BK virus (Polyomavirus): Isolation in cell culture; Molecular diagnosis (PCR). 

Information 10-12-2019.

BK virus infection occurs mainly in childhood, and establishes persistent infections within the renal tubular cells and the urothelium, with minimal clinical implications in immunocompetent patients. However, in recent years it has taken center stage as an increasingly large problem in renal transplantation, and is a cause of deterioration or lost of the graft function.

The BK virus was isolated for the first time in 1971 in a kidney transplant patient who eliminated urine with atypical nuclear morphology in the urine, and was named from the initials of that patient. It belongs to Polyomavirus genes in the Polyomaviridae family, like the JC virus which shares a large part of the genome, or the SV-40 virus (simian vacuolationg 40 virus). These are small, naked viruses, with a size of about 42 nm in diameter, with an icosahedral symmetry capsid that houses a double-stranded circular DNA (dsDNA) genome composed of more than 5,000 pairs of nucleotides. Functionally, the genome of this virus can be divided into three regions: an early transcription region, which fundamentally encodes two non-structural proteins, the T-large antigen and the T-short antigen, which regulate viral replication and control the viral lytic cycle; a late region that encodes the three capsid proteins, called VP1, VP2 and VP3, and the agnoprotein, responsible for the assembly of the viral capsid and the release of virons from infected cells; and an intermediate or regulatory region that encodes elements for transcription control.

Four different BK virus genotypes have been described depending on the variation of the VP1 capsid proteins. Genotype I is the most prevalent in humans (distributed worldwide), followed by type IV (present in Europe and Northeast Asia). Type IV is further subdivided into 4 subtypes, although it is not clear whether this is important from the pathogenic point of view. Genotypes II and III are less frequent. These genotypes do not show differences in aggressiveness, but sometimes they cause difficulties for diagnosis, since most laboratory techniques employ the detection of VP1 or long T antigen in the infection monitoring, which primarily is useful to diagnose the serotype I.

The primary infection usually occurs in the first decade of life, being in most cases asymptomatic or with mild respiratory symptoms. Seroprevalence in the adult population is greater than 80%. The source of infection is exclusively human, it is not proven that there are animal reservoirs, and the route of transmission, although not known in detail, can be fecal-oral, respiratory, transplacental and through donated tissues.

After primary infection, the virus colonizes the urinary tract and remains dormant in the tubular and urothelial cells, without usually causing any complications in immunocompetent hosts. However, when a decrease in immune defenses occurs, the virus begins to replicate in the epithelial cells of the kidney, ureter and bladder. Apparently, the virus initially replicates in the cells of the distal tubular epithelium, which causes necrosis and initiates a process of local injury and inflammation, and when the virus is released, it is eliminated with urine. Then there is denudation and rupture of the basal tubular membrane, with which the infection reaches the intertubular space and the peritubular capillaries, producing viremia. In addition to replicating in transplanted patients, it has also been described in individuals infected with human immunodeficiency virus (HIV), disseminated lupus erythematosus and other pathologies related to decreased immunity.

In renal transplantation, the prevalence of poliomavirus BK-associated nephropathy (PBKAN) ranges from 1% to 10%, depending on the immunosuppressive regimen and the diagnostic methods used. Once the diagnosis of PBKAN has been established, viremia monitoring is the method that best reflects the severity of tissue damage by the virus. In the temporary course of infection, viruria precedes viremia. In addition, when viruria appears, it has a diagnostic value predictive of viremia, especially when it is sustained (2 or more consecutive determinations separated between 1 and 2 months). Patients with sustained viruria have a higher viral load than those with transient viruria, and a positive correlation between viral load in urine and viral load in blood has been shown.

The importance of diagnosing patients with viruria or viremia early when nephropathy occurs, the risk of graft lost increases twice.

The main diagnostic methods used in the clinic are:

  • Urine cytology: microscopic observation of the urine sediment, which shows the presence of urothelial epithelial cells infected by the virus, with large basophilic nuclei with nuclear viral inclusions. This technique does not allow to differentiate the infection between the BK virus and the JC virus. This procedure has the disadvantage of requiring sufficient experience to be able to differentiate these cells.
  • Viral load in blood or urine: consists of the detection of viral DNA by amplification by real-time polymerase chain reaction (rt-PCR). Most PCR assays are based on probes against genotype I and, although they can also detect other genotypes, they are much less sensitive. It is assumed that the cut-off point to consider significant viruria is 107 copies, while the viremia cut-off point to predict nephropathy has been set at 104 copies.
  • Renal biopsy: the histopathology of BK virus nephropathy presents as tubular necrosis and focal cell infiltration and/or fibrosis, which can be confused with acute cell rejection. Infected cells can be stained by immunohistochemical methods with monoclonal antibodies against SV-40 (Simian Vacuolating virus 40).