Longitudinal Analysis Reveals Distinct Antibody and Memory B Cell Responses in SARS-CoV2 Naïve and Recovered Individuals Following mRNA Vaccination

Abstract

Novel mRNA vaccines for SARS-CoV2 have been authorized for emergency use and are currently being administered to millions of individuals worldwide. Despite their efficacy in clinical trials, there is limited data on vaccine-induced immune responses in individuals with a prior SARS-CoV2 infection compared to SARS-CoV2 naïve subjects. Moreover, how mRNA vaccines impact the development of antibodies as well as memory B cells in COVID-19 experienced versus COVID-19 naïve subjects remains poorly understood. In this study, we evaluated antibody responses and antigen-specific memory B cell responses over time in 33 SARS-CoV2 naïve and 11 SARS-CoV2 recovered subjects. mRNA vaccination induced significant antibody and memory B cell responses against full-length SARS-CoV2 spike protein and the spike receptor binding domain (RBD). SARS-CoV2 naïve individuals benefitted from both doses of mRNA vaccine with additional increases in antibodies and memory B cells following booster immunization. In contrast, SARS-CoV2 recovered individuals had a significant immune response after the first dose with no increase in circulating antibodies or antigen-specific memory B cells after the second dose. Moreover, the magnitude of the memory B cell response induced by vaccination was lower in older individuals, revealing an age-dependence to mRNA vaccine-induced B cell memory. Side effects also tended to associate with post-boost antibody levels, but not with post-boost memory B cells, suggesting that side effect severity may be a surrogate of short-term antibody responses. The frequency of pre-vaccine antigen-specific memory B cells in SARS-CoV2 recovered individuals strongly correlated with post-vaccine antibody levels, supporting a key role for memory B cells in humoral recall responses to SARS-CoV2. This observation may have relevance for future booster vaccines and for responses to viral variants that partially escape pre-existing antibodies and require new humoral responses to be generated from memory B cells. Finally, post-boost antibody levels were not correlated with post-boost memory responses in SARS-CoV2 naïve individuals, indicating that short-term antibody levels and memory B cells are complementary immunological endpoints that should be examined in tandem when evaluating vaccine response. Together, our data provide evidence of both serological response and immunological memory following mRNA vaccination that is distinct based on prior SARS-CoV2 exposure. These findings may inform vaccine distribution in a resource-limited setting.

Figure 1.. Antibody responses following mRNA vaccination in SARS-CoV2 naïve and SARS-CoV2 recovered individuals.

A) UPenn Immune Health COVID vaccine study design. B) Concentration of anti-spike and anti-RBD IgG antibodies in vaccinated individuals over time. Dotted lines indicate the limit of detection for the assay. Blue = SARS-CoV2 naïve + mRNA vaccine, red = SARS-CoV2 recovered + mRNA vaccine. Statistics were calculated using unpaired Kruskal-Wallis or Wilcoxon test with adjustment for multiple comparisons.

Figure 2.. Antigen-specific memory B cell responses following mRNA vaccination in SARS-CoV2 naïve and SARS-CoV2 recovered individuals.

A) Gating strategy and representative plots for flow cytometric analysis of SARS-CoV2-specific B cells. Cells were stained with fluorescently labeled SARS-CoV2 full-length spike protein, SARS-CoV2 spike receptor binding domain (RBD), and influenza hemagglutinin (HA). Memory B cells were identified as live, CD3⁻, CD19⁺, non-naïve (≠ IgD⁺ CD27⁻), CD20⁺ CD38^lo/int, decoy⁻ cells. Spike⁺ HA⁻ cells were subsequently analyzed for binding to RBD, as well as immunoglobulin class (IgG vs. IgM). B) Frequency of spike⁺, spike⁺/RBD⁺, and HA⁺ memory B cells over time in vaccinated individuals. Data are represented as frequency of antigen-specific cells in the total memory B cell compartment. C) Frequency of antigen-specific IgG⁺ memory B cells over time in vaccinated individuals. Data are represented as frequency of antigen-specific IgG⁺ cells in the total memory B cell compartment. D) Frequency of IgG and IgM isotypes over time in the antigen-specific memory cell compartments. RD = non-vaccinated, SARS-CoV2 recovered donors. Dotted lines indicate the mean at baseline in SARS-CoV2 naïve and SARS-CoV2 recovered individuals. Blue = SARS-CoV2 naïve + mRNA vaccine, red = SARS-CoV2 recovered + mRNA vaccine, purple = SARS-CoV2 recovered. Statistics were calculated using unpaired Kruskal-Wallis or Wilcoxon test with adjustment for multiple comparisons.

Figure 3.. Age-associated decreases in antigen-specific B cell responses following mRNA vaccination.

A, B) Concentration of anti-spike and anti-RBD IgG antibodies over time compared with sex and age in SARS-CoV2 naïve individuals. Dotted lines indicate the limit of detection for the assay. C, D) Frequency of spike⁺ and spike⁺/RBD⁺ memory B cells over time compared with sex and age in SARS-CoV2 naïve individuals. Data are represented as frequency of spike⁺ and spike⁺/RBD⁺ cells in the total memory B cell compartment. Pre-boost indicates samples collected at timepoint 2 (~15 days post-primary vaccination). Post-boost indicates samples collected at timepoint 4 (~7 days post-secondary vaccination). Dotted lines indicate the mean frequency of cells at baseline. Statistics for sex were calculated using Wilcoxon test. Associations with age were calculated using Spearman correlation.

Figure 4.. Antigen-specific memory cells are a distinct measure of vaccine efficacy and correlate with antibody recall responses.

A) Association of post-boost (timepoint 4) antibody levels with post-boost (timepoint 4) antigen-specific memory cell frequencies in SARS-CoV2 naïve individuals. B) Association of baseline (timepoint 1) antibody levels with baseline (timepoint 1) antigen-specific memory cell frequencies in SARS-CoV2 recovered individuals. C) Association of baseline (timepoint 1) antibody levels with post-primary vaccination (timepoint 2) antibody levels in SARS-CoV2 recovered individuals. D) Association of baseline (timepoint 1) antigen-specific memory cell frequencies with post-primary vaccination (timepoint 2) antibody levels in SARS-CoV2 recovered individuals. Associations between immunological parameters were calculated using Pearson correlation.