Immunosuppression 

Immunosuppression after solid organ transplantation is complex. Over the past 50 years, the medical community has witnessed great advances in the care of patients receiving organ transplants. Improved therapeutic strategies have been associated with better patient and graft survival rates; however, the adverse effects associated with these agents and the risks of long-term immunosuppression present a number of challenges for the clinician. With all the successes of immunosuppressive therapies come the obligations to tailor treatments to meet the individual patient's characteristics and to balance the risks and benefits of these medications.

The image below depicts the points of action of immunosuppressive drugs.

The ability to prolong life by transplanting organs had long been a dream of medical practitioners. Early efforts at transplantation were unsuccessful because of inadequacies in surgical technique and lack of fundamental knowledge of the immune system.

Surgical techniques progressed with the unsuccessful attempt at transplanting a cadaveric kidney in 1933. Initial attempts at immunosuppression were with total body radiation, but all the patients died. Steroids alone were then used, also without success. With the development of 6-mercaptopurine (Purinethol), followed by azathioprine (Imuran) in the early 1960s, pharmacological immunosuppression became the standard of care. After the first initially successful series of transplantations performed between 1962 and 1964 in Denver, Colorado, the combination of azathioprine and steroids came into widespread use and became part of the primary immunosuppressive regimen for the next 20 years.

As knowledge of the immune system evolved, therapy targeted to specific immunoregulatory sites became possible. The first polyclonal antilymphocyte globulin was used in 1967 and spawned the development of other polyclonal and monoclonal antibodies. Introduced in the 1980s, cyclosporine (Sandimmune and, later, Neoral), a calcineurin inhibitor, was used in combination with azathioprine and steroids and was credited with a dramatic improvement in graft survival.

The next advance came in 1994 with the introduction of mycophenolate mofetil (MMF [CellCept]). After tacrolimus (another calcineurin inhibitor) became available in 1994, debate followed regarding which calcineurin inhibitor was superior. Tacrolimus has gradually supplanted cyclosporine in many centers. In contrast, mycophenolate mofetil rapidly replaced azathioprine almost universally. To expand the armamentarium further, sirolimus (Rapamune), a macrolide antibiotic, was recently developed and released. Its exact role is still being defined.

While the short-term outcomes provided with these medications have continued to improve, the consequences of their administration have become the subject of intense scrutiny as comorbid conditions, drug toxicities, and adverse effects keenly affect both patient and graft survival.

Azathioprine

Azathioprine is a derivative of 6-mercaptopurine. It functions as an antimetabolite to decrease DNA and RNA synthesis and is used for maintenance immunosuppression.

Adverse effects include leukopenia, thrombocytopenia, GI difficulties, hepatitis, cholestasis, and alopecia. Monitor CBC counts and pancreatic and liver enzyme levels. Decrease the dose if azathioprine is administered with allopurinol. Myelosuppression can improve with drug discontinuation. Azathioprine is compatible with cyclosporine and tacrolimus. Watch CBC counts carefully when withdrawing steroids.

Corticosteroids

Corticosteroids prevent interleukin (IL)–1 and IL-6 production by macrophages and inhibit all stages of T-cell activation. This agent is used for induction, maintenance immunosuppression, and acute rejection.

Adverse effects include Cushing disease, bone disease (eg, osteoporosis, avascular necrosis), cataracts, glucose intolerance, infections, hyperlipidemia, and growth retardation.

Cyclosporine

Cyclosporine is a polypeptide of 11 amino acids of fungal origin and is active against helper T cells, preventing the production of IL-2 via calcineurin inhibition (binds to cyclophilin protein). This agent is used for induction and maintenance immunosuppression.

Adverse effects include nephrotoxicity with 3 stages: (1) immediate, secondary to renal ischemia; (2) 2-3 weeks after transplantation, secondary to renal vasoconstriction; and (3) chronic, secondary to interstitial nephritis. Other adverse effects include hyperkalemia, hypomagnesemia, nausea, vomiting, diarrhea, hypertrichosis, hirsutism, gingival hyperplasia, hyperlipidemia, glucose intolerance, infection, malignancy, and hyperuricemia. Hypertrichosis and hirsutism can be alleviated by switching from cyclosporine to tacrolimus, provided the patient is carefully monitored. Multiple drug interactions are possible, primarily with agents affecting the cytochrome P-450 system.

