Advances in Neuroimmune Biology

In this paper, I outline briefly the concepts that led to the evolution of Reproductive Immunology. The history starts in 1971, with the “Medawar paradigm”, and as a consequence in the 70 s–90 s, the placental barrier and immunosuppression concepts were proposed. The first conceptual breakthrough started with the “vaccination against abortion in mice”, leading to immunotrophysm, and the importance of cytokines emerged, which lead to the disapproval of the Th1:Th2 paradigm. It was necessary to revise this approach, and, notably, it was recognized that NK cells were not only “bad guys”, but in fact were required for successful pregnancy. NK cells are involved in the transformation of the local vessels. We are interest in the control of NK activation by various interleukins (notably the Il-12, IL-18, NK tripod) as well as tumour necrosis factor-related weak inducer of apoptosis (TWEAK) in mice and human. One must now recognize “suppression” as Tregs emerged, and recognize signals of the embryo. We also discuss here FF G-CSF and sHLA-G. Microarrays, as well as studies on innate immunity, which are amongst our latest interests, while combining studies in mice and human, with spin off consequences for the clinician.

In reproduction, in which there is an exposure to paternal alloantigens at different stages, the female genital tract develops a tolerogenic response during implantation and pregnancy. We summarize data from the literature which support an important contribution of seminal plasma to the maternal immune tolerance to the conceptus and to the establishment of pregnancy. Seminal plasma has for long been considered as a medium for the survival and transport of sperm. However, other important roles related to early pregnancy in the female have been described. Immunoregulatory molecules present in seminal plasma, like prostaglandins and different cytokines, induce throught different mechanisms tolerance to paternal/trophoblast antigens and prepare the uterine endometrial tissues for embryo implantation. A switch from an early pro-inflammatory to an anti-inflammatory state is partly induced by seminal plasma molecules at implantation. In this review we focus on the importance of prostaglandins and TGF-β. This cytokine is one of the most important driver for the induction of T regulatory cells proliferation. It can be stated that the decrease in any of the immunomodulatory molecules from seminal plasma reduces the immunotolerance mechanisms during embryo implantation.

The maternal-fetal interface of pregnant mammals is characterized by a sensitive balance between hormones, cytokines, humoral factors and cellular interactions. Progesterone activity leads to the transformation of endometrial cells into the decidual phenotype and ensures the integrity of the maternal-fetal interface during trophoblast invasion and placenta maturation. Communication between the genomic and non-genomic progesterone-regulated signaling pathways could be of critical importance for the establishment of a correct endocrine-immune interaction in the human endometrium during the establishment and maintenance of pregnancy. Cytolytic cells (NK and T lymphocytes) infiltrating the decidua of pregnancy are heavily equipped with cytolytic molecules perforin and granulysin. These molecules have very important role(s) in the maternal tolerance of fetoplacental unit, control of trophoblast invasion, defense against infective agents and malignancies, as well as immunomodulatory functions. Mucins MUC-1 and TAG-72 are important features of secretory phase decidual reaction, while their disappearance from the surface of the epithelial cells enables apposition, adhesion and infiltration of high quality blastocysts at the precise area at the time of the implantation. Muc-1 and TAG-72 induce antiinflammatory orientation of the decidual antigen presenting cells.

The foetal-maternal interaction results in the induction of a local inflammatory response and a state of systemic inflammation. Several factors are involved in successful embryonic implantation, including hormones, growth factors, cytokines, chemokines, adhesion molecules, extracellular matrix components, and matrix-degrading enzymes. The enriched cytokine milieu associated to implantation is likely to control trophoblast migration and differentiation, leukocyte influx and activation, complement activation, as well as angiogenic and angiostatic processes in the implantation site. Finally, these mediators play a key role in tuning the immune responses to protect the foetus from infections as well as from maternal rejection. Here, the role of two new players involved in regulating inflammation at the maternal-foetal interface will be discussed: the long pentraxin PTX3 and the decoy receptor for inflammatory chemokines D6.

The complement system is one of the major components of humoral innate immunity, acting as one of the first lines of defence against microbes, but new roles in inflammatory and immunological processes are emerging. The placenta undergoes an intense process of tissue remodelling which leads to the activation of the complement (C) system resulting in the release of potentially destructive activation products that need to be neutralized. The protection of the foetus against maternal C activation products is achieved by the surface expression of regulators. The liver is the main source of the plasma C components although extra-hepatic synthesis in several tissues and organs has been documented. The data collected recently indicate that trophoblast cells are able to secrete C3 and C4 and the recognition molecule C1q, contributing to the local synthesis at the placental level. Besides trophoblast cells, decidual endothelial cells acquire the ability to synthesize and express C1q on the cell surface. All these observations support the role of C1q in the placental development and its importance in trophoblast endovascular and interstitial invasion. In conclusion it is increasingly evident that a new role of complement and in particular C1q in the processes of tissue homeostasis as well as is in inflammation and infection is now emerging.

