The blockade of PD-1/PD-L1 pathway promotes the apoptosis of CD19+CD25+ Bregs and suppresses the secretion of IL-10 in patients with allergic rhinitis

PD-1/PD-L1 pathway is crucial to immune regulation by controlling the balance between T cell tolerance and activation. However, the association between PD-1/PD-L1 pathway and regulatory B cells has not been fully investigated in allergic rhinitis. In this study, we detected the number of peripheral CD19⁺CD25⁺ Bregs and the expression of IL-10 on this cell subset in healthy control and patients with allergic rhinitis using flow cytometry. Then, we evaluated the level of PD-L1 in CD19⁺CD25⁺ Bregs and investigated the correlation between PD-L1 and CD4⁺ follicular T helper cells. Finally, we studied the effects of anti–PD-L1 on the apoptosis of Bregs and the production of IL-10. Comparing with healthy controls, the percentage of CD19⁺CD25⁺ Bregs and the expression of IL-10 were both significantly decreased in AR group. In addition, the expression of PD-L1 on CD19⁺CD25⁺ Bregs was also lower in allergic rhinitis patients. Interestingly, a negative correlation was found between the expression of PD-L1⁺ Bregs and CD4⁺CXCR5⁺ follicular T helper cells. In vitro assay revealed that anti–PD-L1 promoted Bregs apoptosis and inhibited the expression of IL-10 in CD19⁺CD25⁺ Bregs. Collectively, these results suggest that PD-L1 expressed on CD19⁺CD25⁺ Bregs may be a potential regulator in the treatment of allergic rhinitis. Blockade of PD-1/PD-L1 pathway might be a valuable pathogenic target for allergic rhinitis through inhibiting the secretion of immunosuppressive cytokine and promoting CD19⁺CD25⁺ Bregs apoptosis.     

LncRNA XLOC_003810 promotes T cell activation and inhibits PD-1/PD-L1 expression in patients with myasthenia gravis-related thymoma

This study aimed to investigate the effect of long non-coding RNA XLOC_003810 on the activation of CD4⁺ T cells and expression of PD-1/PD-L1 in patients with myasthenia gravis-related thymoma (MG-T). Thymus specimens and thymic mononuclear cells were obtained from MG and MG-T patients or cardiac surgery patients undergoing thoracotomy who were selected as negative controls (NC). XLOC_003810 expression was examined using quantitative real-time PCR (qRT-PCR). Frequency of CD4⁺ T cells and proportion of CD4⁺PD-1⁺ T cells and CD14⁺PD-L1⁺ monocytes were quantified by flow cytometry. The release of inflammatory cytokines was measured by qRT-PCR and enzyme-linked immunosorbent assay. Compared with the NC group, expression of XLOC_003810, frequency of CD4⁺ T cells and the production of inflammatory cytokines were increased in patients with MG and MG-T. XLOC_003810 overexpression significantly increased the frequency of CD4⁺ T cells, facilitated the production of inflammatory cytokines and decreased the proportion of CD4⁺PD-1⁺ T cells and CD14⁺PD-L1⁺ monocytes in the thymic mononuclear cells. In contrast, XLOC_003810 knockdown exerted the opposite effect. Together, XLOC_003810 promotes T cell activation and inhibits PD-1/PD-L1 pathway in patients with MG-T.     

Type 1 interferon-induced IL-7 maintains CD8+ T-cell responses and homeostasis by suppressing PD-1 expression in viral hepatitis

Type 1 interferon (IFN-I) promotes antigen-presenting cell maturation and was recently shown to induce hepatic IL-7 production during infection. Herein, we further explored the underlying mechanisms used by IFN-I to orchestrate antiviral immune responses in the liver. Acute viral hepatitis was induced by i.v. injection of adenovirus (Ad) in IFN-α receptor knockout (IFNAR^−/−) and control mice. To disrupt signaling, monoclonal antibodies (mAbs) against IL-7 receptor alpha (IL-7Rα) or PD-L1 were i.p. injected. We found that CD8⁺ T cells in IFNAR^−/− mice were less effective than those in control mice. The reduced T-cell function was accompanied by increased levels of PD-1 expression, apoptosis and decreased IFN-γ production. The lack of IFN-I signaling also impaired the expression of accessory molecules in both intrahepatic dendritic cell (DCs) and hepatocytes. PD-L1 was comparably and highly expressed on hepatocytes in both IFNAR^−/− and control mice. Injection of PD-L1-specific mAb in IFNAR^−/− mice reversed the compromised immune responses in the liver. Further investigation showed that hepatic IL-7 elevation was less pronounced in IFNAR^−/− mice compared to the controls. A treatment with recombinant IL-7 suppressed PD-1 expression on CD8⁺ T cells in vitro. Accordingly, blocking IL-7R signaling in vivo resulted in increased PD-1 expression on CD8⁺ T cells in Ad-infected mice. Collectively, the results suggest that IFN-I-induced hepatic IL-7 production maintains antiviral CD8⁺ T-cell responses and homeostasis by suppressing PD-1 expression in acute viral hepatitis.     

