Residency · Residency · Nephrology

Kidney Transplantation - Immunosuppression and Rejection

Introduction

Kidney transplantation is the optimal treatment for end-stage renal disease, offering superior patient survival, quality of life, and long-term cost-effectiveness compared to maintenance dialysis. Approximately 25,000 kidney transplants are performed annually in the United States, yet the demand far outstrips supply, with nearly 100,000 patients awaiting transplantation on the national waitlist. Living donor transplantation yields better outcomes than deceased donor transplantation, and preemptive living donor transplantation performed before the initiation of dialysis represents the best possible outcome scenario. One-year graft survival rates now approach 98 percent for living donor and 95 percent for deceased donor transplants. However, ten-year graft survival remains more modest at 65 to 70 percent for living donor and 50 to 55 percent for deceased donor grafts, reflecting the ongoing challenge of long-term graft attrition. Chronic antibody-mediated rejection and chronic allograft nephropathy have supplanted acute rejection as the principal threats to long-term graft survival, representing the frontier of current transplant nephrology research.

Immunology of Transplant Rejection

Allorecognition

The immune recognition of transplanted tissue proceeds through three distinct pathways of allorecognition. In the direct pathway, recipient T cells recognize intact donor HLA molecules displayed on the surface of donor antigen-presenting cells that accompany the transplanted organ. This pathway generates a robust early immune response and is the predominant mechanism driving acute cellular rejection. In the indirect pathway, recipient antigen-presenting cells internalize, process, and present donor HLA-derived peptide fragments in the context of self-MHC molecules to recipient CD4-positive T cells. This pathway is slower to develop but is more durable and is the predominant mechanism driving chronic rejection and the generation of donor-specific antibodies through T cell-dependent B cell activation. The semi-direct pathway is a more recently described mechanism in which recipient antigen-presenting cells acquire intact donor HLA molecules through direct cell-to-cell contact or via extracellular vesicles, a process termed cross-dressing. This pathway may contribute to both acute and chronic rejection, though its full clinical significance remains an area of active investigation.

HLA System

The human leukocyte antigen system, encoded by the major histocompatibility complex on chromosome 6, is the primary target of the alloimmune response. HLA class I molecules (HLA-A, HLA-B, and HLA-C) are expressed on the surface of all nucleated cells and present intracellular peptides to CD8-positive cytotoxic T lymphocytes. HLA class II molecules (HLA-DR, HLA-DQ, and HLA-DP) have a more restricted expression pattern, primarily on antigen-presenting cells, and present extracellular peptides to CD4-positive helper T lymphocytes. In clinical transplantation, matching at the HLA-A, HLA-B, and HLA-DR loci has the strongest influence on outcomes, and a zero-antigen mismatch at these loci (0-ABDR mismatch) confers the best deceased donor graft survival. HLA-DQ has received increasing attention in recent years as a major target for de novo donor-specific antibody development, with anti-DQ antibodies being the most common de novo DSA detected in transplant recipients.

Sensitization and Crossmatch

Sensitization refers to the presence of pre-formed anti-HLA antibodies in a potential transplant recipient, typically resulting from prior transplantation, pregnancy, or blood transfusion. The degree of sensitization is quantified by the calculated panel reactive antibody (cPRA), which estimates the percentage of the potential donor population against which the recipient has antibodies. A higher cPRA translates to a longer expected wait time for a compatible organ.

Crossmatch testing is performed before transplantation to detect the presence of recipient antibodies directed against the specific donor's HLA antigens. The complement-dependent cytotoxicity (CDC) crossmatch is the historical standard: donor lymphocytes are incubated with recipient serum and complement, and cell lysis indicates the presence of complement-fixing donor-specific antibodies. A positive CDC crossmatch represents an absolute contraindication to transplantation because of the near-certain risk of hyperacute rejection. The flow cytometry crossmatch offers greater sensitivity by detecting lower-level antibodies through fluorescence-based detection of antibody binding to donor cells; a positive T-cell flow crossmatch is associated with significantly higher rejection risk. The virtual crossmatch, an increasingly utilized approach, uses computer-based algorithms to predict compatibility by comparing the recipient's known antibody profile against the donor's HLA typing, facilitating organ allocation decisions before the organ arrives at the transplant center.

