Renal Transplantation Rejection

Etiology

Certain factors correlate with an increased risk of rejection of the renal allograft after the transplantation. These factors are:

• Prior sensitization - high panel reactive antibodies
• Type of transplant: A deceased donor has a higher rejection than a living transplant
• The advanced age of the donor
• Prolonged cold ischemia time
• Prolonged warm ischemia time[3]
• HLA mismatch
• Positive B cell crossmatch
• ABO incompatibility
• Recipient’s age: Younger recipients have more rejection than older ones
• Recipient’s race: Black race greater than White race
• Delayed graft function
• Therapy non-compliance
• Previous episodes of rejections
• Inadequate immunosuppression[2][4][5] 

Epidemiology

The overall incidence and prevalence of acute allograft rejection have decreased, and graft survival has improved with more advanced and potent immunosuppressive agents for induction and maintenance therapy. The incidence of acute rejection within the first year is around 7.9%. Overall, the rate of acute rejection is lower in living-donor than deceased-donor kidney transplants; this is likely related to better matching and less cold ischemia time.[6]

Pathophysiology

Renal transplant rejection, as stated earlier, is an immunological response that leads to inflammation with specific pathological changes in the allograft due to the recipient’s immune system recognizing the non-self (foreign) antigen in the allograft. There are different mechanisms postulated depending on the type of rejection, as follows:

1. Hyperacute rejection: It is related to preexisting circulating antibodies in the recipient’s blood against the donor antigen (usually ABO blood group or HLA antigen), which is present at the time of transplantation. These antibodies attack and destroy the transplanted organ as soon as or within a few hours after the allograft is revascularized.
2. Acute T cell-mediated rejection: In which the recipient’s lymphocytes become activated by recognition of foreign (non-self) donor antigens in the transplanted organ by antigen-presenting cells (APC) through direct, semi-direct or indirect pathways, which leads to activation and infiltration of the T cells and damage to the allograft.
3. ABMR: It is related to antibodies against foreign (non-self) donor antigens, mainly HLA antigen, which leads to damage to the allograft through activation of the complement-dependent pathway as well as independent mechanisms recruiting natural killer cells (NK) cells, polymorphonuclear cells, platelets, and macrophages to attack the allograft. These antibodies can either preexist at a low level before the transplant or be synthesized de novo post-transplant.
4. Chronic rejection is related to both immune and nonimmune-mediated factors. The primary risk factor for chronic rejection is non-compliance with immunosuppressive medication. It can be either chronic antibody-mediated rejection, mainly related to the presence of donor HLA-antigens donor-specific antibody (DSA), or chronic cellular rejection, which is uncommon.

Histopathology

The standard way to detect rejection is a renal allograft biopsy, which accurately grades the severity of rejection, differentiates between different types and guides the treatment. There are 2 major classifications for the histopathological diagnosis of renal allograft biopsy: The Banff classification system and the Cooperative Clinical Trials in Transplantation (CCTT). Later, both were incorporated into the Banff 97 classification to standardize the histopathological diagnosis of renal allograft biopsy. Subsequently, Banff has had updates at regular intervals; the last 1 was in 2017. When performing a kidney biopsy, it should have adequate tissue to give a definitive interpretation. Adequate core biopsy must contain 10 glomeruli and 2 arteries and a section thickness of 3 to 4 microns (marginal if 7 to 10 glomeruli and 1 artery; unsatisfactory if less than 7 glomeruli or no arteries).[7] The histological characteristics of each type of rejection are as follows:

• Hyperacute Rejection: The transplanted kidney turns mottled, dusky, and black as soon as it is revascularized. Severe endothelial injury, polymorphonuclear (PMN) infiltration, whispered thrombosis, and ischemic tissue necrosis would appear on the biopsy.
• ABMR: Histological features of ABMR include arteriolar fibrinoid necrosis, fibrin thrombi in glomerular capillaries, glomerulitis, peritubular capillaritis, and interstitial hemorrhage. Also, the presence of peritubular capillary linear staining for C4d, a degradation product of the complement pathway that binds covalently to the endothelium, is highly suggestive of ABMR.
• Acute T Cell-Mediated Rejection: Characterized by diffuse lymphocytic infiltration in the tubule, interstitium of the kidney, and, in severe cases, vessels of the allograft
• Chronic Rejection Lesions: These include interstitial fibrosis, tubular atrophy, vascular fibrous intimal thickening, glomerular basement membrane double contouring (transplant glomerulopathy), arteriolar hyalinosis, and hyaline arteriolar thickening.