Upregulation of Immune Checkpoint Molecules,PD-1 and LAG-3, on CD4+ and CD8+ T Cells after Gastric Cancer Surgery

Background

Surgery has been reported to suppress cell-mediated immunity; however, the detailed mechanisms responsible for this remain unclear. This study determined the expression of lymphocyte activation gene 3 (LAG-3) and programmed cell death 1 (PD-1) in lymphocytes following surgery for gastric cancer.

Methods

LAG-3 and PD-1 expression on both CD4+ and CD8+ T cells obtained pre- and post-operatively from gastric cancer patients were evaluated by multicolor flow cytometry.

Results

The total lymphocyte count decreased rapidly from preoperative levels, reaching a minimum on postoperative day 1 and remaining significantly decreased on days 3 and 7. PD-1⁺CD4⁺ T cells significantly increased, reaching a maximum on postoperative day 1 and remaining significantly elevated on day 3. PD-1⁺CD8⁺ T cells significantly increased and reached a maximum on day 7 before returning to the preoperative level on day 30. There were no statistically significant differences in the frequency of LAG-3⁺CD4⁺ or LAG-3⁺CD8⁺ T cells after surgery. There were significant positive correlations between PD-1 and LAG-3 expression on both CD4⁺ andCD8⁺ T cells.

Conclusion

PD-1 and LAG-3 expression on both CD4⁺ and CD8⁺ T cells was up-regulated and might be related to impaired cell-mediated immunity after surgery for gastric cancer.     

Prognostic Values of CD38+CD101+PD1+CD8+ T Cells in Pancreatic Cancer

Programmed death-1 (PD-1), a key immune checkpoint molecule, has been developed as an oncotherapy target for various carcinomas. However, treatment with anti-PD-1 elicited only a minimal effect in pancreatic ductal adenocarcinoma (PDAC). Subsequent studies revealed the existence of a subset of PD-1⁺ T cells coexpressing CD38 and CD101, representing a fixed dysfunctional subpopulation that are not able to be rescued by anti-PD-1 immunotherapy. However, whether this subpopulation of PD-1 expressing CD8⁺ T cells could be useful in predicting PDAC stage or prognosing survival is unknown. In this study, we used flow cytometry and immunofluorescence assay to analyze the expression of CD38 and CD101 in 183 clinical PDAC samples, including 84 of peripheral blood and 99 of surgical tissues. High coexpression of CD38/CD101 on peripheral PD-1⁺CD8⁺ T cells or tumor-infiltrating lymphocytes (TILs) was found to be most significantly correlated with Tumor/Node/Metastasis (T/N/M) classification and clinical stage, in contrast PD-1⁺CD8⁺ T cells could not correlate with T classification. CD38/CD101 co-repression on TILs also correlated with the poor survival in these PDAC patient samples. Our data suggest that CD38/CD101 might represent a more helpful biomarker than PD-1 alone for diagnosis and prognosis of PDAC.     

Human CXCR5+PD-1+ CD8 T cells in healthy individuals and patients with hematologic malignancies

Immune checkpoint blockade (ICB) has revolutionized cancer therapy, but varying response rates illustrate the need for biomarkers of response. Studies in mice have identified a subset of CD8 T cells that is essential for response to PD-1 ICB. These CD8 T cells co-express CXCR5, PD-1 and Tcf1, and provide effector T cells upon PD-1 ICB. It is unknown whether similar T cells play a role in PD-1 ICB in humans. We studied human peripheral blood and lymph nodes (LNs) for the frequency, phenotype, and functionality of CXCR5⁺PD-1⁺ CD8 T cells. We find that CXCR5⁺PD-1⁺ CD8 T cells are memory-like cells, express Tcf1, and lack expression of effector molecules. CXCR5⁺PD-1⁺ CD8 T cells produce cytokines upon stimulation, but have limited proliferative capacity. We studied patients with hematologic malignancies with varying response rates to PD-1 ICB. Specifically in chronic lymphocytic leukemia, in which PD-1 ICB does not induce clinical responses, CXCR5⁺PD-1⁺ CD8 T cells show loss of the memory phenotype and increased effector differentiation. In conclusion, we identified CXCR5⁺PD-1⁺ CD8 T cells in human peripheral blood and LN, which could play a similar role during PD-1 ICB. Future studies should analyze CXCR5⁺PD-1⁺ CD8 T cells during PD-1 ICB and their importance for therapeutic response.     

