Abstract
Pseudoparticle virus neutralization (ppNT) and a conventional microneutralization (MN) assays are specific for detecting antibodies to MERS coronavirus (MERS-CoV) when used in sero-epidemiological studies in animals. Genetically diverse MERS-CoV appear antigenically similar in MN tests. We confirm that MERS-CoV has been circulating in dromedaries in Saudi Arabia in 1993. Preliminary data suggests that feral Australian dromedaries may be free of MERS-CoV but larger confirmatory studies are needed.
Keywords: sero-epidemiology, animal reservoirs, animal vectors, cross-reaction
Introduction
MERS is an emerging respiratory disease of global public health concern. As of 9th May 2014, 536 confirmed human cases have been reported to WHO with 145 deaths [1]. The current epidemiology MERS is one of zoonotic transmission, sometimes followed by chains of limited human-to-human transmission for limited periods of time within families or health care facilities. This is reminiscent of the emergence of Severe Acute Respiratory Syndrome (SARS) in late 2002 [2]. It is therefore critically important to identify the sources of zoonotic transmission, so that evidence based interventions to minimise such infections can be implemented, as for example has been used to minimise the human health risk from highly pathogenic avian influenza H5N1 and SARS [3,4].
Sero-epidemiology is an invaluable tool in such investigations. Many sero-epidemiological studies on domestic livestock have reported high sero-prevalence in dromedary camels in the Arabian peninsula and Africa [5–8]. The detection of MERS-CoV virus by reverse transcription polymerase chain reaction (RT-PCR) and virus isolation supports these sero-epidemiological findings and the contention that dromedaries are a natural host for MERS-CoV [9–11]. But it is not clear if dromedaries are the main source of human infection.
We had previously reported a MERS-CoV pseudoparticle neutralization assay (ppNT) that can be used to detect antibody to MERS-CoV without the need for Biosafety Level-3 (BSL-3) containment that is required for conventional MERS-CoV microneutralisation (MN) tests [6]. In this study, we systematically investigate potential cross reactions that may confound the use of these two assays in sero-epidemiological studies in animals.
Methods
Viruses
MERS-CoV EMC strain was provided by Dr Ron Fouchier, Erasmus Medical Centre, Rotterdam. The virus strains dromedary MERS-CoV Al-Hasa KFU-HKU13 2013 (Al-Hasa 13) and dromedary MERS-CoV Egypt NRCE-HKU270 2013 (Egypt 270) were isolated in our laboratory as previously described [10,12]. The viruses were cultured and titrated in Vero cells (ATCC CCL-81).
Sera
Immune sera specific for alphacoronaviruses (porcine respiratory coronavirus, feline infectious peritonitis virus, canine coronavirus and porcine transmissible gastroenteritis virus), betacoronaviruses (mouse hepatitis virus: strains JHM and A59, SARS coronavirus, BCoV) and gammacoronavirus (infectious bronchitis virus) were obtained from BEI-Resources (animal CoV reagents supplied to BEI by Dr Linda Saif) (http://www.beiresources.org/About/BEIResources.aspx) or generated by Dr Linda Saif or Dr Stanley Perlman, as indicated in table 1. The homologous antibody titres to the immunising virus was also obtained from the respective sources supplying these antisera (Table 1).
Table 1.
