Modulation of liver tolerance by conventional and nonconventional antigen-presenting cells and regulatory immune cells

The liver is a tolerogenic organ with exquisite mechanisms of immune regulation that ensure upkeep of local and systemic immune tolerance to self and foreign antigens, but that is also able to mount effective immune responses against pathogens. The immune privilege of liver allografts was recognized first in pigs in spite of major histo-compatibility complex mismatch, and termed the ＂liver tolerance effect＂. Furthermore, liver transplants are spontaneously accepted with only low-dose immunosuppression, and induce tolerance for non-hepatic co-transplanted allografts of the same donor. Although this immunotolerogenic environment is favorable in the setting of organ transplantation, it is detrimental in chronic infectious liver diseases like hepatitis B or C, malaria, schistosomiasis or tumorigenesis, leading to pathogen persistence and weak anti-tumor effects. The liver is a primary site of T-cell activation, but it elicits poor or incomplete activation of T cells, leading to their abortive activation, exhaustion, suppression of their effector function and early death. This is exploited by pathogens and can impair pathogen control and clearance or allow tumor growth. Hepatic priming of T cells is mediated by a number of local conventional and nonconventional antigen-presenting cells （APCs）, which promote tolerance by immune deviation, induction of T-cell anergy or apoptosis, and generating and expanding regulatory T cells. This review will focus on the communication between classical and nonclassical APCs and lymphocytes in the liver in tolerance induction and will discuss recent insights into the role of innate lymphocytes in this process.

A LIM Domain Protein from Tobacco Involved in Actin-Bundling and Histone Gene Transcription

The two LIM domain-containing proteins from plants （LIMs） typically exhibit a dual cytoplasmic-nuclear dis-tribution, suggesting that, in addition to their previously described roles in actin cytoskeleton organization, they partici-pate in nuclear processes. Using a south-western blot-based screen aimed at identifying factors that bind to plant histone gene promoters, we isolated a positive clone containing the tobacco LIM protein WLIM2 （NtWLIM2） cDNA. Using both green fluorescent protein （GFP） fusion-and immunology-based strategies, we provide clear evidence that NtWLIM2 local-izes to the actin cytoskeleton, the nucleus, and the nucleolus. Interestingly, the disruption of the actin cytoskeleton by latrunculin B significantly increases NtWLIM2 nuclear fraction, pinpointing a possible novel cytoskeletal-nuclear crosstalk. Biochemical and electron microscopy experiments reveal the ability of NtWLIM2 to directly bind to actin filaments and to crosslink the latter into thick actin bundles. Electrophoretic mobility shift assays show that NtWLIM2 specifically binds to the conserved octameric cis-elements （Oct） of the Arabidopsis histone H4A748 gene promoter and that this binding largely relies on both LIM domains. Importantly, reporter-based experiments conducted in Arabidopsis and tobacco proto-plasts confirm the ability of NtWLIM2 to bind to and activate the H4A748 gene promoter in live cells. Expression studies indicate the constitutive presence of NtWLIM2 mRNA and NtWLIM2 protein during tobacco BY-2 cell proliferation and cell cycle progression, suggesting a role of NtWLIM2 in the activation of basal histone gene expression. Interestingly, both live cell and in vitro data support NtWLIM2 di/oligomerization. We propose that NtWLIM2 functions as an actin-stabilizing protein, which, upon cytoskeleton remodeling, shuttles to the nucleus in order to modify gene expression.

Antigen presentation,autoantibody production,and therapeutic targets in autoimmune liver disease

The liver is an important immunological organ that controls systemic tolerance.The liver harbors professional and unconventional antigen-presenting cells that are crucial for tolerance induction and maintenance.Orchestrating the immune response in homeostasis depends on a healthy and well-toned immunological liver microenvironment,which Is maintained by the crosstalk of liver-resident antigen-presenting cells and intrahepatic and liver-infiltrating leukocytes.In response to pathogens or autoantigens,tolerance is disrupted by unknown mechanisms.Intrahepatic parenchymal and nonparenchymal cells exhibit unique antigen-presenting properties.The presentation of microbial and endogenous lipid-,metabolite-and peptide-derived antigens from the gut via conventional and nonconventional mechanisms can educate intrahepatic immune cells and elicit effector responses or tolerance.Perturbation of this balance results in autoimmune liver diseases,such as autoimmune hepatitis,primary biliary cholangitis,and primary sclerosing cholangitis.Although the exact etiologies of these autoimmune liver diseases are unknown,it is thought that the disruption of tolerance towards self-antigens and microbial metabolites and lipids,as well as alterations in bile acid composition,may result in changes in effector cell activation and polarization and may reduce or impair protective antiinflammatory regulatory T and B cell responses.Additionally,the canonical and noncanonical transmission of antigens and antigen:MHC complexes via trogocytosis or extracellular vesicles between different(non)immune cells in the liver may play a role in the induction of hepatic inflammation and tolerance.Here,we summarize emerging aspects of antigen presentation,autoantibody production,and the application of novel therapeutic approaches in the characterization and treatment of autoimmune liver diseases.