Pregnancy is personally special to every woman expecting a child, but is also interesting from the perspective of an immunologist. During a physiological pregnancy, the mother's immune system decides to tolerate and foster an incorporated, non-self, non-dangerous organism. Whether the maternal reaction stems from deciphering the foreigness or safeness of this new individual, it is the general consensus that there is a foeto-maternal, bidirectional “dialogue” occurring and that the “messages” that are “spoken” are relayed through signaling mediators, which are capable of transmitting a functional command to a target cell. Much information dedicated to this theme has been gleaned in the past decade; however, the complex nature of cytokine networks jeopardizes clarity. In this review, we touch upon a list of mediators that are vital for reproduction. These factors are divided according to their receptor family, because this elucidates the characteristic signal transducing pathway, which is expected to mediate their signal within the target cell. The target cells of interest are the trophoblast, upon which we focus for several reasons: 1. the trophoblast represent the foetal compartment while participating in foeto-maternal interplay (e.g. while invading the decidua, trophoblasts are in constant communication with uterine, maternal immunocytes, which check and contain this function), 2. trophoblasts are responsible for foetal well-being (e.g. nutrition, protection from the environment) and 3. dysfunctional trophoblast function results in several pregnancy complications (e.g. preeclampsia, intrauterine growth retardation, miscarriage, preterm delivery). We summarize what is described in the literature on how these mediators are distributed within the reproductive tract, which cells are expressing their respective receptors (especially which trophoblast subsets) and how they modify trophoblast function (namely invasion, proliferation, differentiation and apoptosis). Furthermore, we unearth for which mediator the signal transducing pathway is verifiably used in trophoblast (ic) cells. Finally, we correlate actual biological importance of the mediator for reproduction by comparing murine knockout phenotypes and known positive and negative associations of these mediators with human pregnancy pathologies (as listed above). We expect this concise review to be useful to both basic researchers and clinicians who wish to obtain an overview of the reproductive cytokine network in respect to the trophoblast.

In the frame of EMBIC study, maternal KIR and trophoblastic HLA-C matching was estimated in 39 randomly selected cases of women who were undergoing vacuum uterine curettage for therapeutic termination of first trimester missed pregnancy (Group A = 19) or elective termination of normal pregnancy (Group C = 20). DNA from decidual cells was used for maternal KIR genotyping by Luminex, while DNA from trophoblastic cells was used for foetal HLA C typing by PCR-SSP. HLA-C alleles were grouped as C1 and C2. According to the results, the women in group A were found to possess fewer inhibitory KIR genes than the women in group C (31.6% of A women had all 3 inhKIR -2DL1, 2DL2, 2DL3- receptors vs. 80% of the Controls), while no differences were found in the number of actKIR genes detected in the two groups. The inhKIR2DL1 was statistically significant decreased in group A compared to the controls (p = 0.0043). The percentage of cases possessing the KIR2DL1-C2 combination, which is associated with increased inhibition, was found to be lower in group A in comparison to group C, while the combination KIR2DL3-C1, which is associated with increased activation, was found in higher percentage in group A than in controls. Our findings support the suggestion that uNK cells of women predisposed to miscarriage may have a higher activating potential and that both inhibitory and activating KIR receptors on uNK cells are important for the maintenance of pregnancy.

Paternal HLA-C antigens expressed by trophoblast, which liken the trophoblast to a semiallograft, could be presented by the maternal APCs to the specific maternal CD4⁺ T helper cells, which could release various cytokines in response to these alloantigens. On the basis of the cytokines produced the effector CD4⁺ T helper lymphocytes can be classified in Th1, Th2 and Th17 cells. We focused here the possible role of these 3 human effector CD4⁺ T helper subpopulations, known to be involved either in allograft rejection (Th1 and Th17 cells) or in allograft tolerance (Th2 cells). Th1 and Th17 cells may compromise pregnancy and be responsible for miscarriage and Th2 cells may be responsible for the success and maintenance of pregnancy. Interestingly, our recent findings showed a beneficial role of Th17 cells, whose major role is the protection against extracellular bacteria and thus may promote adequately the response required to protect the mother against dangerous extracellular pathogens. The chronology of action of Th17 cells remains to be investigated to understand completely the mechanisms by which pregnancy development could or could not be affected by Th17 cells.

Establishment of pregnancy relies not only on peripheral adaptations of the immune system to provide a suitable environment whereby the growing embryo escapes rejection, but also on adaptations of the neuroendocrine system that optimally prepares maternal tissues to effectively support embryo development. This review gives an outline of the reciprocal and necessary interactions between the two systems that underpins pregnancy establishment. Both immune balance and neuroendocrine secretion are readily altered by varying internal and/or external environmental conditions. Therefore, we also give an overview of the impact of stress exposure on each system and on their delicate reciprocal control in early pregnancy, linking to potential mechanisms of stress-induced neuro-endocrine-immune-mediated pregnancy failure.

The immunology of pregnancy has been mostly investigated in the mouse, although this animal model is clearly inappropriate for several critical aspects of the studies. The contribution of alternative and less known models such as ovine and bovine species should not be neglected. The uterine morphology and the important resistance to invasive surgery, the synepitheliochorial placentation, the long peri-implantation phase associated to a specific signal of maternal recognition – interferon-tau- and the availability of original experimental models including somatic cloning offer unique opportunities of investigation for answering fundamental and applied biological questions. The low and high-throughput analyses of cell and tissue phenotypes have taken advantages from genome annotation and powerful genomic tools including microarrays and functional exploration of a target gene function. Considering present reports and future directions, ruminants can undoubtedly contribute in a relevant way to a better knowledge of the immunological process taking place in a successful, perturbed or failed pregnancy.