Immunostaining

of a GFAP immunostained section of a brain tumour. In biochemistry, immunostaining is any use of an antibody-based method to detect a specific protein in a sample. The term "immunostaining" was originally used to refer to the immunohistochemical staining of tissue sections, as first described by Albert Coons in 1941. However, immunostaining now encompasses a broad range of techniques used in histology, cell biology, and molecular biology that use antibody-based staining methods.

Techniques

Flow cytometry

A flow cytometer can be used for the direct analysis of cells expressing one or more specific proteins. Cells are immunostained in solution using methods similar to those used for immunofluorescence, and then analysed by flow cytometry. Flow cytometry has several advantages over IHC including: the ability to define distinct cell populations by their size and granularity; the capacity to gate out dead cells; improved sensitivity; and multi-colour analysis to measure several antigens simultaneously. However, flow cytometry can be less effective at detecting extremely rare cell populations, and there is a loss of architectural relationships in the absence of a tissue section. Flow cytometry also has a high capital cost associated with the purchase of a flow cytometer.

Western blotting

Western blotting allows the detection of specific proteins from extracts made from cells or tissues, before or after any purification steps. Proteins are generally separated by size using gel electrophoresis before being transferred to a synthetic membrane via dry, semi-dry, or wet blotting methods. The membrane can then be probed using antibodies using methods similar to immunohistochemistry, but without a need for fixation. Detection is typically performed using peroxidase linked antibodies to catalyse a chemiluminescent reaction. Western blotting is a routine molecular biology method that can be used to semi-quantitatively compare protein levels between extracts. The size separation prior to blotting allows the protein molecular weight to be gauged as compared with known molecular weight markers.

Enzyme-linked immunosorbent assay

The enzyme-linked immunosorbent assay or ELISA is a diagnostic method for quantitatively or semi-quantitatively determining protein concentrations from blood plasma, serum or cell/tissue extracts in a multi-well plate format (usually 96-wells per plate). Broadly, proteins in solution are absorbed to ELISA plates. Antibodies specific for the protein of interest are used to probe the plate. Background is minimised by optimising blocking and washing methods (as for IHC), and specificity is ensured via the presence of positive and negative controls. Detection methods are usually colorimetric or chemiluminescence based.

Immuno-electron microscopy

Electron microscopy or EM can be used to study the detailed microarchitecture of tissues or cells. Immuno-EM allows the detection of specific proteins in ultrathin tissue sections. Antibodies labelled with heavy metal particles (e.g. gold) can be directly visualised using transmission electron microscopy. While powerful in detecting the sub-cellular localisation of a protein, immuno-EM can be technically challenging, expensive, and require rigorous optimisation of tissue fixation and processing methods. Protein biotinylation in vivo was proposed to alleviate the problems caused by frequent incompatibility of antibody staining with fixation protocols that better preserve cell morphology.

Methodological overview

In immunostaining methods, an antibody is used to detect a specific protein epitope. These antibodies can be monoclonal or polyclonal. Detection of this first or primary antibody can be accomplished in multiple ways. The primary antibody can be directly labeled using an enzyme or fluorophore. The primary antibody can be labeled using a small molecule which interacts with a high affinity binding partner that can be linked to an enzyme or fluorophore. The biotin-streptavidin is one commonly used high affinity interaction. The primary antibody can be probed for using a broader species-specific secondary antibody that is labeled using an enzyme, or fluorophore. In the case of electron microscopy, antibodies are linked to a heavy metal particle (typically gold nanoparticles in the range 5-15nm diameter). As previously described, enzymes such as horseradish peroxidase or alkaline phosphatase are commonly used to catalyse reactions that give a coloured or chemiluminescent product. Fluorescent molecules can be visualised using fluorescence microscopy or confocal microscopy.

Applications

The applications of immunostaining are numerous, but are most typically used in clinical diagnostics and laboratory research. Clinically, IHC is used in histopathology for the diagnosis of specific types of cancers based on molecular markers. In laboratory science, immunostaining can be used for a variety of applications based on investigating the presence or absence of a protein, its tissue distribution, its sub-cellular localisation, and of changes in protein expression or degradation.

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