T cell allorecognition and MHC restriction--A case of Jekyll and Hyde?

Decoupling peptide binding from T cell receptor recognition with engineered chimeric MHC-I molecules

Major Histocompatibility Complex class I (MHC-I) molecules display self, viral or aberrant epitopic peptides to T cell receptors (TCRs), which employ interactions between complementarity-determining regions with both peptide and MHC-I heavy chain 'framework' residues to recognize specific Human Leucocyte Antigens (HLAs). The highly polymorphic nature of the HLA peptide-binding groove suggests a malleability of interactions within a common structural scaffold. Here, using structural data from peptide:MHC-I and pMHC:TCR structures, we first identify residues important for peptide and/or TCR binding. We then outline a fixed-backbone computational design approach for engineering synthetic molecules that combine peptide binding and TCR recognition surfaces from existing HLA allotypes. X-ray crystallography demonstrates that chimeric molecules bridging divergent HLA alleles can bind selected peptide antigens in a specified backbone conformation. Finally, in vitro tetramer staining and biophysical binding experiments using chimeric pMHC-I molecules presenting established antigens further demonstrate the requirement of TCR recognition on interactions with HLA framework residues, as opposed to interactions with peptide-centric Chimeric Antigen Receptors (CARs). Our results underscore a novel, structure-guided platform for developing synthetic HLA molecules with desired properties as screening probes for peptide-centric interactions with TCRs and other therapeutic modalities.

Immune Escape after Hematopoietic Stem Cell Transplantation (HSCT): From Mechanisms to Novel Therapies

Acute myeloid leukemia (AML) is the most common type of acute leukemia in adults. Recent advances in understanding its molecular basis have opened the way to new therapeutic strategies, including targeted therapies. However, despite an improvement in prognosis it has been documented in recent years (especially in younger patients) that allogenic hematopoietic stem cell transplantation (allo-HSCT) remains the only curative treatment in AML and the first therapeutic option for high-risk patients. After allo-HSCT, relapse is still a major complication, and is observed in about 50% of patients. Current evidence suggests that relapse is not due to clonal evolution, but instead to the ability of the AML cell population to escape immune control by a variety of mechanisms including the altered expression of HLA-molecules, production of anti-inflammatory cytokines, relevant metabolic changes and expression of immune checkpoint (ICP) inhibitors capable of “switching-off” the immune response against leukemic cells. Here, we review the main mechanisms of immune escape and identify potential strategies to overcome these mechanisms.

Structure and Function of Molecular Chaperones that Govern Immune Peptide Loading

Major histocompatibility class I (MHC-I) molecules bind peptides derived from cellular synthesis and display them at the cell surface for recognition by receptors on T lymphocytes (TCR) or natural killer (NK) cells. Such recognition provides a crucial step in autoimmunity, identification of bacterial and viral pathogens, and anti-tumor responses. Understanding the mechanism by which such antigenic peptides in the ER are loaded and exchanged for higher affinity peptides onto MHC molecules has recently been clarified by cryo-EM and X-ray studies of the multimolecular peptide loading complex (PLC) and a unimolecular tapasin-like chaperone designated TAPBPR. Insights from these structural studies and complementary solution NMR experiments provide a basis for understanding mechanisms related to immune antigen presentation.

Simultaneous Recognition of Allogeneic MHC and Cognate Autoantigen by Autoreactive T Cells in Transplant Rejection

The autoimmune condition is a primary obstacle to inducing tolerance in type 1 diabetes patients receiving allogeneic pancreas transplants. It is unknown how autoreactive T cells that recognize self-MHC molecules contribute to MHC-disparate allograft rejection. In this report, we show the presence and accumulation of dual-reactive, that is autoreactive and alloreactive, T cells in C3H islet allografts that were transplanted into autoimmune diabetic NOD mice. Using high-throughput sequencing, we discovered that T cells prevalent in allografts share identical TCRs with autoreactive T cells present in pancreatic islets. T cells expressing TCRs that are enriched in allograft lesions recognized C3H MHC molecules, and five of six cell lines expressing these TCRs were also reactive to NOD islet cells. These results reveal the presence of autoreactive T cells that mediate cross-reactive alloreactivity, and indicate a requirement for regulating such dual-reactive T cells in tissue replacement therapies given to autoimmune individuals.

