BK Virus in Solid Organ Transplant Recipients

Authors

  • H. H. Hirsch,

    Corresponding author
    1. Transplantation Virology, Department of Biomedicine, Institute for Medical Microbiology, University of Basel, Basel, Switzerland
    2. Infectious Diseases and Hospital Epidemiology, University Hospital, Basel, Switzerland
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  • P. Randhawa,

    1. Transplantation Pathology, Department of Pathology, Thomas Starzl Transplantation Institute, University of Pittsburgh, Pittsburgh, PA
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  • the AST Infectious Diseases Community of Practice

    1. Transplantation Virology, Department of Biomedicine, Institute for Medical Microbiology, University of Basel, Basel, Switzerland
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* Corresponding author: Hans H. Hirsch, hans.hirsch@unibas.ch

Introduction

The human polyomavirus BK (BKV) is linked to two major complications in transplant recipients, polyomavirus-associated nephropathy (PyVAN) (1–4) and polyomavirus-associated hemorrhagic cystitis (PyVHC) (5,6). PyVAN and PyVHC have been encountered in a variety of immunodeficient patients (7,8), but most cases of PyVAN arise in kidney transplant patients at rates of 1–10%, while PyVHC preferentially affects allogeneic hematopoietic stem cell transplant patients at rates of 5–15% (7,9). BKV has been less frequently associated with other pathologies, such as ureteric stenosis, pneumonitis, hemophagocytic syndrome, encephalitis, retinitis, multiorgan failure and polyomavirus-associated multifocal leukoencephalopathy (PyVML) (10–12), a complication of the central nervous system mostly caused by the closely related polyomavirus JC (JCV) (13). The objective of this section is to update previous recommendations (14) regarding diagnosis and management of BKV replication and PyVAN in solid organ transplantation (SOT) using the grading proposed by Gross et al. (15).

BKV belongs to the family Polyomaviridae together with five other polyomaviruses detected in human specimens, that is JCV, KI virus, WU virus, Merkel cell carcinoma virus and Simian virus 40 (16,17). The virions are nonenveloped particles of 42 nm diameter and fairly resistant to environmental inactivation (18). The circular double-stranded DNA genome of ∼5100 base pairs contains the noncoding control region (NCCR) which regulates expression of viral early and late genes encoding the nonstructural proteins small T- and large T-antigen in one direction, and the capsid proteins VP1, VP2, VP3 and the agnoprotein in the other direction, respectively (7). Research has attributed key roles of clinical relevance to: (i) the large T-antigen: for activation of BKV replication, oncogenicity, cellular immune recognition, genotyping and diagnostic immunohistochemistry (7,19–28); (ii) the capsid VP1 protein: for defining BKV serotypes, cellular immune recognition, neutralizing antibodies and diagnostic antibody responses (24,29–35); (iii) the NCCR: for determining the viral replication rate and cytopathology (7,20).

Epidemiology of BKV Infection and Replication

Primary infection with BKV occurs mostly without specific symptoms or signs in the first decade of life as indicated by a shift from negative to positive serostatus in more than 90% of children and young adults worldwide (30,36). Natural transmission is not conclusively resolved, but likely occurs via the respiratory or oral route (7). Subsequently, following a presumed viremic phase, a state of nonreplicating (latent) infection is established in the renourinary tract as the principal site (37).

In healthy BKV seropositive immunocompetent individuals, reactivation and asymptomatic urinary shedding of BKV is detectable in up to 10%, with low urine BKV loads of <5 log10 genome equivalents (geq.)/mL (38). In individuals with impaired immune functions, particularly after SOT or HCT, high-level BKV replication is detectable defined by urine BKV loads of >7 log10 geq/mL (3,39–41). At these high rates of urinary BKV replication, sloughing of altered epithelial cells with nuclear inclusions (‘decoy cells’) appear in urine cytology preparations, and virions can be visualized by negative staining electron microscopy (42–44). In nonkidney SOT recipients, BKV viruria is thought to result from reactivating the endogenous BKV which only exceptionally progresses to clinical disease (40,45–48). In kidney transplant recipients, BKV reactivation most likely results from reactivating BKV in tubular epithelial cells of the donor kidney (49) which is then amplified in the urothelial cell layer (41,42).

