Booster Immunization Improves Memory B Cell Responses in Older Adults Unresponsive to Primary SARS-CoV-2 Immunization

Abstract

The generation of a specific long-term immune response to SARS-CoV-2 is considered important for protection against COVID-19 infection and disease. Memory B cells, responsible for the generation of antibody-producing plasmablasts upon a new antigen encounter, play an important role in this process. Therefore, the induction of memory B cell responses after primary and booster SARS-CoV-2 immunizations was investigated in the general population with an emphasis on older adults. Participants, 20-99 years of age, due to receive the mRNA-1273 or BNT162b2 SARS-CoV-2 vaccine were included in the current study. Specific memory B cells were determined by ex vivo ELISpot assays. In a subset of participants, antibody levels, avidity, and virus neutralization capacity were compared to memory B cell responses. Memory B cells specific for both Spike S1 and receptor-binding domain (RBD) were detected in the majority of participants following the primary immunization series. However, a proportion of predominantly older adults showed low frequencies of specific memory B cells. Booster vaccination resulted in a large increase in the frequencies of S1- and RBD-specific memory B cells also for those in which low memory B cell frequencies were detected after the primary series. These data show that booster immunization is important for the generation of a memory B cell response, as a subset of older adults shows a suboptimal response to the primary SARS-CoV-2 immunization series. It is anticipated that these memory B cells will play a significant role in the immune response following viral re-exposure.

Keywords: SARS-CoV-2; antibodies; booster; coronavirus; memory B cells; older adults; primary vaccination.

Figure 1

SARS-CoV-2 Spike S1- and RBD-specific memory B cells are present in a subset of vaccinees after the primary mRNA immunization series. SARS-CoV-2 Spike S1- (A) and RBD-specific (B) memory B cells were quantified by ELISpot (N = 87). Samples within the grey box are below the cut-off and are considered low-responders. The lower and upper hinges of the boxplot indicate the 25th and 75th percentile, with the middle line indicating the median. The whiskers extend up to 1.5 ×the inter-quartile range. A correlation between Spike S1- and RBD-specific spots at P28 is shown for participants with a response above the cut-off value for both RBD and Spike S1 (N = 42) (C). The strength of the correlation was estimated with a Pearson correlation. The correlation shown in (D) includes all participants in (A/B), where the grey boxes contain participants classified as a low-responder for one of the two antigens. PBMCs = peripheral blood mononuclear cells; RBD = receptor-binding domain; P28 = 28 days after completion of the primary immunization series.

Figure 2

A subset of older adults does not generate a good memory B cell response after the SARS-CoV-2 primary immunization series. The SARS-CoV-2 Spike S1 memory B cell response 28 days after completion of the primary immunization series was compared between Comirnaty (N = 59) and Spikevax (N = 28) immunization (A), male (N = 35) or female (n = 52) participants (B), and between participants split into two different age groups (N = 26 for 20–50 and N = 61 for 50+) (D). Samples within the grey box are below the cut-off and are considered low-responders. The lower and upper hinges of the boxplot indicate the 25th and 75th percentile, with the middle line indicating the median. The whiskers extend up to 1.5 × the inter-quartile range. A potential correlation between SARS-CoV-2 Spike S1 memory B cells and age was further investigated (N = 87) (C). Groups were compared using a Mann–Whitney U test. Correlations were investigated using a Pearson correlation. PBMCs = peripheral blood mononuclear cells; ns = not significant; *** = p < 0.001.