The somatically generated portion of T cell receptor CDR3α contributes to the MHC allele specificity of the T cell receptor

Mature T cells bearing αβ T cell receptors react with foreign antigens bound to alleles of major histocompatibility complex proteins (MHC) that they were exposed to during their development in the thymus, a phenomenon known as positive selection. The structural basis for positive selection has long been debated. Here, using mice expressing one of two different T cell receptor β chains and various MHC alleles, we show that positive selection-induced MHC bias of T cell receptors is affected both by the germline encoded elements of the T cell receptor α and β chain and, surprisingly, dramatically affected by the non germ line encoded portions of CDR3 of the T cell receptor α chain. Thus, in addition to determining specificity for antigen, the non germline encoded elements of T cell receptors may help the proteins cope with the extremely polymorphic nature of major histocompatibility complex products within the species.

Thymic Origins of T Cell Receptor Alloreactivity

Major histocompatibility complex (MHC) restriction is a unique feature of T cell antigen recognition. Mature T cells respond to antigenic nonself peptides bound to self-MHC molecules, but a sizeable fraction of peripheral T cells can also respond to nonself peptide-MHC (pMHC) complexes in the context of transplantation. MHC specificity of the T cell receptor (TCR) repertoire is shaped during thymic development. Two hypotheses have been proposed to explain MHC specificity of T cells. It has been suggested that MHC specificity is an intrinsic feature of TCR structure, mediated by the germline-encoded regions of the TCR sequence. In support of this model, an estimated 15% to 30% of preselection TCR repertoire is estimated to be MHC-specific. Moreover, structural studies have shown some degree of conserved binding topology for TCR-peptide MHC complexes. However, there is also evidence that MHC restriction can be imposed on the TCR repertoire during thymic development, and it has been proposed that the interaction of the Lck kinase with CD4 or CD8 coreceptors is critical for generation of MHC specificity. This review will discuss recent work on assessment of the preselection of TCR repertoire, molecular evidence for the germline encoded TCR bias for MHC, and for the coreceptor sequestration model in the context of alloreactivity and transplantation.

The pharmacokinetics and pharmacodynamics of TOL101, a murine IgM anti-human αβ T cell receptor antibody, in renal transplant patients

BACKGROUND AND OBJECTIVES

TOL101 is a highly selective murine anti-αβ T cell receptor (TCR) IgM antibody and has recently completed phase II testing in primary renal transplant patients. This study was undertaken to determine the pharmacokinetic, pharmacodynamic, and immunogenic profile of TOL101.

METHODS

Nine cohorts of two to six patients received at least five daily doses (of, or combination of, 0.28, 1.4, 7, 14, 28, or 42 mg) of TOL-101 administered at successively higher doses. Semi-logarithmic graphs of serum TOL101 concentration versus time supported the use of a one-compartment intravenous infusion pharmacokinetic model. The model was parameterized in terms of serum clearance (CL) and volume of distribution (V d).

RESULTS

There was a trend toward a decrease in serum CL as the dose increased from 1.4 to 28 mg. However, the mean values for CL and V d were consistent across the cohorts that received 28, 32, and 42 mg. The mean ± standard deviation half-lives for these five cohorts ranged from 15.1 ± 7.35 to 28.6 ± 8.46 h, with an overall mean of 23.8 h, supporting both daily as well as fixed (i.e., not based on weight) dosing. Using CD3+ ≤25 cells/mm(3) as the primary pharmacodynamic marker, all non-responders were in the 0.28, 1.4, or 7 mg cohorts, suggesting that starting doses above 14 mg are required. Finally, one patient out of 36 was found to have anti-drug antibody.

CONCLUSIONS

Together, the data show that while TOL101 is a highly potent anti-TCR antibody, its pharmacological profile is somewhat versatile, allowing for daily dosing without immunogenicity concerns.

Identification of a naturally processed HLA-Cw7-binding peptide that cross-reacts with HLA-A24-restricted ovarian cancer-specific CTLs

Here, we describe an human leukocyte antigen (HLA)-A*24:02-restricted cytotoxic T-lymphocyte (CTL) clone, 1G3, established from naïve CD8(+) T-lymphocytes obtained from a healthy donor stimulated with HLA-modified TOV21G, an ovarian cancer cell line. The 1G3 clone responds not only to ovarian cancer cells in the context of HLA-A*24:02 but also to allogeneic HLA-Cw*07:02 molecules through cross-reactive T-cell receptor recognition. Expression screening using a complementary DNA library constructed from TOV21G messenger RNA revealed that this alloreactivity was mediated through the nine-mer peptide VRTPYTMSY, derived from RNA-binding motif protein 4. To our knowledge, this study presents the first example of the allorecognition of an HLA-Cw molecule by HLA-A-restricted T-cells, thereby revealing a naturally processed epitope peptide. These findings provide the structural bases for the allorecognition of human T-cells. In addition, this study suggests that unexpected alloresponses occur in certain HLA combinations, and further study is needed to understand the mechanisms of alloreactivity for better prediction of alloresponses in clinical settings.

