Negative T cell costimulation and islet tolerance

Basic Understanding of Liver Transplant Immunology

The liver is a specialized organ and plays an important role in our immune system. The liver constitutes parenchymal cells which are hepatocytes and cholangiocytes (60-80%) and non-parenchymal cells like liver sinusoidal endothelial cells (LSECs), hepatic satellite/Ito cells, Kupffer cells, neutrophils, mononuclear cells, T and B lymphocytes (conventional and non-conventional), natural killer cells, and natural killer T (NKT) cells. The liver mounts a rapid and strong immune response, under unfavorable conditions and acts as an immune tolerance to a variety of non-pathogenic antigens. This delicate and dynamic interaction between different kinds of immune cells in the liver maintains a balance between immune screening and immune tolerance. The liver allografts are privileged immunologically; however, allograft rejection is not uncommon and is classified as cell or antibody-mediated. Advancements in transplant immunology help in the prevention of allografts rejection by immune reactions of the host thus leading to better graft and host survival. Fewer patients may not require immunosuppression due to systemic donor-specific T-cell tolerance. The liver tolerance mechanism is poorly studied, and LSEC and unconventional lymphocytes play an important role that dampens T cell response either by inducing apoptosis of cells or inhibiting co-stimulatory pathways. Newer cell-based therapy based on Treg, dendritic cells, and mesenchymal stromal cells will probably change the future of immunosuppression. Various invasive and non-invasive biomarkers and artificial intelligence have also been investigated to predict graft survival, post-transplant complications, and immunotolerance in the future.

Toward Small-Molecule Inhibition of Protein-Protein Interactions: General Aspects and Recent Progress in Targeting Costimulatory and Coinhibitory (Immune Checkpoint) Interactions

Protein-Protein Interactions (PPIs) that are part of the costimulatory and coinhibitory (immune checkpoint) signaling are critical for adequate T cell response and are important therapeutic targets for immunomodulation. Biologics targeting them have already achieved considerable clinical success in the treatment of autoimmune diseases or transplant recipients (e.g., abatacept, belatacept, and belimumab) as well as cancer (e.g., ipilimumab, nivolumab, pembrolizumab, atezolizumab, durvalumab, and avelumab). In view of such progress, there have been only relatively limited efforts toward developing small-molecule PPI inhibitors (SMPPIIs) targeting these cosignaling interactions, possibly because they, as all other PPIs, are difficult to target by small molecules and were not considered druggable. Nevertheless, substantial progress has been achieved during the last decade. SMPPIIs proving the feasibility of such approaches have been identified through various strategies for a number of cosignaling interactions including CD40-CD40L, OX40-OX40L, BAFFR-BAFF, CD80-CD28, and PD-1-PD-L1s. Here, after an overview of the general aspects and challenges of SMPPII-focused drug discovery, we review them briefly together with relevant structural, immune-signaling, physicochemical, and medicinal chemistry aspects. While so far only a few of these SMPPIIs have shown activity in animal models (DRI-C21045 for CD40-D40L, KR33426 for BAFFR-BAFF) or reached clinical development (RhuDex for CD80-CD28, CA-170 for PD-1-PD-L1), there is proof-of-principle evidence for the feasibility of such approaches in immunomodulation. They can result in products that are easier to develop/ manufacture and are less likely to be immunogenic or encounter postmarket safety events than corresponding biologics, and, contrary to them, can even become orally bioavailable.

Small-Molecule Inhibitors of the CD40-CD40L Costimulatory Protein-Protein Interaction

Costimulatory interactions are required for T cell activation and development of an effective immune response; hence, they are valuable therapeutic targets for immunomodulation. However, they, as all other protein-protein interactions, are difficult to target by small molecules. Here, we report the identification of novel small-molecule inhibitors of the CD40-CD40L interaction designed starting from the chemical space of organic dyes. For the most promising compounds such as DRI-C21045, activity (IC50) in the low micromolar range has been confirmed in cell assays including inhibition of CD40L-induced activation in NF-κB sensor cells, THP-1 myeloid cells, and primary human B cells as well as in murine allogeneic skin transplant and alloantigen-induced T cell expansion in draining lymph node experiments. Specificity versus other TNF-superfamily interactions (TNF-R1-TNF-α) and lack of cytotoxicity have also been confirmed at these concentrations. These novel compounds provide proof-of-principle evidence for the possibility of small-molecule inhibition of costimulatory protein-protein interactions, establish the structural requirements needed for efficient CD40-CD40L inhibition, and serve to guide the search for such immune therapeutics.

