Immunosupression in liver transplant.

Editor's Notes

• #6: 1.(APC) migrates from local tissue to lymphoid organs. 2.In the paracortex of the lymphoid organ, the APC meets a naive T-cell. If the naive T-cell has a T-cell receptor (TCR) that binds the antigen as it is presented by a MHC on the APC. 3.T-cell-APC interactions are likely to follow. This includes T-cell CD4 binding to the MHC on an APC, as well as costimulatory signals via extracellular receptors CD28 or CD40. 4. After this set of T-cell-APC interactions begins, a set of intracellular signals follow towards the nucleus of the T-cell. 5.The naive T-cell is then activated, and begins to replicate. This replication is called clonal expansion, and produces a population of T-cells that eventually migrate back to the tissue that contains antigen. AP-1: Activator protein 1; CD: Cluster of differentiation; CDK: Cyclin-dependentkinase; IKK: Inhibitor of kappa-B kinase; IL: Interleukin; Jak3: Janus-associated kinase 3; PI3K: Phosphatidyl inositide 3-kinase; MAPK: Mitogen-activated protein kinase; mTOR: Mechanistic target of rapamycin; NFAT: Nuclear factor of activated T-cells; NF-kB: Nuclear factor kappa-light-chain-enhancer of activated B cells; TCR: T-cell receptor; MHC: Major histocompatibility complex.
• #7: 1.Upon reperfusion of the liver allograft, passenger leukocytes migrate to recipient lymphoid tissues where they trigger a rapid and vigorous alloimmune response. Host T cells are activated by means of direct antigen presentation pathway and proliferate in the draining lymph tissues, leading to marked increase in Interleukin-2 (IL-2) and Interferon-γ (IFN-γ) expression within the first day after transplantation. 2.This paradoxical early immune activation is more vigorous and greater in magnitude than the immune responses observed during rejection. 3.Host T cells activated in this manner are unable to initiate rejection and instead undergo apoptosis within the recipient lymphoid tissues. 4.The remaining activated T lymphocytes travel back to the liver allograft, where they also undergo programmed cell death  5.The end result of this process is the clonal deletion of donor-reactive T lymphocytes, and the induction of hyporesponsiveness towards donor-specific antigens . 6.Donor microchimerism, or the persistence of donor cells and nucleic acid in the blood and tissues of the recipient, has been postulated to promote long-term graft survival.  HIGH DOSE ANTIGEN EFFECT 1.The liver is approximately 10 times larger than the heart or the kidney, and its large tissue mass has been postulated to dilute cytokines and alloreactive T lymphocytes leading to exhaustion of the host immune response.  SOLUBLE AND CELL BOUND MHC CLASS 1 MOLECULES 2.The liver allograft releases large quantities of soluble MHC class I molecules, which persist in the recipient circulation at high concentrations as long as the graft is functional  Soluble MHC class I molecules can interact directly with the T-cell receptor on alloreactive CD8+ T lymphocytes. MHC induces T cell apoptosis rather than activation Soluble MHC class I molecules may neutralize lymphocytotoxic alloantibodies by direct binding.
• #9: 1. Liver allograft : hepatocytes, nonparenchymal cells such as Kupffer Cells (KC), Liver Sinusoidal Endothelial Cells (LSEC), resident DCs, and stellate cells. Organized into an unique structure within the sinusoids which enables direct interaction between the hepatic cells and circulating lymphocytes . 2.The fenestrated endothelium of liver sinusoids, along with the low velocity of blood flow, allows for antigen presentation by hepatocytes and other nonparenchymal liver cells to T lymphocytes that pass through the liver 3.Hepatocytes constitutively express MHC I molecules at high levels, while MHC II expression can be induced following inflammation. 4.Hepatocytes act as an efficient (APC) for naïve CD4+ and CD8+ T lymphocytes, but such interactions appear to be tolerogenic as they result in the loss of cytolytic function and premature T cell death  5.KCs are hepatic macrophages resided within the sinusoidal lumen KCs express MHC class II and co-stimulatory molecules, and can function as APCs for allospecific T cells. Shortly after liver allograft reperfusion, KCs secrete massive amounts of IL-10, a dominant cytokine within the liver which exerts multiple immunomodulatory effects . 6.KCs also release nitric oxide and prostaglandins which may suppress T cell activation 7.KCs can initiate apoptosis of alloreactive T effector cells via the Fas/ Fas ligand (FasL) pathway. 8.LSECs plays a critical role in hepatic immune surveillance by clearing antigens in the blood, often in the form of immune complexes . LSECs express MHC class II molecules and possess the ability to present antigens to CD4+ and CD8+ T lymphocytes. 10.Resident DCs in the liver inhibits CD8+ T cell effector function and facilitates Th1 cell apoptosis while promoting Th2 generation and Treg development in an IL-10-dependent manner.
