Chimeric Coronavirus Spike Combining SARS-CoV-2 RBD and Scaffold Domains from HKU-1 Elicits Potent T Helper Cell and Neutralising Antibody Responses in Murine and Non-Human Primate Models 4642

The receptor binding domain (RBD) of the SARS-CoV-2 spike is the major target for neutralising antibodies elicited by current vaccines. However, RBD subunit vaccines are poorly immunogenic, due to insufficient CD4 T follicular helper (TFH) cell epitopes, consequently driving poor germinal centre (GC) responses. We engineered a novel chimeric trimer-RBD (CTR) glycoprotein, replacing the RBD of human coronavirus HKU-1 spike with SARS-CoV-2 RBD (Wuhan-Hu-1 or BA.2). This strategy leverages HKU-1 scaffold CD4 epitopes while preserving SARS-CoV-2 RBD neutralising targets, with antigenicity confirmed by monoclonal antibodies. In C57BL/6 mice, CTR-BA.2 prime-boost vaccination elicited high anti-BA.2-RBD IgG titres (∼104) and BA.2 neutralisation (IC50: 4.0x102), matching native spike responses. GC B cells elicited by CTR-BA.2 were predominantly Wuhan-Hu-1+/BA.2+ cross-reactive, while 25.3% of TFH cells, measured by CD154+, recognised HKU-1 epitopes, demonstrating scaffold-directed T cell help. Macaques prime-boost immunised with CTR-Wuhan-Hu-1 elicited high anti-RBD IgG (∼104) and neutralising responses (IC50: 7.5x102), comparable to native spike (n = 5/group). RBD-specific GC B cells (BCL-6+CD95+) were present at elevated levels in draining lymph nodes of native (1.94%) or CTR spike-vaccinated (1.62%) animals vs non-draining sites. Our findings establish the chimeric glycoprotein design as a strategy to overcome the poor immunogenicity of the SARS-CoV-2 RBD by engaging CD4 TFH cells. Supported by the National Health and Medical Research Council (Australia) (2009711, 2026762, 2009308) and Medical Research Future Fund (Australia) (2013870). Vaccines and Immunotherapy (VAC)