- This study characterized the immune functions of monoclonal antibody CR3022, which binds to a site on the SARS-CoV-2 spike protein not fully blocked in the presence of ACE2
- In vitro, CR3022 bound to Fc receptors and drove cellular phagocytosis, natural killer cell activation, complement deposition and neutrophil activation
- However, a functionally enhanced Fc variant of CR3022 showed divergent virologic effects in mouse and hamster models and resulted in increased pathology in both
- Strategic Fc engineering of monoclonal antibodies against SARS-CoV-2 will be crucial to maximize protection and minimize pathology
Scientists are keenly interested in monoclonal antibodies as a class of therapeutics for SARS-CoV-2, the virus that causes COVID-19, because in general, they can neutralize (block) infection. However, it's unclear whether neutralization alone will be sufficient. To fully prevent disease it may be essential to clear the virus from cells in the upper respiratory tract that have already become infected.
Another potential problem is a phenomenon called antibody-dependent enhancement of infection. For example, as reported in 2019 in JCI Insight, treating macaques with neutralizing antibodies against SARS-CoV (the virus that caused the SARS epidemic of 2002–2004) actually worsened disease by inducing inflammatory responses.
Therefore, caution is needed in developing monoclonal antibody therapeutics for SARS-CoV-2. Caroline Atyeo, PhD candidate, and Galit Alter, PhD, principal investigator at the Ragon Institute of Massachusetts General Hospital, MIT and Harvard, and colleagues are studying the ability of antibodies to target infected cells through Fc receptors, key immune regulatory receptors that connect antibody-mediated (humoral) immune response to cellular effector functions.
In a recent report in JCI Insight, they report experiments with a first-in-class monoclonal antibody called CR3022, which provided information about the potential immune-protective versus the immunopathological role of antibodies for SARS-CoV-2.
Background on CR3022
CR3022 was initially cloned from a SARS-CoV–infected individual. It is of interest because it can bind to the SARS-CoV receptor-binding domain (part of the spike protein) even in the presence of angiotensin-converting enzyme 2 (ACE2), the major protein that lets SARS-CoV-2 enter cells.
Therefore, CR3200 should offer therapeutic benefits even after the infection has been initiated. By studying this antibody, the researchers were able to explore Fc-dependent effects on SARS-CoV-2 without confounding by neutralization.
CR3022 Drives Fc Effector Functions
In vitro, CR3022 bound to Fc receptors and drove innate immune activities: cellular phagocytosis, natural killer (NK) cell activation, complement deposition and neutrophil activation.
Even in the presence of ACE2, CR3022 drove cellular phagocytosis and complement deposition. When compared with certain neutralizing antibodies and other non-neutralizing antibodies, its immune activity was similar or superior.
Further Fc Engineering
To gauge the therapeutic benefits of CR3022, the researchers generated CR3022 Fc variants that could selectively augment phagocytosis or NK cell, complement or neutrophil activity. Along with wild-type CR3022 immunoglobulin G (WT), two variants were carried into in vivo experiments:
- An antibody that knocked out all four Fc effector functions
- An enhanced antibody that exhibited slightly better Fc function than WT did
In Vivo Experiments
The researchers infected BALB/c mice and Syrian golden hamsters with SARS-CoV-2, then treated them with the WT, Fc-knockout or Fc-enhanced CR3022 antibody. The hamster model is ultrasensitive to SARS-CoV-2, becoming readily infected and developing severe disease.
In mice, researchers found that:
- Viral load in the lung was higher with WT than with a control but was reduced with the Fc-enhanced antibody
- Both WT and the Fc-enhanced antibody produced significant morbidity (signaled by weight loss), perhaps by promoting an inflammatory response
- The Fc-knockout antibody reduced viral load and produced minimal to no weight loss
In hamsters, researchers found that:
- WT again had no impact on viral load
- The Fc-enhanced antibody increased viral load and increased weight loss
- The Fc-knockout antibody showed no benefit
Hope for the Future
The disconnect between viral load and pathogenesis in animal experiments highlights the critical importance of strategic Fc engineering of monoclonal antibodies to maximize protection and minimize pathology.
Based on these animal studies, Fc enhancement would seem to be a liability. However, many Fc modifications can selectively improve NK cell activation, phagocytosis, neutrophil activation or complement deposition, offering potentially more precise mechanisms to control infection.
Whether these immune functions can clear SARS-CoV-2 from infected cells and prevent disease is still unknown. Rapid discovery of novel neutralizing antibodies and pan–cross-reactive CoV antibodies, along with rapid Fc engineering, will provide critical clues about how to generate therapeutics with the highest clinical benefit.
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