First-in-human study of the safety and efficacy of TOL101 induction to prevent kidney transplant rejection

TOL101 is a murine IgM mAb targeting the αβ TCR. Unlike other T cell targets, the αβ TCR has no known intracellular signaling domains and may provide a nonmitogenic target for T cell inactivation. We report the 6-month Phase 2 trial data testing TOL101 in kidney transplantation. The study was designed to identify a dose that resulted in significant CD3 T cell modulation (<25 T cell/mm(3) ), to examine the safety and tolerability of TOL101 and to obtain preliminary efficacy information. Thirty-six patients were enrolled and given 5-10 daily doses of TOL101; 33 patients completed dosing, while three discontinued after two doses due to a self-limiting urticarial rash. Infusion adjustments, antihistamines, steroids and dose escalation of TOL101 reduced the incidence of the rash. Doses of TOL101 above 28 mg resulted in prolonged CD3 modulation, with rapid recovery observed 7 days after therapy cessation. There were no cases of patient or graft loss. Few significant adverse events were reported, with one nosocomial pneumonia. There were five biopsy-confirmed acute cellular rejections (13.9%); however, no donor-specific antibodies were detected. Overall TOL101 was well-tolerated, supporting continued clinical development using the dose escalating 21-28-42-42-42 mg regimen.

Predisposed αβ T cell antigen receptor recognition of MHC and MHC-I like molecules?

The diverse αβ T cell receptor (TCR) repertoire exhibits versatility in its ability to generate antigen (Ag) receptors capable of interacting with polymorphic Major Histocompatibility Complex (MHC) molecules and monomorphic MHC-I like molecules, including the CD1 and MR1 families. Collectively, these evolutionarily related Ag-presenting molecules present peptides, lipids and vitamin B metabolites for T cell surveillance. Interestingly, whilst common TCR gene usage can underpin recognition of these distinct classes of Ags, it is unclear whether the 'rules' that govern αβTCR-Ag MHC interactions are shared. We highlight recent observations in the context of TCR biases towards MHC and MHC-I like molecules.

Mucins help to avoid alloreactivity at the maternal fetal interface

During gestation, many different mechanisms act to render the maternal immune system tolerant to semi-allogeneic trophoblast cells of foetal origin, including those mediated via mucins that are expressed during the peri-implantation period in the uterus. Tumour- associated glycoprotein-72 (TAG-72) enhances the already established tolerogenic features of decidual dendritic cells with the inability to progress towards Th1 immune orientation due to lowered interferon (IFN)-γ and interleukin (IL)-15 expression. Mucine 1 (Muc 1) supports alternative activation of decidual macrophages, restricts the proliferation of decidual regulatory CD56⁺ bright natural killer (NK) cells, and downregulates their cytotoxic potential, including cytotoxic mediator protein expression. Removing TAG-72 and Muc 1 from the eutopic implantation site likely contributes to better control of trophoblast invasion by T cells and NK cells and appears to have important immunologic advantages for successful implantation, in addition to mechanical advantages. However, these processes may lead to uncontrolled trophoblast growth after implantation, inefficient defence against infection or tumours, and elimination of unwanted immunocompetent cells at the maternal-foetal interface. The use of mucins by tumour cells to affect the local microenvironment in order to avoid the host immune response and to promote local tumour growth, invasion, and metastasis confirms this postulation.

