Applications of immunohistochemistry.

Immunohistochemistry (IHC) is an important application of monoclonal as well as polyclonal antibodies to determine the tissue distribution of an antigen of interest in health and disease. IHC is widely used for diagnosis of cancers; specific tumor antigens are expressed de novo or up-regulated in certain cancers. This article deals with the various applications of IHC in diagnosis of diseases, with IHC playing an important role in diagnostic and research laboratories.

Immunohistochemistry (IHC), the utilization of monoclonal and polyclonal antibodies for the detection of specific antigens in tissue sections, is an extraordinarily powerful tool in the armamentarium of the diagnostic surgical pathologist. IHC is an important application of monoclonal as well as polyclonal antibodies to determine the tissue distribution of an antigen of interest in health and disease. It is widely used for diagnosis of cancers because specific tumor antigens are expressed de novo or up-regulated in certain cancers. IHC plays an important role in pathology, particularly in the subspecialties of oncologic pathology, neuropathology, and hematopathology. Utilization studies are rare,[1] but several authors have reviewed the diagnostic utility of IHC in surgical pathology.[2-4] In autopsy pathology while basic histologic examination of tissue is considered a useful and necessary component IHC may provide a greater insight.[56]

IHC requires the availability of biopsies; these are processed into sections with a microtome and then the sections are incubated with an appropriate antibody. The site of antibody binding is visualized under an ordinary or fluorescent microscope by a marker such as fluorescent dye, enzyme, radioactive element, or colloidal gold, which is directly linked to the primary antibody or to an appropriate secondary antibody.

The Principle of IHC

The principle of IHC has existed since the 1930s, but it was not until 1941 that the first IHC study was reported.[7] Coons and his colleagues used Fluorescein isothiocyanate (FITC)-labeled antibodies with a fluorescent dye to localize pneumococcal antigens in infected tissues. With the expansion and development of IHC technique, enzyme labels have been introduced, such as peroxidase[89] and alkaline phosphatase.[10] Colloidal gold[11] label has also been discovered and used to identify immunohistochemical reactions at both light and electron microscopy levels. Other labels include radioactive elements, and the immunoreaction can be visualized by autoradiography. The aim of IHC is to perform most IHC staining by causing least damage on the cell or tissue, and by using least amount of antibody, it finds a way in the tumor typing and tumor markers.

Applications

Since IHC involves specific antigen–antibody reactions, it has apparent advantage over traditionally used special enzyme staining techniques that identify only a limited number of proteins, enzymes, and tissue structures. Therefore, IHC has become a crucial technique and is widely used in many medical research laboratories as well as clinical diagnostics.[12]

Prognostic Markers in Cancer

To predict the prognosis of tumors by identification of enzymes, tumor-specific antigens, oncogenes, tumor suppressor genes, and tumor cell proliferation markers. Analysis of tumors by these methods is a significant improvement over the conventional prognostic considerations by clinical staging and histologic grading. IHC is used for disease diagnosis, drug development, and biological research. Using specific tumor markers, physicians use IHC to diagnose a cancer as benign or malignant, determine the stage and grade of a tumor, and identify the cell type and origin of a metastasis to find the site of the primary tumor. IHC is also used in drug development to test drug efficacy by detecting either the activity or the up- or down-regulation of disease targets.[13]

Tumors of Uncertain Histogenesis

IHC methods have brought about a revolution in approach to diagnosis of tumors of uncertain origin, primary as well as metastatic from unknown primary tumor. A panel of antibodies is chosen to resolve such diagnostic problem cases. The selection of antibodies being made is based on clinical history, morphological features, and results of other relevant investigations. Immunohistochemical stains for intermediate filaments are expressed by tumor cells (keratin, desmin, vimentin, neurofilaments, and glial fibrillary acidic proteins).[13]

Prediction of Response to Therapy

IHC is widely used to predict therapeutic response in two important tumors, i.e. carcinoma of breast and prostate. Both these tumors are under the growth regulation of the hormones estrogen and androgen, respectively. The specific receptors for these growth regulating hormones are located on respective tumor cells. Tumors expressing high level of receptor positivity would respond favorably to removal of the endogenous source of such hormones or hormonal therapy is administered to lower their levels – estrogen therapy in prostate cancer and androgen therapy in breast cancer.[13]

Infections

Immunohistochemical methods are also being applied to confirm infectious agent in tissues by use of specific antibodies against microbial DNA or RNA, e.g. in Cytomegalo virus, Hepatitis B virus, Hepatitis C virus, etc. The application is used routinely in validation of disease targets as it allows visualizing expression of the target in the affected tissue during the disease process. The concept was introduced as early as the 1940s when fluorescein dye (visible under ultraviolet light) was tagged to antibodies directed against pneumococci for identification of this organism with specific anti-serum.[7] This method, often abbreviated IFA for “immunofluorescence assay”, has been widely used for the detection of specific pathogens, viral as well as bacterial and protozoal, in “fresh”/unfixed tissues in both human and veterinary medicine.

Another important advantage of IHC is that it can also be used to detect organisms in cytological preparations such as fluids, sputum samples, and material obtained from fine needle aspiration procedures. This can be very helpful in certain situations such as detection of pneumocystis from the sputum of an immunocompromised patient who needs rapid and precise confirmation of infection in order to begin immediate and appropriate therapy.

In Genetics

IHC can also be used to determine the function of specific gene products in fundamental biological processes such as development and apoptosis. Using a custom made monoclonal antibody against p53 homologue of the pro-apoptotic pathways of p53 was identified.

Neurodegenerative Disorders

Degenerative disorders of the nervous system include a wide range of diseases characterized by the dysfunction and death of specific, selectively vulnerable populations of nerve cells. It has played an increasingly important role in the subclassification of neurodegenerative disorders and the development of consensus criteria for their diagnosis.

Brain Trauma

In the last few years, immunohistochemical staining for beta amyloid precursor protein has been validated as a method to detect axonal injury within as little as 2–3 h of head injury.[14] Immunohistochemical detection of axonal injury can be useful in establishing timing of a traumatic insult in medico-legal settings.

IHC in Muscle Diseases

Specific diagnosis of muscular dystrophy is important because of the genetic counseling implications of inherited disease and accurate prognostication. In recent years, abnormalities in several muscle proteins have been identified in muscular dystrophies. Such abnormalities involve proteins located in the sarcolemma, extracellular matrix, cytosol, nucleus, and other sites within muscle fibers.[15] Skeletal muscle biopsy can play a main role in differentiating vascular dystrophy from non-dystrophic disorders and IHC can assist in establishing a specific diagnosis of the dystrophies for which specific protein abnormalities are known.

Research Application

Much of the current research into the causes of neurodegenerative diseases is directed at identifying the factors that result in the formation of paired helical filaments, the deposition of beta amyloid, cytoplasmic accumulations of alpha synuclein, etc. Consequently, studies to localize and quantify the abnormal proteins that constitute reasons of neurodegenerative diseases are of central importance. IHC using antibodies to beta amyloid, alpha synuclein, ubiquitin, huntingtin, polyglutamine, and others has become a routine tool for a sensitive detection and quantification of these abnormal proteins in both human tissues and in experimental animals that are used to model some of the features of these diseases. IHC is an important tool in diagnostic and research laboratories.