Placental Abnormalities in the Humanized Mouse Model of Sickle Cell Disease

Introduction: Sickle Cell Disease (SCD) is a group of inherited red blood cell disorders commonly characterized by abnormal hemoglobin production resulting in the deformation of red blood cells into a rigid, sickle-shape under conditions of hypoxia, acidosis, and increased physiological stress such as pregnancy. Pregnancy in SCD severely increases the likelihood of obstetric complications including veno-thromboembolism, preterm labor, intrauterine growth restriction, and maternal and fetal mortality. During placental development, the impact of SCD becomes evident as vaso-occlusion and vasculopathy resulting from sickling of red blood cells contribute to hemolysis, endothelial dysfunction, pro-inflammatory responses, and increased cellular adhesion. These mechanisms lead to diminished placental health and impaired nutrient and oxygen exchange to the fetus. Through our previous studies, we have identified worsened pregnancy outcomes in the Townes humanized mouse model of SCD. This study now aims to characterize the ramifications of SCD on the placenta using the same translational model. Methods: Townes SCD (SS) females underwent timed breeding with healthy (AA) males along with AA male and female breeding pairs as controls. At the end of gestation (18.5 days post conception (dpc)) dams were sacrificed and placentas collected and processed for histology. Frozen sections underwent appropriate staining (H&E, PAS, Masson's trichrome) and were digitized and coded for blind assessment by a pathologist. Identified pathologies were scored for severity using the scale 0.0 - Absent, 1.0 - Mild, 2.0 - Moderate, and 4.0 - Severe. For all studies we analyzed one (1) placenta per litter for seven (7) SS and seven (7) AA dams. Data is reported as mean ± SD and p-values were calculated with α = 0.05 and significant threshold set at p < 0.05 denoted by (*). Highly significant p-values were further denoted as: p < 0.01 (**), p < 0.001 (***), and p < 0.0001 (****); ns = not significant. Results: When compared to healthy controls, placentas from mice with SCD display significant evidence of vascular trauma including maternal and fetal vessel ectasia (AA 1.3 ± 1.1 vs SS 2.6 ± 0.8**), fibrosis (AA 0.4 ± 1.1 vs SS 1.9 ± 1.2**), thrombosis (AA 0.4 ± 0.8 vs SS 1.6 ± 0.9*), hemorrhage (AA 1.1 ± 1.1 vs SS 2.9 ± 1.4*), acute necrosis (AA 0.3 ± 1.4 vs SS 1.7 ± 1.5*), and calcification (AA 0.6 ± 0.6 vs SS 1.6 ± 0.6**). Furthermore, SCD placentas displayed irregularities in overall structure such as decreased labyrinth area (AA 55.6 ± 8.6% vs SS 44.7 ± 6.0%*), placental architectural loss (AA 0.4 ± 1.4 vs SS 2.9 ± 1.3****), and invasion of glycogenated cells (AA 0.4 ± 1.2 vs SS 1.7 ± 0.9*) and spongiotrophoblasts (AA 1.3 ± 0.9 vs SS 2.1 ± 0.0*) into the labyrinth acting to damage the placenta's ability to effectively participate in maternal-fetal nutrient, gas, and waste exchange. Additionally, placentas from mice with SCD displayed an increased number of decidual natural killer cells which are cytotoxic to the maternal-fetal interface and implicated in preterm delivery (Peak Section Count AA 5.6 ± 2.8 vs SS 9.1 ± 3.3**) Conclusion: With the growing adoption of improved care plans along with increased availability of interventions such as hydroxyurea, blood transfusion therapy, and recent anti-sickling agents the overall health and survival of individuals with SCD has improved significantly, allowing more women to reach sexual maturity and consider reproduction. It is the objective of our investigations to address the severe obstetric disparities faced by these women to ensure a safer childbearing experience. This study helps to further characterize pregnancy in the humanized mouse model of SCD by identifying placental pathologies which likely contribute to worsened maternal and fetal outcomes. These abnormalities are highly reflective of the systemic inflammatory and ischemic environment of SCD and highlight the unique milieu's ability to compromise the integrity and functionality of the placenta. We believe these studies establish the importance of the maternal-fetal interface in overall pregnancy outcomes in SCD and support continued investigation of placental insufficiency in both clinical and translational applications. We further believe these results support the utility of the Townes mouse model in preclinical investigations of pregnancy in SCD, a condition where clinical vulnerabilities remain high.