The SARS-CoV-2 spike protein S2 subunit plays an essential role in the virus-host cell membrane fusion process. Therefore, the subject of this study was to characterize the gamma-immunoglobulin (IgG) response, in a group of COVID-19 convalescent patients, against the S2 subunit with eight linear peptides to generate a monoclonal antibody (mAb) against the immunodominant linear peptide to be used for therapeutic and diagnostic purposes. Results of antibody percentages against assessed linear peptides were 100% for A21P73, A21P74, A21P75, A21P76, M20P51, M20P65, M20P83, and 66.7% for M20P85. Plasma samples were also used for purifying IgG to corroborate specificity against the same linear peptides, where results reproduced those applying plasmas directly to ELISA-plates. Within these peptides, A21P75 was chosen as immunodominant (100% of recognition with higher absorbance). The A21P75 linear peptide showed poor immunogenicity in mice (1:4000–8000 after four doses), allowing the generation of a CB.HS2A21P75 hybridoma for mAb production that recognized the A21P75 linear peptide with middle-to-high affinity constant (Kaff) (0.8×108 M-1).
This study concludes that the A21P75 linear peptide is the assessed immunodominant linear peptide for this COVID-19 convalescent patient group. This peptide is located in the HR1 region that plays an important role in SARS-CoV-2 host cell membrane fusion process and is highly conserved between SARS-CoV-2 and SARS-CoV. Thus, due to CB.S2A21P75 mAb specificity and Kaff, it might be the proper reagent to study inhibition of virus-host cell membrane fusion, and as a diagnostic reagent for coronavirus. Finally, the combination of A21P75 linear peptide with other peptides (e.g., receptor binding domain [RBD]) could be suitable reagents for the development of vaccines and therapeutic antibodies with virus infection-blocking capacity.