- Volume 77, Issue 12, 1996

Epstein—Barr virus (EBV) initiates infection of normal B lymphocytes by binding to CD21, a complement receptor. Since EBV, unlike most viruses, preferentially infects resting (non-activated) cells, the present studies were undertaken to evaluate the hypothesis that intracellular signalling pathway(s) triggered by EBV binding to CD21 activate the expression of certain cellular genes, as well as the initially expressed viral genes, and thus enable EBV to infect resting B cells. Experiments with non-transforming EBV, recombinant virus ligand and anti-CD21 MAbs show that EBV binding to CD21 on resting B cells increases CD23 mRNA levels independently of viral gene expression. A panel of five protein kinase C (PKC) and tyrosine kinase (PTK) inhibitors, all with different modes of action, exhibited a distinctive pattern of effects on the EBV induced induction of CD23 expression, ranging from nearly complete inhibition to no influence. The results suggest that distinct PKC isoforms and PTKs are involved in the signalling pathway(s) triggered by EBV binding to CD21. Significantly, the five inhibitors showed the same pattern of effects on the earliest stages of infection (EBNA-2 transcription) and B cell transformation (mitogenesis and colony formation). The identical pattern of effects of these PKC and PTK inhibitors with diverse mechanisms of action on the EBV induced increase in both CD23 and EBNA-2 mRNA levels strongly suggests that their transcription is mediated by an intracellular signalling pathway which shares, at least in part, common members.

A polyclonal antiserum, raised against a pp71 fusion protein, was prepared in order to investigate the biosynthesis and localization of the matrix protein pp71 of human cytomegalovirus (HCMV), the UL82 gene product, during the HCMV infectious cycle in human fibroblasts. Transcription of the pp71-specific bicistronic 4.0 kb mRNA and pp71 biosynthesis exhibited a biphasic pattern during one round of the HCMV infectious cycle, with a first peak at 12 h and a second at 72 h post-infection (p.i.). Cycloheximide treatment of infected human fibroblasts revealed that the presence of pp71 in total cell extracts prior to 3 h p.i. was due to the input virus inoculum. Transcription of the two specific pp71 mRNAs commenced 5–7 h p.i. as shown by Northern blot analysis of total cellular RNA. Western blot analysis of isolated nuclei and indirect immuno-fluorescence experiments indicated that pp71, like the major tegument protein pp65, is present in the nucleus shortly after infection as well as during the late phase of viral morphogenesis. Also, after transient transfection of UL82 into U373MG cells, pp71 was found to be present in the nucleus of the transfected cells. By immunogold labelling, pp71 was detected in the nucleoplasm in association with nucleocapsids in electron-dense nuclear skein structures at late stages of the infection cycle. These findings suggest functions of pp71 in viral maturation in addition to that as an early transactivator of viral gene transcription described recently.

The cellular sites and mechanisms of human cytomegalovirus (HCMV) latency are still poorly defined. Although evidence suggests that peripheral blood monocytes are one site of latency in the healthy carrier, it is unlikely that monocytes represent a site of primary HCMV infection. Consequently, we have analysed CD34+ bone marrow progenitors, precursors of monocytes, to determine whether they are a site of HCMV carriage in normal virus carriers. For the first time, we demonstrate the presence of endogenous HCMV within bone marrow progenitors in the absence of HCMV lytic gene expression. These findings are consistent with previous evidence showing that the permissiveness of myeloid cells for HCMV is critically dependent on the differentiation state of the cell.

To evaluate the role of the OKT4 epitope in human herpesvirus 7 (HHV-7) infection, we studied the susceptibility to HHV-7 infection of CD4+ T cells isolated from two individuals with OKT4 epitope deficiency. HHV-7-infected OKT4−Leu3a+ T cells exhibited the characteristic cytopathic effect, reactivity with HHV-7-seropositive serum by immuno-fluorescence and down-modulation of surface CD4 in a manner similar to HHV-7-infected OKT4+Leu3a+ T cells. A semiquantitative PCR revealed that the amounts of HHV-7 replicated in OKT4+Leu3a+ T cells and OKT4− Leu3a+ T cells were not significantly different. Although it has been reported that OKT4 monoclonal antibody efficiently inhibits HHV-7 infection, the present study demonstrated that the interaction of HHV-7 with CD4+ T cells does not require participation of the epitope defined by OKT4 monoclonal antibody.

The enzyme dUTPase catalyses the hydrolysis of dUTP to dUMP and pyrophosphate, thereby suppressing incorporation of uracil into DNA and providing a pool of dUMP, the precursor of dTTP. Hydrolysis of other nucleotides similar in structure to dUTP would conceivably be physiologically detrimental and high specificity of the reaction seems to be necessary. In this work, we characterize the substrate specificity of the dUTPases from herpes simplex virus type 1 (HSV-1) and mouse mammary tumour virus (MMTV) in comparison to the Escherichia coli enzyme. We tested dCTP, dTTP, UTP and dUDP as substrates. Significantly higher reactivity was observed for the HSV-1 enzyme with dCTP and dTTP and for the MMTV enzyme with dTTP and UTP. The lower substrate specificity of the two virus enzymes compared with the bacterial enzyme is discussed in relation to the DNA precursor metabolism during virus replication.

