Description
Mesothelin is a glycoprotein located on the cell surface that stimulates T cell
responses, initially identified as the antigenic target of the K1 monoclonal antibody,
which was developed using an ovarian cell line as the immunogen. In normal
tissues, mesothelin expression is predominantly confined to mesothelial cells.
Nonetheless, positivity for this marker has also been documented in some epithelial
cells of the trachea, tonsil, kidney, and fallopian tube . Mesothelin is a highly
sensitive biomarker for mesothelioma, reportedly expressed in nearly all epithelioid
mesotheliomas and some carcinomas, especially serous carcinomas of the ovary
and pancreatic carcinomas. Epithelioid mesotheliomas and adenomatoid tumors
consistently exhibit mesothelin expression, typically characterized by a robust and
diffuse reactivity along the cell membrane. This contrasts with sarcomatoid
mesotheliomas, which typically test negative for this marker . Most pancreatic
adenocarcinomas (86%-100%) and nonmucinous ovarian carcinomas, such as
serous carcinomas (93%-100%), clear cell carcinomas (43%-75%), and transitional
cell carcinomas (100%), are known to be mesothelin positive. About 40% to 50% of
pulmonary adenocarcinomas and 15% to 30% of squamous cell carcinomas of the
lung have been shown to exhibit this marker with a localized and cytoplasmic
staining pattern, unlike the membranous pattern observed in mesotheliomas .
Despite the low specificity for mesothelioma, the prevalent robust membranous
reactivity in epithelioid mesotheliomas should be considered a significant indicator
against the diagnosis of mesothelioma when a negative stain is obtained .
Furthermore, mesothelin expression has been documented as either absent or
infrequently faintly positive in certain carcinomas, including renal cell carcinomas,
and may therefore be incorporated into the panel of markers utilized to differentiate
these tumors from epithelioid mesotheliomas.
