Transplant rejection

Transplant rejection occurs when the immune system of the recipient of a transplant attacks the transplanted organ or tissue. This is because a normal healthy human immune system can distinguish foreign tissues and attempts to destroy them, just as it attempts to destroy infective organisms such as bacteria and viruses.

Types of rejection

Hyperacute rejection

Hyperacute rejection is a complement-mediated response in recipients with pre-existing antibodies to the donor (for example, ABO blood type antibodies). Hyperacute rejection occurs within minutes and the transplant must be immediately removed to prevent a severe systemic inflammatory response. Rapid coagulation of the blood occurs. This is a particular risk in kidney transplants, and so a prospective cytotoxic crossmatch is performed prior to kidney transplantation to ensure that antibodies to the donor are not present. For other organs, hyperacute rejection is prevented by transplanting only ABO-compatible grafts. Hyperacute rejection is the likely outcome of xenotransplanted organs.

Acute rejection

Main article: Histocompatibility

Acute rejection is generally acknowledged to be mediated by T cell responses to proteins from the donor organ which differ from those found in the recipient. Unlike antibody-mediated hyperacute rejection, development of T cell responses first occurs several days after a transplant if the patient is not taking immunosuppressant drugs. Since the development of powerful immunosuppressive drugs such as cyclosporin, tacrolimus and rapamycin, the incidence of acute rejection has been greatly decreased, however, organ transplant recipients can develop acute rejection episodes months to years after transplantation. Acute rejection episodes can destroy the transplant if it is not recognized and treated appropriately. Episodes occur in around 60-75% of first kidney transplants, and 50 to 60% of liver transplants. A single episode is not a cause for concern if recognised and treated promptly and rarely leads to organ failure, but recurrent episodes are associated with chronic rejection of grafts. The bulk of the immune system response is to the Major Histocompatibility Complex (MHC) proteins. MHC proteins are involved in the presentation of foreign antigens to T cells, and receptors on the surface of the T cell (TCR) are uniquely suited to recognition of proteins of this type. MHC are highly variable between individuals, and therefore the T cells from the host recognize the foreign MHC with a very high frequency leading to powerful immune responses that cause rejection of transplanted tissue. Identical twins and cloned tissue are MHC matched, and are therefore not subject to T cell mediated rejection. The first successful organ transplant was performed between identical twins by Dr. Joseph Murray at the Peter Bent Brigham Hospital in Boston. This transplant was successful because no T cell mediated responses were generated to the transplanted organ. Dr. Murray later received a Nobel prize for his work.

Chronic rejection

Chronic rejection was a term used to describe all long term loss of function in organ transplants associated with fibrosis of the internal blood vessels of the transplant, but this is now termed chronic allograft vasculopathy and the term chronic rejection is reserved for those cases where the process is shown to be due to a chronic alloreactive immune response. It can be caused by a member of the Minor Histocompatibility Complex such as the H-Y gene of the male Y chromosome. This usually leads to need for a new organ after a decade or so.

Rejection Mechanisms

Rejection is an adaptive immune response and is mediated through both T cell mediated and humoral immune (antibodies) mechanisms. The number of mismatched alleles determines the speed and magnitude of the rejection response. Different grafts usually have a proclivity to a certain mechanism of rejection.

Organ/tissue	Mechanism
Blood	Antibodies (isohaemagglutinins)
Kidney	Antibodies, CMI
Heart	Antibodies, CMI
Skin	CMI
Bonemarrow	CMI
Cornea	Usually accepted unless vascularised, CMI

Prevention of rejection

Rejection is prevented with a combination of drugs including:

• Calcineurin inhibitors
  • Ciclosporin
  • Tacrolimus
• mTOR inhibitors
  • Sirolimus
  • Everolimus
• Anti-proliferatives
  • Azathioprine
  • Mycophenolic acid
• Corticosteroids
  • Prednisolone
  • Hydrocortisone
• Antibodies
  • Monoclonal anti-IL-2Rα receptor antibodies
    • Basiliximab
    • Daclizumab
  • Polyclonal anti-T-cell antibodies
    • Anti-thymocyte globulin (ATG)
    • Anti-lymphocyte globulin (ALG)

Generally a triple therapy regimen of a calcineurin inhibitor, an anti-proliferative, and a corticosteroid is used, although local protocols vary. Antibody inductions can be added to this, especially for high-risk patients and in the United States. mTOR inhibitors can be used to provide calcineurin-inhibitor or steroid-free regimes in selected patients.

A bone marrow transplant allows the chimeric body's immune system to adapt and accept a new organ. This requires that the bone marrow, which produces the immune cells, be from the same person as the organ donation (or an identical twin or a clone). Bone marrow is not attacked by the body's immune system, and is the only known type of transplant that has this quality. However, there is a risk of graft versus host disease (GVHD) in which the immune cells arising from the bone marrow transplant recognise the host tissues as foreign and attack and destroy them accordingly.

An FDA approved immune function test from Cylex has shown effectiveness in minimizing the risk of infection and rejection in post-transplant patients by enabling doctors to tailor immunosuppressant drug regimens. By keeping a patient's immune function within a certain window, doctors could adjust drug levels to prevent organ rejection while avoiding infection. Such information could help physicians reduce the use of immunosuppressive drugs, lowering drug therapy expenses while reducing the morbidity associated with liver biopsies, improve the daily life of transplant patients, and could prolong the life of the transplanted organ.

Treatment of rejection

Acute rejection is normally treated initially with a short course of high-dose methylprednisolone, which is usually sufficient to treat successfully. If this is not enough, the course can be repeated or ATG can be given. Acute rejection refractory to these treatments may require plasma exchanges to remove antibodies to the transplant.

The monoclonal anti-T cell antibody OKT3 was formerly used in the prevention of rejection, and is occasionally used in treatment of severe acute rejection, but has fallen out of common use due to the severe cytokine release syndrome and late post-transplant lymphoproliferative disorder, which are both commonly associated with use of OKT3; in the United Kingdom it is available on a named-patient use basis only.

Acute rejection usually begins after the first week of transplantation, and most likely occurs to some degree in all transplants (except between identical twins). It is caused by mismatched HLA antigens that are present on all cells. HLA antigens are polymorphic therefore the chance of a perfect match is extremely rare. The reason that acute rejection occurs a week after transplantation is because the T-cells involved in rejection must differentiate and the antibodies in response to the allograft must be produced before rejection is initiated. These T-cells cause the graft cells to lyse or produce cytokines that recruit other inflammatory cells, eventually causing necrosis of allograft tissue. Endothelial cells in vascularized grafts such as kidneys are some of the earliest victims of acute rejection. Damage to the endothelial lining is an early predictor of irreversible acute graft failure. The risk of acute rejection is highest in the first 3 months after transplantation, and is lowered by immunosuppressive agents in maintenance therapy. The onset of acute rejection is combatted by episodic treatment.

Chronic rejection is irreversible and cannot be treated effectively. The only definitive treatment is re-transplantation, if necessary. This would typically be ten years after a transplant, and this may entail returning to a transplant queue.

External links

• The Immune Tolerance Network