Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) messenger RNA
(mRNA)-based vaccines are ~95% efective in preventing coronavirus disease 20191–5.
The dynamics of antibody secreting plasmablasts (PBs) and germinal centre (GC) B
cells induced by these vaccines in humans remain unclear. We examined
antigen-specifc B cell responses in peripheral blood (n=41) and draining lymph nodes
(LNs) in 14 individuals who received two doses of BNT162b2, an mRNA-based vaccine
encoding full-length SARS-CoV-2 spike (S) gene1. Circulating IgG- and IgA-secreting
PBs targeting the S protein peaked one week after the second immunization then
declined, becoming undetectable three weeks later. These PB responses preceded
maximal levels of serum anti-S binding and neutralizing antibodies to an early
circulating SARS-CoV-2 strain as well as emerging variants, especially in individuals
previously infected with SARS-CoV-2, who produced the most robust serologic
responses. By examining fne needle aspirates (FNAs) of draining axillary LNs, we
identifed GC B cells that bound S protein in all participants sampled after primary
immunization. Remarkably, high frequencies of S-binding GC B cells and PBs were
sustained in these draining LNs for at least twelve weeks after the booster
immunization. S-binding GC B cell-derived monoclonal antibodies predominantly
targeted the receptor binding domain of the S protein, with fewer clones binding to
the N-terminal domain or to epitopes shared with the S proteins of the human
betacoronaviruses OC43 and HKU1. The latter cross-reactive B cell clones had higher
levels of somatic hypermutation compared to those that only recognized SARS-CoV-2
S protein, suggesting a memory B cell origin. Our studies demonstrate that
SARS-CoV-2 mRNA-based vaccination of humans induces a persistent GC B cell
response, enabling the generation of robust humoral immunity.
The concept of using mRNAs as vaccines was introduced over 30 years
ago6,7. Key refinements that improved the biological stability and translation
capacity of exogenous mRNA enabled development of these molecules as vaccines8,9.
The emergence of SARS-CoV-2 in December, 2019 and the ensuing pandemic has unveiled
the potential of this platform9–11. Hundreds of millions of people have received
one of the two SARS-CoV-2 mRNA-based vaccines that were granted emergency use
authorization by the FDA in December, 2020. Both vaccines demonstrated remarkable
immunogenicity in phase 1/2 studies and efficacy in phase 3 studies1–4,12–14.
Whether these vaccines induce robust and persistent germinal center (GC) reactions
that are critical for generating high-affinity
and durable antibody responses has not been examined in humans.
To address this question, we conducted an observational
study of 41 healthy adults (8 with history of confirmed SARS-CoV-2
infection) who received the Pfizer-BioNTech SARS-CoV-2 mRNA vaccine
(BNT162b2) (Extended Data Tables 1, 2). Blood samples were collected
at baseline and at weeks 3 (pre-boost), 4, 5, 7, and 15 after the first
immunization (Fig. 1a). FNAs of the draining axillary LNs were collected from 14
participants (none with history of SARS-CoV-2 infection) at weeks 3
(pre-boost), 4, 5, 7, and 15 after the first immunization (Fig. 1a).
Antibody-secreting PBs in blood that bound SARS-CoV-2 S protein
were measured by enzyme-linked immune absorbent spot (ELISpot)
assay. SARS-CoV-2-S-specfic IgG- and IgA-secreting PBs were detected
three weeks after primary immunization in 24 of 33 participants with
no history of SARS-CoV-2 infection but 0 of 8 participants previously
infected with SARS-CoV-2. PBs peaked in blood during the first week
after boosting (week 4 after primary immunization), with frequencies
varying widely from 3 to 4,100 S-binding PBs per 106 PBMC (Fig. 1b, c).
Plasma IgG antibody titers against S measured by ELISA increased in all
participants over time, reaching peak geometric mean half-maximal
binding titers (GMBTs) of 5,567 and 15,850 5 weeks after immunization
among participants without and with history of SARS-CoV-2
infection, respectively, with a subsequent decline by 15 weeks after
immunization. Anti-S IgA titers and IgG titers against the receptor
binding domain (RBD) of S showed similar kinetics, reaching peak
GMBTs of 172 and 739 for anti-S IgA and 4,501 and 7,965 for anti-RBD IgG
among participants without and with history of SARS-CoV-2 infection,
respectively before declining. IgM responses were weaker and more
transient, peaking 4 weeks after immunization among participants
without history of SARS-CoV-2 infection with a GMBT of 78 and were
undetectable in all but two previously infected participants (Fig. 1d,
Extended Data Fig. 1a).
The functional quality of serum antibody was measured
using high-throughput focus reduction neutralization tests15 on
Vero-TMPRSS2 cells against three authentic infectious SARS-CoV-2
strains with sequence variations in the S gene16,17: (a) a Washington strain
(2019n-CoV/USA) with a prevailing D614G substitution (WA1/2020
D614G); (b) a B.1.1.7 isolate with signature changes in the spike gene18,
including the 69–70 and 144–145 deletions and N501Y, A570D, D614G
and P681H substitutions; and (c) a chimeric SARS-CoV-2 with a B.1.351
spike gene in the Washington strain background (Wash B.1.351) that
contained the following changes: D80A, 242-244 deletion, R246I,
K417N, E484K, N501Y, D614G and A701V. Serum neutralizing titers
increased markedly in participants without history of SARS-CoV-2
infection following boosting, with geometric mean neutralization
titers (GMNTs) against WA1/2020 D614G of 58 three weeks after primary
immunization and 572 two or four weeks after boost (five or seven
weeks after primary immunization). Neutralizing titers against B.1.1.7
and B.1.351 variants were lower, with GMNTs of 49 and 373 against
B.1.1.7 and 36 and 137 against B.1.351 after primary and secondary
immunization, respectively. In participants with a history of prior
SARS-CoV-2 infection, neutralizing titers against all three viruses
were detected at baseline (GMNTs of 241.8, 201.8, and 136.7 against
WA1/2020 D614G, B.1.1.7, and B.1.351 respectively). In these participants,
neutralizing titers increased more rapidly and to higher levels
after immunization, with GMNTs of 4,544, 3,584, and 1,897 against
WA1/2020 D614G, B.1.1.7, and B.1.351, respectively after primary
immunization, and 9,381, 9,351, and 2,749 against WA1/2020 D614G, B.1.1.7,
and B.1.351 respectively after secondary immunization. These GMNTs
were 78-, 73-, and 53-fold higher after primary immunization and
16-, 25-, and 20-fold higher after boosting against WA1/2020 D614G,
B.1.1.7, and B.1.351, respectively than participants without history of
SARS-CoV-2 infection. (Extended Data Fig. 1b).