Highly efficient immune response in asymptomatic SARS-CoV-2 patients

Researchers in Singapore have investigated the role of SARS-CoV-2-specific antibodies and T-cells in asymptomatic individuals suffering from COVID-19. They found that, contrary to some initial hypotheses, asymptomatic individuals mount an adaptive immune response on par with symptomatic individuals, demonstrating that T cell response does not appear to be correlated with symptom severity.

The full article can be read online at the Journal of Experimental Medicine.

The COVID-19 pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), continues to spread around the globe. Countries such as the UK and USA are making strides in suppressing the virus with mass vaccine rollouts, but characterizing factors in disease severity between individuals remains important.

SARS-CoV-2 has been, in part, able to rapidly spread through populations due to a large number of asymptomatic cases. Initial studies appeared to suggest a positive correlation between the severity of SARS-CoV-2 symptoms and the degree of COVID-specific antibodies, although this is yet to be proven.

The researchers sought to investigate this further, comparing the magnitude of T-cell and SARS-CoV-2-specific antibody responses in symptomatic and asymptomatic sufferers of the disease.

The research team studied a group of male foreign workers who resided in a Singapore dormitory, following up from a group recruited by an earlier study investigating the prevalence of SARS-CoV-2 amongst migrant workers.

T-cells are specialized antibodies designed to target specific antigens, such as SARS-CoV-2. T-cells can provide long-term immunity against diseases, as they remain in the body’s system and can be rapidly produced to mount an aggressive response against recurring pathogens.

It had previously been suggested that individuals suffering from less severe symptoms of SARS-CoV-2 produced T-cells in lower volumes, relying instead on general antibodies to mount an immune response. If this were the case, asymptomatic individuals might experience SARS-CoV-2 to a much harsher degree if experiencing future infections of the virus, as they lack the sufficient T-cell memory to deal with the virus rapidly.

Through examination of blood plasma in the study group, the researchers found that both sets of individuals in fact, had mounted sufficient virus-specific T-cell responses, indistinguishable in magnitude from one another.

T-cells in asymptomatic individuals appeared to produce more significant quantities of IFN-y and IL-2. These cytokines inhibit viral growth and are also able to coordinate the proinflammatory and regulatory cytokines in symptomatic individuals.

Although nearly identical in magnitude in recently infected individuals, the researchers also found that SARS-CoV-2-specific T-cell levels decline more rapidly in asymptomatic individuals. This seems to confer with previous studies suggesting that T-cell level in asymptomatic individuals was lower than in symptomatic patients, as these studies were conducted up to three months after the initial infection had passed.

This seems to be the first published article describing similar magnitudes of T-cells in both symptomless and symptomatic individuals exposed to COVID-19 and promisingly shows that, although T-cell levels decline faster in symptomless patients, both groups show sufficient immune response with persistent T-cell memory. Although these results are encouraging, the authors warn that the sample was entirely male, and almost all participants were either of Indian or Bangladeshi origin. T-cells of different ethnic groups and females must be addressed in future studies.

Successful vaccine campaigns have gradually started to make an impression in countries like the UK and the USA. However, characterizing the immune response of infected individuals remains a critical aspect of understanding and thus controlling the spread of the virus. This article can be accessed online at the Journal of Experimental Medicine. The authors go into far greater detail about their findings and the effects of T cells, B cells, and monocytes.

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