Approximately 30%–50% of kidney transplant patients with high-level viruria progress to BKV viremia and histologically and clinically manifest PyVAN (3,50–52). The imbalance between BKV replication and BKV-specific immune control are viewed as a key element of pathogenesis. Duration and extent of this imbalance appears to drive the direct cytopathic loss of >7 log10 tubular epithelial cells per day. Inflammation elicited by necrosis and denudation of the tubular basement membrane is followed by infiltrating lymphocytes, tubular atrophy and fibrosis (1,41,42,53). With the advent of effective screening techniques that permit identification of kidney transplant patients early before extensive tissue damage has occurred, the rate of graft loss has been reduced to 10% or less (54,55).

Risk Factors of PyVAN

The preference of PyVAN in kidney transplants compared to other nonkidney SOT is striking, and suggests the importance of factors associated with transplanting this organ of BKV latency, which include the tissue BKV load, reactivation by ischemia-reperfusion injury as well as qualitative and quantitative mismatch with BKV-specific T-cells. Similar PyVAN frequencies of 2%–8% have been observed in pediatric kidney transplant recipients, but the overall outcome appears to be more favorable with less graft loss (56). Depending on center and study, risk factors of PyVAN include donor characteristics (such as female gender, deceased donation, ischemia-reperfusion injury, high BKV-specific antibody titers as a marker for recent exposure, HLA-mismatches, African-American ethnicity), recipient characteristics (such as older age, male gender, low or absent BKV-specific T-cell activity) and posttransplant factors (such as acute rejection and antirejection treatment, cumulative steroid exposure, lymphocyte depleting antibodies, higher immunosuppressive drug levels, tacrolimus-combinations compared to cyclosporine or mTOR inhibitor-combinations (3,14,57–61).

Although various risk factors have been identified, general changes of organ allocation practices or of immunosuppressive protocols cannot be recommended at this time [III]

Though multiple and only partly overlapping in different centers, the various entities may indeed reflect partly complementing factors in PyVAN pathogenesis, the central feature being a disrupted balance between BKV reactivation in renal tubular epithelial cells and BKV-specific cellular immune control (27,28,62). Given the worldwide distribution of BKV (63), geographic variability of PyVAN incidence is mostly attributed to local differences in transplant programs particularly regarding immunosuppression.

Diagnosis of PyVAN

The presentation of PyVAN is initially inconspicuous, with no clinical or laboratory signs other than high-level viruria as defined by decoy cell shedding, urine BKV loads >7 log geq/mL and BKV viremia (3,53). Other markers associated with PyVAN are BKV VP1 mRNA of >6 log copies/ng total urine RNA (64) or the detection urine PyV aggregates by electron microscopy (65). PyVAN passes through a series of histologically defined patterns that appear to result from the extent and duration of viral damage and corresponding immune responses in the allograft kidney. A creeping increase of serum creatinine is frequently the first clinical sign of PyVAN and results from an advanced stage of damage caused by extensive virus-mediated tubular epithelial cell damage and corresponding inflammation, which together with higher chronicity scores in kidney allograft biopsy determines a poorer outcome (42,44,53,66).

The definitive diagnosis of PyVAN should be sought by demonstrating PyV cytopathic changes in renal allograft tissue, and be confirmed by an ancillary technique such as immunohistochemistry for BKV proteins, or by in situ-hybridization for BKV nucleic acids (‘definitive PyVAN’) [II-1]

For immunohistochemistry, most centers use cross-reacting antibodies raised against the large T-antigen of the Simian virus 40, but specific staining for BKV VP1 or agnoprotein has also been described.

