Virology tidbits

Virology tidbits

Tuesday, 22 April 2014

SARS-CoV v. MERS-CoV: differences and similarities, what do we know?

Coronaviruses are important animal and human pathogens and are the causative agent of 30-40% community acquired upper respiratory tract infections, most of them mild diseases, although severe diarrhea has been observed in children and infants. Besides relatively benign infections, the infection of infants and children has been implicated in some cases to acute asthmatic attacks and the onset of croup (whizzing cough). 
Main differences between SARS-CoV and MERS-CoV
With the identi-fication of SARS-CoV in 2003 became associ-ated with more severe pulmonary disease particu-larly in immuno-compromised individuals. SARS -CoV caused se-vere lower re-spiratory disease with nearly 10% mortality and evidence of systemic spread mostly in Southeast Asian countries .
                     Pathology of SARS-CoV v. MERS-CoV

In contrast to SARS-CoV, the infection with MERS-CoV is leading to a higher fatality rate (50% v. 10%), although both viruses cause severe pneumonia and multiorgan dysfunction.  To understand the pathogenesis, it is vital to compare various aspects of the disease, including but not limited to the receptor distribution, viral entry and affected organs as well the interference with antiviral signaling.
Genomes of SARS-CoV and MERS-CoV: whilst similar in size, SARS-CoV encodes additional genes

                             MERS-CoV and cytokines

Both SARS-CoV and MERS-CoV inhibit the secretion of interferon (IFN)-α and IFN-β and induce the expression of pro-inflammatory tumour necrosis factor (TNF)-α and Interleukin-6, thus inducing inflammation of surrounding tissue (and potentially necrosis). MERS-CoV also induces the expression of IL-12,  IFN-γ and chemokines (e.g. RANTES/CCL-5, Il-8, IP-10/CXCL-10, MCP-1/CCL-2, MIP-1α/CCL-3) in significantly higher levels than SARS-CoV, which are required for the recruitment of T- lymphocytes to sites of inflammation. Antiviral signaling is inhibited via the inhibition of TLR mediated induction of IFN-β by the orf 4a and 4b proteins of MERS-CoV by interfering with RIG-1 and MDA5/PACT mediated signaling (whether the nucleocapsid protein is also involved has not been investigated) (see also previous blog entry).
In contrast to SARS-CoV, MES-CoV can infect and replicate in human monocyte–derived macrophages (MDM) and the aberrant induction of cytokines in these cells might contribute to disease pathogenesis. Furthermore, in MDM MERS-CoV increases the expression of MHC-class I and co-stimulatory molecules leading to an activation of the immune response. 
The severity of MERS might be enhanced by the immunological status of the infected individual since symptoms are generally more severe in the elderly and immunocompromised.

    SARS-CoV and MERS-CoV: receptors and cell tropism

Both SARS-CoV and MERS-CoV have been shown to infect a range of human, primate, porcine and bat derived cell lines, including but not limited to commonly used cell lines such as Vero cells and human airway epithelia cells. In the case of MERS-CoV, and in contrast to SARS-CoV, in vivo target cells include type II alveolar cells and non-ciliated cells epithelial cells (Clara cells) whereas ACE2 expressing ciliated epithelial cells (which are infected by SARS-CoV) are not susceptible to MERS-CoV infection. In addition, MERS-CoV but not SARS-CoV is capable of infecting endothelial cells as well. The receptor for MERS-CoV, first identified in Huh7 and primary human bronchial epithelial cells, was identified as dipeptidyl peptidase 4 (DDP4/CD26) and confirmed by transfecting non-permissive Cos7 cells both with bat (Pipistrellus pipistrellus) and human derived DPP4 followed by infection with MERS-CoV. The application of antibodies binding DPP4 to Huh7 and primary human bronchial epithelial cells prior to infection successfully prevented cells from MERS-CoV infection, thus further validating DPP4 being the receptor for MERS-CoV. DPP4 also specifically co-purified with the S1 subunit of MERS-CoV Spike protein.  Besides being the receptor for MERS-CoV, DPP 4 has many diverse functions in glucose homeostasis, T-cell activation, neurotransmitter function, and modulation of cardiac signaling, but the enzymatic function of DPP4 is not required for viral entry (similarly the function of ACE2 is not required for SARS-CoV entry). The susceptibility for MERS-CoV of Vero cells is increased by the presence of a cell surface lung protease, TMPRSS2, as well as the presence of low-affinity receptors. During the entry of Coronaviruses into the host cell, the type II transmembrane protease TMPRSS2 activates the spike (S) protein by cleaving the mature S protein into two subunits (S1 and S2) thereby increasing the fusogenicity with the host cell receptor. In the absence of TMPRSS2, Coronavirus particles enter the cell via the endosomal pathway, which is dependent on Cathepsin L. Both SARS-CoV and HCoV-NL63 have been shown to enter the host cell via both pathways, thus suggesting that MERS-CoV might be similar and -if this is the case offer some potential options for successful treatment or prevention. Vero cells expressing TMPRSS2 show larger syncytia at 18 hrs p.i. compared to control cells, which can be blocked by the application of Camostat, a Serine protease inhibitor - Camostat however only partially blocks viral entry. This indicates that MERS-CoV, as other Coronaviruses, enters Vero cells via two independent pathways; indeed the application of both Camostat and  a Cathepsin L inhibitor ((23,25)-trans-epoxysuccinyl-L-leucylamindo-3-methylbutane ethyl ester or EST) not only blocks MERS-COV but also HCOV-NL63 and SARS-CoV entry into Vero-TMRSS cells. In MERS-CoV infected  human bronchial submucosal gland-derived cells (Calu-3) cells , treatment with both EST and Camostat nor in combination with leupeptin is more efficacious than treatment with Camostat alone (in contrast to HCoV-Nl63 and SARS-CoV). These inhibitors were also not efficacious against MERS-CoV infection of lung derived MRC-5 and WI-38 cell lines (both are however different from mature pneumocytes, suggesting that a single treatment with Camostat is sufficient to block MERS-CoV entry into differentiated lung-derived cell lines. In the context of the infection of humans with MERS-CoV, the presence of low-affinity receptors as well as the presence of TMPRSS2 (or another S-cleaving protease) on the cell surface might sensitize cells to MERS-CoV infection. In addition, the presence of both a receptor for MERS-CoV and a S cleaving protease in a variety of animals present in the Middle East might determine potential animal reservoirs and sources of recurring transmission to humans. Since the MERS-CoV receptors in human, horse and camel are equally effective -with goat and bat receptors less effective- it might be worthwhile to extent screening beyond camels, especially in the light of the increase interest in racehorses among wealthy Arabians.

