A human cell is not completely defenceless against the novel coronavirus (SARS-CoV-2). In order to defend itself against the pathogen, the cell produces various antiviral effectors that attach to the genetic material of the virus, for example, and cut it up. Defence mechanisms such as this could play a role in new COVID-19 treatments. Led by Professor Frank Kirchhoff, researchers from the fields of virology and microbiology at the Ulm University Medical School have now succeeded in characterising a promising cellular effector. In cooperation with colleagues from King’s College in London, they describe the effects of “ZAP” in the journal “mBio”.
The first author of the study, Rayhane Nchioua, didn’t expect such an antiviral effect: “We were surprised at how effective this cellular effector was at restricting SARS-CoV-2 and hope that our results will help to improve immune therapies for this virus”, says the doctoral student at the Institute of Molecular Virology at the Ulm University Medical Centre.The first author of the study, Rayhane Nchioua, didn’t expect such an antiviral effect: “We were surprised at how effective this cellular effector was at restricting SARS-CoV-2 and hope that our results will help to improve immune therapies for this virus”, says the doctoral student at the Institute of Molecular Virology at the Ulm University Medical Centre.
SARS-CoV-2 has already adapted to humans
The cellular defence strategy works like this: a protein by the name of ZAP (zinc finger antiviral protein) recognises CpG dinucleotides in ribonucleic acids (RNA), which are essential for the production of viral and cellular proteins. Here, a nucleotide with the base of guanine directly follows one with cytosine. In human RNA, such sites are much less common than with most viruses and bacteria. A human cell can thus distinguish between which RNA molecules are their “own” and which ones are “foreign” – and should thus be cut up. A comparison with the sequences of more than 200 other coronaviruses showed that SARS-CoV-2 and its closest relatives, which have been isolated in bats, have unusually few CpG sites. Thus, the coronavirus which is responsible for the current pandemic was already adapted to human beings.
“Viruses try to imitate their hosts”, explains Kirchhoff, senior author of the study. As a result, they are in less danger of being attacked by the weapons of the immune system, such as the cell’s molecular scissors. However, they are not completely successful. In experiments with human lung cell lines, the researchers were able to show that ZAP inhibits the reproduction of SARS-CoV-2. And this is the case even though the virus only offers the protein a few points for attack. “The crucial factor is not the average, but the weakest link in the chain”, says Kirchhoff. This means that not only is the average number of CpG sites on the viral RNA important, but also where they are located. With SARS-CoV-2, they apparently appear more frequently in areas that are essential for the reproduction of the virus.
Interferons increase antiviral effect
The antiviral effect of ZAP was even more pronounced when researchers added interferons to the cells. These proteins are a central component of the innate immune system. They trigger numerous defence mechanisms – such as the production of ZAP, for instance, as the team of researchers observed. However, when they restricted the production of ZAP using so-called siRNA, the virus was better able to reproduce in the cell cultures. This indicates that ZAP plays an important role in the body’s immune response.
Interferons are already being used in medications to treat viral infections, such as hepatitis B and C. It was also already known that SARS-CoV-2 reacts sensitively to interferons. Up to now, however, it has not been clear which type of interferon attacks the virus most effectively or which factors are involved. The virologists observe the strongest effects with the interferon gamma, which also led to the greatest increase in ZAP production. For this reason, the researchers suggest that this molecule should be considered in developing medication to treat Covid-19.
Contributors to the study included teams from Ulm University’s Institute of Molecular Virology led by Professor Frank Kirchhoff, Dr Konstantin Sparrer, Professor Jan Münch and Junior Professor Dr Daniel Sauter. Professor Steffen Stenger (Institute of Medical Microbiology and Hygiene, Ulm University Medical Centre) and researchers from King’s College were also involved.
The current study was conducted primarily with funding from the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG), and in particular from the Collaborative Research Centre 1279 at Ulm University (Exploiting the Human Peptidome for Novel Antimicrobial and Anticancer Agents). Further funding came from the EU project Fight-nCoV, from the Ministry of Science, Culture and the Arts’ fund for Covid-19 research and from projects sponsored by the Federal Ministry of Education and Research (Restrict SARS-CoV-2, protACT).
Text: Dr. Annika Röcker
Nchioua R, Kmiec D, Müller JA, Conzelmann C, Groß R, Swanson CM, Neil SJD, Stenger S, Sauter D, Münch J, Sparrer KMJ, Kirchhoff F. 2020. SARS-CoV-2 is restricted by zinc finger antiviral protein despite preadaptation to the low-CpG environment in humans. mBio 11:e01930-20. https://doi.org/10.1128/mBio.01930-20.