A minimum of 2 biopsy cores should be taken, preferentially containing medullary tissues [II-1]

Because of the focal nature of PyVAN and the possibility of sampling error in at least 10%–30% (53), negative biopsy results cannot rule out early focal PyVAN with certainty, even when ancillary techniques have been applied. Many centers consider a rebiopsy in cases of high suspicion, for example in cases with sustained BKV viremia and a negative initial biopsy (‘presumptive PyVAN’).

The histopathology of PyVAN is characterized by intranuclear polyomavirus inclusion bodies in tubular epithelial and/or glomerular parietal cells, often associated with epithelial cell necrosis and acute tubular injury. These alterations may affect only few nephrons and can be associated with varying degrees of inflammatory cell infiltrates, tubular atrophy and fibrosis. Cytopathic viral changes are often associated with epithelial cell necrosis and the denudation of tubular basement membranes. BKV-induced tubular injury is viewed as a major morphological correlate for allograft dysfunction.

The histological presentation of PyVAN should be semi-quantitatively assessed regarding the extent of viral cytopathic changes and the inflammatory infiltrates, tubular atrophy and fibrosis according to the Banff working classification [II-3]

Standardized assessment and reporting is viewed as an important cornerstone to improve the comparability of case series and clinical studies. In line with the poor prognosis of PyVAN-C, the index of fibrosis and tubular atrophy in conjunction with PyVAN may be the most important predictor of a poor outcome (54) (Table 1). However, the classification into three categories of PyVAN-A, -B and -C may not be sufficient to provide sufficient statistical discriminatory power for clinical studies. Most biopsies with PyVAN fall into category B, and subgroups B1, B2, and B3 defined by the percentage of biopsy area affected may be more informative (Table 1) (53).

Table 1.  Histological patterns of PyVAN and graft loss modified after (14,53,125)
PatternDescriptionExtent of biopsy coreGraft functionRisk of graft loss
PyVAN-A
 Viral cytopathic changesMild to moderate≤25%Mostly baseline<15%
 Interstitial inflammationMinimal≤10%  
 Tubular atrophyMinimal≤10%  
 Interstitial fibrosisMinimal≤10%  
PyVAN-B
 Viral cytopathic changesVariable11–>50%Mostly impaired50%
 Interstitial inflammationVariable11–>50%  
 Tubular atrophyMild<25%  
 Interstitial fibrosisMild<25%  
 B1
  Viral cytopathic changesModerate11–25%Often slightly above baseline25%
  Interstitial inflammationMild11–25%  
  Tubular atrophyMinimal≤10%  
  Interstitial fibrosisMinimal≤10%  
 B2
  Viral cytopathic changesExtensive26–50%Significantly impaired50%
  Interstitial inflammationModerate26–50%  
  Tubular atrophyMild11–25%  
  Interstitial fibrosisMild11–25%  
 B3
  Viral cytopathic changesExtensive>50%Significantly impaired75%
  Interstitial inflammationSevere>50%  
  Tubular atrophyModerate26–50%  
  Interstitial fibrosisModerate26–50%  
PyVAN-C
 Viral cytopathic changesVariableVariableSignificantly impaired progressive failure>80%
 Interstitial inflammationVariableVariable  
 Tubular atrophySevere>50%  
 Interstitial fibrosisSevere>50%  

Acute rejection concurrent with PyVAN should be diagnosed, the diagnosis is only considered secure if one finds endarteritis, fibrinoid vascular necrosis, glomerulitis, or C4d deposits along peritubular capillaries [II-3]

