Immune regulation during pregnancy and host-pathogen interactions in infectious abortion

The immunological mechanisms that govern the success of pregnancy in outbred mammals are complex. During placental formation the invasion of fetal cells into maternal tissue must be controlled to prevent damage to the mother. Equally, maternal recognition of pregnancy must be such that allorejection of the fetus does not occur. Despite the complexity of this phenomenon, it is clear that cytokines play a crucial role at the maternofetal interface and in the periphery to ensure that pregnancy proceeds successfully. Inflammatory cytokines such as tumour necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) can exert detrimental effects in the placenta and tend to be present at low concentrations, whereas the regulatory cytokines interleukin (IL)-10 and tranforming growth factor-beta (TGF-beta) are beneficial and tend to predominate. This means that infection with pathogens that target the placenta and that elicit inflammatory responses may cause abortion by giving rise to a detrimental combination of cytokines that causes damage but does not control the disease. Infectious abortion is discussed in the context of the modulation of host immune responses during pregnancy, taking into account the different placental structures present in human beings, rodents and ruminants.