Acknowledgments
I am grateful for the excellent photomicrographs provided by Dr. Rajathurai Murugasa and Professor Ranjit S. Nanra of John Hunter Hospital, Newcastle, NSW, Australia.
Introduction
Epidemiology
Despite the success of modern kidney transplantation where early acute rejection rates are less than 15% and 1-year graft survivals exceed 90%, yearly graft-loss rates have not greatly changed at 4%. Short-term improvements have incompletely translated into better long-term survival, although increased numbers of marginal donors are being transplanted into higher-risk recipients generating potential survival bias. Kidney transplantation still offers substantial survival benefit compared with remaining waitlisted for deceased donor transplantation on dialysis. Progressive transplant structural deterioration causes renal dysfunction and graft failure, an event accompanied by substantial morbidity and mortality. This chapter will evaluate the causes of chronic transplant failure, its pathophysiology and diagnostic pathology, and present a therapeutic approach.
Pathophysiology of Chronic Allograft Damage
Historical Concepts of Allograft Failure
Originally, kidney allograft damage and subsequent allograft failure was simply attributed in registry data to “chronic rejection,” with the corresponding pathology of interstitial lymphocytic infiltration and tubulitis with tubular atrophy and chronic interstitial fibrosis (IF/TA), often accompanied by vascular or glomerular abnormalities and redesignated as chronic allograft nephropathy (CAN). Although common in the prednisolone-azathioprine immunosuppression era (early acute rejection rates have fallen from 70% to 80% to less than 15%), the clinical presentation (fever, graft tenderness, oliguria) and pattern of histologic injury have altered with the introduction of potent modern regimes, incorporating the calcineurin inhibitors (CNIs), cyclosporine and tacrolimus, and with better immunologic matching. Acute and chronic rejection nowadays presents with a rising serum creatinine, although episodes remaining are clinically more severe. Rejection remains a clinically relevant problem for immunologically active patients, those with iatrogenic underimmunosuppression, and noncompliant individuals.
Clinical Risk Factors of Allograft Loss
The risk factors for chronic allograft loss include donor organ quality and ischemia-reperfusion injury manifest by delayed graft function (DGF); immune factors, including human leukocyte antigens (HLA) mismatch, donor-specific and non-HLA antibodies, episodes of acute and chronic rejection; and recipient factors such as hypertension, proteinuria, smoking, and medication nonadherence Table 27.1 . Posttransplant risk factors for graft failure are subcategorized into immune and nonimmune causes.
Nonimmune donor risks
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Proteinuria (from 0.5 g/day) is a powerful composite risk for graft loss and allograft dysfunction. It originates from either glomerular leakage (glomerular proteinuria) or inadequate tubular reabsorption (from atrophic tubules within IF/TA; tubular proteinuria). Direct injury from tubular resorption of filtered toxic substances, cytokines, or other mediators accompanying proteinuria can cause further injury.
Posttransplant hypertension is associated with graft failure and death, by registry analysis, and occurs in 70% to 90% of recipients. Contributors include pretransplant hypertension and dialysis vascular disease, glucocorticoid and cyclosporine therapy, allograft dysfunction with impaired excretion of dietary sodium, or transplant renal artery stenosis. Allograft hypertensive changes include fibrointimal thickening of small muscular arteries with duplication of the internal elastic lamina, arteriolar hyalinosis, and ischemic glomerulosclerosis. Other adverse factors for graft deterioration include recipient smoking, uncontrolled dyslipidemia, and diabetes mellitus (either preexisting or new-onset posttransplant).
A Unified Model of Chronic Transplant Injury
The pathophysiology of chronic allograft failure is best conceptualized as the end result of several immune and nonimmune mechanisms, which cause progressive damage to individual nephrons and eventually compromise the internal structure of the organ. The transplanted kidney can be progressively injured from a number of non-mutually exclusive pathologic insults and stresses, that occur over the allograft’s lifetime. Cells of local anatomic compartments (tubular, interstitial, microvascular, and glomerular) have a limited repertoire of response to injury, expressed by stereotypical gene patterns and a limited number of histologic phenotypes. Different drivers of injury are expressed at different times, and the relative mixture of disease and pathology prevalence alters with time posttransplantation, but they can operate simultaneously and be observed within the same biopsy.
The histopathology of a chronically failing allograft typically shows chronic interstitial fibrosis, tubular atrophy, glomerulosclerosis, and vascular abnormalities. Chronic alloimmune activity is manifest by persistent cellular interstitial inflammation in scarred and nonscarred areas, fibrointimal arterial hyperplasia, and transplant glomerulopathy associated with circulating donor-specific antibody and microvascular tissue C4d. These features are end-pathway phenotypes caused by the cumulative effects of tissue injury and its fibrotic healing response. They are influenced and modulated in turn by alloimmunity, noncognate inflammation, immunosuppression, and against intrinsic donor, the resilience ( Fig. 27.1 ; see Table 27.1 ).
The cumulative damage hypothesis assumes that chronic allograft damage is the end result of multiple, time-dependent immune and nonimmune insults, which cause irreversible loss of a finite number of transplanted nephrons. Early events include procurement, preservation, and implantation injury, followed by cellular and antibody-mediated alloimmunity, and additional nonimmune processes such as CNI nephrotoxicity, infection, acute kidney injury, hypertension, and diabetes. Destroyed nephrons cannot be replaced once lost, although hypertrophy of tubular cells within remaining nephrons may partially compensate for losses by hyperfiltration. The kidney gradually fails from the incremental loss of individual nephrons, combined with internal structural derangements contributing to organ malfunction. Multiple alloimmune, ischemic, and inflammatory stimuli all cause lethal or sublethal cellular injury, which provokes a profibrotic chronic healing response. (see Table 27.1 , Fig. 27.2 ).
Mechanistic Theories of Chronic Allograft Injury
Several other hypotheses of chronic allograft failure have been advanced, although they may be better described as pathophysiologic mediators, or pathways of injury, rather than as principal etiologic causes (see Fig. 27.2 ).
Inflammatory Cytokines
The cytokine excess theory postulates that acute and repeated tissue injury induces excessive cytokine production (e.g., interferon-γ), resulting in interstitial and vascular fibrosis (mediated by TGF-β1). Cytokines and chemokines can recruit, activate and traffic inflammatory cells. Upregulated or altered expression of VEGF, endothelin-1, plasminogen-activating factor-1, MCP-1, PDGF-A and B, RANTES, BMP 7, hepatocyte growth factor, CTGF, and advanced glycation end-products, have all been described in experimental and human “chronic rejection.”
Reactive Oxygen Species and Glomerular Hyperfiltration
Excessive or uncontrolled production of reactive oxygen species (ROS) from tubular cell mitochondria can cause cellular injury, apoptosis, and senescence. Interstitial inducible nitric oxide synthase protein expression, nitrotyrosine, and ex vivo ROS production are increased in CAN.
The hyperfiltration theory postulates that an increased share of the kidney’s metabolic and protein reabsorption load falls to a diminishing number of remaining nephrons, causing glomerular hypertension and secondary glomerulosclerosis. However, evidence in human transplantation is contradictory, with many registry studies showing no effect on graft survival in donor-recipient size-mismatched pairs. The classical hyperfiltration lesions of secondary focal segmental glomerulosclerosis (FSGS) are uncommon. Hyperfiltration is relevant with substantial nephron loss, or when an infant kidney is transplanted into a large adult.
Failed Resolution of Chronic Inflammation
Wound healing after acute injury is normally self-limited, with complete resolution of inflammation and fibrogenesis. Allografts may be repeatedly subject to episodic acute injury, where resolution of inflammation is partial or incomplete. A self-perpetuating cycle of tubular injury, enhanced allorecognition, and further immune-mediated injury may result.
The total inflammatory burden, irrespective of location, is a strong predictor of graft survival and superior to the Banff i-score alone. Intragraft Foxp3+ T (regulatory, Treg) cells have been associated with better graft function, suggestive of a protective effect, although other data report mixed results. Tregs may be important for transplant tolerance; however, their ability to recognize allo- or self-antigens is unclear. Inflammation within IF/TA areas (i-IFTA) is common, and follows prior subclinical and acute T-cell-mediated rejection (TCMR). i-IFTA is associated with functional impairment, greater structural damage by sequential histology, and death-censored graft failure.
Epithelial-Mesenchymal Transition–Induced Fibrosis
Epithelial-mesenchymal transition (EMT) describes a sequence of phenotypical changes where tubular epithelial cells are transformed into spindle-shaped mesenchymal or myofibroblast-like cells. Excepting distal collecting ducts, tubular cells originate from fetal mesenchyme and transition to an epithelial phenotype during embryonal development. They retain their ability to back-differentiate into mesenchymal cells with appropriate stimuli, such as sublethal tubular hypoxic injury, TGF-β1, or IL-1. Clinical drivers of EMT include subclinical rejection, cyclosporine nephrotoxicity, and oxidative stress. EMT is also reversible; surviving cells can repopulate injured and denuded tubules by mesenchymal-to-epithelial transition. The genetically programmed steps of EMT begin with impaired cell-to-cell adhesion, loss of tight and adherent junctions, desmosomes, and E-cadherin (an epithelial marker); this is followed by reorganization of F-actin stress fibers and expression of smooth muscle actin (a mesenchymal marker), filopodia, and lamellipodia capable of cellular migration ( Fig. 27.3 ). Transitioned myofibroblasts can then secrete interstitial matrix proteins, collagen, and fibronectin.
Evidence that EMT provides the interstitial fibroblasts (type II EMT) responsible for IF/TA is limited. These fibroblasts are predominantly of recipient descent consistent with resident or infiltrating fibroblast origin (using Y chromosomal DNA analysis of sex-mismatched donor-recipient pairs). Cell lineage tracing studies even suggest some fibroblasts originate from recipient capillary endothelial cells (via endothelial-to-mesenchymal transition). EMT is not uncommon in cross-sectional studies, with transitional tubular cells expressing both epithelial and mesenchymal markers (e.g., β-catenin and vimentin). Sequential histologic studies correlating EMT with subsequent progression of interstitial fibrosis are mixed. However, the EMT is likely to be incomplete in many cases, where transformed tubular cells without cell-adhesion molecules are sloughed into the tubular lumen, instead of crossing into the interstitial compartment.
Donor age and Replicative Senescence
Replicative senescence is the normal cellular aging process that ultimately leads to irreversible growth arrest and has been postulated to cause allograft failure and an explanation for reduced survival from older donor kidneys. Preexisting structural abnormalities including vascular disease and glomerulosclerosis, and the reduced regenerative capacity of older kidneys also contribute. Senescent cells are enlarged and flattened from altered cytoskeletal collagen, with lipofuscin deposition and age-dependent p53 and p16 gene activation. Cultured somatic cells stop cycling after a fixed number of doublings (Hayflick limit), which in humans is regulated by telomeres that shorten after each mitotic division. Although shorter telomeres are found in older native and transplanted kidneys, the “mitotic clock” (and p53 tumor-suppressor pathway) is not accelerated by transplantation nor responsible for IF/TA-associated graft failure. A separate telomere-independent (p16 INK4a ) senescence pathway operates dependent on oxidative stress. The senescent phenotype of chronic allograft nephropathy resembles aging and reflects intrinsic alterations of cell-cycling pathways.
The input-stress model describes the interplay between donor organ quality subject to ongoing stress factors from immune-mediated or nonimmune (load) mechanisms, including hypertension, hyperfiltration, proteinuria, dyslipidemia, nephrotoxic drugs, and infection. Stressors were theorized to drive normal cells into a senescent state with exhausted repair, and this would lead to graft failure. Subsequent research has disproved replicative senescence as the cause of allograft failure.
Cortical Ischemia and Angioregression
Tubular epithelial cells are metabolically active with abundant mitochondria powering electrolyte pumps and endocytotic protein reabsorption. Supplied by a network of peritubular capillaries (PTC) downstream from glomerular efferent arterioles, they are susceptible to ischemia from upstream vascular and glomerular disease. Partial or total glomerulosclerosis, CNI-induced vasoconstriction, arteriolar hyalinosis, and small arterial vascular disease (from donor disease, hypertension, or fibrointimal hyperplasia), all reduce postglomerular capillary blood flow. Injured capillary endothelial cells display nuclear swelling, fenestrae loss, and apoptotic detachment from the PTC basement membrane. Hypoxic tubular cells switch to anaerobic glycolysis and activate antiapoptotic molecular programs as an adaptive survival response. Hypoxia-inducible factors (e.g., HIF1α) regulate and activate multiple genes that control glucose metabolism, cellular proliferation and survival, angiogenesis, chemotaxis of inflammatory cells, and regulate extracellular matrix via profibrotic cytokines, such as TGFα, PDGF, CTGF, and VEGF.
The microvascular capillary and small muscular arterial networks become progressively attenuated with chronic damage and have been reported in chronic TCMR, C4d + chronic antibody-mediated rejection (AMR), and sclerosing CAN. Capillary rarefaction with loss of PTC surface area correlated with IF/TA, allograft dysfunction, and proteinuria. Cross-sectional transplant studies limit causal inferences of angioregression-associated tubular ischemia, versus microvascular loss being a consequence of reduced supportive angiogenic factors from injured tubules.
Internal Structural Organ Failure
Transplant renal function is reduced by the summated damage to individual nephrons, or by disruption of the kidney’s internal structure, which is essential to form urine. Local compromise along the length of the intact nephron (such as tubular collapse or luminal obstruction from debris) causes functional failure of the whole unit. Global or partial glomerulosclerosis, and transplant glomerulopathy, all limit ultrafiltrate generation. Atubular glomeruli develop after disconnection of their downstream collapsed tubules. Internal disruption reduces the kidney’s ability to modify tubular ultrafiltrate into concentrated and acidified urine. A segmentally injured glomerulus with synechiae attached to the Bowman’s capsule can misdirect ultrafiltrate into paraglomerular or paratubular channels, leading to the interstitial space. Inflammatory necrosis with obliterative fibrosis destroys the integrity of the tubule. Allograft renal failure can be considered as the summated losses of individual nephrons combined with disturbance of the organ’s internal organization.
Time Course of Allograft Damage
The pathway of progression from implanted donor kidney to failed transplant comprises a series of time-dependent insults causing progressive histologic injury ( Figs. 27.4 and 27.5 , see Table 27.1 ). In addition to inherited donor disease, two broad overlapping patterns of allograft damage can be discerned by sequential biopsy studies. Tubulointerstitial injury dominates the early posttransplant period, followed by microvascular and glomerular abnormalities and slowly increasing IF/TA.
Substantial numbers of tubules are lost early posttransplant from ischemia-reperfusion injury, early severe acute rejection, persistent subclinical rejection (SCR), or polyomavirus (BK virus allograft nephropathy [BKVAN]). Subsequent tubular injury is often less intense and driven by residual alloimmunity and inflammation, accompanied by glomerular and microvascular changes. Later disease drivers include late acute rejection from nonadherence, CNI nephrotoxicity, chronic AMR or chronic-active CA-TCMR, recurrent glomerulonephritis, acute kidney injury (AKI) from intercurrent illness, and pathologic effects of hypertension, dyslipidemia, and diabetes mellitus ( Table 27.2 ). Chronic histologic damage and specific diseases exert additive and independent effects on graft outcome.
Structural/infective
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Donor Quality and Procurement Injury
Inherited donor abnormalities strongly influence posttransplant histology, function, and response to injury which ultimately determine long-term graft survival. Implantation biopsy histology accurately defines the contribution of donor disease relative to subsequent factors in later biopsy samples and is recommended. Important donor pathologic features include the extent of glomerulosclerosis (>20% is severe, and these kidneys are often discarded), glomerulomegaly (with implied nephron loss and hyperfiltration), and microvascular disease (a persistent abnormality associated with donor age, diabetes, hypertension, and death from cerebrovascular disease). Semiqualitiative scoring systems help determine potential suitability for transplantation, with fair predictive capacity against outcomes. Kidney Donor Profile Index (KPDI) is a single numerical measure reflecting 10 (adverse) clinical donor disease and demographic parameters, which summarizes relative organ quality (actually inferiority), with a modest C-statistic. Matching recipients for longevity with the most optimal KPDI kidneys is used within the US allocation algorithm. The projected graft half-life of KPDI kidneys scoring 86% to 100% (the worst) is half that of a living- or standard-criteria deceased donors (SCD) scoring 0% to 20% (the best kidneys). Kidneys from donors with preexisting diabetes or hypertension display marginally lower aggregate survivals (but still provide benefit over waitlisting), however they require careful vetting before acceptance.
Brain death indirectly influences graft outcome by potentiation of graft immunogenicity and other nonspecific pathophysiologic changes. Registry survival rates of living unrelated and one haplotype-matched living related donor kidneys are identical, despite greater genetic and HLA differences, and superior compared with deceased donor transplants. The transplanted organ is immunologically altered by proinflammatory mediators released during donor brain death, resulting in increased acute rejection rates. Injured tissues express innate danger receptors or Toll receptor system ligands (normally signaling intracellular infection), which promotes immune cell maturation, activation, and rejection. Experimental brain death releases a cascade of chemokines, cytokines, proinflammatory lymphokines (TNF-α, interferon-γ), and adhesion molecules, with increased expression of major histocompatibility complex (MHC) antigens, which amplify the host’s alloimmune response.
The “autonomic storm” from donation after brain death (DBD) causes chaotic blood pressure fluctuation. An early hypertensive phase from brainstem herniation and massive circulating catecholamine release, is followed by hypotension from hypothalamic-pituitary dysfunction (and reduced thyroid and cortisol levels), central diabetes insipidus with electrolyte disturbances, and hypothermia from core temperature dysregulation. Systemic hypotension, cardiovascular instability, adrenergic vasoconstriction, and coagulopathies cause donor AKI. Histologic abnormalities associated with brain death include glomerular hyperemia, glomerulitis, periglomerulitis, endothelial cell proliferation, tubular vacuolation (from administered osmotic agents such as mannitol), followed by tubular degeneration, acute tubular necrosis (ATN; seen on implantation biopsy), and tubular atrophy. Transplant dysfunction is worst from hemodynamically unstable donors with prolonged hypotension after brain death. The clinical sequela is DGF with the need for posttransplant dialysis. Long-term transplant outcomes are comparable for kidneys with and without AKI, provided cortical necrosis is excluded.
Delayed Graft Function and Ischemic Injury
DGF, defined by need for dialysis within the first posttransplant week, has gradually increased from 15% to about 22% over two decades as kidneys from older deceased donors with vascular comorbidity (expanded criteria donors, ECD) or after circulatory death (DCD, or nonheartbeating donors) are being transplanted because of organ shortages. Although “marginal” kidneys result in inferior initial function rates and patient and allograft survival relative to standard criteria donors (SCD), the overall recipient survival is better than remaining on dialysis. DGF incidence correlates with donor age, organ size and quality (older donor, diabetes, AKI, and hypertension), and donor hypotension or ischemic times (prolonged from shipping or perioperative surgical reperfusion times). DGF increases long-term graft loss by up to 41% using systematic meta-analysis.
Renal tubular epithelial cells are metabolically active and vulnerable to ischemia where hypoxia reduces cellular metabolism and Na/K ATPase exchanger function. Reperfusion injury from oxygenated blood generates ROS causing DNA breakdown, lipid peroxidation, apoptosis and necrosis of tubular cells, and vascular endothelial cell injury. Limiting procurement and perioperative ischemic injury may improve organ quality and ameliorate graft immunogenicity. Better organ preservation techniques and solutions, optimal intensive care unit management, pulsatile ex vivo machine perfusion, rapid transfer of organs, and prompt implantation reduce organ stresses. Ischemic preconditioning using vasodilators or antiapoptotic molecules is investigational.
Ischemic tubules can recover if sufficient residual tubular cells survive to replenish nephron losses, and the basement membrane remains intact. Repair mechanisms are initiated by inflammatory and fibrogenic signals, causing interstitial infiltration of fibroblasts, mononuclear cells, and macrophages. Fibroblast proliferation with extracellular matrix deposition remodels tissue contributing to the IF/TA pattern: the sequela of tubular injury combined with an injury-repair response. IF/TA is accompanied by low-level proteinuria, hypertension, allograft dysfunction, and reduced graft survival.
Early Tubulointerstitial Damage
Tubular cell injury during the early posttransplant period results from ischemia-reperfusion injury with ATN, acute severe rejection and persistent SCR, BKVAN, and CNI nephrotoxicity, superimposed on donor disease. Acute functional CNI nephrotoxicity causes tubular cell injury characterized by isometric vacuolization, microcalcification, and rarely, by cytoplasmic inclusion bodies (from giant abnormal mitochondria). The histologic features of tubular atrophy are reduced tubular cell height, nuclear loss, and tubular luminal dilation. Alloimmune mononuclear infiltration increases profibrotic factors including TGF-β and the TIMP family of enzymes, further increasing interstitial fibrosis.
The extracellular matrix is a dynamic network of proteins and proteoglycans, which accumulate from increased synthesis and/or decreased breakdown. Mediators include TGF-β1, angiotensin, and immunosuppression. Cyclosporine increases profibrotic cytokines (TGF-β1 and TIMP-1) in humans and in experimental models of CNI nephrotoxicity with interstitial fibrosis. Angiotensin II blockade is a potential treatment target, but it is not used in clinical practice. Cell cycle inhibitors (including mycophenolate and mammalian target of rapamycin [mTOR] inhibitors) reduce myofibroblast proliferation and collagen deposition in vivo and in experimental chronic rejection. Sirolimus may limit tubular atrophy, vascular hyperplasia, and interstitial fibrosis, although a confounding increase in early alloimmune injury (compared with potent CNI controls) make conclusions less certain.
Acute Rejection and Alloimmune Mechanisms
Acute rejection is a risk factor of graft half-life and actuarial graft survival (especially for deceased donors), and although its incidence has decreased with modern immunosuppression, those rejection episodes remaining are more severe.
Other alloimmune risk factors for graft loss include recipient sensitization and HLA matching (see Chapter x). Registry graft survival is reduced by HLA mismatching, despite more potent immunosuppression. Pretransplant antibodies specific to donor HLA antigens (DSA) can result from blood transfusion, pregnancy/miscarriage, or prior transplantation. Posttransplant de novo DSA formation correlates with allograft failure from chronic rejection in retrospective and prospective studies. A causal role for antibody-mediated graft loss is supported by microvascular inflammation with C4d and transplant glomerulopathy on biopsy. Rarely, DSA against nonclassical HLA antigens (e.g., endothelial cells as AECA; AT1 receptor; perecan, a heparin sulfate; MICA, a polymorphic nonclassical class I antigen; and basement membrane molecules, collagen, vimentin, Kα-tubulin) are pathogenic.
The effect of rejection depends on its type, timing, severity, and persistence. When diagnosed and treated promptly, acute interstitial cellular rejection usually resolves without sequelae. In contrast, episodes of vascular or steroid-resistant rejection, recurrent rejection episodes, untreated SCR, chronic-active CA-TCMR, or late (mixed) rejection causes substantial allograft damage. Persistent interstitial CA-CMR increases later IF/TA, clinical or subclinical AMR may result in transplant glomerulopathy, and vascular rejection causes arterial vasculopathy.
Subclinical Rejection
SCR is defined as histologically proven acute or borderline rejection, but without concurrent functional deterioration. It is only diagnosed by protocol biopsy and is clinically distinct from acute rejection, with functional impairment prompting an indication-biopsy. It can be present in the interstitial or microvascular compartments and mediated by T cells or DSA, respectively. The reported variation in prevalence relates to differences in transplant population, HLA mismatch, prior acute rejection, ethnicity, baseline immunosuppression protocol, transplant era, and biopsy timing. The prevalence of acute subclinical TCMR (Banff grade 1A) in 3-month protocol biopsy specimens ranged from 3% to 31%, with borderline SCR occurring in 11% to 41%.
Interstitial SCR has been associated with IF/TA, allograft dysfunction, and reduced graft survival ( Fig. 27.6 ). SCR is followed by IF/TA mediated by TCMR and can cause persistent CA-TCMR with i-IFTA in some, mediated by lymphocytes and activated macrophages, inflammatory mediators, and profibrotic signals (IL-1, IL-6, TNF-α, adhesion molecules, and TGF-β). More potent immunosuppression is associated with fewer acute rejection episodes, reduced SCR, and subsequent IF/TA in cohort studies.
Several controlled trials have evaluated treatment of interstitial SCR. The first randomized controlled trial (RCT) demonstrated that corticosteroid therapy significantly decreased acute rejection episodes and 6-month IF/TA scores and improved 2-year function and trend to 4-year survival. The cyclosporine-based immunosuppression produced an SCR prevalence of 30%. A repeat trial using tacrolimus in low-immune risk patients could not demonstrate benefit of treatment (SCR prevalence was 4.7%). Another study using cyclosporine or tacrolimus (28% SCR prevalence) showed better eGFR at 6 and 12 months with treatment.
Subclinical AMR rejection in protocol biopsies (with microvascular inflammation and C4d) is more common in desensitized positive-crossmatch or untreated sensitized recipients, and after late AMR treatment. Sublethal microcirculatory injury from circulating DSA and complement activates glomerular endothelial cells, produces a widened subendothelial space with fibrillary deposition, glomerular basement membrane (GBM) reduplication and eventually, chronic transplant glomerulopathy. PTC multilamination develops below PTC endothelial cells.
Bk Virus Nephropathy
BK virus is an endemic polyomavirus infection of high prevalence, low morbidity, and long latency that can reactivate in immunocompetent individuals. After primary childhood infection, BK virus can establish a lifelong, subclinical infection within the renal cortex and medulla, and it can be transmitted within a transplanted kidney. Asymptomatic reactivation occurs in 10% to 68% of recipients using CNI-based immunosuppression, where 1.1% to 10.3% progress to BKVAN. Severe BKVAN with tubulointerstitial nephritis leads to progressive renal dysfunction and graft loss (5% incidence, where 46% fail). Polyomavirus allograft infection encompasses BK virus (but rarely JC virus cases). Asymptomatic BK viremia may occur within 2 to 6 months posttransplant, evolving into a stage of renal impairment and clinical disease.
Viral replication within tubular epithelial cells forms intranuclear inclusions, which enlarge with smudgy nuclear chromatin, cellular atypia, and anisocytosis ( Fig. 27.7 ). Cells degenerate with rounding, detachment, and apoptosis or necrosis, then slough into the tubular lumen—visualized as diagnostic urinary “decoy” cells. Multifocal viral activation advances and provokes an acute antiviral cytopathic inflammatory response of monocytes, polymorphonuclear leukocytes, and plasma cells, which resembles acute TCMR (but lacks arteritis, C4d deposition, or HLA-DR expression). Chronic-active tubulointerstitial inflammation with persistent SV40T and tubular necrosis ensues, and this is commonly followed by progressive nephron destruction and functional deterioration.
Suspicious pathology should be confirmed by tissue SV40T staining and viremia quantification ( Fig. 27.8 ). Characteristic 35 to 38 nm intranuclear paracrystalline viral arrays by electron microscopy are distinguished from adenovirus (70–90 nm), cytomegalovirus (CMV), and enveloped herpes simplex (120–160 nm). Late viral infection demonstrates chronic tubulointerstitial scarring, dystrophic microcalcification, and sometimes low-grade (plasma cell rich) inflammation that is SV40T-negative.
