Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) emerged as a “pneumonia of unknown etiology” in Wuhan, China in 2019. The disease caused by SARS-CoV-2 was termed coronavirus disease 2019 (COVID-19) and was declared a pandemic by the World Health Organization (WHO). COVID-19 infections can range from asymptomatic to severe which can ultimately lead to the death of older individuals and those with comorbidities.
Previous studies on mild and severe SARS-CoV-2 infections have indicated that the activation of antibody responses is required to recover from COVID-19. Higher IgG antibodies were identified in patients with severe infections as compared to those with mild infections. The spike (S) glycoprotein and nucleocapsid (N) proteins have been identified to be major immunogens. One study also showed that the levels of S-specific or receptor-binding domain (RBD)-specific IgG antibodies were higher in patients who were not admitted to the intensive care unit (ICU) while the levels of N-specific IgG were higher in patients admitted to the ICU.
Several studies have also reported that the IgG and IgM antibody titers peaked by day 18 of SARS-CoV-2 infection followed by a gradual decline. The decline of IgM occurred within the first month while the levels of IgG remained elevated for several months and declined approximately 7 months following SARS-CoV-2 infection. However, the cause for the decline of the antibody titers remains unclear and requires further research.
Furthermore, multiple conserved epitopes have been identified on S and N proteins which could have a protective role against SARS-CoV-2 infection. Therefore, these epitopes can be used as a target for the development of vaccines. Additionally, multiple epitopes were identified from serum samples obtained from the United States, Switzerland, Singapore, and China that could be used as markers for infection as well as disease severity.
A new study published in Frontiers in Microbiology aimed to demonstrate the reactivity of COVID-19 serum obtained from patients in Tatarstan, Russia with the S and N peptides that were previously identified as immunogenic. The study also aimed to determine the correlations of S and N peptide reactivity with the severity of lung damage, clinical and demographic characteristics, duration of symptoms, and age along with the cross-reactivity of COVID-19 convalescent sera with human endemic coronavirus (eCoV) peptides.
About the study
The study involved the collection of convalescent serum samples from 138 COVID-19 patients and 39 controls between March and December 2020. Additionally, 22 control serum samples that were collected during 2015-2016 were included in the study. Acute serum samples were collected from 14 COVID-19 patients. The serum samples were collected at different time points post-infection. Blood samples were also collected from 17 COVID-19 patients and 9 controls.
Thereafter, S and N peptides from SARS-CoV-2 as well as other eCoVs such as NL63, OC43, HKU1, and 229E were synthesized followed by the alignment of amino acid sequences of SARS-CoV-2 N and S peptides with each eCoV. SARS-CoV-2 specific antibody titers were measured using an enzyme-linked immunosorbent assay (ELISA) followed by peptide reactivity with serum antibodies of infected patients as well as controls. Finally, ELISpot analysis was carried out using peripheral blood mononuclear cells (PBMCs) obtained from the blood samples.
The results indicated 105 mild COVID-19 cases, 43 moderate cases, and 4 severe cases. 39 patients with moderate disease and 4 patients with serve disease required hospitalization. Lung damage was found to be less than 20 percent, 20 to 40 percent, and greater than 40 percent in 117, 31, and 4 patients, respectively while fever was reported in all patients.
Peptides that were essential for virus replication and binding to the ACE2 receptor were selected for the study. Moreover, two mutations were identified in the S protein peptides and five mutations were identified in the N protein peptides from the recently circulating delta strain.
The results also reported that S1, S7, S18, and N6 peptides had higher reactivity with convalescent serum compared to the serum collected from controls in 2020. However, more SARS-CoV-2 peptides from control samples collected in 2015 were found to have higher reactivity with convalescent serum. The results were similar for acute serum as well.
Furthermore, the peptides S1, S2, S7, S14, S19, and N6 were reported to have high reactivity in serum samples collected ≤ 3 months post-recovery as compared to 2020 controls, and most of them were reported to remain active 4–6 months post-recovery. However, the reactivity of half of these peptides was lost ≥ 7 months post-recovery with only S7, S18, and N6 peptides remaining reactive.
An increased serum reactivity with peptides S1, S7, S18, and N6 was observed in male COVID-19 patients while for females the reactivity was higher for peptides S7 and N6 as compared to the respective controls. Also, younger COVID-19 patients were found to have higher reactivity with more peptides as compared to older patients. Moreover, SARS-CoV-2 severity was positively correlated with lung damage, increasing age, high fever, as well as the duration of fever. Out of all the peptides that showed reactivity, only S1 and S18 were reported to correlate with clinical parameters with S1 being negatively correlated with disease severity and S18 being positively correlated with fever.
The results also identified that T-cell reactivity to S6, N6, and N19 was higher in COVID-19 patients as compared to controls. Also, reactivity to S4 and S15 was observed to be higher in older patients as compared to younger patients. Furthermore, the highest degree of similarity of SARS-CoV-2 S protein was found with NL63.S1 alphacoronavirus while for other eCoVs it was less than 65b percent. Analysis of antibody reactivity with other eCoVs indicated that prior infection with them could not protect against severe COVID-19.
Therefore, the current study demonstrated early activation and circulation of anti-SARS-CoV-2 antibodies which gradually declined with time post-infection. Several S and N peptides were identified that were immunogenic and could indicate disease severity. The study also highlighted that COVID-19 serum had limited cross-reactivity with other eCoV peptides. Thus, this study is important in identifying T-cell epitopes that can be used for the development of prophylactic and therapeutic measures.