<image>Diagram of the immunologic mechanisms of kidney transplant rejection showing three pathways of allorecognition. Direct pathway: donor APC presenting intact donor MHC to recipient T cell via TCR, leading to acute cellular rejection. Indirect pathway: recipient APC processing and presenting donor HLA peptides in the context of self-MHC to recipient CD4+ T cell, leading to chronic rejection and alloantibody production. Semi-direct pathway: recipient APC with acquired donor MHC molecules. Show downstream effector mechanisms: CD8+ cytotoxic T cells (tubulitis, endothelialitis), CD4+ T cells activating B cells (antibody production, DSA), macrophages (interstitial inflammation), and complement activation (C4d deposition on endothelium). Include the three-signal model of T-cell activation: Signal 1 (TCR-MHC), Signal 2 (CD28-B7 costimulation), Signal 3 (IL-2 and cytokine-driven proliferation), with drug targets at each step.</image>

Immunosuppressive Agents

Induction Therapy (Perioperative)

Induction therapy provides intense immunosuppression during the perioperative period when the risk of acute rejection is highest. Basiliximab (Simulect) is a chimeric monoclonal antibody directed against CD25, the alpha subunit of the interleukin-2 receptor, expressed on activated T cells. By blocking IL-2-mediated signaling, basiliximab prevents T-cell proliferation without causing T-cell depletion. It is administered as 20 mg intravenously on day 0 and day 4 post-transplant, is well tolerated with minimal side effects, and is the standard induction agent for immunologically low-risk recipients.

Rabbit anti-thymocyte globulin (rATG, Thymoglobulin) is a polyclonal antibody preparation derived from rabbits immunized with human thymocytes. It contains antibodies against multiple T-cell surface molecules, causing profound T-cell depletion through complement-mediated lysis, antibody-dependent cellular cytotoxicity, and apoptosis. The standard dosing is 1.5 mg/kg/day administered intravenously over 3 to 7 days. rATG is the preferred induction agent for high immunologic risk recipients, including those who are sensitized, undergoing re-transplantation, or at high risk for delayed graft function. Side effects include cytokine release syndrome during the first infusion (mitigated by premedication with corticosteroids, acetaminophen, and diphenhydramine), leukopenia, thrombocytopenia, and increased risk of opportunistic infections, particularly CMV and BK polyomavirus.

Alemtuzumab (Campath) is a humanized monoclonal antibody against CD52, a glycoprotein expressed on the surface of virtually all mature lymphocytes, monocytes, and some granulocytes. A single 30 mg intravenous dose produces profound and prolonged lymphocyte depletion lasting months to years. It is used off-label in transplantation and has been particularly employed in steroid-avoidance and minimization protocols, though the depth and duration of immunodepletion raise concerns about increased infectious and malignant complications.

Maintenance Immunosuppression (Triple Therapy Standard)

Calcineurin Inhibitors (CNIs)

Tacrolimus (Prograf, Envarsus) is the first-line calcineurin inhibitor in modern transplantation. It binds to the intracellular immunophilin FKBP-12, and the resulting complex inhibits calcineurin, a phosphatase required for the dephosphorylation of the nuclear factor of activated T cells (NFAT). Inhibition of calcineurin blocks the transcription of interleukin-2 and other cytokines essential for T-cell activation and proliferation. Target trough levels are typically maintained at 8 to 12 ng/mL during the first 3 months post-transplant, reduced to 6 to 8 ng/mL from months 3 to 12, and further reduced to 4 to 6 ng/mL beyond the first year. The principal toxicities of tacrolimus include nephrotoxicity, manifesting acutely as afferent arteriolar vasoconstriction with reduced GFR and chronically as interstitial fibrosis and tubular atrophy; new-onset diabetes after transplant (NODAT), resulting from direct beta-cell toxicity and insulin resistance; neurotoxicity including tremor and the posterior reversible encephalopathy syndrome (PRES); hyperkalemia from impaired potassium secretion in the collecting duct; hypomagnesemia from renal magnesium wasting; and alopecia. Extended-release formulations, including Envarsus XR and Astagraf XL, allow once-daily dosing and may improve medication adherence, which is a critical determinant of long-term graft survival.

