BK virus in cell culture: infectivity quantitation and sequential expression of antigens detected by immunoperoxidase staining

Phosphorylation of Ser-80 of VP1 and Ser-254 of VP2 is essential for human BK virus propagation in tissue culture

BK virus (BKV) infection may cause polyomavirus-associated nephropathy in patients with renal transplantation. Recently, the phosphorylated amino acids on the structural proteins VP1, VP2 and VP3 of BKV have been identified by liquid chromatography-tandem mass spectrometry in our laboratory. In this study, we further analysed the biological effects of these phosphorylation events. Phosphorylation of the BKV structural proteins was demonstrated by [(32)P]orthophosphate labelling in vivo. Site-directed mutagenesis was performed to replace all of the phosphorylated amino acids. The mutated BKV genomes were transfected into Vero cells for propagation analysis. The results showed that expression of the early protein LT and of the late protein VP1 by the mutants VP1-S80A, VP1-S80-133A, VP1-S80-327A, VP1-S80-133-327A and VP2-S254A was abolished. However, propagation of other mutants was similar to that of wild-type BKV. The results suggest that phosphorylation of Ser-80 of VP1 and Ser-254 of VP2 is crucial for BKV propagation.

Utilization of vero cells for primary and chronic BK virus infection

BK virus (BKV) nephropathy has a poor prognosis for renal allograft survival with 30% to 60% risk of allograft loss over 1 year. In the past decade, BKV nephropathy has occurred in 1% to 10% of renal transplant patients, with higher rates observed in patients with increased immunosuppression exposure and renal allograft injury. Vero cells (Green monkey kidney cell origin) were optimized for BKV primary and chronic infection inclusive of culture requirements for 60-day growth and monolayer confluence. Quantification of BKV replication in the culture supernatant (SN) and cells was by real-time polymerase-chain reaction (PCR) using the Roche Lightcycler 2.0. Primary BKV infection of Vero cells is achieved by 2 hour incubation with 6.5 x 10(5) BKV copies with subsequent washing of cells leading to steady-state cellular infection of 10(2) to 10(3) BKV copies. Primary infection is demonstrated within 7 to 10 days by a >10-fold increase of BKV copies in SN. Thereafter, a BKV viral load reduction in SN to a chronic/latent level (<10(2) BKV copies in SN) is observed by 14 days. Vero cells with chronic low-level BKV infection (10(2)-10(3) BKV copies in cells) exhibited reactivation (>10(5) BKV copies in SN) in >72% of late culture wells after 40 days. Vero cells can accommodate primary and chronic BKV infection followed by viral reactivation in late culture. The performance characteristics of 3 different pathogenic BKV strains obtained from patients with BKV nephropathy had infectivity profiles that correlated well the relative clinical profile in this Vero cell culture system.

Impact of a polyomavirus (BKV) infection on mRNA expression in human endothelial cells

Polyomavirus BK-associated nephropathy (PVAN) is an emerging cause of early renal transplant failure. In order to learn more about the cellular response to BK virus, microarrays were used to study its effect on mRNA expression in human endothelial cells. The oligo-based, 35k arrays used cover the predicted 25,000 human protein-expressing genes, and distinguish between a number of alternatively spliced mRNAs. Four parallel experiments were performed for each of two time-points (24 and 40 h) during the first round of the 48 h viral replicative cycle. Immunoperoxidase staining demonstrated that the pulse exposure to virus caused infection in at least 75% of the cells. At 24 h, 55 genes were more than doubly up-regulated and 249 genes were similarly down-regulated; at 40 h, the numbers were 242 and 104, respectively. Gene ontology analyses suggested that immune/defence response genes were selectively down-regulated. Genes involved in cell division and DNA replication tended to be up-regulated, which may reflect an attempt on behalf of the virus to promote viral replication. Genes associated with PVAN were not induced, suggesting that these genes are not required for viral replication, but rather reflect circumstances specific for the disease. Only a few immuno-related genes were turned on, including the interferon response genes G1P2 and IFIT3. However, some of the up-regulated genes of unknown function may be involved in viral defence.

BKV and SV40 infection of human kidney tubular epithelial cells in vitro

The interaction of BKV with its natural target cells, human kidney epithelial cells, has not been studied. In vitro infections of human primary kidney epithelial cells were performed to investigate a BKV infection in its natural host cell. BKV undergoes a lytic replication cycle in this system: high levels of T antigen expression were first detected at 36 h postinfection, while viral DNA replication, capsid protein expression, and progeny virus were observed at 48 h postinfection. It was observed that the related polyomavirus SV40 is incapable of infecting human kidney epithelium except in the presence of the GM1 ganglioside, recently reported to be an SV40 receptor.