Cross neutralization antibody titers for MERS-CoV and BCoV in anti-sera raised against different coronaviruses
| Genus | Antisera | Homologous Ab titre by ELISA unless otherwise specified | MERS-CoV MN titre | MERS-CoV ppNT titre | BCoV MN titre |
|---|---|---|---|---|---|
| Alpha coronavirus | Gnotobiotic pig antiserum to Porcine Respiratory coronavirus - NR 460 | 1:1200# | <1:10 | <1:10 | <1:10 |
| Guinea Pig antiserum to Feline infectious Peritonitis virus - NR 2518 | 1:2000# | <1:10 | <1:10 | <1:10 | |
| Guinea pig antiserum to Canine Coronavirus - NR 2727 | 1:4094ϕ | <1:10 | <1:10 | <1:10 | |
| Gnotobiotic pig antiserum to Porcine transmissible gastroenteritis virus - NR 458 | 1:1400# | <1:10 | <1:10 | <1:10 | |
| Beta coronavirus | Guinea Pig anti-SARS-CoV - NR 10361 | 1:2560 | <1:10 | <1:10 | <1:10 |
| Rabbit anti serum for SARS-CoV S protein (Zero titer) - NRC 769 | <1:10 | <1:10 | <1:10 | <1:10 | |
| Rabbit anti serum for SARS-CoV S protein (low titer) - NRC 770 | 1:80 | <1:10 | <1:10 | <1:10 | |
| Rabbit anti serum for SARS-CoV S protein (medium titer) - NRC 771 | 1:160 | <1:10 | <1:10 | <1:10 | |
| Rabbit anti serum for SARS-CoV S protein (high titer) - NRC 772 | 1:640 | <1:10 | <1:10 | <1:10 | |
| Mouse hepatitis virus (JHM strain) hyper immunized mouse dam 1 | 1:1778¶ Neutralization titre |
<1:10 | <1:10 | <1:10 | |
| Mouse hepatitis virus (JHM strain) hyper- immunised mouse dam 2 | 1:363¶ Neutralization titre |
<1:10 | <1:10 | <1:10 | |
| Mouse hepatitis virus (A59 strain) infected mouse | 1:1000¶ Neutralization titre |
<1:10 | <1:10 | <1:10 | |
| BCoV antisera from guinea pig - GP 9910 | 1:20480ϕ | <1:10 | <1:10 | 1:160 | |
| BCoV antisera from germfree calf - NR 456 | 1:10000# | <1:10 | <1:10 | 1:40 | |
| BCoV antisera from germfree calf - C3816 | 1:580ϕ Neutralization titre |
<1:10 | <1:10 | 1:640 | |
| Gamma coronavirus | Guinea Pig anti serum to infectious bronchitis virus - NR 2515 | 1:50000# | <1:10 | <1:10 | <1:10 |
Homologous antibody titre data from
BEI resources;
Linda Saif;
Stanly Perlman. All other antibody titres are obtained as part of this study. All homologous antibody titres are ELISA titres except for antisera to mouse hepatitis virus and BCoV antiserum C3816 which are neutralizing antibody titres.
Abbreviations: MERS-CoV: Middle East respiratory syndrome coronavirus; BCoV: bovine coronavirus; SARS-CoV: Severe acute respiratory syndrome coronavirus; MN: Microneutralisation; ppNT: pseudoparticle neutralization.
Sera from 25 adult dromedary camels were collected in 2014 in Australia, 17 being from feral camels from central Australia gathered and transported to an abattoir in Caboolture, Queensland, while the other 8 sera originated from a camel farm in Coominya, Queensland. Dromedary sera from Egypt were collected from abattoirs in Egypt in 2014. Archived dromedary sera collected in 1993 from Al Hasa, Eastern province (n=27), As Sulayyil, Ar Riyad province (n=30), Hafar Al-Batin (Eastern province) (n=45) and Medina, Al Medinah province (n=29) were retrieved from the serum archive at the Department of Microbiology and Parasitology, College of Veterinary Medicine, King Faisal University, Saudi Arabia. Paired acute and convalescent sera from three dromedary calves who had RT-PCR confirmed MERS-CoV infection in a dromedary farm in Al-Hasa, Saudi Arabia in December 2013 are included in this study. The epidemiological and virological data on these three animals as well as the serological responses to MERS-CoV has been reported previously [12].
Serological tests
The methods for the ppNT and MN neutralization test for MERS-CoV, and for the MN test for BCoV have been previously reported [6,13]. We used serial two-fold dilutions of heat inactivated (56°C for 30 minutes) sera with an entry dilution of 1:10. Titres of ≥1:40 are reported as positive and those 1:10–1:20 regarded as indeterminate.