Progressive and Late-Stage Chronic Allograft Damage
Late phenotypical changes develop in the glomerular and microvascular compartments, overlaying progressive tubulointerstitial damage. Drivers for ongoing tubular injury (see Fig. 27.1 ) include residual CA-TCMR, late acute rejection (often associated with DSA), CNI nephrotoxicity, BK viral infection, and episodes of late AKI from an intercurrent illness. Acute late rejection often results in severe inflammatory tubular loss and initiation of persistent chronic inflammation, followed by progressive renal dysfunction and graft failure (see Fig. 27.1 ). The causes of late glomerular and vascular disease includes CNI, chronic AMR with transplant glomerulopathy, CA-TCMR, recurrent glomerulonephritis, diabetic nephropathy, hypertensive nephrosclerosis, and vascular effects of smoking and dyslipidemia (see Fig. 27.1 ).
Chronic-Active T-Cell-Mediated Rejection
The expanded definition of CA-TCMR now includes two phenotypes: grade 1 as persistent interstitial mononuclear infiltration with i-IFTA, and uncommonly, a vascular variant of arterial fibrointimal hyperplasia (grade 2).
Interstitial CA-TCMR (type involves T cells, CD4 + or CD8 + ) and macrophages in the interstitial compartment resulting in tubulitis and inflammatory injury characterized by i-IFTA (>25% i-IFTA areas with tubulitis). i-IFTA itself strongly predicts graft failure independent of renal function, IF/TA, and inflammation scores, and associated with progressive IF/TA with tissue injury/repair-associated molecular transcripts. i-IFTA is preceded by TCMR and underimmunosuppression, and it is followed by progressive IF/TA, fibrointimal hyperplasia, renal dysfunction, and allograft loss. Persistent i-IFTA is the pathologic consequence of CA-TCMR, although it is a nonspecific phenotype of active tubular destruction (where other causes such as BKVAN, pyelonephritis, and obstruction should be excluded).
Vascular chronic TCMR (grade 2) presents as medial fibrointimal hyperplasia within small muscular arteries, associated with focal destruction of the internal elastic lamina, smooth muscle infiltration forming a “neointima” with vascular narrowing (the Banff cv score; Figs. 27.9 and 27.10 ). Donor disease, past vascular rejection, hyperlipidemia, hypertension, and smoking also cause similar arterial changes (distinguished by subendothelial hyalinosis, elastic lamina reduplication, and medial hyperplasia in small arteries on histology and by clinical verification; Fig. 27.11 )
Calcineurin Inhibitor Nephrotoxicity
The introduction of cyclosporine revolutionized kidney transplantation, markedly improving 1-year graft survival rates and permitting transplantation of nonrenal solid organs. CNIs are generally well tolerated and underpin modern immunosuppression regimens (see Chapters 17), but are pleomorphic nephrotoxins, causing multiple histologic abnormalities—a significant diagnostic and management issue for long-term use.
CNI exposure to patients with autoimmune diseases, nonrenal transplants, or rodent models of nephrotoxicity all produce a stereotypical constellation of renal histologic abnormalities. The classically described histologic features of CNI nephrotoxicity include de novo or increasing arteriolar hyalinosis ( Fig. 27.12 ), striped cortical fibrosis ( Fig. 27.13 ), isometric tubular vacuolization ( Fig. 27.14 ), and tubular microcalcification (unrelated to other causes, such as ATN or hyperparathyroidism). Other reported abnormalities include peritubular and glomerular capillary congestion (diagnostically unreliable), diffuse interstitial fibrosis (important but nonspecific), toxic tubulopathy (in high-dose cyclosporine therapy), and juxtaglomerular hyperplasia (rare and nonspecific). Tacrolimus and cyclosporine show similar histology, although lesion frequencies vary by CNI. Mild acute arteriolopathy, striped interstitial fibrosis, glomerular congestion, and tubular microcalcification are more frequent with cyclosporine (compared with tacrolimus). Chronic moderate to severe arteriolar hyalinosis was comparable between CNI and very common by 10 years posttransplant, being irreversible once established. Tacrolimus offers more potent immunosuppression with comparable nephrotoxicity.
The pathologic diagnosis of CNI nephrotoxicity is hindered by the paucity of specific and reliable diagnostic markers. Striped fibrosis (cortical fibrosis demarcated against normal adjacent cortex in a striped pattern) likely originates from watershed infarction of interlobular or arcuate arteries, but lacks diagnostic sensitivity and specificity (see Fig. 27.13 ). Tubular microcalcification is described in chronic cyclosporine nephrotoxicity, but can also be secondary to residual hyperparathyroidism or localized cellular necrosis from any cause (including severe rejection and ATN). Isometric vacuolation of proximal tubular cells (corresponding to dilated and stressed endoplasmic reticulum in proximal straight tubules) was frequent with high-dose cyclosporine, and it can still represent acute CNI toxicity (although tubular vacuolation also occurs in ATN). Cytoplasmic inclusion bodies from abnormal giant mitochondria with deranged cristae are rare. Chronic diffuse tubulointerstitial damage from CNI is common in humans and experimental models (but diagnostically nonspecific).
