Antibodies are some of nature’s miracle molecules. While perhaps best known for their ability to prevent infections before they occur through viral particle neutralization, some antibodies can exert protective effects after a cell has been breached by a pathogen.
They do this by glomming onto a target expressed on the infected cell’s surface and signaling to the immune system—specifically, to natural killer (NK) cells via the antibody’s Fc region—to eliminate the infected cell. This process is called antibody-dependent cellular cytotoxicity (ADCC), and, along with pathogen neutralization, is one of several ways that antibodies protect against viruses.
So, what particular features allow an antibody to facilitate ADCC or neutralize a pathogen?
Members of the Overbaugh Lab in the Human Biology Division are experts in antibody-virus interactions. In a recent study in PLoS Pathogens, they began to investigate this question using antibodies against SARS-CoV-2 as a model.
“This study brings more attention to ADCC, an often-overlooked antibody function that is increasingly recognized as important for disease outcome,” says Dr. Delphine Depierreux, a post-doctoral researcher in the Overbaugh Lab and lead author on the study.
Dr. Depierreux and colleagues took advantage of a large panel of antibodies isolated from a single person after who had hybrid immunity to SARS-CoV-2 (i.e., they had been vaccinated and then experienced post-vaccination infections). All antibodies in the panel were directed against Spike, the glycoprotein which mediates viral entry into host cells. Some of these could neutralize infection, some could mediate ADCC, and some could do both.
The authors were first curious whether being a neutralizer made an antibody more likely to have ADCC functionality. When they looked at the correlation between neutralization and ADCC, the authors found no relationship: neutralization potency does not affect ADCC ability and vice versa.
“Antibodies can mediate both ADCC and neutralization, with no clear trade-off between these functions,” Dr. Depierreux explains.
There are many factors that impact an antibody’s activity and ability to bind its target. The first is where on the protein the antibody binds. It makes sense that something blocking the receptor-binding domain (RBD) would likely neutralize the virus. Indeed, when the authors mapped which domains of Spike were bound by each antibody, that’s exactly what they found: most of the neutralizers bound the RBD.
In contrast, the antibodies able to facilitate ADCC were more split: some targeted the RBD, but most targeted the region of Spike closest to the viral membrane called Subunit 2 (S2).
“These findings indicate that ADCC-mediating Abs recognize a broader range of epitopes than neutralizing Abs and that those targeting the S2 domains are more likely to induce potent ADCC,” the authors write.
The other factors that impact antibody function are distinct but related: first, how strongly does the antibody bind the antigen? Next, how much recursive mutation in the germline genes was needed to generate this antibody? Reminder: this recursive refinement is called somatic hypermutation, and it is a key mechanism by which our bodies diversify our antibody repertoire. Finally, how much antigen exposure was needed to generate the antibody?
The authors systematically investigated each of these factors. First, through flow cytometry, they interrogated how strongly each antibody bound the original version of Spike (Wuhan-1). They found that there is a minimum level of binding required for ADCC but that the strength of binding doesn’t correlate with the magnitude of the ADCC response.
In general, antibody affinity is improved over time through repeated exposure to the antigen as the cells repeatedly mutate the regions that determine complementarity to the antigen. Therefore, looking at the degree of somatic hypermutation compared to the germline gene can inform us how many rounds of optimization the antibody underwent.
The authors next asked whether high somatic hypermutation is associated with either neutralization or ADCC ability. They found that more mutation seems to lead to better neutralizers but does not appear to correlate with ADCC potency.
A unique aspect of this study was its built-in longitudinal component, as the panel of antibodies were isolated from two donations at different times—and after repeat SARS-CoV-2 infections.
This allowed the authors to ask whether additional antigen exposure could boost ADCC activity. Again, they observed that repeated antigen exposure enhances the potency of neutralizing antibodies but doesn’t impact ADCC potency.
“The study’s major contribution is the systematic identification of antibody features that enable NK cell-mediated ADCC,” concludes Dr. Depierreux. “By analyzing a large panel of SARS-CoV-2 monoclonal antibodies and dissecting their Fab-domain features, we show that ADCC is shaped primarily by where an antibody binds on Spike, particularly the membrane-proximal S2 domain, rather than by binding strength or the degree of somatic hypermutation.”
“These findings have practical implications for antibody selection as therapeutics and for vaccine design,” she continues. “They suggest that selecting antibodies solely based on strong antigen binding may not be the best strategy to identify potent ADCC mediators.”
In the future, the authors are interested in investigating why the membrane-proximal region of SARS-Cov-2 Spike seems to be a prime epitope for favorable ADCC. Dr. Depierreux has a few hypotheses, including “the angle of antibody binding, the distance between the effector and target cell membranes, or other structural features that promote productive engagement of Fc receptors on NK cells,” she says.
This work has implications not just for SARS-CoV-2 but for vaccine efforts towards a broad range of pathogens. ADCC antibodies are suggested to be correlates of protection against a variety of viral infections, and they are implicated in faster viral clearance and overall better outcomes.
“More broadly, this work raises important questions about how vaccines could be designed to elicit antibodies that combine multiple antiviral functions, including neutralization and Fc-mediated effector activity,” Dr. Depierreux reflects.
The more we learn about these magical molecules, the more we’ll be able to make use of them—not just for viral neutralization, but in all of the roles that antibodies play in the immune system.
The spotlighted research was funded by the National Institutes of Health and the Washington Research Foundation.
Depierreux DM, Ruiz F, Lilly M, Guenthoer J, Chohan V, Overbaugh J. Determinants of natural killer cell-mediated antibody dependent cellular cytotoxicity in SARS-CoV-2 antibodies. 2026. PLoS Pathog. doi: 10.1371/journal.ppat.1014026.
Hannah Lewis is a postdoctoral research fellow with Jim Boonyaratanakornkit’s group in the Vaccine and Infectious Disease Division (VIDD). She is developing screens to find rare B cells that produce protective antibodies against human herpesviruses. She obtained her PhD in molecular and cellular biology from the University of Washington.