Serological tests are assays that allow us to detect immunoglobulins in our body, which are antibodies that bind to pathogens in order to fight them. We can identify different types of immunoglobulins, the immunoglobulins most commonly used for diagnosis are IgG and IgM.
Immunoglobulins M (IgM) are generated at the beginning, indicating an acute infection process by the virus, and the more abundant Immunoglobulins G (IgG) indicate the protective antibodies that have been generated in response to the infection. In addition, at the beginning of the pandemic, Immunostep developed a test that also detects IgA antibodies, which have been shown during this time to have prognostic value in the early stages of infection. Data from our studies suggested that IgA detection significantly improves the diagnostic outcome in the early stages of infection.
On the other hand, we can use different techniques to detect the presence of antibodies, the most commonly used in the context of this pandemic are the Enzyme-linked immunoadsorption assay (ELISA), assays based on the antigen-antibody reaction, and lateral flow tests, also known as immunochromatographic tests, which are devices developed to detect the presence of antigen-antibody complexes in a liquid sample and whose basis method is very similar to pregnancy tests. At Immunostep, we have also been working on finding techniques that provide a wider range of information and for that we have developed a new flow cytometry test, a method that has been more used in the area of haematology and immunology as it allows a better understanding of the pathophysiology of oncohaematological diseases and the immune system.
In a very general way, we could classify the methods used for the detection of Covid-19 infection into three types: CRPs, which are tests to detect the genetic material of the virus, viral antigen tests, which detect virus proteins, and serological or antibody tests, which detect antibodies generated against different parts of the virus. (+ information about each type of test).
For early detection of infection, the CRP has proven to be a very reliable test that can identify the virus in the early stages of respiratory infection, a positive result in this test confirms infection and if there is no genetic material of the virus in the body, it means that the individual is not infected. However, some serological tests have also been shown to provide valuable information regarding active infection as well as previous infection and can be a powerful mass diagnostic tool for SARS-CoV-2.
The SARS-CoV-2 genome encodes 4 structural proteins: the E protein (envelope), the M protein (membrane), the N protein (nucleocapsid) and the S protein (spike protein), where we also find the RBD domain. The SARS-CoV-2 genome also expresses 27 non-structural proteins (which are released after cell infection).
The spike protein (Spike) is therefore a protein that is on the surface of the virus and is what the virus uses to enter human cells. A specific part of this protein, the receptor-binding domain (RBD), binds to proteins in human cells called ACE2, which act as a gateway into the human cell.
When the virus genome enters through this gateway, infection begins. The fact that this protein contains the receptor binding domain (RBD), which recognises and binds to the host receptor (ACE2), has made it the main target for vaccines.
The way in which tests detect antibodies varies depending on the technology used in the assay. For example, one of the most commonly used techniques in this pandemic, the ELISA technique, uses an immobilised antigen that is detected by an antibody linked to an enzyme in a way that generates a detectable product, such as a colour change, which will reveal whether an immune response to the virus has been generated.
Another common question is, if antibodies are detected in this test, does that mean I will always have them? How long do the antibodies last? Based on numerous studies carried out so far, having generated antibodies provides a level of immunity that would be reliable enough to fight the disease, but this does not mean total immunity against the virus and therefore against contagion and transmission. There are also many studies on possible reinfections and on the longevity of the antibodies generated, for which there are still no conclusive answers.
On the other hand, antibodies are not only generated as a result of natural infection, but are also generated through vaccination, which is why several tests have also been developed in addition to ELISA/CLIA tests to detect and differentiate between the two types of antibodies. In fact, Immunostep recently developed a test capable of differentiating both types of antibodies with an estimated accuracy of 99%.
This is a flow cytometry bead-based immunological assay in which we have beads coated with the SARS-CoV-2 proteins, so that when we add the sample, the antibodies in that sample bind to the magnetic beads. In this way, when the solution is added to a cytometer, a colour signal is emitted which can be observed with graphs on a computer indicating the presence of antibodies of each type for each protein that have linked to the microsphere. (+ info).
Research is still ongoing on the immune response to the different strains, and no conclusive answers have been given to this question so far, but research published by scientists at Oregon Health & Science University showed that antibodies generated by SARS-CoV-2 reacted to other strains of Covid-19 and vice versa.
Recent research also found out that variants from the UK and South Africa contained mutations that could elude neutralising antibodies. However, neutralising potency of two specific types of antibodies targeting the virus receptor binding domain (RBD) of the viral spicule was unaffected by these variants, adapting and using different mechanisms to fight them.
Through science, we are able to develop antibodies that target specific antigens; these are monoclonal antibodies. Monoclonal antibodies are immune system proteins that are created in the laboratory, in order to act like human antibodies in the immune system. Many monoclonal antibodies are used as a type of targeted therapy for cancer.
In the context of Covid-19, neutralising monoclonal antibodies are being used to target and label the virus to help the immune response eliminate it. Some of the new therapeutic targets are based on these antibodies and aim to treat patients with mild to moderate coronavirus infections.
Yes, there are antibodies test that are able to detect both types of antibodies, and even differentiate them.
By identifying antibodies to different proteins, we can differentiate individuals who have been naturally infected from those who have generated antibodies as a result of vaccination, because the same proteins are not expressed when an individual suffers from natural infection as when antibodies are generated by vaccination.
For example, our assays are able to identify four proteins, all of which generate an immune reaction when SARS-CoV-2 enters the body naturally. However, most vaccines are designed so that our immune system generates antibodies against only two of them: the viral spike (Spike) and its receptor binding domain (RBD), so if the assay only identifies those antibodies have been generated against these two proteins, it means that no natural infection has occurred. If, on the other hand, antibodies against the Mpro and nucleocapsid proteins are detected, it means that there has been a natural infection, since these are not expressed with vaccination.