Characterization of the baseline SARS-CoV-2 S-reactive B cell repertoire

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources

A preprint version of the study is available on the bioRxiv* server while the article undergoes peer review.

The study

The researchers investigated samples from ten individuals who had never been exposed to SARS-CoV-2 or received a vaccine. They interrogated these individuals' baseline spike protein reactive B cell repertoire using a combinatorial B cell staining approach with labeled antigenic probes. This novel method allowed for the simultaneous identification of B cells reactive to six different pathogens in a single sample.

SARS-CoV-2 reactive B cells were then compared to those that recognized pre-encountered antigens, including influenza A hemagglutinin fusion protein, tetanus toxoid, HIV-envelope glycoprotein, and hepatitis C virus envelope protein. The B cells specific for SARS-CoV-2 were present at a relatively high frequency in unexposed individuals, around 10-30 fold higher than other unencountered antigens. The level of anti-spike B cells was more similar to the level seen against previously encountered antigens.

The scientists then analyzed both the naïve and memory subsets of the specific B cells and subdivided them into four populations according to the surface expression of IgD and CD27. These were naive, classical memory, CD27- memory, and unswitched memory.

As expected, there were nearly no naive B cells for previously encountered antigens (~0.001%), but there was a significant amount for the spike protein, around 0.11%. The researchers suggest that this could indicate a de novo B cell response from the naive compartment playing a role in infection with SARS-CoV-2.

For the classical memory compartment, B cells against seasonal infections or against vaccine targets were much more common than those against the spike protein. However, the SARS-CoV-2 spike protein reactive B cells were still seen more often than those against unencountered antigens. The majority of these were IgG+, while B cells that targeted vaccine components were normally IgM+ and those against seasonal infections were IgM-.

SARS-CoV-2 spike protein reactive memory B cells, peripheral blood mononuclear cells (PBMCs) were then stimulated with resiquimod, IL2, and IL-21, allowing the differentiation of memory B cells into plasmablasts.

Then, the Spike protein- reactive Ig secretion could be assessed against a set of control antigens, revealing that high antibody levels were seen against common seasonal infections compared to relatively low levels against SARS-CoV-2, previously unencountered antigens as well as other coronaviruses such as SARS-CoV and MERS-CoV.

However, the reaction against SARS-CoV-2 spike protein was still higher than the other coronaviruses and unencountered antigens. In eighty percent of samples from unexposed individuals, the specific antibodies against SARS-CoV-2 were similar to those against other coronaviruses.

The B cells that were reactive against SARS-CoV-2 were single-cell sorted. The vast majority of these were IgD+/CD27- and belonged to the naive B cell compartment, confirming previous observations. One hundred thirty-two heavy chain sequences paired with 101 light chain sequences were obtained. 101, 20, and 5 of the sequences from the heavy chains came from naive, unswitched, and classical memory B cells, respectively.

In 75% of the donors that had samples taken for single-cell sorting, IGHV1-69 was the most dominant heavy chain gene used by cells reactive against the spike protein, and in the remaining 25%, it was the second most dominant. The IGVK3 family made up 36% of the light chains in the light chain sequence compared to 8% in an unselected naïve repertoire. Considering the pairing of the heavy and light chains, 13% of all pairs were IGHV1-69/IGKV2-11.

Conclusion

The authors highlight that the number of B cells reactive to the SARS-CoV-2 spike protein is much higher than expected, with far more reactivity than other previously unencountered antigens and a significantly increased proportion of naive B cells showing reactivity. There was also an existing pool of memory B cells able to recognize SARS-CoV-2 in unexposed individuals.

The researchers suggest that the significant presence of IGHV1-69/IGKV3-11 antibodies may recognize conserved/aberrant spike protein conformation and argue for further research to investigate whether the generation of these antibodies should be promoted during vaccination.

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources