Chronic Kidney Transplant Rejection

Etiology

CKTR can be due to cell-mediated or humoral immune response and usually occurs in patients with insufficient immunosuppression or medication nonadherence.[7] Acute rejection (AR) is one of the risk factors for late kidney allograft loss. El Ters et al. studied the effect of AR on graft histology in a cohort of 797 renal transplant patients without donor-specific antibodies (DSA) during the time of transplant. AR was the etiology in 15.2% of patients. One and 2-year biopsies of patients with a history of AR were associated with more inflammation, fibrosis, transplant glomerulopathy (TG), and early allograft loss.[8] Lorentz et al. further studied the effect of immunosuppression nonadherence on graft histology. Non-adherence with immunosuppressive therapy at five years post-transplant was associated with increased fibrosis and inflammation but not TG.[9]

Non-immune risk factors for late allograft loss include delayed graft function, immunosuppressive medication toxicity, recurrence of primary kidney disease, diabetes, hypertension, and hyperlipidemia.[10][11] These factors can potentiate the normal aging process of transplanted kidneys, exacerbating chronic injury, and further contributing to graft loss.

Epidemiology

Alloimmunity is one of the most frequent causes of graft loss. Nankivell et al. reported a 25.8% incidence of subclinical rejection at 1-year post-transplant.[12] The Deterioration of Kidney Allograft Function Study (DeKAF) group biopsied 173 subjects (7.3 +/- 6.0 years post-transplant). Subjects who were positive for DSA, complement component C4d deposition on biopsy (discussed later), or both had an increased risk of kidney allograft failure two years post-transplant.[13] Sellares et al. studied the causes of allograft loss in 60 patients with failure out of a total cohort of 315 patients.[2] The incidence of antibody-mediated rejection increased over time in those with failure, especially after five years post-transplantation. Protocol biopsies by Stegall et al. reported a prevalence of moderate-severe fibrosis in 13% and 17% of patients at one and 5-years post-transplant, respectively. Moreover, 23% of allografts who had a biopsy at one and 5-years post-transplant showed progression in fibrosis from mild to severe forms.[14] Only 5% of tacrolimus treated patients, however, showed evidence of TG, a lesion characteristic of chronic antibody-mediated rejection, suggesting that the use of tacrolimus may help to prevent CKTR.[14]

Pathophysiology

CKTR is, by definition, immune-mediated and generally divides into chronic active antibody-mediated rejection (CAAMR) and chronic active T cell-mediated rejection (CATMR).[7] CAAMR occurs due to DSA against human leukocytic antigens (HLA) and non-HLA antigens. DSAs can damage the endothelium both directly and indirectly through complement-mediated activation and inflammatory cell recruitment. An extended alloreactive immune response over a prolonged period leads to microvascular remodeling of both the glomerular and peritubular capillaries, microvascular inflammation, and arterial intimal fibrous thickening. Complement activation, identified by C4d deposition in the peritubular capillaries, also contributes to microvascular inflammation. However, C4d positivity was eliminated as a requirement for the diagnosis of CAAMR after the emergence of C4d negative antibody-mediated kidney rejection.[6][7]

Cell-mediated injury can involve both the renal tubulointerstitial or arterial components. Antigen-presenting cells (APC) present donor antigens to T cells, which then cross the microcirculation of the donor's kidney and enter the interstitium. Several cytokines are then produced, including interferon-gamma (IFN-γ) and transforming growth factor-beta (TGF-beta), triggering a cascade of inflammation leading to tubulitis, the hallmark feature of CATMR.[15] T cell-mediated injury can also involve the arteries, leading to arterial inflammation and intimal fibrosis.[6] Ultimately progressive IFTA may be a late consequence of CATMR.

Histopathology

At the histopathological level, CKTR affects all parts of the kidneys, including the arteries, interstitium, glomeruli, and tubules. CAAMR leads to microvascular remodeling in both the glomerular or peritubular capillaries. Glomerular microvascular remodeling leads to TG, which is characterized by double contouring of glomerular capillary walls. Other histopathological features of antibody-mediated injury include peritubular capillary basement membrane multilayering and arterial intimal fibrosis.[6][7]

CATMR involves mainly the renal interstitium and arteries, leading to tubulitis and chronic allograft arteriopathy, respectively. Tubular inflammation leads to IFTA. Chronic allograft arteriopathy manifests primarily as arterial intimal fibrosis. Since DSAs in CAAMR can stimulate fibrosis of the arterial intima, it is challenging to differentiate arteriopathy secondary to CAAMR and CATMR on the histological level.[6][7]

History and Physical

The diagnosis of CKTR starts with clinical evaluation through a thorough history taking and a comprehensive physical exam. Important items to ask for during history taking include medication nonadherence, recurrence of the original cause of nephropathy, previous transplantation, prior AR, and baseline HLA sensitization. Unexplained decreases in immunosuppression levels can also point towards nonadherence as a cause for CKTR.[16] Medication nonadherence can also be due to health insurance problems. It is, therefore, crucial to ask about insurance coverage of immunosuppression medications. Medication history is also essential, as many drugs can affect the metabolism of immunosuppression medications, decreasing or increasing blood levels leading to rejection and toxicity, respectively. Physical examination findings are usually nonspecific but may include hypertension, lower extremity edema, and/or fatigue. Symptoms of severe AR, such as fever or graft tenderness, are typically absent in CKTR. More progressive stages of rejection can manifest as signs of kidney failure and uremia, including oliguria, nausea, vomiting, a metallic taste, pericardial friction rub, and asterixis.

Evaluation

Early diagnosis of CKTR is imperative for early and successful treatment. As discussed above, the diagnosis of CKTR starts with clinical evaluation. The Kidney Disease Improving Global Outcomes (KDIGO) guidelines recommend biweekly clinic visits 3 to 6 months post-transplant, monthly visits 7 to 12 months post-transplant, and every 2 to 3 months after that.[17]

Laboratory tests can help differentiate different causes of allograft dysfunction. Kidney allograft function assessed by serum creatinine (Cr) and estimated glomerular filtration rate (eGFR) requires measurement at or before each visit. The eGFR is suggested to be a more accurate indicator and predictor of graft function and long-term graft loss, respectively.[18][19] Iothalamate GFR and cystatin C can also be used to evaluate graft function, especially in situations where Cr may be inaccurate due to extremes of muscle mass. Proteinuria over 500 mg/day may be an early marker of chronic kidney allograft dysfunction.[20]

DSA is typically measured in an HLA laboratory using flow cytometry and the single antigen bead technique. Positive DSA is a relatively good marker for CAAMR. A decrease or disappearance of DSA can be used to monitor response to treatment.[18] DSA, however, may not always correlate with tissue injury. In the Deterioration of Kidney Allograft Function (DeKAF) trial, C4d positive biopsies showed an equal risk of graft failure regardless of the presence or absence of DSA.[13] Denovo-DSA (dnDSA) forming after transplantation has been implicated as a major cause of chronic graft loss and can be detected before graft dysfunction ensues. Prospective monitoring for dnDSA can provide an opportunity for early treatment before the establishment of irreversible graft injury.[21]

Doppler ultrasonography (US) is a non-invasive and relatively inexpensive tool to assess kidney allograft vasculature. Resistance indices over 0.8 at three months have links to deterioration in graft function.[22] Contrast-enhanced US (CES) can help detect a decrease in graft function before the resistance index increases.[23] CES uses gas microbubbles to determine vascular perfusion. After intravenous contrast application, a flush with an increased mechanical index leads to the detection of kidney perfusion through “burst imaging.” One analysis showed that allograft perfusion was related to serum creatinine levels. CES evaluation of blood flow was also more sensitive, specific, and accurate than determining blood flow through conventional indices.[23]

A biopsy is imperative for diagnosing CKTR. Graft histology (as described previously) provides visual evidence of the underlying pathology of graft dysfunction. C4d complement fragment deposition in the peritubular capillaries is a marker for antibody-mediated tissue injury.[16] Although C4d complement fragment deposition can help diagnosis, the emergence of C4d negative antibody-mediated rejection led to the removal of C4d as a diagnostic criterion in the Banff Classification Criteria for CAAMR (described later).[24] Genetic analysis of biopsy tissue has also been suggested to aid the diagnosis of allograft rejection in conjunction with histology. Researchers have identified increased expression of genes primarily related to natural killer cells and microvascular inflammation in both antibody-mediated and T cell-mediated rejection. Immunostaining can help differentiate antibody and T cell-mediated rejection, with CD56 and CD68 positivity linked more to antibody-mediated rejection.[25]

The Banff classification, originally founded in 1991 and later updated in 2007, 2009, 2013, and 2017 established specific criteria for the diagnosis of kidney allograft rejection.[6] Based on the 2017 revised Banff criteria, CAAMR and CATMR are diagnosed and classified as follows:

I) CAAMR (all criteria must be present):

1. Histological evidence of chronic tissue injury (one or more of the following):

• Transplant glomerulopathy without evidence of thrombotic microangiopathy or glomerulonephritis
• Severe multilayering of the glomerular basement membrane on electron microscopy
• New-onset arterial intimal fibrosis

2.Evidence of antibody interaction with vascular endothelium (one or more of the following):

• Linear C4d deposition of peritubular capillaries
• Moderate or severe microvascular inflammation in the absence of glomerulonephritis
• Increased gene expression of gene transcripts strongly suggests antibody-mediated rejection