Cyclosporine (Neoral, Gengraf) binds to cyclophilin and similarly inhibits calcineurin, but it has been largely supplanted by tacrolimus. The SYMPHONY trial definitively established the superiority of tacrolimus-based regimens over cyclosporine for both graft survival and acute rejection rates. Cyclosporine-specific side effects include hirsutism, gingival hyperplasia, hypertension, and dyslipidemia.

Antimetabolites

Mycophenolate mofetil (CellCept) and its enteric-coated derivative mycophenolic acid (Myfortic) inhibit inosine monophosphate dehydrogenase (IMPDH), the rate-limiting enzyme in the de novo pathway of purine synthesis. Because lymphocytes lack the salvage pathway for purine synthesis that is available to most other cell types, mycophenolate selectively inhibits lymphocyte proliferation while having relatively limited effects on other rapidly dividing cells. Standard dosing is mycophenolate mofetil 1000 mg twice daily or mycophenolic acid 720 mg twice daily. Common side effects include diarrhea and gastrointestinal intolerance, leukopenia, and increased susceptibility to CMV and BK polyomavirus infections. Mycophenolate is absolutely teratogenic, causing characteristic facial clefts and ear and limb abnormalities, and must be switched to azathioprine at least three months before planned conception.

Azathioprine (Imuran) is a purine analog that is less potent than mycophenolate and is now used primarily in pregnancy, where it has an established safety profile, and in patients intolerant of mycophenolate. Before initiating azathioprine, testing for thiopurine methyltransferase (TPMT) and NUDT15 genotype is recommended, as deficiency in either enzyme predisposes to severe myelosuppression. A critical drug interaction exists with allopurinol, which inhibits xanthine oxidase and thereby dramatically increases azathioprine levels, leading to potentially fatal pancytopenia. When the combination cannot be avoided, the azathioprine dose must be reduced by 50 to 75 percent with close monitoring.

Corticosteroids

High-dose corticosteroids are administered perioperatively, typically as methylprednisolone 500 mg intravenously, and are rapidly tapered to a maintenance dose of prednisone 5 mg daily by one to three months post-transplant. Long-term low-dose prednisone reduces the risk of rejection but contributes to a substantial burden of side effects, including NODAT, osteoporosis and fractures, cataracts, weight gain, and skin fragility. Steroid withdrawal and avoidance protocols have been successfully implemented in immunologically low-risk patients who receive potent induction therapy with rATG or alemtuzumab combined with tacrolimus and mycophenolate maintenance. The BENEFIT trial further supported steroid avoidance in the context of belatacept-based regimens.

Alternative Agents

mTOR Inhibitors

Sirolimus (Rapamune) and everolimus (Zortress) bind to FKBP-12, similar to tacrolimus, but the resulting complex inhibits the mammalian target of rapamycin (mTOR) rather than calcineurin. By blocking the mTOR signaling pathway, these agents prevent IL-2-driven T-cell proliferation at a step downstream of calcineurin. mTOR inhibitors are used in strategies aimed at minimizing or withdrawing calcineurin inhibitors to reduce chronic CNI nephrotoxicity, in patients who develop post-transplant malignancies because of their anti-proliferative properties, and in cases of CNI-induced thrombotic microangiopathy. Side effects include impaired wound healing, which precludes their use in the early post-transplant period, proteinuria, dyslipidemia, oral ulcers, pneumonitis, and cytopenias. The TRANSFORM trial demonstrated that everolimus with reduced-dose CNI was not superior to mycophenolate with standard-dose CNI at 12 months for the composite of treated rejection, graft loss, or death.

Belatacept (Nulojix)

Belatacept is a CTLA4-Ig fusion protein that selectively blocks the CD80/CD86 costimulatory pathway on antigen-presenting cells, thereby preventing the delivery of Signal 2 required for full T-cell activation. The BENEFIT trial, with seven-year follow-up data published in 2016, demonstrated that belatacept was superior to cyclosporine for long-term graft function, as reflected by significantly higher eGFR, and for patient survival. Although belatacept was associated with a higher rate of early acute rejection, the long-term outcomes were clearly superior, making belatacept an increasingly attractive option for CNI-free regimens, particularly in patients with established CNI toxicity. Belatacept is administered as a monthly intravenous infusion after an initial loading phase, which may enhance adherence by eliminating the need for daily oral medication. An important absolute contraindication is the use of belatacept in Epstein-Barr virus seronegative recipients, who face a substantially elevated risk of post-transplant lymphoproliferative disorder.

