Effects of the estrous cycle and early pregnancy on uterine expression of Mx protein in sheep (Ovis aries)

Conceptuses of ruminant ungulates produce large amounts of a type I interferon, interferon-tau (IFNτ), which is the signal for maternal recognition of pregnancy. Induction of cellular Mx proteins is an important component of the response to type I interferon in the immune system, but Mx regulation and function have not been studied in the uterus. This study examined temporal and spatial alterations in ovine uterine Mx expression during the cycle and early pregnancy using immunohistochemistry, in situ hybridization, and Northern and slot-blot analysis. Sheep uterine endometrium expressed a single ~2.5-kilobase Mx mRNA transcript that was detectable at all stages of the estrous cycle and early pregnancy examined. In cyclic ewes, mRNA abundance in endometrium increased from Day 1 to peak levels at Day 13 and then declined to Day 15. In pregnant ewes, steady-state levels of Mx mRNA were first detected above the level in cyclic ewes at Day 13 postmating, were greater than 10-fold higher at Day 15, and remained elevated at Day 19. Expression of Mx mRNA in the myometrium did not change during the estrous cycle but increased ~23-fold between Days 11 and 15 of pregnancy. Immunohistochemical and in situ hybridization analysis revealed a similar temporal pattern of Mx expression. In cyclic ewes, Mx protein and mRNA were initially localized to the luminal epithelium at Days 1 and 3, increased from Days 5 to 13, especially in the shallow uterine glands, and then declined at Day 15. Pregnancy resulted in up-regulation of Mx expression in the luminal and glandular epithelium, stroma, and myometrium. Punctate Mx immunostaining and Mx mRNA concentrations were greatest when progesterone production was maximal during the estrous cycle and were strongly upregulated by the conceptus across the entire uterine wall. It is suggested that a cascade of induction of Mx gene expression proceeds from the luminal epithelium to the outer longitudinal myometrium and that transcriptional activation of the promoter may involve both soluble cytokines (i.e., IFNτ) and steroid hormones (i.e., progesterone).