Syngeneic homograft of framework regions enhances the affinity of the mouse anti-human epidermal receptor 2 single-chain antibody e23sFv

e23sFv is a HER2-targeted single-chain variable fragment (scFV) that was characterized as the targeting portion of a HER2-targeted tumour proapoptotic molecule in our previous study. In vitro antibody affinity maturation is a method to enhance antibody affinity either by complementarity-determining region (CDR) mutagenesis or by framework region (FR) engraftment. In the present study, the affinity of e23sFv was enhanced using two strategies. In one approach, site-directed mutations were introduced into the FRs of e23sFv (designated EMEY), and in the other approach e23sFv FRs were substituted with FRs from the most homologous screened antibodies (designated EX1 and EX2). Notably, EX1 derived from the FR engraftment strategy demonstrated a 4-fold higher affinity for HER2 compared with e23sFv and was internalized into HER2-overexpressing cells; however, EMEY and EX2 exhibited reduced affinity for HER2 and decreased internalization potential compared with EX1. The 3D structure of EX1 and the HER2-EX1 complex was acquired using molecular homology modelling and docking and the HER2 epitopes of EX1 and the molecular interaction energy of the EX1-HER2 complex were predicted. In the present study, it was demonstrated that scFv affinity improvement based on sequence alignment was feasible and effective. Moreover, the FR grafting strategy was indicated to be more effective and simple compared with site-directed mutagenesis to improve e23sFv affinity. In conclusion, it was indicated that the affinity-improved candidate EX1 may present a great potential for the diagnosis and treatment of HER2-overexpressing tumours.