Rejection

Hyperacute Rejection

Hyperacute rejection occurs within minutes to hours of transplant reperfusion and is mediated by pre-formed donor-specific antibodies that activate the complement cascade, causing widespread endothelial injury, thrombotic microangiopathy, and rapid graft thrombosis. This catastrophic event has been virtually eliminated by the routine performance of crossmatch testing prior to transplantation. When it does occur, the only treatment is graft nephrectomy.

Acute T-Cell Mediated Rejection (TCMR)

Acute T-cell mediated rejection can occur at any point from days to months post-transplant and is particularly associated with medication non-adherence. The histologic features and severity grading are defined by the Banff classification system. Borderline rejection describes findings suspicious for rejection that do not meet formal diagnostic criteria. Grade IA rejection is characterized by significant interstitial inflammation involving more than 25 percent of the cortex with moderate tubulitis, defined as four or more mononuclear cells per tubular cross-section. Grade IB shows similar interstitial inflammation with severe tubulitis of ten or more cells per tubular cross-section. Grade IIA demonstrates mild intimal arteritis with lymphocytes beneath the vascular endothelium, while grade IIB shows severe intimal arteritis with greater than 25 percent luminal narrowing. Grade III represents the most severe form, with transmural arteritis and fibrinoid necrosis of the vessel wall.

Treatment of TCMR is stratified by severity. Borderline rejection and grades IA through IB are typically treated with pulse methylprednisolone 500 mg intravenously daily for three days, along with optimization of maintenance immunosuppression including verification of adherence and therapeutic drug levels. Grades IIA and IIB, reflecting vascular rejection, are initially treated with pulse steroids, but if steroid-resistant, require escalation to rATG at 1.5 mg/kg/day for 7 to 14 days. Grade III rejection with transmural arteritis carries a high risk of graft loss even with aggressive therapy, and rATG-based treatment is indicated with realistic counseling regarding prognosis.

Acute Antibody-Mediated Rejection (ABMR)

Acute antibody-mediated rejection is driven by donor-specific antibodies that target endothelial HLA molecules, activating the complement cascade and causing endothelial injury. The Banff diagnostic criteria require the presence of circulating donor-specific antibodies, histologic evidence of microvascular inflammation (characterized by glomerulitis with inflammatory cells in glomerular capillaries and peritubular capillaritis with neutrophils and monocytes in peritubular capillaries), C4d deposition along peritubular capillaries on immunofluorescence or immunohistochemistry, and increasingly, molecular classifiers based on gene expression profiling.

Treatment of acute ABMR is challenging and supported by limited evidence. Current approaches include plasmapheresis for 5 to 7 sessions to remove circulating antibodies, high-dose intravenous immunoglobulin at 2 g/kg divided over 2 to 4 days, rituximab at 375 mg/m2 for anti-CD20 B-cell depletion, pulse methylprednisolone, and optimization of tacrolimus trough levels. Emerging therapies for severe or refractory acute ABMR include tocilizumab, an anti-IL-6 receptor antibody, and imlifidase, an IgG-cleaving enzyme derived from Streptococcus pyogenes that rapidly degrades circulating IgG including donor-specific antibodies.

Chronic Active ABMR

Chronic active antibody-mediated rejection has emerged as the leading cause of late graft loss and represents the most formidable unsolved problem in transplant nephrology. The hallmark histologic finding is transplant glomerulopathy, characterized by glomerular basement membrane duplication visible as double contours on electron microscopy. C4d staining may be negative in chronic active ABMR, and the concept of C4d-negative antibody-mediated rejection is now well established. Molecular diagnostics using the Banff molecular classifier system have significantly improved diagnostic accuracy for this condition.

Treatment of chronic active ABMR remains largely unsatisfactory, as no therapy has demonstrated consistent efficacy in randomized trials. The BORTEJECT trial showed that bortezomib, a proteasome inhibitor targeting plasma cells, did not improve outcomes, and the RITUX-ERAH trial found no benefit of rituximab for chronic ABMR. The most effective approach remains prevention through rigorous adherence monitoring, surveillance for de novo donor-specific antibodies, and maintenance of adequate immunosuppression.

