T Lymphocyte Characteristics and Immune Tolerance During Human Pregnancy

Abstract

To ensure a fruitful and healthy pregnancy, every maternal organ system needs to adapt to the novel physiological needs raised by the developing fetus. The maternal immune system is no exception. Since the conceptus is half of foreign origins, presenting paternal antigens, it is considered a semi-allograft to maternal immunity. Therefore, an immune tolerance must develop to avoid immunological rejection of the fetus. In the normal course of pregnancy, the mother extends her ‘definition of self’ for 40 weeks on the foreign antigens of the fetus, and the conceptus is accepted by the mother’s immune system. The impairment of this tolerance and the development of an abnormal immune response directed at the fetus play a major role in adverse pregnancy outcomes, including spontaneous abortion, preterm labour and preeclampsia. In recurrent abortion and preeclampsia, abnormal maternal immune reactions have an autoimmune character, and the disorders resemble many features typically seen in autoimmune diseases, or in association with autoimmune reactions. Although this does not mean that recurrent abortion or preeclampsia should be considered autoimmune conditions, it still suggests that abnormal autoimmune processes play an important role in their pathogenesis. In this regard, preeclampsia mimics autoimmune responses observed in both allograft rejection and graft-versus-host disease. Several aspects of the development of the pregnancy-specific immune tolerance have been described recently. Initially, the contact between maternal and fetal cells is taking place on a local level and is restricted to the decidua, but during the second trimester of pregnancy, it is extended to the entire body of the mother. Both the innate and adaptive arms of immunity are involved in these events. In this chapter we will focus on the role of T lymphocytes, the adaptive cellular elements of the immune system. We will discuss the characteristic alterations of T lymphocyte subsets in prevalence and functionality in healthy and pathologic development of the immune tolerance in human pregnancy.