The adverse consequences (eg, hypertension, renal impairment) of long-term cyclosporine use for solid organ transplant rejection have prompted exploration of various treatment regimens. Gallagher et al studied long-term graft survival by comparing 3 immunosuppressive regimens in 489 patients with a median follow-up of 20.6 y.^[1] The regimens included azathioprine and prednisolone (AP), long-term cyclosporine alone (Cy), or cyclosporine initiation followed by withdrawal at 3 mo and replacement with azathioprine and prednisolone (WDL).

Mean graft survival (censoring deaths) was 14.8 y in the WDL group, 12.4 y (P = 0.01) in the AP group, and 12.5 y (P = 0.01) in the Cy group. Graft survival without censoring deaths was 9.5 y in the WDL group, 6.7 y (P = 0.04) in the AP group, and 8.5 y (P = 0.06) in the Cy group. Patient survival did not differ between the 3 groups. Renal function was superior in the AP group at 1, 10, and 15 years posttransplantation and in the WDL group at 1, 5, 10, 15, and 20 years compared with the Cy group.^[1]

Tacrolimus

Tacrolimus is a macrolide antibiotic and is active against helper T cells, preventing the production of IL-2 via calcineurin inhibition (binds to tacrolimus-binding protein instead of cyclophilin protein). This agent is used for maintenance immunosuppression and for rescue therapy in patients with refractory rejection under cyclosporine-based therapy.

Adverse effects include nephrotoxicity, neurotoxicity, glucose intolerance, and QT prolongation (rare). Tacrolimus causes fewer cosmetic effects than cyclosporine, but it can cause reversible alopecia. Multiple drug interactions are possible, primarily with agents affecting the cytochrome P-450 system.

One study evaluated the long-term safety and efficacy of tacrolimus in orthotopic liver transplantation (OLT). The study reviewed 1000 primary OLTs performed between August 1989 and December 1992 and maintained with tacrolimus-based immunosuppression therapy. After 17- to 20-year follow-up, the results found patient survival rates at 35.8% and graft survival rates at 32.6%, with significantly better survival among children. Graft loss was attributed to age-related complications, recurrence of primary disease, and malignancy; rarely was graft loss related to immunologic reasons. The data conclude that tacrolimus is a potent immunosuppressive agent in OLT.^[2]

Mycophenolate Acid

Mycophenolate acid (MCA) inhibits the enzyme inosine monophosphate dehydrogenase (required for guanosine synthesis) and impairs B- and T-cell proliferation, sparing other rapidly dividing cells (because of the presence of guanosine salvage pathways in other cells). This agent is used for maintenance immunosuppression and chronic rejection. Mycophenolate mofetil was the first available form of MCA. More recently, enteric-coated mycophenolate acid was introduced with fewer reports of gastrointestinal adverse effects. Clinical trials, including the myfortic Prospective Multicenter Study (myPROMS), have begun to demonstrate similar clinical outcomes (eg, rejection rates) between the 2 drugs.^[3]

Adverse effects include nausea, vomiting, diarrhea, leukopenia, anemia, and thrombocytopenia. Do not administer with azathioprine because of hematologic toxicity. Trough levels and area-under-the-curve values are increased with concurrent administration of tacrolimus.

Sirolimus

Sirolimus is a macrolide antibiotic that binds the FK-binding protein, but its mechanism of action is via the "target of Rapamune," or TOR. It inhibits G1- to S-phase cell division and, therefore, cell proliferation. This agent is used for maintenance immunosuppression and chronic rejection.

Adverse effects include hyperkalemia, hypomagnesemia, hyperlipidemia, hypertriglyceridemia, leukopenia, anemia, impaired wound healing, and joint pain. Sirolimus can be used concomitantly with tacrolimus, cyclosporine, or mycophenolate mofetil. Multiple drug interactions are possible, especially because of the extremely long half-life.

