Membrane antigens associated with infection, transformation, and tumorigenesis by polyoma virus

Infection, transformation, or tumorigenesis by Py virus leads inter alia to modifications in the membrane antigens of the affected cells. The modifications include antigenic gains or losses or quantitative changes in both directions. Although there is a pronounced common denominator in the antigenic alterations in the three distinct Py-induced biological processes, it is nontheless possible that each of them is characterized by specific antigenic modifications. This possibility has yet to be analyzed. Specific antigenic modifications, if they occur, are probably the result of different selective processes and adaptions to these pressures.

In this brief review, we have attempted to survey the literature pertinent to this aspect. While doing so, we discovered that most researchers have not considered the possibility that differences could exist between antigens of cells infected by Py, cells transformed by this virus, and Py-induced tumor cells. We feel that a comprehensive antigenic comparison between these cells utilizing well-defined reagents is an essential prerequisite to understanding of the successful immunological surveillance against Py-induced malignancy.

T-antigen expression by polyoma mutants with modified RNA splicing

Polyoma virus mutants were constructed that could not express all the three T-antigens. The mutagenesis was directed to the two 5' splice sites utilized in the maturation of early RNA. The mutant bc1051 had a base change at the splice site of large T-antigen mRNA, and the mutants dl1061 and dl1062 had deletions at the corresponding splice point of small and middle T-antigen mRNA. The site was removed in mutant dl1061 and altered by fusion to upstream sequences in mutant dl1062. Analysis of viral RNA showed that dl1061 and dl1062 formed only large T-antigen mRNA, whereas bc1051 did not produce this RNA-species. However, the biological properties of dl1062 suggested that it also produced mRNA directing the synthesis of a small T-antigen-related polypeptide, at least in low amounts. Only mutant bc1051 could induce transformation of rat cells. In mouse 3T3 cells dl1062 multiplied to a limited extent, while bc1051 and dl1061 failed to produce virus. However, dl1061 DNA was synthesized at a low rate which could be increased to normal levels by co-transfection with mutant bc1051. This result suggests that polyoma small and middle T-antigen have a previously unrecognized function in the early phase of the infection process, or in viral DNA-synthesis.

Antibodies against a nonapeptide of polyomavirus middle T antigen: cross-reaction with a cellular protein(s)

Antibodies were raised against the sequence Glu-Glu-Glu-Glu-Tyr-Met-Pro-Met -Glu, which represents a part of the middle T antigen of polyomavirus that is considered to be important in inducing the phenotype of transformed cells. The antibodies reacted with native as well as denatured middle T antigens. In addition, the antibodies immunoprecipitated a cellular protein with an apparent molecular weight of 130,000 (130K) from mouse and rat cells. In some cases, a 33K protein was also immunoprecipitated. Immunoprecipitation of middle T antigen as well as 130K and 33K proteins was blocked by the peptide. The antibodies labeled microfilaments of untransformed mouse, rat, human, and chicken cells by immunofluorescence. This labeling was also blocked by the peptide. The labeling pattern and distribution under a variety of conditions were indistinguishable from those of anti-actin antibodies, although no evidence has been obtained to indicate that the anti-peptide antibodies react with actin. The 130K protein migrated in sodium dodecyl sulfate-polyacrylamide gel electrophoresis slightly slower than chicken gizzard vinculin (130K) and slightly faster than myosin light-chain kinase of chicken smooth muscle (130K). Neither of these proteins absorbed the anti-peptide antibodies. The 33K protein does not seem to be tropomyosin (32K to 40K).

