middle East Respiratory Syndrome Virus Pathogenesis
date: 2016-08-03
journal: Semin Respir Crit Care Med
DOI: 10.1055/s-0036-1584796
sha: b69353a7338ec3fe6c9e5fdfda059b3016762620
doc_id: 977798
cord_uid: 9zd88mba

Coronaviruses (CoVs) are enveloped RNA viruses that infect birds, mammals, and humans. Infections caused by human coronaviruses (hCoVs) are mostly associated with the respiratory, enteric, and nervous systems. The hCoVs only occasionally induce lower respiratory tract disease, including bronchitis, bronchiolitis, and pneumonia. In 2002 to 2003, a global outbreak of severe acute respiratory syndrome (SARS) was the seminal detection of a novel CoV (SARS-CoV). A decade later (June 2012), another novel CoV was implicated as the cause of Middle East respiratory syndrome (MERS) in Saudi Arabia. Although bats might serve as a reservoir of MERS-CoV, it is unlikely that they are the direct source for most human cases. Severe lines of evidence suggest that dromedary camels have been the major cause of transmission to humans. The emergence of MERS-CoV has triggered serious concerns about the potential for a widespread outbreak. All MERS cases were linked directly or indirectly to the Middle East region including Saudi Arabia, Jordan, Qatar, Oman, Kuwait, and UAE. MERS cases have also been reported in the later phases in the United Kingdom, France, Germany, Italy, Spain, and Tunisia. Most of these MERS cases were linked with the Middle East. The high mortality rates in family-based and hospital-based outbreaks were reported among patients with comorbidities such as diabetes and renal failure. MERS-CoV causes an acute, highly lethal pneumonia and renal dysfunction. The major complications reported in fatal cases are hyperkalemia with associated ventricular tachycardia, disseminated intravascular coagulation, pericarditis, and multiorgan failure. The case-fatality rate seems to be higher for MERS-CoV (around 30%) than for SARS-CoV (9.6%). The combination regimen of type 1 interferon + lopinavir/ritonavir is considered as the first-line therapy for MERS. Antiviral treatment is generally recommended for 10 to 14 days in patients with MERS-CoV infection. Convalescent plasma therapy has shown some efficacy among patients refractory to antiviral drugs if administered within 2 weeks of the onset of the disease.

decade after the outbreak of the SARS-CoV epidemic in 2002-2003. 7,8 The inefficient spread between infected patients suggested the strong possibility of zoonotic transmission, but the exact source of infection is still not known in human population. The Tylonycteris and Pipistrellus bats have been recently reported as the reservoir of MERS-CoV. 9 Although bats might serve as a reservoir of MERS-CoV, it is unlikely that they are the direct source for most of the human cases because human contacts with bats are not very common. The phylogenetic studies revealed the presence of CoV RNA sequences in the bat fecal samples in Europe, Africa, and Asia, including the Middle East. Some of the RNA sequences have been reported to be closely related to MERS-CoV sequences. [10] [11] [12] [13] [14] The MERS-CoV spilled over in human population either from bats or through other animals as intermediate hosts. 10 However, other evidences indicated the origin of MERS-CoV from dromedary camels. High levels of neutralizing antibodies, viral RNA, and infectious viruses have been reported in dromedary camels, suggesting a potential for camels to be the source for human transmission. 15, 16 The presence of MERS-CoV neutralizing antibodies in dromedary camels suggested either a past infection with MERS-CoV or a closely related virus in camels. 17, 18 Unpasteurized camel milk has also been suggested as a potential source because the virus has been detected in the camel milk. 8, 19, 20 The World Health Organization (WHO), Saudi Arabia, and Qatar have issued recommendation for not consuming unpasteurized camel milk and to be cautious in cases of close association with dromedary camels. 20 The full-genome sequencing data have shown the sequence similarity in the MERS-CoV isolated from a man and his camel in Saudi Arabia. Such sequence similarity indicated camels as a strong possible link in the cases of camelto-human transmission. 21 The phylogenetic studies provided the evidence of interspecies jumps of CoVs. 22, 23 The emergence of MERS-CoV has triggered serious concerns about the potential for a widespread outbreak. All MERS cases were linked directly or indirectly to the Middle East region including Saudi Arabia, Jordan, Qatar, Oman, Kuwait, and UAE. 11, 24 The MERS cases have also been reported in the later phases in the United Kingdom, France, Germany, Italy, Spain, and Tunisia. 25 Most of these MERS cases were linked with the Middle East. 25 The high mortality rates in family-based and hospital-based outbreaks were reported among patients with comorbidities such as diabetes and renal failure. 24, 26, 27 