Joannidès et al found that a cyclosporine A–free immunosuppressant regimen based on sirolimus reduced aortic stiffness, plasma endothelin-1, and oxidative stress in renal recipients enrolled in the CONCEPT trial.^[4]

Biologic Agents

Polyclonal antibodies (eg, antithymocyte globulins)

These agents are derived by injecting animals with human lymphoid cells, then harvesting and purifying the resultant antibody. Polyclonal antibodies induce the complement lysis of lymphocytes and uptake of lymphocytes by the reticuloendothelial system and mask the lymphoid cell-surface receptors. Preparations include horse antithymocyte globulin (Atgam) and rabbit antithymocyte globulin (Thymoglobulin). These agents are used for induction and acute rejection.

Adverse effects include fever, chills, thrombocytopenia, leukopenia, hemolysis, respiratory distress, serum sickness, and anaphylaxis. Some adverse effects are ameliorated with steroids, acetaminophen, and diphenhydramine.

A high average dose of antithymocyte immunoglobulin has been associated with an increased risk of non-Hodgkin lymphoma.^[5]

Muromonab-CD3

Muromonab-CD3 (OKT3) is a murine monoclonal antibody of immunoglobulin 2A clones to the CD3 portion of the T-cell receptor. It blocks T-cell function and has limited reactions with other tissues or cells. This agent is used for induction and acute rejection (primary treatment or steroid-resistant).

Adverse effects include cytokine release syndrome (ie, fever, dyspnea, wheezes, headache, hypotension) and pulmonary edema. Avoiding administration in hypervolemic patients is especially important, although pulmonary edema can occur in euvolemic patients. Infuse muromonab-CD3 after premedication with steroids (first 2 doses only), acetaminophen, and diphenhydramine to avoid cytokine release syndrome. Monitor effects of therapy using a CD3 antigen assay.

Monoclonal Anti-CD25 antibody

Basiliximab (Simulect) and daclizumab (Zenapax) are humanized antimonoclonal antibodies that target the IL-2 receptor. Clinically, both agents are very similar, and both are used for induction.

These agents have a very low prevalence of adverse effects, although hypersensitivity reactions have been reported with basiliximab (Simulect), albeit rarely.

Monoclonal Anti-CD20 antibody

Rituximab (Rituxan) is a monoclonal antibody directed against the CD20 antigen on B cells. Its use is currently being studied in treatment of some forms of antibody-mediated rejection. Another area of possible utility is in the hypersensitized patient awaiting transplant. A single phase I trial on 9 dialysis patients obtained a significant reduction in anti-HLA antibodies in 2 of the patients and a loss of antibody specificity in 5 other patients. Further studies are necessary to confirm these results as well as the safety of the drug's use in this arena.^[6]

Phases

Immunosuppressive treatment of the transplantation patient begins with the induction phase, perioperatively and immediately after transplantation. Maintenance therapy then continues for the life of the allograft. Induction and maintenance strategies use different medicines at specific doses or at doses adjusted to achieve target therapeutic levels to give the transplantation patient the best hope for long-term graft survival.

Maintenance immunosuppression is the key to prevention of acute and chronic rejections throughout the life of the graft.

Primary Immunosuppressive Agents

Calcineurin inhibitors

These agents combine with binding proteins to inhibit calcineurin activity. This works to inhibit IL-2, which is a critical link in the proliferation of helper T cells. Calcineurin normally exerts phosphatase activity on the nuclear factor of activated T cells. This factor then migrates to the nucleus to start IL-2 transcription. Studies have shown that cyclosporine and tacrolimus were associated with similar rates of graft survival, but several studies showed lower rates of rejection episodes with tacrolimus.^{[7, 8]}

Recent United Network for Organ Sharing data revealed that more than 50% of patients with new transplants are started on tacrolimus (80% with liver and pancreas transplants, 65% with kidney transplants, < 50% with heart and lung transplants). Although little information compares the long-term graft survival in patients treated with cyclosporine versus patients treated with tacrolimus, recent data demonstrated a less rapid decline in glomerular filtration rate (GFR) in patients receiving tacrolimus compared with patients receiving cyclosporine (Neoral). Whether this translates into improved graft survival has yet to be proven.^[9]

