Science Bite (3 minute oral presentation with PPT in live session and poster) - Students, ECRs and EMCRs only 16th Lorne Infection and Immunity 2026

Characterising the influence of bispecific IgG4 on SARS-CoV-2 vaccine-induced Fc effector functions (132104)

Carissa Aurelia 1 , Robyn Esterbauer 1 , Andrew Kelly 1 , Simone Richardson 2 3 , Jennifer Juno 1 , Oanh Nguyen 1 , Katherine Kedzierska 1 , Adam Wheatley 1 , Stephen Kent 1 4 , Kevin Selva 1 , Amy Chung 1
  1. The Peter Doherty Institute for Infection and Immunity, University of Melbourne, Melbourne, Australia
  2. Centre for HIV and STIs, National Institute for Communicable Diseases of the National Health Laboratory Service, Johannesburg, South Africa
  3. South African Medical Research Council Antibody Immunity Research Unit, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa
  4. Melbourne Sexual Health Centre, Alfred Health, Monash University, Melbourne, Australia

Repeated COVID-19 mRNA vaccination (>3 doses) induces elevated levels of anti-spike IgG4. We have previously demonstrated using anti-spike IgG4 monoclonal antibodies (mAbs) that elevated IgG4 can reduce vaccine-induced antibody-dependent cellular cytotoxicity (ADCC). As Fc functions help maintain protection against SARS-CoV-2, this has raised concerns regarding reduced vaccine effectiveness. However, IgG4 possesses the unique ability to undergo Fab arm exchange, forming half-molecules which randomly recombine, so that the majority of IgG4 in vivo is thought to be bispecific. Currently, studies examining the influence of SARS-CoV-2 bispecific IgG4 on vaccine-induced Fc-mediated responses are limited.

To address this, we generated a panel of bispecific IgG4 mAbs targeting SARS-CoV-2 spike and an irrelevant antigen (influenza hemagglutinin) using controlled Fab arm exchange. Bispecificity was confirmed by multiplex and biolayer interferometry assays. To assess the influence of bispecific IgG4 on vaccine-induced antibody functions, a cocktail of bispecific or monospecific IgG4 was added into post-dose 2 vaccine plasma samples. Bispecific IgG4 significantly reduced ancestral spike-specific FcγRIIIa binding (1.4-fold, p<0.01), the main Fcγ Receptor involved in ADCC, however, not to the same extent as monospecific IgG4 addition (1.8-fold decrease, p<0.001). These differences were also confirmed with ADCC functional assays. Likewise, bispecific IgG4 resulted in smaller reductions in FcγRIIIa binding against Omicron subvariants compared to monospecific IgG4 (p<0.0001). Moreover, bispecific IgG4 displayed reduced ability to bind all tested SARS-CoV-2 variants (1.6-2.3-fold decrease) compared to monospecific anti-spike IgG4, contributing to less competition with more functional IgG subclasses (IgG1 and IgG3).

Collectively, our study characterises the influence of bispecific IgG4 on vaccine-induced IgG subclass and ADCC responses and suggests that elevated bispecific IgG4 is less detrimental for ADCC compared to monospecific IgG4. Ongoing studies will further characterise the influence of bispecific IgG4 on other antibody functions to provide a more physiologically relevant understanding on the role of IgG4 on antibody-mediated protection.