• #15: Activation signal 1 results from binding of T-cell receptors (TCRs) with processed peptide antigens presented by professional antigen-presenting cells (APCs) in the antigen- binding grooves of MHC class II (CD4-restricted) and I (CD8-restricted) molecules. Costimulatory signal 2 is transduced by the binding of several different receptors on T cells with costimulatory ligands expressed by APCs. T-cell activation requires co-stimulation, a process in which a number of ligands on the APC bind to a variety of T-cell receptors, including CD28, CD154, CD2, CD11a, and CD54. The T-cell receptor complex is internalized and binds to immunophilin. Immunophilin stimulates calcineurin, which activates (NFAT) The activated NFAT then translocates to the nucleus where it drives interleukin (IL)-2 transcription. STEP 3 Signal III is required for T-cell clonal proliferation and maturation and is mediated by receptors for mitogenic cytokines and growth factors Newly synthesized IL-2 is secreted by T cells and binds to IL-2 receptors (IL-2R) on the cell surface in an autocrine fashion, stimulating a burst of cell proliferation.  Inflammation — T-cell proliferation is associated with cell-mediated cytotoxicity and secretion of cytokines, chemokines, and adhesion molecules. The secreted mediators recruit additional inflammatory cells to the graft. The result is an inflammatory milieu with additional toxic and vasoactive mediators. 
• #17: Corticosteroids Corticosteroids have immunosuppressive and antiinflammatory properties at the level of T-cell activation by APCs . By reducing CD4 T-cell secretion of IL-2, they inhibit T-cell activation and clonal proliferation. They also inhibit secretion of the proinflammatory cytokines, IL-1, IL-6, and TNF- by APCs, reducing the efficiency of antigen presentation and the proinflammatory milieu (Fig. 1). Ursodeoxycholic Acid Among its multiple mechanisms of action, UDCA is weakly immunosuppressive, immuno- modulatory, and antiapoptotic Calcineurin Inhibitors Both cyclosporine and tacrolimus inhibit signal 1 of T-cell activation. This signal 1 is mediated by Ca2+-dependent activation of calcineurin, which dephosphorylates transcription factors, such as nuclear factor of activated T cells (NFAT), required for transcription of essential mitogenic cytokines and growth factors, including IL-2, IL-3, IL-4, TNF-, IFN-, and granulocyte-macrophage colony-stimulating factor (GM-CSF) (42). As a result, both block cell cycle progression from phase G0 to G1 (Fig. 4). Intravenous Immunoglobulin Intravenous immunoglobulin (IVIG) contains IgG antibodies from pooled human serum with a multitude of antigen specificities, including natural autoantibodies and antiidiotypes. IVIG has concurrent immunosuppressive, immunomodulatory, and antiinflammatory effects on dendritic cells, B cells, T cells, antibodies, cell proliferation, apoptosis, production of TGF- and IL-4, fibrogenesis, and transendothelial migration of leukocytes into tissues. Antiidiotypic antibodies in IVIG inhibit antigen-specific TCRs, preventing T-cell activation. IVIG also inhibits effector mechanisms (Fig. 1) mediated by autoantibodies, immune complexes, C activation, and proinflammatory cytokines. Adverse events are usually well tolerated and include headache, fever, chills, anemia, back pain, temporary hypotension, nausea, perspiration, and venous thromboses (59). IVIG has not been used to treat acute liver failure, alcoholic hepatitis, or auto- immune or alloimmune liver diseases; however, its mechanisms of action provide the rationale for pilot feasibility studies. HUMANIZED MONOCLONAL ANTIBODIES Several monoclonal antibodies (MAbs) have been developed to inhibit signal 1 of T-cell activation. Further clinical studies are