Split tolerance in horse: importance in the immune balance at the materno-fetal interface

Abstract

Pregnancy induces a state of split tolerance, a situation where although a pregnant female is capable of developing an immune response to fetal antigens, it´s tolerant to those antigens in the pregnant uterus. The pregnant uterus is an “immunological privileged site” where there is a physical barrier more or less inaccessible for the maternal immune system and a maternal immune-modulation at the interface, fundamental for the establishment of the immune balance that must ensure the survival of the fetus in the uterine environment. In this review we highlight the contribution of the horse in the understanding of the pregnancy immune balance mechanisms, because it is the best example of split tolerance. It is analyzed both equine epitheliocorial placenta and it´s parallelism with human placenta, and the immune-modulation at the interface in which are involved: endometrial and trophoblast secreted hormones; cytokines appropriate for the establishment of the tolerant microenvironment; antigen presenting cells with an unusual behavior; T lymphocytes inhibited and induced to apoptosis; and the important presence of T regulatory lymphocytes TregFoxP3+.