Protocol

Immunohistochemistry Protocol for Proteinase Antibodies

Step-by-step IHC protocol tuned for proteinase antibody detection on formalin-fixed paraffin-embedded (FFPE) tissue. Covers antigen retrieval choices, dilution starting points, controls, and interpretation of proteinase-specific staining patterns.

Immunohistochemistry for proteinases presents a specific set of challenges that generic IHC protocols do not address. Proteinases in fixed tissue may be present as multiple functional forms (zymogen, mature, cleaved, complexed with inhibitor), and each form may have differential epitope accessibility depending on retrieval conditions. This protocol establishes reproducible starting points for each of the four major proteinase families and provides a diagnostic framework for troubleshooting when the initial protocol does not resolve the target.

Materials

Tissue

FFPE tissue sections, 4-5 µm thickness, cut onto positively-charged slides. For proteinase detection, sections should be used within 3 months of cutting — longer storage results in progressive epitope loss on proteinase targets, more so than for many other protein families.

Antigen retrieval buffers

Citrate buffer, 10 mM, pH 6.0 (standard heat-induced retrieval) and EDTA buffer, 1 mM, pH 9.0 (alternative for hard-to-retrieve targets). Prepared fresh weekly.

Blocking reagents

Normal serum (10%) from the species of the secondary antibody host (typically goat serum for anti-rabbit secondaries), 3% hydrogen peroxide in methanol (endogenous peroxidase block for HRP-based detection), and 2% BSA in TBS-T for background reduction.

Detection reagents

Polymer-based anti-rabbit HRP secondary (recommended over biotin-streptavidin systems for reduced endogenous biotin background), DAB substrate kit (or AEC as alternative chromogen), and hematoxylin counterstain.

Day 1: Section preparation, retrieval, and primary antibody

1. Deparaffinize and rehydrate

Bake sections at 60°C for 1 hour to increase adherence, then dewax through three changes of xylene (5 minutes each), followed by graded ethanol series: 100% (2× 3 minutes), 95% (3 minutes), 70% (3 minutes), then wash in dH2O for 5 minutes.

2. Antigen retrieval — the most important step

For proteinase antibodies, antigen retrieval choice is critical and target-dependent. As a starting point:

  • Citrate pH 6.0 (default): Works for most catalytic-domain and C-terminal antibodies. Standard heat-induced epitope retrieval: transfer slides to buffer in a plastic Coplin jar, microwave at full power to boiling (~3 minutes), then reduce power to maintain sub-boiling temperature for 15 minutes. Cool at room temperature in the buffer for 20 minutes. Do not rinse in cold buffer — thermal shock causes tissue detachment.
  • EDTA pH 9.0 (alternative): Better for propeptide-directed antibodies and some cathepsins, particularly on heavily-fixed archival tissue. Same heat protocol as citrate.
  • Enzymatic retrieval (proteinase K, trypsin): Avoid for proteinase antibodies unless specifically necessary. Enzymatic retrieval can cleave endogenous proteinases in the section, artificially creating "cleaved" bands and confounding the interpretation of zymogen vs mature detection. If a target absolutely requires enzymatic retrieval, use the lowest effective enzyme concentration and include untreated adjacent sections as controls.

3. Endogenous peroxidase blocking (HRP detection only)

Incubate sections in 3% H2O2 in methanol for 20 minutes at room temperature. This eliminates endogenous peroxidase activity from red blood cells, neutrophils, and other heme-containing cells that would otherwise produce false-positive DAB signal. Skip this step for fluorescence detection.

4. Blocking

Block in 10% normal serum + 2% BSA in TBS-T for 60 minutes at room temperature in a humidity chamber. Use serum from the host species of your secondary antibody (goat serum for goat-anti-rabbit secondary, etc.). Extended block time can reduce background on tissues with high endogenous immunoglobulin.

5. Primary antibody incubation

Dilute the primary antibody in blocking buffer at the recommended starting dilution. As general guidance for rabbit polyclonal antibodies in IHC:

  • Starting range 1:100 to 1:500 for tissues with expected high expression (tumours, hepatocytes, macrophages)
  • Starting range 1:50 to 1:200 for tissues with expected low expression (normal brain, skin, kidney glomeruli)
  • IHC dilutions are typically 5-10× more concentrated than the corresponding Western blot dilution for the same antibody

Apply approximately 100 µL of diluted antibody per section, ensuring the entire tissue is covered. Incubate overnight at 4°C in a humidity chamber — this is the reference protocol and produces the most reproducible results. A 60-minute room-temperature incubation is acceptable for high-abundance targets, but for proteinases the overnight-cold protocol resolves more forms and gives lower background.

Day 2: Detection, chromogen development, and mounting

6. Wash

Three washes in TBS-T, 5 minutes each, on a rocking platform at room temperature. Insufficient washing is the most common source of background staining in IHC — do not shorten this step.

7. Secondary antibody

Apply polymer-based anti-rabbit HRP secondary (ready-to-use) for 30 minutes at room temperature. Polymer-based detection is preferred over biotin-streptavidin systems for proteinase IHC because many target tissues (liver, kidney, brain) have significant endogenous biotin that would otherwise create false-positive signal.

8. Wash again

Three washes in TBS-T, 5 minutes each.

9. Chromogen development

Apply DAB working solution (prepared per manufacturer instructions) and monitor colour development under a microscope. Typical development time is 3-5 minutes; stop the reaction by rinsing in dH2O when the specific signal is clearly visible but before the background darkens. Development time should be identical across all sections in a comparison series — use a timer, not eyeballing.

For dual-chromogen work or when DAB oxidation may quench a companion fluorophore, AEC (red chromogen) is an acceptable substitute; it is water-soluble and requires aqueous mounting.

