Remodeling Antibodies from the Inside Out: Functional Engineering of Full-Length Antibodies in the Cytoplasm of Bacteria

Abstract

Antibodies are indispensable tools in many research, diagnostic, and clinical applications. Current methods for producing immunoglobulin G (IgG) antibodies in engineered cells often require refolding steps or secretion across one or more biological membranes. In this work, we describe a robust expression platform for biosynthesis of full-length IgG antibodies in the Escherichia coli cytoplasm. IgGs with clinically relevant antigen- and effector-binding activities are readily produced in the E. coli cytoplasm by grafting antigen-specific variable heavy and light domains into a cytoplasmically stable framework and remodeling the fragment crystallizable domain with amino acid substitutions that promote binding to Fcγ receptors. The resulting cytoplasmic IgGs —named “cyclonals”— effectively bypass the potentially rate-limiting steps of membrane translocation and glycosylation. Standard antibody discovery techniques often require multiple labor intensive and technically challenging steps that limit the pace at which valuable antibodies can be developed. We addressed this challenge by adapting the cyclonal platform for identification of specific antibody-antigen binding by implementing two different survival selections. Finally, we describe isolation of high-affinity full-length IgGs from combinatorial libraries after just a single round of selection. Taken together, these results show that our E. coli-based platform constitutes a simple yet powerful alternative for rapidly engineering full-length IgG antibodies.