With inputs from Dr Monica Vasudev

1046: A spurt of new studies has shown that a large proportion of the population — at some places, around 20-50% of people — might carry T cells that identify the new coronavirus despite having never encountered it before. Although it’s too early to ascertain how helpful they might be, but even a slight influence on immune response could make the disease milder.

While the new coronavirus was unknown until 8 months back, yet to some human immune cells, it was already something familiar.

This could be a case of family resemblance. For the immune system, pathogens with common roots can look alike, such that when a similar pathogen comes to call, the body may already have a clue of its intentions.

The presence of T cells has fascinated the experts, who state that it is too early to be able to tell if the cells will play a helpful, harmful or negligible role against the new coronavirus.

However, if these T cells exert even a modest influence on the body’s immune response, the disease might become milder. This could, in part, explain why some people become very sick while others don’t. (New York Times Excerpt)

SARS-CoV-2-specific T cell immunity in COVID-19, SARS and uninfected controls

Memory T cells that are induced by previous pathogens can build the susceptibility to, and clinical severity of, subsequent infections. There is limited information about the presence of pre-existing memory T cells in humans with the potential to recognize SARS-CoV-2.

In a recent paper published in Nature, researchers assessed T cell responses to structural (nucleocapsid protein, NP) and non-structural (NSP-7 and NSP13 of ORF1) regions of SARS-CoV-2 in 36 COVID-19 convalescents. Investigators noted the presence of CD4 and CD8 T cells recognizing multiple regions of the NP protein in all of them. Twenty three SARS-recovered patients were still found possess long-lasting memory T cells that were reactive to SARS-NP nearly 17 years after the 2003 outbreak, showing strong cross-reactivity to SARS-CoV-2 NP.

SARS-CoV-2 specific T cells were also identified in individuals with no history of SARS, COVID-19 or contact with SARS/COVID-19 patients (n=37).

SARS-CoV-2 T cells detected in uninfected donors had a different pattern of immunodominance, frequently targeting the ORF-1-coded proteins NSP7 and 13 as well as the NP structural protein.

Epitope characterization of NSP7-specific T cells exhibited recognition of protein fragments with low homology to ‘common cold’ human coronaviruses but it was conserved amongst animal beta-coranaviruses.

Therefore, infection with beta-coronaviruses tends to induce multispecific and long-lasting T cell immunity to the structural protein NP.

Understanding how pre-existing NP- and ORF-1-specific T cells present in the general population affect the vulnerability and pathogenesis of SARS-CoV-2 infection is important for the management of the COVID-19 pandemic.

[Bert NL, et al. Nature (2020)]

Dr KK Aggarwal

President CMAAO, HCFI and Past National President IMA