Highly divergent T-cell receptor binding modes underlie specific recognition of a bulged viral peptide bound to a human leukocyte antigen class I molecule

Human leukocyte antigen (HLA)-I molecules can present long peptides, yet the mechanisms by which T-cell receptors (TCRs) recognize featured pHLA-I landscapes are unclear. We compared the binding modes of three distinct human TCRs, CA5, SB27, and SB47, complexed with a “super-bulged” viral peptide (LPEPLPQGQLTAY) restricted by HLA-B*35:08. The CA5 and SB27 TCRs engaged HLA-B*35:08^LPEP similarly, straddling the central region of the peptide but making limited contacts with HLA-B*35:08. Remarkably, the CA5 TCR did not contact the α1-helix of HLA-B*35:08. Differences in the CDR3β loop between the CA5 and SB27 TCRs caused altered fine specificities. Surprisingly, the SB47 TCR engaged HLA-B*35:08^LPEP using a completely distinct binding mechanism, namely “bypassing” the bulged peptide and making extensive contacts with the extreme N-terminal end of HLA-B*35:08. This docking footprint included HLA-I residues not observed previously as TCR contact sites. The three TCRs exhibited differing patterns of alloreactivity toward closely related or distinct HLA-I allotypes. Thus, the human T-cell repertoire comprises a range of TCRs that can interact with “bulged” pHLA-I epitopes using unpredictable strategies, including the adoption of atypical footprints on the MHC-I.

Immunology and the challenge of transplantation

Transplantation of tissues or organs between individuals who are not genetically related often leads to rejection by the recipient. The human genes responsible for this process are located on the short arm of the chromosome 6 and are called Major Histocompatibility Complex (MHC). Six main loci have been identified in the human MHC: HLA-A, HLA-B and HLA-C belong to the HLA class I, while HLA-DP, HLA-DQ and HLA-DR belong to HLA class II. The physiological function of MHC molecules is to present peptides to the T cells. Indeed, they are integral components of the ligands that recognise most T cells, since the receptor of the T cell (TCR) has specificity for complexes of foreign antigenic peptides, and self-MHC molecules. Thus the proteins of the MHC are responsible for the body being able to distinguish between its own and foreign cells, known as self-tolerance and consequently are the proteins which determine the evolution of transplants. The special case of foreign MHC antigen recognition is known as allorecognition and consists of the capacity of T cells to recognise peptide/MHC complexes with which they have not been in contact during the process of maturation in the thymus. There are two mechanisms of allorecognition, direct and indirect; both can lead to rejection of the transplant. Direct recognition prevails during the first few weeks or months after transplantation, and is caused by the APCs of the donor. These cells start disappearing from the transplanted organ and indirect recognition becomes important. There is evidence that the indirect pathway is sufficient to mediate both acute and chronic rejection. In this chapter we will describe fundamental aspects of the MHC system, as well as, specifically, its involvement in the allogenic response of the immune system against organ transplants.

A single autoimmune T cell receptor recognizes more than a million different peptides

The T cell receptor (TCR) orchestrates immune responses by binding to foreign peptides presented at the cell surface in the context of major histocompatibility complex (MHC) molecules. Effective immunity requires that all possible foreign peptide-MHC molecules are recognized or risks leaving holes in immune coverage that pathogens could quickly evolve to exploit. It is unclear how a limited pool of <10(8) human TCRs can successfully provide immunity to the vast array of possible different peptides that could be produced from 20 proteogenic amino acids and presented by self-MHC molecules (>10(15) distinct peptide-MHCs). One possibility is that T cell immunity incorporates an extremely high level of receptor degeneracy, enabling each TCR to recognize multiple peptides. However, the extent of such TCR degeneracy has never been fully quantified. Here, we perform a comprehensive experimental and mathematical analysis to reveal that a single patient-derived autoimmune CD8(+) T cell clone of pathogenic relevance in human type I diabetes recognizes >one million distinct decamer peptides in the context of a single MHC class I molecule. A large number of peptides that acted as substantially better agonists than the wild-type "index" preproinsulin-derived peptide (ALWGPDPAAA) were identified. The RQFGPDFPTI peptide (sampled from >10(8) peptides) was >100-fold more potent than the index peptide despite differing from this sequence at 7 of 10 positions. Quantification of this previously unappreciated high level of CD8(+) T cell cross-reactivity represents an important step toward understanding the system requirements for adaptive immunity and highlights the enormous potential of TCR degeneracy to be the causative factor in autoimmune disease.