Isolation and characterization of polyoma nucleoprotein complexes

A method for the isolation of polyoma nucleoprotein complexes has been developed using neuraminidase treatment of infected cell lysates. At least three distinct forms of polyoma virion intermediates were identified by their [3H]thymidine labeling kinetics and sedimentation coefficients: a rapidly labeled 95 S "replicating complex" which chases to a 75 S minichromosome and then to a 240 S virion structure. The general properties of these distinct intermediates were similar to those found for SV40. In contrast to SV40, however, a continuum of labeled polyoma viral DNA sedimented between 240 S and 95 S. These complexes were characterized by their release from cell debris with neuraminidase, precipitation with antivirion antibody, complete disruption in 1 M NaCl, and association with hemagglutinating (HA) activity. These intermediates may represent incremental capsid protein additions to the 75 S minichromosome, hypothesized in the current models for SV40 assembly. The ability to isolate a complete complement of polyoma subviral complexes provides a basis for studying the growth defect of polyoma host-range mutants, and the properties of neuraminidase release, hemagglutination, and specific immunoprecipitation suggest purification steps for further characterization of these virion assembly intermediates.

An analysis of transformed clones obtained by coinfections with hr-t and ts-a mutants of polyoma virus

Clones of stably transformed rat cells from the F-111 (Fisher rat embryo) and NRK (normal rat kidney) cell lines have been obtained by complementation using pairs of nontransforming polyoma mutants of the hr-t and ts-a classes. A total of 78 clones were isolated and studied, 21 from the NRK line, and 57 from the F-111 line. These "complementation transformed clones" were then examined for the presence and expression of each of the parental mutant viruses. The expression of viral T (tumor) antigens was analyzed by immunoprecipitation. Every one of 54 clones examined express the 22K (small) and 56K (middle) T antigens which are the products of the hr-t viral gene. This indicates a requirement for the presence of a wild-type hr-t allele contributed by the ts-a mutant parent. Ten clones lack detectable large T antigen, while four show thermolabile large T antigen. These results support the conclusion that middle and small but not large T antigen(s) are essential for the maintenance of the transformed phenotype. Most of the 57 F-111 clones are virogenic when fused to mouse cells under permissive conditions and yield both mutant types. Surprisingly, wild-type recombinants have been recovered from 40 of the F-111 clones. Three clones show evidence of retention of only the ts-a mutant genome. Virus cannot be rescued from the majority of the 21 NRK clones. The few clones which are virogenic, however, yield both hr-t and ts-a mutants. Integration patterns of several clones analyzed by Southern blotting confirm the expectations based on viral T antigen(s) and virus rescue analyses. Hr-t mutant genomes, though not required for the maintenance of the transformed state, are frequently retained in complementation transformed clones. Tandem integration of the two parental mutants is clearly demonstrated in one clone and implicated in the other eight out of nine virogenic F-111 complementation transformed clones examined. This observation provides the basis of a model for the generation of wild-type recombinants following fusion of F-111 clones.

Polyomavirus large T antigen binds independently to multiple, unique regions on the viral genome

To map the polyomavirus large T antigen binding sites on the viral genome we employed a quantitative immunoassay. Defined, radiolabeled fragments of the viral genome were reacted with crude nuclear extracts prepared from lytically infected mouse 3T6 cells, and the fragments bound by large T antigen were immunoprecipitated with anti-T serum and Formalin-fixed Staphylococcus aureus. The immunoprecipitated DNA was then analyzed by gel electrophoresis and autoradiography. By the use of a variety of restriction endonuclease-generated fragments of wild-type and mutant viral DNAs, the region of high-affinity binding was localized to a 153-base-pair stretch between nucleotides 5292 and 152. At least two independent binding sites lie within this region, one upstream and the other downstream of the Bg/I site at nucleotide 87. One of the binding sites is located within sequences required in cis for DNA replication; the other overlaps the TATA box and cap sites of the early transcription unit. The two sites share a common sequence, A/TGAGGC-N4/5-A/TGAGGC, which may serve as the recognition sequence for large T antigen.