The MERS-CoV genome consists of 30,119 nucleotides and 11 open reading frames (ORFs). 2, 28 The single positive-stranded RNA genome has 5′-UTR (278 nucleotides) and 3′-UTR (300 nucleotides) (►Fig. 1). The genes located at the 5′-UTR play an important role in viral replication, whereas the genes at the 3′-UTR encode accessory and structural proteins. 13 The 5′-UTR has two overlapping ORFs (ORF1a and ORF1b), which gets translated to two polyproteins (polyprotein 1a [pp1a] and polyprotein 1ab [pp1ab]) (►Fig. 1). 28 These polyproteins are cleaved into 16 functional nonstructural proteins (nsps) through proteolytic activity of two viral proteases called papain-like protease (PLpro) and 3C-like protease (3CLPro) after their self-cleavage from pp1ab (►Fig. 1). 2, 25, 29, 30 PLpro and 3CLpro are located on nsp3 and ns5, respectively. 31 Other nsps encoded by ORFs are nsp12 (for RNA-dependent RNA polymerase activity), nsp14 (for exoribonuclease activity), nsp15 (for exoribonuclease activity), and nsp16 (for methyl transferase activity) (►Fig. 1). 2, 32, 33 The nsp14 plays an important role in proofreading activity to monitor the rate of mutation. 33 The other genes downstream to ORF1ab encode for structural proteins such as Spike (S), envelope (E), membrane (M), and nucleocapsid (N) and accessory proteins. 25 The accessory proteins might help the virus in immune evasion by interfering with the innate immune response. MERS-CoV has five accessory proteins and SARS-CoV has eight different accessory proteins. 32 These differences might lead to the differences in the effects on induction and signaling of type 1 interferons (IFNs), 32 which could explain the greater sensitivity of IFN to MERS-CoV than SARS-CoV. 34, 35 The viral RNA is encapsidated in the N protein and transported to the endoplasmic reticulum-Golgi intermediate compartment (ERGIC), the site of assembly. Viral RNA encapsidated in the N protein and then buds into vesicles lined with the S, M, and E proteins. 32 The dipeptidyl-peptidase 4 (DPP4/CD26) was identified as the host-cell receptor for the entry of MERS-CoV. 13, 36 Both MERS-CoV and SARS-CoV bind to large ectopeptidases (DPP4 and ACE2, respectively) to invade the cells. 36, 37 The aminopeptidase N (APN) has been reported as entry receptors for several α-CoVs. 38 All hCoV receptors reported so far are exopeptidases, although their proteolytic activity is not necessary for the virus to bind to the receptors, or for entering in the host cells. 5,39-41 MERS-CoV can bind to the DPP4 of the several species, but the binding site of DPP4 is different in different species, explaining the variation in the susceptibility of MERS-CoV infections in various animals. 42 The S glycoprotein located on the surface of the MERS-CoV virion interacts with the receptor binding domain of DPP4 to enter in the host cells. 2, 36, 43 The S protein consists of a globular S1 domain at the N-terminal region, an S2 domain with two heptad repeats (HR1 and HR2), and a transmembrane domain. 3 Lower rate of mutation has been reported so far in the MERS-CoV during transmission in human populations in the recent outbreaks.

