Question 20 of 20 Advanced Concept
Why does Fc engineering change what a therapeutic antibody does?
Short answer
The Fc region decides which immune receptors and proteins an antibody engages and how long it lasts in the body. Mutations or glycan changes can boost killing of target cells, silence it, or extend half-life, without touching what the antibody binds.
Why
The Fc region binds Fc gamma receptors on natural killer cells and macrophages, the complement protein C1q, and the neonatal Fc receptor FcRn. Fc gamma receptors drive antibody-dependent cellular cytotoxicity and phagocytosis. C1q starts complement-dependent killing.
FcRn rescues IgG from breakdown inside cells and gives it a half-life of weeks. Saunders reviews how each can be tuned. Removing core fucose from the N297 glycan raises binding to Fc gamma RIIIa and makes cell killing up to about 50 times more potent, which suits antibodies meant to deplete tumor cells.
Silencing mutations such as the L234A and L235A pair, or removing the glycan, cut receptor binding for antibodies that should only block, such as checkpoint inhibitors or agonists that should not kill the cells they bind. Mutations such as YTE or LS increase binding to FcRn at acidic pH and extend half-life, allowing less frequent dosing. Choices interact.
Silencing may remove crosslinking that some agonist antibodies need to signal. Engineered changes can also add immunogenicity risk. For example, an anti-CD20 antibody benefits from enhanced killing, while an anti-PD-1 antibody that killed activated T cells would undermine itself.
A strong answer covers
- Fc gamma receptors, C1q and FcRn as Fc partners
- Afucosylation enhances cell killing
- Silencing mutations for blocking antibodies
- FcRn-enhancing mutations extend half-life
- Trade-offs such as lost crosslinking for agonists
Common mistakes
- Assuming Fc changes affect antigen binding
- Silencing the Fc of an agonist antibody without checking whether crosslinking is needed
- Choosing an effector-competent Fc for an antibody that targets immune cells you want to keep
The interviewer might ask next
How would you test whether an Fc-silenced antibody still engages any Fc gamma receptor?