Keratin 17: Immunohistochemical mapping of its distribution in human epithelial tumors and its potential applications

The distribution of keratin 17 (K17), molecular weight (MW) 46 kd, was immunohistochemically evaluated in a series of >500 formaldehyde-fixed, paraffin-embedded human epithelial and selected nonepithelial tumor specimens using a specific monoclonal antibody (clone E3). In normal epithelial tissues, K17 reactivity was found in a subset of hair shaft cells, sebaceous gland reserve cells, and respiratory and prostate basal epithelial cells. Furthermore, variable and inconsistent K17 expression was found in myoepithelia of sweat, salivary, and mammary glands. Although normal epidermis and other squamous epithelia were K17-negative, regenerative epithelia (e.g., at the site of ulceration or acute inflammation) were positive. Furthermore, K17 was present in dysplastic but not in normal squamous epithelia of skin and internal organs. Most squamous cell carcinomas showed K17-positive cells that were more numerous in more well-differentiated tumors. About 50% of pulmonary and >50% of pancreatic adenocarcinomas showed K17-positive cells. Although K17-positive cells, in some cases, reflected adenosquamous differentiation, this was not the case in all K17-positive adenocarcinomas. Approximately one third of mammary ductal carcinomas showed K17-positive cells, whereas lobular carcinomas were invariably K17-negative. In situ lobular and ductal carcinomas often showed prominent K17-positive myoepithelial cells; such cells showed even greater positivity when present with in situ carcinoma or proliferative conditions, as compared with normal ducts. Other adenocarcinomas often showing K17-positive cells included endometrial, ovarian, and, to a lesser degree, gastric carcinomas. K17-negative tumors, almost without exception, included colon, prostate, renal, and hepatocellular carcinomas and neuroendocrine tumors. Synovial sarcomas typically showed K17-positive epithelial cells in biphasic but not in monophasic spindle cell tumors. The observed distribution of K17 indicates that this marker may be helpful in evaluating abnormalities of squamous epithelia and in the subtyping of epithelial tumors.