Biochemical studies on structural and nonstructural proteins of the African green monkey B-lymphotropic papovavirus (LPV)

Quantitative detection of Merkel cell virus in human tissues and possible mode of transmission

Merkel Cell Virus (MCV) is a newly discovered polyomavirus, recently found in a rare skin cancer, Merkel cell carcinoma (MCC). However, MCV has also been detected in some normal tissue samples. We tested and compared the relative quantity of the MCV in a set of diverse human tissue samples with the MCC samples. The levels of MCV in MCCs were over 60 times higher than the highest values in all other tissues. Low quantities of MCV were detected in diverse tissue samples independently of malignant or benign histologic status. Higher levels of the virus were found in the upper aerodigestive tract, digestive system, and saliva compared to the lung and genitourinary system samples. These results confirm that MCV is widespread in the human body and suggest a possible fecal-oral transmission route similar to the Hepatitis A virus. Despite widespread presence of the virus, it appears that only neuroendocrine skin cells are susceptible to transformation by MCV.

Lymphotropic polyomavirus is detected in peripheral blood from immunocompromised and healthy subjects

BACKGROUND

Lymphotropic Polyomavirus (LPV) was isolated from a B-lymphoblastoid cell line of an African green monkey. This virus shares some characteristics with human polyomaviruses, but it is antigenically distinct from BK Virus (BKV) and JC Virus (JCV). Seroepidemiological studies revealed that human sera react in the presence of LPV antigens, and, recently, the viral genome was amplified in the peripheral blood from patients affected with HIV-related leukoencephalopathies.

OBJECTIVES

The aims of the study were to investigate and compare the presence of LPV DNA with that of JCV and BKV in different biological samples and patient groups.

STUDY DESIGN

LPV, JCV and BKV DNA were searched and quantified in peripheral blood and CSF from HIV+ patients and in peripheral blood from healthy subjects.

RESULTS

The LPV genome was detected in peripheral blood of both HIV+ patients and healthy subjects, with a prevalence of 7.2% and 4.7% respectively, but not in CSF. However, its presence was less frequent than that of JCV and BKV.

CONCLUSIONS

The amplification of LPV genome from human peripheral blood confirms the fact that LPV can infect the human population. LPV DNA was amplified from patients affected with HIV-related leukoencephalopathies but also from HIV patients without neurological disorders and from healthy subjects. Therefore, the results do not support the hypothesis of an association between LPV infection and any neurological disease. However, given their high similarity, it is possible that LPV, as well as BKV and JCV, could establish latency in humans and cause disease only in rare circumstances.

Selection against large T-antigen expression in cells transformed by lymphotropic papova virus

Large T-antigen (T-Ag) in hamster cells transformed by the lymphotropic papova virus (LPV) exhibits similar properties as the T-Ag of simian virus 40 (SV40) with regard to its interaction with cellular targets. However, in contrast to SV40-transformed cells, LPV-transformed cells in cell culture select against high expression of LPV T-Ag. Southern analysis revealed that this selection process was accompanied by drastic changes at the DNA level, involving the loss of most of the integrated viral DNA copies. These changes probably were responsible for an approximately 100-fold downregulation of LPV T-Ag transcription. To elucidate the biological significance of this phenomenon, we studied the effects of the expression of LPV and SV40 T-Ag, respectively, in a variety of cells. Our data suggest that LPV T-Ag, like SV40 T-Ag, acts as an immortalizing and transforming protein. However, in contrast to SV40 T-Ag, high-level expression of LPV T-Ag seems to be detrimental to the establishment and maintenance of LPV-transformed cells in vitro.

Detection of individual virus-infected cells by filter in situ hybridization

The diagnosis of virus infection by nucleic acid hybridization represents an alternative to classical virological diagnostic methods. One special technique termed 'filter in situ hybridization' consists of fixation of intact cells to nitrocellulose filters followed by hybridization with a labelled DNA probe. We demonstrate that filter in situ hybridization can be a simple and sensitive method for the detection of virus infection in cells. In an in vitro model system using a human B-lymphoma cell line infected by the lymphotropic papovavirus (LPV), it is shown that individual virus replicating cells can be detected by this method. Infection can be diagnosed even if only one out of 20,000 cells in a culture contains replicating virus. This assay may be of value as a diagnostic tool in other viral systems.

A purine-rich DNA sequence motif present in SV40 and lymphotropic papovavirus binds a lymphoid-specific factor and contributes to enhancer activity in lymphoid cells

The enhancer of simian virus 40 (SV40) is active in a wide variety of tissues and hosts and thus has been called a general enhancer. Previous work has established that this region contains a number of subsegments, each with a different cell type specificity. By analyzing enhancer deletion mutants of SV40 with a restricted cell type specificity, we have identified a purine-rich DNA motif (Pu box) that is essential for activity of the mutant enhancer in lymphoid cells and also obtained evidence for an independent element that confers activity in kidney CV-1 cells. The Pu box element is positively regulated since binding of a cognate factor(s) was only observed in lymphoid cell extracts. The Pu box is also present in the 63-bp enhancer repeat of lymphotropic papovavirus (LPV), and competition experiments show that the SV40 and LPV sequences bind the same factor(s). Thus, the Pu box is likely to be a major determinant of the lymphotropic host range of LPV. In SV40, however, the Pu box is one of several enhancer elements with different cell specificities. We suggest that such an arrangement reflects a diversification strategy of SV40 to maximize its chance of adaptation to various types of host cells.