Levels of both cyclosporine and tacrolimus must be carefully monitored. Trough levels seem to correlate well with drug exposure in patients receiving tacrolimus. Initially, levels can be kept in the range of 10-20 ng/mL, but, after 3 months, levels are kept lower (5-10 ng/mL) to reduce the risk of nephrotoxicity. Controversy continues about the best method of monitoring cyclosporine levels. Trough levels have been used for some time; however, current data suggest that levels 2 hours after a dose (C2 monitoring) may be more reflective of drug exposure, particularly with the microemulsion formulation (Neoral).

One investigation studied living donor transplants, comparing cyclosporine and mycophenolate mofetil to tacrolimus and mycophenolate mofetil. The 2-year graft survival rate favored the cyclosporine arm, but the actual graft survival rate difference was only 2.1%. The Collaborative Transplant Study reported a 3-year graft survival rate of more than 80%, including both cyclosporine-based and tacrolimus-based regimens.^[10]

Adjuvant agents

These agents are usually combined with a calcineurin inhibitor and include steroids, azathioprine, mycophenolate mofetil, and sirolimus. Currently, most protocols use a calcineurin inhibitor and steroids with or without mycophenolate mofetil. The use of adjuvant agents allows clinicians to achieve adequate immunosuppression while decreasing the dose and toxicity of individual agents.

Mycophenolate mofetil in kidney transplant recipients has assumed an important role in immunosuppression after several clinical trials have shown a markedly decreased prevalence of acute cellular rejection compared with azathioprine and a reduction in 1-year treatment failures. Ongoing long-term studies suggest mycophenolate mofetil also reduces the prevalence of chronic rejection.^{[7, 11]}

Sirolimus has shown great promise for its potential to combat acute cellular rejection and to provide rescue immunosuppression. Current work shows that sirolimus causes a significant decrease in acute rejection and improvement in patient and graft survival compared with azathioprine. The combination of sirolimus and mycophenolate mofetil is currently undergoing investigation.^[11] A single-center trial of cyclosporine, sirolimus, and prednisone in living, related renal transplant patients demonstrated a reduced rate of acute rejection compared with historical data. Rates of acute rejection were reported at 7.5%, with a 12% rate in African American patients.^[12]

The 2 types of induction strategies used to avoid early acute rejection are (1) antibody-based therapy and (2) aggressive early immunosuppression.

• Antibody-based therapy: This therapy uses monoclonal (eg, muromonab-CD3) or polyclonal antibodies or anti-CD25 antibodies (eg, basiliximab, daclizumab) and is administered in the early posttransplant period (up to 8 wk). Antibody-based therapy allows for avoidance or dose reduction of calcineurin inhibitors, possibly reducing the risk of nephrotoxicity. All agents are effective for preventing acute rejections, although the anti-CD25 antibodies may require concurrent administration of calcineurin inhibitors. The adverse effect profile of the polyclonal and monoclonal antibodies limits their use in some patients. Patients at highest risk of rejection may receive rabbit antithymocyte globulin (Thymoglobulin).
• Aggressive early immunosuppression: This therapy uses maintenance drugs at higher doses to achieve the strongest immunosuppressive effect soon after transplantation. Approximately 50% of patients do not receive antibody therapy at the time of transplantation. The highest doses of calcineurin inhibitors place patients at increased risk of nephrotoxicity and may not be the best strategy for patients at the highest risk for rejection.

Rejection

Acute

The 3 agents used to treat acute rejection are (1) steroids, (2) antithymocyte globulin, and (3) muromonab-CD3.

