Optimization of Soluble Expression of CTA1-(S14P5)4-DD and CTA1-(S21P2)4-DD Fusion Proteins as Candidates for COVID-19 Intranasal Vaccines

Developing intranasal vaccines against pandemics and devastating airborne infectious diseases is imperative. The superiority of intranasal vaccines over injectable systemic vaccines is evident, but the challenge in developing effective intranasal vaccines is more substantial. Fusing a protein antigen with the catalytic domain of cholera toxin (CTA1) and the two-domain D of staphylococcal protein A (DD) has significant potential for intranasal vaccines. In the present study, we constructed two fusion proteins containing CTA1, tandem repeat linear epitopes of the SARS-CoV-2 spike protein (S14P5 or S21P2), and DD. The in silico characteristics and solubility of the fusion proteins CTA1-(S14P5)4-DD and CTA1-(S21P2)4-DD were analyzed when overexpressed in Escherichia coli. Structural predictions indicated that each component of the fusion proteins was compatible with its origin. Both fusion proteins were predicted by computational tools to be soluble when overexpressed in E. coli. Contrary to these predictions, the constructs exhibited limited solubility. The solubility did not improve even after lowering the cultivation temperature from 37{degrees}C to 18{degrees}C. Induction with IPTG at the early log phase, instead of the usual mid-log phase growth, significantly increased soluble CTA1-(S21P2)4-DD but not CTA1-(S14P5)4-DD. The solubility of overexpressed fusion proteins significantly increased when a non-denaturing detergent (Nonidet P40, Triton X100, or Tween 20) was added to the extraction buffer. In a scale-up purification experiment, the yields were low, only 1-2 mg/L of culture, due to substantial losses during the purification stages, indicating the need for further optimization of the purification process.