Regeneron: A Model for Understanding Antibodies & Immunity

Monoclonal Antibody Therapies can show us a lot about our Immune System

Due to the goings on in Florida a lot of talk has surfaced once again around monoclonal antibodies. More importantly, the conversation has surrounded the therapeutic Regeneron. Because of the conversation it seems important to discuss monoclonal antibodies, how they operate, and how it relates to our own immune system.

Monoclonal Antibodies

First off, we should define what monoclonal antibodies are. During an infection our B cells come into contact with antigens from pathogens such as viruses, bacteria, and parasites, that cause a cascade of events to occur. Our B cells differentiate into plasma cells that produce antibodies specific to the antigen. The constant division of these cells is referred to as clones since all of these cells produce the same antibody.

A big issue when it comes to large scale manufacturing of antibodies is that B cells do not divide forever meaning that antibody production would be very limited. Therefore, if there was a way to cause B cells to divide indefinitely it would allow for larger scale production of antibodies. This can be solved by combining cancerous B cells, which divide indefinitely, with antibody-producing B cells, creating a cell line called a hybridoma. Note that mouse models are not the only source of antibodies. Humanized mice which exhibit predominately human Mabs are used as well in order to reduce reactions against mouse proteins.

The antibodies produced by these hybridomas specialized for a single cell line (clones) are labeled as monoclonal antibodies (Mabs for short, note that all monoclonal antibodies contain the suffix -mab).

An Example Model for Producing Monoclonal Antibodies. Note the isolation of the antibodies of concern (in blue).

In order to produce the needed antibodies it’s important to know which antigen is being targeted. In the case of SARS-COV2 the spike protein acts as the best antigen to target since it is required in order to gain entry into cells. More importantly, antibodies should be developed to target the receptor binding domain (RBD) of the spike protein. The RBD is the vital portion of the spike antigen since this region interacts directly with the ACEII receptor. Therefore, an antibody that binds to the RBD of the spike protein will be able to block the virus from binding to the ACEII receptor, and thus “neutralize” it. This is what is referred to as a neutralizing antibody.

With that in mind we can look at Regeneron. Regeneron, also called REGN-COV2 or REGN-COV, is a monoclonal antibody therapy that is comprised of two different antibodies; Casirivimab (REGN-10933) and Imdevimab (REGN-10987).

Regeneron

As indicated by studies from Regeneron Pharmaceuticals, Inc. thousands of antibodies originally derived from humanized mice and human patients previously infected with SARS-COV2 went through several screening procedures to see which ones would work best against the SARS-COV2 spike.

The takeaways from the Hansen et al., 2020 and Baum et al., 2020 papers are as follows:

• Neutralizing antibodies, antibodies that target the RBD of the SARS-COV2 spike protein, are the most effective antibodies.

• Neutralizing antibodies with high affinity to the SARS-COV2 spike protein, meaning less antibodies are needed to neutralize, are highly preferred.

• Cross-neutralizing antibodies that can neutralize both SARS-COV1 and SARS-COV2 may be weakly neutralizing; antibodies should specially cater to one viral antigen.

• Antibodies with mediator effector function, meaning they can interact with macrophages and monocytes to remove viral particles, can increase protection against infection.

• Antibody cocktails (therapies utilizing 2 or more Mabs) should not contain competing antibodies; antibodies that may have overlapping binding regions to the spike antigen would be susceptible to the same escape mutation and would therefore become ineffective.

• Antibody cocktails place less selective pressure on viruses to mutate than single antibody therapies. Therefore, cocktails of antibodies would be preferred.

From these tests scientists at Regeneron Pharmaceuticals, Inc. found that the 2 best candidates to serve as an antibody cocktail was Casirivimab and Imdevimab.