In PyVAN, C4d deposits have been observed in the tubular basement membranes, but not in peritubular capillaries (65,67). As a point of consideration, a single case of PyVAN with signs of intimal arteritis has been reported which improved following reduction of immunosuppression alone (68). There is also an exceptional case of generalized polyomavirus vasculopathy where B endothelial cells in the skeletal muscle were infected with BKV (69). Determining whether interstitial infiltration and tubulitis is directed against viral or tubular antigens cannot be reliably done by light microscopy. Tubulitis out of proportion to inflammation or occurring away from tubules with obvious viral cytopathology has also been proposed, but requires further research. Increased expression of MHC class II antigens by the tubular epithelium has been regarded as a marker for rejection, which is absent in PyVAN biopsies with acute viral tubular necrosis, but requires further validation (70). The presence of MHC class II upregulation, however, may not be specific for acute rejection, since it may also be a response to influx of inflammatory cells secondary to BKV-induced parenchymal injury. The primary diagnostic value of staining for MHC class II antigens may reside in demonstrating the absence of expression in biopsies with PyVAN-B, where acute rejection is a consideration in the differential diagnosis (70,71).

Treatment of PyVAN

Immunosuppression should be reduced in kidney transplant patients with definitive PyVAN [II-1]

The mainstay of therapy for PyVAN in kidney transplant patients without concurrent acute rejection is to improve BKV-specific immunity by reducing or discontinuing immunosuppressive drugs (28,62). The following strategies and their combinations have been reported:

Strategy 1. First dose reduction of the calcineurin inhibitor by 25%–50%; followed by reducing the antiproliferative drug by 50%; followed by discontinuing the latter if necessary (72,73).

Strategy 2. First reducing the antiproliferative drug by 50% followed by reducing calcineurin inhibitors by 25%–50% followed by discontinuing the antiproliferative drug if necessary (74).

Oral prednisone is typically tapered to 10 mg or less daily dose. Immunosuppression is further adapted according to the course of serum creatinine concentration and the plasma viral loads, but responses may require several weeks.

Tacrolimus trough levels are commonly targeted to <6 ng/mL [II-3], cyclosporine trough levels to <150 ng/mL [II-3], sirolimus trough levels of <6 ng/mL [II-3], and mycophenolate mofetil daily dose equivalents of ≤1000mg [II-3]

Even further reduction may be appropriate in individual patients. Recent studies suggest that lower calcineurin inhibitor levels, that is targeting trough levels for tacrolimus of 3 ng/mL and cyclosporine of 100 ng/mL may be considered as a first step (38).

Additional strategies have been switching from tacrolimus to low-dose cyclosporine or switching from mycophenolic acid to leflunomide, to low-dose sirolimus, or from calcineurin inhibitor to low-dose sirolimus [III]

Use of other adjunctive treatments, such as cidofovir, leflunomide, immunoglobulins and fluoroquinolones is discussed further.

The primary goal of therapy is to achieve control of BKV replication and preserve renal allograft function which can be monitored by serum creatinine concentration, BKV load and allograft histology. As a rule of thumb, a follow-up would consist of monitoring serum creatinine once–twice per week, plasma BKV load every 2 weeks, and allograft biopsy after two months or in case of an unsatisfactory further course. The time needed for BKV-specific immune recovery is highly variable, and may take from 4 to 8 weeks. Patients responding effectively to treatment are first identified by a decline in plasma BKV loads followed by declining urine BKV loads with a variable delay to reach curtailing of intrarenal BKV replication by >80% (41). In many patients, urine BKV load may persist at lower levels of <5 log10 geq./mL. Depending on the individual risk of acute rejection, immunosuppression may have to be reincreased.

Reducing immunosuppression should also be considered for kidney transplant patients with presumptive PyVAN defined by sustained plasma BKV loads of >4 log10 geq/mL for >3 weeks [II-2]

Strategy 1 has been applied to pediatric patients reducing the calcineurin inhibitors by 15%–20%, followed by reducing or discontinuing the anti-proliferative drugs if viremia persisted (75). Strategy 2 has been applied to adult patients discontinuing the anti-proliferative drug mycophenolic acid or azathioprine followed by reducing the calcineurin inhibitor as a second step (39). Both protocols appear to be safe and effective in adult and pediatric patients for preventing PyVAN and clearing BKV viremia without excessive rates of acute rejection during follow-up (39,75). Although no randomized controlled trials have evaluated any of these strategies, a number of observational studies and case series have reported success rates of >85% (54,76–80).

