Pregnancy is characterized by three distinct immunological phases: 1) implantation and early placentation associated with a pro-inflammatory response (i.e., implantation resembles an ‘open wound’); 2) the second trimester and most of the third trimester of pregnancy when immune homeostasis is achieved through a predominantly anti-inflammatory state at the maternal-fetal interface; and 3) parturition, a pro-inflammatory state1-7. Immune cells play an important role in the regulation of the inflammatory response at the maternal-fetal interface where their abundance and localization change throughout pregnancy6-9.
In humans, the maternal-fetal interface represents an area of direct contact between maternal (decidua) and fetal (chorion or trophoblast) tissues. This interface includes: 1) the decidua parietalis that lines the uterine cavity not covered by the placenta and is in juxtaposition to the chorion laeve; and 2) the decidua basalis, located in the basal plate of the placenta where it is invaded by interstitial trophoblasts10 (Figure 1). The intimacy of these areas of contact creates conditions for fetal antigenic exposure to the maternal immune system11-13. Not surprisingly, leukocytes comprise up to 30-40% of the decidual cells8,9,14,15 in addition to typical stromal-type cells and glandular cells8,14,16. The role of leukocytes at the maternal-fetal interface encompasses multiple processes that include the limitation of trophoblast invasion17, remodeling of spiral arteries18,19, maintenance of maternal tolerance12,20, and initiation of labor21-26. Leukocytes of both the adaptive and innate limbs of the immune system, i.e., T cells, macrophages, neutrophils, B cells, dendritic cells, and NK cells, have been identified in the decidual tissues, and their proportions and activation status have been shown to vary spatially and temporally throughout gestation6-10,12,14,24,27-30. Perturbations in the leukocyte population and/or function are associated with spontaneous abortion31, preeclampsia32, intrauterine growth restriction32,33, and preterm labor7,24. Therefore, the study of the phenotypic characteristics and functionality of leukocytes at the human maternal-fetal interface will facilitate the elucidation of the immunological pathways dysregulated in pregnancy disorders.
One of the most powerful tools used to determine the phenotype and functional properties of leukocytes is flow cytometry, technology that allows the quantitative analysis of multiple parameters simultaneously34-36. To analyze leukocytes by flow cytometry, isolation of the leukocytes in a single-cell suspension is required. Therefore, a method to separate infiltrating leukocytes from the maternal-fetal interface is needed to study their phenotypic and functional properties.
Several methods have been described to isolate leukocytes from the human maternal-fetal interface10,14,25,27,37-39. While some apply mechanical disaggregation10,25,27,38, others use enzymatic digestion37,40 for tissue dissociation. Because mechanical disaggregation produces a lower yield and reduced viability41, and enzymatic dissociation can affect viability and cell surface marker retention42, the method described herein combines gentle mechanical dissociation with enzymatic pre-treatment to increase the yield of isolated leukocytes without compromising cell viability. A similar combination of methods has been demonstrated to be effective in the isolation of leukocytes from the decidual tissues at the maternal-fetal interface39. Therefore, the protocol described herein involves mechanical disaggregation with an automatic tissue dissociator that increases consistency while saving time and labor when compared to traditional mincing with opposing scalpels, razor blades, or surgical scissors10,28. The enzyme chosen for tissue dissociation was Accutase. Unlike commonly used collagenase43, dispase44, and trypsin45, Accutase (a cell detachment solution) combines both general proteolytic and collagenolytic activities that contribute to efficient yet gentle dissociation46,47. After dissociation, the leukocytes are enriched from the total population of the decidual cells by density gradient centrifugation. Various density gradient media have been previously utilized, the most common of which are Percoll (a suspension of colloidal silica particles)48 and Ficoll (a polymer of sucrose with a high synthetic molecular weight)49. The superior efficiency of isolation by the sucrose polymer has been previously shown50, and the protocol described herein further proves that this density gradient media produces a sufficiently high purity of mononuclear leukocytes.
Hence, the protocol described herein combines the mechanical tissue disaggregation with an automatic tissue dissociator, enzymatic digestion with a cell detachment solution, and leukocyte separation with a density gradient media (1.077 + 0.001 g/ml) to isolate leukocytes from human decidual tissues. This protocol has been proven to preserve cell surface antigens along with cell viability. The isolated leukocytes can be used for multiple applications that include immunophenotyping with flow cytometry and functional studies in vitro.