MERS CoV spike pseudoparticle neutralization (ppNT) assay
A codon optimized spike gene was designed based on MERS-CoV genome sequence (GenBank: JX869059.1), synthesized in Genecust (Luxembourg) and subcloned into pcDNA3.1+ vector to generate pcDNA-S. To produce HIV/MERS spike pseudoparticles, 10 & mu;g pNL Luc E- R-_and 10 μg pcDNA-S were co-transfected into 4 × 106 293T cells. Supernatants of transfected cells were harvested 48h later and quantified for HIV p24 viral protein using a p24 ELISA Kit (Cell Biolabs, INC, San Diego, CA, USA) [6].
HIV/MERS pseudoparticles containing 5ng HIV p24 was used to infect Vero E6 cells (ATCC CRL-1586) in a single well (96 well plate format; 1 × 104 cells/well). Infected cells were lysed in 20 μl lysis buffer and 100 μl of luciferase substrate at two days postinfection (Promega Corporation, Madison, WI, USA). Luciferase activity was measured in a Microbeta luminometer (PerkinElmer, Waltham, MA, USA). For the ppNT assay, HIV/MERS pseudoparticles (5ng of p24) were pre-incubated with serially diluted sera for 30 min at 4°C and then added to cells in triplicate. Residual virus replication was assayed at two days postinfection, as described above. The highest serum dilution giving a 90% reduction of luciferase activity was regarded as the ppNT antibody titre.
Microneutralization (MN) tests: MERS-CoV (strain: EMC) and bovine coronavirus (ATCC BRCV-OK-0514-2) were used. Vero cells (ATCC CCL-81) were used for MERS-CoV and HRT-18G cells (obtained from ATCC) for BCoV. Serum dilutions were mixed with equal volumes of 200 tissue culture infective dose (TCID)50 of virus and incubated for one hour at 37°C. The virus–serum mixture was then added in quadruplicate to cell monolayers in 96-well microtitre plates. After one hour of adsorption, the virus-serum mixture was removed and 150μl of fresh culture medium was added to each well and the plates incubated at 37°C in 5% CO2 in a humidified incubator. A virus back-titration was performed without immune serum to assess input virus dose. Cytopathic effect (CPE) was read at three days post infection for MERS-CoV and four days post infection for BCoV. The highest serum dilution that completely protected the cells from CPE in half of the wells was defined as the neutralising antibody titre. Positive and negative control sera were included in each assay [13].
Results
We tested immune sera to a range of animal alpha-, beta- and gamma- coronaviruses and found no cross reaction to MERS-CoV in either the MERS-CoV ppNT or MN assays (Table 1). Specifically, we demonstrated that calf and guinea pig immune sera to BCoV do not cross-react in the MERS-CoV ppNT or MN assays.
Of the archived dromedary sera collected in 1993, 26 of 27 sera from Al Hasa, 22 of 30 sera from As Sulayyil, 43 of 45 sera from Hafar Al-Batin and 27 of 29 sera from Medina had detectable (≥1:40) ppNT antibody titres to MERS-CoV, with antibody titres ranging from 1:40 to ≥1:5120. Data from representative sera are shown in table 2. Many, but not all of the MERS-CoV antibody positive sera were also positive for BCoV antibody in MN tests.
Table 2.
Serological reactions to MERS-CoV and B-CoV in selected dromedary sera collected from camels in Egypt (2014), Saudi Arabia (1993) and Australia (2014).
| Location of serum collection (year) | Serum identity number | MERS-CoV MNT | MERS-CoV ppNT | BCoV MNT |
|---|---|---|---|---|
| Egypt (2014) | E2 | 1:320 | 1:640 | 1:40 |
| E4 | 1:160 | 1:320 | 1:40 | |
| E5 | 1:40 | 1:160 | 1:40 | |
| E6 | 1:40 | 1:160 | 1:40 | |
| E7 | 1:80 | 1:320 | <1:10 | |
| E8 | 1:40 | 1:80 | 1:40 | |
| E9 | 1:320 | 1:640 | 1:160 | |
| Australia (2014) (selected from 25) | A1 | <1:10 | <1:10 | <1:10 |
| A2 | <1:10 | <1:10 | 1:160 | |
| A3 | <1:10 | <1:10 | 1:160 | |
| A4 | <1:10 | <1:10 | 1:160 | |
| A5 | <1:10 | <1:10 | 1:320 | |
| A6 | <1:10 | <1:10 | 1:320 | |
| Saudi Arabia (1993) (selected from 131) | S1 | 1:320 | 1:1280 | 1:80 |
| S2 | 1:320 | 1:2560 | 1:40 | |
| S3 | 1:640 | 1:1280 | 1:160 | |
| S4 | >1:1280 | >1:5120 | 1:160 | |
| S5 | 1:40 | 1:160 | <1:10 | |
| S7 | 1:80 | 1:80 | <1:10 | |
| S8 | 1:640 | 1:1280 | 1:320 | |
| S9 | <1:10 | <1:10 | <1:10 |
MERS-CoV: Middle East respiratory syndrome coronavirus; BCoV:Bovine coronavirus; MNT: Microneutralisation test; ppNT: pseudoparticle neutralization test.