In patients with sustained high-level plasma BKV load despite adequately reduced immunosuppression, the adjunctive use of agents with antiviral activity may be considered [III]

However, there are no randomized controlled trials providing evidence that adjunctive use of agents with antiviral activity is superior over timely reduced immunosuppression.

  • 1Cidofovir, trade name Vistide® (Gilead), is a nucleoside analog, licensed by The Food & Drug Administration for the treatment of cytomegalovirus retinitis. Cidofovir has been administered intravenously for PyVAN in doses from 0.25 mg/kg to 1.0 mg/kg at 1–3 weekly intervals, without probenecid. The patients should be followed closely by frequent evaluation of serum creatinine concentration, leukocyte counts, eye symptoms and vision, as well as bi-weekly plasma BKV load. In vitro studies indicate an BKV IC-50 or IC-90 between 30 and 40 μg/mL (81,82). The literature on the clinical efficacy of cidofovir for BKV replication is conflicting, with some studies reporting apparent stabilization of function (83,84), while others report no demonstrable benefit (54,85–87). A comparison of cidofovir-treated with nontreated patients has been described but not in a randomized controlled trial (88,89). The latter study used a dose of 1 mg/kg, administered weekly as a slow intravenous infusion after hydration, for up to 10 weeks. While the cidofovir treated group had better outcome (graft loss 15% vs. 73%), the rate of urine and plasma BKV clearance was in fact not different between the treated and untreated groups. Maximal blood levels of 5 ug/mL were reached (88) which are below the IC-90 of BKV replication in vitro (82). Significant nephrotoxicity was not observed at the lower doses used, but anterior uveitis occurred in 12–35% cases in other series (89,90).
  • 2Leflunomide, trade name Arava® (Aventis) is orally administered as a replacement for discontinued mycophenolic acid with a loading dose of 100 mg for 5 days, followed by an initial maintenance dose of 40 mg. Regular blood counts and liver function tests are advisable once a month for all patients on leflunomide treatment, as well as plasma BKV loads once every 2 weeks. Leflunomide is a malonitrilamide compound inhibiting the pyrimidine synthesis that has been approved as in anti-inflammatory agent for rheumatoid arthritis. Leflunomide has immunosuppressive (38) as well as anti-viral properties, which makes it conceptually an appealing option to treat patients with BKV infection with suspected superimposed acute rejection. However, investigators that have found it to be useful also simultaneously with reduced immunosuppression. This makes it difficult to evaluate how much of the benefit could be attributed to the drug. BKV response to leflunomide has been reported to increase blood levels between 40 and 100 μg/mL in some studies (85,91,92), but not in others (87,93). Significant toxic effects have been described including hepatitis, hemolysis, thrombotic microangiopathy, bone marrow suppression and fungal pneumonia.
  • 3Intravenous immunoglobulins (IVIG) has been administered in doses commonly ranging from 0.2 to 2.0 g/kg in conjunction with reduced immunosuppression. The rationale behind IVIG therapy is that human immunoglobulin preparations contain anti-BKV antibodies, which may favorably affect the course of the disease. IVIG has been used empirically for the treatment of PyVAN. In one study, 8 renal transplant recipients received 2 g/kg IVIG with simultaneous reduction of immunosuppression. After a mean follow up of 11.4 months, 7 patients still had functioning grafts, although renal function continued to be impaired. No control arm was included in the study (94).
  • 4Fluoroquinolones may inhibit BKV replication. Some case studies suggest reduced BKV viruria levels in HCT recipients, but since these drugs are frequently given and even in vitro résistance has been reported, recommendations have to await the results of randomized controlled trials (87).

