A novel chimeric coronavirus spike vaccine combining SARS-CoV-2 RBD and scaffold domains from HKU-1 elicits potent neutralising antibody responses

Sep 28, 2026

SARS-CoV-2 spike receptor binding domain (RBD) is the dominant neutralising antibody target, yet RBD subdomains can limit CD4 T follicular helper (TFH) availability and reduce immunogenicity. A stabilised chimeric trimeric RBD (CTR) glycoprotein was engineered by substituting HKU-1 spike RBD with SARS-CoV-2 RBD, while retaining an HKU-1 scaffold to provide CD4 help. In mice and macaques, CTR designs induced strong RBD-directed IgG and neutralising titres, with germinal centre B cell cross-reactivity and scaffold-directed TFH recognition.

LanguageEnglish

CategoryHealthcare

FormatPDF

Pages14

Reading Time35 min

AccessFree to download

Background

RBD as a neutralising antibody target

Limitations of monomeric or isolated RBD immunogenicity

Vaccine design

Chimeric trimeric RBD (CTR) glycoprotein

Immunogenicity in animal models

Mouse findings

Macaque findings

npj | vaccines Article | Published in partnership with the Sealy Institute for Vaccine Sciences | | | --- | --- | | [https://doi.org/10.1038/s41541-025-01323-6](https://doi.org/10.1038/s41541-025-01323-6) | | | A novel chimeric coronavirus spike vaccine combining SARS-CoV-2 RBD and scaffold domains from HKU-1 elicits potent neutralising antibody responses
Check for updates | | | Veronica P. Zoest1, Wen Shi Lee1, Lydia Murdiyarso1, Lauren Burmas1, Phillip Pymm2, Robyn Esterbauer1, Andrew Kelly1, Hannah G. Kelly1,3, Isaac Barber-Axthelm1,4, James P. Cooney2,5, Kathryn C. Davidson2,5, Merle Dayton2, Courtney E. McAleese6, Marianne Gillard6, Karen Hughes6, Martina L. Jones6,
Marc Pellegrini2,5,7, Wai-Hong Tham2,5,8, Ben Hughes6,9, Stephen J. Kent1,10, Adam K. Wheatley1 , Jennifer A. Juno1 & Hyon-Xhi Tan1 | | The SARS-CoV-2 spike receptor binding domain (RBD) is the major target for neutralising antibodies. However, subdomains like RBD may constrainthe availability of CD4 Tfollicular helper (TFH) cells and impact immunogenicity. We engineered a chimeric trimeric RBD (CTR) glycoprotein, replacing the RBD of HKU-1 spike with SARS-CoV-2 RBD (ancestral WT/Omicron BA.2) . This maintains trimerised RBD, while providing CD4 help via the HKU-1 scaffold. In C57BL/6 mice, CTR-BA.2 elicited high antiBA.2-RBD IgG and neutralising titres, matching native spike responses. Germinal centre B cells were predominantly WT+/BA.2+ cross-reactive, and TFH predominantly recognised HKU-1 epitopes, demonstrating scaffold-directed help. In macaques, CTR-WT elicited comparable anti-RBD IgG, antispike IgG and neutralising responses to native spike, with elevated RBD-speciﬁc GCB cells in draining lymph nodes. Macaque TFH responses targeted RBD, NTD/S2 or HKU-1 peptides. This chimeric design overcomes poor RBD immunogenicity by engaging CD4 TFH, maintaining neutralising responses that is non-inferior to native spike. The SARS-CoV-2spike(S)glycoprotein is the primary immunogen inmost COVID-19 vaccines and is highly effective at eliciting neutralising antibodies. Embedded within the spike protein, the receptor binding domain (RBD) mediates viral engagement with cellular angiotensin-converting enzyme 2 (ACE2) receptors and subsequent cell entry. While neutralising antibodies can target distal regions across the spike, including the N-terminal domain (NTD)1 and S2 domain2, the majority of potent neutralising activity is associated with antibodies that directly block RBD-ACE2 interactions3–6. Vaccines that preferentially focus antibody responses toward theRBDare therefore a potential pathway to maximising protective vaccine efﬁcacy. The RBD subunit itself can be used as a vaccine immunogen7,8. However, many studies have demonstrated that monomeric RBD is poorly immunogenic in pre-clinical models, especially compared to full-length spike9–12. We have linked this poor immunogenicity, in part, to suboptimal levels of CD4 T cell help13. To augment the low immunogenicity of small protein subdomains while concurrently minimising repetitive boosting of non-neutralising epitopes, vaccine antigens can be embedded into scaffolds 1Department of Microbiology and Immunology, University of Melbourne, Peter Doherty Institute for Infection and Immunity, Melbourne, VIC, Australia. 2The Walter and Eliza Hall Institute of Medical Research, Parkville, VIC, Australia. 3Department of Immunology and Infectious Disease, John Curtin School of Medical Research, The Australian National University, Canberra, ACT, Australia. 4Department of Comparative Medicine, University of Washington, Seattle, WA, USA. 5Department of Medical Biology, University of Melbourne, Melbourne, VIC, Australia. 6National Biologics Facility, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, QLD, Australia. 7Centenary Institute, The University of Sydney, Camperdown, NSW, Australia. 8Research School of Biology, The Australian National University, Ca

FAQs

Why focus vaccine designs on the SARS-CoV-2 RBD?

RBD antibodies are largely responsible for potent neutralisation by blocking RBD–ACE2 interactions. Focusing responses on the RBD can improve protective vaccine efficacy.

What problem does the HKU-1 scaffold address?

Isolated RBD subunits can be poorly immunogenic due to suboptimal CD4 T cell help. The HKU-1 scaffold is used to engage CD4 TFH cells and support the RBD-specific response.

How do the CTR vaccines perform in preclinical models?

CTR designs elicited high anti-RBD IgG and neutralising titres in C57BL/6 mice, and comparable RBD- and spike-specific IgG and neutralising responses in macaques, with elevated RBD-targeted germinal centre B cells.

Sep 28, 2026

Uploaded by Alex Sinclair