Protein A-antigen conjugates enhance production of specific antibodies following intranasal administration

The objective of our work was to study immunogenic properties of protein A conjugates with the receptor-binding domain (RBD) of the SARS-CoV-2 spike protein (B.1.617.2, Delta variant) and their ability to induce a specific humoral immune response following intranasal administration. Recombinant RBD (SARS-CoV-2, Delta variant) conjugates with Staphylococcus aureus protein A were prepared using EDC or Sulfo-SMCC (1:1 molar ratio) followed by gel filtration purification. Immunization procedure was performed in 80 six-week-old Balb/c mice, divided into groups of 10 animals. Experimental groups received intranasal administration of 20 µL conjugate (50 µg RBD) twice at a 14-day interval, while control groups received intramuscular RBD or saline. Blood was collected 10 days after boosting. Specific IgG titers were determined by ELISA using RBD as the antigen. Statistical significance was assessed using the Mann–Whitney U test (GraphPad Prism 8.0, p < 0.05). Intranasal administration of RBD-protein A conjugates (Con-S and Con-E) induced high specific IgG titers (up to 10⁵), comparable to intramuscular RBD immunization. Both conjugates showed statistically significant enhancement of the immune response compared to intranasal administration of free RBD (p ≤ 0.05). However, the response proved to be heterogenous among animals, with some mice exhibiting a weak immune response, likely due to intranasal delivery variability. RBD-protein A conjugates elicit a robust IgG response upon intranasal administration, comparable to intramuscular immunization, confirming the adjuvant properties of protein A. Both conjugation methods (Sulfo-SMCC and EDC) were equally effective. Despite response variability due to different to mucosal delivery, these findings support the potential of protein A in developing intranasal vaccines against SARS-CoV-2.