needed to determine their safety and efficacy. HuMax-CD4 Two humanized anti-CD4 MAbs (HuMax- CD4 and TRX1) have reduced immunogenicity, and alteration of their Fc domains prevents depletion of CD4 T cells. By disrupting activation, HuMax-CD4 produced dose-dependent decreases in memory CD4 T cells in psoriasis vulgaris (60), while TRX1 prevented humoral responses in baboons, and repeated doses produced tolerance (61). These results provided proof of principle that inhibition of memory CD4 T cells can ameliorate chronic disease. Efalizumab Signal 1 of T-cell activation can also be prevented by disrupting adhesion between ICAM-1 (CD54) on APCs and lymphocyte function antigen-1 (LFA-1; heterodimer of CD11a and [L]-CD18 [2] integrin) expressed on T cells. Efalizumab, an injectable humanized MAb against the CD11a monomer of T-cell LFA-1 (62), was effective in prolonged therapy of psoriasis (63), oral lichen planus (64), dermato- myositis (65), and cutaneous systemic lupus erythematosus (SLE) (66). Mild adverse events occurring in 1 to 2% included headache, chills, fever, nausea, and myalgia after the first two injections and arthralgia, asthenia, and edema later (67). The relative contributions of inhibition of signal 1 in memory T cells and transendothelial T-cell migration into tissues are unclear, but it is likely that both mechanisms are involved. INHIBITION OF T-CELL COSTIMULATION Signal 2 provides the costimulatory signaling required for functional activation and clonal expansion of antigen-specific T cells. Multiple receptors transduce costimulatory signals (Fig. 2), including CD28, CD154 (also termed CD40-ligand [CD40L]), CD11a (portion of the LFA-1 heterodimer), CD2, CD137, and CD152. Therapeutic agents have been developed to inhibit each of these costimulatory receptors. Although such inhibition is most appealing for prevention of alloimmune acti- vation mediating allograft rejection or GVHD, the repetitive process of T-cell activation required for clonal proliferation of memory T cells mediating chronic diseases suggests that inhibition of signal 2 might be beneficial in the treatment of established hepatic diseases. For example, in autoimmune and IMIDs, it could reduce the activation and maturation of antigen-specific memory T cells responsible for immuno- pathology. Inhibition of signal 2 at the time of organ or hematopoieitic stem cell transplantation could theoretically induce polyclonal T-cell anergic tolerance. FUSION PROTEIN INHIBITORS CTLA4-Ig Inhibitors Both CD28 and cytotoxic T-lympho- cyte antigen-4 (CTLA-4, also termed CD152) are T cell receptor for the B7.1 (CD80) and B7.2 (CD86) ligands expressed on APCs. In contrast to the activating costimulatory signal trans- duced by CD28, CTLA-4 signaling is immunosuppressive (70). A CD28-Ig fusion protein has been developed to block CD80/CD86, but it has not been used clinically (71). INHIBITION OF CLONAL T-CELL PROLIFERATION AND DIFFERENTIATION Signal 3 of T-cell activation results in clonal expansion and maturation of T cells. The mitogenic cytokine IL-2 stimulates clonal proliferation by binding to IL-2 receptors (IL-2R [CD25]). In addition, growth factors induce receptor-mediated signaling required for differentiation of T-cell effector functions. Two approaches have been used to inhibit signal 3: (1) blocking of IL-2Rs with MAbs during the initiation of T-cell activation and (2) prevention of signaling by inhibiting the mammalian target of rapamycin (mTOR). Sirolimus Sirolimus (also termed rapamycin) forms complexes with cytosolic FKBP12 that inhibit mTOR signal- ing (90). This inhibition results in reduced translation of proteins and disruption of cell cycle transition, which pre- vent clonal T-cell proliferation. Everolimus (also