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Temperature controls are performed on people who arrive for the Covid-19 antibody test at no cost, organized by … [+]
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There is notable news about the immune response to SARS-CoV-2. It is now clear that the virus mutates to evade neutralizing antibody responses. New results suggest that this is not the whole story. When it comes to another branch of immunity, T-cell immunity, a high response to one form of the virus means a high response to all. Here we will review what that could mean for the future of Covid-19.
In the months since SARS-CoV-2 variants came to the fore in the Covid-19 discussion, the average person is left with a series of questions. Will the vaccine my grandmother just received protect her from emerging strains? What about my previous infection during the summer? How long will that protection last?
Vaccines and previous infections confer antibodies, which are proteins made by B cells to counteract a specific antigen. Imagine a customized haute couture for you. The fabric lines up perfectly with every inch of your torso. An antibody is like a custom-made protein that adapts to a specific pathogen, in this case, SARS-CoV-2. However, extensive research indicates that these antibodies fade fairly quickly over time and may not protect as well against some variants. Although it turns out that B cells and antibodies are only half of the adaptive immunity puzzle.
The other piece is the T cell. These are types of immune system white blood cells that act as front-line soldiers against disease. Its purpose is to eliminate invading pathogens and clean up dead cell debris. They do this by remembering past infections, hoping to find short sequences of virus amino acids called peptides, and then killing the virus when it reappears. While antibodies are proteins custom-made for a specific virus, T cells attack a variety of different pathogens, like a suit bought off the shelf. It may not fit exactly, but close enough will do and will fit many others as well.
The T cell is another tool in the arsenal against Covid-19, but the questions above need to be translated in reference to this secondary weapon. Will T cells protect against rapidly emerging variants of the virus that show signs of antibody resistance? Research from the La Jolla Institute of Immunology, led by Dr. Alessandro Sette and Dr. Shane Crotty, sought to clarify these unknowns.
Their findings were encouraging. By exposing human T cells from a wide range of hosts previously infected with Covid-19 to various variants of SARS-CoV-2, the T cell response was well maintained. Responses decreased by a maximum of 30% relative to that cell’s response to wild type SARS-CoV-2. In some cases, such as the South African variant, the T cells produced a response nearly equal to the variant than the non-mutated virus.
The same can be said for samples taken from people vaccinated against Covid-19. Their T cells responded in the same or slightly less volume compared to the wild type. This is a huge victory, confirming that T cells have some effect in slowing down mutated variants, but to what extent?
There are strengths and weaknesses in the T cell response. Strength, as these researchers demonstrated, is the overall coverage they have on the variants. The virus is adaptive and mutative, changing over time. When we introduce virus-specific antibodies, the ability to develop mutations in the receptor-binding domain, the n-terminal domain, or elsewhere has been shown, making the antibody less effective. With T cells, because they all have their own unique hereditary set rather than a specific developed antibody from a vaccine or previous infection, the virus cannot adapt.
However, the weakness is that the responses of T cells to SARS-CoV-2 are limited. T cells are not designed to prevent infection, only to kill cells once they are in the body. The researchers confirm this by concluding that “while circulating memory T cells are not expected to be effective in preventing SARS-CoV-2 infection, it is plausible that they may reduce the severity of Covid-19.”
Given all this, it may be the case that SARS-CoV-2 is allowing a certain level of response from T cells. Variants evolved while T cells were present, leading to the idea that viruses are modulating the response of T cells at levels low enough for them to continue to spread. While primarily speculative, it seems possible that the virus may have a regulatory influence on the T cell.
Regardless, the reactivity of T cells to variant forms of SARS-CoV-2 is an absolute positive. The implication is that those vaccinated or previously infected will have at least some form of defense against the newly circulating variants, even if they are capable of evading antibodies. This is much-needed positive news in the face of the growing variant threat.
