Aims Cytokeratin 5 (CK5) is a surrogate maker of progenitor cells and early glandular and myoepithelial cells (MECs) in the breast, and CK5 expression in breast MECs varies from ducts to lobules, and from normal to diseased tissue. However, the mechanisms underlying immunophenotypic alterations of CK5 expression in MECs remain unclear.

Methods CK5 expression in MECs of 20 normal breast samples, 58 ductal carcinoma in situ (DCIS; including 21 DCIS with extensive lobular involvement), 11 atypical ductal hyperplasia (ADH), 18 non-invasive lobular neoplasia consisting of 11 atypical lobular hyperplasia (ALH) and 7 lobular carcinoma in situ (LCIS), 20 cystic lobules and 10 usual ductal hyperplasia (UDH) involving lobules were observed to evaluate the effects of contact with benign hyperplastic or cancerous luminal cells and pressure of dilated glands on CK5 expression.

Results CK5 expression in normal ductal MECs was exclusively positive, whereas most normal lobular MECs were negative. In DCIS, cancerous ducts were primarily surrounded by CK5-positive MECs (91.0%), as were lobular acini involved by DCIS (89.2%), while the remaining normal acini maintained CK5-negative. CK5-positive MECs were found in 57.5% of acini in ALH and were more prevalent in LCIS (70.7%). CK5 expression was occasionally positive in both cystic lobules (16.7%) and lobules involved by UDH (14.3%), while an increase of CK5-positive MECs was found in ADH (38.2%).

Conclusions These results suggest that CK5 expression in lobular MECs may be altered by contact with cancerous luminal cells rather than benign hyperplastic luminal cells or pressure from dilated glands.