TNF superfamily protein-protein interactions: feasibility of small- molecule modulation

The tumor necrosis factor (TNF) superfamily (TNFSF) contains about thirty structurally related receptors (TNFSFRs) and about twenty protein ligands that bind to one or more of these receptors. Almost all of these cell surface protein-protein interactions (PPIs) represent high-value therapeutic targets for inflammatory or immune modulation in autoimmune diseases, transplant recipients, or cancers, and there are several biologics including antibodies and fusion proteins targeting them that are in various phases of clinical development. Small-molecule inhibitors or activators could represent possible alternatives if the difficulties related to the targeting of protein-protein interactions by small molecules can be addressed. Compounds proving the feasibility of such approaches have been identified through different drug discovery approaches for a number of these TNFSFR-TNFSF type PPIs including CD40-CD40L, BAFFR-BAFF, TRAIL-DR5, and OX40-OX40L. Corresponding structural, signaling, and medicinal chemistry aspects are briefly reviewed here. While none of these small-molecule modulators identified so far seems promising enough to be pursued for clinical development, they provide proof-of-principle evidence that these interactions are susceptible to small-molecule modulation and can serve as starting points toward the identification of more potent and selective candidates.

Rodent models for investigating the dysregulation of immune responses in type 1 diabetes

Type 1 diabetes (T1D) is an autoimmune disease mediated by T cells that selectively destroy the insulin-producing β cells. Previous reports based on epidemiological and animal studies have demonstrated that both genetic factors and environmental parameters can either promote or attenuate the progression of autoimmunity. In recent decades, several inbred rodent strains that spontaneously develop diabetes have been applied to the investigation of the pathogenesis of T1D. Because the genetic manipulation of mice is well developed (transgenic, knockout, and conditional knockout/transgenic), most studies are performed using the nonobese diabetic (NOD) mouse model. This paper will focus on the use of genetically manipulated NOD mice to explore the pathogenesis of T1D and to develop potential therapeutic approaches.

Human hepatocyte transplantation: state of the art

Hepatocyte transplantation is making its transition from bench to bedside for liver-based metabolic disorders and acute liver failure. Over eighty patients have now been transplanted world wide and the safety of the procedure together with medium-term success has been established. A major limiting factor in the field is the availability of good quality cells as hepatocytes are derived from grafts that are deemed unsuitable for transplantation. Alternative sources of cell, including stem cells may provide a sustainable equivalent to primary hepatocytes. There is also a need to develop techniques that will improve the engraftment, survival and function of transplanted hepatocytes. Such developments may allow hepatocyte transplantation to become an accepted and practical alternative to liver transplantation in the near future.

Expression of a CD200 transgene is necessary for induction but not maintenance of tolerance to cardiac and skin allografts

CD200, a type 2 transmembrane molecule of the Ig supergene family, can induce immunosuppression in a number of biological systems, as well as promote increased graft acceptance, following binding to its receptors (CD200Rs). Skin and cardiac allograft acceptance are readily induced in transgenic mice overexpressing CD200 under control of a doxycycline-inducible promoter, both of which are associated with increased intragraft expression of mRNAs for a number of genes associated with altered T cell subset differentiation, including GATA-3, type 2 cytokines (IL-4, IL-13), GITR, and Foxp3. Interestingly, some 12-15 days after grafting, induction of transgenic CD200 expression can be stopped (by doxycycline withdrawal), without obvious significant effect on graft survival. However, neutralization of all CD200 expression (including endogenous CD200 expression) by anti-CD200 mAb caused graft loss, as did introduction of an acute inflammatory stimulus (LPS, 10 microg/mouse, delivered by i.p. injection). We conclude that even with apparently stably accepted tissue allografts, disruption of the immunoregulatory balance by an intense inflammatory stimulus can cause graft loss.

