KERATIN-7 REACTIVITY IN ENDOTHELIAL-CELLS - A POTENTIAL PITFALL IN DIAGNOSTIC IMMUNOHISTOCHEMISTRY

Human vascular endothelial cells are usually regarded as being vimenti n-positive and keratin-negative. However, in lower animals such as fro gs and fish, endothelial cells commonly contain simple epithelial (low -molecular-weight) keratins. Observation of keratin 7 (K7) immunoreact ivity in normal endothelial cells of soft tissue prompted us to examin e the possible keratin reactivity of nonneoplastic endothelial cells i n more detail, using formaldehyde-fixed, paraffin-embedded tissue and antibodies recognizing K7 and other keratin polypeptides. Small venule s and lymphatics in peripheral soft tissues, intestinal mucosa, uterin e ectocervix, and lymphoid tissue were variably positive for K7, but a rterioles and arteries including aorta showed only sporadic K7+ endoth elial cells. Endothelial cells were negative for K8, but vascular smoo th muscle cells, especially in arteries, showed K8 (CAM5.2) reactivity in some cases. However, our results do not disagree with previous fin dings of CAM5.2 negativity of endothelial cells that are keratin-posit ive with other antibodies reactive with K8 and K18. No reactivity for K17 or K19 was seen in endothelia or in vascular smooth muscle. These findings indicate that subsets of vascular and lymphatic endothelial c ells may contain K7. K7 immunoreactivity in endothelial cells must be considered in the differential diagnosis of epithelial neoplasms, such as keratin-positive soft tissue tumors (i.e., synovial sarcomas) and metastatic carcinomas.