Both hACE2 and DPP4 have been reported to shed from cell surface. 38, 44 The loss of hACE2 results in more severe pulmonary disease. 45 The DPP4 has been reported as a neutrophil chemo repellent; therefore, the variations in DPP4 shedding during MERS-CoV infection could influence the composition of the immune infiltrate, and ultimately to the outcome of the infection. DPP4 has also been reported to be expressed on immune cells, including T cell lymphocytes, and required for optimal function of these cells. 38 The events occurring immediately after MERS-CoV binding with the DPP4 receptor appear to be similar to other CoVs. Cleavage of the surface glycoprotein is one of the necessary steps, which exposes the fusion peptide and facilitates the fusion of virus-cell membrane, which results into the release of the nucleocapsid in the cell cytoplasm. Several host proteases, including cathepsin B and members of the transmembrane protease, serine 2 (TMPRSS) family, have been implicated in this process. 38, 46 Immunopathogenesis of MERS-CoV MERS-CoV causes an acute, highly lethal pneumonia and renal dysfunction. Renal dysfunctions take place either due to hypoxic damage or direct infection of the kidney. The entry receptor (DPP4) for MERS-CoV is expressed at high levels in the kidney. 47 Neutrophil and macrophage infiltration and alveolar edema have been reported in infected lung tissues in the MERS animal models. 32 The cell-line susceptibility study indicated that the MERS-CoV may infect several human cell lines, including lower respiratory, kidney, intestinal, and liver cells, as well as histiocytes. 48 The range of MERS-CoV tissue tropism in vitro was found to be broader than any other HCoV.

MERS-CoV has been reported to elicit attenuated innate immune responses with delayed proinflammatory cytokine induction in in vitro and in vivo, [49] [50] [51] which might lead to immune dysfunctions. Similar findings have been reported among SARS patients. 32, 52 The ineffective B cell and T cell responses with prolonged cytokine expression have also been observed among the MERS patients during advanced stages of the disease. Clinical symptoms include fever, cough, sore throat, myalgia, chest pain, and gastrointestinal symptoms, such as diarrhea, vomiting, and abdominal pain. 5, 53, 54 Respiratory symptoms indicate the lower respiratory tract complications (dyspnea, cough, and fever); however, the upper respiratory tract complications are rare. The virus is detected in the upper respiratory tract only during early course of the infections, and lower respiratory tract during late stages of infections. 21, [55] [56] [57] In some cases, the virus is detected in urine and blood of the patients, which indicates the possibility of systemic infection. The multiple specimens should be collected from different sites at different time points to increase the possibility of detecting MERS-CoV. Priority should be given to the respiratory specimens (lower tract if obtainable and in all cases of severe disease; upper tract if the disease is mild and lower tract specimens cannot be obtained). Potential risk factors include obesity, diabetes mellitus, end-stage renal disease, cardiac disease, hypertension, lung disease including asthma and cystic fibrosis, and immunosuppressive conditions. 24, 26, 53, 54 The major complications reported in fatal cases are hyperkalemia with associated ventricular tachycardia, disseminated intravascular coagulation, pericarditis, and multiorgan failure. 5 Coinfections have also been reported frequently in the cases of MERS. 5, 27, 54 Hematological manifestations include higher leukocyte counts and lymphopenia, while a few cases showed lymphocytosis, thrombocytopenia, and coagulopathy. 5, 27 Elevated levels of creatinine, lactate dehydrogenase, alanine aminotransferase, and aspartate aminotransferase are indicative of renal and liver dysfunctions. 27, 58, 59 The median incubation period for MERS-CoV (5.2 days) is slightly longer than the incubation period of SARS-CoV (4 days). MERS and SARS are respiratory viruses, but gastrointestinal manifestations have been reported among patients suffering from SARS-CoV and MERS-CoV infections. The case-fatality rate seems to be higher for MERS-CoV (around 30%) than for SARS-CoV (9.6%). 60 The lesions and opacities in chest radiographs of the MERS patients have been reported to closely resemble the pneumonia patients infected with H1N1pdm09. 5, 59 The lesions in MERS patients are different from the lesions in SARS patients showing fibrocellular intraalveolar organization with a bronchiolitis obliterans organizing pneumonia (BOOP)-like pattern. 60,61 MERS-CoV has been reported to induce greater dysregulation of the host response to infection than SARS-CoV. MERS-CoV has been reported to downregulate the genes involved in the antigen presentation, as an immune evasion strategy. 62 The decreased expression of IFN-α, retinoic inducible acid gene (RIG)-1, melanoma differentiation associate (MDA5), and IFN regulatory factors IRF-3 and IRF-7 have been reported in bronchoalveolar lavage (BAL) and serum of patients in advanced stages of MERS. However, the higher levels of CXC-motif chemokines ligand (CXCL) 10 and interleukin (IL)-10 have been reported among patients severely suffering from MERS, which might have resulted in the lower IFN-γ expression and higher levels of IL-17A and IL-23. 63

