An international study led by virologists at The University of Hong Kong (HKU) has shown that geographically and genetically distinct Middle East respiratory syndrome coronaviruses (MERS-CoV) from Africa have lower replication competence in human and mouse lung than those from the ME.
These findings, reported in the June 7, 2021, online edition of Proceedings of the National Academy of Sciences, may explain the observed absence of severe zoonotic MERS disease in Africa and suggest that ME MERS-CoV may have increased human pathogenic potential.
"MERS-CoV from West Africa had previously been shown to have lower replication competence than ME viruses, but most dromedary camels in the ME are imported from East Africa," noted study co-leader Malik Peiris, a professor of virology in the School of Public Health at HKU.
"Therefore the present study included West, North and East Africa, in order to investigate if this biological difference was representative across the African continent and most importantly in East Africa," Peiris said.
"These findings clearly have major implications for public health policy, namely the risk of MERS-CoV emerging as a zoonotic or pandemic threat from Africa," he told BioWorld Science.
As seen with COVID-19, coronaviruses (CoVs) can cross species barriers to cause pandemic zoonotic diseases, including the 2003 SARS epidemic from bats, while dromedary camels were the source of the 2012 MERS-CoV outbreak.
MERS-CoV is prevalent in camels in the ME, where zoonotic disease has been reported, and in Africa, where over 70% of MERS-CoV-infected camels are found but no MERS disease has been detected.
Phylogenetically, there are three clades of MERS-CoVs, A, B and C. Of these, Clade A, the earliest MERS-CoV virus detected in Saudi Arabia, is no longer detectable in humans or camels.
Clade B viruses are dominant in camels in the ME and cause sporadic zoonotic disease, sometimes leading to clusters of human transmission.
However, viruses detected in camels in East, North and West Africa all belong to different sublineages of clade C.
Preliminary studies comparing replication competence of clade A and clade B MERS-CoV from Saudi Arabia with a clade C virus from West Africa found that the latter had lower replication competence in human bronchus and lung cell cultures.
The West African clade C1.1 viruses had ORF4b gene deletions, possibly contributing to reduced replication competence, but it is unclear whether viruses from other African regions, where ORF4b deletions are not found, also share this reduced replication phenotype.
This warrants a more extensive phenotypic comparison of MERS-CoV from different sublineages and regions of Africa with ME viruses.
The new study co-led by Peiris and Chris Ka Pun Mok, an honorary assistant professor in the HKU-Pasteur Research Pole and an assistant professor in The Jockey Club School of Public Health at the Chinese University of HK compared replication competence of ME clade A and B MERS-CoV with clade C viruses from East, North and West Africa.
African viruses were shown to have significantly lower replication competence in ex vivo human lung cell cultures and in lungs of experimentally infected human-DPP4 (hDPP4) genetic knock-in mice.
"The reduced replication competence in the lung with MERS-CoV from Africa was substantial, being 10- to 100-fold lower than that seen with the ME viruses," Peiris told BioWorld Science.
However, he noted that there is no ideal small animal model to study MERS experimentally, with mice not being susceptible to MERS-CoV because they lack the DPP4 viral receptor.
Therefore, "the hDPP4 receptor molecule has been 'knocked in' by genetic engineering, making these mice susceptible to MERS-CoV infection so viral infection and replication can be studied in this model, which reinforces our findings with human lung and bronchus."
Previous experiments have shown that MERS-CoV viruses from Africa replicate less efficiently that those of the ME in human lung, so the next step was to try to understand the mechanisms contributing to this difference using lentiviruses.
Lentivirus pseudotypes expressing MERS-CoV spike protein from Saudi Arabian clade A prototype strain (EMC) or African clade C1.1 viruses were used to show that the clade C1.1 spike protein was associated with reduced viral entry into Calu-3 human respiratory epithelial cell lines.
"We used an HIV lentivirus with many of its genes stripped out, so it cannot infect or replicate in cells or cause disease, then provided these defective lentiviruses with the MERS-CoV spike protein, so they can attach to and infect cells but still cannot replicate or cause disease," explained Peiris.
"These lentiviruses are also engineered to carry a luciferase gene used to detect and measure how efficiently the MERS-CoV spike protein helps cell entry, allowing comparison of spike proteins from Africa versus ME MERS-CoV."
Moreover, isogenic EMC viruses with spike protein from EMC or clade C1.1 generated by reverse genetics showed that the clade C1.1 spike was associated with reduced virus replication competence in Calu-3 cells in vitro, in ex vivo human bronchus, and in lungs of hDPP4 knock-in mice in vivo.
These findings suggest that MERS disease may not occur in Africa due to less pathogenic viral variants being found there and that if the more pathogenic ME pathogenic clade B viruses are introduced into Africa, the risk to humans may change.
"Previous studies suggest that ME clade B viruses out-compete MERS-CoV viruses from Africa, even in dromedary camels," said Peiris.
"Thus their introduction to Africa may lead to this being the dominant virus there and be associated with adverse health impacts and an increased pandemic threat," he said.
"Given this pandemic threat, we need to assess the risks associated with this virus and, since most MERS-CoV-infected camels are found in Africa, we believe it is important to investigate humans there for evidence of disease and whether African MERS-CoV can mutate to become more pathogenic."