Programmed death-1 (PD-1)-dependent functional impairment of CD4+ T cells in recurrent genital papilloma

Genital papilloma is caused by human papilloma virus (HPV) infection and recurs frequently. Although T cells are known to play a critical role in the control of HPV infection and papilloma development, the function and phenotype of these cells in the lesion remain to be elucidated. In the present study, we examined the function and phenotype of CD4⁺ T cells isolated from the lesions of primary (n = 9) and recurrent (n = 11) genital papillomas. In recurrent papillomas, the frequency of proliferating (Ki-67⁺) CD4⁺ T cells was significantly reduced compared with primary papillomas. Cytokine production was evaluated by intracellular cytokine staining in anti-CD3/anti-CD28-stimulated CD4⁺ T cells. CD4⁺ T cells from recurrent lesions showed impaired production of IL-2, IFN-γ, and TNF-α. Of interest, the frequency of cytokine-producing CD4⁺ T cells significantly correlated with the frequency of Ki-67⁺CD4⁺ T cells. We also studied expression of programmed death-1 (PD-1), a T-cell exhaustion marker. The frequency of PD-1⁺CD4⁺ T cells was significantly increased in recurrent lesions and inversely correlated with the frequency of cytokine-producing CD4⁺ T cells. The functional significance of PD-1 expression was determined in blocking assays with anti-PD-L1, which restored cytokine production of CD4⁺ T cells from recurrent lesions. Taken together, in recurrent genital papilloma lesions, proliferation, and cytokine production by CD4⁺ T cells are impaired and the PD-1/PD-L1 interaction is responsible for the functional impairment of CD4⁺ T cells.     

Combined Env- and Gag-specific T cell responses in relation to programmed death-1 receptor and CD4+ T cell loss rates in human immunodeficiency virus-1 infection

Additional progression markers for human immunodeficiency virus (HIV) infection are warranted. In this study we related antigen-specific responses in CD4⁺ and CD8⁺ T cells to CD38, reflecting chronic immune activation, and to CD4⁺ T cell loss rates. Clones transiently expressing CD107a (CD8⁺) or CD154 (CD4⁺) in response to Gag, Env and Nef overlapping peptide pools were identified, along with their expression of the inhibitory programmed death-1 receptor (PD-1) in fresh peripheral blood mononuclear cells (PBMC) from 31 patients off antiretroviral treatment (ART). HIV-specific CD8⁺ T cell responses dominated over CD4⁺ T cell responses, and among CD8⁺ responses, Gag and Nef responses were higher than Env-responses (P < 0·01). PD-1 on CD8⁺ HIV-specific subsets was higher than CMV-specific CD8⁺ cells (P < 0·01), whereas PD-1 on HIV-specific CD4⁺ cells was similar to PD-1 on CMV-specific CD4⁺ cells. Gag and Env CD8⁺ responses correlated oppositely to the CD4 loss rate. Env/Gag CD8⁺ response ratios, independently of PD-1 levels, correlated more strongly to CD4 change rates (r = −0·50 to −0·77, P < 0·01) than the total number of Gag-specific CD8⁺ cells (r = 0·44–0·85, P ≤ 0·02). The Env/Gag ratio performed better than CD38 and HIV-RNA in logistic regression analysis predicting CD4 change rate as a measure of progression. In conclusion, HIV-specific CD8⁺CD107a⁺ Env/Gag response ratio was a stronger predictor for progression than CD38 and HIV-RNA. The Env/Gag ratio may reflect the balance between possibly beneficial (Gag) and detrimental (Env) CD8⁺ T cell responses and should be explored further as a progression marker.     

