Kidney transplantation is the replacement therapy of choice for end-stage renal disease (ESRD). Regardless of age (1,2), race (3), or gender (4), renal transplantation results in a significantly greater survival advantage over the various dialysis modalities. The history of renal transplantation has tightly coupled improved clinical outcomes to an expanding knowledge of the details of transplant immunology. These increases in knowledge and experience have resulted in incredibly low acute rejection rates, tremendously improved short-term graft survivals and ultimately longer 5-year graft survival rates (5). Ironically, some of the immunosuppressants used to prevent acute rejection may be a primary factor in the development of chronic allograft nephropathy. Chronic allograft nephropathy, in addition to death with a functioning graft (6), has become the most common cause of allograft loss (7). Other side effects of immunosuppressants contribute to increased morbidity and mortality. To reduce morbidity and mortality a chemotherapeutic approach to immune suppression has been adopted. By using multiple agents that attack at various points in the immune response, the efficacy of overall immunosuppression is increased while reducing the dose of each drug thereby minimizing the dose-dependent side effects of each individual agent. This chapter intends to introduce each currently employed immunosuppressant, the evidence that supports their efficacy, and their known untoward side effects.

The risk of acutely rejecting a renal allograft is greatest during the first 3 months following engraftment. With time, the host accommodates the graft such that late rejections are more often secondary to noncompliance than immunologic incompatibility. The development of protocols that maximize immunosuppression during this high immunologic risk period, a concept known as induction, is sound, as acute rejection is associated with long-term graft dysfunction and premature loss (8,9). Antibodies that target single (monoclonal) or multiple (polyclonal) antigenic targets have been developed that provide profound immunosuppression in the perioperative period.

Protocols that include induction are found in approximately 50% of U.S. transplant centers and significantly less than half of transplant centers worldwide, although the percentage of centers utilizing these agents is growing. Centers against their use cite the increased upfront cost of the drugs and often present data that question whether the lower rates of rejection seen with induction truly translate into increased graft survival. Those who advocate their use will show data that induction will on average extend a transplanted kidney’s life (10) and that with the increased rates of rejection and hospitalization, there is not a significant cost savings to avoiding induction.

Currently available polyclonal antibodies to T lymphocytes are derived from the immunologic response of the target species from exposure to human thymus tissue. The commonly utilized animal species are horse and rabbit. The drugs are commercially known as ATGAM (horse) and Thymoglobulin (rabbit). In truth, polyclonal antibodies target many cell types in addition to T cells and address an array of cell determinants beyond those that are commonly activated
in acute rejection. These agents are powerful immunosuppressants, which are cytotoxic to many target cells, especially the CD8 cytotoxic/suppressor variety leading to a reversal of the CD4/CD8 ratio for up to 1 year. In addition, they may deplete other peripheral white blood cells. Absolute CD3-positive white cell counts can be measured as a measure of their efficacy. Absolute CD3-positive cell counts below 50 render the possibility of acute rejection remote. Intermittent dosing of Thymoglobulin has been achieved with significant cost savings and adequate immunosuppression provided when CD3+ cell counts are measured daily and maintained below 20 cells/mm³, with redosing of the polyclonal antibody when lymphocyte counts rebound above this level (11). One can also follow peripheral white blood counts as a surrogate measure of the CD3 cell count.

Prior exposure to these agents (previous transplant or treated acute rejection) or to the host species (e.g., farm exposure to rabbits or horses) often leads to species-specific blocking antibody formation by the recipient. Repeat exposure to the antibody by administration of multiple doses over several days can also lead to blocking antibodies. These species-specific, antiheterologous protein antibodies can be measured if their presence is suspected. The presence of blocking antibodies in high titer should lead one to change to an agent derived from a different species. The presence of these antibodies renders the polyclonal agent inert. Higher doses of the polyclonal antibody would have to be given to exert an effect. Continued use after species-specific antibody formation puts the patient at risk for serum sickness, which typically manifests as fever, arthralgia, pruritus, and rash. Serum sickness can appear up to 1 month after utilizing these agents. Although usually self-limited, morbidity from this condition may last for 2 weeks without treatment. Treatment options include cessation of the polyclonal antibody, high dose steroids, and plasmapheresis.