As illustrated in the figure below, both antibodies bind to different regions of the spike protein. Because they are not competing for the same binding region an escape mutation that renders one antibody ineffective would not affect the other. This is important, since 2 antibodies that bind to different regions would require escape mutations in both antibody binding regions in order to render them both ineffective; an extremely rare scenario. It also means that monotherapies can force selective pressure on SARS-COV2 to form escape mutants. Evidence also indicates that Casirivimab and Imdevimab elicit strong mediator effector function which would provide even more immune functionality.

The complex formed between Casirivimab (REGN-10933, colored in green and cyan), Imdevimab (REGN-10987, colored in red and yellow), and the SARS-COV2 spike RBD (colored in dark blue). Note the lack of overlap between both antibodies. Taken from Hansen et al., 2020.

As noted by Baum et al., 2020:

The data described herein strongly support the notion that cocktail therapy may provide a powerful way to minimize mutational escape by SARS-CoV-2; in particular, our studies point to the potential value of antibody cocktails in which two antibodies were chosen so as to bind to distinct and non-overlapping regions of the viral target (in this case, the RBD of the spike protein), and thus require the unlikely occurrence of simultaneous mutations at two distinct genetic sites for viral escape. A clinical candidate selection criterion for broad potency that includes functional assessment against naturally circulating sequence variants, as well as inclusion of multiple antibodies with non-overlapping epitopes, may provide enhanced protection against loss of efficacy.

Relating to Immunity and Therapy

It’s strange to see people discussing monoclonal antibodies and Regeneron as mostly an exogenous, synthetic compound. As we have indicated here monoclonal antibodies look and operate the same way as our own antibodies do because they are just like our antibodies! This means we can infer many things about antibodies and how it relates to our immune system.

We know that both vaccine-induced immunity and natural immunity produce a robust array of antibodies, meaning there can be broad spectrum protection from both viral infection as well as mutant strains. However, because the vaccines target specifically the spike protein many of the antibodies produced by vaccination may produce antibodies that share overlapping binding regions. This would become an issue with escape mutants since that may render many antibodies ineffective. Regardless, as we have seen with Regeneron, a collection of antibodies that target many different, nonoverlapping regions should still be able to reduce infection and neutralize the virus.

This also indicates that the creation of neutralizing antibodies would be the most effective way to destroy the virus. With breakthrough cases on the rise, it does become cause for concern to elucidate the structures of both natural and vaccine-induced antibodies to see which ones provide the best protection.

With a strong preference for cocktail antibody therapies over monotherapies we should be concerned with large scale usage of monotherapies. As can be seen in case studies of immunocompromised individuals the usage of monoclonal antibodies, as well as antiviral therapeutics, can lead to the creation of many mutant strains, especially after long term use.

This will cause a large predicament with what we are seeing in Florida. Disagreements, to put it mildly, between the Biden administration and Florida Governor Ron DeSantis has lead Governor DeSantis to source monoclonal antibody treatment from GlaxoSmithKline. Their therapy, Sotrovimab (AKA Xevudy or it’s precursor S309), is a monotherapy meaning there is a high chance that large scale usage of this therapeutic could put selective pressure to mutate and thus create escape variants. This will become a big concern, and research into this possibility should be conducted. It would be highly preferably to see a cocktail therapeutic utilized to reduce such a possibility.

Here we have outlined monoclonal antibodies and their relationship to the antibodies we produce, how Mabs function, and what we should be concerned about when they are being used as a therapeutic.

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In Text Citations:

1. Hansen, J. et al., 2020. Studies in humanized mice and convalescent humans yield a SARS-CoV-2 antibody cocktail. Taken from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7299284/

2. Baum, A. et al., 2020. Antibody cocktail to SARS-CoV-2 spike protein prevents rapid mutational escape seen with individual antibodies. Taken from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7299283/

3. Kemp, S. A. et al., 2021. SARS-CoV-2 evolution during treatment of chronic infection. Taken from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7610568/

4. Pinto, D. et al., 2020. Cross-neutralization of SARS-CoV-2 by a human monoclonal SARS-CoV antibody. Taken from https://www.nature.com/articles/s41586-020-2349-y