MERS-CoV seems more sensitive to prophylactic and therapeutics in vitro than SARS-CoV. 36 Immunofluorescence assays have been used to detect MERS-CoV antibodies. 64 IgG and IgM antibodies in serum samples could be determined using an anti-MERS-CoV indirect immunofluorescence assay. 16, 65 In addition, the enzyme-linked immunosorbent assays (ELISA), protein microarray technology, and micro-neutralization (MN) assays have also been used to detect MERS-CoV antibodies. 65, 66 The real-time PCR (RT-PCR) assays targeting RNA upstream of the E gene (upE) and ORF-1b and ORF-1a have been also used for detection. 32 According to the WHO, the screening using RT-PCR targeting upE gene should be conducted on samples from suspected MERS patients. All positive samples should undergo confirmatory testing by targeting ORF-1a, ORF-1b, or N genes. 16, 67 These RT-PCR assays have been reported to be very specific without any cross-reactivity with other respiratory viruses including hCoVs. 32 Respiratory samples should be collected at least every 2 to 4 days to confirm virus clearance in the cases where two consecutive results are negative. 32, 68 Western blotting is also useful in the diagnosis of MERS-CoV. 68, 69 The samples from lower respiratory tract such as BAL and tracheal aspirates are better suited for the testing due to the higher viral loads in lower respiratory tract samples. 70 Antiviral treatment under the supervision of medical experts is recommended for MERS patients with or without comorbidities, with clear symptoms of pneumonia. 71 The efficacy of lopinavir/ritonavir was found to be similar to IFN-β1b in experimental trials using marmosets. 72 IFN-β1b or lopinavir/ritonavir can be considered for treating patients, in whom the use of ribavirin is not possible due to renal dysfunction or other adverse effects. 71 The combination regimen of type 1 IFN þ lopinavir/ ritonavir may be considered as the first-line drug for MERS patients. 71 One must monitor creatinine clearance according to renal function during the use of ribavirin. 71 Antiviral treatment is generally recommended for 10 to 14 days in patients with MERS-CoV infection. 71 The levels of hemoglobin, bilirubin, haptoglobin, and reticulocyte must be monitored carefully during the use of ribavirin. The use of lopinavir/ritonavir has been recommended instead of ribavirin in cases of hemolytic anemia. 71 Since ribavirin is teratogenic, patients are advised not to plan for pregnancy for 6 months after treatment. 73 Convalescent plasma therapy has been shown some efficacy among patients refractory to antiviral drugs if administered within 2 weeks of the onset of the disease. 71,74 Convalescent plasma therapy could help some patients with severe disease. 10,32,75 MERS-CoV-specific peptide fusion inhibitors have been reported as a novel approach to treat MERS patients. 71

There is a need to understand the framework of establishment of the MERS-CoV infection in the society to develop control measures for the larger outbreaks. The involvement of pattern recognition receptors and downstream signaling at the cellular level is required to understand the molecular pathogenesis of MERS-CoV. The role of accessory proteins of MERS-CoV should be dissected out to understand their diverse roles in the immune evasion strategies adopted by MERS-CoV to inhibit IFN-β expression. There is need to develop suitable animal models to study the MERS-CoV pathogenesis for testing antiviral drugs and vaccines. The threat of potential pandemics is always there because pathogenic CoVs continue to spill over from animals to the human population. The specific diagnostics and control measures are required to avoid the spread among healthcare workers and nosocomial infections among other patients. The 

