Question 13 of 20 Intermediate Concept
How is antibody half-life extended through the neonatal Fc receptor?
Short answer
FcRn rescues IgG from degradation by binding it in acidic endosomes and releasing it back outside the cell at neutral pH. Substitutions such as M252Y, S254T and T256E, known as YTE, raise binding at acidic pH without raising it at neutral pH, which lengthens half-life.
Why
Most proteins taken up by cells are degraded. IgG escapes because FcRn binds the Fc at the acidic pH of the endosome, below about 6.5, and carries it back to the surface, where neutral pH releases it. That recycling is why IgG1, IgG2 and IgG4 circulate for about 21 days.
Engineering exploits the pH switch. Scanning the FcRn interface around residues 252 to 256 and 433 to 436 produced the YTE variant, M252Y with S254T and T256E, which binds human FcRn about fourfold more strongly at acidic pH with no increase at neutral pH, so it is still released efficiently. Nirsevimab, an anti-RSV antibody carrying YTE, was approved for preventing disease in young children and has a reported serum half-life of 71 days.
Other variants follow the same principle, including M428L with N434S and H433K with N434F, the latter reported to bind FcRn about 16-fold better at endosomal pH. The rule is the switch, not raw affinity. A variant that also binds tightly at neutral pH is not released at the cell surface, so it can be cleared faster rather than slower.
A strong answer covers
- FcRn recycles IgG out of the endosome instead of letting it degrade
- Binding must be strong at acidic pH and weak at neutral pH
- YTE raises acidic-pH binding about fourfold
- Nirsevimab with YTE has a reported half-life of 71 days
- Tight binding at neutral pH can shorten rather than extend half-life
Common mistakes
- Treating higher FcRn affinity at any pH as better
- Forgetting that target-mediated clearance can dominate half-life
- Assuming animal half-life data transfer directly to humans
The interviewer might ask next
How would you measure the pH dependence of FcRn binding for a new variant?