Sixteen of the 25 dromedary sera collected in Australia in 2014 had BCoV antibody titres ranging from 1:40 – 1: 320 but none of them had any antibody reactivity to MERS-CoV in either pNT or MN assays. Representative results are shown in Table 2.
Comparative MN tests were carried out using a clade B dromedary MERS CoV isolate from Al Hasa (Al-Hasa 13), a clade A human MERS CoV isolate from Saudi Arabia (EMC) and a genetically diverse MERS-CoV isolate from Egypt (Egypt 270) using an acute and convalescent serum from the calf 13 from which Al-Hasa 13 MERS-CoV was isolated [12]. Sera from two other adult dromedaries from Saudi Arabia and two from Egypt were included. All three MERS-CoV were neutralised to comparable titres by the convalescent sera from calf 13 and the four adult dromedaries. The paired sera from calf 13 did not show an antibody response to BCoV (Table 3) and two other calves (Nos 15 and 19 (reported in ref 12) who seroconverted to MERS-CoV also failed to seroconvert to BCoV (data not shown).
Table 3.
Comparative antibody titres of dromedary sera to different isolates of MERS-CoV and to an isolate of BCoV
| Dromedary sera | Reciprocal microneutralisation antibody titres (MN) | |||
|---|---|---|---|---|
| MERS-CoV | Bovine CoV (BCoV) | |||
| Al-Hasa 13/2013 | Egypt270/2013 | EMC/2012 | ||
| Pre-infection calf1 | <10 | <10 | <10 | <10 |
| Post infection calf1 | 80 | 40 | 80 | <10 |
| Saudi adult 1 | 640 | 320 | 320 | 80 |
| Saudi adult 2 | 640 | 640 | 640 | 40 |
| Egypt adult 1 | 640 | 320 | 640 | <10 |
| Egypt adult 2 | 640 | 640 | 1280 | 40 |
Acute and convalescent serum from dromedary calf infected with Al-Hasa 13/2013 MERS-CoV (described in ref 12. Note that titres in ref 12 were ppNT titres and the ppNT assay is more sensitive than MN assays).
Adult sera were selected dromedary sera from Saudi Arabia and Egypt known to be seropositive to MERS-CoV.
Discussion
Antisera to alpha-, beta- or gamma- coronaviruses (other than MERS-CoV) had high homologous antibody titres but failed to cross react with MERS-CoV in MN or ppNT tests. Amongst the studied serum panel, the lack of cross reaction with SARS coronavirus is of note since this virus is phylogenetically more closely related to MERS-CoV.
Many dromedary camel sera have antibody to both MERS-CoV and BCoV and it is important to establish whether this represents separate infections with the two viruses or serological cross reactions. Some previous studies have addressed this problem by testing for multiple viruses in parallel and demonstrating some sera with MERS-CoV reactivity in the absence of BCoV (or closely related OC43) reactivity [5,13,14,15,16]. In the present study, the lack of MERS-CoV ppNT or MN antibody reactivity in BCoV immune calf or guinea pig sera (Table 1) confirms the specificity of these two serological assays to discriminate between these two viruses. However, dromedaries have unusual single heavy chain immunoglobulins [17] and it is conceivable that these single-chain Ig sera may have unusually broad cross-reactivity, although there is no direct evidence for this hypothesis. The observation that 18 out of 25 dromedary sera from Australia have antibody to BCoV (titres up to 1:320) without any cross reactivity to MERS-CoV in the ppNT and MN assays is an important confirmation that these assays discriminate between the two viruses in dromedaries as well. Finally, we had three acute and convalescent sera from dromedary calves who had RT-PCR confirmed MERS-CoV infection and they showed significant (more than four-fold) increases in antibody to MERS-CoV without any change in titre to BCoV. Collectively, these data conclusively demonstrate that ppNT or MN antibody titres to MERS-CoV in any animals species is strongly suggestive of MERS-CoV infection. This does not exclude the hypothetical possibility that a hitherto unknown coronavirus more closely related to, but distinct from MERS-CoV, may give cross-reactive antibodies in sero-surveillance studies.
