Expression of Ly‐6D on the Surface of Normal and Neoplastic Mammary Epithelial Cells of the Mouse

Rat were immunized with mouse mammary epithelial cells and monoclonal antibodies (MAbs) were obtained to identify antigens stably expressed on the surface of both normal and neoplastic mammary epithelial cells of the mouse. Examination of the reactivities of the MAbs by immunofluorescence staining of tissue sections showed that one of the antibodies, MAb 2A2, reacted with luminal epithelium of the mammary gland and spontaneous mammary carcinomas of SHN mice. Further examination of the tissue lysates by western blot analysis revealed that MAb 2A2 reacts with a 17‐kDa antigen expressed in normal mammary epithelial cells and mammary carcinoma cells. The antigen recognized by MAb 2A2 was absent in the lysates of liver, lung, salivary gland, kidney, small intestine, ovary and uterus. After immunoaffinity purification of the MAb 2A2‐ recognized antigen and determination of its N‐terminal amino acid sequence, we identified the antigen as Ly‐6D, also known as ThB, which belongs to a family of glycosyl‐phosphatidylinositol‐anchored cell surface proteins. Northern blot analysis further demonstrated that Ly‐6D mRNA is expressed in the mammary gland. Based on these observations we concluded that Ly‐6D is stably expressed on the surface of both normal and neoplastic mammary epithelial cells of the mouse. Ly‐6D will serve as a useful epithelial cell surface marker for the study of mammary gland development, as well as for breast cancer research.