Localization of ISG15 and Conjugated Proteins in Bovine Endometrium Using Immunohistochemistry and Electron Microscopy

The interferon-stimulated gene ISG15, a ubiquitin homolog, becomes conjugated to and regulates uterine proteins in response to conceptus-derived interferon-τ on d 18 of pregnancy. It was hypothesized here that cellular localization of ISG15 within endometrial cells might provide insight regarding function. Uteri were collected from cows (∼21-d estrous cycle) on d 17–21/0 of the estrous cycle and pregnancy and d 23, 45, and 50 of pregnancy. Intracellular ISG15 and its conjugates were present on d 17 of pregnancy, peaked to highest levels from d 18 to 23 and then declined to low but detectable levels by d 45 (P < 0.05) based on Western blotting. ISG15 and its conjugates were not detected on d 50 of pregnancy or during the estrous cycle. Immunohistochemistry revealed that ISG15 was localized throughout the endometrium on d 18–23, with heaviest staining in the sublumenal stratum compactum and the glandular epithelium throughout the stratum spongiosum. By d 45 and 50, ISG15 was lightly stained only in the stra...