Evidence for a bovine origin of the polyomavirus detected in foetal rhesus monkey kidney cells, FRhK-4 and -6

Rabbit antisera to the stump-tailed macaque polyomavirus (STMV) which had been shown by immunoelectron microscopy and indirect immunofluorescence to react with the polyomavirus found in FRhK-4 cells (FRKV), also gave precipitin lines in counter-immunoelectrophoresis (CIE) and double diffusion in gel (GD) when reacted with FRKV. The reactions in GD showed identity with that of a rabbit antiserum to FRKV. Naturally occurring antibody to FRKV (anti-FRKV) was found by CIE in 48 per cent of 353 cattle, 1/106 pigs and 1/20 goats but not in any of 13 other species including 45 rhesus monkeys and 97 humans. Each of 9 anti-FRKV positive samples from cattle, the goat serum, but not the pig serum gave a line of identity with the rabbit antiserum to FRKV in GD against FRKV. Detection of anti-FRKV in colostrum deprived newborn calves and in commercial foetal calf sera (FBS) indicates that intra-uterine infection of cattle with FRKV may occur. FRKV adapted readily to growth in secondary calf kidney cultures and grew more rapidly and to higher titres than in the FRhK-4 cultures. We conclude that FRKV is probably another strain of STMV and that the natural hosts of these viruses are cattle and not primates. Evidence of intra-uterine infection of cattle implies that infectious FRKV may be present in some FBS and may thus have gained entry into various susceptible cell lines, particularly primate kidney.

Enhancement of polyoma virus middle T antigen tyrosine phosphorylation by epidermal growth factor

Polyoma virus codes for three proteins involved in host cell transformation: the large, middle and small T antigens. Middle T antigen is a major transforming protein which is responsible for the induction of the phenotype of transformed cells and, without it, transformation does not occur (reviewed in refs 1-4). Middle T antigen alone can transform established cell lines, although large, and possibly small, T antigens are also required for the full expression of the phenotype of transformed cells in media with a low concentration of serum. A subfraction of middle T antigen is associated with a protein kinase activity which phosphorylates middle T antigen in vitro on tyrosine. There is a strong correlation between the level of this kinase activity and the degree of expression of the phenotype of transformed cells. We report here that epidermal growth factor (EGF) stimulates tyrosine phosphorylation of middle T antigen, suggesting the possibility that mitogenic growth factor(s) regulates this phosphorylation activity.

Transforming activity of polyoma virus middle-T antigen probed by site-directed mutagenesis

The ability of polyoma virus to transform cells results primarily from the action of one of the virus-coded early proteins, called middle-T antigen. Middle-T has an associated tyrosine-specific protein kinase activity that can be measured in vitro and results in the phosphorylation of middle-T itself. Almost all mutants so far tested that lack the ability to transform cells, also lack associated kinase activity. Attempts to map within middle-T the tyrosine residue(s) that are phosphorylated in vitro suggest that a likely site of phosphorylation is tyrosine 315 (refs 8-10 and unpublished results). The amino acid sequence preceding Tyr 315 includes a tract of six contiguous glutamic acid residues and bears some homology with that preceding the tyrosine phosphorylated in vivo in pp60v-src, the transforming protein of Rous sarcoma virus, and with a region in the polypeptide hormone, gastrin, preceding a tyrosine that is sulphated. Furthermore, although surprisingly large tracts of middle-T may be removed without affecting its transforming activity, mutants that lack the sequences corresponding to amino acids 311-318 inclusive are transformation defective. Because the likely site of phosphorylation, the homology with pp60v-src and gastrin and the sequence apparently required for transformation all overlap, it has generally been accepted that this region of middle-T may form part of an essential region, possibly an active site on the protein. Here we have used techniques of site-directed and site-specific mutagenesis to probe the sequence requirements in more detail. Contrary to expectation, the results obtained strongly suggest that Tyr 315 and conservation of the surrounding amino acid sequence are not essential for transformation.

