Frozen-section immunofluorescence/immunohistochemistry

• Use OCT or comparable cryo-embedding medium for frozen section support;
• Use phosphate buffer/PBS-type wash buffer for removing unbound reagents;
• Use acetone or paraformaldehyde fixation only when compatible with the antigen, because acetone improved ganglioside detection in cryosections whereas 4% paraformaldehyde preserved GFP localization when applied directly to unfixed cryosections at 30-37°C.

• Use a validated primary antibody against the target antigen, species-matched fluorescent secondary antibody or other detection reagent, nuclear counterstain when needed, and anti-fade mounting medium for fluorescence imaging;
• For mouse primary antibodies on mouse tissue, include a serum block plus anti-mouse IgG blocking reagent to reduce mouse-on-mouse background.

• Use a cryostat for cutting frozen sections, glass slides suitable for cryosections, humidified staining chamber, fluorescence or confocal microscope for IF, and a whole-slide or multispectral fluorescence scanner when multiplex phenotyping or whole-section quantification is required.

• Collect tissue rapidly, embed or freeze according to the antigen-preservation strategy, and cut cryosections using a cryostat;
• Fixed/frozen spleen sections, murine cochlear cryosections, and human or mouse frozen tissues have been used successfully for immunofluorescence protocols.

• For unfixed GFP-positive frozen brain tissue, omit drying of cryosections and directly post-fix with 4% paraformaldehyde pre-warmed to 30-37°C to reduce GFP leakage.

• Prepare antibody dilutions and blocking conditions according to the validated antibody system;
• When mouse antibodies are applied to mouse tissue, perform conventional serum blocking followed by anti-mouse IgG blocking before primary antibody incubation.

• Mount cryosections on slides, fix using the antigen-compatible condition selected from the literature-supported options, wash with phosphate buffer/PBS-type buffer, block nonspecific binding, incubate with primary antibody, wash, incubate with fluorescent secondary antibody or detection reagent, wash again, counterstain nuclei if required, mount, and image.

• For acetone-sensitive antigen retrieval in paraformaldehyde-fixed nervous tissue, acetone etching retrieved ganglioside immunoreactivity while preserving neuronal-marker immunoreactivity.

• For rapid frozen-section IF, antibody-coated fluorescent ferrite beads under magnetic promotion reduced EGFR staining of frozen xenograft sections to a 1-minute reaction plus 1-minute wash, but this specialized method should not be substituted for conventional IF unless the magnetic bead system and target antibody are validated.

• Acquire images with exposure, laser power, and detector settings kept comparable across experimental groups;
• Multispectral frozen-section fluorescence can detect up to six markers in one section and can be analyzed with machine-learning phenotyping software for quantitative cell classification.

• Include no-primary-antibody controls or secondary-only controls to evaluate nonspecific secondary-antibody signal, and include tissue or cell populations expected to express the marker as positive controls when reported for the marker system.

High background when using mouse primary antibodies on mouse frozen sections:

Possible cause
Anti-mouse secondary antibody binding endogenous mouse immunoglobulins.
Solution
Add a double-blocking approach with conventional serum blocking plus anti-mouse IgG blocking reagent before applying the mouse primary antibody.

Background caused by endogenous human immunoglobulin in human frozen tissue sections:

Possible cause
Secondary anti-immunoglobulin reagents detecting endogenous immunoglobulin.
Solution
Preincubation with Fab fragments against endogenous immunoglobulin to reduce or abolish endogenous-Ig-related background in frozen human tissue sections.

Antigen-independent human IgM binding on frozen epithelial tissue sections:

Possible cause
Secretory component mediating antigen-independent binding of human IgM and dimeric IgA.
Solution
Block endogenous immunoglobulin and preincubate with anti-secretory-component antibody before applying human IgM monoclonal antibody.

Loss or diffusion of GFP signal in unfixed snap-frozen cryosections:

Possible cause
GFP leaking when membrane integrity is disrupted during cryosection handling.
Solution
Omit drying and directly post-fix cryosections with 4% paraformaldehyde pre-warmed to 30-37°C.