Study explores boost with variant-derived COVID vaccine

In a recent study posted on Bio Rxiv*, Researchers at the Washington University School of Medicine in St. Louis demonstrated that a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine booster induced a strong germinal center (GC) B-cell response.

Several studies have reported that coronavirus disease 2019 (COVID-19) vaccine boosters enhance immune responses to ancestral SARS-CoV-2 and feared emerging variants. In addition, new vaccines based on circulating SARS-CoV-2 variants are being developed to enhance antibody responses.

In addition, recent evidence suggests that beta-variant-based booster doses result in higher titers Neutralizing antibody (nAbs) against beta and Omicron variants over wild-type-based boosters or wild-type and Omicron-encoding bivalent vaccines spike proteinNevertheless, it remains unclear whether booster doses induce GC responses.

study: SARS-CoV-2 Omicron boosting induces de novo B cell responses in humansImage credit: NIAID

Research and Findings

In this study, investigators evaluated immune responses in healthy, vaccinated adults who had no history of SARS-CoV-2 infection. The participant completed primary vaccination with her BNT162b2 from Pfizer or mRNA-1273 vaccine from Moderna. They were boosted with a single dose of mRNA-1273 or bivalent mRNA-1273.213 (based on beta and delta mutant spike proteins). 46 people were recruited. Seven were boosted with mRNA-1273 and 31 received the variant-specific vaccine.

An enzyme-linked immunosorbent spot (ELISpot) assay was used to quantify circulating spike-specific plasmablasts. The authors detected spike-specific IgA- and IgG-producing plasmablasts in all mRNA-1273 recipients one week after the booster immunization. Similarly, they observed IgG-producing plasmablasts against the spike protein of the ancestral strain, beta, and delta mutants. Additionally, fine needle aspirates (FNA) and bone marrow aspirates were collected from some individuals after boost.

FNA samples were stained with fluorescently labeled spike protein probes from ancestral strains, beta, delta, and omicron (BA.1) variants. Spike-binding GC B lymphocytes and follicular T helper cells were detected in all his FNA samples from all participants after 2 weeks. Spike-specific memory B cells (MBC) were present in all participants before booster vaccination and were maintained at similar frequencies after boosting.

Three participants from the mRNA-1273 and mRNA-1273.213 cohorts were selected to characterize the MBC repertoire.clonally different antigenSpecific monoclonal antibodies (mAbs) were generated from 6 participants and evaluated for spike binding using a multiplex bead binding assay. The authors noted that 94% and 92% of MBC-derived mAbs from the mRNA-1273 and mRNA-1273.213 cohorts recognized spikes from the ancestral strain, beta, delta and BA.1 variants.

Single-cell RNA sequencing was performed on these six plasmablast and FNA samples. Most spike-specific plasmablasts identified after boosting were clonally related to GC B cells, MBCs and plasma cells induced by primary vaccination. There were multiple spike-specific plasmablast clones after administration of the second and booster doses. Representatives of these clones in the booster response showed significantly higher somatic hypermutation (SHM) frequencies.

Spike-specific plasmablast clones were identified in all six GC responses. Her SHM for spike-specific MBC was significantly higher after boost than after primary vaccination. Both booster vaccines induced strong GC responses and MBC maturation, but variant-specific antibodies (against beta and delta spikes) were not isolated.

In addition, eight individuals were recruited to assess whether antigenically more diverse vaccine boosters elicit immune responses to novel epitopes. These double-vaccinated subjects received an mRNA-1273.529 vaccine (booster) encoding the BA.1 spike. MBC were sorted from peripheral blood samples of participants 17 weeks after boost. Nearly all mAbs (99%) showed cross-reactivity and bound spikes from ancestral strains, beta, delta, and BA.1 variants.

Next, we examined the neutralizing capacity of mAbs from the three cohorts in a high-throughput assay. Researchers detected 131 mAbs that neutralized infection by at least 80%. These mAbs were tested for neutralization against a panel of authentic SARS-CoV-2 particles. Most mAbs inhibited infection by 90% against ancestral strain, beta, and delta variants. However, relatively few mAbs were effective against the BA.1 and BA.5 variants in each cohort.

Finally, the research team isolated a mAb that (specifically) recognizes the BA.1 spike that does not bind to the ancestral spike. This approach isolated 78 mAbs. Of these, 57 mAbs were still cross-reactive (to the ancestral spike). One mAb recognized the BA.1 spike, 12 mAbs recognized the BA.1 and Beta/Delta spikes, and 8 mAbs did not bind antigen above background. None of these 13 mAbs neutralized the ancestral strain. Seven (54%) mAbs neutralized BA.1, one mAb neutralized BA.5, and no mAb neutralized beta or delta variants.

Characterization of BA.1-specific mAbs. (a) gating strategy for sorting BA.1⁺ WA1/2020⁻ MBC after 17 weeks of boosted PBMC. (b) mAb binding from BA.1⁺ WA1/2020⁻ We sorted MBCs into the indicated strains of SARS-CoV-2 S measured by multiplexed bead binding arrays. (c) Overview of mAb binding. (d) neutralizing activity of BA.1⁺ WA1/2020⁻ Binding mAbs against the indicated strains of authentic SARS-CoV-2 virus. Numbers above each virus are for mAbs below the 90% infection reduction threshold. (e) IGHV mutation frequencies in clones associated with mAbs from participants 382-54 and 382-55 that neutralize D164G (left) and BA.1 but not D614G (right). Black lines indicate median values. Each symbol represents a sequence. n = 39 for D614G⁺ n = 7 for BA.1⁺ D614G⁻.What) Plaque assay in Vero E6 cells using the indicated mAbs in the overlay to isolate escape mutants (red arrows). Images are representative of three experiments per mAb. (g) The structure of the RBD with the hACE2 footprint highlighted in brown, the BA.1 mutation highlighted in blue, and the amino acids whose substitutions confer resistance to the mAbs shown in the plaque assay highlighted in red.

Conclusion

This study showed that boosting with an ancestral spike-based or bivalent (beta/delta) vaccine induced a strong spike-specific GC response in the axillary lymph nodes of all participants. Spike-specific GC B cells and plasmablasts were primarily derived from pre-existing clonal lineages.In particular, the researchers showed that boosting with an antigenically distant monovalent Omicron-based vaccine (mRNA-1273.529) was induced Also B-cell responses targeting novel epitopes of the Omicron spike.

*Important Notices

Bio Rxiv We publish a non-peer-reviewed, preliminary scientific report and should not be taken as conclusive, to guide clinical practice/health-related actions, or to be treated as established information.