Human polyomavirus in pregnancy. A model for the study of defence mechanisms to virus reactivation

We have carried out a longitudinal study of human polyomavirus infection in 71 pregnant women and correlated the virological findings with changes in the defence system in the same patients. As reactivation of human polyomaviruses generally occurred late in the second trimester it was possible to distinguish between the immunological changes which preceded the onset of reactivation and those which were secondary to the infection. Evidence of reactivation was detected in 26 women; all had high or rising antibody titres against BK or JC virus, but only five of these developed viruria. A positive correlation was observed between a high monocyte count in early pregnancy and subsequent virus reactivation. The virus excretors had significantly lower neutrophil counts than the women who had no evidence of virus reactivation. In contrast, women with serological evidence of virus activity but no viruria has significantly higher neutrophil counts than the non-activators. They also had stronger lymphocyte responses to PHA than the virus excretors. Virus activators were found to have a significant lymphopenia in the third trimester compared to the non-activators. High antibody levels did not appear to inhibit virus excretion. These findings suggest that monocytosis may predispose to reactivation of human polyomaviruses in pregnancy. On the other hand, ability to contain the virus once it has been activated, was associated with neutrophilia, and relatively vigorous in vitro reactivity of lymphocytes to PHA. Persistent lymphopenia was probably secondary to virus reactivation. The model on which this study is based could be adapted to investigate the causes of reactivation of other viruses. It may also help to identify risk factors in patients who are particularly susceptible to infection with opportunistic viruses.

Immunocytochemical localization of T antigen in cells of BK virus-induced hamster brain tumor

The localization of both the large T and small t tumor (T) antigens in cultured cells (Vn 12 cells) of hamster brain tumors induced with BK virus (BKV), a new human papovavirus, was studied by an enzyme labelled antibody method at both the light and electron microscopic levels. Under the light microscope, BKV T antigen was observed in the nucleus, except for the nucleoli, of cells in interphase, and under the electron microscope it was observed in the nucleus except for the nucleoli and nuclear membrane. BKV T antigen appears to be closely associated with nuclear chromatin as previously reported for simian virus 40 tumor antigen (SV40 T antigen). The intracellular localization of BKV T antigen was the same as that of SV40 T antigen. In metaphase, BKV T antigen seems to be distributed diffusely throughout the cytoplasm except for the chromosomes. In telophase, BKV T antigen transfers from the cytoplasm to the nucleus. The migration of BKV T antigen during the cell cycle is thought to be related to the function of T antigen.

Tubulo-interstitial nephritis associated with polyomavirus (BK type) infection

We studied viral injury to the kidney in a six-year-old boy with hyperimmunoglobulin M immunodeficiency who presented with irreversible acute renal failure and eventually died after five months of dialysis. Renal biopsy at the time of his presentation revealed a predominantly tubulo-interstitial process with numerous viral inclusions that were identified as polyomavirus. Urine cultures showed a massive viruria with BK-type, polyomavirus. The kidney disease was end stage, with persistence of BK virus identified by morphologic techniques and by culture. DNA hybridization analysis showed virus in low concentration in the lymph nodes, spleen, and lungs. The marked viruria, the high concentration of BK virus, and the extensive distribution of viral antigen throughout the kidney all suggest that infection with BK virus was the basis of the severe renal parenchymal injury.

Deletion mapping of a short polyoma virus middle T antigen segment important for transformation

Viable polyoma virus mutants were constructed that had small deletions in the early region of the genome. The deletions together removed most of the segment missing from the genome of the nontransforming mutant dl23 (N. Smolar and B. E. Griffin, J. Virol. 38:958-967, 1981). The transformation properties, as measured by colony formation in soft agar, of mutants with overlapping or contiguous deletions showed that part or all of the middle T antigen segment, consisting of the short amino acid sequence Glu4-Tyr-Met-Pro-Met, was essential for the activity of the protein in transformation. However, the segment could be deleted without significant effect on the in vitro protein kinase activity associated with the middle T antigen.

