With the use of an accurate high throughput assay, researchers from the Weizmann Institute of Science and Magen David Adom Blood Services in Israel showed substantial inter-individual variability and antibody cross-reactivity against seasonal coronaviruses upon SARS-CoV-2 infection, with important implications for immune surveys and vaccines. The study is currently available on the medRxiv* preprint server.
The coronavirus disease (COVID-19) pandemic, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has spread around the world with alarming speed and enormous human health toll.
Alongside SARS-CoV-2, six more coronaviruses are known to infect humans – including SARS-CoV-1 responsible for the original SARS outbreak in 2003 and MERS-CoV (Middle East respiratory syndrome). Moreover, four seasonal endemic human coronaviruses (OC43, HKU1, NL63, 229E) circulate among people and cause the common cold.
Consequently, previous exposures to seasonal human coronaviruses may prompt immunological memory with potential benefits in the course of SARS-CoV-2 infections. It has already been shown that up to 60% of individuals unexposed to SARS-CoV-2 have T cells that recognize its epitopes, as well as cross-reactivity against seasonal coronaviruses.
In this novel study, a research group led by Dr. Thomas Vogl from the Department of Computer Science and Applied Mathematics of the Weizmann Institute of Science in Israel applied a high-resolution antibody assay to appraise cross-reactive antibody responses against SARS-CoV-2 and seasonal common cold coronaviruses.
A phage displayed antigen library (a) of human Coronaviruses’ peptide antigens (b) applied to serum samples of unexposed individuals and recovered COVID-19 patients (c) detects a high serum prevalence of seasonal hCoVs, interindividual variability of antibody repertoires against SARS-CoV-2, and cross-reactive antibody responses from SARS-CoV-2 infection (d-k). The numbers of proteins per strain in panel b include polyproteins being split into 14 separate proteins. d-k, All bound antigens of the hCoV library are shown in panel d, the following panels depict binding against peptides of each hCoV strain separately. The illustration of the SARS-CoV-2 virion is reproduced from CDC PHIL #23312 released as public domain (CDC/ Alissa Eckert, MSMI; Dan Higgins, MAMS), the phylogenetic tree is reproduced form Wu et al. (1) [open access].
Building a robust coronavirus library
In short, the researchers have utilized the aforementioned high-resolution antibody assay to test for binding against 1,539 peptide antigens that cover all known proteins of all human coronaviruses.
More specifically, phage immunoprecipitation sequencing (PhIP-Seq) that relies on the display of synthetic oligo libraries on T7 phages has been used for that purpose. This method enables rational selection of displayed antigens and allows probing for hundred of thousands of antigens in parallel.
Consequently, they have generated a PhIP-Seq library with all open reading frames of human coronaviruses as 64 amino acid sections with 20 amino acid overlaps between adjacent peptides. This library also included important positive and negative controls to interpret the obtained results adequately.
The scientists were able to test IgG antibody binding against this library with 32 serum samples of individuals unexposed to SARS-CoV-2, collected in 2013/2014 before the COVID-19 outbreak, comparing it to 32 serum samples of recovered COVID-19 patients obtained in April and May 2020.
A majority of analyses were based on antibody responses against 57 human coronavirus peptides from 32 different coronavirus proteins – shared by more than five individuals in either group and seven peptides with significantly different abundances between the groups.
Antibody repertoires and cross-reactivities
"We have detected a high seroprevalence of seasonal human coronaviruses in up to 75% of individuals both unexposed to SARS-CoV-2 and recovered from COVID-19, variability in antibody repertoires against SARS-CoV-2, and cross-reactivity against seasonal coronaviruses upon SARS-CoV-2 infection", study authors summarize their findings in this medRxiv paper.
More specifically, unexposed individuals demonstrated copious antibody responses against all seasonal human coronaviruses. Binding against peptides of hCoV-NL63 has been detected in 75% of unexposed individuals, while against hCoV-HKU1, hCoV-229E and hCoV-OC43 in 66%, 63%, and 38% of them, respectively.
Very similar frequencies were observed in those recovered from COVID-19, mainly originating from spike protein or nucleocapsid protein. Importantly, no convergence of antibody responses against the same peptide was detected, suggesting substantial inter-individual variation in antibody response against SARS-CoV-2.
Serum samples from those that had COVID-19 also showed common binding against SARS-CoV-1, indicating detection of cross-reactivity of antibodies targeting SARS-CoV-2. In addition, approximately 50% of recovered patients had responses against unique epitopes of seasonal hCoV-OC43, which was not detectable in unexposed individuals.
Implications for immune surveys and vaccines
Overall, the findings from this study point towards the conclusions that significant cross-reactivity between all human coronaviruses observed for T cells also extends to antibody responses against seasonal coronaviruses.
"We show that infection with SARS-CoV-2 mounts cross-reactive antibodies against hCoV-OC43 antigens", say study authors. "However, the reverse direction of preexisting antibody responses targeting seasonal human coronaviruses recognizing SARS-CoV-2 is more difficult to assess", they add.
Still, considering the affordability of processing phage-displayed libraries in parallel, the high accuracy of the technique, as well as its excellent suitability for robot automation, serological testing based on this library may be broadly applicable to appraise preexisting immunity at population-scale.
Consequently, this will have implications towards protecting the population and generating adequate herd immunity against SARS-CoV-2, but also for effective stratification of vaccine trial costs for candidates that are currently tested.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.