Direct immunofluorescence (DIF) is the go-to diagnostic test when evaluating vesiculobullous eruptions, connective tissue disease, and vasculitis. This specialized test allows visualization of autoantibodies and their reaction products in the epidermis and dermis (skin) and epithelium and subepithelium (mucosa). Indirect immunofluorescence (IIF) and enzyme-linked immunosorbent assay (ELISA) are additional tests that can help in the diagnosis of autoimmune blistering disease. In the blistering autoimmune diseases, the autoantibodies target components in skin and mucous membranes that are essential for cell-cell and cell-matrix adhesion causing separation within or beneath the epidermis, depending on where the target components are located. This article is intended to serve as a helpful primer for immunofluorescence testing in dermatology, with an overview of the tests available as well as pragmatic tips for optimal biopsy sites and specimen transport.
Direct Immunofluorescence
Immunofluorescence techniques date back to 1941 when Albert Coons, an American physician, pathologist, and immunologist, fluorescently labelled antibodies to visualize pneumococcal antigens in infected tissues.1-3 In dermatology, similar methodology was used to visualize the deposition of immunoglobulins and complement in the skin of patients with systemic lupus erythematosus in 1963.4 Basement membrane zone antibodies were first visualized via DIF in bullous pemphigoid in 1967.5 This elegant test utilizes specific antibodies labeled with fluorophores that are then incubated with the patient’s tissue, ultimately forming antibody-antigen conjugates that can be visualized with a fluorescent microscope. Antibodies usually include IgG, IgM, IgA, fibrinogen, and C3. Some institutions also evaluate for IgG4.
Transport medium is critical for proper evaluation of tissues using DIF. Inappropriate storage of tissue can degrade the antigen and confuse the interpretation of specimens. An acceptable medium for DIF includes Michel transport medium, which allows tissue to be stored for days while being transported at ambient temperature without loss of signal.6,7 Zeus medium also can be used and is more readily available. Alternatively, biopsy tissue can be snap frozen using liquid nitrogen. Specimens also may be stored on saline gauze but should be analyzed within 24 to 48 hours.8 Most importantly, do not place the specimen in formalin; even a brief soak in formalin can greatly alter results, especially when trying to diagnose pemphigus.9 Proper transport conditions are critical to prevent autolysis, mitigate putrefaction, and preserve morphology while maintaining antigenicity.10
Indirect Immunofluorescence
Indirect immunofluorescence can be helpful for detecting antibodies circulating in patient serum. Indirect immunofluorescence can be used to help diagnose pemphigoid, pemphigus, epidermolysis bullosa acquisita, bullous lupus erythematosus, and dermatitis herpetiformis. Serum testing also can be a helpful alternative when obtaining tissue is difficult, such as in children.
Indirect immunofluorescence is a 2-part technique that takes a bit longer to assay than DIF.11 The first step involves incubating prepared tissue substrates with patient serum. Unlabeled antibodies in the patient serum are allowed to bind to antigens in the substrate tissue for about 30 minutes. Doubling dilutions of patient serum can be performed to titer antibody levels. The second step uses fluorescein-labeled antihuman antibodies to recognize the antigen-antibody conjugates. Normal whole tissues (eg, monkey esophagus for pemphigus vulgaris, rat bladder for paraneoplastic pemphigus, salt-split normal human skin substrate for pemphigoid and epidermolysis bullosa) are the usual substrates for testing.11,12 Again, this test requires serum and should be collected in a red-top tube or serum-separator tube. Usually, a minimum of 0.5 mL is required for testing, but check with your preferred immunodermatology send-out laboratory before collecting.13
Indirect immunofluorescence usually involves an initial screening panel using 1 or 2 tissue substrates followed by individual antigen-specific assays that correspond to the clinical suspicion and IIF screening results.11 Salt-split skin is used to localize basement membrane zone autoantibodies to either the epidermal (roof) or dermal (floor) side. Although many dermatopathology laboratories offer DIF testing, IIF is more specialized and may be a send-out test at your institution.
Enzyme-linked Immunosorbent Assays
Another tool in the immunodermatology armamentarium is ELISA. Commercial ELISA systems are available for the detection of autoantibodies against bullous pemphigoid (BP) antigen 180, BP230, type VII collagen, desmoglein (Dsg) 1, Dsg3, and envoplakin.11 This test allows semiquantitative measurement of antibody levels and thus can be used to monitor response to treatment or identify relapse and treatment failure.11 For example, in BP, significantly increased baseline anti-BP180 IgG levels correlate with 1-year mortality rates (P=.001) and relapse rates (P=.041).14,15 Numerous additional studies support the observation that monitoring anti-BP180 as a potential marker of disease relapse can be helpful.16,17 In pemphigus, the presence or increase of autoantibodies at remission, either anti-Dsg3 or anti-Dsg1, may be a useful tool in predicting disease relapse.18 It is important for physicians to be aware of this to be able to offer guidance on prognosis.