Serum antibodies to papova viruses (BK and SV 40) in subjects from the area with Balkan endemic nephropathy

The presence of antibodies to BK virus and SV40 was investigated in 63 patients with Balkan endemic nephropathy (BEN) and in 83 apparently healthy subjects from the endemic area. Serum antibodies to BK virus were detected in 95.2% of the former and in 74.7% of the latter, high antibody levels being prevalent in the age groups 41-60 years. Antibodies to SV40 were absent in the BEN patients and their frequency in the healthy subjects (27.7%) was much lower than that previously recorded in healthy persons from other zones of Romania (40%). The results obtained plead for a prevalence of BK virus infection in the endemic area with BEN.

Virus specific antigens in BK virus-transformed cells detected by cell-mediated immune reactions

Mouse cell lines transformed by BK virus (BKV) were used for immunizations and as targets in the 51Cr release assay to detect BKV specific cell surface antigens. When BKV-transformed cells from C57BL/6J mice were used to immunize syngeneic animals and the effector cells were stimulated in vitro, cytotoxic lymphocytes were generated which lysed syngeneic BKV-transformed cells but not control cells transformed by UV irradiation or 3-methylcholanthrene. The reaction was apparently H-2-restricted. A cross reaction was observed between cell lines transformed by BKV or SV40. The BKV specific antigen detected by the cell-mediated immune reaction presumably corresponds to the tumour specific transplantation antigen previously detected in BKV-transformed cells.

Loss of polyoma virus infectivity as a result of a single amino acid change in a region of polyoma virus large T-antigen which has extensive amino acid homology with simian virus 40 large T-antigen

The polyoma virus (Py) transformed cell line 7axB, selected by in vivo passage of an in vitro transformed cell, contains an integrated tandem array of 2.4 genomes and produces the large, middle, and small Py T-antigen species, with molecular weights of 100,000, 55,000, and 22,000, respectively (Hayday et al., J. Virol. 44:67-77, 1982; Lania et al., Cold Spring Harbor Symp. Quant. Biol. 44:597-603, 1980). The integrated viral and adjacent host DNA sequences have been molecularly cloned as three EcoRI fragments (Hayday et al.). One of these fragments (7B-M), derived from within the tandem viral sequences, is equivalent to an EcoRI viral linear molecule. Fragment 7B-M has been found to be transformation competent but incapable of producing infectious virus after DNA transfection (Hayday et al.). By constructing chimerae between 7B-M and Py DNA and by direct DNA sequencing, the mutation responsible for the loss of infectivity has been located to a single base change (adenine to guanine) at nucleotide 2503. This results in a conversion of an aspartic acid to a glycine in the C-terminal region of the Py large T-antigen but does not appear to affect the binding of the Py large T-antigen to Py DNA at the putative DNA replication and autoregulation binding sites. The mutation is located within a 21-amino acid homology region shared by the simian virus 40 large T-antigen (Friedmann et al., Cell 17:715-724, 1979). These results suggest that the mutation in the 7axB large T-antigen may be involved in the active site of the protein for DNA replication.

Serologic studies of papovavirus infections in pregnant women and renal transplant recipients

Serologic studies were performed to determine if BKV and JCV are transmitted congenitally and to assess if the virus present in donor kidney contributes to virus activity in renal transplant recipients. Fourteen of 100 normal women showed antibody rise to BKV or JCV during pregnancy; in all instances, these were reactivation infections occurring in initially seropositive women. BKV-and JCV-specific antibodies of the IgM class were not detected in over 300 umbilical cord sera. Thus, there was no evidence of congenital transmission of BKV or JCV. BKV-seronegative renal transplant recipients who received kidneys from BKV-seropositive donors had a frequency of BKV infection which was about four times greater than that in BKV-seronegative recipients receiving kidneys from BKV-seronegative donors. These data suggest that BKV in donor kidney contributes to primary BKV infections in renal transplant recipients.

The roles of individual polyoma virus early proteins in oncogenic transformation

The expression in normal rat cells of modified polyoma virus genomes, separately encoding large T, middle T or small T antigens, has allowed the investigation of the roles of these proteins in oncogenic transformation. Middle T is sufficient to transform cells of established lines but the transformants are serum dependent. Large T lacks intrinsic oncogenic potential but can relieve the serum dependence of normal and transformed cells. Middle T alone cannot transform primary rat embryo fibroblasts.