No evidence-based recommendations can be made for the treatment of PyVAN and concurrent acute rejection [III]

The clinical management of patients with PyVAN and a perceived component of concurrent acute cellular rejection is controversial. One school of thought, propagated by the Pittsburgh group (83) contends that PyVAN is itself an indication that the patient has received excessive immunosuppression, and that further steroid treatment will promote further viral replication. Another school of thought advocated by the Basel group (3,95), is to administer a limited course of steroids to treat the rejection prior to reducing immunosuppression. Clearly, the patient history, the relative predominance of signs of acute rejection such as endarteritis, fibrinoid vascular wall necrosis, glomerulitis, or C4d deposits versus those of PyVAN and the evolution of BKV loads may enter an individual treatment decision. Rapidly acting and efficacious anti-BKV drugs are needed to resolve this currently insoluble problem.

Acute rejection following reduced immunosuppression for presumptive or definitive PyVAN should be treated according to standard protocols [III]

If acute rejection is diagnosed in allograft biopsies, in the absence of PyVAN by histology and adjunct diagnostic tools and undetectable plasma BKV DNA, anti-rejection treatment is indicated and a judicious increase in maintenance immunosuppression be considered. Administration of lymphocyte depleting agents should be done with utmost caution. However, a case has been reported wherein alemtuzumab was unwittingly administered to a patient with PyVAN, who was nonetheless able to clear viremia in due course of time (96).

Anti-rejection treatment administered in patients with a history of BKV replication and/or PyVAN should be accompanied by at least bi-weekly monitoring of the urine and plasma viral load [III]

Approximately 5–40% of patients suffer from transient or persistent graft dysfunction after a diagnosis of PyVAN which can be due to PyVAN, immune reconstitution, rejection and/or increased fibrosis. Periodic measurement of donor specific antibodies has been proposed since late antibody mediated rejection can occur in patients with PyVAN maintained on low immunosuppression for extended periods of time (97).

A frequent scenario is the recurrence of viruria or viremia with interstitial inflammation, and tubulitis, but no viral inclusions, and negative in situ hybridization for viral DNA. If BKV viremia reappears, recurrence of PyVAN deserves consideration (98). On the other hand, if viruria alone is present without viremia, the interpretation of inflammation as acute rejection, viral interstitial nephritis, or an anti-viral immune reconstitution syndrome is not possible. Empirically, some centers consider to administer steroids if, in the absence of viremia, urinary viral load is <7 log10 geq/mL (98).

Prevention and Prophylaxis

Kidney transplant recipients should be screened for BKV replication to identify patients at increased risk of PyVAN [II-2]

Screening for BKV replication should be performed at least once every 3 months during the first 2 years posttransplant, and then annually until the fifth year posttransplant (Figure 1) (14). Testing urine for BKV replication allows to rule out PyVAN with a high positive predictive value of >95% and can be performed by cytology for decoy cells or by quantitative PCR for BKV DNA or VP1 mRNA (14,42,43,51,52,64,99). Patients with high-level urinary BKV replication (decoy cells or urine BKV loads >7 log geq/mL) should be tested for plasma BKV DNA load. In patients with sustained plasma BKV DNA loads of >4 log10 geq/mL for >3 weeks, a diagnosis of presumptive PyVAN is made and an allograft biopsy should be considered for a diagnosis of definitive PyVAN (14). Increases of serum creatinine from baseline are not required for the diagnosis of presumptive PyVAN. Several prospective observational studies have documented that 20–40% of kidney transplant patients develop high-level viruria (>7 log geq/mL) during the first two years posttransplant (3,39,51,52,59,75,100–102). These screening recommendations represent a compromise between costs and screening efficiency by applying, at a relatively low frequency of every 3 months, a test of very high sensitivity which also provides an average lead time of approximately 6 weeks (range 4–12 months) until histological and functional manifestation of PyVAN. Using this strategy, at least 80–90% patients at risk for PyVAN can be identified before significant histological and functional impairment occurs. Given the low positive predictive value of new onset high-level viruria, most centers do not reduce immunosuppression based solely on this observation. The positive predictive value for PyVAN in patients with high-level urinary BKV replication (BKV load >7 log10 geq/mL or decoy cells) persisting for >2 months (39,41,51), or plasma BKV loads of >4 log10 geq/mL is more than 50% (51,52,103), and further increases if high-level viremia persists for >3 weeks. Given the focality of PyVAN, the increasing detection rate with load and time likely reflects PyVAN progression. In retrospective studies, the positive predictive value of plasma BKV DNA loads was over 90%, but this evaluated mostly patients with allograft dysfunction as an indication for biopsy (104). A recent retrospective study reported that detecting three-dimensional PyV aggregates in urine by electron microscopy increased the positive and negative predictive values of PyVAN to >90% (65). For practical purposes, some centers screen for plasma BKV load, which is associated with a higher positive predictive value than urinary screening. However, given the shorter lead-time to onset of PyVAN, monthly testing for the first six months may be more appropriate (Figure 1, dotted arrow).

