Persistent B cell memory after SARS-CoV-2 vaccination is functional during breakthrough infections

Abstract

Breakthrough SARS-CoV-2 infections in fully vaccinated individuals are considered a consequence of waning immunity. Serum antibodies represent the most measurable outcome of vaccine-induced B cell memory. When antibodies decline, memory B cells are expected to persist and perform their function, preventing clinical disease. We investigated whether BNT162b2 mRNA vaccine induces durable and functional B cell memory in vivo against SARS-CoV-2 3, 6, and 9 months after the second dose in a cohort of health care workers (HCWs). While we observed physiological decline of SARS-CoV-2-specific antibodies, memory B cells persist and increase until 9 months after immunization. HCWs with breakthrough infections had no signs of waning immunity. In 3-4 days, memory B cells responded to SARS-CoV-2 infection by producing high levels of specific antibodies in the serum and anti-Spike IgA in the saliva. Antibodies to the viral nucleoprotein were produced with the slow kinetics typical of the response to a novel antigen.

Keywords: COVID-19; SARS-CoV-2; breakthrough infections; mRNA vaccine; memory B cells; mucosal immunity; salivary IgA; waning immunity.

Graphical abstract

Figure 1

Flowchart showing the experimental plan

Figure 2

SARS-CoV-2 antibody responses and specific memory B cells in vaccinated HCWs (A) Anti-Trimeric Spike-specific IgG, (B) total anti-RBD antibody levels, (C) percentage of Spike-specific MBCs, and (D) frequency of RBD-specific cells, identified inside the total Spike-positive MBCs, measured at different time points in vaccinated HCWs: before vaccine administration (T0), 7 days (1w), 3 (3mo), 6 (6mo), and 9 (9mo) months after the second dose (n = 34). (E) Simple linear regression models for Spike-specific MBCs and time (red line), and for anti-Trimeric Spike-specific IgG and time (blue line). Antibody titer is shown on a log₁₀ scale. Medians are indicated, and statistical significance was determined using Wilcoxon matched pairs signed rank test. ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001, ^∗∗∗∗p < 0.0001.

Figure 3

Levels of specific antibodies and frequency of Spike-specific memory B cells in vaccinated HCWs who never had a positive NPS and in HCWs with breakthrough infections For a Figure360 author presentation of this figure, see https://doi.org/10.1016/j.chom.2022.01.003. (A) Anti-Trimeric Spike-specific IgG, (B) total anti-RBD antibodies, and (C) neutralizing antibodies. The values measured in SARS-CoV-2-negative HCWs 3 and 6 months after vaccination (n = 34) are compared to those of HCWs with breakthrough infections that occurred either 3 (3mo) (n = 8) or 6 (6mo) (n = 14) months after the second dose. (D) Plot depicts the percentage of Spike-specific MBCs in vaccinees with and without breakthrough infections. (E) Pie charts show the distribution of switched and IgM+ Spike-specific MBCs in fully vaccinated HCWs without and with breakthrough infections 3 and 6 months after vaccination. In (F), the percentage of RBD+ cells among Spike-specific MBCs in the two groups of HCWs is shown. Medians of the antibody values at different time points showing the kinetics of anti-Trimeric Spike-specific IgG (G), anti-RBD Ig (H), and anti-N Ig (I) are shown. (L) Percentage of Spike-specific MBCs, (M) RBD-specific MBCs, identified among Spike-specific MBCs, and (N) Spike-specific plasmablasts in the peripheral blood of vaccinated HCWs with breakthrough infections (n = 33). Samples were analyzed after the first positive NPS at the following time points: 0–3 days, 4–6 days, 7–10 days,11–14 days, and > 15days after the first positive NPS. Empty circles represent HCWs for which only one sample was collected at a known time point after the first positive NPS (n = 12). Antibody titer is shown on a log₁₀ scale. Neutralizing antibodies are expressed as the reciprocal of the highest serum dilution inhibiting at least 90% of virus-induced cytopathic effect (MNA90) and values ≥10 were considered positive. Dashed line indicates the cut-off of the test (MNA90 < 10). Medians are indicated, and statistical significance was determined using unpaired Mann-Whitney t test. ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001.

Figure 4

HCWs salivary IgA levels (A) Detection of salivary IgA levels in 81 HCWs who previously had COVID-19, 34 vaccinated HCWs, and 33 HCWs with breakthrough infections measured at different time points after the first positive NPS (0–3 days, 4–6 days, 7–10 days, 11–14 days, >15 days, and 5–8 months). Empty circles refer to HCWs for which only one sample was collected at a known time point after the first positive NPS (n = 19). Statistical significances are reported in the table besides the graph. Medians are indicated, and statistical significance was determined using unpaired Mann-Whitney t test. ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001; ^∗∗∗∗p < 0.0001. (B) Graphical model of the possible mechanism by which SARS-CoV-2 infection turns vaccine-induced systemic immunity into local immunity. In vaccinated individuals, specific antibodies reach mucosal sites by transudation (on the left). After infection, memory T and B cells reach the site of viral entry. MBCs differentiate into plasma cells secreting large amounts of specific IgA (on the right).