The Receptor-Binding Domain (RBD) of the Spike (S) protein from Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) has glycosylation sites which can limit the production of reliable antigens expressed in prokaryotic platforms, due to glycan-mediated evasion of the host immune response. However, protein regions without glycosylated residues capable of inducing neutralizing antibodies could be useful for antigen production in systems that do not carry the glycosylation machinery. To test this hypothesis, the potential antigens NG06 and NG19, located within the non-glycosylated S-RBD region, were selected and expressed in Escherichia coli, purified by FPLC and employed to determine their immunogenic potential through detection of antibodies in serum from immunized rabbits, mice, and COVID-19 patients. IgG antibodies from sera of COVID-19-recovered patients detected the recombinant antigens NG06 and NG19 (A450 nm = 0.80 ± 0.33; 1.13 ± 0.33; and 0.11 ± 0.08 for and negatives controls, respectively). Also, the purified antigens were able to raise polyclonal antibodies in animal models evoking a strong immune response with neutralizing activity in mice model. This research highlights the usefulness of antigens based on the non-N-glycosylated region of RBD from SARS-CoV-2 for candidate vaccine development.

The pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has affected millions of people worldwide. Public health strategies to reduce viral transmission are based on widespread diagnostic testing to detect and isolate contagious patients. Several reverse transcription (RT)-PCR tests, along with other SARS-CoV-2 diagnostic assays, are available to attempt to cover the global demand. Loop-mediated isothermal amplification (LAMP) based methods have been established as rapid, accurate, point of care diagnostic tests for viral infections; hence, they represent an excellent alternative for SARS-CoV-2 detection. The aim of this study was to develop and describe molecular detection systems for SARS-CoV-2 based on RT-LAMP. Recombinant DNA polymerase from Bacillus stearothermophilus and thermostable engineered reverse transcriptase from Moloney Murine Leukemia Virus were expressed using a prokaryotic system and purified by fast protein liquid chromatography. These enzymes were used to set up fluorometric real time and colorimetric end-point RT-LAMP assays. Several reaction conditions were optimized such as reaction temperature, Tris-HCl concentration, and pH of the diagnostic tests. The key enzymes for RT-LAMP were purified and their enzymatic activity was determined. Standardized reaction conditions for both RT-LAMP assays were 65°C and a Tris-HCl-free buffer at pH 8.8. Colorimetric end-point RT-LAMP assay was successfully used for viral detection from clinical saliva samples with 100% sensitivity and 100% specificity compared to the results obtained by RT-qPCR based diagnostic protocols with Ct values until 30. The developed RT-LAMP diagnostic tests based on purified recombinant enzymes allowed a sensitive and specific detection of the nucleocapsid gene of SARS-CoV-2.