The Banff system uses scores to assess the presence and the degree of histopathological changes in the different compartments of renal transplant biopsies. It focuses mostly, but not exclusively, on the diagnostic features seen in rejection. The staging is below according to the scoring of the various lesions described above.

• Category 1: Normal biopsy or nonspecific changes
• Category 2: Antibody-mediated rejection - AMR: depending on the features of the lesion, further divided into acute AMR, chronic AMR, or active chronic AMR
• Category 3: Suspicious (borderline) for acute T cell-mediated rejection - TCMR.
• Category 4: TCMR. Depending on the score of chronic lesions, it further divides into acute TCMR, chronic TCMR, or acute chronic TCMR
• Category 5: Interstitial fibrosis and tubular atrophy - IFTA
• Category 6: Other changes not considered to be the result of acute or chronic rejection[8]

History and Physical

Most patients who have acute rejection episodes are asymptomatic and have abnormal allograft dysfunction, as evidenced by routine blood evaluation. When serum creatinine suddenly rises to more than 25% of the baseline value, the clinician should suspect allograft rejection. Even when creatinine does not trend down as expected in the early post-transplant phase, the possibility of rejection should be considered. Any new-onset or worsening proteinuria and new-onset or worsening hypertension should raise the suspicion of rejection. Sometimes a rejecting graft may present with fever, pain at the graft site, hematuria, dysuria, hypertension, fluid retention, and decreased urine output.[2]

Evaluation

The approach for elevated serum creatinine in a renal transplant recipient would be the same as evaluating for acute kidney injury (AKI) with added workup for the rejection. The specific workup for evaluating allograft dysfunction should include the following:

• Rule out prerenal causes: Check orthostatic vital signs, blood pressure, and volume status
• Rule out post-renal causes, mainly obstructive uropathy in older adults, by bladder scan, renal ultrasound (US)
• Complete blood count: Look for anemia and thrombocytopenia to rule out thrombotic microangiopathy (TMA)
• Electrolyte abnormality related to chronic kidney disease (CKD) and AKI
• Urinalysis (UA) and urine culture: It is essential to rule out infection as a cause of AKI
• Check for proteinuria: Either urine protein/creatinine ratio (UPCR) or 24-hour urine collection as nephrotic range proteinuria correlates with the presence of extensive transplant glomerulopathy
• Check for BK polyomavirus, cytomegalovirus (CMV) polymerase chain reaction (PCR) in clinically indicated patients
• Testing for donor-specific antibodies
• Transplant renal ultrasound with Doppler for renal arterial and venous indices
• Many transplant centers use testing for donor-derived free DNA testing. This test can be positive even before the actual rise in serum creatinine, suggesting possible rejection.[9] 

Treatment / Management

The treatment plan determination uses multiple factors, including the type of rejection, the severity of the histological lesion, the chronicity score, and the recipient's comorbidity. What is discussed is a general guideline, but tailoring medical treatment to individual characteristics is needed.[10][11](A1)

1. Hyperacute Rejection: No effective therapy usually leads to early allograft nephrectomy, so prevention is the key by assuring the following:

• ABO compatibility between donor and recipient. Sometimes, it is advisable to address ABO incompatibility under specific criteria, and careful pre-transplant preparation of the recipient with the removal of anti-ABO antibodies by plasmapheresis is an option, intravenous immunoglobulin (IVIG) with or without rituximab
• Pre-transplant cross-match (complement cell cytotoxicity test): Recipient serum is added to donor lymphocytes. If the test is positive (the recipient has an antibody that reacts with the donor HLA antigens on lymphocytes), one should not proceed with transplant unless these antibodies are removable pre-transplant.

2. Antibody-Mediated Rejection:

The treatment of acute antibody-mediated rejection also depends on the antibody levels. Higher antibody levels need plasma exchange for the removal of the antibodies. The following are the different modalities used for AMR:

• Plasma exchange: 3 to 5 sessions daily on every other day are used for antibody removal, followed by IVIG and rituximab
• IVIG: IV immunoglobulin (100 to 200 mg/kg) is used, followed by the last session of plasma exchange when combined with plasmapheresis or a higher dose of 2g/kg after the final plasmapheresis. 
• Rituximab: Anti CD20 cell antibody rituximab (375 mg/m^2) is combined with IVIG, followed by plasma exchange
• Bortezomib: Plasma cell inhibitor bortezomib (1.3 mg/m^2) is also used in combination with plasma exchange and IVIG
• Splenectomy: A splenectomy is very rarely an option, but there are anecdotal reports of successful treatment of refractory rejections
• Optimize the dose and the level of the maintenance immunosuppressive drugs.

