The potential interplay between the innate immune system and the microbiota in uterine related pathology

Uterine mucosa is an important tissue barrier whose main function is to offer protection against pathogens and other toxic factors, while maintaining a symbiotic relationship with commensal microbes. Compared to other districts the uterine mucosa is unique since it changes cyclically during the menstrual cycle as well as pregnancy. The immune system, besides its role in the defense process, plays crucial roles in reproduction as it ensures local immune tolerance to fetal/paternal antigens, trophoblast invasion, and vascular remodeling. The human endometrium contains a high number of immune cells phenotypically distinct from the cells present in the peripheral blood. The number and the phenotype of these cells change during the menstrual cycle. Furthermore, in the endometrium it is possible to identify few lymphoid aggregates comprising B cell and CD8+ T cells. The microbiota can influence the cell phenotype and function of the local immune system. Immune cells can sense the presence of microbes through their pattern recognition receptors, setting up host-microbe interaction. The microbiota exerts an appropriately controlled defense mechan- ism by competing for nutrients and mucosal space with pathogens. Alteration in the uterine microbiota is likely to play a role in diseases such as endometriosis. There has been an increasing interest in the understanding of the potential relationship between the microbiota and the immune system present in the uterine environment for its potential impact on fertility and pregnancy.