Increasing the melting temperature of VHH with the in silico free energy score

Abstract

VHH, the antigen-binding fragment of a heavy chain-only antibody, is a useful component of antibody-based therapeutics. Thermal stability, represented by the melting temperature (Tm), is one of the key factors affecting the developability of antibody-based therapeutics. In this study, we examined whether the in silico free energy score dStability can be used to design mutants with improved Tm compared to the anti-lysozyme VHH, D3-L11. After verifying that exhaustive mutagenesis was inefficient for improving Tm, we performed a two-round rational approach that combined dStability calculations with a small number of experiments. This method improved the Tm by more than 5 °C in several single mutants including A79I. It reduced the affinity for the antigen by less than 1.6-fold. We speculate that stabilization of A79I required exquisite compatibility among neighboring residues to fill in the internal cavity in the protein. Given that we identified only one mutation that could simultaneously improve Tm and almost maintain affinity, we concluded that achieving both is extremely difficult, even with single mutations that are not located in the paratope. Therefore, we recommend using a variety of approaches when trying to achieve such a feat. Our method will be a useful complementary approach to other existing methods.