Figure 3

SARS-CoV-2 memory B cell non-responders show reduced antibody levels and functionality. Anti-Spike S1 antibody levels were compared between Spike S1 memory B cell non-responders (N = 27) and responders (N = 60) 28 days after completion of the primary immunization series (A). Samples within the grey box are below the cut-off for antibody positivity. Anti-Spike S1 antibody avidity was compared between Spike S1 memory B cell responders (N = 26) and non-responders (N = 14) 28 days after completion of the primary immunization series (B). Virus neutralization titers were compared between Spike S1 memory B cell non-responders (N = 15) and responders (N = 27) 28 days after completion of the primary immunization series (C). Samples within the grey box were below the lower limit of detection. All samples below the lower limit of detection were assigned a titer of 5. In both cases, a Mann–Whitney U test was used to investigate differences between groups. The lower and upper hinges of the boxplot indicate the 25th and 75th percentile, with the middle line indicating the median. The whiskers extend up to 1.5 × the inter-quartile range. Red dots indicate participants aged 20–50, while blue dots indicate participants over 50 years of age. AI = avidity index; BAU = binding antibody units; VNT = virus neutralization titer; *** = p < 0.001, ** = p < 0.01.

Figure 4

SARS-CoV-2 mRNA booster immunization increases the memory B cell response. SARS-CoV-2 Spike S1-specific (A) and RBD-specific (B) memory B cell responses are shown at different timepoints after mRNA immunization. N = 87, 44, and 58 at P28, B0, and B1, respectively. The lower and upper hinges of the boxplot indicate the 25th and 75th percentile, with the middle line indicating the median. The whiskers extend up to 1.5 × the inter-quartile range. Samples within the grey box are below the cut-off for positivity. Changes in memory B cell responses over time in paired samples are shown for SARS-CoV-2 Spike S1 (C) and RBD (D). N = 43 for samples paired between P28 and B0. N = 41 for samples paired between B0 and B1. Red dots indicate participants aged 20–50, while blue dots indicate participants over 50 years of age. Groups were compared with a paired Wilcoxon signed-rank test. P28 = 28 days after the primary immunization series; B0 = before booster immunization; B28 = 28 days after booster immunization; PBMCs = peripheral blood mononuclear cells; RBD = receptor-binding domain; ns = not significant, * p < 0.05, *** = p < 0.001.

Figure 5

SARS-CoV-2 booster immunization decreases age differences in the memory B cell response. A correlation between Spike S1- and RBD-specific spots at B28 in the same participants is shown for participants positive for both RBD and Spike S1 (N = 54) (A). A correlation between SARS-CoV-2 Spike-S1-specific memory B cells at P28 and B28 is shown (N = 58) (B) The strength of the correlations was estimated with a Pearson correlation. The SARS-CoV-2 Spike S1 memory B cell response 28 days after completion of the booster immunization was compared between different vaccine combinations (C) and between participants split into two different age groups (D). (N = 15, 28, and 15 for CCC, CCS, and SSC, respectively (C indicates Comirnaty, S indicates Spikevax)). (N = 12 and N = 46 for the 20–50 and 50+ age groups, respectively). The lower and upper hinges of the boxplot indicate the 25th and 75th percentile, with the middle line indicating the median. The whiskers extend up to 1.5 × the inter-quartile range. Samples within the grey box are below the cut-off for positivity. The strength of the correlation was estimated with a Pearson correlation. Red dots indicate participants aged 20–50, while blue dots indicate participants over 50 years of age. P28 = 28 days after completion of the primary vaccination series; B28 = 28 days after booster immunization; PBMCs = peripheral blood mononuclear cells; RBD = receptor-binding domain; ns = not significant.

Figure 6

The amount of SARS-CoV-2 Spike S1-specific memory B cells is comparable to SARS-CoV-2 S1 Omicron-specific memory B cells. SARS-CoV-2 Spike S1- and Spike S1 Omicron BA.1-specific memory B cells are compared within the same samples at P28 ((A), N = 8), at B0 ((B), N = 9), and at B1 ((C), N = 8). Samples within the grey box are below the cut-off for positivity. Differences between groups were investigated with a Mann–Whitney U test. Red dots indicate participants aged 20–50, while blue dots indicate participants over 50 years of age. P28 = 28 days after the primary immunization series; B0 = before booster immunization; B28 = 28 days after booster immunization; PBMCs = peripheral blood mononuclear cells; ns = not significant.