Molluscum contagiosum virus (MCV) and vaccinia virus (VV) are serologically unrelated poxviruses with a disparate genome composition (MCV, 66% G + C; VV, 33% G + C). Molecular studies of MCV have been hindered by the inability to propagate the virus in cells cultured in vitro. We sequenced 7765 bp of MCV DNA cloned from four widely spaced regions throughout the MCV genome and identified a total of 11 potential open reading frames (ORF), designated CX1-11. These include MCV homologues of the VV genes encoding protein kinase 2, structural protein VP8, RNA polymerase 35 kDa subunit and 3β-hydroxysteroid dehydrogenase. The position and orientation of the MCV ORFs was collinear to the VV genome, with the exception of the region around ORF CX11 which is inverted in the MCV genome.

Using a series of new insertion/expression vectors, we constructed a set of recombinant vaccinia viruses (recVV) encoding the murine T cell costimulatory molecules mB7-1 or mB7-2, or both together in the same construct. On infection with replication incompetent and non-cytopathic recVV, several tumour cell lines expressed the respective molecules and bound to CTLA-4. The highest binding capacity was found when both mB7 molecules were co-expressed. Mouse B16.F10 melanoma cells expressing mB7-1 or mB7-2 provided effective costimulation for proliferation of resting CD4+ T cells in the presence of concanavalin A and plate-bound anti-T cell receptor antibodies, respectively. If mB7-1 and mB7-2 were delivered together on the same cell, the proliferative response of CD4+ T cells increased further. The costimulatory effect could be blocked with CTLA-4, the soluble ligand for B7 molecules. The possibility of engineering tumour cells using recVV holds implications for the future design of vaccination strategies.

We have cloned and analysed the transcriptional properties of two closely linked genes, p39 and cg30, of Bombyx mori nuclear polyhedrosis virus (BmNPV). These genes encode a structural polypeptide and a putative transcriptional regulator of the virus, respectively. The cg30 gene is transcribed prior to and after DNA replication from a site located within the ORF of the adjacent p39 gene. Its transcription product, a 1.3 kb mRNA, is polyadenylated at a site containing consensus eukaryotic polyadenylation signals and mapping 87 bp downstream of the translation termination codon for CG30. During the later stages of infection, two additional RNAs, 2.2 and 6.5 kb, are also transcribed through the cg30 gene. The 2.2 kb RNA, representing the mRNA that encodes P39, is initiated from three relatively closely spaced sites located upstream of the P39 ORF. The 6.5 kb RNA is apparently transcribed from the promoter sequences of another gene located further upstream of the p39 gene. The 2.2 and 6.5 kb transcripts have two polyadenylation sites. The first site is the same as the one used to generate the cg30 gene-specific transcripts. The second is located 4 bp downstream of the CG30 translation termination codon. Transient expression assays show that the p39 gene sequences immediately upstream of the CG30 ORF can direct expression of a reporter gene when the latter is co-transfected with the gene encoding the early baculovirus trans-activator IE1. Thus, these sequences behave as a delayed-early baculovirus gene promoter.

Two pea seedborne mosaic potyvirus (PSbMV) isolates, P-1 DPD1 (P-1), which is highly seed-transmitted, and P-4 NY (P-4), which is rarely seed-transmitted, and chimeras between P-1 and P-4 were analysed to map the viral genetic determinants of seed transmission. Infectivity of chimeric viruses was evaluated by inoculating Pisum sativum with RNA transcribed in vitro from recombinant full-length cDNA clones. The chimeric viruses that were used demonstrated that a genomic segment encoding the 49 kDa protease and putative RNA polymerase was responsible for symptom induction. Attempts to determine transmission of the chimeric viruses in P. sativum cultivars known to transmit P-1 at high frequencies showed that seed transmission is a quantitative character influenced by multiple viral determinants. Seed transmission frequency did not correlate with accumulation of virus in vegetative tissue. The 5′ 2.5 kb of the 10 kb PSbMV genome had a major influence on the seed transmission frequency and was analysed further. This showed that, while the helper-component protease was a major determinant of seed transmission, the potyviral P1-protease exerted no measurable influence.

The complete nucleotide sequence of the genome segment 5 (S5) of a Thai isolate of rice ragged stunt virus (RRSV) was determined. The 2682 nucleotide sequence contains a single long open reading frame capable of encoding a polypeptide with a molecular mass of ∼ 91 kDa. Polypeptides encoded by various truncated cDNAs of S5 were expressed using the pGEX fusion protein vector and the highest level of fusion protein was obtained from a construct encoding a hydrophilic region of S5 protein. Antibodies raised against this fusion protein recognized a minor polypeptide, with a molecular mass of ∼ 91 kDa, that was present in purified preparations of RRSV particles, infected insect vectors and infected rice plants. This indicates that RRSV S5 encodes a minor structural protein. Comparing the RRSV S5 sequence with sequences of other reoviruses did not reveal any significant sequence similarities.