Humoral Immune Responses in Patients with Severe COVID-19: A Comparative Pilot Study between Individuals Infected by SARS-CoV-2 during the Wild-Type and the Delta Periods

Since the onset of the COVID-19 pandemic, humanity has experienced the spread and circulation of several SARS-CoV-2 variants that differed in transmissibility, contagiousness, and the ability to escape from vaccine-induced neutralizing antibodies. However, issues related to the differences in the variant-specific immune responses remain insufficiently studied. The aim of this study was to compare the parameters of the humoral immune responses in two groups of patients with acute COVID-19 who were infected during the circulation period of the D614G and the Delta variants of SARS-CoV-2. Sera from 48 patients with acute COVID-19 were tested for SARS-CoV-2 binding and neutralizing antibodies using six assays. We found that serum samples from the D614G period demonstrated 3.9- and 1.6-fold increases in RBD- and spike-specific IgG binding with wild-type antigens compared with Delta variant antigens (p < 0.01). Cluster analysis showed the existence of two well-separated clusters. The first cluster mainly consisted of D614G-period patients and the second cluster predominantly included patients from the Delta period. The results thus obtained indicate that humoral immune responses in D614G- and Delta-specific infections can be characterized by variant-specific signatures. This can be taken into account when developing new variant-specific vaccines.

Tracing Human IgE B Cell Antigen Receptor-Bearing Cells With a Monoclonal Anti-Human IgE Antibody That Specifically Recognizes Non-Receptor-Bound IgE

Up to 30% of the population suffers from immunoglobulin E (IgE)-mediated allergies. Despite current stepwise gating approaches, the unambiguous identification of human IgE-producing cells by flow cytometry and immunohistology remains challenging. This is mainly due to the scarcity of these cells and the fact that IgE is not only expressed in a membrane-bound form on the surface of IgE-producing cells in form of the B cell antigen receptor (BCR), but is more frequently found on various cell types bound to the low and high affinity receptors, CD23 and FcϵRI, respectively. Here we sought to develop a sequential gating strategy for unambiguous detection of cells bearing the IgE BCR on their surface. To that aim we first tested the monoclonal anti-IgE antibody omalizumab for its ability to discriminate between IgE BCR and receptor-bound IgE using cells producing IgE or bearing IgE bound to CD23 as well as basophils exhibiting FcϵRI receptor-bound IgE. Using flow cytometry, we demonstrated that omalizumab recognized IgE producing cells with a high sensitivity of up to 1 IgE+ cell in 1000 human peripheral blood mononuclear cells (PBMCs). These results were confirmed by confocal microscopy both in cell suspensions as well as in nasal polyp tissue sections. Finally, we established a consecutive gating strategy allowing the clear identification of class-switched, allergen-specific IgE+ memory B cells and plasmablasts/plasma cells in human PBMCs. Birch pollen specific IgE+ memory B cells represented on average 0.734% of total CD19+ B cells in allergic patients after allergen exposure. Thus, we developed a new protocol for exclusive staining of non-receptor bound allergen-specific IgE+ B cell subsets in human samples.

Pattern of circulating SARS-CoV-2-specific antibody-secreting and memory B-cell generation in patients with acute COVID-19

Objectives

To predict the spread of coronavirus disease (COVID‐19), information regarding the immunological memory for disease‐specific antigens is necessary. The possibility of reinfection, as well as the efficacy of vaccines for COVID‐19 that are currently under development, will largely depend on the quality and longevity of immunological memory in patients. To elucidate the process of humoral immunity development, we analysed the generation of plasmablasts and virus receptor‐binding domain (RBD)‐specific memory B (Bmem) cells in patients during the acute phase of COVID‐19.

Methods

The frequencies of RBD‐binding plasmablasts and RBD‐specific antibody‐secreting cells (ASCs) in the peripheral blood samples collected from patients with COVID‐19 were measured using flow cytometry and the ELISpot assay.

Results

The acute phase of COVID‐19 was characterised by the transient appearance of total as well as RBD‐binding plasmablasts. ELISpot analysis indicated that most patients exhibited a spontaneous secretion of RBD‐specific ASCs in the circulation with good correlation between the IgG and IgM subsets. IL‐21/CD40L stimulation of purified B cells induced the activation and proliferation of Bmem cells, which led to the generation of plasmablast phenotypic cells as well as RBD‐specific ASCs. No correlation was observed between the frequency of Bmem cell‐derived and spontaneous ASCs, suggesting that the two types of ASCs were weakly associated with each other.

Conclusion

Our findings reveal that SARS‐CoV‐2‐specific Bmem cells are generated during the acute phase of COVID‐19. These findings can serve as a basis for further studies on the longevity of SARS‐CoV‐2‐specific B‐cell memory.