Unclear prospects: attempts to create an anti-Covid-19 drug based on cross-reactive antibodies may prove futile

US, WASHINGTON (ORDO NEWS) — The SARS-CoV-2 virus has certain structural similarities with other coronaviruses: in particular, the causative agent of SARS-CoV SARS and the virus of the Middle East respiratory syndrome. This leads researchers to the idea that antibodies against earlier pathogens can be used to create an effective tool for the treatment of Covid-19.

A group of scientists who for several years studied the structures and virulence of various coronaviruses is now testing monoclonal antibodies as code S309. An article on the work of researchers is published in the publication Nature .

S309 were isolated from the blood of a person who contracted the SARS-CoV virus in 2003. The authors of the work claim that this antibody effectively binds to the main antigen of coronavirus – the spiky protein, which helps the pathogen to contact the cell and penetrate through its membrane. S309 recognizes a portion of the protein that is present not only in SARS-CoV-2, but also in its sister viruses, which are combined into a subgenus of sarbekoviruses.

The effectiveness of the binding of monoclonal antibodies to coronavirus has been tested in vitro. The authors of the study hope that S309 – alone or in combination with other types of antibodies – will become the basis for the prevention or postexposure therapy of Covid-19.

However, the above approach to finding a cure for coronavirus pneumonia can be fruitless. The article, published in Cell Reports , claims that the cross-reactivity of antibodies (their ability to bind to antigens from closely related bacteria or viruses) does not provide cross-protection against several pathogens.

The authors of this work took blood samples from 15 people at various stages of infection (from the second to the 22nd day since the onset of the first symptoms of Covid-19). Antibodies of five patients, whose biological material was taken no earlier than on the 11th day after the first signs of infection were detected, showed cross-reactivity to spiky proteins of both SARS-CoV and SARS-CoV-2.

The researchers also analyzed blood samples taken from seven patients three to six months after infection with the causative agent of SARS. The immunoglobulins of these people were able to bind to SARS-CoV-2 proteins, although the immune system of patients did not specifically deal with this virus.

Scientists then conducted studies on cell cultures to better understand how cross-reactivity could be beneficial. All blood samples taken on the 11th day after the onset of symptoms or later in patients who had Covid-19 had antibodies that neutralized SARS-CoV-2 particles well and prevented infection. But only in one sample were they able to neutralize the earlier SARS-CoV virus, moreover, this ability was very weakly expressed.

Similarly, five blood samples from patients infected with SARS-CoV had neutralizing antibodies against this virus, but none could cross-neutralize SARS-CoV-2. Additional experiments on mice confirmed the findings of scientists.

Thus, the prospects for using cross-reactive antibodies as anti-Covid-19 agents remain unclear. It is possible that in the end, such immunoglobulins can provide cross-protection against viruses in a complex organism, even if they do not protect the cell culture.

On the other hand, they can have the opposite effect – the so-called antibody-dependent increase in infection, in which the binding of the virus to incompletely neutralizing antibodies enhances its penetration into the cells of the host body. Further study of this issue will be crucial for the development of a safe and effective universal anti-coronavirus drug.

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