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Pregnancy involves adaptations of the cellular composition in utero to establish a functioning fetal-maternal interface. Different subsets of leukocytes populate the endometrium and contribute to tolerance of the fetal allograft while protecting it from potentially threatening infections or rejection. ¬¬Innate lymphoid cells are recently discovered immune cells that, besides the gut, lung and skin, possess immunoregulatory functions in the female reproductive tract, especially during gestation. Although present at the fetal-maternal interface, the dynamics of ILC migration during pregnancy remains poorly investigated. The involvement of homing receptors in ILC migration to the uterus was the main subject of the present work.
First, the expression of homing receptors on ILCs from miscellaneous organs was assessed across the course of murine pregnancy in vivo by means of flow cytometry. Then, their migratory capacity towards pregnancy-relevant chemokines was investigated in vitro. The impact of pregnancy related hormones on the migration and homing of ILCs was then analysed in vitro via migration assays.
The results confirm altered proportions of ILCs in utero and the altered expression of homing receptors in ILCs in pregnancy. Different murine lymphoid organs showed augmented expression of chemokine receptors and decreased levels of homing integrin α4β7 in the first trimester, suggesting enhanced migration patterns of ILCs during early pregnancy. Subsequently, migration assays were used to demonstrate the role of different chemokine ligands in enhancing ILC migration.
Eventually, the alterations in homing receptor expression were correlated with female pregnancy hormones. Progesterone treatment caused similar effects on homing receptor expression in ILCs as observed during early gestation. These results represent the first study evaluating the effect of sex steroid hormones on ILC chemokine receptor distribution.
Taken together, our results indicate the involvement of pregnancy-relevant chemokines, including CCL4, CCL20 and CCL28, in the recruitment of ILCs to the uterus during pregnancy. The data highlight an endocrinological-immune crosstalk in the regulation of ILC homing to the female reproductive tract. Gestation alters chemokine receptor expression in order to regulate the access of immune cell subsets to the fetal-maternal interface. An adequate regulation is highly needed, as a lack or abundance of different subgroups could result in pregnancy complications, including fetal loss, pre-eclampsia or pre-term birth. Thus, the role of ILC chemotaxis to the pregnant uterus and its regulation are of interest in the understanding, prevention and treatment of the clinically relevant obstetric diseases.
The success of pregnancy depends on precisely adjusted, local immune mechanisms. In early pregnancy, fetal trophoblast cells implant into the endometrium to build and anchor the placenta. Simultaneously, they mediate fetal tolerance and defense against infections. To cover these versatile requirements, local immune factors must be in balance. A too tolerogenic milieu can lead to an inadequate placentation; while a too inflammatory milieu can cause rejection of the semi-allogenic fetus. Bacterial infections can provoke these inflammatory pregnancy complications as well. Therefore, the pregnant uterus was long thought to be sterile. Descriptions of a placental microbiome opened a scientific discourse, which is unsolved due to contrary studies. The colonization of the non-pregnant endometrium is, however, confirmed. It is supposed to affect both, uterine pathologies and fertility. Precise data are lacking. Aim of this work was to assess if and under which circumstances a bacterial colonization would be tolerable.
One of the described species in placental and endometrial samples is Fusobacterium nucleatum. It is an opportunistic bacterium, which is known from the human oral cavity and associated with the development of colon carcinomas. F. nucleatum supports tumorigenesis by the induction of epithelial proliferation, survival, migration and invasion as well as angiogenesis and tumor tolerance. Since similar processes are required for implantation and placentation, F. nucleatum might support these as well. In this work, the effects of F. nucleatum on leukocyte-trophoblast-interactions, especially of macrophages and innate lymphoid cells type 3 (ILC3), were assessed.
The monocytic cells (THP-1) were differentiated into inflammatory M1 (IFN-γ) or tissue-repairing and tolerogenic M2a (IL-4) and M2c (TGF-β) macrophages. Inactivated F. nucleatum, LPS or E. coli was added. Only small concentrations of inactivated bacteria were used (bacteria:leukocyte ratio of 0.1 or 1), since it was not the aim to analyze infections. Conditioned medium of treated leukocytes was added to trophoblastic cells (HTR-8/SVneo). Migratory, invasive and tube formation behavior of trophoblastic cells was quantified.
Treated M1 macrophages impaired trophoblast function, whereas M2a macrophages induced trophoblast invasion. M2c macrophages supported trophoblast migration and tube formation if treated with the smaller, but not with the higher concentration of F. nucleatum. This treatment induced the accumulation of HIF-1α and the secretion of VEGF-A in M2c macrophages as well. Moreover, the higher concentration of F. nucleatum caused rather inflammatory responses (NF-κB activation and cytokine expression). The activation of the HIF-1α-VEGF-A axis under the influence of TGF-β might serve as a mild immune stimulation by low abundant commensal bacteria supporting placentation.
