Griffin:Antigen Retrieval Technique
Antigen Retrieval is a proven and effective method for optimizing tissue antigen detection of formalin-fixed, paraffin-embedded tissue sections. Pretreatment with an antigen retrieval reagent (Antigen unmasking) that reverses protein cross-links formed by formalin fixation allows greater exposure for the antibody to associate with antigenic sites. There are heat-based (Microwave Oven, Pressure Cooker or Steamer/Roce Cooker) are the most commonly used heating devices. Certain antigenic determinants are masked by formalin fixation and paraffin embedding and may be exposed. Below are the most popular and effective methods from researchers performing these experiments.
10-20 minute heat treatment is a common range that is widely effective. The cooling process taking ~20 minutes. The main mechanism of heat-induced antigen retrieval is disruption of the cross-links and pH is also an essential factor for proper refolding of epitopes. For this reason a low pH and high pH buffer is a good contrast in optimizing the staining protocol. Below are 3 options to consider;
- OPTION 1: 10 mM sodium citrate buffer, pH 6.0.
- OPTION 2: 50 mM glycine-HCl buffer, pH 3.5, with 0.01% (w/v) EDTA.
Summary: Heat at 95º C for 5 minutes. Top off with fresh buffer and heat at 95º C for 5 minutes (optimal incubation time may vary for each tissue type). Allow slides to cool in the buffer for approximately 20 minutes. Wash in deionized H2O three times for 2 minutes each. Aspirate excess liquid from slides.
Materials: Citrate or Glycine-EDTA buffer, a microwaveable plastic container with slide holder and a microwave with a rotating dish.
I) For consistency, fill the slide holder with slides even if only a couple of slides with tissues are being stained. Use clean slides with no tissue for that.
II) Fill the container with the citrate buffer, which is kept at ambient temperature.
III) Place the container with the slides and the buffer in the microwave and bring the buffer to a boil with the microwave at full power. This may take around 2 minutes depending on the microwave used. As soon as the buffer begins to boil the power is reduced to about 30% to allow intermitent boiling of the buffer. Keep the slides for 7 minutes under these conditions. This is done so the tissues don't come off the slides due to continuous boiling. Note: the amount of power used for this step will need to be determined for each microwave.
IV) The container is left at ambient temperature to cool down.
V) Once the buffer is near ambient temperature the slides with tissues are removed, washed briefly with 1xPBS and we proceed with the peroxidase blocking and antibody staining.
- OPTION 3: 0.05% citraconic anhydride solution, pH 7.4
Formaldehyde reacts predominantly with the lysyl residues and forms intramolecular cross-links. Citraconic anhydride reacts with the free amino groups of proteins and replaces the positively charged NH3 groups of lysyl residues with negatively charged carboxyl groups. Citraconic anhydride solution efficiently releases formaldehyde cross-links. Heat deparaffinized formalin fixed tissue sections in 0.05% citraconic anhydride solution, pH 7.4, at 98C for 45 min., followed by 3 wash steps.
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- Trypsin: Incubate sections for 10–20 minutes in 0.1% Trypsin PBS at room temperature. Wash slides several times in deionized H2O. Aspirate excess liquid from slides.
- Pepsin: Incubate sections for 10–20 minutes in 0.1% pepsin in 0.01 N HCl at room temperature. Wash slides several times in deionized H2O. Aspirate excess liquid from slides.
- Saponin: Incubate sections for 30 minutes in 0.05% saponin in deionized H2O at room temperature. Wash at least three times in PBS. Aspirate excess liquid from slides.