Infectious Disease Pathology
Case 2 -
BK Polyomavirus Nephritis
BK Associated Hemorrhagic Cystitis
David H. Myerson
Fred Hutchinson Cancer Research Center
University of Washington
Click on each slide thumbnail image for an enlarged view
A 56 year old male, 163 days post-marrow transplant for refractory follicular lymphoma. He was
prepared with fludarabine, total body irradiation (TBI), and cyclophosphamide, and engrafted from his HLA
haplo-identical brother. His course was complicated by bacterial, viral, metabolic and neoplastic
disease. Ten days post-transplant he developed microscopic hematuria, with urine BK polyomavirus
detected at 9x107 copies/ml. Thirty-eight days post-transplant his condition deteriorated
with gross hematuria and passage of clots. Concurrent urine BKV level was 8 x 108 genomes/ml.
He became free of gross hematuria after a month, with a urine BKV level of 3 x 105 genomes/ml.
Serum PCR detected 2400, 390, and 1100 BK genomes/ml respectively. Renal function remained slightly
compromised but adequate, with a creatinine up to 2.0. Other complications included graft-versus-host
disease (GVHD). Ultimately recurrent malignancy and a Zygomycetes intervened. The kidney was obtained
The H&E slide of the kidney shows a slight interstitial lymphocytic infiltrate. There is a single
disrupted tubule with epithelial cells containing large hyperchromatic nuclei, and with epithelial cell
swelling and necrosis. This tubule shows BK polyomavirus DNA by in situ hybridization.
Case 2 - Figure 1 - BKV nephropathy. Several hyperchromatic enlarged nuclei are present in the tubular epithelium of a single isolated tubule.
Case 2 - Figure 2 - In situ hybridization with digoxigenin-labeled BKV probe shows the same tubule with heavily staining cells, indicating many copies of BKV. The same result is seen under stringent conditions, avoiding potential detection of other polyomaviruses (e.g. JCV, SV40).
Viruses which may be histologically associated with nephritis in the bone marrow transplant recipient
are BK polyomavirus, adenovirus and CMV.
Diagnosis: BK polyomavirus nephritis, and BK associated hemorrhagic cystitis.
BK virus is a polyomavirus. Polyomaviruses were first isolated by Ludwig Gross in 1953 in studies of
mouse leukemia and cause solid tumors at multiple sites. The Simian Virus 40 polyomavirus was discovered
in 1960 by Sweet and Hillman in rhesus monkey kidney cells, contaminating the Salk and Sabin polio
vaccines. SV40 has been extensively studied in the interim, expressing oncogenic proteins including
large T and small t. SV40 was the first organism to be genetically mapped, and one of the first to be
BK polyomavirus (BK, BK virus, BKV) was isolated in 1971 from the urine of a renal transplant
recipient, and named after the initials of the individual. It's genomic sequence is 69% homologous to
SV40 and 75% homologous to the related JC polyomavirus (also named after the initials of an individual),
the cause of progressive multifocal leukoencephalopathy (PML). 
The polyomaviruses were originally classified with the papillomavirses as Papovaviridae but are now
placed in a separate family, the Polyomaviridae. There are 13 known members, including 2 human members,
BKV and JCV, and one member that has infected humans presumably through contaminated polio vaccine, SV40.
Host range is narrow.
They are small nonenveloped viruses with an icosahedral shell, 45 nm in diameter. The genome is
double stranded circular supercoiled DNA encoding 5-6 proteins. The capsid is composed of 72 pentamers,
each pentamer made up of 5 molecules of VP1. There is one copy of VP2 or VP3 per pentamer, not
surface-exposed. The pentamer may contact the usual 5 neighbors, or it may contact 6 neighbors acting
like a hexamer. The virion contains 12 pentamers in the pentameric configuration and 60 in the hexameric
configuration. The BKV genome has a length of 5153 base pairs (Dunlop Strain), and is complexed with
cellular histones. There are 4 subtypes of BK viruses, separated by antigenic differences with
concurrent differences in DNA sequence. Rearrangement of the regulatory regions have been found in some
cases of polyomavirus disease, of possible biological significance.
