The SARS-CoV-2 receptor-binding domain expressed in Pichia pastoris as a candidate vaccine antigen

The effort to develop vaccines based on economically accessible technological platforms available by developing countries vaccine manufacturers is essential to extend the immunization to the whole world population and to achieve the desired herd immunity, necessary to end the COVID-19 pandemic. Here we report on the development of a SARS-CoV-2 receptor-binding domain (RBD) protein, expressed in yeast Pichia pastoris. The RBD was modified with the addition of flexible N- and C-terminal amino acid extensions aimed to modulate the protein/protein interactions and facilitate protein purification. Fermentation with yeast extract culture medium yielded 30-40 mg/L. After purification by immobilized metal ion affinity chromatography and hydrophobic interaction chromatography, the RBD protein was characterized by mass-spectrometry, circular dichroism, and binding affinity to angiotensin-converting enzyme 2 (ACE2) receptor. The recombinant protein shows high antigenicity with convalescent human sera and also with sera from individuals vaccinated with the Pfizer-BioNTech mRNA or Sputnik V adenoviral-based vaccines. The RBD protein stimulates IFN{gamma}, IL-2, IL-6, IL-4, and TNF in mice secreting splenocytes from PBMC and lung, CD3+ enriched cells. Immunogenicity studies with 50 g of the recombinant RBD formulated with alum, induce high levels of binding antibodies in mice and non-human primates, assessed by ELISA plates covered with RBD protein expressed in HEK293T cells. The mouse sera inhibited the RBD binding to ACE2 receptor in an in-vitro test and show neutralization of SARS-CoV-2 infection of Vero E6 cells. These data suggest that the RBD recombinant protein expressed in yeast P. pastoris is suitable as a vaccine candidate against COVID-19.