In contrast to macrophages, the function of ILC3s during pregnancy is still unknown. In general, ILC3s are located in mucosal tissue, such as the gut. They participate in tolerance mechanisms and form the local micromilieu by the secretion of cytokines and the presentation of antigens. In order to characterize local, uterine ILC3s, murine ILC3s were compared to peripheral, splenic ILC3s. Uterine ILC3s were more activated and produced higher levels of IL-17 compared to splenic ILC3s. However, uterine ILC3s barely expressed MHCII on their surface. A reduced antigen presentation potential was confirmed in human ILC3s differentiated from cord blood stem cells by the addition of TGF-β or hCG. The treatment with bacteria increased MHCII expression, but not to the initial level. The higher bacterial concentration induced IL-8 secretion and led to an increased trophoblast invasion. ILC3s were less sensitive to bacterial stimulation than macrophages.
Recent studies on the uterine or placental presence of bacteria during pregnancy are discrepant. The results of this project indicate that bacteria or bacterial residues might serve as a mild stimulus under certain circumstances to support implantation without negative effects. The current discussion must therefore not only be expanded by additional studies, but especially include differentiated local conditions. In this context, the sheer presence of bacteria or bacterial components must not be equated with an infection representing a known hazard.
IL-21 gehört zur Gruppe der proinflammatorischen Zytokine und ermöglicht als lös-licher Botenstoff eine Interaktion zwischen verschiedenen Zellen innerhalb des Im-munsystems. Unlängst wurde die Rolle von IL-21 bei der Entstehung von Autoim-munerkrankungen beschrieben. Dabei kommt es zur Fehlregulation des Immunsys-tems, so dass infolge körpereigene Strukturen bekämpft werden. Während einer Schwangerschaft kommt es zu umfassenden Anpassungsvorgängen innerhalb des Immunsystems, wobei mögliche Fehlregulationen sowohl für die Mutter als auch für den Fötus verschiedene Komplikationen mit sich bringen kann. Inwieweit IL-21 in-nerhalb dieser Vorgänge und letzten Endes für den erfolgreichen Ausgang einer Schwangerschaft von Bedeutung ist, ist bislang unbekannt und ist Gegenstand der in dieser Arbeit dargestellten Untersuchungsergebnisse.
Die Ergebnisse lassen erkennen, dass B-Zellen der verschiedenen Mausgruppen (NPM=nicht schwanger, GPOM=normale Schwangerschaften und PPOM=komplizierte Schwangerschaften) unterschiedlich sensibel auf IL-21 reagie-ren. Diese Beobachtung fußt auf unterschiedlicher Verteilung des IL-21R. Demnach exprimieren B-Zellen der GPOM-Gruppe niedrigere Level an IL-21R, deren Beein-flussung durch das Zytokin IL-21 als gering zu beurteilen ist und deshalb nachfol-gend relativ seltener in die Apoptose eintreten. Die größten Differenzen bezüglich der Rezeptorexpression konnten innerhalb der MZ B-Zellen beobachtet werden. MZ B-Zellen wird bereits in früheren Arbeiten eine wichtige Rolle für eine erfolgreiche Schwangerschaft zugeschrieben.
Mittels immunologischer Methoden wurde weiterhin die Produktion von IL-10 unter-sucht, die bei GPOM am höchsten war. Dieses Ergebnis deutet darauf hin, dass in dieser Situation B-Zellen sich durch eine Herunterregulierung ihrer IL-21R dem Ein-fluss des Zytokins entziehen und nachfolgend für eine vermehrte Produktion von IL-10 zur Verfügung stehen. Auch IL-10 konnte bereits als ein für eine erfolgreiche Schwangerschaft förderlicher Faktor identifiziert werden.
Als mögliche Quelle von IL-21 und aufgrund der räumlichen Nähe zu den MZ B-Zellen wurden Tfh-Zellen mittels Durchflusszytometer analysiert. Hierbei wurde festgestellt, dass diese Zellen gehäuft in der PPOM-Gruppe vorkommen, wodurch auch diese T-Zell Subpopulation für den Ausgang einer Schwangerschaft mit ver-antwortlich sein kann. Alle Ergebnisse dieser Arbeit sprechen in der Zusammen-schau dafür, dass es zu einer Reduktion des IL-21/IL-21R-Systems kommen muss, um einen erfolgreichen Ausgang der Schwangerschaft zu gewährleisten. Aus die-sem Grund sollte auf der einen Seite das IL-21/IL-21R-System verstärkt in den Fo-kus künftiger Forschung rücken und auf der anderen Seite ist gleichzeitig eine Er-weiterung des für eine Schwangerschaft bestehenden Th1/Th2/Th17/Treg Para-digmas um Tfh-Zellen angezeigt.