10. Counterstain

Counterstain with Mayer's or Harris's hematoxylin for 30-60 seconds, followed by tap water rinse until nuclei are blue. Overstaining hematoxylin masks weak specific signal; understaining loses tissue architecture. 30 seconds is a good starting point.

11. Dehydrate and mount

Dehydrate through graded ethanol (70%, 95%, 100% each 2 minutes), clear in xylene (2× 3 minutes), and mount with a xylene-compatible mounting medium (e.g., DPX). AEC-stained sections require aqueous mounting (glycerol-based or aqueous polyvinyl alcohol) — xylene will dissolve AEC signal.

Controls to run in parallel

Positive control tissue

A section of tissue with known target expression. For BACE1, adult mouse or human brain cortex. For MMPs, invasive tumour margin or wound-healing tissue. For cathepsins, spleen (macrophages) or thyroid follicles. Signal on the positive control confirms the protocol worked.

Negative control tissue

A section from tissue where the target is genetically absent (knockout mouse tissue) or biologically absent (target not expressed in that tissue). Absence of signal on the negative control confirms the antibody is not cross-reacting with an unrelated antigen.

No-primary control

Serial adjacent section run through the full protocol except the primary antibody incubation (replaced by blocking buffer alone). Any DAB signal on this section is coming from the secondary detection system, not the target — if signal appears here, investigate before interpreting the primary-stained sections.

Isotype control

A section stained with a rabbit IgG at the same total protein concentration as your primary antibody, replacing the specific antibody. Distinguishes non-specific IgG binding from specific antigen recognition. Optional but recommended for publication-quality IHC.

Interpreting proteinase-specific staining patterns

Proteinases are not distributed uniformly through cells or tissues. Understanding the expected staining pattern for your target class helps distinguish real signal from artefact.

Membrane-anchored proteinases (BACE1, BACE2, ADAM10, ADAM17, TMPRSS family)

Expected pattern is membrane-bound punctate staining concentrated at the cell periphery and (for polarized epithelial cells) at the basolateral or apical surface. Cytoplasmic staining suggests trafficking-compartment (ER, Golgi) localization — if this dominates, the sample may have been captured before the enzyme reached the surface, or your fixation trapped the trafficking pool. Nuclear staining is almost always artefact for membrane proteinases — investigate.

Lysosomal proteinases (cathepsins B, D, K, L, S)

Expected pattern is punctate cytoplasmic staining corresponding to lysosomes. In macrophages, dendritic cells, and thyroid follicular cells, punctate signal is very intense. In non-professional secretory cells, expect fainter but still punctate signal. Diffuse cytoplasmic staining suggests lysosomal disruption during processing.

Secreted proteinases (MMPs, ADAMTS, kallikreins)

Expected pattern varies with cellular state. In resting cells, secreted proteinases are stored in secretory granules or trafficked through the secretory pathway — expect Golgi-region and vesicular staining. In activated/stimulated cells, expect membrane and extracellular matrix staining as the secreted enzyme deposits in the pericellular space. In cell-free stroma, staining reflects prior secretion and matrix retention.

Cytosolic proteinases (calpains, granzymes at rest)

Expected pattern is diffuse cytoplasmic staining. Upon activation, calpains translocate to specific compartments (membrane for calpain-1, cytoskeleton for calpain-2). Granzymes concentrate in cytotoxic granules of activated T cells.

Domain-specific antibody strategies for functional-form resolution

Where multiple antibodies are available targeting non-overlapping epitopes of the same proteinase (for example, propeptide, catalytic domain, and cytoplasmic tail), a multi-antibody strategy on serial adjacent sections resolves distinct functional pools of the target that single-antibody IHC cannot distinguish. For IHC specifically:

  • Propeptide-directed antibody: stains the ER-resident latent zymogen pool. In quiescent cells, expect ER-region staining; in activated cells, expect reduced propeptide signal as the propeptide is cleaved.
  • Catalytic-domain antibody: stains total target (zymogen + mature). Highest sensitivity single-antibody detection.
  • C-terminal / cytoplasmic tail antibody: for membrane-anchored proteinases, distinguishes membrane-bound from soluble/shed forms. Loss of C-terminal signal with retention of catalytic-domain signal indicates ectodomain shedding — the enzyme has been released.

Serial adjacent sections stained separately with each domain antibody allow you to build a spatial map of protein form distribution across the tissue.

Troubleshooting

No signal on the positive control

  1. Retrieval was inadequate — try EDTA pH 9.0 instead of citrate pH 6.0, or extend citrate retrieval time to 20 minutes.
  2. Primary antibody too dilute — increase concentration by 2-5×.
  3. Chromogen development too short — extend to 8-10 minutes and monitor.
  4. Secondary antibody expired or misapplied — verify with a control section stained with just the secondary + DAB.

Signal on the negative (no-primary) control

  1. Endogenous peroxidase not adequately blocked — extend H2O2 block or use a commercial peroxidase blocker.
  2. Non-specific binding of the polymer secondary — extend serum blocking or try a different manufacturer's polymer.
  3. Endogenous biotin (if using biotin-streptavidin system) — switch to polymer-based detection.

Diffuse background on tissue

  1. Insufficient washing — add a 4th wash step.
  2. Primary too concentrated — dilute 2-5× further.
  3. Autofluorescence (for IF; not relevant for chromogenic IHC) — consider a chemical autofluorescence quench.

Staining is present but at the wrong subcellular location

  1. Fixation duration — over-fixation traps mature enzyme in the trafficking compartment; under-fixation loses cytoplasmic signal.
  2. Cell type differences — the expected pattern may vary with cellular state; check the published literature for your specific tissue.
  3. Wrong antibody clone — run individual domain antibodies (if a multi-antibody set is available) to see which domain is producing the aberrant signal.

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