Could this immunosuppressive activity play a role in the case of syncytins?
TH: A third hypothesis that we had not introduced in our 1998 PNAS paper concerns the syncytins: being env genes from endogenous retroviruses, they are expected to be not only fusogenic -in relation with syncytiotrophoblast formation- but also immunosuppressive. This actually proved to be the case, for at least one of them in both the primate and rodent lineage, leading to the working hypothesis that this property could play a role -or could have played a role in evolution- in the establishment of maternal-fetal tolerance, and even be associated with the emergence of placental mammals from egg-laying animals. Actually, such a hypothesis still remains a working model, and a series of experiments are underway in our laboratory using for instance knocked-in mice in which we have selectively abolished the immunosuppressive activity of the syncytins without altering their fusogenic activity, via specific mutations. Whatever the outcome of these experiments, it is already clear that endogeno