Accumulation of CD5<Superscript>+</Superscript>CD19<Superscript>+</Superscript> B lymphocytes expressing PD-1 and PD-1L in hypertrophied pharyngeal tonsils

Programmed death-1 (PD-1) is one of the most important inhibitory co-receptors expressed predominantly on activated T and B lymphocytes whose expression could be sustained by permanent antigenic stimulation accompanying chronic or recurrent tonsillitis. The expression of PD-1 and PD-1L was analyzed using flow cytometry on hypertrophied tonsils collected from 57 children. We observed high expression of PD-1 and PD-1L on certain lymphocytes subpopulations of hypertrophied tonsils; among T cells, the expression of PD-1 on protein level was higher on CD4⁺ cells (70.3 %) than on CD8⁺ cells (35 %). Interestingly, a limited expression of PD-1 was observed on CD19⁺ B lymphocytes (6.5 %), while CD5⁺CD19⁺ B cells overexpressed PD-1 (52.5 %). Moreover, the expression of PD-1L was also higher on CD5⁺CD19⁺ B cells (16.5 %) than on CD19⁺ B cells (3.5 %) and on CD4⁺ T cells (20 %) than on CD8⁺ T cells (10 %). PD-1 and PD-1L expressions correlated only on CD5⁺CD19⁺ cells. In conclusion, high expression of PD-1 and PD-1L on T and B cells could represent hallmark of immune system adaptation to chronic antigenic exposition in patients with tonsillitis.     

Defective positioning in granulomas but not lung-homing limits CD4 T-cell interactions with Mycobacterium tuberculosis-infected macrophages in rhesus macaques

Protection against Mycobacterium tuberculosis (Mtb) infection requires CD4 T cells to migrate into the lung and interact with infected macrophages. In mice, less-differentiated CXCR3⁺ CD4 T cells migrate into the lung and suppress growth of Mtb, whereas CX3CR1⁺ terminally differentiated Th1 cells accumulate in the blood vasculature and do not control pulmonary infection. Here we examine CD4 T-cell differentiation and lung homing during primary Mtb infection of rhesus macaques. Mtb-specific CD4 T cells simultaneously appeared in the airways and blood ∼21–28 days post exposure, indicating that recently primed effectors are quickly recruited into the lungs after entering circulation. Mtb-specific CD4 T cells in granulomas display a tissue-parenchymal CXCR3⁺CX3CR1⁻PD-1^hiCTLA-4⁺ phenotype. However, most granuloma CD4 T cells are found within the outer lymphocyte cuff and few localize to the myeloid cell core containing the bacilli. Using the intravascular stain approach, we find essentially all Mtb-specific CD4 T cells in granulomas have extravasated across the vascular endothelium into the parenchyma. Therefore, it is unlikely to be that lung-homing defects introduced by terminal differentiation limit the migration of CD4 T cells into granulomas following primary Mtb infection of macaques. However, intralesional positioning defects within the granuloma may pose a major barrier to T-cell-mediated immunity during tuberculosis.     

Programmed death-1 upregulation is correlated with dysfunction of tumor-infiltrating CD8+ T lymphocytes in human non-small cell lung cancer

T-cell tolerance is an important mechanism for tumor escape, but the molecular pathways involved in T-cell tolerance remain poorly understood. It remains unknown whether the inhibitory immunoreceptor programmed death-1 (PD-1) plays a role in conditions of human non-small cell lung cancer (NSCLC). In this study, we detected PD-1 expression on CD8⁺ T cells from healthy control peripheral blood mononuclear cells (PBMCs) and the PBMCs of NSCLC patients as well as NSCLC tissues. Results showed that tumor-infiltrating CD8⁺ T cells had increased PD-1 expression and impaired immune function, including reducing cytokine production capability and impairing capacity to proliferate. Blockade of the PD-1/PD-L1 pathway by the PD-L1-specific antibody partially restored cytokine production and cell proliferation. These data provide direct evidence that the PD-1/PD-L1 pathway is involved in CD8⁺ T-cell dysfunction in NSCLC patients. Moreover, blocking this pathway provides a potential therapy target in lung cancer.     

Differing HLA types influence inhibitory receptor signalling in CMV-specific CD8 T cells

The dysregulated immune response to CMV constitutes a major force driving T cell immunosenescence and growing evidence suggests that it is not a benign virus in old age. We show here that the PD-1/L pathway defines a reversible defect in CMV specific CD8⁺ T cell proliferative responses in both young and old individuals. More specifically, highly differentiated CD45RA⁺CD27⁻ CMV-specific CD8⁺ T cells exhibit a proliferative deficit compared their central and effector memory counterparts, which is reversed following PD-L blockade. However, we also report that HLA-B^∗07/TPR specific CD8⁺ T cells express higher levels of PD-1 than HLA-A^∗02/NLV specific cells and HLA-A^∗02 individuals show a higher proliferative response to PD-L blockade, than HLA-B^∗07 individuals, which we postulate may be due to the differing functional avidities for these two CMV-specific CD8⁺ T cells populations. Nevertheless data presented here demonstrate that CMV-specific CD8⁺ T cells can be functionally enhanced by perturbation of the PD-1/L signalling pathway, whose manipulation may provide a therapeutic modality to combat age-associated immune decline.     