3. Positive DSA antibodies to HLA and non-HLA antigens.

II) CATMR is classified as follows (after ruling out other causes of IFTA):

• Grade IA: More than 25% interstitial inflammation of the cortex with “moderate tubulitis” in 1 or more tubules, excluding severely atrophic tubules.
• Grade IB: Greater than 25% interstitial inflammation of the cortex with “severe tubulitis” in 1 or more tubules, excluding severely atrophic tubules
• Grade II: Chronic allograft arteriopathy indicated by neointima formation, intimal arterial fibrosis, and mononuclear infiltration

Treatment / Management

The management of CKTR remains challenging, mainly due to irreversibility at the time of diagnosis. Management, therefore, focuses on the prevention and early management of AR rather than treating CKTR. Adequacy of immunosuppression and patient adherence are pivotal for preventing AR, which later translates into a lower incidence of CKTR. Optimizing HLA matching reduces the chances of early allograft injury, further decreasing the risk of chronic allograft loss.[26] Moreover, early treatment of acute antibody-mediated rejection with intravenous immunoglobulin, plasmapheresis, or steroids will also reduce the risk of chronic allograft loss.[8]

Most immunosuppressive regimens in the United States include a CNI, an antimetabolite, and corticosteroids. Although extremely effective, CNIs carry a high risk of chronic nephrotoxicity. Two methods that were suggested to balance efficacy and toxicity are (1) Guiding dosage by monitoring blood drug levels and (2) CNI sparing strategies. The four main approaches to minimize CNI exposure are CNI minimization, conversion, withdrawal, and avoidance.

CNI Minimization: Minimization refers to lowering target blood trough levels of CNIs, with or without another immunosuppressive agent. A systematic review and meta-analysis showed that CNI minimization was associated with a relatively low risk of AR and overall improved allograft function.[27] The timing of CNI minimization was also studied. CNI minimization during the first six months post-transplant reduced the incidence of rejection compared to reducing CNI doses in the second 6 months post-transplant. No head to head trials, however, were conducted to compare early and late minimization directly.(A1)

Combining low dose CNI with mycophenolic acid (MPA) preparations also reduced the risk of AR with no difference in mortality. Pairing CNI minimization with a mammalian target of rapamycin (mTOR) inhibitor (such as sirolimus or everolimus) did not increase the risk of biopsy-proven AR. It led to an improvement in kidney function in some studies. It is worth noting, however, that full dose CNI plus mTOR inhibitor therapy increases the risk of nephrotoxicity.[27](A1)

CNI Conversion: Conversion refers to switching CNI to another maintenance drug. Converting from CNI to an mTOR inhibitor showed improvement in kidney function, which was more observed with the conversion from cyclosporine compared to tacrolimus.[27] Conversion to an mTOR inhibitor was also associated with a lower risk of cytomegalovirus (CMV) infection.[27] Conversion to sirolimus showed better outcomes in patients with GFR exceeding 40 ml/min with less proteinuria, suggesting that conversion should occur before significant parenchymal damage.[28] Grimbert et al. suggested that early conversion to mTOR inhibitors within one year was associated with increased production of dnDSA, which increased the risk of antibody-mediated rejection. Therefore, conversion to mTOR inhibitor therapy with the elimination of CNI therapy should be performed with great caution and may increase the risk of CKTR. Late conversion after one year was not associated with increased dsDNA.[29] Evidence from studies of conversion to azathioprine, mycophenolate sodium, and belatacept was insufficient to draw conclusions.[27](A1)

CNI Withdrawal: Withdrawal refers to tapering CNIs until completely discontinued. CNI withdrawal with either MPA or mTOR inhibitor-based regimens was associated with an increased risk of rejection. Early withdrawal (<6 months post-transplant) was associated with an increased risk of graft loss, with insufficient evidence for both rejection and a decrease in renal function. Late withdrawal with the continuation of MPA preparations was associated with an overall greater risk of rejection.[27] CNI withdrawal from azathioprine-based regimens was also associated with increased rejection.[28](A1)

CNI Avoidance: Avoidance refers to CNI free regimens planned from the start. Initial trials to avoid CNIs while using daclizumab or anti-thymocyte globulin were associated with an increased risk of AR, which required reintroduction of CNIs in some patients.[28] Sirolimus-based immunosuppression regimens were also compared to CNI based regimens. Comparing sirolimus to tacrolimus in MPA-based regimens showed an increased risk of graft loss. Sirolimus, however, was associated with improved kidney function and reduced risk of CMV infection.[27](A1)