The rabbit-derived antithymocyte globulin (Thymoglobulin) is typically dosed between 1 and 1.5 mg/kg daily, rounded off to the nearest 25 mg aliquot. Although peripheral infusions of polyclonal antibodies have been successfully completed (12), the fastest and safest way (avoiding phlebitis and catheter infiltration) to infuse these agents is via a central line over 3 to 6 hours. The horse-based antithymocyte globulin (ATGAM) is dosed between 10 to 20 mg/kg and infused slowly through a central intravenous access. Duration of dosing varies by protocol. Most programs will deliver between 7 and 14 days of the polyclonal agent when treating acute rejection episodes. Continued administration of these agents beyond 14 days increases the risk of lymphoproliferative disease to unacceptably high levels. Induction regimens are shorter and typically range between 4 and 7 days. In patients with delayed graft function, early introduction of calcineurin inhibitors (discussed later) might prolong recovery of the graft secondary to the vasoconstrictive side effect of these agents. Polyclonal antibody preparations have been shown to provide adequate rejection prophylaxis (13) in patients in whom calcineurin inhibitors are held and outcomes are ultimately improved versus no induction (14).

Polyclonal antibodies are to be used with some trepidation (Table 10.1). Most recipients of these agents experience a “first-dose” effect that may include fever, rigors, diaphoresis, and hypotension as cytokines are released when leukocytes are destroyed. Symptoms range from mild to life threatening and can be temporized by premedication with high-dose steroids, antihistamines, and antipyretics. Some advocate delivering a small test dose to discover hypersensitivity reactions before they occur and to avoid anaphylaxis. Leucopenia and thrombocytopenia are common side effects of both antibodies. When leucopenia (4,000 cells/mL³) or thrombocytopenia (<100,000 plts/mL³) becomes severe, the dose of antibody is usually reduced or held until the white cell or platelet count returns to safer levels. Prolonged use certainly increases the recipient’s risk for developing hematologic malignancies (posttransplant lymphoproliferative disease, leukemias) as well as life-threatening opportunistic infections (cytomegalovirus [CMV], Pneumocystis carinii pneumonia) (15). Shorter induction regimens, typically 4 to 6 days, have resulted in lower rates of the aforementioned complications (16). In addition, greater utilization of more efficacious prophylactic pharmaceuticals has greatly reduced the rates of opportunistic infections.

Several studies comparing Thymoglobulin (rabbit) versus ATGAM (horse) have been conducted attempting to discern if one agent has a clear advantage over the other. The consensus of the available data suggests that Thymoglobulin
provides better protection during induction (17) and may be more successful in treating acute rejections (18).

The commercially available monoclonal antibody product that is suitable both as an induction agent as well as an agent against acute rejection, Orthoclone (OKT3), is derived from hybridoma technology using a murine (mouse) vector. This manufacturing technique generates a pure product of immunoglobulin targeting a single epitope. The binding of this antibody to the CD3 complex results in endocytosis of the T-cell receptor with eventual removal by the reticuloendothelial system. In addition, little crossreaction with platelets occurs such that thrombocytopenia is not experienced. Blocking antibody formation first to the idiotype and then to mouse residues itself commonly forms, especially after repeated exposure. Therefore, it is prudent to save this powerful immunosuppressant for serious rejection episodes and avoid using it for induction. It is also wise to monitor the efficacy of the antibody by monitoring CD3 counts. Unlike the polyclonal agents, OKT3 does not affect the large number of leukocytes; therefore, one should not monitor peripheral white cell counts as a surrogate of efficacy. Moreover, since the agent is not cytotoxic to T cells, CD3 counts must be interpreted with caution.