MERS-CoV from different geographic regions (Saudi Arabia and Egypt) are genetically diverse [10]. For example, two serotypes of feline coronaviruses show limited serological cross reactivity [18]. The question arises whether MERS-CoV ppNT and MN assays using one MERS-CoV will detect antibodies to these genetically diverse viruses. We find that genetically diverse MERS-CoV strains (clade A EMC, clade B Al-Hasa 13 and genetically distant Egypt 270) give comparable MN antibody titres in a calf seroconverting to Al-Has 13 clade B virus. Similarly adult dromedaries from Saudi Arabia and Egypt each gave comparable (within two fold) titres to all three MERS-CoV. The data provide a-priori evidence that a single MERS-CoV isolate is likely to be sufficiently representative for MERS-CoV sero-epidemiology studies. It also suggests that genetically diverse MERS-CoV may be antigenically conserved.
Although we have not carried out studies using specific immune sera to exclude cross reactivity to currently endemic human 229E, OC43, HKU-1 and NL63 coronaviruses, we have so far tested human sera from Egypt and Hong Kong by the MERS-CoV MN tests (n=1343) and ppNT tests (n=394) [6,10] with negative results. Since these human coronaviruses are ubiquitous with high sero-prevalence in human adults worldwide [19,20], it is very likely that antibodies to 229E, OC43, NL63 and HKU-1 do not cross react with MERS-CoV in these assays.
Serological evidence of MERS-CoV in dromedaries has been previously reported in archived sera dating back over past decades [7,11,16]. Our data with serological assays that have been demonstrated to be free of cross reaction with BCoV and other CoV’s reconfirms that MERS-CoV was circulating in dromedaries in Saudi Arabia as early as 1993.
Although adult dromedaries in the Middle East and in North and East Africa (e.g. Egypt, Nigeria, Tunesia, Ethiopia, Kenya) have very high sero-prevalence to MERS-CoV (>90%) [6,8,21], we found that the sera from adult dromedary camels in Australia were uniformly seronegative. Given the small number of sera tested in this study, a larger sero-epidemiological study would be needed to confirm that Australia is indeed MERS-CoV free. On the other hand, the BCoV-like virus so common in the Middle East is also prevalent in Australia. Dromedaries were imported into Australia between 1840 and 1907 to serve as means of transport but are now largely found as feral animals (http://www.environment.gov.au/system/files/resources/2060c7a8-088f-415d-94c8-5d0d657614e8/files/feral-camel-action-plan.pdf). The dromedary population in Australia is now estimated to be around 450,000 (Al Jassim – unpublished data).
We conclude that the MERS-CoV ppNT and MN tests reported here do not detect cross-reactive antibodies to other animal coronaviruses including the BCoV-like virus that are common in dromedaries. Thus these two serological assays can be used with confidence in sero-epidemiological studies to identify animal species that may serve as reservoirs or vectors of MERS-CoV. We also confirm that MERS-CoV or a very closely related virus has been circulating in dromedaries in Saudi Arabia for many decades.
Acknowledgments
We thank the Deanship of Research of the College of Veterinary Medicine, King Faisal University for their support. This research was funded by a grant from the European Community Seventh Framework Program (FP7/2007–2013) under project European management Platform for Emerging and Re-emerging Disease entities (Grant agreement No. 223498) (EMPERIE) and a research contract from the National Institute of Allergy and Infectious Diseases, National Institutes of Health (contract no. HHSN266200700005C).
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