T-cells play a huge role in cellular immunity of the body, by injecting toxic substances in the pathogen to kill it. A COVID-19 infection can lead to decreased T-cell levels. New studies suggest that a unique structure on the COVID-19 spike protein fuses with the T-cell when the two get in contact, thereby disrupting its natural function. The virus upon entering the T-cell prevents it from performing its phagocytic function and can lead to decreased immunity. 

State University of New York at Buffalo

- CD8+ T-cells: Directly attacks and kills virus-infected cells
- CD4+ T-cells: Primes CD8+ T cells and antibody-producing B cells, responsible for cytokine production
- T-cell exhaustion and reduced functional diversity lead to severe disease

T-Cell Immunity in COVID-19

Dr. Tim Sandle. "COVID-19 and the immune system", Pharmaceutical Microbiology, 6 October 2020. https://www.pharmamicroresources.com/2020/10/covid-19-and-immune-system.html


COVID-19 and the immune system

Any viral infection leads to a decrease in lymphocyte levels but the change varies from virus to virus. In the case of COVID-19, it has been found out that CD8+ cells (Cytotoxic T cells) play an important role in predicting the severity of the infection and therefore are crucial in designing a proper treatment method. It has been observed that the levels of CD4+ and CD8+ cells decrease drastically in severe cases but not so much in milder ones. Even though a change in CD4+ to CD8+ ratio is not seen. It was observed that the CD4+ and CD8+ levels start rising as the body responds to treatment. 

Effect of COVID-19 on T-cell levels in infected patients

Mechanism of viral attack on T-cells

Studies show the activation of a different kind of T-cell called MAIT cell, which is an anti-bacterial cell but can also fight against certain viruses. These MAIT cells are seen to be strongly activated in the people who suffer from severe COVID-19, their number also seems to be declining indicating to the fact that they are involved in the fight against the virus. They are also seen to accumulate in the airways of the patient to a greater extent than a normal healthy person. 

Strong activation of antibacterial T cells or MAIT cells in severe COVID-19

- CD4+ T cell responses are more responsible for controlling SARS-CoV-2 infection than CD8+ T cells
- induction of antibodies also depend on spike-specific CD4+ T cells
- in a study examining CD4+ T cell responses to all SARS-CoV-2 proteins, CD4+ T cell responses were detectable for all but three of the smallest proteins
- S spike protein, M, and nucleocapsid were the main targets of CD4+ T cells
- virus specific CD4+ T cells differentiate into Th1 cells and T follicular helper cells (Tfh)

CD4+ T cell response in SARS-CoV-2 infection

- presence of SARS-CoV-2 specific CD8+ T cells has been associated with better COVID-19 outcomes
- CD8+ T cells are specific for a range of SRS-CoV-2 antigens including S spike protein, M, nucleocapsid and ORF3a
- in acute COVID-19, SARS-CoV-2 specific CD8+ T cells exhibit high levels of association with effector functions such as IFNy, granzyme B, perforin, and CD107a

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