Human polyomavirus BK (BKV) transiently transforms and persistently infects cultured osteosarcoma cells

Human polyomavirus BK (BKV) DNA and proteins have been detected in a number of bone tumours. We therefore investigated whether BKV infection might initiate transformation of human anchorage-dependent osteosarcoma cells in vitro. Infection of the osteosarcoma cell line U-2OS with a naturally occurring BKV strain resulted in soft agarose competent cell clones. In a subclone, designated U-2OS15E, approximately 10-20% of the cells contained episomal BKV genomes. A corresponding proportion of cells expressed BKV proteins and produced viral progeny. This proportion was not increased by BKV superinfection. Furthermore, U-2OS15E cells were resistant to SV40 infection. The transformed status of U-2OS15E cells lasted only for a few passages. However, the persistently infected cells produced infectious virions for more than 300 generations. In addition to representing a model system for persistent BKV infection, the uninfected and persistently BKV-infected cell cultures are useful tools for control and calibration of in situ BKV nucleic acid and protein detection methods.

Lactoferrin and cyclic lactoferricin inhibit the entry of human cytomegalovirus into human fibroblasts

Lactoferrin is mainly produced by polymorphonuclear leukocytes and has been demonstrated in mammalian milk and external secretions. Lactoferrin is an iron-binding, multifunctional protein and may play an important role in immune regulation and in defense mechanisms against bacteria, fungi and viruses. Lactoferricin is a potent antimicrobial peptide generated from the N-terminal part of lactoferrin by pepsin cleavage. We demonstrate that lactoferrins from different species and its N-terminal peptide lactoferricin (particularly the cyclic form) inhibit expression of early and late antigens, as well as production of infectious viral progeny during human cytomegalovirus (HCMV) infection in vitro. Iron-saturated lactoferrin did not affect HCMV antigen expression. Heparin had the same effects as iron-depleted lactoferrin. Yet, mixtures of lactoferrin and heparin did not inhibit HCMV multiplication i.e. lactoferrin and heparin seemed to mutually block each other's antiviral activities. HCMV-infected cells exposed to lactoferrin and cyclic lactoferricin contained less intracellular virus than unexposed cells. The antiviral activity of cyclic lactoferricin was more than seven-fold weaker than that of the maternal molecule. Lactoferrin and cyclic lactoferricin prevented HCMV entrance into the host cell.

The agnogene of the human polyomavirus BK is expressed

Primate polyomavirus genomes all contain an open reading frame at the 5' end of the late coding region called the agnogene. A simian virus 40 agnoprotein with unknown functions has previously been demonstrated. We now show that a BK virus agnoprotein appears in the perinuclear area and cytoplasm late in the infectious cycle. It is phosphorylated in vivo and coimmunoprecipitates with a subset of host cell proteins.

The human polyomavirus BK T antigen induces gene expression in human cytomegalovirus

Co-infections or co-habitations of cells by two or more viruses may occur in the human organism. Human cytomegalovirus (HCMV) and the human polyomavirus BK (BKV) have common host cells and may both establish lifelong latency/persistence following primary infection. Both viruses may become reactivated by immunosuppression or other conditions which upset host-virus balance, and they encode gene products with the inherent potential of acting as heterologous transacting factors for expression of cellular or viral genes. It has been shown that HCMV induces gene expression and replication of primate polyomaviruses. We now demonstrate that BKV is able to enhance the expression of HCMV immediate early (IE1 and 2) as well as the early (E) protein pp65 during double infections in semi-permissive cells. By transfection experiments it was established that the phenomenon is due to heterologous transcriptional transactivation of the HCMV major IE promoter (MIEP) by the BKV large T antigen, without contribution from the small t antigen.

In vivo expression of a single viral DNA-binding protein generates systemic lupus erythematosus-related autoimmunity to double-stranded DNA and histones

Although the origin of autoimmune antibodies to double-stranded DNA is not known, the variable-region structures of such antibodies indicate that they are produced in response to antigen-selective stimulation. In accordance with this, results from experiments using artificial complexes of DNA and DNA-binding polypeptides for immunizations have indicated that DNA may induce these antibodies. Hence, the immunogenicity of DNA in vivo may depend upon other structures or processes that may render DNA immunogenic. We report that in vivo expression of a single DNA-binding protein, the polyoma virus T antigen, is sufficient to initiate production of anti-double-stranded DNA and anti-histone antibodies but not a panel of other autoantigens. Expression of a mutant, non-DNA-binding T antigen did result in strong production of antibodies to the T antigen, but only borderline levels of antibodies to DNA and no detectable antibodies to histones. Nonexpressing plasmid DNA containing the complete cDNA sequence for T antigen did not evoke such immune responses, indicating that DNA by itself is not immunogenic in vivo. The results represent a conceptual advance in understanding a potential molecular basis for initiation of autoimmunity in systemic lupus erythematosus.

