Elevated SARS-CoV-2 IgG4 in plasma and mucosa following repeated mRNA boosters impact antibody functions to Omicron and sarbecoviruses.

BACKGROUND: Elevated SARS-CoV-2 IgG4 levels following repeated COVID-19 mRNA boosters may impact blood and mucosal antibody functions against Omicron variants and sarbecoviruses. This study characterised the rise of IgG4 in blood and saliva following four consecutive COVID-19 mRNA doses and examined how IgG4 modulated neutralising and non-neutralising responses. METHODS: Paired plasma and saliva samples were collected pre- and post-mRNA boosters from homologous mRNA vaccinees (2 primary mRNA + 2 mRNA booster) or adenovirus-vector primed vaccinees (2 Vaxzevria + 4 mRNA booster). The breadth and proportion of IgG4 responses towards Omicron variants and sarbecoviruses were measured using multiplex bead arrays. Epitope blocking and depletion assays assessed immune imprinting. Live virus assays established IgG4's contribution towards neutralisation. FINDINGS: Plasma and salivary IgG4 antibodies binding Omicron variants and sarbecoviruses increased post-booster across both cohorts. Although non-neutralising responses significantly improved post-booster, they negatively correlated with IgG4 against all variants tested. Immune imprinting biased IgG4 responses towards the ancestral receptor binding motif, limiting neutralising IgG4 antibodies towards Omicron variants. Increased proportions of class-switched IgG4 also corresponded with decreased concentrations of spike-specific IgG1, particularly against the RBD, dampening neutralisation. INTERPRETATION: Repeated COVID-19 mRNA boosters improved neutralising and non-neutralising activity across viral variants for both cohorts, though these responses were dampened by increasing IgG4 antibodies. This work emphasises the potential longitudinal effects of booster-induced IgG4 subclass switching. FUNDING: This study was supported by the VC2 Research Fund and an NHMRC Investigator grant #2008092. Sample collection was supported by WHO Unity funds (2020/1085469-0) and WEHI Philanthropic Funds.