3. T Cell-Mediated rejection: They receive treatment with the following agents based on the severity of the lesion.

• Methyl prednisone IV (250 to 1000 mg daily) targeting T cells, B cells, and macrophages; given for 3 to 5 days
• rATG - rabbit anti-thymocyte globulin IV (1 to 1.5 mg/kg) targeting T cell receptors. The duration varies among different transplant centers, but generally, it is for 7 to 14 doses based on the response and Cd3 level.
• Optimize the dose and the level of the maintenance immunosuppressive drugs. 

4. Chronic rejection: Since the antibody-mediated rejection mechanism is a major cause of chronic rejection, the same therapy as ABMR has been used, but generally, these measures are ineffective when serum creatinine is over 3 mg/dl or heavy proteinuria is present.

Differential Diagnosis

While dealing with renal allograft dysfunction, equal weight should be given to find possible etiologies other than the rejection. The following are the most common reasons for allograft dysfunctions other than allograft rejection:

A. Immediate Post Transplant (less than 1 week)

1. Postischemic acute tubular necrosis or ischemia-reperfusion injury.
2. Volume depletion leading to pre-renal AKI
3. Surgical complications:
   1. Fluid collection – urinoma, perinephric hematoma, or lymphocele
   2. Vascular thrombosis - arterial and venous
   3. Multiple renal arteries from the donor's kidney - infarction of the part of the allograft or necrosis of the ureter leading to urinary obstruction or urinary leak
   4. Aeroembolism
   5. Calcium oxalate crystals deposits in renal allograft

B. Early (1 week to 3 months) and Late Post Transplant (over 3 months)

1. Volume depletion
2. Acute tubular necrosis
3. Calcineurin inhibitor nephrotoxicity - manifesting as acute azotemia as well as chronic progressive renal disease.
4. Urinary obstruction
5. Infections
   1. Bacterial pyelonephritis
   2. Viral infections - BK polyomavirus and CMVAcute and chronic interstitial nephritis
6. Recurrent primary glomerular diseases 
   1. Focal segmental glomerulosclerosis (FSGS)
   2. Primary membranous nephropathy
   3. Diabetic nephropathy
   4. Ig A nephropathy
   5. C3 Glomerulonephritis (C3 GN)
7. De novo glomerular disease
8. Thrombotic microangiopathy
   1. In patients with a prior history of thrombotic thrombocytopenic purpura (TTP), hemolytic uremic syndrome (HUS), antiphospholipid antibody syndrome
   2. Associated with calcineurin inhibitor nephrotoxicity
9. Transplant renal artery stenosis
10. Post-transplant lymphoproliferative disease[12] 

Pertinent Studies and Ongoing Trials

Pertinent studies and ongoing trials on renal transplant rejection include:

• Small trials are ongoing for cell therapies to modulate the immune system.
• AntiCD40 monoclonal antibody (CFZ533) has the potential to block the co-stimulatory CD40–CD154 pathway and is a possible upcoming clinical treatment for acute rejection.
• A few clinical trials are underway to check the safety and efficacy of immunomodulatory medicines for desensitization and antibody-medicated rejections.[13]

Prognosis

The acute rejections predispose to chronic graft dysfunction. T-cell-mediated rejections have better graft survival, especially when they respond to therapy and the serum creatinine reaches near the previous baseline after the treatment. Acute rejections occurring after 3 months, vascular rejections, and the rejections not responding to therapy (serum creatinine not reaching 75% of baseline value) are associated with poor graft survival. The appearance of de novo DSAs at any time post-transplant is associated with 5% poor graft outcome per year compared to recipients without these antibodies. Due to the above-mentioned graft outcomes, each acute rejection episode must be treated and monitored thoroughly by optimizing the maintenance immunosuppressive regimen.[14][15]

Complications

The complication of acute rejection is graft failure if not treated appropriately and promptly. Even after the treatment, there is an association between poor graft survival after each rejection episode. The treatment of acute rejection, as described above, results in a severely immunocompromised state and puts the recipient at an increased risk from the side effects of the agents. There are many side effects associated with these immunosuppressive agents:

• Increased cardiovascular risk
• Development of post-transplant diabetes mellitus
• Dyslipidemia
• Various malignancies (squamous cell carcinoma and post-transplant lymphoproliferative disease are the more common among them)
• Various opportunistic infections (viral- CMV, BK; fungal- histoplasmosis, coccidioidomycosis, etc, and other atypical organisms)

CMV infection itself is also associated with an increased risk of rejection. Managing immunosuppressive agents after rejection episodes is always a double-edged sword. The clinician has to manage wisely while weighing the risks and benefits.