Next-Generation Techniques for Discovering Human Monoclonal Antibodies

Preclinical animal models to evaluate therapeutic antiviral antibodies

Despite the availability of effective preventative vaccines and potent small-molecule antiviral drugs, effective non-toxic prophylactic and therapeutic measures are still lacking for many viruses. The use of monoclonal and polyclonal antibodies in an antiviral context could fill this gap and provide effective virus-specific medical interventions. In order to develop these therapeutic antibodies, preclinical animal models are of utmost importance. Due to the variability in viral pathogenesis, immunity and overall characteristics, the most representative animal model for human viral infection differs between virus species. Therefore, throughout the years researchers sought to find the ideal preclinical animal model for each virus. The most used animal models in preclinical research include rodents (mice, ferrets, …) and non-human primates (macaques, chimpanzee, ….). Currently, antibodies are tested for antiviral efficacy against a variety of viruses including different hepatitis viruses, human immunodeficiency virus (HIV), influenza viruses, respiratory syncytial virus (RSV), severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and rabies virus. This review provides an overview of the current knowledge about the preclinical animal models that are used for the evaluation of therapeutic antibodies for the abovementioned viruses.

Therapeutic Antibodies: An Overview

Polyclonal immunoglobulin (Ig) preparations have been used for several decades for treatment of primary and secondary immunodeficiencies and for treatment of some infections and intoxications. This has demonstrated the importance of Igs, also called antibodies (Abs) for prevention and elimination of infections. Moreover, elucidation of the structure and functions of Abs has suggested that they might be useful for targeted treatment of several diseases, including cancers and autoimmune diseases. The development of technologies for production of specific monoclonal Abs (MAbs) in large amounts has led to the production of highly effective therapeutic antibodies (TAbs), a collective term for MAbs (MAbs) with demonstrated clinical efficacy in one or more diseases. The number of approved TAbs is currently around hundred, and an even larger number is under development, including several engineered and modified Ab formats. The use of TAbs has provided new treatment options for many severe diseases, but prediction of clinical effect is difficult, and many patients eventually lose effect, possibly due to development of Abs to the TAbs or to other reasons. The therapeutic efficacy of TAbs can be ascribed to one or more effects, including binding and neutralization of targets, direct cytotoxicity, Ab-dependent complement-dependent cytotoxicity, Ab-dependent cellular cytotoxicity or others. The therapeutic options for TAbs have been expanded by development of several new formats of TAbs, including bispecific Abs, single domain Abs, TAb-drug conjugates, and the use of TAbs for targeted activation of immune cells. Most promisingly, current research and development can be expected to increase the number of clinical conditions, which may benefit from TAbs.

Perspectives on passive antibody therapy and peptide-based vaccines against emerging pathogens like SARS-CoV-2

The current epidemic of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is raising awareness of the need to act faster when dealing with new pathogens. Exposure to an emerging pathogen generates an antibody response that can be used for preventing and treating the infection. These antibodies might have a high specificity to a target, few side effects, and are useful in the absence of an effective vaccine for treating immunocompromised individuals. The approved antibodies against the receptor-binding domain (RBD) of the viral spike protein of SARS-CoV-2 (e.g., regdanvimab, bamlanivimab, etesevimab, and casirivimab/imdevimab) have been selected from the antibody repertoire of B cells from convalescent patients using flow cytometry, next-generation sequencing, and phage display. This encourages use of these techniques especially phage display, because it does not require expensive types of equipment and can be performed on the lab bench, thereby making it suitable for labs with limited resources. Also, the antibodies in blood samples from convalescent patients can be used to screen pre-made peptide libraries to identify epitopes for vaccine development. Different types of vaccines against SARS-CoV-2 have been developed, including inactivated virus vaccines, mRNA-based vaccines, non-replicating vector vaccines, and protein subunits. mRNA vaccines have numerous advantages over existing vaccines, such as efficacy, ease of manufacture, safety, and cost-effectiveness. Additionally, epitope vaccination may constitute an attractive strategy to induce high levels of antibodies against a pathogen and phages might be used as immunogenic carriers of such peptides. This is a point worth considering further, as phage-based vaccines have been shown to be safe in clinical trials and phages are easy to produce and tolerate high temperatures. In conclusion, identification of the antibody repertoire of recovering patients, and the epitopes they recognize, should be an attractive alternative option for developing therapeutic and prophylactic antibodies and vaccines against emerging pathogens.

Considerations for the Design of Antibody-Based Therapeutics

Antibody-based proteins have become an important class of biologic therapeutics, due in large part to the stability, specificity, and adaptability of the antibody framework. Indeed, antibodies not only have the inherent ability to bind both antigens and endogenous immune receptors but also have proven extremely amenable to protein engineering. Thus, several derivatives of the monoclonal antibody format, including bispecific antibodies, antibody-drug conjugates, and antibody fragments, have demonstrated efficacy for treating human disease, particularly in the fields of immunology and oncology. Reviewed here are considerations for the design of antibody-based therapeutics, including immunological context, therapeutic mechanisms, and engineering strategies. First, characteristics of antibodies are introduced, with emphasis on structural domains, functionally important receptors, isotypic and allotypic differences, and modifications such as glycosylation. Then, aspects of therapeutic antibody design are discussed, including identification of antigen-specific variable regions, choice of expression system, use of multispecific formats, and design of antibody derivatives based on fragmentation, oligomerization, or conjugation to other functional moieties. Finally, strategies to enhance antibody function through protein engineering are reviewed while highlighting the impact of fundamental biophysical properties on protein developability.