Do anti-nucleocapsid antibodies affect COVID-19 immune responses?
Do anti-nucleocapsid antibodies affect COVID-19 immune responses?

Do anti-nucleocapsid antibodies affect COVID-19 immune responses?

Coronavirus disease 2019 (COVID-19) first appeared in Wuhan, China, and has since spread around the world. The rapid rate of transmission forced many countries to impose costly and restrictive measures in the hope of controlling the disease, including mandatory face masks, laws on social distancing and even full closure and orders to stay at home.

Examination: Nucleocapsid-specific humoral responses enhance the control of SARS-CoV-2. Image credit: Andrii Vodolazhskyi / Shutterstock

The introduction of vaccines made it possible to dismantle these in many countries, but with new variants continuing to evade both vaccine-induced and natural immunity, more research is needed to ensure long-term protection.

Researchers from Northwestern University have investigated the potential of vaccines and treatments against the nucleocapsid protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

A preprint of the paper can be found on bioRxiv* preprint server while the survey undergoes peer review.

The study

SARS-CoV-2 has four major structural proteins – peak protein, nucleocapsid protein, membrane protein and coat protein. The nail protein is the key to the pathogenicity of the disease, as the S1 subunit, after cleavage of a host protein, binds to angiotensin-converting enzyme 2 (ACE2) to allow viral cell penetration, while the S2 subunit mediates membrane fusion.

This is the target of most monoclonal antibody treatments as well as most vaccines, but it is also a highly variable region showing significant differences in conformation between different variants. The nucleocapsid protein is currently not targeted by any approved vaccine, and testing for anti-nucleocapsid antibodies tends to be used to test for previous infection in vaccinated individuals. There is increased interest in developing vaccines against alternative proteins to reduce the effect of new variants.

Initially, C57BL / 6 mice were primed by intramuscular injection with an adenovirus serotype 5 vector expressing the nucleocapsid protein of SARS-CoV-2 at a dose of 10 ^ 11 PFU. A 100 μg booster of nucleocapsid protein was provided three weeks later. Control mice were immunized with a vector without nucleocapsid protein and a PBC booster. Nucleocapsid-specific immune response was measured two weeks after the booster, revealing robust responses in CD8 T cellsmemory B cells and antibodies. To help confirm the effects, these tests were followed using focus reduction neutralization titers (FRNT) using live virus. Mice that had received a tip-based adenovirus vaccine were used as a control. The sera from mice receiving the spike protein-based vaccine successfully prevented infection, but unfortunately there was no antiviral effect from the nucleocapsid-based vaccine.

However, neutralization is not the only way that immune response can help protect against virus-based disease, and to investigate whether non-spike immune response could help fight COVID-19 through alternative mechanisms, the researchers conducted a passive immunization study. They transferred 500 μl of nucleocapsid-specific sera to otherwise naive transgenic mice expressing hACE2. After transfer, the mice were challenged intranasally with 10 ^ 3 PFU of SARS-CoV-2 and quantified viral load in the lungs on day four. The mice receiving the injections showed a significantly lower amount of virus than the control mice.

The conclusion

Further studies could attempt to identify the specific mechanism by which anti-nucleocapsid immune response helps protect against the disease, potentially by examining the effects of CD8 T cells or memory B cells on disease progression alone, rather than the overall immune response.

Nevertheless, the research presented here provides strong evidence that anti-nucleocapsid protein can help protect the host from SARS-CoV-2 infection, as well as showing that neutralization of the nucleocapsid protein does not block infection.

While a previous study suggested that a nucleocapsid-based vaccine was effective for a limited effect, the authors explain this as a dose difference – the mice in this study received a typical physiological dose of 10 ^ 3 PFU, whereas the previous study used a much higher dose – 5×10 ^ 4.

It is important to investigate the effect of the immune response against proteins other than the spike protein, as new variants continue to show significant mutability and changes in conformation that reduce the effect of anti-spike treatments and vaccinations. This information may help future vaccine manufacturers and researchers to investigate alternative treatments for COVID-19.

*Important message

bioRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered as crucial, guide clinical practice / health-related behavior or be treated as established information.

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