Intrahepatic Expression of Programmed Death-1 and its Ligands in Patients with HBV-Related Acute-on-Chronic Liver Failure

Hepatitis B virus (HBV) infection is a major public health problem, and HBV-related acute-on-chronic liver failure (HBV-ACLF) has an extremely poor prognosis due to a lack of understanding of pathogenesis as well as a lack of effective treatments. Signals from the inhibitory receptor programmed death-1 (PD-1) have been demonstrated to be involved in regulating the pathogenesis of infectious diseases. However, the expression of PD-1 and its ligands in HBV-ACLF patients has yet to be evaluated. In this study, the expression of PD-1 and its ligands, PD-L1 and PD-L2, in liver biopsies from HBV-ACLF as well as chronic hepatitis B (CHB) patients were analyzed by immunohistochemistry. The results showed that all three molecules were observed in the HBV-ACLF samples and their levels were significantly higher than they were in CHB. Immunofluorescence double-staining showed that PD-1 was found on CD3⁺, CD8⁺ T cells, CD56⁺ NK cells, CD68⁺ macrophages, CK-18⁺ epithelial cells, and CD16⁺ monocytes. The PD-L1 expression was observed on all cell types detected and the PD-L2 was chiefly on CK-18⁺ epithelial cells and CD31⁺ endothelial cells. Interestingly, high levels of virus-induced procoagulant molecule fibrinogen-like protein 2 (FGL2) were observed in liver sections from HBV-ACLF, and PD-L1 and PD-L2 expression was also observed on FGL2⁺ cells in these patients. Our combined results suggest that the expression of PD-L1 and PD-L2 may be biomarkers to identify and diagnose ACLF, and a clear understanding of their functional roles should further elucidate the pathogenesis of this disease.     

CD4+ Th1 cells promote CD8+ Tc1 cell survival, memory response, tumor localization and therapy by targeted delivery of interleukin 2 via acquired pMHC I complexes

The cooperative role of CD4⁺ helper T (Th) cells has been reported for CD8⁺ cytotoxic T (Tc) cells in tumor eradication. However, its molecular mechanisms have not been well elucidated. We have recently demonstrated that CD4⁺ Th cells can acquire major histocompatibility complex/peptide I (pMHC I) complexes and costimulatory molecules by dendritic cell (DC) activation, and further stimulate naïve CD8⁺ T cell proliferation and activation. In this study, we used CD4⁺ Th1 and CD8⁺ Tc1 cells derived from ovalbumin (OVA)-specific T cell receptor (TCR) transgenic OT II and OT I mice to study CD4⁺ Th1 cell''s help effects on active CD8⁺ Tc1 cells and the molecular mechanisms involved in CD8⁺ Tc1-cell immunotherapy of OVA-expressing EG7 tumors. Our data showed that CD4⁺ Th1 cells with acquired pMHC I by OVA-pulsed DC (DC_OVA) stimulation are capable of prolonging survival and reducing apoptosis formation of active CD8⁺ Tc1 cells in vitro, and promoting CD8⁺ Tc1 cell tumor localization and memory responses in vivo by 3-folds. A combined adoptive T-cell therapy of CD8⁺ Tc1 with CD4⁺ Th1 cells resulted in regression of well-established EG7 tumors (5 mm in diameter) in all 10/10 mice. The CD4⁺ Th1’s help effect is mediated via the helper cytokine IL-2 specifically targeted to CD8⁺ Tc1 cells in vivo by acquired pMHC I complexes. Taken together, these results will have important implications for designing adoptive T-cell immunotherapy protocols in treatment of solid tumors.     

