A recombinant SARS-CoV-2 RBD antigen expressed in insect cells elicits immunogenicity and confirms safety in animal models

COVID-19 pandemic has accelerated the development of vaccines against its etiologic agent, SARS-CoV-2. However, the emergence of new variants of the virus requires new immunization strategies in addition to the current vaccines approved for human administration. In the present report, the immunological and safety evaluation in mice and hamsters of a subunit vaccine based on the RBD sub-domain with two adjuvants of oil origin is described. The RBD protein was expressed in insect cells and purified by chromatography until >95% purity. The protein was shown to have the appropriate folding as determined by ELISA and flow cytometry binding assays to its receptor, as well as by its detection by hamster immune anti-S1 sera under non-reducing conditions. In immunization assays in mice and hamsters, the purified RBD formulated with adjuvants based on oil-water emulsifications and squalene was able to stimulate specific neutralizing antibodies and confirm the secretion of IFN-γ after stimulating spleen cells with the purified RBD. The vaccine candidate was shown to be safe, as demonstrated by the histopathological analysis in lungs, liver and kidney. These results demonstrate the potential of the purified RBD administered with adjuvants through an intramuscular route, to be evaluated in a challenge against SARS-CoV-2 and determine its ability to confer protection against infection.