Characterization and complete genome sequence of a novel coronavirus, coronavirus HKU1, from patients with pneumonia

Middle East respiratory syndrome coronavirus: transmission, virology and therapeutic targeting to aid in outbreak control

Coronavirus pathogenesis and the emerging pathogen severe acute respiratory syndrome coronavirus

A previously undescribed coronavirus associated with respiratory disease in humans

Pathogenesis of Middle East respiratory syndrome coronavirus

Epidemiology and clinical presentations of the four human coronaviruses 229E, HKU1, NL63, and OC43 detected over 3 years using a novel multiplex real-time PCR method

New coronavirus outbreak. Lessons learned from the severe acute respiratory syndrome epidemic

Emergence of MERS-CoV in the Middle East: origins, transmission, treatment, and perspectives

Genetic relatedness of the novel human group C betacoronavirus to Tylonycteris bat coronavirus HKU4 and Pipistrellus bat coronavirus HKU5

Middle East respiratory syndrome coronavirus (MERS-CoV): announcement of the Coronavirus Study Group

Molecular diversity of coronaviruses in bats

Circulation of group 2 coronaviruses in a bat species common to urban areas in Western Europe

Genomic characterization of a newly discovered coronavirus associated with acute respiratory distress syndrome in humans

Close relative of human Middle East respiratory syndrome coronavirus in bat

Middle East respiratory syndrome coronavirus in dromedary camels: an outbreak investigation

Middle East respiratory syndrome coronavirus "MERS-CoV": current knowledge gaps

Middle East respiratory syndrome coronavirus neutralising serum antibodies in dromedary camels: a comparative serological study

Seroepidemiology for MERS coronavirus using microneutralisation and pseudoparticle virus neutralisation assays reveal a high prevalence of antibody in dromedary camels in Egypt

Reply to "Concerns about misinterpretation of recent scientific data implicating dromedary camels in epidemiology of Middle East respiratory syndrome (MERS)

Evidence for camel-tohuman transmission of MERS coronavirus

Evolutionary history of the closely related group 2 coronaviruses: porcine hemagglutinating encephalomyelitis virus, bovine coronavirus, and human coronavirus OC43

MERS coronavirus: data gaps for laboratory preparedness

KSA MERS-CoV Investigation Team. Hospital outbreak of Middle East respiratory syndrome coronavirus

MERS: emergence of a novel human coronavirus

Clinical course and outcomes of critically ill patients with Middle East respiratory syndrome coronavirus infection

Epidemiological, demographic, and clinical characteristics of 47 cases of Middle East respiratory syndrome coronavirus disease from Saudi Arabia: a descriptive study

Current advancements and potential strategies in the development of MERS-CoV vaccines

Ligand-induced Dimerization of Middle East Respiratory Syndrome (MERS) Coronavirus nsp5 Protease (3CLpro): IMPLICATIONS FOR nsp5 REGULATION AND THE DEVELOPMENT OF ANTIVIRALS

Crystal structure of the Middle East respiratory syndrome coronavirus (MERS-CoV) papain-like protease bound to ubiquitin facilitates targeted disruption of deubiquitinating activity to demonstrate its role in innate immune suppression

Coronaviruses post-SARS: update on replication and pathogenesis

Middle East respiratory syndrome

Coronaviruses lacking exoribonuclease activity are susceptible to lethal mutagenesis: evidence for proofreading and potential therapeutics

Efficient replication of the novel human betacoronavirus EMC on primary human epithelium highlights its zoonotic potential

Human cell tropism and innate immune system interactions of human respiratory coronavirus EMC compared to those of severe acute respiratory syndrome coronavirus

Dipeptidyl peptidase 4 is a functional receptor for the emerging human coronavirus-EMC

Angiotensin-converting enzyme 2 is a functional receptor for the SARS coronavirus

The Middle East respiratory syndrome-how worried should we be?

