SARS-CoV-2 nucleocapsid antigen as a possible COVID-19 severity marker

Coronavirus disease (COVID-19) has been the focus of molecular research over the past 18 months. Despite the efforts behind the development of vaccines against the severe acute respiratory distress syndrome coronavirus (SARS-CoV-2), the relationship and dynamics between host and viral proteins and their impact on disease severity remain unclear.

Study: SARS-CoV-2 nucleocaspid antigen in urine of hospitalized patients with Covid-19. Image Credit: WildMedia/ Shutterstock

Studies focusing on viral antigenic load in respiratory fluids have shown a high concentration of viral RNA in the initial phase, followed by a rapid decrease in the second week after the onset of symptoms and a low or undetectable level of RNA later. The viral loads of asymptomatic or mild forms of SARS-CoV-2 infections being similar to severe disease forms make it difficult to understand the dynamics of the host-virus relationship.

The levels of SARS-CoV-2 RNA in blood plasma are generally low, and to date, the virus has not been isolated from peripheral blood. Hence, markers for severe disease in COVID-19 patients remain unknown.

Scientists have now focused on the nucleocapsid antigen (N-Ag) as one of the key topics for studying pathology mechanisms of the SARS-CoV-2 among hospitalized patients, detected in their blood, respiratory fluids, and saliva thus far.

Previous studies have suggested the presence of N-Ag to be associated with early-stage disease in hospitalized patients. Hence, a major part of the research concerning COVID-19 now focuses on studying different body fluids for markers of severe infection.

In a recent study, French researchers used a highly sensitive and specific nucleocapsid-Ag assay to explore the relationship between N-Ag in urine during COVID-19 and the severity of the disease in hospitalized patients. They also assessed the relationship between N-Ag levels in blood and urine and its significance. A preprint version of the study is available on the medRxiv* server while the article undergoes peer review.

Study details

Researchers collected plasma, urine, and nasopharyngeal samples from 82 consenting SARS-CoV-2 infected patients admitted to Montpellier University hospitals between March 2020 and May 2021.

The researchers also recorded the estimated date of the onset of symptoms. It ranged from one to 35 days before hospital admission. The SARS-CoV-2 infected patients were allocated in two groups: those hospitalized in an intensive care unit (ICU) and those not in ICU. Controls for the study were consenting COVID-19 negative patients.

Samples were analyzed using RT-PCR for viral RNA and ELISA for COVID-19 specific antibodies, IgG and IgA, and N-Ag in blood and urine samples.

N-Ag was present in the urine of patients hospitalized with SARS-CoV-2 infection. Urinary N-Ag levels were high in samples collected during the first and second week after the onset of symptoms and decreased sharply during the third week. Blood N-Ag levels also correlated well with the number of days after the onset of symptoms.

Levels of blood N-Ag were low in N-IgG positive patients compared to N-IgG negative patients. However, most patients (81.36%) who tested positive for SARS-CoV-2 nucleocapsid IgG remained positive for blood N-Ag. The level of N-Ag in the urine was also significantly lower in patients who tested positive for circulating anti-N IgG compared to N IgG negative patients. Still, there was a less pronounced difference between the two groups than in blood. Patients who tested negative for nucleocapsid antibodies in plasma also tested negative for both N-IgA and N-IgG in urine.

These observations highlighted that urine and blood N-Ag were crucial markers for initial onset disease, which decreased considerably on the production of antibodies as the disease progressed. However, despite antibodies being produced, the N-Ag remained detectable in urine.

Patients had higher urinary N-Ag levels when hospitalized in intensive care units (ICU) compared to general medical wards. This was crucial to understanding the importance of N-Ag as markers for severe disease and hospitalizations.

Study implications

The study showed that high urinary N-Ag levels were associated with admission in intensive care units, high C-reactive protein levels, lymphopenia, eosinopenia, high lactate dehydrogenase (LDH), and absence of SARS-CoV-2 nucleocapsid-IgG.

The study results demonstrated that N-Ag is present in the urine of patients hospitalized for COVID-19. Urinary and blood N-Ag were direct markers linked to the dissemination of viral compounds in the body and probably SARS-CoV-2 replication. Predicting the adverse evolution of SARS-CoV-2 infections would need further studies. N-Ag can then be a potential target for drug and vaccine development for COVID-19.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behaviour, or treated as established information.

Written by

Sreetama Dutt

Sreetama Dutt has completed her B.Tech. in Biotechnology from SRM University in Chennai, India and holds an M.Sc. in Medical Microbiology from the University of Manchester, UK. Initially decided upon building her career in laboratory-based research, medical writing and communications happened to catch her when she least expected it. Of course, nothing is a coincidence.