Figure 1.

Screening and treating BKV replication and disease in kidney transplant patients.

The caveats of the different strategies reside in suboptimal performance and timing of urine cytology, intra- and interlaboratory variability of PCR testing, and graft histology, particularly if performed outside of dedicated expert laboratories with implemented quality assurance (52,105,106). This affects the identification and management of patients at risk when scoring urine for decoy cells, or quantifying BKV DNA in urine and plasma specimens with reference to previously mentioned cut-off values. The interpretation of the center-specific PyVAN rate requires biopsies of sufficient quality, appropriate confirmatory assays such as immunohistochemistry or in situ hybridization, as well as expert histology evaluation. The screening strategy should also consider transplant center-specific expertise, the incidence of PyVAN the preferred intervention strategy and outcomes. In a simulation model assuming an 80% efficacy to clear PyVAN and a 10% risk of precipitating acute rejection following reduced immunosuppression, screening appeared to be cost-effective for PyVAN incidence rates of more than 2.1% (107). However, in two prospective nonrandomized intervention studies of adult and pediatric kidney transplant patients, no progression to PyVAN or excess of acute rejection was observed following reduced immunosuppression for presumptive PyVAN (39,75) indicating that a benefit may be obtained at lower incidence rates and more frequent monthly plasma screening. A cost analysis suggested that preventing PyVAN by screening and reducing immunosuppression may be cost saving after the second year posttransplant (108).

Despite prolonged high-level viruria, nosocomial transmission has not been formally documented. Hence, the role of infection control for prevention of BKV infection is undefined.

Retransplantation can be considered for patients after loss of a first kidney allograft due to PyVAN [II-3]

To minimize the likelihood of recurrent BKV replication and PyVAN, a period of reduced immunosuppression should precede retransplantation to allow mounting of sufficient BKV-specific immunity. Plasma and urine BKV loads may serve as surrogate markers and should be undetectable or decreasing >2 log10 geq/mL from baseline. The extent of this modification is limited by the risk of allo-sensitization and the need of immunosuppression for other organ transplants, for example pancreas. Surgical removal of the first kidney graft is not required unless preemptive retransplantation is considered during ongoing BKV replication. Induction therapy is not contraindicated, but extended periods of intense maintenance immunosuppression should be avoided. Prospective screening for BKV replication and early intervention should be implemented as recommended for first time transplants. Recipient and donor should be informed about the potential risks of retransplantation as well as PyVAN recurrence and graft loss. Retransplantation after kidney allograft due to PyVAN has been successfully performed in the approximately 80–90% of reported cases (109). The factors associated with success have not been conclusively determined and are thought to depend on mounting of sufficiently robust BKV-specific immunity, organs from living, preferably related, donors, immunosuppression in the lower range of normal and close surveillance and intervention for BKV replication posttransplant. Surgical removal of the primary transplant has been performed in approximately half of all cases, but did not protect against recurrent BKV replication and PyVAN (109). Primary graft nephrectomy should be considered in case of preemptive retransplantation with ongoing BKV replication and detectable plasma BKV loads (109–111).