Cytokine storm after the first few days is almost universal with OKT3 use, which can be profound and, rarely, even fatal (Table 10.1). Fever, rigors, hypotension, electrolyte shifts, pulmonary edema, meningitis, diarrhea, and even death have been reported. It is recommended that the recipient of OKT3 be within 5% of their dry weight to reduce the severity of cytokine-induced pulmonary edema, a complication which can be encountered even in patients at their dry weights. There are four humanized versions of OKT3 currently under development. The manufacturers of these molecules hope to deliver equivalent immunosuppression without the first-dose effects experienced with murine OKT3.

Similar to polyclonal antibodies, OKT3 is typically administered for 7 to 14 days when the goal is to reverse acute rejection episodes and 4 to 7 days for induction. A standard, non-weight-based dose of 5 mg is typically delivered through a peripheral line as an intravenous push. The ability to safely and efficaciously administer OKT3 through a peripheral line is a select advantage of OKT3 when compared to polyclonal antibodies. Two monoclonal antibodies have been developed to target cell determinant (CD) 25, the alpha chain of the interleukin-2 (IL-2) receptor on activated T lymphocytes, thereby affecting only those lymphocytes that would be involved in response to alloantigen. IL-2 binding to its receptor initiates a cascade in which signal proteins such as the mammalian target of rapamycin (see mTOR inhibitors below) assist in the translation of new proteins that allow the cell cycle to progress from the G1 (growth) to the S (synthesis) phase resulting in lymphocyte proliferation. Basiliximab (Simulect) and daclizumab (Zenapax) are the two commercially available anti-CD 25 agents. Both derived from mouse, they have been genetically altered to be either chimeric (basiliximab) or humanized (daclizumab) and therefore are either 70% or 90% human, respectively. As a result of adding human components, hypersensitivity reactions (19) are rare, and a first-dose effect is not experienced. Infection rates, including CMV, do not appear to be elevated (20). Clinically relevant blocking-antibody formation and serum sickness have not been described. The efficacy of these agents in reducing acute rejections when used as an induction agent is well established (21, 22, 23, 24). These molecules must be used in concert with maintenance immunosuppressive therapy in order to provide adequate prophylaxis against acute rejection episodes. In patients with delayed graft function, where calcineurin inhibitors may be held until allograft recovery occurs, IL-2R antagonists have not been shown to effectively forestall acute rejection alone and should be used in a three-drug regimen (with sirolimus) (25) or avoided (26). In addition, they are ineffective in attempting to reverse an established acute rejection; therefore, they should only be used as induction agents. There is a suggestion that IL-2R agents appear inferior to polyclonal antibody induction when the recipients are of high immunologic risk (e.g., sensitized) (27).

Maintenance immunosuppression is the science (or art) of delivering adequate immunosuppression to prevent acute rejection while progressively reducing the serum levels or delivered dose of a given immunosuppressant such that the risks of infection, malignancy, and chronic allograft nephropathy are minimized. Rapid and excessive tapering of immunosuppression increases the risk of acute rejection. Late acute rejections, those that occur during the maintenance phase, are detrimental and carry a worse prognosis than early acute rejections (28). Chronic underdosing of immunosuppression not only increases the risk of acute rejection but chronic allograft nephropathy (CAN), as antidonor antibodies are more likely to form (29). Many have advocated the use of protocol biopsies to monitor for subclinical rejection that will eventually lead to chronic rejection of the allograft if the level of immunosuppression is not increased (30). Whether or not a renal transplant recipient receives induction immunosuppression during the perioperative period, the great majority of patients will require some degree of lifelong immunosuppression to prevent acute rejection. Even recipients of a two haplotype-matched kidney are at risk for acute rejection without maintenance immunosuppression, since there are unmeasured minor histocompatibility antigens that the recipient has likely mismatched to the donor. There are anecdotal reports of transplant recipients who appear to have attained functional tolerance, an ideal immunologic state where the graft is accepted as self by the host and therefore there is no longer a need for immunosuppression. These patients typically have a history of complete cessation of their immunosuppression secondary to noncompliance and continue to possess a working transplant. Most patients who stop their maintenance regimen experience
acute or more commonly CAN. Even those who seem to have attained tolerance remain at jeopardy for acute rejection as the recipient’s exposure to immunostimulating viruses can upregulate their major histocompatibility antigens with resultant acute rejection.