Increased Coexpression of PD-1, TIGIT,and KLRG-1 on Tumor-Reactive CD8+ T Cells During Relapse after Allogeneic Stem Cell Transplantation

Allogeneic stem cell transplantation (allo-SCT) can be a curative treatment for patients with a hematologic malignancy due to alloreactive T cell responses recognizing minor histocompatibility antigens (MiHA). Yet tumor immune escape mechanisms can cause failure of T cell immunity, leading to relapse. Tumor cells display low expression of costimulatory molecules and can up-regulate coinhibitory molecules that inhibit T cell functionality on ligation with their counter-receptors on the tumor-reactive T cells. The aim of this explorative study was to evaluate immune checkpoint expression profiles on T cell subsets and on cytomegalovirus (CMV)- and/or MiHA-reactive CD8⁺ T cells of allo-SCT recipients using a 13-color flow cytometry panel, and to correlate these expression patterns to clinical outcomes. MiHA-reactive CD8⁺ T cells exhibited an early differentiated CD27⁺⁺/CD28⁺⁺ phenotype with low KLRG-1 and CD57 expression. These T cells also displayed increased expression of PD-1, TIM-3, and TIGIT compared with total effector memory T cells and CMV-specific CD8⁺ T cells in healthy donors and allo-SCT recipients. Remarkably, high coexpression of PD-1, TIGIT, and KLRG-1 on MiHA-reactive CD8⁺ T cells was associated with relapse after allo-SCT. Taken together, these findings indicate that MiHA-specific CD8⁺ T cells of relapsed patients have a distinctive coinhibitory expression signature compared with patients who stay in remission. This phenotype may serve as a potential monitoring tool in patients. Moreover, these findings suggest that PD-1 and TIGIT play important roles in regulating T cell-mediated tumor control, providing a rationale for immunotherapy with blocking antibodies to treat relapse after allo-SCT.     

Cytoskeletal proteins bound to heat‐shock protein 70 may elicit resistance to simian immunodeficiency virus infection of CD4+ T cells

This study is based on the evidence that immunization of macaques with human CD4⁺ T cells elicits prevention of simian immunodeficiency virus (SIV) infection. We hypothesized that heat‐shock protein 70 (HSP70) isolated from CD4⁺ T cells may act as a chaperone and carry the protective host proteins. Two moieties of HSP70 were affinity‐purified from human CD4⁺ T cells; an ADP preparation with HSP70‐bound proteins (ADP‐HSP) and an ATP control preparation. Immunization of rhesus macaques with these preparations showed significant inhibition of SIVmac251 infectivity ex vivo in CD4⁺ T cells only with the ADP‐HSP (P = 0·01). Proteomic analysis identified three cytoskeletal elements, cofilin, profilin and γ‐actin, exclusively in the ADP‐HSP preparation. Investigation of the mechanism of prevention of SIV replication suggests that antibodies to the cytoskeletal proteins may inhibit actin depolymerization and facilitate viral degradation by the innate antiviral APOBEC3G. As cytoskeletal proteins are critical in the formation of virological and immunological synapses, finding specific antibodies and anti‐SIV/human immunodeficiency virus (HIV) factors suggests a novel insight into HIV‐1 immunopathogenesis.     

Immune tolerance: What is unique about the liver

The ‘liver tolerance effect’ mediates local and systemic tolerance to self and foreign antigens and has been attributed to specialized resident cells expressing anti-inflammatory mediators and inhibitory cell surface ligands for T cell activation. Non-parenchymal liver cells responsible for the tolerogenic properties of the liver are the resident dendritic cells (DCs), which comprise myeloid as well as plasmacytoid DCs, liver sinusoidal endothelial cells (LSECs), Kupffer cells (KCs) as well as hepatic stellate cells (HSCs), also known as Ito cells. These cells mediate immunosuppression by production of anti-inflammatory cytokines such as IL-10 and TGFβ as well as by expression of the negative co-stimulator for T cell activation programmed cell death ligand-1 (PD-L1). An interesting observation in this context is that knockout of IL-10 or PD-L1 (or the receptor PD-1) does not necessarily result in inflammatory liver damage whereas transgenic inhibition of TGFβ signaling induces liver disease in mice resembling chronic cholangitis. However, depending on the mouse model and on the type of injury, e.g. autoimmune disease, allograft rejection or viral infection, IL-10 or TGFβ and/or PD-1 as well as cytotoxic T lymphocyte antigen-4 (CTLA-4) contribute to the immunosuppressive mechanisms of CD4⁺CD25⁺Foxp3⁺ regulatory T cells (Tregs), which seem to be converted in the liver from infiltrating conventional naïve CD4⁺ T cells and/or effector CD4⁺ T cells to control the disease. Finally, hepatocytes also contribute to the ‘liver tolerance effect’ by expression of MHC class II molecules, probably low levels of co-stimulatory molecules and high levels of the co-inhibitory molecule PD-L1.