There are currently no data to recommend testing of BKV-specific antibodies in kidney transplant donors or recipients [III]

The detection of BKV-specific IgG identifies donors and recipients who have been exposed to, and are hence infected with, BKV. However, there are no standardized serological assays validated for routine clinical use in transplantation. Using different assays, researchers observed decreasing seroprevalence, decreasing IgG titers and reversion to seronegative status in older individuals, in patients with terminal renal failure on hemodialysis, and in immunosuppressed patients (26,30,36). Single-center studies report that enzyme immunoassays (EIA) using BK virus-like particles (BKVLP) assembled in vitro from recombinant VP1 may be more sensitive but equally specific compared to hemagglutination inhibition and neutralization tests (26,34,112). However, direct comparisons of assays in clinical studies of diagnosis and their role on guiding the management have not been conducted. Potential roles of BKV serology stem from the following observations: High BKV-specific IgG titers in the donor have been associated with a higher incidence of viruria in the recipient, compared to seronegative donors (49). Recipients with low or undetectable BKV antibodies may be at higher risk of BKV viremia than seropositive recipients (49,113,114), as are recipients failing to mount a BKV-specific IgA response at week 1 posttransplant (115). Longitudinal studies in kidney transplant patients indicate that BKV IgG and IgM increase as markers of recent or ongoing BKV exposure, but are not associated with identifiable protection (26,34,75,115–119). Overall hypogammaglobulinemia has not been studied formally, but it is not likely to be a risk factor given the BKV antibody responses.

There are currently no data to recommend testing of BKV-specific T-cell responses in kidney transplant donors or recipients [III]

BKV-specific T-cells can be quantified in peripheral blood mononuclear cells (PBMC) of kidney transplant patients after stimulation with BKV antigens, peptides or peptide pools using flow cytometry and MHC-labeling, intracellular cytokine staining, or by cytokine release assays such as enzyme linked immunospot assays. However, no unequivocal thresholds have been established that allow discriminating kidney transplant patients at increased risk of PyVAN from those that are protected. Potential roles of BKV-specific T-cell assays have been obtained from cross-sectional studies identifying significantly higher responses in patients clearing BKV viremia and PyVAN than in patients with increasing and persisting BKV replication. Besides the diverse assay formats and lack of standardization, further limitations stem from the fact that BKV-specific T-cell responses measured after direct stimulation of PBMC are approximately 10- to 50-fold lower than cytomegalovirus- or Epstein-Barr virus-specific responses (33). Larger responses are obtained after 9- or 21-day in vitro culture following antigen-specific stimulation. However, these responses may no longer be representative of the actual antiviral immune control exerted in patients at the time of blood sampling.

Screening of nonrenal transplant recipients for BKV replication is not recommended [II-2]

In heart, liver and lung transplant recipients have a 5%–25% incidence of viruria, but viremia is uncommon and PyVAN extremely rare (45,46,120–124). When PyVAN is diagnosed in the autologous kidneys, treatment by reducing immunosuppression is limited by the threat of rejection of a vital graft, with no possibility for artificial replacement (47,48). In most cases, treatment with cidofovir was not effective (48).

Acknowledgments

Supported by grants R01 AI 51227, R01 AI 63360 and N01 AI30044 to PR; and Swiss National Fund grant 320080-11004011 to HHH.

Disclosures

Hirsch H.H.: Consultant, Norvatis, Astellas, Wyeth, Chimerix, Pfizer, Roche; Lecture fees, Novartis and Roche. Randhawa, P.: The author has nothing to disclose.

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