Enhancement of antigen-specific humoral immune responses and protein solubility through conjugation of bacterial flagellin, Vibrio vulnificus FlaB, to the N-terminus of porcine epidemic diarrhea virus surface protein antigen S0

Oh, Seo-ho; Kim Cho, Young-Saeng; Lee, Ho-Bin; Lee, Sang-Mok; Kim, Whee-Soo; Hong, Liang; Cho, Chong-Su; Choi, Yun-Jaie; Kang, Sang-Kee

Flagellin has been frequently used as a molecular adjuvant in conjugation with various antigens [11] [12] [13] [14] ; however, the adjuvant effect of flagellin varies depending on the target antigen or the position of its conjugation [13, 16] . In this study, we conjugated flagellin to the N-or C-terminus of PEDV antigen S0 and evaluated its ability to enhance both solubility and the antigen-specific humoral immune response. Expression of recombinant proteins in an E. coli system has many advantages in terms of cost, yield and scalability; however, expression in E. coli often leads proteins of interest to form biologically inactive aggregates known as inclusion bodies (IBs) [23] . Despite enclosing neutralizing epitopes, PED subunit vaccine candidate antigens, including the S0 (25-229 amino acid), S1D (636-789 amino acid) and COE (499-638 amino acid) regions of the PEDV spike protein, are prone to be expressed as IBs [8, 9] . Although IBs can be refolded by several methods, it is better to express recombinant proteins in a soluble form and prevent IB formation in the first place [23] [24] [25] . There are various approaches to enhancing the solubility of recombinant proteins, including conjugation of a solubility-enhancing fusion partner protein or optimization of protein expression by adjusting the physiological condition of the host cells [23, 26, 27 ]. Here, we tested whether a flagellin fusion could enhance the solubility of the recombinant PEDV antigen S0 in the presence or absence of the trigger factor chaperone tig.