Question 5 of 6 Advanced Concept
Why may an antibody block a receptor in one assay but activate it in another?
Short answer
Antibody binding can prevent ligand access in one setting yet cluster receptors in another. Its function may depend on epitope, valency, Fc receptor engagement, cell context and receptor density. Binding affinity alone does not tell you whether it is an antagonist or agonist.
Why
Consider an anti-CD40 antibody. In an assay with low Fc-receptor engagement, its binding may block the natural ligand or show little signaling. In a culture containing FcγR-positive accessory cells, antibody-mediated clustering can assemble CD40 into a signaling-competent complex and produce agonism.
Similar context dependence can occur for other receptor families, although mechanisms differ by target. Examine receptor occupancy, ligand competition, downstream signaling, receptor internalization and the presence or absence of FcγR-bearing cells. Compare intact IgG with an Fc-silent variant or monovalent fragment when experimentally appropriate.
A positive readout in a mixed-cell assay does not show that the antibody intrinsically activates the receptor without additional crosslinking. For drug discovery, characterize function across relevant biological systems and document Fc-mediated effector functions separately from target-receptor signaling.
Common mistakes
- Predicting agonism or antagonism from binding strength alone or ignoring Fc receptor-mediated crosslinking
The interviewer might ask next
How could an Fc-silent control test whether CD40 agonism requires accessory cells?