CTLA4-Ig-modified dendritic cells inhibit lymphocyte-mediated alloimmune responses and prolong the islet graft survival in mice

The induction of antigen specific tolerance is critical for prevention and treatment of allograft rejection. In this study, we transfected CTLA4-Ig gene into dendritic cells (DCs), and investigated their effect on inhibition of lymphocyte activity in vitro and induction of immune tolerance on pancreatic islet allograft in mice. An IDDM C57BL/6 murine model induced by streptozotocin is as model mouse. The model mice were transplanted of the islet cells isolated from the BALB/c mice to their kidney capsules, and injected of CTLA4-Ig modified DCs (mDCs). The results showed that mDCs could significantly inhibit T lymphocyte proliferation and induce its apoptosis; whereas, unmodified DCs (umDCs) promoted the murine lymphocyte proliferation. Compared with injection of umDCs and IgG1 modified DCs, the injection of mDCs prolonged IDDM mice's allograft survival, and normalized their plasma glucose (PG) levels within 3 days and maintained over 2 weeks. The level of IFN-gamma was lower and the level of IL-4 was higher in mDCs treated recipient mice than that in control mice, it indicated that mDCs led to Th1/Th2 deviation. After 7 days of islet transplantation, HE stain of the renal specimens showed that the islets and kidneys were intact in structure, and islet cells numbers are increased in mDCs treated mice. Our studies suggest that DCs expressing CTLA4-Ig fusion protein can induce the immune tolerance to islet graft and prolong the allograft survival through the inhibition of T cell proliferation in allogeneic mice.

Transplantation immunology: what the clinician needs to know for immunotherapy

The liver is unique among transplanted organs with respect to its interaction with the host immune system. There is evidence, both anecdotal and documented, that some liver recipients who cease taking immunosuppressive drugs maintain allograft function, suggesting robust tolerance is in place. Moreover, recipients of human liver allografts require less immunosuppression than do other organ recipients, and liver transplants confer protection on other organ grafts from the same donor. Hence, the liver shows features of immune privilege. Still, the liver can display destructive immunologic processes such as rejection in approximately one quarter of patients. The understanding of the cellular and molecular mechanisms operant in tolerance vs allograft rejection is important for developing new agents to improve long-term outcome, minimize infectious complications (including recurrence of hepatotropic viruses), and deliver immunosuppression without long-term toxicity. This review describes the unique aspects of the hepatic immune response, the pathways involved in T-cell activation and alloantigen recognition, effector cells and pathways mediating liver allograft rejection, the role of regulatory T cells, and targets of current and future immunosuppressive agents.

Cell-mediated rejection results in allograft loss after liver cell transplantation

Liver cell transplantation in humans has been impeded by invariable loss of the graft. It is unclear whether graft loss is due to an immune response against donor hepatocytes. Transplantation with ABO-matched liver cells was performed in a patient with Crigler-Najjar type 1. After successful engraftment, there was a gradual loss of graft function. Solid-phase enzyme immunoassay testing and cell-complement cytotoxicity assays detecting preformed antibodies directed toward class I and/or class II human leukocyte antigen (HLA) molecules were negative. In contrast, a striking host alloresponse to either the HLA-B39 or C7 antigen was found, suggesting that a vigorous response to a defined mismatched HLA antigen contributed to graft loss in our patient. This study provides evidence that a T-cell-mediated immune mechanism could be responsible for human liver cell transplant graft loss. This finding warrants confirmation in future liver cell transplants in humans.

