By Stefan Dübel
While antibodies have been indispensable tools for drug discovery for decades, recent developments open doors to a number of novel applications and approaches. Key to these developments is the availability of a rapidly increasing number of recombinant antibodies.
Other than the polyclonal and monoclonal antibodies well known to everybody, recombinant antibodies are typically isolated by animal-independent in vitro selection systems, most prominently phage display. The different process of their generation improves the reliability of the research results, as these antibodies are unambiguously defined by their complete sequence right from the start. This is not the case for conventional antibody reagents, which recently led to a call for better standardization of antibodies after serious problems in clinical studies when animal-derived antibodies emerged, not speaking of an estimated waste in time and money ($350 million annually in the United States alone) due to poorly characterized antibodies. But besides this, the different approach used for the generation of recombinant antibodies also provides completely new opportunities for drug discovery and development.
Recombinant antibody providers have significantly matured their capabilities within the last decade. Both in the U.S. and Europe, significant investments into respective methods have been made, resulting in robust pipelines capable of delivering large numbers of high affinity antibodies to almost every target in a very short time. Consequently, a set of sequence-defined antibodies to every human protein could be made within a few years, and for a fragment of the cost that would be needed for hybridoma antibodies. So, whether researchers will get such a biobank of well defined antibodies to the complete human proteome now rather depends on financial and political decisions.
But scientists can benefit from other favorable properties of recombinant antibodies already today. Since the generation of any recombinant antibody always delivers the gene encoding it right away, new applications have been developed that use this additional resource. Recently, it has been shown for the first time that endoplasmatic reticulum retained antibodies (“intrabodies”) can induce a knock down phenotype in transgenic mice. For this purpose, the gene encoding the antibody is cloned into transgenic mice, with the addition of a short peptide that retains the antibody—together with the antigen it binds—inside the cell (see figure). In the published example, vascular cell adhesion molecule 1 (VCAM-1), a cell surface mediator of immune functions, was removed from the cell surface in the bone marrow of mice, resulting in a strong phenotype of aberrant distributions of immature B cells in blood and bone marrow. This demonstrates a novel approach to achieve protein inactivation (knock out) in vivo.
The availability of this novel method to assess the function of a protein in its naive setting in the living organism, combined with the rapidly growing number of available antibody genes, will spark a new level for the functional study of membrane and plasma proteins in vivo. This holds great promises for drug discovery, as many new drugs against cancer, autoimmune, and infectious diseases are targeting membrane proteins. The capability to selectively knock these out right on the protein level in a living organism will open up new approaches for target discovery and validation. It will further be valuable for generating mouse models for drug development that more closely resemble disease states than classic genetic knock outs can do.