Transcriptional 'enhancers' from SV40 and polyoma virus show a cell type preference

Transcriptional "enhancers" have been identified in both the monkey virus SV40 and in the mouse polyoma virus. Here we report that these enhancers show a cell type preference. This was done, (i) by assaying for T antigen expression and viral DNA replication of polyoma DNA which contains its own enhancer or the enhancer of SV40 virus, and (ii) by linking either of the two enhancer elements to the rabbit beta 1-globin gene and measuring transient globin expression in mouse and primate cells. The results were consistent in all the assays: In mouse cells the polyoma enhancer is slightly more effective than the SV40 enhancer. However, in primate cells the SV40 enhancer induces a four to six fold higher level of gene expression than does the polyoma enhancer.

The relation between polyoma T-antigen and increased 5S RNA synthesis in cell-free extracts from polyoma-infected mouse kidney cell cultures

In polyoma-infected mouse kidney cell cultures 5S RNA synthesis began to increase around 16 h, i.e. 7-9 h after the onset of polyoma T-antigen synthesis. The rate of polyoma-induced 5S RNA synthesis reached a maximum plateau around 25 h when it was 1.8-2.0 times higher than in mock-infected parallel cultures. Stimulation of 5S RNA synthesis in vivo thus coincided in time with the increase in total cellular RNA and protein. Cell-free extracts (S100) prepared at 15 h from mock-(S100-M) or polyoma-infected (S100-Py) mouse kidney cell cultures were indistinguishable with respect to protein concentration and 5S RNA synthesis, using a cloned somatic Xenopus borealis 5S gene as template. S100-Py extracted 25 h after infection contained 30% more protein and synthesized 1.5-2.0 times more 5S RNA than S100-M. Complete removal of the polyoma T-antigens from S100-Py by 3 cycles of immunoprecipitation with hamster anti-T serum remained without effect on stimulated 5S RNA synthesis. However, a linear relationship between 5S RNA synthesis and protein concentration of S100-M and S100-Py was observed.

mlt Mutants of polyoma virus

New mlt deletion mutants of polyoma virus were isolated, and their abilities to produce a lytic response in mouse cells or to transform rat cells were assessed. Their properties were analyzed in terms of the sequences deleted and their effects upon the structure and functions of the viral middle and large T-antigens.

Organization of polyoma virus DNA in mammary tumours of athymic mice

Restriction mapping of polyoma virus DNA in mammary tumours of athymic mice gave patterns that varied with the tumour examined. These reflected differences in both the organization and the state of integration of virus genomes in the host chromosomes. All tumours contained tandemly integrated full-length and defective virus genomes. Some tumours also contained unintegrated virus DNA molecules, some full-length and others defective. The deletions were localized in the virus genomic sequences coding for the distal part of the large T antigen. After the first transplantation, the organization of polyoma virus genomes in tumours remained essentially unchanged through four successive transplantations. The tumour cells that initially contained free virus DNA molecules continued to possess such molecules during serial transplantations. The virus DNA molecules in transplanted tumours lacking unintegrated virus genomes were more methylated than in tumours containing unintegrated virus genomes.

Antibody to the nonapeptide Glu-Glu-Glu-Glu-Tyr-Met-Pro-Met-Glu is specific for polyoma middle T antigen and inhibits in vitro kinase activity

Middle T antigen of polyoma virus has an associated tyrosine kinase activity which phosphorylates tyrosine residue 315 on middle T in immunoprecipitates. A peptide representing the sequence of middle T from residue 311 to 319 has been synthesized. This peptide acts as a weak inhibitor of the kinase reaction. An antiserum has been raised against this peptide after conjugation to bovine serum albumin. The antibody is middle T-specific. Middle T antigen precipitated by this serum is largely inactive in the kinase reaction. Dissociation of the immune complex with peptide releases middle T in a kinase-active form.