Presence of SARS-CoV-2 S-protein specific BMEMORYcells in convalescent COVID-19 patients
Unlabelled SARS-CoV-2 S-protein in class-switched BMEMORY cells could significantly block binding in COVID-19 patients, but not healthy donor samples (Figure 2b, Figure 3c). SARS-CoV-2 specific BMEMORY cells correlated moderately with anti-S-protein IgG antibodies (Figure 3b). Overall, the frequency of S-protein specific BMEMORY cells was significantly higher in the COVID-19 disease cohort compared to healthy individuals (Figure 3c). The difference between COVID-19 patients and healthy individuals was restricted to BMEMORY cells and was not observed for other B cell subsets such as naïve or unswitched BMEMORY cells (Figure 3d-f). This has two important implications: First, it demonstrates the specificity of the protocol for the detection of SARS-CoV-2 specific BMEMORY cells, since it is expected that only COVID-19 convalescent, but not unexposed individuals generate SARS‑CoV-2 specific B cell memory. Second, it demonstrates that the humoral immune response in the analysed patient cohort could be mediated by long-lived memory B cells. We did not observe substantial amounts of SARS-CoV-2 specific plasmablasts (Figure 3e).
The longevity of B cell memory can also be demonstrated by analysing the frequency of SARS-CoV-2 specific BMEMORY cells concerning the time of sample collection after COVID-19 diagnosis (Fig. 3g). While there was a tendency of lower frequencies in samples collected at later time points, SARS-CoV-2 specific BMEMORY cells remained detectable also 6 months after COVID-19 diagnosis.