Internalization assays
For an antibody drug conjugate, an enzyme delivery molecule or anything else that needs to act inside a cell, binding is necessary and not sufficient. The molecule has to be internalized, reach the right compartment, and release what it is carrying. Internalization assays measure the first part of that chain.
The methods in common use
pH-sensitive dyes. Conjugating the antibody to a dye that is dark at neutral pH and bright in acidic compartments turns internalization into a direct signal, because only the molecules that reach endosomes and lysosomes light up. It works in live cells, it can be read by imaging or by flow cytometry, and it needs no wash steps to separate surface from internal.
Surface stripping or quenching. Label the antibody, let the cells take it up at thirty-seven degrees, then remove or quench what is left on the surface with an acid wash or an anti-label quencher. What remains is internal. Simple, and sensitive to how complete the stripping is.
Imaging. Fixed or live cell imaging with compartment markers shows where the antibody went, not only that it went somewhere. Slower, and the only method that answers the trafficking question directly.
Functional readout. For a payload-bearing molecule, cell killing is the end-to-end measurement, and it aggregates internalization, trafficking, release and payload potency into one number.
The temperature control is the experiment
Every internalization protocol depends on the same comparison: parallel samples at four degrees, where binding occurs and uptake is blocked, and at thirty-seven degrees, where both happen. The difference between the two is internalization. Without the cold arm, a strong binder looks like an internalizing binder.
What else to control
- A non-internalizing binder against a surface protein known to stay put, as a negative.
- A known internalizing reference, ideally against the same target, as a positive.
- Conjugation matched across candidates. Dye to protein ratio changes signal and can change behavior, so measure it and keep it consistent.
- Receptor density. Uptake scales with the number of receptors engaged, so comparisons belong on one cell line.
- Time course rather than an endpoint. Internalization rates differ, and a single time point can rank candidates in the opposite order from their rates.
Interpreting the result
Internalization is not a property of the antibody alone. It is a property of the antibody, the epitope, the receptor and the cell. Binding a receptor that recycles quickly can deliver a large total amount of payload over time even if the amount inside at any moment is modest. Binding a receptor that stays at the surface can give a beautiful binding curve and no delivery at all.
Where the epitope sits on the receptor can also change the outcome, because some epitopes trigger or block the receptor's own trafficking. That is one of the reasons epitope diversity in a panel is worth having before the internalization screen rather than after.