Structural Design and Immunogenicity of a Novel Self-Adjuvanting Mucosal Vaccine Candidate for SARS-CoV-2 Expressed in Plants

Mucosal vaccination for COVID-19 to boost preexisting though insufficient systemic and local/mucosal immunity remains an attractive prospect but there are currently no licensed mucosal vaccines against this infection. Here, using a plant expression system, we developed a novel mucosal vaccine platform for respiratory viruses and demonstrated its application in the context of SARS-CoV-2 infection. In addition to the antigen itself, the PCF (Platform CTB-Fc) vaccine candidate incorporates two molecular adjuvants, the IgG-Fc antibody fragment and the nontoxic cholera toxin B subunit (CTB), with the first targeting the vaccine to IgG receptors on antigen-presenting cells, and the second providing local adjuvanticity by targeting cellular gangliosides in the mucosae. We demonstrated that this vaccine candidate is highly immunogenic in mice, inducing virus-neutralising systemic and mucosal antibodies as well as tissue resident memory T cells in the lungs. We also demonstrated that SRBD-PCF is recognised by immune cells from exposed or vaccinated individuals, and that circulating antibodies also bind to the antigen within the vaccine, forming immune complexes (IC). Finally, with a view of respiratory delivery, we demonstrated that the vaccine can be aerosolised without loss of material or biological activity, and that it is noncytotoxic and nonhaemolytic to human cells. Furthermore, we demonstrate that the plant expression system represents a suitable platform to produce these complex, multifunctional macromolecules capable of simultaneously binding to multiple targets. Our data strongly support the case for a safe, self-adjuvanting mucosal COVID-19 vaccine development, as means to boosting both systemic and mucosal immunity.