The most reliable diagnostic abnormality is de novo or increasing arteriolar hyalinosis, classically described as peripheral and nodular pattern (vs. subendothelial and diffuse), but imperfect. CNI arteriolopathy is from arteriolar smooth muscle cell loss and replacement by hyaline deposits. Early acute arteriolar hyalinosis is often mild and patchy, and reversible with dosage reduction, but alternatively, may be from age-related donor disease. Late arteriolar hyalinosis lesions are more severe and less reversible, with microvascular narrowing causing ischemic glomerulosclerosis and associated IF/TA ( Fig. 27.15 ). Hyalinized arterioles are unable to maintain their structural integrity, collapsing inwards with luminal narrowing and reduced blood flow (to 20% of normal). Arteriolar hyalinosis correlates with downstream glomerulosclerosis from ischemia.
Arteriolar hyalinosis on biopsy is assessed for a nodularity (imperfect for CNI etiology) and histologic progression (compared with prior or baseline histology; see Fig. 27.12 ). Donor disease, ischemic arteriolar injury, dyslipidemia, diabetes mellitus, and hypertensive vascular disease are alternative causes of hyalinosis. Categorizing arteriolopathy by circular or noncircular involvement marginally improved reproducibility, but at the expense of usable clinical grading. Even nodular arteriolopathy (thought to be specific to CNI toxicity) can be observed in 28% without cyclosporine exposure (and is likely related to vascular aging or other causes).
CNI nephrotoxicity with hyalinosis and glomerulosclerosis is a common secondary diagnosis (in 30% of indication biopsies), and it becomes increasingly common after chronic CNI exposure and use of older donor kidneys. The contribution of CNI nephrotoxicity to chronic graft injury is suggested by unchanged long-term graft attrition rates despite suppression of early acute rejection, appearance of characteristic histologic lesions from longitudinal histopathology studies, and by data from clinical trials of CNI avoidance, early withdrawal and dose reduction, or late withdrawal; which demonstrate functional, structural, and survival benefits.
Progressive Glomerular Abnormalities
Microvascular and glomerular compartments are becoming more commonly affected in late histology with worsening arteriolar hyalinosis and severe vascular narrowing from CNI nephrotoxicity, diabetes mellitus, hypertension, dyslipidemia, smoking, or vascular aging. Glomerular abnormalities include ischemic glomerular loss, formation of atubular glomeruli formation, recurrent glomerular disease, and transplant glomerulopathy (see later).
Morphometric analysis of CAN finds two separate populations of glomeruli. Smaller glomeruli display ischemic wrinkling and extracapillary fibrotic material are contrasted with larger, hyperfiltering glomeruli. Late vascular or endothelial cell injury from CNI, hypertension, or alloimmune injury results in vascular narrowing of afferent arterioles, causing downstream ischemic glomerulosclerosis. Glomerular ischemia and podocyte injury may also lead to proteinuria and glomerulosclerosis.
Severe tubular injury can disconnect the downstream proximal tubule from its perfused glomerulus, designated as “atubular glomeruli.” They are often small or contracted within an enlarged glomerular cyst surrounded by periglomerular fibrosis. Bowman’s space is filled by inspissated proteinaceous material from residual glomerular filtration and local reabsorption. Atubular glomeruli are common in native tubulointerstitial kidney diseases (e.g., lithium and cisplatin nephrotoxicity), and comprise 1% to 2% of glomeruli from living and deceased donor implantation biopsies, 17% to 18% in CAN, and 29% with cyclosporine nephrotoxicity.
Transplant Glomerulopathy and Chronic-Active Antibody-Mediated Rejection
Chronic transplant glomerulopathy is the principal morphologic expression of persistent CA-AMR within the glomerulus. Its features by light microscopy include capillary activation, capillaritis, double contour formation on silver staining, and mesangial interposition ( Figs. 27.16 and 27.17 ). It is semiquantitated, scored by the Banff schema by worst involved peripheral capillary loop (designated cg), and reported in 5% to 15% of failing and failed grafts. Electron microscopy (EM) demonstrates glomerular endothelial swelling and activation, subendothelial widening with deposition of flocculent or fibrillary material GBM duplication, and mesangial matrix expansion (and designated cg1a if only on EM; Fig. 27.18 ) Transplant glomerulopathy is associated with proteinuria, renal impairment, and reduced graft survival.