Patients with chronic pancreatitis have islet progenitor cells in their ducts, but reversal of overt diabetes in NOD mice by anti-CD3 shows no evidence for islet regeneration

Monoclonal antibodies to T-cell coreceptors have been shown to tolerise autoreactive T-cells and prevent or even reverse autoimmune pathology. In type 1 diabetes, there is a loss of insulin-secreting beta-cells, and a cure for type 1 diabetes would require not only tolerance induction but also recovery of the functional beta-cell mass. Although we have previously shown that diabetic mice have increased numbers of ductal progenitors in the pancreas, there is no evidence of any increase of insulin-secreting cells in the ducts. In contrast, in the adult human pancreas of patients with chronic pancreatitis, we can demonstrate, in the ducts, increased numbers of insulin-containing cells, as well as cells containing other endocrine and exocrine markers. There are also significantly increased numbers of cells expressing the homeodomain protein, pancreatic duodenal homeobox-1. Anti-CD3 has been shown to reverse overt diabetes in NOD mice; thus, we have used this model to ask whether monoclonal antibody-mediated inhibition of ongoing beta-cell destruction enables islet regeneration to occur. We find no evidence that such monoclonal antibody therapy results in either regeneration of insulin-secreting beta-cells or of increased proliferation of islet beta-cells.

T cell tolerance induced by therapeutic antibodies

Ever since the discovery of Medawar, over 50 years ago, that immunological tolerance was an acquired phenomenon that could be manipulated in neonatal mice, the ability to induce therapeutic tolerance against autoantigens, allergens and organ grafts has been a major driving force in immunology. Within the last 20 years we have found that a brief treatment with monoclonal antibodies that block certain functional molecules on the surface of the T cell is able to reprogramme the established immune repertoire of the adult mouse, allowing indefinite acceptance of allografts or effective curing of autoimmune diseases. We are only now just beginning to define many of the regulatory mechanisms that induce and maintain the tolerant state with the aim of being able to safely and reliably apply these technologies to human clinical situations.

PD-L1 is expressed by human renal tubular epithelial cells and suppresses T cell cytokine synthesis

T cell activation is affected by both stimulatory and inhibitory co-signaling. MHC class II-expressing renal tubular epithelial cells (TEC) can function as APC for T cells. To study the influence of inhibitory ligands on TEC-mediated T cell activation, we examined the expression of programmed death ligand-1 (PD-L1) on human TEC line HK-2 cells, as well as in normal and diseased kidney samples. RT-PCR, FACS, and immunocytochemistry showed that PD-L1 is constitutively expressed on HK-2 cells, and is dramatically upregulated by IFN-gamma. In situ hybridization and immunohistochemical staining revealed constitutive low expression of PD-L1 on proximal tubules at both mRNA and protein levels in normal kidneys, but much higher expression in kidneys with type IV lupus nephritis. In vitro, pretreatment of IFN-gamma-stimulated HK-2 cells with anti-PD-L1 significantly enhanced IL-2 secretion from cocultured, mitogen-activated Jurkat or human peripheral blood T cells. These results suggest that the PD-L1:PD-1 pathway negatively regulates T cell activation by TEC, and may play an inhibitory role in TEC-mediated immune activation and immunopathology in the kidney.

The balance of immune responses: costimulation verse coinhibition

Many of the B7 superfamily members (e.g., B7-1, B7-2, ICOS-L, B7-H1, B7-DC) were initially characterized as T cell costimulatory molecules. However, more recently it has become clear they can also coinhibit T cell responses. We review many of the B7 family members, with a particular focus on B7-H1, and examine their role in autoimmunity, transplant rejection, and cancer pathogenesis. It is crucial to understand that many B7 family members have opposing effects on an immune response. This cautions against using clinical immunotherapeutic reagents targeted against these molecules until we gain a better understanding of the circumstances that regulate the outcomes of the T cell response.

