Pregnancy is one of the strongest inducers of immunological tolerance. Disease activity of many autoimmune diseases including multiple sclerosis (MS) is temporarily suppressed by pregnancy, but little is known about the underlying molecular mechanisms. Here, we investigated the endocrine regulation of conventional and regulatory T cells (Tregs) during reproduction. In vitro, we found the pregnancy hormone progesterone to robustly increase Treg frequencies via promiscuous binding to the glucocorticoid receptor (GR) in T cells. In vivo, T-cell-specific GR deletion in pregnant animals undergoing experimental autoimmune encephalomyelitis (EAE), the animal model of MS, resulted in a reduced Treg increase and a selective loss of pregnancy-induced protection, whereas reproductive success was unaffected. Our data imply that steroid hormones can shift the immunological balance in favor of Tregs via differential engagement of the GR in T cells. This newly defined mechanism confers protection from autoimmunity during pregnancy and represents a potential target for future therapy.multiple sclerosis | autoimmunity | pregnancy | Treg | steroid hormones R eproduction is fundamental to the maintenance and evolution of species. To ensure successful pregnancy, mothers have to establish robust immunological tolerance toward the semiallogeneic conceptus providing a secure niche for fetal development. Multiple mechanisms have evolved to prevent fetus-directed immune responses and alloreactive infiltration of the fetomaternal interface (1). These include creating a privileged local microenvironment that hampers T-cell priming and infiltration (2-4) but also imply global modulation of the immune system by pregnancy hormones and the shedding of fetal antigen into the mothers circulation (5).Intriguingly, pregnancy is also well known to suppress the inflammatory activity of a number of cell-mediated autoimmune diseases, including rheumatoid arthritis (6, 7), autoimmune hepatitis (8), and multiple sclerosis (MS) (9, 10). However, this beneficial effect is limited to the period of gestation and usually followed by a rebound of disease activity postpartum. In the case of MS, third trimester pregnancy leads to a remarkable reduction of the MS relapse rate (11), which exceeds the effects of most currently available disease-modifying drugs. Similarly, pregnancy as well as treatment with pregnancy hormones protect rodents from experimental autoimmune encephalomyelitis (EAE), a widely used animal model of MS (12) in both SJL/J and C57BL/6 mice (13-16), underpinning an interaction between pregnancy-related immune and endocrine adaptations and central nervous system (CNS) autoimmunity (17).The sensitive balance between conventional effector T cells (Tcons) and regulatory T cells (Tregs) has transpired as a common theme that connects reproductive biology and autoimmunity on a mechanistic level (18)(19)(20)(21). Tregs are characterized by the transcription factor forkhead box P3 (Foxp3) and effectively control effector responses mounted by Tcons in...

Indoleamine 2,3-dioxygenase (IDO), an enzyme which is activated by pro-inflammatory cytokines, has been suggested as a potential link between neuroinflammatory processes in neurodegenerative diseases (like Alzheimer's disease) and depression. The present study aimed to determine whether neuroinflammation-induced increased IDO levels in the mammalian brain will lead to depressive-like behavior. Neuroinflammation was initiated in mice by a single intracerebroventricular injection of lipopolysaccharide (LPS). Cerebral inflammation was monitored 1, 2, 3 and 4 days after the injection with small-animal positron emission tomography (PET) using the inflammatory marker [(11)C]-PK11195. In the presence or absence of systemically applied 1-methyl-tryptophan (1-MT), a competitive IDO-inhibitor, we assessed the development of depressive-like behavioral symptoms in parallel with IDO expression and activity. The PK11195 PET signal reached a highly significant peak 3 days after LPS injection, while these animals displayed a significant increase of depressive-like behavior in the forced swim test compared to vehicle-injected animals. These findings were paralleled by a significant increase of IDO in the brainstem, and an increased kynurenine/tryptophan ratio in the serum. Moreover, we report here for the first time, that inhibition of IDO by 1-MT in centrally induced neuroinflammation under experimental conditions can prevent the development of depressive-like behavior.

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