<image>Banff classification visual guide for kidney transplant rejection. Show a comparison of three biopsy patterns: (1) Acute T-cell mediated rejection: interstitial inflammation with lymphocyte-rich infiltrate invading tubular epithelium (tubulitis, marked with arrows), and intimal arteritis with lymphocytes beneath the endothelium of an artery. (2) Acute antibody-mediated rejection: glomerulitis (inflammatory cells in glomerular capillaries), peritubular capillaritis (neutrophils/monocytes in peritubular capillaries), C4d staining along peritubular capillaries on immunofluorescence (linear pattern), and DSA detected in serum. (3) Chronic active ABMR: transplant glomerulopathy showing GBM duplication (double contours visible on PAS/silver stain and EM), interstitial fibrosis, tubular atrophy, and arteriopathy with intimal fibrosis. Label each finding with the corresponding Banff lesion score.</image>

Desensitization and Incompatible Transplantation

ABO-Incompatible Transplantation

ABO-incompatible kidney transplantation has become feasible at experienced centers through desensitization protocols that include plasmapheresis or immunoadsorption to remove anti-A or anti-B isoagglutinins, rituximab for B-cell depletion, and intravenous immunoglobulin. The target is to reduce anti-A or anti-B antibody titers to 1:8 or below before proceeding with transplantation. With adherence to established protocols, graft survival in ABO-incompatible transplantation is now comparable to ABO-compatible transplantation at experienced centers.

HLA-Incompatible Transplantation

For highly sensitized patients with a cPRA exceeding 85 percent, HLA-incompatible transplantation may represent the best option for achieving transplantation within a reasonable timeframe. Desensitization protocols typically employ high-dose intravenous immunoglobulin at 2 g/kg combined with rituximab, with or without plasmapheresis, to reduce the level of anti-HLA antibodies. Kidney paired donation programs have dramatically expanded access for patients with willing but incompatible living donors. In these programs, incompatible donor-recipient pairs are matched with other incompatible pairs to create compatible exchanges, sometimes involving chains of multiple donor-recipient pairs facilitated by altruistic non-directed donors.

Key Clinical Pearls

• Tacrolimus-based triple therapy (tacrolimus + MMF + prednisone) remains the standard maintenance regimen; SYMPHONY trial established tacrolimus superiority over cyclosporine, sirolimus, and low-dose combinations
• Belatacept (BENEFIT trial) offers superior long-term graft function and survival compared to CNI-based regimens; consider for patients with CNI nephrotoxicity; contraindicated in EBV-seronegative recipients
• Chronic active ABMR is the leading cause of late graft loss; no proven effective treatment exists; the best strategy is prevention through adherence monitoring, DSA surveillance, and adequate immunosuppression
• Medication non-adherence is the most common preventable cause of late graft loss, particularly in adolescents and young adults; non-adherence risk assessment and interventions should be part of routine post-transplant care
• Donor-specific antibody (DSA) monitoring is critical; de novo DSA development (especially anti-DQ) predicts chronic rejection and graft loss

References

1. Ekberg H, Tedesco-Silva H, Demirbas A, et al. Reduced Exposure to Calcineurin Inhibitors in Renal Transplantation (SYMPHONY). N Engl J Med. 2007;357(25):2562-2575.
2. Vincenti F, Rostaing L, Grinyo J, et al. Belatacept and Long-Term Outcomes in Kidney Transplantation (BENEFIT). N Engl J Med. 2016;374(4):333-343.
3. Loupy A, Haas M, Roufosse C, et al. The Banff 2019 Kidney Meeting Report: Revised Diagnostic Criteria for Chronic Active T Cell-Mediated Rejection, Antibody-Mediated Rejection, and Prospects for Integrative Endpoints for Next-Generation Clinical Trials. Am J Transplant. 2020;20(1):16-30.
4. Hart A, Lentine KL, Smith JM, et al. OPTN/SRTR 2020 Annual Data Report: Kidney. Am J Transplant. 2022;22(S2):21-136.
5. Montgomery RA, Lonze BE, King KE, et al. Desensitization in HLA-Incompatible Kidney Recipients and Survival. N Engl J Med. 2011;365(4):318-326.

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