termed RAD) is a derivative of sirolimus with lesser affinity for FKBP12 (91). Both sirolimus and everolimus prevent G1 to S cell cycle transition in T cells by inhibiting Ca2+-independent activation of T cells by IL-2, IL-4, and IL-6 (Figs. 2 and 4) and costimulatory effects of B7. Monoclonal Anti-IL-2R (CD25) Antibodies Daclizumab and basiliximab are therapeutic MAbs that bind to the Tac/ CD25 -subunit of the IL-2 receptor (IL-2R, [CD25]). Both are approved for use as induction therapy to prevent renal allograft rejection (100,101). The combination of blockade of IL-2R-mediated mitogenesis and inhibition of IL-2 synthesis with cyclosporine or tacrolimus effectively inhibits both signals 2 and 3 of T-cell activation. PURINE SYNTHESIS INHIBITION Mycophenolate mofetil Mycophenolate mofetil is a prodrug of mycophenolic acid, which is a non-competitive, reversible inhibitor of the rate-limiting enzyme for de novo purine synthesis, inosine monophosphate dehydrogenase (Fig. 2) (118). Mycophenolic acid preferentially inhibits T-and B-cell proliferation because they require de novo purine synthesis to compensate for deficient purine salvage pathways. Mycophenolic acid has multiple immunosuppressive effects, which make it difficult to assess the mechanisms involved in clinical efficacy. Its immunosuppressive effects include: (1) decreased expression of IL-2R, HLA-DR, transferrin receptors, and chemokines involved in signals 1 and 3 and leukocyte migration; (2) decreased glycosylation of adhesion molecules required for signal 1 and cytotoxic effector functions; and (3) decreased T-cell-independent B-cell secretion of Ig. Mycophenolic acid, however, does not reduce secretion of proinflammatory IL-1, IL-6, or TNF- by APCs, or T cell secretion of IL-2, IL-4, and IL-13, or neutrophil superoxide or chemotaxis. PYRIMIDINE SYNTHESIS INHIBITION Development of more lymphocyte-specific antiproliferative agents is now focused on pyrimidine synthesis inhibition. Two promising drugs being studied are leflunomide and FK778, which not only inhibit pyrimidine synthesis but also exhibit additional immunosuppressive properties and antiviral activity against cytomegalovirus (CMV)
• #19: Corticosteroids have immunosuppressive and anti-inflammatory properties at the level of T-cell activation by APCs . By reducing CD4 T-cell secretion of IL-2, they inhibit T-cell activation and clonal proliferation. They also inhibit secretion of the proinflammatory cytokines, IL-1, IL-6, and TNF- by APCs, reducing the efficiency of antigen presentation and the proinflammatory milieu Glucocorticoids suppress antibody and complement binding, upregulate interleukin (IL)-10 expression, and downregulate IL-2, IL-6, and interferon-gamma synthesis by T cells Four steroid formulations are used commonly in transplantation: hydrocortisone, prednisone, prednisolone, and methylprednisolone.
• #28: Neurological toxicity may include altered mental status, polyneuropathy, dysarthria, myoclonus, seizures, hallucinations, and cortical blindness
• #36: Inhibiting mTOR, controls cell cycle transition from G1 to S phase limit cellular proliferation, proliferation and activation of lymphocytes.
• #41: The long term use of CNI leads to renal dysfunction. In that case Early conversion tp SRL results in a profound improvement in eGFR.
• #43: Everolimus facilitates early tac minimization.
• #44: Inosine monophosphate dehydrogenase
• #45: Most mammalian cells are able to maintain GMP levels through the purine salvage pathway. However, lymphocytes lack a key enzyme of the guanine salvage pathway (hypoxanthine-guanine phosphoribosyltransferase), and cannot overcome the MPA-induced block.
• #46: Undergoes enterohepatic recycling before it is ultimately excreted in the urine