Repeated COVID-19 mRNA vaccination results in IgG4 class switching and decreased NK cell activation by S1-specific antibodies in older adults

Abstract

Background: Previous research has shown that repeated COVID-19 mRNA vaccination leads to a marked increase of SARS-CoV-2 spike-specific serum antibodies of the IgG4 subclass, indicating far-reaching immunoglobulin class switching after booster immunization. Considering that repeated vaccination has been recommended especially for older adults, the aim of this study was to investigate IgG subclass responses in the ageing population and assess their relation with Fc-mediated antibody effector functionality.

Results: Spike S1-specific IgG subclass concentrations (expressed in arbitrary units per mL), antibody-dependent NK cell activation, complement deposition and monocyte phagocytosis were quantified in serum from older adults (n = 38-50, 65-83 years) at one month post-second, -third and -fifth vaccination. Subclass distribution in serum was compared to that in younger adults (n = 64, 18-47 years) at one month post-second and -third vaccination. Compared to younger individuals, older adults showed increased levels of IgG2 and IgG4 at one month post-third vaccination (possibly related to factors other than age) and a further increase following a fifth dose. The capacity of specific serum antibodies to mediate NK cell activation and complement deposition relative to S1-specific total IgG concentrations decreased upon repeated vaccination. This decrease associated with an increased IgG4/IgG1 ratio.

Conclusions: In conclusion, these findings show that, like younger individuals, older adults produce antibodies with reduced functional capacity upon repeated COVID-19 mRNA vaccination. Additional research is needed to better understand the mechanisms underlying these responses and their potential implications for vaccine effectiveness. Such knowledge is vital for the future design of optimal vaccination strategies in the ageing population.

Keywords: Ageing; Booster; Complement; Fc-effector functions; IgG subclasses.

Fig. 1

SARS-CoV-2 spike S1-specific total IgG levels in younger and older adults following mRNA vaccination. A Schematic overview of the vaccination and sampling scheme in which participants received original monovalent (1st to 4th dose) and bivalent (5th dose) mRNA vaccines. SARS-CoV-2 spike S1-specific total IgG concentrations (BAU/mL) following mRNA vaccination were measured in serum by multiplex immunoassay for B older adults (n = 50) up to one month after the fifth vaccination and for C younger (n = 64) and older (n = 50) adults up to approximately one month after the third vaccination. Dotted lines indicate the cut-off for seropositivity. SARS-CoV-2-infected individuals (based on N seropositivity) are indicated with open circles. Differences between groups were assessed using a non-parametric (unpaired) Wilcoxon signed rank test with Bonferroni’s correction for multiple testing. *** P < 0.001; **** P < 0.0001. BAU, binding antibody units; N, nucleoprotein; ns, not significant

Fig. 2

SARS-CoV-2 spike S1-specific IgG subclasses in older adults following repeated mRNA vaccinations. SARS-CoV-2 spike S1-specific IgG1, IgG2, IgG3 and IgG4 concentrations (AU/mL) following mRNA vaccination were measured in serum by multiplex immunoassay for older adults (n = 50) up to one month after the fifth vaccination. Dotted lines indicate the cut-off for seropositivity. SARS-CoV-2-infected individuals (based on N seropositivity) are indicated with open circles. Differences between groups were assessed using a non-parametric Wilcoxon signed rank test with Bonferroni’s correction for multiple testing. ** P < 0.01, **** P < 0.0001. AU, arbitrary units; N, nucleoprotein

Fig. 3

S1-specific total IgG and IgG subclasses in older adults with and without SARS-CoV-2 infection prior to the start of COVID-19 vaccination. SARS-CoV-2 spike S1-specific total IgG, IgG1, IgG2, IgG3 and IgG4 concentrations following the fifth mRNA vaccination were measured in serum by multiplex immunoassay for older adults with (n = 4) and without (n = 50) SARS-CoV-2 infection prior to the start of vaccination. Dotted lines indicate the cut-off for seropositivity. Differences between groups were assessed using a non-parametric Wilcoxon signed rank test with Bonferroni’s correction for multiple testing. * P < 0.05. AU, arbitrary units; BAU, binding antibody units

Fig. 4

Comparison of SARS-CoV-2 spike S1-specific IgG subclasses following mRNA vaccination between younger and older adults. SARS-CoV-2 spike S1-specific IgG1, IgG2, IgG3 and IgG4 concentrations (AU/mL) following mRNA vaccination were measured in serum by multiplex immunoassay for younger (n = 64) and older (n = 50) adults up to one month after the third vaccination. Dotted lines indicate the cut-off for seropositivity. SARS-CoV-2-infected individuals (based on N seropositivity) are indicated with open circles. Differences between groups were assessed using a non-parametric unpaired Wilcoxon signed rank test with Bonferroni’s correction for multiple testing. **** P < 0.0001. AU, arbitrary units; N, nucleoprotein; ns, not significant

Fig. 5

SARS-CoV-2 spike S1-specific Fc-mediated effector functions upon repeated vaccination in older adults. The absolute capacity (A) and the capacity relative to S1-specific total IgG levels (B) of SARS-CoV-2 spike S1-specific serum antibodies for mediating ADNKA, ADCD and monocyte ADCP following mRNA vaccination in older adults (n = 38) up to one month after the fifth vaccination. Dotted lines indicate the level of the negative control (pre-pandemic serum sample). Solid lines were made using locally estimated scatterplot smoothing (LOESS). SARS-CoV-2-infected individuals (based on N seropositivity) are indicated with open circles. Differences between groups were assessed using a non-parametric Wilcoxon signed rank test with Bonferroni’s correction for multiple testing. *** P < 0.001; **** P < 0.0001. ADCD, antibody-dependent complement deposition; ADCP, antibody-dependent cellular phagocytosis; ADNKA, antibody-dependent natural killer cell activation; BAU, binding antibody units; iMFI, integrated median fluorescence intensity; N, nucleoprotein

Fig. 6

Association between Fc-mediated effector functions and IgG4/IgG1 ratios or IgG avidity in older adults. The capacity of spike S1-specific serum antibodies to mediate ADNKA, ADCD and monocyte ADCP following mRNA vaccination in older adults (n = 38) at approximately 1 month after the third and fifth vaccinations relative to S1-specific total IgG levels at the same timepoint. Color scales indicate the IgG4/IgG1 ratio (A) or avidity index (B) of the SARS-CoV-2 spike S1-specific antibodies. Dotted lines indicate the level of the negative control (pre-pandemic serum sample). Solid grey lines were made using linear regression (straight lines) or locally estimated scatterplot smoothing (LOESS). SARS-CoV-2-infected individuals (based on N seropositivity) are indicated with open circles. ADCD, antibody-dependent complement deposition; ADCP, antibody-dependent cellular phagocytosis; ADNKA, antibody-dependent natural killer cell activation; BAU, binding antibody units; iMFI, integrated median fluorescence intensity; N, nucleoprotein