Immunologie de la grossesse : faits nouveaux

[Impact of maternal obesity and diabetes on placental function]

Located at the feto-maternal interface, the placenta is involved in exchange, endocrine and immune functions, which impact fetal development. In contact with the maternal environment, this organ is sensitive to metabolic disorders as over-nutrition, obesity or diabetes. The alteration of blood parameters associated with these pathologies affects placental histology, vascularization and nutrient transfers and, according to the types of troubles, induces local inflammation or hypoxia. These placental changes lead to disturbance of development and fetal growth, which increase the risk of pathologies in offspring in adulthood. The placenta thus appears as a crucial player in the fetal programming.

[Syncytins - retroviral envelope genes captured for the benefit of placental development]

In the past, germline infections by retroviruses have led to vertical transmission of "endogenized" retroviruses. Escaping genetic drift, some of the viral genes have been conserved until now because of beneficial effects on their host. Here we present the syncytin genes that encode envelope proteins from endogenous retroviruses. Syncytins have inherited fusogenic properties from their infectious ancestor and are specifically expressed in the placenta. Both properties have suggested their involvement in the formation of the syncytiotrophoblast, a multinucleated layer that mediates feto-maternal exchanges in the placenta. The capture of syncytin genes occurred on several independent occasions during evolution of mammals. Knock-out experiments of syncytins in the mouse definitively confirmed the role of these genes in placentation. Finally, a second function for syncytins, i.e. an immunosuppressive activity, could contribute to materno-fetal immune tolerance. This constitutes a remarkable example of convergent evolution where the properties of retroviral envelope genes are subverted to play a major physiological role.

The danger hypothesis applied to idiosyncratic drug reactions

The danger hypothesis has had a profound effect on the way immunologists view the immune response. This hypothesis proposes that the major determinant of whether an immune response is mounted against some agent is determined by whether that agent causes some type of cell damage. Assuming that most idiosyncratic drug reactions (IDRs) are immune-mediated, this hypothesis also has the potential to explain many aspects of the mechanism of these adverse drug reactions. For example, most IDRs appear to be caused by chemical metabolites rather than the parent drug, but not all drugs that form reactive metabolites are associated with a significant incidence of IDRs. Therefore, using the danger hypothesis, one feature of a drug candidate that may predict whether it causes an IDR is whether the drug, or more likely its reactive metabolites, cause cell damage. Although the range of molecules that can act as danger signals is unknown, the most attractive candidates are high mobility group box 1 protein (HMGB1), heat shock proteins, and S100 proteins. These molecules act through the same receptors (toll-like receptors) as pathogen-associated molecules that stimulate the immune system. Therefore, other environmental factors such as infections or trauma might determine which patients would be at increased risk for IDRs. Although there are examples where this appears to be the case, in most cases there are no obvious environmental factors that determine IDR risk. In addition, in animal models of immune-mediated reactions, stimulation of toll-like receptors often does not increase the immune response, and depending on the timing, it can actually be protective. Therefore, there may be additional unknown control mechanisms that are involved. A better understanding of these fundamental immune mechanisms has the potential to have a significant impact on many areas of medicine.