Overview
Cytokeratin 8 (CK8) and Cytokeratin 18 (CK18) are low molecular weight cytokeratins (approximately 52.5–53.5 kDa) that are usually expressed as a paired complex. They are components of the intermediate filament cytoskeleton of epithelial cells. CK8 and CK18 are known constituents of nuclear inclusions in rhabdoid tumors, where they are tangled with vimentin, and are also present in Mallory bodies.
Mutations or altered expression of CK8/18 have been reported in malignant rhabdoid tumors, chronic pancreatitis (around 9%), and liver disease. Beyond structural support, CK8–18 play important roles in key cellular processes such as apoptosis, mitosis, cell cycle progression, and cell signaling. They influence tumor cell behavior and protect hepatocytes from oxidative stress. During certain infections, such as Chlamydia infection, CK18 is cleaved to permit vacuole expansion necessary for bacterial replication.
For diagnostic interpretation, CK8 or CK18 expression is considered positive only when more than 50% of tumor cells show expression; otherwise, it should be reported as negative. CK8-18
Symptoms
CK8–18 expression itself does not produce symptoms. Its clinical relevance lies in the evaluation of epithelial tumors and associated diseases. Symptoms observed in patients are related to the underlying pathological condition, such as adenocarcinomas, squamous cell carcinomas, liver disease, or pancreatic disorders. The marker assists in correlating histopathological findings with clinical manifestations rather than indicating a specific symptom profile.
Causes
Altered CK8–18 expression is associated with epithelial differentiation and malignant transformation. CK8–18 is widely used to identify epithelial cells and to differentiate epithelial tumors from non-epithelial tumors. It is useful in identifying most adenocarcinomas and some squamous cell carcinomas.
Expression patterns of CK8/18 are particularly helpful in classifying tumors of unknown origin and poorly differentiated carcinomas. CK8–18 is also used in the evaluation of sentinel lymph nodes in colon carcinoma. Reduced or absent staining intensity is commonly seen as tumors become poorly differentiated or dedifferentiated, reflecting loss of epithelial characteristics during tumor progression.
Risk Factors
CK8–18 expression patterns are associated with tumor behavior and prognosis. In squamous cell carcinomas of the oral cavity, CK8/18 expression indicates a poor prognosis. In duct cell carcinoma of the breast, CK8/18 immunostaining shows that loss of the normal membranous staining pattern in malignant tumors is significantly associated with higher tumor grade, increased mitotic activity, and negative HER2/neu status.
As tumors progress toward higher grade and poorer differentiation, CK8–18 staining intensity may decrease or become absent, which is an important risk indicator for aggressive disease and unfavorable outcomes.
Prevention
There is no direct prevention related to CK8–18 expression, as it is a biological and diagnostic marker rather than a disease entity. However, accurate diagnosis depends on correct laboratory technique and interpretation. CK8–18 testing is primarily performed using immunohistochemistry on formalin-fixed, paraffin-embedded tissue, and in selected settings, fluorescence in situ hybridization (FISH) may also be used.
CK8–18 typically demonstrates diffuse, granular cytoplasmic staining, with intensity varying depending on tissue type and cellular characteristics. In normal epithelial cells, staining is usually moderate to strong. Internal positive controls are essential and include epithelial cells from tissues where CK8–18 expression is normally expected.
Correct interpretation of CK8–18 results, in conjunction with histology and other immunomarkers, helps ensure accurate tumor classification, prognostic assessment, and appropriate clinical management.