• Steroids: These agents are the mainstay of therapy for acute rejection episodes, preventing release of IL-1 by macrophages and blocking synthesis of IL-2 by helper T cells. Steroids also have anti-inflammatory properties. The typical dosage is 3-5 mg/kg/d for 3-5 days, which is then tapered to a maintenance dose. Steroids reverse 60-75% of rejection episodes.
• Antithymocyte globulin: This agent binds all circulating T and B lymphocytes, which are then lysed or phagocytosed by the reticuloendothelial system. Antithymocyte globulin has efficacy similar to that of muromonab-CD3. It is reserved for steroid-resistant acute rejection secondary to cost, toxicity, and the development of drug antibodies.
• Muromonab-CD3: This agent displaces the T3 molecule from antigen receptors, captures all mature T cells, and prevents alloantigen recognition. The reversal rate of first acute rejection episodes is 94%. Muromonab-CD3 is sometimes used as the first-line agent for severe vascular rejections. The development of human antimurine antibodies allows for the reappearance of CD3 T cells, which may decrease the efficacy of muromonab-CD3 and necessitate higher doses (increasing risk of infection). A second course of muromonab-CD3 may be given for recurrent rejection, although repeat treatment may be associated with complications from the development of antimouse antibodies. The success rate in recurrent episodes is approximately 40-50%.

Chronic

Unless inadequate immunosuppression is the cause of rejection, changes in immunosuppressive therapy are generally not effective in reversing chronic rejection. The addition of sirolimus to mycophenolate mofetil is currently being studied to determine efficacy.^[11] Long-term data on transplanted patients treated with sirolimus demonstrated a chronic rejection rate of 14%, which is much lower than rates traditionally reported in cyclosporine-based regimens.^[13]

Control of blood pressure, treatment of hyperlipidemia, and management of diabetes are the current mainstays of treatment for graft preservation.

Steroid versus steroid-free protocols

The known toxicity of long-term steroid exposure has prompted the development of steroid-free immunosuppressive regimens. Benefits of the withdrawal or avoidance of steroids include normal growth in children, improved lipid profiles, improved blood pressures, better glycemic control, and lower risk of bone disease.

The development of cyclosporine prompted attempts to develop steroid-free protocols. Initially, patients were doing well with cyclosporine monotherapy. Over time, 50% of these patients required steroids, usually for acute rejection. More recently, a follow-up study of 100 patients in Denmark who underwent transplantation on steroid-free protocols showed a 1-year graft survival rate of 97% and a 4-year rate of 82%. Strong randomized studies are undoubtedly needed to prove both efficacy and safety of these protocols.^[14]

Steroid withdrawal has been used as a strategy to avoid adverse steroid effects in transplantation patients. Recent data show the risk of rejection is higher in patients withdrawn from steroids on a cyclosporine-based protocol. After tacrolimus became available, protocols with this drug showed that withdrawal of steroids after 6 months was successful 80% of the time. More recently, studies involving rapid steroid withdrawal (over 1-2 wk) in patients taking tacrolimus show similar graft survival rates compared with patients withdrawn after 3-6 months.

In African American patients, the risk of acute rejection is high; therefore, steroid-tapering regimens are prohibited. The use of sirolimus with tacrolimus followed by a steroid taper at 3 months has resulted in acceptable rejection rates in African Americans in one early study.^[15] The role of sirolimus and mycophenolate mofetil in steroid-free protocols has yet to be definitively determined, although the future looks promising for greater use of steroid-free protocols.

Calcineurin inhibitor-free protocols

Because of the risk of both acute and chronic nephrotoxicity attributed to calcineurin inhibitors, the development of protocols free of these agents is desirable.^[16] The use of sirolimus, mycophenolate mofetil, and anti-CD25 antibodies has been studied to determine whether graft survival and acute rejection rates can be maintained at the present rates in the absence of a calcineurin inhibitor.

The withdrawal of cyclosporine has been investigated in several trials. While the long-term graft survival rates were similar in patients withdrawing from cyclosporine compared with those maintained on it, the incidence of acute rejection in the withdrawal group was higher. The addition of sirolimus has been used in these withdrawal protocols, with a suggestion of improved renal function at 2-year follow-up. Higher rates of acute rejection were again noted in the withdrawal group.

Many other protocols that minimize exposure to calcineurin inhibitors have been studied. Promising protocols include sirolimus, mycophenolate mofetil, and steroids or the combination of anti-CD25 antibodies, sirolimus, mycophenolate mofetil, and steroids. One study has shown that belatacept plus mycophenolate mofetil or belatacept plus sirolimus provide primary immunosuppression with acceptable rates of acute rejection, improved renal function compared to a TAC-based regimen, and may avoid the need for calcineurin inhibitors and corticosteroids.^[17] These protocols have shown acceptable graft survival rates and acute rejection rates, although the studies are small and further research is warranted. In short, multiple regimens have been shown to be effective.

