While common sense may suggest that antibodies that do not neutralize the SARS-CoV-2 virus are of little value, recent studies show they can still confer protection against infection. Monoclonal antibodies have been a very effective method for preventing and treating Covid-19. Unfortunately, the current generation of virus variants has mutated to a degree that currently approved monoclonal antibodies that were previously successful are no longer effective.
There is a clear need for new approaches to antibody therapies. One method we have described is the use of combination antibodies that target highly conserved sites. Here we outline a different approach: using antibodies that bind, but are non-neutralizing, yet still protective in high doses. Part one will describe how this method was uncovered, and part two will describe more recent progress in this approach.
A recent study from Swedish scientists Bahnan et al. was the first to show the potential of non-neutralizing antibodies. What is a protective non-neutralizing antibody? These antibodies bind the virus spike, but do not prevent infection in vitro. However, they do protect from infection in live animal models.
How can that be? The virus still enters the cells as the antibody does not block infection, but the infected cells are killed before the virus spreads throughout the body. Bahnan et al. found that high levels of non-neutralizing monoclonal antibodies mediate the interactions between infected cells and immune system monocytes. The following study describes this discovery.
Isolation of Spike-Reactive Human Monoclonal Antibodies
They isolated the antibodies found in the sera of 20 Covid-19 patients. Ten of the 96 they isolated were reactive when exposed to the in vitro spike protein. Those most notably reactive were Ab11, 57, 59, 66, 77, 81, 94, and 95.
They identified the binding epitopes using cryo-electron microscopy to narrow down those that may be neutralizing antibodies. Of those most strongly reactive, they identified six antibodies that bound the spike receptor-binding domain (Ab11, 57, 59, 66, 77, 81) and one that binds both the receptor-binding and N-terminal domains (Ab94). However, they note that only Ab59’s epitope allows for virus neutralization. The rest were non-neutralizing.
FIGURE 1: The binding sites for RBD-binding Ab66 and RBD/NTD-binding Ab94.Models for Ab66 binding the Spike protein in both its open and closed conformations as well as Ab94 are shown.
BAHNAN ET AL.
The most important factor in our typical search for monoclonal antibody treatments is their neutralizing capacity. As suggested by their binding epitopes, only Ab59 showed a reduction of virus titers.
FIGURE 2: Neutralization assays identify one monoclonal as blocking the ACE2-Spike protein interaction. (A) SPR analysis of the binding of monoclonal antibodies to the RBD domain of the Spike protein. PBS served as a negative control, and the intact
BAHNAN ET AL.
High Levels of Non-Neutralizing Antibodies Can Protect Against SARS-CoV-2 Infection
Given the varying levels of neutralization among the 96 isolated antibodies, Bahnan et al. introduced each antibody individually to spike-monocyte interactions to determine the role of antibodies as mediators of immune interactions.
They found that modulated interactions were correlated with the dosage of monoclonal antibodies involved in the interaction. The highest dosage yielded a reduction in spike-monocyte interaction. Ab59, the only neutralizing antibody among the set of 96, showed the same trend of modulating monocyte interaction as other non-neutralizing antibodies, suggesting that this phenomenon is independent of neutralizing capacity.
FIGURE 3: High levels of human monoclonal antibodies reduce Spike-monocyte interaction.
BAHNAN ET AL.
Using humanized ACE2 mice subjects, they found that subjects treated with a large dose (250 µg) of non-neutralizing Ab94 were better protected from Covid-19 using weight loss as the measure as compared to the calculated protective dose (100 µg) of neutralizing Ab59.
FIGURE 4: Non-neutralizing antibodies can protect against SARS-CoV-2 infection. (B) Body weights relative to each individual mouse over the time course of viral infection and treatment.
BAHNAN ET AL.
Discussion
While neutralizing monoclonal antibodies will continue to be the focus of Covid-19 treatment, this discovery unlocks a new world of potential for antibody treatment. There have been dozens, if not hundreds, of antibody candidates in the past three years that either were not completely neutralizing initially, or lost neutralization as emerging variants developed. At the right dosage, some of these antibodies may prove to be valuable protective treatments.
In part two, we will describe another study that explores the use of monoclonal antibodies outside of their traditional role as neutralizing agents against the SARS-CoV-2 spike. With any luck, this new avenue may yield new treatments for those continuing to suffer from Covid-19 sooner rather than later.