Retention and tolerance of autoreactive CD4(+) recent thymic emigrants in the liver

Reinterpreting recent thymic emigrant function: defective or adaptive?

Recent thymic emigrants (RTEs) are those peripheral T cells that have most recently completed thymic development and egress. Over the past decade, significant advances have been made in understanding the cell-extrinsic and cell-intrinsic requirements for RTE maturation to mature naïve (MN) T cells and in detailing the functional differences that characterize these two T cell populations. Much of this work has suggested that RTEs are hypo-functional versions of more mature T cells. However, recent evidence has indicated that rather than being defective T cells, RTEs are exquisitely adapted to their cellular niche. In this review, we argue that RTEs are not flawed mature T cells but are adapted to fill an underpopulated T cell compartment, while maintaining self tolerance and possessing the capacity to mount robust immune responses.

Newly Generated CD4+ T Cells Acquire Metabolic Quiescence after Thymic Egress

Mature naive T cells circulate through the secondary lymphoid organs in an actively enforced quiescent state. Impaired cell survival and cell functions could be found when T cells have defects in quiescence. One of the key features of T cell quiescence is low basal metabolic activity. It remains unclear at which developmental stage T cells acquire this metabolic quiescence. We compared mitochondria among CD4 single-positive (SP) T cells in the thymus, CD4⁺ recent thymic emigrants (RTEs), and mature naive T cells in the periphery. The results demonstrate that RTEs and naive T cells had reduced mitochondrial content and mitochondrial reactive oxygen species when compared with SP thymocytes. This downregulation of mitochondria requires T cell egress from the thymus and occurs early after young T cells enter the circulation. Autophagic clearance of mitochondria, but not mitochondria biogenesis or fission/fusion, contributes to mitochondrial downregulation in RTEs. The enhanced apoptosis signal-regulating kinase 1/MAPKs and reduced mechanistic target of rapamycin activities in RTEs relative to SP thymocytes may be involved in this mitochondrial reduction. These results indicate that the gain of metabolic quiescence is one of the important maturation processes during SP-RTE transition. Together with functional maturation, it promotes the survival and full responsiveness to activating stimuli in young T cells.

Mechanisms of liver-induced tolerance

PURPOSE OF REVIEW

To highlight the results of the ongoing research on the mechanisms of liver-induced tolerance focusing on results from the last year.

RECENT FINDINGS

The liver is exposed to a massive antigenic burden of dietary and commensal products from the gastrointestinal tract via portal vein, most of which are necessary for survival. To prevent the immune system from destroying these foreign yet beneficial elements, the liver has developed unique mechanisms to suppress immune responses. It is thought that these mechanisms of acquired tolerance may also underlie the spontaneous acceptance of liver allografts observed after transplantation in many species. The fact that isolated hepatocyte transplants are acutely rejected, suggests that nonparenchymal liver cells play a critical role in spontaneous liver allograft acceptance. IFN-γ, a key inflammatory cytokine produced by T effector (Tef) cells, is paradoxically compulsory for spontaneous liver allograft acceptance. Analysis of IFN-γ signaling points to liver mesenchymal nonparenchymal liver cell that eliminate infiltrating Tef cells via expression of B7-H1, IL-10, and tumor growth factor-β, as well as the enhancement of Tregs and MDSCs. Thus, liver mesenchymal cells are thought to promote tolerance by eliminating alloreactive Tef cells and enhancing suppressor cells (T and B).

SUMMARY

The research during last year offered some key insights into the mechanisms of liver-induced tolerance. Through interactions with activated T cells and B cells via IFN-γ/B7-H1 pathways, liver mesenchymal cells have been shown to be critical components of liver-specific tolerance induction.

Adaptive immunity in the liver

The anatomical architecture of the human liver and the diversity of its immune components endow the liver with its physiological function of immune competence. Adaptive immunity is a major arm of the immune system that is organized in a highly specialized and systematic manner, thus providing long-lasting protection with immunological memory. Adaptive immunity consists of humoral immunity and cellular immunity. Cellular immunity is known to have a crucial role in controlling infection, cancer and autoimmune disorders in the liver. In this article, we will focus on hepatic virus infections, hepatocellular carcinoma and autoimmune disorders as examples to illustrate the current understanding of the contribution of T cells to cellular immunity in these maladies. Cellular immune suppression is primarily responsible for chronic viral infections and cancer. However, an uncontrolled auto-reactive immune response accounts for autoimmunity. Consequently, these immune abnormalities are ascribed to the quantitative and functional changes in adaptive immune cells and their subsets, innate immunocytes, chemokines, cytokines and various surface receptors on immune cells. A greater understanding of the complex orchestration of the hepatic adaptive immune regulators during homeostasis and immune competence are much needed to identify relevant targets for clinical intervention to treat immunological disorders in the liver.