                                   Acute renal failure

The pathology of patients infected with MERS-CoV include not only respiratory disease but also acute renal failure. Camels infected with MERS-CoV might shed viral particles in the urine, thus (potentially!) contributing to viral transmission. Infection and replication of kidneys with MERS-CoV might therefore not only lead to acute renal failure but also to shedding and transmission of MERS-CoV in urine - thus leading to new cases not only via airborne transmission but also under favorable conditions via contaminated drinking water. Indeed, DPP4 is present on the surface of both cells derived from a healthy human kidney and in primary kidney cell lines (as is ACE2, the receptor for SARS-CoV but not receptors for other HCoV). In addition to primary human kidney cells, MErS-CoV also replicates with high titers in kidney epithelial cells derived from bats, pigs, and monkey (such as LLC-MK2, Vero, and 769-P cell lines).
Although acute renal failure is a relatively late complication well after the onset of first symptoms- shedding of viral particles might partially explain familial clusters of infections. In contrast to SARS-CoV, the infection of primary kidney cells with MERS-CoV induces a more severe cytopathic effect and in higher viral titers, not only when compared with SARS-CoV but also when compared to human bronchial epithelial cells.  Acute renal failure however is absent in rhesus macaques. Unfortunately, to my knowledge no post-mortem data are available from diseased patients, so at present it is not clear if the infection of kidneys causes tissue necrosis.  In the case of SARS-CoV, histopathological findings revealed mainly acute tubular necrosis without abnormal pathology of the glomeruli, being the result of a systemic inflammatory response rather than a specific effect of viral infection of the kidney. 


Based on the experience from SARS-CoV related outbreaks and epidemiological data, MERS-CoV is thought to be transmitted by

  • Large particle respiratory droplets (by air; requires close contact).
  • Contact with contaminated surfaces.
  • Oral-fecal route.
  • Hospital procedures associated the generation of aerosol.

As a disclaimer, it should be noted that the precise mechanism of transmission has not been established.

Finally, the question remains how to treat patients infected with MERS-CoV? So far no specific treatment in the form of replication inhibitors exist. Treatment relies mainly on supportive care, alleviating the symptoms. Experimental treatment includes treatment of patients with Interferon-α2b and ribavirin, thus limiting viral replication. A future vaccine will most likely be based on the Spike protein and be a DNA vaccine rather than an live or an inactivated (attenuated) vaccine, similar to the experimental SARS-CoV vaccine. It should be noted that any vaccine developed might be used to vaccine animals rather than humans simply because animal vaccines are easier to be approved of.
In the meantime it is important to identify the reservoir and use this information to prevent further cases.

