A systematic mutational analysis identifies a 5-residue proline tag that enhances the in vivo immunogenicity of a non-immunogenic model protein

Poor immunogenicity of small proteins is a major hurdle in developing vaccines or producing antibodies for biopharmaceutical usage. Here, we systematically analyzed the effects of 10 solubility controlling peptide tags (SCP-tags) on the immunogenicity of a non-immunogenic model protein, bovine pancreatic trypsin inhibitor (BPTI-19A; 6 kDa). CD, fluorescence, DLS, SLS, and AUC measurements indicated that the SCP-tags did not change the secondary structure content nor the tertiary structures of the protein nor its monomeric state. ELISA results indicated that the 5-proline (C5P) and 5-arginine (C5R) tags unexpectedly increased the IgG level of BPTI-19A by 240- and 73-fold, respectively, suggesting that non-oligomerizing SCP-tags may provide a novel method for increasing the immunogenicity of a protein in a highly specific manner.