Cell surface display of functional human MHC class II proteins: yeast display versus insect cell display

Reliable and robust systems for engineering functional major histocompatibility complex class II (MHCII) proteins have proved elusive. Availability of such systems would enable the engineering of peptide-MHCII (pMHCII) complexes for therapeutic and diagnostic applications. In this paper, we have developed a system based on insect cell surface display that allows functional expression of heterodimeric DR2 molecules with or without a covalently bound human myelin basic protein (MBP) peptide, which is amenable to directed evolution of DR2-MBP variants with improved T cell receptor (TCR)-binding affinity. This study represents the first example of functional display of human pMHCII complexes on insect cell surface. In the process of developing this pMHCII engineering system, we have also explored the potential of using yeast surface display for the same application. Our data suggest that yeast display is a useful system for analysis and engineering of peptide binding of MHCII proteins, but not suitable for directed evolution of pMHC complexes that bind with low affinity to self-reactive TCRs.

Identification by random forest method of HLA class I amino acid substitutions associated with lower survival at day 100 in unrelated donor hematopoietic cell transplantation

The identification of important amino acid substitutions associated with low survival in hematopoietic cell transplantation (HCT) is hampered by the large number of observed substitutions compared to the small number of patients available for analysis. Random forest analysis is designed to address these limitations. We studied 2,107 HCT recipients with good or intermediate risk hematologic malignancies to identify HLA class I amino acid substitutions associated with reduced survival at day 100 post-transplant. Random forest analysis and traditional univariate and multivariate analyses were used. Random forest analysis identified amino acid substitutions in 33 positions that were associated with reduced 100 day survival, including HLA-A 9, 43, 62, 63, 76, 77, 95, 97, 114, 116, 152, 156, 166, and 167; HLA-B 97, 109, 116, and 156; and HLA-C 6, 9, 11, 14, 21, 66, 77, 80, 95, 97, 99, 116, 156, 163, and 173. Thirteen had been previously reported by other investigators using classical biostatistical approaches. Using the same dataset, traditional multivariate logistic regression identified only 5 amino acid substitutions associated with lower day 100 survival. Random forest analysis is a novel statistical methodology for analysis of HLA-mismatching and outcome studies, capable of identifying important amino acid substitutions missed by other methods.

T cell allorecognition via molecular mimicry

T cells often alloreact with foreign human leukocyte antigens (HLA). Here we showed the LC13 T cell receptor (TCR), selected for recognition on self-HLA-B( *)0801 bound to a viral peptide, alloreacts with B44 allotypes (HLA-B( *)4402 and HLA-B( *)4405) bound to two different allopeptides. Despite extensive polymorphism between HLA-B( *)0801, HLA-B( *)4402, and HLA-B( *)4405 and the disparate sequences of the viral and allopeptides, the LC13 TCR engaged these peptide-HLA (pHLA) complexes identically, accommodating mimicry of the viral peptide by the allopeptide. The viral and allopeptides adopted similar conformations only after TCR ligation, revealing an induced-fit mechanism of molecular mimicry. The LC13 T cells did not alloreact against HLA-B( *)4403, and the single residue polymorphism between HLA-B( *)4402 and HLA-B( *)4403 affected the plasticity of the allopeptide, revealing that molecular mimicry was associated with TCR specificity. Accordingly, molecular mimicry that is HLA and peptide dependent is a mechanism for human T cell alloreactivity between disparate cognate and allogeneic pHLA complexes.

HLA-A disparities illustrate challenges for ranking the impact of HLA mismatches on bone marrow transplant outcomes in the United States

HLA disparity between hematopoietic stem cell donors and recipients is one of the most important factors influencing transplant outcomes, but there are no well-accepted guidelines to aid in selecting the optimal donor among several HLA mismatched donors. In this report, HLA-A is used as a model to illustrate factors that are barriers to delineating the relationship between specific HLA mismatches and transplant outcomes in the United States. Patients in this investigation received transplants for hematologic malignancies that were facilitated by the National Marrow Donor Program (NMDP) between 1990 and 2002 (n = 4226). High-resolution HLA typing was performed for HLA-A, -B, -C, -DRB1, -DQA1, -DQB1, -DPA1, and -DPB1. HLA-A mismatches were observed in 745 donor-recipient pairs and 62% of these pairs also had disparities at HLA-B, -C, and/or -DRB1. The HLA-A mismatches involved 190 different combinations of HLA-A alleles and 51% of these were observed in only 1 pair. Addition of a single HLA-A disparity when HLA-B, -C, and -DRB1 were matched (n = 282) was associated with increased mortality (odds ratio [OR] = 1.32, confidence interval [CI] 1.07-1.63). When HLA-B, -C, and -DRB1 were matched, the most frequent HLA-A mismatches were HLA-A*0201:0205 (n = 28), HLA-A *0301:0302 (n = 15), HLA-A *0201:0206 (n = 15), HLA-A *0201:6801 (n = 12), HLA-A*0101:1101 (n = 11), and HLA-A*0101:0201 (n = 10). There were no statistically significant relationships between any of these disparities and transplant outcomes (engraftment, acute and chronic graft-versus-host disease [aGVHD, cGVHD] relapse, treatment-related mortality [TRM], or overall survival [OS]) when adjustments for multiple comparisons were considered. Achieving 80% power to detect an effect of any 1 of these 6 HLA-A disparities on survival is estimated to require a total transplant population of 11,000 to more than 1 million U.S. donor-recipient pairs depending upon the HLA disparity. Thus, alternative approaches are required to develop a clinically relevant ranking system for specific HLA disparities in the United States.

