SAT-196 Maternal immunity: Preimplantation Preparation

The maternal uterus in mammals undergoes extensive remodeling in preparation for implantation of the semi-allogeneic embryo. Activation of the T cell homing receptor, CCR7, regulates multiple aspects of adaptive immunity. Genetic deletion of CCR7 reduces T regulatory (Treg) cell migration into mouse uteri and decreases embryo implantation. CCL19 and CCL21 are the sole ligands for CCR7. We hypothesized that CCL19 and/or CCL21 expression could attract Treg cells into the pre-implantation uterus and provide local immune suppression prior to implantation. Sprague Dawley rat uteri were removed from pregnant rats at Days 3-6. RNA was isolated and real time PCR was used to measure CCL19 and CCL21 expression. To determine the spatial distribution of these ligands, rat uteri were fixed, embedded in paraffin and uterine sections were analyzed by immunocytochemistry. The influence of sex steroids on the spatial distribution of CCL19 was evaluated in uterine sections from steroid treated rats. Hormonal control of CCL19 and CCL21 expression was investigated in a rat uterine stromal cell line treated with female sex steroids. Expression of CCL19 and CCL21 peaked at Day 4 of pregnancy compared with Days 3, 5 or 6 (Mann Whitney, p <0.05). Interestingly, expression of both ligands was significantly higher (p < 0.05) in the inter-site regions compared with the implantation sites at Day 6 of pregnancy. At Day 3 of pregnancy, CCL21 expression was limited to the glandular epithelium. Expression appeared in the antimesometrial uterine stroma at Day 4 of pregnancy. The distribution of CCL21 was similar between Days 4 and 5 of pregnancy but expression was less robust at Day 5. CCL21 expression was lost from the glands at Day 6 but was maintained in the antimesometrial stroma of the inter-site regions. CCL19 was expressed in the luminal and glandular epithelia of ovariectomized rat uteri. Progesterone pretreatment (2 mg daily, 3 days) stimulated expression of CCL19 in the periluminal stroma, which extended into the antimesometrial regions. Estradiol (0.2 µg) administration to progesterone pretreated rats, increased the expression of CCL19 but did not change its spatial distribution. Both ligands were detected in a rat uterine stromal cell line. CCL19 expression increased in response to estradiol (10 nM) at 6 h after hormone addition. Taken together, these results suggest that the maternal preparation for implantation includes a peak in the expression of CCL19 and CCL21 at Day 4 of pregnancy, one day prior to implantation in the rat. Unlike the constitutive expression of these ligands in primary and secondary lymphoid tissue, the spatial distribution and expression of CCL19 and CCL21 is regulated by female sex steroids in the uterus. The data support the concept that immune-mediated events, necessary for successful implantation and pregnancy, are induced in females prior to implantation. Funded by NIH.