HEp-2, which stands for Human Epithelial type 2, is a laboratory-grown cell line used as a foundational tool in medical diagnostics. It helps identify markers for various autoimmune conditions. These cells provide a standardized substrate for detecting specific antibodies in patient samples, particularly in tests screening for autoimmune disorders.
The HEP-2 Cell Line: A Closer Look
The HEp-2 cell line originated from human laryngeal carcinoma cells in 1954. Despite a later reclassification, HEp-2 cells remain widely utilized in laboratories due to their characteristics. They possess a native protein array that presents numerous antigens, making them an excellent substrate for detecting antinuclear antibodies (ANA). Their large nuclei and abundant cellular components allow for clear visualization of autoantibodies, which are antibodies that mistakenly target the body’s own tissues.
These cells serve as a standardized “target” because they contain a wide variety of cellular components, including those found in the nucleus, cytoplasm, and structures involved in cell division. This broad range of antigens enables the detection of over 100 different autoantibodies. The presence of mitotic figures, or dividing cells, within the HEp-2 cell preparation further enhances their utility, aiding in the recognition of various antibody patterns.
The ANA Test and HEP-2 Cells
HEp-2 cells are primarily utilized in the Antinuclear Antibody (ANA) test, specifically through a method called Indirect Immunofluorescence Assay (IFA). This assay is considered a reference method for ANA testing. The process involves incubating a patient’s serum, which may contain autoantibodies, with HEp-2 cells grown on a slide. If autoantibodies are present in the serum, they bind to the corresponding cellular components within the HEp-2 cells.
After a washing step to remove unbound serum components, a second antibody is added. This secondary antibody binds to human autoantibodies attached to the HEp-2 cells and is labeled with a fluorescent dye. When viewed under a fluorescence microscope, areas where autoantibodies have bound to the HEp-2 cells will glow, allowing for the visualization of these autoantibodies and the identification of distinct staining patterns. The comprehensive antigen presentation of HEp-2 cells is a key reason why IFA using these cells is effective in screening for a wide range of autoantibodies.
Interpreting ANA Test Results
ANA test results on HEp-2 cells are reported in two main parts: the titer and the fluorescence pattern. The titer indicates the concentration of autoantibodies in the patient’s blood, expressed as a ratio such as 1:40, 1:80, or 1:160. Higher titers, often 1:160 or above, are more likely to be considered clinically significant.
The fluorescence pattern describes how the autoantibodies stain the HEp-2 cells, providing clues about which specific cellular components are being targeted. Common patterns include:
Homogeneous, where the entire nucleus shows even staining.
Speckled, characterized by fine or coarse dots across the nucleus.
Nucleolar, which stains the nucleoli within the nucleus.
Centromere, which shows distinct dots corresponding to the centromeres of chromosomes.
Cytoplasmic patterns, which stain components outside the nucleus.
Each pattern suggests different types of autoantibodies, guiding further investigation.
Clinical Significance and Test Limitations
A positive ANA test result, especially with certain patterns and higher titers, can suggest the presence of various autoimmune diseases, such as Systemic Lupus Erythematosus, Sjögren’s Syndrome, or Scleroderma. However, a positive ANA test alone is not sufficient to diagnose a specific autoimmune disease. It functions as a screening tool, indicating the possibility of an autoimmune process that requires further clinical evaluation and additional specific tests.
A low-titer positive ANA result can occur in healthy individuals; up to 30% of healthy adults show a positive ANA at a 1:40 titer and around 15% at 1:80. The prevalence of positive ANA also tends to increase with age. Additionally, certain medications, infections, or other medical conditions can lead to a positive ANA test without an autoimmune disease. Conversely, some individuals with autoimmune conditions may have a negative ANA result. Medical professionals consider the ANA test results alongside a patient’s symptoms and other laboratory findings for an accurate diagnosis.