Emerging human middle East respiratory syndrome coronavirus causes widespread infection and alveolar damage in human lungs

MERS-coronavirus replication induces severe in vitro cytopathology and is strongly inhibited by cyclosporin A or interferon-α treatment

Aminopeptidase N is a major receptor for the entero-pathogenic coronavirus TGEV

Adenosine deaminase acts as a natural antagonist for dipeptidyl peptidase 4-mediated entry of the Middle East respiratory syndrome coronavirus

A predicted receptor-binding and critical neutralizing domain in S protein of the novel human coronavirus HCoV-EMC

Angiotensinconverting enzyme 2 (ACE2) in disease pathogenesis

Dipeptidyl peptidase IV is a human and murine neutrophil chemorepellent

The spike protein of the emerging betacoronavirus EMC uses a novel coronavirus receptor for entry, can be activated by TMPRSS2, and is targeted by neutralizing antibodies

Dipeptidylpeptidase IV from bench to bedside: an update on structural properties, functions, and clinical aspects of the enzyme DPP IV

Preliminary epidemiological assessment of MERS-CoV outbreak in South Korea

Tropism of and innate immune responses to the novel human betacoronavirus lineage C virus in human ex vivo respiratory organ cultures

Active replication of Middle East respiratory syndrome coronavirus and aberrant induction of inflammatory cytokines and chemokines in human macrophages: implications for pathogenesis

Delayed induction of proinflammatory cytokines and suppression of innate antiviral response by the novel Middle East respiratory syndrome coronavirus: implications for pathogenesis and treatment

Canadian SARS Research Network. Interferon-mediated immunopathological events are associated with atypical innate and adaptive immune responses in patients with severe acute respiratory syndrome

Middle East respiratory syndrome coronavirus: a case-control study of hospitalized patients

Middle East respiratory syndrome coronavirus disease in children

MERS-CoV Study Group. Clinical features and viral diagnosis of two cases of infection with Middle East Respiratory Syndrome coronavirus: a report of nosocomial transmission

Clinical features and virological analysis of a case of Middle East respiratory syndrome coronavirus infection

Prevalence of MERS-CoV nasal carriage and compliance with the Saudi health recommendations among pilgrims attending the 2013 Hajj

Hospital-associated outbreak of Middle East respiratory syndrome coronavirus: a serologic, epidemiologic, and clinical description

Middle East respiratory syndrome coronavirus (MERS-CoV) infection: chest CT findings

Lung pathology of fatal severe acute respiratory syndrome

Pulmonary pathological features in coronavirus associated severe acute respiratory syndrome (SARS)

Cell host response to infection with novel human coronavirus EMC predicts potential antivirals and important differences with SARS coronavirus

Distinct immune response in two MERS-CoV-infected patients: can we go from bench to bedside?

Serological assays for emerging coronaviruses: challenges and pitfalls

Virological and serological analysis of a recent Middle East respiratory syndrome coronavirus infection case on a triple combination antiviral regimen

Human-dromedary camel interactions and the risk of acquiring zoonotic Middle East respiratory syndrome coronavirus infection

Detection of Middle East respiratory syndrome coronavirus using reverse transcription loop-mediated isothermal amplification (RT-LAMP)

Identification of a receptor-binding domain in the S protein of the novel human coronavirus Middle East respiratory syndrome coronavirus as an essential target for vaccine development

Identification of protein receptors for coronaviruses by mass spectrometry

Respiratory tract samples, viral load, and genome fraction yield in patients with Middle East respiratory syndrome

Antiviral treatment guidelines for Middle East respiratory syndrome

Treatment with lopinavir/ritonavir or interferon-β1b improves outcome of MERS-CoV infection in a nonhuman primate model of common marmoset

What the infectious disease physician needs to know about pegylated interferon and ribavirin

Convalescent Plasma Study Group. The effectiveness of convalescent plasma and hyperimmune immunoglobulin for the treatment of severe acute respiratory infections of viral etiology: a systematic review and exploratory meta-analysis

Severe acute respiratory syndrome vs. the Middle East respiratory syndrome

This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited.knowledge about the dynamics and molecular characteristics of MERS-CoV and other closely related CoVs in circulation is very important to understand their emergence and to strengthen disease surveillance and disease preparedness.