Transcription map of the African green monkey lymphotropic papovavirus

The S1 endonuclease mapping technique was employed to characterize polyadenylated RNA molecules encoded by the African green monkey lymphotropic papovavirus (LPV) in B-lymphoblastoid cells of human origin infected with the virus. Two groups of stable transcripts of opposite polarities were identified and mapped. By analogy with other papovaviruses, these molecules were designated as "early" and "late" RNAs. The early transcripts include two RNA molecules consisting of a colinear 3' "main body" of about 1.9 kb spliced to 5' "leaders" of 0.26 and 0.60 kb, respectively. These molecules are presumably mRNAs encoding the LPV large T and small t antigens. Shorter, less abundant early RNA molecules have also been detected but have not been mapped. The late RNAs include three molecules, of which two consist of a common leader of 0.22 kb spliced to main bodies of 1.2 and 1.85 kb, respectively. The third late transcript of 2.3 kb is colinear. These late transcripts are presumably mRNAs encoding the LPV capsid proteins. A survey of the LPV DNA sequence published by M. Pawlita, A. Clad, and H. zur Hausen (1985, Virology 143, 196-211) allowed tentative assignment of the termini and splice sites of the various RNAs to nucleotides in the DNA sequence. The LPV transcription map is similar but not entirely analogous to those of polyoma virus and simian virus 40. These distinctions support a previous classification of LPV as a member of a new subgroup of the polyomaviruses.

Mutation in the VP-1 gene is responsible for the extended host range of a monkey B-lymphotropic papovavirus mutant capable of growing in T-lymphoblastoid cells

Monkey B-lymphotropic papovavirus (LPV) replicates only in B-lymphoblastoid cells, whereas the LPV mutant 76 (LPV-76) can grow in either B- or T-lymphoblastoid cells. The nucleotide sequence of the wild-type LPV PstI B segment was compared with that of LPV-76 PstI-B, within which the mutation responsible for the extended host range had been located. The VP-1 coding region of LPV-76 was found to have mutations, single-base substitutions causing three amino acid substitutions.

Monkey B-lymphotropic papovavirus genome: the entire DNA sequence and variable regions

DNA from monkey B-lymphotropic papovavirus (LPV) adapted to growth in human B-lymphoblastoid cell line BJA-B consisted of three classes of molecules of 5.1, 5.0, and 4.9 kilobases (kb), and the entire DNA sequence of 5.1-kb molecule was determined. The three types of LPV DNA yielded infectious virus upon transfection to BJA-B cells. Comparison of the structures among these clones and clone K38 sequenced by Pawlita et al. (1985) suggests that the transcriptional control region, the small-T antigen gene, and the VP-1 gene are variable in the LPV genome, and that the 5.0-kb LPV resembles the parent virus, from which all the others had evolved by duplication and deletion during passages in BJA-B cells, probably for adaptation to better growth in human cells.

Complete DNA sequence of lymphotropic papovavirus: prototype of a new species of the polyomavirus genus

Lymphotropic papovavirus (LPV) is a new member of the polyomavirus genus. Its host range in vitro is restricted to transformed cells of B-lymphocyte origin. Here the complete 5270-bp DNA sequence of LPV is presented. The LPV early region can encode a large T and a small t antigen but no middle T antigen and the late region can encode the three structural proteins VP1, VP2, and VP3. Based on sequence conservation of shared proteins LPV is equally related to both mouse polyomavirus (Py) and simian virus 40 (SV40) and represents a new distinct species of the polyomavirus genus. Sequence comparisons of LPV, SV40, and Py point out essential conserved sequence features of the polyomavirus genus more clearly than the comparison of only SV40 and Py. The most conserved proteins are VP1 with 42% and large T antigen with 28% of the amino acids conserved among the three viruses. Although least conserved the noncoding DNA sequences of LPV show significant homologies both to SV40 and Py (origin of viral DNA replication and putative early promoter). A 63-bp tandem repeat at the late side of the replication origin possibly represents a LPV enhancer element.

Lymphotropic papovavirus transformation of hamster embryo cells

Hamster embryo cells were transformed by African green monkey lymphotropic papovavirus (LPV). The transformed cells contained intranuclear T-antigens demonstrable by fluorescent antibody staining with hamster anti-LPV serum. Analysis of uncloned and cloned lines of transformed cells for LPV sequences revealed that the viral DNA was present as free nonintegrated and integrated genomes; there were approximately 10 copies of free DNA and about one to two copies of integrated genomes per cell. The cells were highly tumorigenic when inoculated into hamsters and produced progressively growing tumors in 100% of newborn or 10-day-old hamsters that were inoculated with LPV-transformed cells. The serum from tumor-bearing hamsters reacted with LPV-transformed cells and also showed a weak reaction with simian virus 40-, BK virus-, and JC virus-transformed cells, thereby showing an antigenic relationship with the T-antigens of other primate polyomaviruses. The large T-antigen of LPV was found to be an 84,000-molecular-weight protein which was immunoprecipitated by hamster anti-LPV (antiviral) as well as by tumor serum.