BKV infection usually occurs early in life, with 50% of children infected by age 3, and 90% or more by
age 10. Infection is believed to be usually asymptomatic or a mild respiratory infection. Latency
continues throughout life. Viruria is present in 0.3%of healthy adults and 3% of pregnant women. BKV is
found and presumed latent in the kidney, renal pelvis, ureters, bladder and B-lymphocytes of normal
In the kidney transplant recipient BKV is associated with BK tubulointerstitial nephritis and renal
failure, as well as ureteral stenosis.
The origin frequently appears to be from the transplanted
kidney. Sensitive but not specific diagnosis may be made from the urine by the detection of cells with
the BKV nuclear inclusions "decoy cells".  30% or more of renal transplant recipients are viruric.
 Plasma BKV levels are highly correlated to failure of the transplanted kidney. The gold standard is
a renal biopsy showing BKV inclusions associated with a tubulointerstitial nephritis, sometimes
delineated by antibody studies, in situ hybridization, or electron microscopy.
demonstrates a mixed inflammatory infiltrate with focal tubular injury. The tubular epithelium shows
marked anisonucleosis, nuclear atypia and basophilic or amphophilic intranculear inclusions. Tubulitis
is frequent. A florid regenerative response with prominent nucleoli may develop. However, "cells
infected with the virus incite a mononuclear or polymorphonuclear interstitial infiltrate and focal
inflammation of the tubules that may closely resemble acute rejection, and distinguishing between viral
cytopathologic effects and other changes related to nuclear degeneration can be difficult. Therefore,
the identification of virally infected cells relies heavily on immunohistochemical analysis or in situ
hybridization."  The etiology of the renal failure appears certainly due to BK, as it is often
effectively treated by lowering immunosuppression or instituting antiviral therapy with cidofovir. This
strongly suggests that BKV is a pathogen rather a bystander to an immune mediated disease, or a bystander
to renal rejection (host-versus-graft reaction). BKV nephritis appears to be a cause of renal failure
and graft loss in the kidney transplant recipient.
BK shows consistent reactivation in bone marrow transplant recipients, with more than 53% of
allografted patients show BKV in the urine by PCR.
Plasma BKV was detected in 34% of the marrow
transplant recipients.  Unlike the kidney transplant recipient, BK viremia in the marrow transplant
recipient is not clearly associated with clinical renal failure. The tissue biopsy in this case report
suggests why this may be so. Only a single nephron appears to be infected in the tissue block analyzed.
Taken across the entire kidney, this suggests that the infection is limited to an insufficiently large
number of nephrons to cause clinical renal failure.
In bone marrow transplant recipients, in contrast to renal transplants, BKV is strongly associated
with late-onset hemorrhagic cystitis. High urine loads of BKV, assessed by quantitative PCR, correlates
with hemorrhagic cystitis.  This supports a causative, rather than a passenger, role for BKV in bladder
disease. Although it is possible that more than one insult is necessary to produce hemorrhagic cystitis,
BKV and busulfan, for example, or BKV and graft-versus-host disease, the data support the proposal that
BKV is an essential part of the etiology of late-onset hemorrhagic cystitis in the hematopoietic stem
cell transplant recipient.
In addition to routine stains in light microscopy, identification of BKV infected cells in kidney
tissue has been performed by immunohistochemistry. Most studies have used one of a variety of antibodies
to SV40 large T antigen or other antibodies of similar cross reactivity. These antibodies generally do
not differentiate between BKV, JCV and SV40, reacting with all. Additional, more specific, testing is
required to confirm the specific presence of BKV. Non-specific background staining of the tubular
epithelial cells is sometimes a problem.
BKV may be detected by in situ hybridization. The commercially available biotin-labeled DNA probe
(Enzo Life Sciences, Inc.) shows good signal, but the intrinsic background of biotin detection systems
may be high. A digoxigenin-labeled DNA probe has been developed for detecting BK virus in
formalin-fixed, paraffin embedded tissue.  The hybridization signal is strong and there is no
perceptible background staining, the second slide showing this result. With the probe, in situ
hybridization conditions can be adjusted for specific BKV detection, or detection of JCV as well as BKV.
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