Detection of BK virus DNA in nasopharyngeal aspirates from children with respiratory infections but not in saliva from immunodeficient and immunocompetent adult patients

Our understanding of important stages in the pathogenesis of the human polyomavirus BK virus (BKV) and JC virus (JCV) infections is limited. In this context, nasopharyngeal aspirates from 201 children with respiratory diseases and saliva from 60 human immunodeficiency virus type 1-infected adults and 10 healthy adult controls were collected and analyzed for the presence of BKV and JCV DNA by PCR. Neither BKV nor JCV DNA was detected in the saliva specimens. We demonstrated BKV DNA, but no infectious BKV, in 2 of 201 nasopharyngeal aspirates. Each sample contained one unique rearranged noncoding control region variant of BKV. The results indicate that (i) BKV and JCV are not regularly associated with respiratory infections in children requiring hospitalization, (ii) nasopharyngeal cells are not an important site for primary replication of human polyomavirus BKV and JCV, and (iii) the salivary glands and oropharyngeal cells seem not to be involved in BKV and JCV persistence. We propose that for the polyomaviruses BKV and JCV the alimentary tract should be considered as a portal of entrance to the human organism.

A steroid hormone response unit in the late leader of the noncoding control region of the human polyomavirus BK confers enhanced host cell permissivity

The effect of steroid hormones on multiplication of the human polyomavirus BK (BKV) was studied. Physiological concentrations of the synthetic glucocorticoid dexamethasone, progesterone R5020, or estrogen 17 beta-estradiol enhanced the permissivity of the host cell for BKV, resulting in an up to 11-fold (dexamethasone), 5-fold (progesterone), or 3-fold (17 beta-estradiol) higher virus yield. The increase in virus yield in dexamethasone-stimulated cells correlated with enhanced steady-state levels of viral transcripts. The late leader sequence of the BKV control region contains a hormone response unit composed of a nonconsensus glucocorticoid and/or progesterone response element (GRE/PRE) and a fully consensus estrogen response element (ERE). DNA-protein binding studies showed that the glucocorticoid receptor and the progesterone receptor bound to this BKV GRE/PRE-like sequence, while the estrogen receptor could bind to the BKV ERE motif. By transient transfection assays, we were able to show that these sequences can mediate steroid hormone-induced gene expression. However, no cooperative transactivation effect between the BKV GRE/PRE-like motif and BKV ERE motif was observed. This BKV hormone response unit may play an important role in vivo by enhancing a productive BKV infection, and perhaps also by reactivating a latent infection, during physiological or pathological conditions accompanied by increased steroid hormone levels.

Antibodies to eukaryotic, including autologous, native DNA are produced during BK virus infection, but not after immunization with non-infectious BK DNA

The contemporary view concerning the origin of anti-dsDNA antibodies is that eukaryotic dsDNA is not immunogenic. Results presented here, however, show (1) that inoculation of rabbits with BK virus elicits antibodies to eukaryotic, including autologous, dsDNA, (2) that the transition from a non-immunogenic to an immunogenic state of autologous dsDNA depends on productive infection with BK virus, and (3) that inoculation with protein-free circular BK dsDNA initiates both infection in vivo and production of antibodies to autologous dsDNA. Non-infectious linearized BK dsDNA did not elicit any anti-dsDNA antibodies, while the same DNA molecule, when complexed with methylated bovine serum albumin, elicited anti-dsDNA antibodies solely recognizing BK dsDNA. Neither of the two linearized BK dsDNA preparations initiated infection. Using two different techniques, we could demonstrate that two separate sets of anti-dsDNA antibodies were produced during viral infection; one recognizing BK dsDNA, and the other recognizing autologous dsDNA. Thus, in contrast to previous assumptions, autologous dsDNA may be immunogenic. Based on the present results, we propose that autologous dsDNA can be rendered immunogenic through complex formation with viral DNA binding protein(s) such as the structural protein VP1 or the tumour antigen T. Such DNA-protein complexes may bypass a putative T-cell tolerance to autologous dsDNA.

