Covid-19 Specific Memory T Cells

• 15 Jul 2020

Why in News

Recent studies have shown that people unexposed to and not infected with Covid-19 (caused by SARS-CoV-2 or Novel Coronavirus) may still exhibit T cell responses specific to other coronaviruses.

Key Points

• A huge number of adults are exposed to four different coronaviruses that cause common cold and studies have shown that 20-50% of healthy people display SARS-CoV-2-specific memory T cells.
  • The healthy people studied were those tested prior to the pandemic or have not been infected with novel coronavirus.
  • Memory T cells protect against previously encountered pathogens.
• It is thought that SARS-CoV-2-specific T cell responses seen in healthy people might arise from memory T cells derived from exposure to ‘common cold’ coronaviruses.
• However, it is not known that the presence of pre-existing immunity from memory T cells offers clinical relevance, when exposed to SARS-CoV-2.
• Measuring pre-existing immunity and correlating it with infection and severity of disease is the only way to find out its role in protection against SARS-CoV-2.
• However, the relationship between the infections by and immunity from common cold coronavirus and age is not well established. That is why it is unclear why children do not show severe symptoms, while older people do.
• These considerations underline how multiple variables may be involved in potential pre-existing partial immunity to Covid-19.
• Pros:
  
  • It is believed that people with a high level of pre-existing memory T cells could mount a faster and stronger immune response upon exposure to the virus and thereby limit disease severity.
    
    • T cells could potentially facilitate an increased and more rapid neutralizing antibody response against the virus.
• Cons:
  
  • The pre-existing immunity can reduce the immune responses that the vaccine causes through a mechanism called the ‘original antigenic sin’.
    
    • Original antigenic sin, also known as the Hoskins effect, refers to the propensity of the body's immune system to preferentially utilize immunological memory based on a previous infection when a second slightly different version of that foreign entity is encountered.
  • It can also lead to antibody-mediated disease enhancement, where antibodies present at sub-neutralising concentrations can actually augment virus infection and cause more severe disease.

T Cells

• T cells are so called because they are predominantly produced in the thymus.
• They recognise foreign particles (antigen) by a surface expressed, highly variable, T cell receptor (TCR).
• There are two major types of T cells: the helper T cell and the cytotoxic T cell.
• As the names suggest, helper T cells ‘help’ other cells of the immune system, whilst cytotoxic T cells kill virally infected cells and tumours.
• The severity of disease can depend on the strength of these T cell responses.

Source: TH