Belatacept, a novel fusion protein that inhibits T cell activation, was also compared to CNI based regimens. Vincenti et al. randomized patients into three groups; a cyclosporine, an intensive belatacept, and a less intensive belatacept-based regimen. Patients were followed for seven years. Patients on belatacept-based regimens showed a 43% reduction in risk of graft loss and death, compared to cyclosporine. Kidney function improved in both belatacept-based regimens, while it declined with cyclosporine.[30] 

Non-immunological management of CKTR includes tight control of blood pressure and lipid levels.[10] The KDIGO recommends maintaining blood pressure of over 130/80 in kidney transplant recipients.[20] Hyperlipidemia control with HMG-CoA reductase inhibitors also improves patient survival in kidney transplant recipients.[31] The data regarding the use of angiotensin-converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARBs) is contradicting, with a possible benefit in patients with chronic allograft dysfunction and proteinuria.[32][33][34][35] ACE inhibitors and ARBs should be used cautiously with CNIs due to an increased risk of hyperkalemia and azotemia. Some authors suggest a possible role for vitamin D in increasing graft survival; however, prospective studies are required to confirm efficacy.[11](A1)

Differential Diagnosis

CKTR requires differentiation from other causes of late kidney allograft dysfunction:

Calcineurin Inhibitor (CNI) Toxicity

CNIs are among the most widely used immunosuppressive medications in kidney transplantation. The introduction of cyclosporine and tacrolimus in the early 1980s and late 1990s, respectively, lead to improved clinical outcomes.[28] Acute CNI toxicity is associated with hypertension, thrombotic microangiopathy, and kidney dysfunction secondary to afferent arteriolar vasoconstriction and up-regulation of fibrotic cytokines such as TGF-beta. CNIs also increase the risk of hypertension, post-transplant type 2 diabetes, and hyperlipidemia, all of which are risk factors for late kidney allograft loss. Histologically, chronic CNI toxicity presents with IFTA, similar to what may present as a consequence of CKTR. Therefore it is imperative to differentiate CKTR from CNI toxicity on biopsy. Histologically, CNI toxicity characteristically demonstrates striped interstitial fibrosis, medial arteriolar hyalinosis, tubular microcalcification, vacuolization, and atrophy. The presence of TG, peritubular capillary inflammation, and C4d deposition are all more specific for CKTR.[28]

BK-Virus Associated Nephropathy (BKVAN)

BKVAN is also a significant cause of late allograft dysfunction and requires differentiation from CKTR. BKVAN occurs when the BK virus, a polyomavirus, propagates in the face of immunosuppression. Most transplant centers screen for BK virus in the bloodstream during the first year post-transplant, and particularly high-level viremia tends to correlate with BKVAN. Histologically, BKVAN can present with tubulointerstitial scarring similar to CKTR. Suspicious biopsy findings need confirmation by polymerase chain reaction (PCR) detection of viral DNA in the blood, characteristic intranuclear viral particles on electron microscopy, and/or BK virus detection using immunohistochemistry and in situ hybridization (e.g., staining for the SV40 large T antigen).[3]

Recurrent or De-Novo Glomerular Disease

Recurrent glomerulonephritis (GN) causes approximately 8.4% of late renal allograft loss.[3] Dense deposit disease and focal segmental glomerulonephritis (FSGS) are associated with a high risk of recurrence after transplantation, with a poor prognosis.[3] Differentiating GN from CKTR can be done by history, laboratory, and histopathology testing. A history of GN pre-transplantation with similar findings on urinary sediment post-transplantation supports a recurrence of GN, particularly if nephrotic range proteinuria is present, while DSA positivity supports CKTR. Histologically, both can be differentiated by light microscopy, electron microscopy, and immunofluorescence.

Prognosis

The prognosis of CKTR and late allograft loss depends on the degree of fibrosis and reversibility of rejection at the time of diagnosis. Denisov et al. suggested that measuring hemoglobin, creatinine, and proteinuria 1-year post-transplant can be beneficial in the prognostication of kidney transplantation.[36] Indeed, a calculator for prognostication was patented and is available on the internet in Russian with a reported 92% accuracy for the prediction of renal graft function three years post-transplant.[36] Further studies are needed, however, to confirm its accuracy.

Complications

The main complication of CKTR is allograft loss, which leads to kidney failure and possibly death, especially in patients who are poor candidates for repeat kidney transplantation. Patient complications include anxiety and depression, with an increased risk of mortality and worse quality of life with dialysis re-initiation. Kaplan et al. reported a less than 40% chance of at least 10-year survival in patients with kidney allograft failure.[37] Cardiovascular disease is the most common cause of death, followed by infection, which is mainly due to prior exposure to immunosuppression medications.[38] The economic burden of rejection and dialysis re-initiation is also detrimental for both the patient and the community.[1]