Immunosuppression - Professor Richard Kirk 2023

Professor Richard Kirk
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Immunosuppression is required for life to prevent rejection. Combinations of drugs are used to target more than one pathway. Many of the complications of transplantation are due to immunosuppression. If it is too little, rejection occurs, if too much, infections and medication side-effects are more pronounced e.g. renal impairment. Unfortunately, we do not have a reliable test to assess the level of activation of the immune system, therefore, adjusting medications relies on "walking the tightrope" between too much and too little, based on the patient's condition.

The strategy to prevent early graft rejection usually includes intraoperative steroids followed either by T cell depletion (thymoglobulin) or preventing Signal 2 of the T cell activation pathway with Basiliximab. Long term the mainstay is a combination of calcineurin inhibition (preventing intracellular cytokine signaling from reaching the nucleus) and cell cycle inhibition - either directly using azathioprine or mycophenolate mofetil (MMF) or through mTOR inhibitors.

  • Steroids principally act by inhibiting gene expression of IL2. They are also powerful anti-inflammatory agents suppressing macrophage function, inhibit lymphocyte migration through vessel walls and induce lymphocyte apoptosis. They also reduce the expression of HLA proteins.
  • Calcineurin inhibitors (CNI): Ciclosporin and Tacrolimus are calcineurin inhibitors. After T cells receptors are activated a signal pathway is enhanced to produce interleukins (amongst other molecules). This pathway requires calcineurin (a calcium calmodulin dependent phosphatase).  Inactivation of this pathway reduces IL2 production and subsequent lymphocyte activation.
  • Cell cycle inhibitors: Azathioprine is a purine analogue whilst MMF blocks purine synthesis by enzyme inhibition. Both impair DNA synthesis and they thus act as antiproliferation agents.
  • Target of Rapamycin Inhibitors (mTOR) are also known as Protein Kinase Inhibitors  (PKIs): Sirolimus and  Everolimus are in clinical usage. TOR is a cytoplasmic enzyme that is involved in the signaling pathway from T cell surface to nucleus required for cell proliferation.
  • Polyclonal antibodies (thymoglobulin) target T cells resulting in apoptosis and therefore lymphocyte depletion.
  • Monoclonal antibodies are more specific as they target specific CD[NBH1]  proteins on the  lymphocytes and hence reduce only certain populations of lymphocytes.  For example, Basiliximab targets CD25 proteins found on T cells, Rituximab targets CD20 proteins found on B cells and Alemtuzumab targets CD52 found on T, B and APCs.
Drug interactions are common so whenever a new medication is proposed it is advisable to check its interaction with immunosuppressants
Figure 1. Adapted from Philip Halloran. N Engl J Med 2004;351:2715-29
Figure 1 includes drugs that are potential targets for transplant immunosuppression. Belatacept is a selective T-cell co-stimulation blocker. It comprises a recombinant extracellular domain of human cytotoxic T lymphocyte  antigen-4 (CTLA-4) and a fragment of a modified Fc portion of human  IgG1. The drug binds to CD 80/86 ligands of antigen-stimulating cells and thereby inhibits the CD-28-mediated T-cell costimulation. Toralizumab, Tofacitinib, FK778 and Fingolimod are not in common usage.
Authors: Richard Kirk, Nathanya Baez Hernandez
Updated: 20 June 2023
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