Local expression of B7-H1 promotes organ-specific autoimmunity and transplant rejection

A number of studies have suggested B7-H1, a B7 family member, inhibits T cell responses. Therefore, its expression on nonlymphoid tissues has been proposed to prevent T cell-mediated tissue destruction. To test this hypothesis, we generated transgenic mice that expressed B7-H1 on pancreatic islet beta cells. Surprisingly, we observed accelerated rejection of transplanted allogeneic B7-H1-expressing islet beta cells. Furthermore, transgenic B7-H1 expression broke immune tolerance, as some of the mice spontaneously developed T cell-dependent autoimmune diabetes. In addition, B7-H1 expression increased CD8+ T cell proliferation and promoted autoimmunity induction in a T cell adoptive transfer model of diabetes. Consistent with these findings, B7-H1.Ig fusion protein augmented naive T cell priming both in vitro and in vivo. Our results demonstrate that B7-H1 can provide positive costimulation for naive T cells to promote allograft rejection and autoimmune disease pathogenesis.

Islet transplantation in patients with diabetes mellitus: choice of immunosuppression

Islet transplantation offers patients with type 1 diabetes mellitus freedom from long-term insulin therapy and a degree of metabolic control that is far superior to injected insulin. The hope is that near-perfect glucose control sustained over time will prevent progression of secondary diabetic complications. The selection of optimal immunosuppressive agents for islet transplantation has been a formidable challenge, given the need to overcome both autoimmune and alloimmune barriers, as well as the potential toxicity of immunosuppressive agents on transplanted islets. Early strategies relied on protocols that had proven success in solid organ transplantation and consisted of azathioprine, cyclosporine and corticosteroids. Under these protocols, fewer than 10% of patients were able to achieve insulin independence. The development of the 'Edmonton Protocol' dramatically transformed clinical outcomes in islet transplantation in recent years through the introduction of a more potent, less diabetogenic, and corticosteroid-free immunosuppressive regimen consisting of sirolimus, low-dose tacrolimus, and induction anti-interleukin-2 receptor antibody. While insulin independence rates under this protocol have been highly successful, patients must be maintained on lifelong immunosuppression. While the risk of malignancy, post-transplant lymphoma and sepsis have been low and diminishing in transplanted patients to date, fears of these complications and a host of drug-related adverse effects have precluded broader application. Patients undergoing islet transplantation today must exchange insulin for chronic immunosuppressive therapy, and therefore the procedure can only be justified in patients with very unstable forms of diabetes, or in those with another solid organ allograft who already endure the risks of immunosuppression. Advances in more specific and less toxic immunosuppressive agents together with progress in better understanding the biology of diabetes will lead to more suitable strategies to control both alloimmune and recurrent autoimmune reactions. These protocols, ultimately aimed at establishing tolerance, are an essential pre-requisite to move towards providing islet transplantation earlier in the course of the disease, including transplantation in children. This review addresses the evolution of immunosuppressive strategies in islet transplantation, and highlights some novel agents in pre-clinical development or in early clinical trials that may offer considerable promise in facilitating the induction of tolerance.

Altering immune tolerance therapeutically: the power of negative thinking

The etiology of most human autoimmune diseases remains largely unknown. However, investigators have identified several negative regulatory mechanisms acting at the level of innate and/or adaptive immunity. Mutations resulting in a deficiency of some key regulatory molecules are associated with systemic or organ-specific inflammatory disorders, which often have a prominent autoimmune component. Genetic studies have implicated the negative regulator cytotoxic T-lymphocyte antigen 4 (CTLA-4) and other regulatory molecules in human autoimmune diseases. In addition to CTLA-4, key inhibitory molecules include programmed death 1 and B and T lymphocyte attenuator. Transforming growth factor beta1 and interleukin-10 also play major anti-inflammatory and regulatory roles. Tumor cells and infectious agents use negative regulatory pathways to escape immunity. The therapeutic blockage of negative signaling (particularly of CTLA-4) increases immunity against tumor antigens but also induces or aggravates autoimmune diseases. It appears that under normal conditions, the immune system is under strong "negative influences" that prevent autoimmunity and that release of this suppression results in disease. Regulation involves communication between the immune system and nonlymphoid tissues, and the latter can deliver inhibitory or stimulatory signals. Recent studies reveal that the generation of negative signals by selective engagement of inhibitory molecules is feasible and is likely to be of therapeutic benefit in autoimmune diseases and allograft rejection.