Liver sinusoidal endothelial cells induce tolerance of autoreactive CD4+ recent thymic emigrants

The liver is a unique lymphoid organ whose microenvironment is biased towards tolerance induction. We previously found that a proportion of CD4(+) autoreactive recent thymic emigrants (RTEs) retained in the liver after thymic egress and acquired IL-10 producing capability. To investigate the tolerance of these liver persisting CD4(+) RTEs in more detail and to study the liver stromal cell types that facilitate the tolerogenic changes in young T cells, the phenotype and function of liver RTEs were further characterized and the impact of liver sinusoidal endothelial cells (LSECs) and Kupffer cells on RTEs were examined using an in vitro co-culture system. More than 70% of CD4(+) CD44(hi) RTEs in the liver acquired Foxp3(-)LAG3(+) CD49b(-) regulatory phenotype and function. But higher ratio of apoptosis with enhanced FasL and Bim expression was also found in these CD4(+) liver RTEs when compared to those in the lymph nodes and spleen. LSECs played an important role in RTEs' acquisition of tolerogenic and regulatory phenotype. These results indicate an important role of liver microenvironment in enforcing peripheral tolerance to CD4(+) thymic emigrants against self- and gut-derived antigens.

Unmet challenges in immune-mediated hepatobiliary diseases

It is ironic that the liver, which serves a critical function in immune tolerance, itself becomes the victim of an autoimmune attack. Indeed, liver autoimmunity and the autoimmune diseases associated with both innate and adaptive responses to hepatocytes and/or cholangiocytes are models of human autoimmunity. For example, in primary biliary cirrhosis, there exists a well-defined and characteristic autoantibody and considerable homogeneity between patients. In autoimmune hepatitis, there are clinical characteristics that allow a rigorous subset definition and well-defined inflammatory infiltrates. In both cases, there are defects in a variety of immune pathways and including regulatory cells. In primary sclerosing cholangitis, with its characteristic overlap with inflammatory bowel disease, there are unique defects in innate immunity and particular important contribution of lymphoid homing to disease pathogenesis. In these diseases, as with other human autoimmune processes, there is the critical understanding that pathogenesis requires a genetic background, but is determined by environmental features, and indeed the concordance of these diseases in identical twins highlights the stochastic nature of immunopathology. Unfortunately, despite major advances in basic immunology and in immunopathology in these diseases, there remains a major void in therapy. The newer biologics that are so widely used in rheumatology, neurology, and gastroenterology have not yet seen success in autoimmune liver disease. Future efforts will depend on more rigorous molecular biology and systems analysis in order for successful application to be made to patients.

The clinical phenotypes of autoimmune hepatitis: A comprehensive review

Autoimmune hepatitis (AIH) fulfills the generally accepted contemporary criteria of an autoimmune liver disease: the presence of autoantibodies and autoreactive T cells, a female gender bias, association with other autoimmune diseases, response to immunosuppressive therapy and strong associations with the major histocompatibility complex HLA loci. It occurs worldwide in both children and adults and is marked by both etiopathogenic and clinical heterogeneity, differing from the other putative autoimmune liver diseases, primary biliary cirrhosis (PBC) and primary sclerosing cholangitis (PSC), albeit occasionally presenting with overlapping features of PBC or PSC. Although diagnostic criteria have been established and validated, there are still major issues to be clarified due to its variability, such as autoantibody-negative AIH, drug-induced AIH, AIH sharing features with PBC or PSC, and post-transplant de novo AIH. In view of the diverse presentations and courses, including classical chronic onset, acute and acute severe onset, cirrhosis and decompensated cirrhosis, individualized management of patients is indicated. Each patient should receive a personalized analysis of the benefits and side effect risks of drugs. Herein we describe a comprehensive review of the clinical phenotypes of AIH underscoring its clinical heterogeneity.