Expression and neutralization capacity of single domain HIV antibody fragments
The discovery of broad and potent neutralizing HIV antibodies (bNAbs) has opened up new opportunities of passive immunization for HIV-1 prevention. In this study, we have engineered CAP256-VRC26.25, a V1V2 bNAb that neutralizes 70% of clade C viruses, as a single domain antibody (sdAb). These small antigen binding entities are derived from naturally occurring heavy chain only antibodies present in members of the dromedary families, and are characterized by the absence of a light chain, long complementarity-determining regions (CDR) heavy (H) chain 3 and high stability. Since CAP256.25 contains a highly charged and protruding CDR-H3 that binds mainly through its heavy chain, we hypothesized that it may function well as an sdAb. Multiple camelization approaches to engineer CAP256.25 as a sdAb were tested in silico utilising structural modelling software. Parameters such as germline sequence homology, hydrophobicity and solubility, folding energy, torsion angles and native conformation of CAP256.25 in complex with its binding epitope were major factors considered during the modelling process. Four CAP256.25 sdAb derivatives were generated from parental antibody, the mut_0 or a wild type (WT), which was used as a base line for downstream optimization. CAP256.25 mut_4 in which residues involved in LC interactions were replaced with residues strongly conserved in camel sdAbs, which minimize hydrophobic interface of the sdAb. Mut_8 variant, which included four additional substitutions to increase solubility and mut_9 contained a single additional mutation at the base of CDR-H3 to improve the energetic landscape of sdAb. All genes were synthesized and sub-cloned into a mammalian expression vector and recombinant proteins expressed in HEK293T cell line, and purified by Immobilized Metal Ion Affinity Chromatography (IMAC) and Fast Protein Liquid Chromatography (FPLC). CAP256.25_mut0 expression was below the detectable level and whilst mut_4 expressed at low levels, it showed no neutralization activity. CAP256.25 sdAb mut_8 and mut_9 expressed at significantly lower levels compared to m36, a previously described sdAb used a positive control. Nevertheless CAP256.25mut_8 sdAb showed neutralization capability although it lost significant potency in comparison to the parental antibody, yet still within the therapeutic window of the VRC01 bNAb. Importantly, CAP256.25 sdAb was unable to neutralize the K169E mutant confirming that it retained specificity for the V2 epitope. These data suggest that camelization of human antibodies is possible although further engineering is required to increase expression and improve stability. As such, sdAb engineering could be an encouraging step for the generation of small antigen binding fragments for future therapeutic purposes including topical delivery at mucosal surfaces, to interrupt or block sexual transmission of HIV.
A dissertation submitted to the Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Medicine in the specialty of Virology, June 2018