Length of treatment

Although induction of tolerance may allow withdrawal of immunosuppression in the future, at this time, immunosuppressive medications appear to be necessary for the life of the transplanted organ. Episodes of acute cellular rejection have occurred after the cessation of medication even 20 years after transplantation. For patients with stable graft function, individual components of the treatment regimen may be gradually diminished or completely discontinued; however, in most patients, some degree of immunosuppression must be continued. Some patients with severe resistant infections or malignancy related to immunosuppressants require the discontinuation of these medicines.

Pregnancy

Current data suggest that protocols involving cyclosporine, azathioprine, and steroids are associated with low rates of birth defects, although patients are treated with high-risk pregnancy strategies. However, also note that children born to parents with previous transplants are often small for gestational age. Preliminary data also suggest the safety of tacrolimus. Mycophenolate mofetil animal data and some early human studies show adverse effects on fetal development. Presently, few data exist regarding sirolimus and pregnancy.

Infection and malignancy issues

An increasingly recognized problem associated with immunosuppression is BK virus nephropathy. This virus, a member of the human papovavirus family, lives in the human genitourinary tract and replicates in some patients who are immunosuppressed, leading to allograft dysfunction. While antiviral agents such as cidofovir and leflunomide are active against the BK virus, the mainstay of therapy is a reduction in immunosuppression. In one study of 178 pancreas-kidney transplant recipients, the incidence of BK virus nephropathy was found to be low (1.1%), and no evidence of pancreatic allograft dysfunction was evident. Concurrent renal allograft rejection was treated with pulse steroid therapy and a reduction in immunosuppression, and, in one patient, the use of leflunomide, meaningful, though not complete, recovery of renal function was realized.^[18] The risk of acute allograft rejection with this reduction in dose is being studied.^[19]

Complications associated with immunosuppression for kidney transplantation are discussed in the video below.

The results of one study found that a combination of monthly screening for polyoma BK virus nephropathy (PVN) using PCR and a modest decrease in immunotherapy is a safe and effective in preventing PVN and may significantly decrease cytomegalovirus and Epstein-Barr virus in renal transplant patients.^[20]

The results of another study found that monthly nucleic acid testing during the first 6 months post renal allograft and immediate reduction of immunosuppression is effective in preventing BK polyomavirus virus nephropathy (BKVN) in viremic patients.^[21]

Opportunistic infections remain an important risk to the immunocompromised patient despite the use of prophylactic measures. Exposure to viruses such as Epstein-Barr virus (EBV), cytomegalovirus (CMV), herpes simplex virus, and human papillomavirus place the donor at risk for infection and, potentially, later malignancy.

The incidence of CMV has been reduced with the use of antiviral prophylaxis in the first 3 months posttransplant; preemptive monitoring and initiation of treatment in the case of significant viremia after discontinuation of prophylaxis remains to be proven as a strategy for reducing the risk of late-onset CMV disease.^[22] The cause of death of approximately 27% of patients who die with a functioning graft is related to infectious or malignant complications.^[23] This highlights the question of the appropriate amount of immunosuppression required to balance the aspects of graft function with complications related to therapy.

Posttransplant lymphoproliferative disease (PTLD) is a growing concern in the transplant population. Most of these are of B-cell origin and linked to EBV infections. Patients present with constitutional symptoms such as night sweats, fevers, and weight loss. An acute rise in creatinine levels, similar to acute allograft rejection, may also be seen. Risk factors for PTLD include primary EBV infection;^[24] the use of cyclosporine, tacrolimus, and MMF; and an exposure to antithymocyte globulin (ATG) or OKT3. Treatment options include reduction or discontinuation of immunosuppression with an increase in prednisone to reduce rejection risk.^[24] Further studies involving the efficacy of sirolimus and rituximab are needed to determine their specific role.