Multiplex Immunofluorescence

Each marker gets its own round of staining, and the rounds stack up.

1. Sequential TSA protocol. Apply the primary antibody, then the HRP secondary, then develop with fluorescent tyramide, which deposits covalently at the binding sites. Strip the antibodies with heat or acid, and repeat for the next marker.
2. Nuclear counterstain. DAPI goes on last as a universal nuclear stain.
3. Multispectral imaging. Acquire through several narrow-band filter sets, or with a tunable filter or spectrometer, to capture each fluorophore's emission profile.
4. Spectral unmixing. Reference spectra separate the overlapping emission mathematically into one clean channel per marker. Labs often record these from single-stained controls; StrataQuest takes them from swatches drawn in the image, or loads them from its Spectral Database.

You end up with 5 to 8 registered, unmixed grayscale images of one field: one per protein, plus DAPI for the nuclei.

Spread six or more fluorophores across the visible spectrum (420 to 720 nm) and their emissions overlap, however you arrange them. Pawley notes that even 0.1% leakage from a strong FITC signal can dominate a weak neighboring channel. Spectral unmixing separates the signals computationally, but it amplifies the noise.

Here is the arithmetic. Split 100 photons across 6 channels and each channel gets about 17, with photon noise of √17 ≈ 4. That is 24% noise per channel, against 10% for the undivided signal. This noise amplification is the fundamental physical cost of multiplexing: the same light is divided among more channels.

Specificity, in Dobrucki's words, "should not be taken for granted," and the strip-and-restain protocol adds risks of its own. Incomplete stripping leaves primary antibody from earlier rounds, which the current secondary can bind. Antibodies raised in the same species, such as two mouse antibodies, can cross-react, so panels are designed with different host species, or with TSA protocols that strip the whole primary-secondary complex.

Pair bright fluorophores (high quantum yield) with weak targets (low expression), and dim fluorophores with strong targets. Spectrally adjacent fluorophores, such as Opal 520 and 540, are the hardest to unmix reliably, and placing a third fluorophore between them improves separation. These constraints tighten as the panel grows, so practical panels rarely exceed 7 to 8 markers on conventional systems.

Here is a 7-plex TSA panel for immuno-oncology research.

• Round 1: CD68 on Opal 520 (macrophages)
• Round 2: CD3 on Opal 540 (T cells)
• Round 3: CD8 on Opal 570 (cytotoxic T subset)
• Round 4: FOXP3 on Opal 620 (regulatory T cells)
• Round 5: PD-L1 on Opal 650 (immune checkpoint)
• Round 6: CK on Opal 690 (tumor epithelium)
• Final: DAPI (all nuclei)

Spectral unmixing turns the acquisition into 7 clean channels. In StrataQuest you then detect nuclei, measure all 6 biomarkers in each cell, gate each marker, define phenotypesLoading... (CD3+CD8+, CK+PD-L1+ and so on) and analyze spatial interactions. One section characterizes the tumor-immune microenvironment.