Test systems from IBH RAS
New antibody test against COVID-19
Maxim Rousseau, Polit.roo
Scientists of the Institute of Bioorganic Chemistry of the Russian Academy of Sciences named after Academicians M. M. Shemyakin and Yu. A. Ovchinnikov of the Russian Academy of Sciences have developed four variants of test systems for detecting antibodies against the SARS-CoV-2 coronavirus. The new test systems are capable of detecting antibodies of three different classes (IgM, IgG, IgA) in human serum or plasma, which significantly increases the specificity of the tests. The development is reported in a joint press release of the IBH RAS and the Ministry of Science and Higher Education of the Russian Federation.
To control the development of the disease and make decisions about the necessary measures, data on the number of infected and, most likely, those who have received immunity are especially important. With COVID-19 coronavirus infection, for which there is still very little information about the mechanism of immunity formation, the situation is further complicated by the fact that a significant number of people carry the infection in a latent form. Reverse transcription polymerase chain reaction (PCR) is used to diagnose coronavirus infection. It detects the presence of RNA virus in the body. But this method has its limitations. Most often, sputum samples or smears from the mucous membrane of the patient's nose and throat are used for PCR. But in many cases, the need for diagnosis arises when viruses are no longer in the cells of the mucous membranes, and it is too difficult and dangerous to do a lung biopsy for each patient for the sake of diagnosis.
Therefore, an alternative method – the detection of antibodies to coronavirus – becomes especially important. It allows you to identify both those in whose body the virus is present, and those who have already been ill. Recall that antibodies, or immunoglobulins, are protein molecules produced by cells of the immune system in response to the penetration of dangerous objects into the body from the outside (bacteria, viruses, toxins). Antibodies consist of four chains of amino acids tightly bound together, so that the molecule resembles the letter Y in shape. In the "vertical stick" of this letter Y is the so-called constant part of the antibody. All antibodies with the same constant part belong to the same isotype or class. Classes are designated by Latin letters IgA, IgD, IgM, IgG, IgE and so on. The opposite part of the antibody molecule is called the variable part. It differs in antibodies of the same class and ensures that the antibody recognizes its antigen. The antigen is, for example, a toxin molecule in the blood, the protein envelope of the virus or surface proteins in the cell membrane (bacterial, tumor or virus-affected).
A fragment of a molecule that directly comes into chemical contact with an antigen is called an antigen-binding site, and the antigen site to which the antibody attaches is called an epitope. Each antibody will respond to its own epitope. Moreover, several different types of antibodies can correspond to the same epitope.
The attachment of an antigen to an antibody is necessary for an immune response. The consequences of this event may be different. Sometimes, binding to an antibody alone is enough to block the harmful effects of the antigen. In other cases, the attached antibody serves as a signal to the cells of the immune system, which are in a hurry to deal with the antigen.
In tests for the SARS-CoV-2 coronavirus, antibodies of three classes IgM, IgG and IgA are determined. IgM antibodies form pentamers when five Y-shaped molecules connect, turning to each other with the "legs of the letter Y". Such an antibody can bind simultaneously to five antigen molecules. Usually, antibodies of this class occur at an early stage of the immune response. IgM can be a sign of both the onset of COVID-19 and a non-specific immune response due to other diseases. Another class – IgG – is represented by antibodies, the molecules of which consist of two Y-legs facing each other. Their presence in the blood serum indicates a previous disease and the possible presence of immunity to coronavirus infection.
Finally, IgA antibodies, with "single" Y-shaped molecules. At the early stages of the study of the SARS-CoV-2 coronavirus, scientists paid little attention to this type of antibodies, and now it is of particular interest. The fact is that it was IgA antibodies that were found in some people who came into contact with COVID-19 patients and had the disease in a latent form (their infection with the virus was confirmed by PCR). It is even suggested that IgA antibodies have caused the asymptomatic course of the disease. Immunoglobulins A are present in secreted fluids: on the surface of mucous membranes and in breast milk.
To create new test systems, IBH RAS scientists obtained a complex of antigens based on a recombinant surface viral spike protein (S-protein) involved in the penetration of the virus into human cells, and a recombinant highly sensitive virus N-antigen (SARS-CoV-2 nucleocapsid protein). The systems were tested at the Central Clinical Hospital of the Russian Academy of Sciences. Employees of the Institute of Bioorganic Chemistry, admitted to work in the red zone, analyzed the sera of more than 400 patients diagnosed with COVID-19. The kit for testing patients for IgG class antibodies has passed state registration, and the registration of the remaining three kits will be completed in the near future. The first experimental batch of kits that have passed clinical trials has already been released.
"The test system we have developed has a number of advantages. First, we have expanded the spectrum of antigenic determinants carrier proteins in the test system to three. Secondly, a mixture of conjugates to the diagnostically important classes of antibodies – IgA, IgG, IgM - is used here. Thirdly, the production of recombinant proteins included in the test system is not associated with mammalian or insect cells, but involves the use of bacterial culture, which is more profitable from an economic point of view," said Academician Alexander Gabibov, Director of the IBH RAS.
Portal "Eternal youth" http://vechnayamolodost.ru