A cell population shifts unexpectedly across several fluorescence channels. How would you troubleshoot?

Why

Biology usually moves one or two markers. A whole population moving diagonally across several detectors points to a cause that touches every channel. Activated or dying cells and myeloid cells are more autofluorescent, which an unstained sample of the same cells shows.

Dead cells and Fc receptors bind antibodies regardless of specificity, as the Cossarizza guidelines explain. Tandem dyes such as PE-Cy7 and APC-Cy7 can break down. The donor then emits in its own channel, so a PE-Cy7 marker appears as false PE signal.

Hulspas and colleagues found PE-Cy7 among the least stable PE tandems, sensitive to light and handling. Le Roy and colleagues showed APC tandems can be degraded by the cells themselves. Compensation set from controls that do not match the sample's tandem lot pushes events diagonally.

Finally, laser delay, a clog or drifting voltages change everything at once.

How to work it through

1. Look at the shift in the unstained and fluorescence-minus-one controls of the same cells to test autofluorescence.
2. Gate on live singlets with a viability dye and check whether the shift is confined to dead or aggregated events.
3. Compare the affected samples with and without Fc block.
4. Check whether the false signal sits in the donor channel of a tandem, and rerun lot-matched single-stain controls.
5. Review the daily bead performance record and the time plot of the acquisition for clogs or drift.

Where the answer stops holding

Several causes can act together, so removing one may only shrink the shift. Some real biology, such as activation, does change several markers and autofluorescence together. A shift seen only in one batch may reflect sample handling before staining, which no control run on the day can reveal.