Cross-recognition of HLA DR4 alloantigen by virus-specific CD8+ T cells: a new paradigm for self-/nonself-recognition

The ability of CD8+ T cells to engage a diverse range of peptide–major histocompatibility complex (MHC) complexes can also lead to cross-recognition of self and nonself peptide-MHC complexes and thus directly contribute toward allograft rejection or autoimmunity. Here we present a novel form of cross-recognition by herpes virus–specific CD8+ cytotoxic T cells that challenges the current paradigm of self/non-self recognition. Functional characterization of a human leukocyte antigen (HLA) Cw*0602-restricted cytomegalovirus-specific CD8+ T-cell response revealed an unusual dual specificity toward a pp65 epitope and the alloantigen HLA DR4. This cross-recognition of HLA DR4 alloantigen was critically dependent on the coexpression of HLA DM and was preferentially directed toward the B-cell lineage. Furthermore, allostimulation of peripheral blood lymphocytes with HLA DRB*0401-expressing cells rapidly expanded CD8+ T cells, which recognized the pp65 epitope in the context of HLA Cw*0602. T-cell repertoire analysis revealed 2 dominant populations expressing T-cell receptor beta variable (TRBV)4-3 or TRBV13, with cross-reactivity exclusively mediated by the TRBV13+ clonotypes. More importantly, cross-reactive TRBV13+ clonotypes displayed markedly lower T-cell receptor binding affinity and a distinct pattern of peptide recognition, presumably mimicking a structure presented on the HLA DR4 allotype. These results illustrate a novel mechanism whereby virus-specific CD8+ T cells can cross-recognize HLA class II molecules and may contribute toward allograft rejection and/or autoimmunity.

Immune-mediated liver diseases: programmed cell death ligands and circulating apoptotic markers

Primary biliary cirrhosis, primary sclerosing cholangitis and autoimmune hepatitis are the three major immune-mediated liver diseases. The etiologies of primary biliary cirrhosis, primary sclerosing cholangitis and autoimmune hepatitis are largely unknown, but seem to be influenced by genetic and environmental factors. Autoantibodies can be found in nearly all patients with primary sclerosing cholangitis and autoimmune hepatitis, and in the vast majority of patients with primary sclerosing cholangitis. In addition, autoimmune hepatitis is associated with high concentrations of serum globulins. Enhanced liver cell death by apoptosis has been described in all of these liver diseases, although the precise mechanisms remain unclear. In general, apoptosis can be initiated via an extrinsic pathway that is triggered by engagement of death receptors on the cell surface, or via an intrinsic pathway that is induced by mitochondrial injury and is influenced by members of the Bcl-2 family. In both pathways, effector caspases are finally activated that cleave and degrade cell structures, resulting in the release of apoptotic products into the circulation. New diagnostic tests can detect these apoptotic markers and programmed cell death ligands such as Fas and Fas-ligands, nucleosomes, caspases, cytokeratin fragments, macrophage migration inhibitory factor, soluble intracellular adhesion molecule, natural killer cells group 2D and programmed death ligands. Several of these markers have been found to be altered in tissue and/or blood of immune-mediated liver diseases, some also in nonimmune-mediated liver diseases. Beyond their potential usefulness as additional diagnostic markers, they may be valuable for the estimation of disease severity and therapy monitoring. This review summarizes current knowledge on apoptotic mechanisms, death receptor ligands and circulating apoptotic markers in immune-mediated liver diseases.