Transformed rat pancreatic islet-cell lines established by BK virus infectionin vitro

Single-cell suspensions of primary rat pancreatic islet cells were infected with BK virus (BKV) prototype (Gardner) and the naturally occurring BKV strain (TU), isolated from human urine. These viral strains have different sequences in their non-coding control regions which contain promoter-enhancer elements and origin of DNA replication. Uninfected cell cultures disintegrated within 3 weeks, while a varying number of virus-infected cultures were immortalized and a majority exhibited focal areas of 2-dimensional growth. A significantly higher proportion of the BKV (TU)- than of the BKV (Gardner)-infected cultures were immortalized. Large tumor (T) antigen expression was evident in virus-infected cultures from day 4 post infection (p.i.), while insulin secretion decreased steadily during the first weeks of culture. Two cell lines were established from BKV (TU)-infected cultures. Line 5A4 was contact-inhibited, growing as a dense monolayer, while 6A3 demonstrated foci of 2-dimensional growth. Both cell lines have retained their morphology and T-antigen expression for approximately 130 passages. At high passages a low level of intracellular insulin, but no secretion into media, was detected. Based on standard biological tests (reduced serum requirements, growth at low cell density, anchorage-independent growth and tumor induction in nude mice) both cell lines have a fully transformed phenotype, but 6A3 appears to be more malignantly transformed. Since both cell lines were established simultaneously from the same primary cells with the same virus batch, they provide an opportunity to study the transforming mechanisms of BKV in a relative context.

Amplification and sequencing of the control regions of BK and JC virus from human urine by polymerase chain reaction

The various strains of BKV and JCV exhibit a remarkable degree of heterogeneity in the noncoding region near the origin of DNA replication. It is of great interest, therefore, to characterize the naturally occurring variants as a first step towards the attainment of an understanding of the origin and the biological significance of this hypervariability. In this paper we report the use of polymerase chain reaction for amplification and sequencing of the control regions of BKV and JCV DNAs from urines of AIDS patients, bone marrow transplantation recipients, and other patient groups. Our results support the conclusion that BK(WW) and its variants constitute the most prevalent strain in the human population tested so far. A new strain, designated BK(TU), was isolated from some patients from Norway. Urine inocula containing BK(WW) gave BK(TU) after propagation in cell culture, while BK(TU) did not change the sequence of its control region during the same procedure. The JCV isolates were almost identical with several strains molecularly cloned from the urine reported by Y. Yogo, T. Kitamura, C. Sugimoto, T. Ueki, Y. Aso, K. Hara, and F. Taguchi (J. Virol., 1990, 64, 3139-3143). This archetypal strain may represent the JCV circulating in the human population, from which various regulatory sequences of JCV isolates could have evolved by deletions and amplifications.

At least two types of control regions can be found among naturally occurring BK virus strains

The various strains of BK virus (BKV) exhibit a remarkable degree of heterogeneity in the transcriptional control region, which may affect the biological characteristics of a BKV strain. We describe the detection and sequencing of BKV control regions directly from urine samples and after propagation in cell culture. A BKV strain [BKV (TU)] with a control region anatomy not described earlier, as well as a BKV (WW)-like strain [BKV (WWT)], was detected in urine samples by direct sequencing of polymerase chain reaction products. Urine inocula containing BKV (WWT) yielded BKV (TU) upon one passage in cell culture, while BKV (TU) did not change its control region during propagation in cell culture. Analysis of the nucleotide sequence of the transcriptional control regions revealed a partial deletion and duplication in BKV (TU) compared with BKV (WWT). In addition, the control region of BKV (TU) contains two point mutations relative to BKV (WWT). This indicates that both virus strains were probably present in the BKV (WWT)-dominated urine inocula, rather than that BKV (WWT) genomes were rearranged into BKV (TU) genomes during cell propagation. The heterogeneity of the control region of BKV strains is discussed in relation to both confirmed and putative transcription factor-binding sites.

An indirect immunofluorescence method for detection of infectious BK virus in urine

An indirect immunofluorescence (IF) method is described for the detection of infectious BK virus in urine within seven days in contrast to up to three months or longer using routine tissue culture. Virus is pelleted from the urine, inoculated onto pre-formed monolayers of human embryo lung (HEL) fibroblasts, and infected cells are detected in an indirect fluorescent antibody test using a human serum. The sensitivity of the IF method is 91%. Positive isolates may be confirmed as BK virus using specific rabbit antisera on the original inoculated cultures. Furthermore, a virus stock may be grown up by passage from the original culture fluids for further studies such as DNA analysis. As a broadly-reactive human serum is used for screening the cultures, other viruses which grow in HEL cells, such as CMV, may also be detected.