mRNA vaccines encoding membrane-anchored receptor-binding domains of SARS-CoV-2 mutants induce strong humoral responses and can overcome immune imprinting (preprint)

To address the limitations of whole-spike COVID vaccines, we explored mRNA vaccines encoding membrane-anchored receptor-binding domain (RBD-TMs), each a fusion of a variant RBD, the transmembrane (TM) and cytoplasmic tail (CT) fragments of the SARS-CoV-2 spike protein. In naive mice, RBD-TM mRNA vaccines against ancestral SARS-CoV-2, Beta, Delta, Delta-plus, Kappa, Omicron BA.1 or BA.5, all induced strong humoral responses against the target RBD. Multiplex surrogate viral neutralization (sVNT) assays indicated broad neutralizing activity against a range of variant RBDs. In the setting of a heterologous boost, against the background of exposure to ancestral whole spike vaccines, sVNT studies suggested that RBD-TM vaccines were able to overcome the detrimental effects of immune imprinting. Omicron BA.1 and BA.5 RBD-TM booster vaccines induced serum antibodies with 12 and 22-fold higher neutralizing activity against the target RBD than their equivalent whole spike variants. Boosting with BA.1 or BA.5 RBD-TM provided good protection against more recent variants including XBB and XBB.1.5. Each RBD-TM mRNA is 28% of the length of its whole-spike equivalent. This advantage will enable tetravalent mRNA vaccines to be developed at well-tolerated doses of formulated mRNA.

Broad immunity to SARS-CoV-2 variants of concern mediated by a SARS-CoV-2 receptor-binding domain protein vaccine (preprint)

The SARS-CoV-2 global pandemic has fuelled the generation of vaccines at an unprecedented pace and scale. However, many challenges remain, including: the emergence of vaccine-resistant mutant viruses, vaccine stability during storage and transport, waning vaccine-induced immunity, and concerns about infrequent adverse events associated with existing vaccines. Here, we report on a protein subunit vaccine comprising the receptor-binding domain (RBD) of the ancestral SARS-CoV-2 spike protein, dimerised with an immunoglobulin IgG1 Fc domain. These were tested in conjunction with three different adjuvants: a TLR2 agonist R4-Pam2Cys, an NKT cell agonist glycolipid alpha-Galactosylceramide, or MF59 squalene oil-in-water adjuvant. Each formulation drove strong neutralising antibody (nAb) responses and provided durable and highly protective immunity against lower and upper airway infection in mouse models of COVID-19. We have also developed an RBD-human IgG1 Fc vaccine with an RBD sequence of the highly immunoevasive beta variant (N501Y, E484K, K417N). This beta variant RBD vaccine, combined with MF59 adjuvant, induced strong protection in mice against the beta strain as well as the ancestral strain. Furthermore, when used as a third dose booster vaccine following priming with whole spike vaccine, anti-sera from beta-RBD-Fc immunised mice increased titres of nAb against other variants including alpha, delta, delta+, gamma, lambda, mu, and omicron BA.1 and BA.2. These results demonstrated that an RBD-Fc protein subunit/MF59 adjuvanted vaccine can induce high levels of broad nAbs, including when used as a booster following prior immunisation of mice with whole ancestral-strain Spike vaccines. This vaccine platform offers a potential approach to augment some of the currently approved vaccines in the face of emerging variants of concern, and it has now entered a phase I clinical trial.