Staining aggregates in tissue
Tissue staining is where an antibody meets the material it was ultimately raised against. It is also where antibodies that performed beautifully on recombinant protein frequently fail, because fixed tissue presents the epitope differently and because everything in a section binds something.
Fixation changes the epitope
Formalin crosslinks proteins, and the crosslinks can mask an epitope entirely. Antigen retrieval, usually by heating in citrate or EDTA buffer, reverses enough of it to restore staining, and the retrieval conditions are a variable to optimize per antibody rather than a fixed protocol step.
Frozen sections avoid crosslinking and give poorer morphology. For aggregate pathology, which is often judged on morphology, that trade matters.
Some conformational antibodies lose their selectivity after fixation and retrieval, which is worth knowing before a program relies on tissue staining as its confirmation.
The controls that decide the result
- A known positive case and a known negative case on the same slide where possible, since staining varies between runs.
- Primary omitted, to show the secondary is not producing the signal.
- An isotype control at matched concentration.
- A reference antibody with established pathology staining, run in parallel. For tau, the phospho-tau antibodies used in neuropathology are the standard comparators.
- Blocking for endogenous peroxidase and, for brain, autofluorescence, because lipofuscin autofluorescence in aged human tissue mimics puncta convincingly.
Reading the pathology
Aggregate pathology has characteristic morphologies: neurofibrillary tangles, neuropil threads and dystrophic neurites for tau; Lewy bodies and neurites for synuclein; cytoplasmic inclusions for TDP-43. A signal that does not take one of the expected forms is more likely to be background than a new finding.
Quantification needs a defined region of interest, a defined threshold and a blinded reader or an automated pipeline validated against one. Counting objects in a few fields chosen by eye produces numbers that cannot be defended.
Double labeling
Pairing a candidate antibody with an established marker on the same section answers the question that single staining cannot: is the candidate labeling the same structures as the reference, or something else that happens to be in the same area? For a conformation-selective candidate, colocalization with a standard pathology marker, plus the absence of staining in control tissue, is the evidence that matters.
Material and permissions
Human brain tissue comes from brain banks with defined consent, diagnostic confirmation and case metadata, and access is a process with a timeline. Model tissue is easier to obtain and answers a narrower question. Either way, case details, including diagnosis, region, post-mortem interval and fixation history, belong with every image, because they explain most of the variation between runs.