Further reading

Coleman, C., & Frieman, M. (2013). Emergence of the Middle East Respiratory Syndrome Coronavirus PLoS Pathogens, 9 (9) DOI: 10.1371/journal.ppat.1003595 

Barlan A, Zhao J, Sarkar MK, Li K, McCray PB Jr, Perlman S, & Gallagher T (2014). Receptor variation and susceptibility to middle East respiratory syndrome coronavirus infection. Journal of virology, 88 (9), 4953-61 PMID: 24554656

Raj, V., Mou, H., Smits, S., Dekkers, D., Müller, M., Dijkman, R., Muth, D., Demmers, J., Zaki, A., Fouchier, R., Thiel, V., Drosten, C., Rottier, P., Osterhaus, A., Bosch, B., & Haagmans, B. (2013). Dipeptidyl peptidase 4 is a functional receptor for the emerging human coronavirus-EMC Nature, 495 (7440), 251-254 DOI: 10.1038/nature12005

Müller MA, Raj VS, Muth D, Meyer B, Kallies S, Smits SL, Wollny R, Bestebroer TM, Specht S, Suliman T, Zimmermann K, Binger T, Eckerle I, Tschapka M, Zaki AM, Osterhaus AD, Fouchier RA, Haagmans BL, & Drosten C (2012). Human coronavirus EMC does not require the SARS-coronavirus receptor and maintains broad replicative capability in mammalian cell lines. mBio, 3 (6) PMID: 23232719

Shirato K, Kawase M, & Matsuyama S (2013). Middle East respiratory syndrome coronavirus infection mediated by the transmembrane serine protease TMPRSS2. Journal of virology, 87 (23), 12552-61 PMID: 24027332

Kawase M, Shirato K, van der Hoek L, Taguchi F, & Matsuyama S (2012). Simultaneous treatment of human bronchial epithelial cells with serine and cysteine protease inhibitors prevents severe acute respiratory syndrome coronavirus entry. Journal of virology, 86 (12), 6537-45 PMID: 22496216

Eckerle I, Müller MA, Kallies S, Gotthardt DN, & Drosten C (2013). In-vitro renal epithelial cell infection reveals a viral kidney tropism as a potential mechanism for acute renal failure during Middle East Respiratory Syndrome (MERS) Coronavirus infection. Virology journal, 10 PMID: 24364985

Chu KH, Tsang WK, Tang CS, Lam MF, Lai FM, To KF, Fung KS, Tang HL, Yan WW, Chan HW, Lai TS, Tong KL, & Lai KN (2005). Acute renal impairment in coronavirus-associated severe acute respiratory syndrome. Kidney international, 67 (2), 698-705 PMID: 15673319

Pacciarini F, Ghezzi S, Canducci F, Sims A, Sampaolo M, Ferioli E, Clementi M, Poli G, Conaldi PG, Baric R, & Vicenzi E (2008). Persistent replication of severe acute respiratory syndrome coronavirus in human tubular kidney cells selects for adaptive mutations in the membrane protein. Journal of virology, 82 (11), 5137-44 PMID: 18367528

Kawase M, Shirato K, van der Hoek L, Taguchi F, & Matsuyama S (2012). Simultaneous treatment of human bronchial epithelial cells with serine and cysteine protease inhibitors prevents severe acute respiratory syndrome coronavirus entry. Journal of virology, 86 (12), 6537-45 PMID: 22496216

Roper, R., & Rehm, K. (2009). SARS vaccines: where are we? Expert Review of Vaccines, 8 (7), 887-898 DOI: 10.1586/erv.09.43

 Zhou J, Chu H, Li C, Wong BH, Cheng ZS, Poon VK, Sun T, Lau CC, Wong KK, Chan JY, Chan JF, To KK, Chan KH, Zheng BJ, & Yuen KY (2014). Active replication of middle East respiratory syndrome coronavirus and aberrant induction of inflammatory cytokines and chemokines in human macrophages: implications for pathogenesis. The Journal of infectious diseases, 209 (9), 1331-42 PMID: 24065148

Payne DC, Iblan I, Alqasrawi S, Al Nsour M, Rha B, Tohme RA, Abedi GR, Farag NH, Haddadin A, Al Sanhouri T, Jarour N, Swerdlow DL, Jamieson DJ, Pallansch MA, Haynes LM, Gerber SI, Al Abdallat MM, & for the Jordan MERS-CoV Investigation Team (2014). Stillbirth During Infection With Middle East Respiratory Syndrome Coronavirus. The Journal of infectious diseases PMID: 24474813

 Zielecki F, Weber M, Eickmann M, Spiegelberg L, Zaki AM, Matrosovich M, Becker S, & Weber F (2013). Human cell tropism and innate immune system interactions of human respiratory coronavirus EMC compared to those of severe acute respiratory syndrome coronavirus. Journal of virology, 87 (9), 5300-4 PMID: 23449793

 Drosten, C. (2013). Is MERS another SARS? The Lancet Infectious Diseases, 13 (9), 727-728 DOI: 10.1016/S1473-3099(13)70159-2

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