Kunito Yoshiike

Studies on DNA from low-density particles of SV40. I. Heterogeneous defective virions produced by successive undiluted passages.

Abstract Successive undiluted passages of SV40 (small plaque) in African green monkey kidney cells result in the production of heterogeneous defective particles of lower densities than that of infectious virions. Band centrifugation in neutral and alkaline solutions and electron microscopy have been used to study DNA preparations from light virions fractionated by cesium chloride density gradient centrifugation. The studies have demonstrated that light virions contain circular DNA molecules smaller than those from plaque formers. Shortening of DNA would account for defectiveness as well as alteration in density of virions.

Human Papillomavirus Type 16 Minor Capsid Protein L2 N-Terminal Region Containing a Common Neutralization Epitope Binds to the Cell Surface and Enters the Cytoplasm

ABSTRACT The first step of papillomavirus infection is believed to be binding of major capsid protein L1 to the cell surface without involvement of minor capsid protein L2, but the viral infectivity can be neutralized either by anti-L1 or anti-L2 antibody. To understand the role of L2 in human papillomavirus (HPV) infection, we examined a segment of HPV type 16 (HPV16) L2, which contains a neutralization epitope common to HPV6, for its involvement in adsorption and penetration of the capsids. Preincubation of monkey COS-1 cells with a synthetic peptide having amino acids (aa) 108 to 120 of HPV16 L2 reduced the susceptibility of COS-1 cells to infection with HPV16 pseudovirions. Confocal microscopy showed that the green fluorescence protein (GFP) fused with the L2 peptide was found to bind to the surface of a HeLa cell, an HPV18-positive human cancer cell line, at 4°C and to enter the cytoplasm after subsequent incubation at 37°C. Flow cytometry showed that fused GFP did not bind to HeLa cells that had been treated with trypsin. Besides COS-1 and HeLa cells, some human and rodent cell lines were detected by flow cytometry to be susceptible to binding with fused GFP, showing a tendency of epithelial cells toward higher susceptibility. Substitutions at aa 108 to 111 inhibited fused GFP from binding to HeLa cells and reduced the infectivity in COS-1 cells of the in vitro-constructed pseudovirions. The results suggest that L2 plays an important role in enhancing HPV infection through interaction between the N-terminal region and a cellular surface protein, facilitating penetration of the virions and determining part of the tropism of HPVs.

Immortalization of primary rat cells by human papillomavirus type 16 subgenomic DNA fragments controlled by the SV40 promoter

We tested the human papillomavirus 16 (HPV 16) early genes for their ability to immortalize or transform primary rat brain cells, using the expression plasmids that contain the neomycin-resistance-inducing unit (pSV2neo) and the transcriptional unit for the HPV 16 subgenomic DNA fragments controlled by the SV40 early promoter. After transfection, drug-resistant colonies were maintained by refeeding and replating for characterization. The E7 gene alone was found to be capable of immortalizing and morphologically transforming primary rat cells, and the transformed cells showed anchorage-independent growth. Although its activity was lower than that of the E7 gene, the E6 gene also immortalized primary rat cells. The immortalized or transformed cells contained HPV 16-specific DNA and mRNA.

Nasal immunization of mice with peptide having a cross-neutralization epitope on minor capsid protein L2 of human papillomavirus type 16 elicit systemic and mucosal antibodies.

A common cross-neutralization epitope for human papillomavirus types 6 and 16 (HPV 6 and 16) is present in the region of amino acids (aa) 108-120 of HPV-16 minor capsid protein, L2. We nasally immunized Balb/c mice with a synthetic peptide with the 13 aa HPV 16 L2 sequence, and examined the antibodies elicited. ELISA showed that the immunization induced predominantly IgG and IgA antibodies cross-binding to L1/L2-capsids of HPVs 6, 16, and 18 in sera and in vaginal secretions, respectively. The serum containing the IgG antibody and the vaginal wash containing the IgA antibody neutralized HPV 16 pseudovirions and HPV 11 authentic virions, as shown by surrogate infectivity assays. From their cross-binding activity for HPV 16 and 18, the peptide-induced antibodies can probably cross-neutralize most of the genital HPVs. The peptide-induced neutralizing activity in vaginal wash was comparable to that induced by nasally immunization with HPV 16 L1-capsids. Unlike Balb/c, C57BL/10, which has different MHC class II, did not respond to the peptide immunization, but aa substitutions in the peptide to fulfill the requirement for the C57BL/10 agretope rendered the modified peptides immunogenic. The results provide a basis for development of a peptide vaccine against broad-spectrum of genital HPVs for humans.

Antigen-forming defective viruses of simian virus 40.

The presence of two kinds of defective virus particles was demonstrated in SV40 preparations obtained by serial undiluted passage in African green monkey kidney (GMK) cell cultures. They were designated T and V particles, respectively. The former produces only T antigen and the latter V antigen in the nuclei of GMK cells, without producing infectious progeny. A tentative method of the titration of each particle is presented. The purified virion preparation obtained by the third undiluted passage was shown to consist mainly of physically heterogeneous defective particles. The average buoyant density of defective virions was significantly lower than that of plaque-forming particles. T particles were present in the light virion population.

Human papillomavirus type 16 E7 protein expressed in Escherichia coli and monkey COS-1 cells: immunofluorescence detection of the nuclear E7 protein.

Human papillomavirus type 16 E7 protein, expressed in Escherichia coli as fusion protein lac alpha peptide-E7 (lac-E7), was purified by electrophoresis and used to immunize rabbits to raise antisera. The anti-lac-E7 sera recognized another bacterially expressed fusion protein trpE-E7 by the Western blot method. The antisera were used to identify E7 protein transiently expressed in monkey COS-1 cells from an SV40-derived expression plasmid containing E7 gene. Like the E7 protein from CaSki cells, the majority of 19K E7 protein in the transfected COS-1 cells was found by immunoprecipitation in the soluble cytoplasmic fraction. By immunofluorescence staining, however, the E7 protein was detectable in the nuclei of the transfected COS-1 cells. The results suggest that the E7 protein expressed in monkey cells is nuclear, although it is readily released from nuclei when the cell structure is broken.

Human papillomavirus type 16 E6 proteins with glycine substitution for cysteine in the metal-binding motif

The human papillomavirus type 16 (HPV 16) E6 is a 151 amino acid protein containing four metal-binding motifs, Cys-X-X-Cys. We constructed and characterized three mutants with Gly substitutions for Cys within the motif; for Cys-66, for Cys-136, and for both, respectively. Zinc binding to bacterially expressed E6 was markedly reduced by the substitution for Cys-66, but DNA binding was unaffected by any of these mutations. Immunofluorescence staining showed that, whereas the E6 expressed in monkey COS-1 cells appeared mostly nuclear, the Cys-66 mutant appeared cytoplasmic. Subcellular fractionation followed by immunoprecipitation showed that the E6 in COS-1 cells was located in the membrane, nuclear, and nuclear-wash fractions, but not in the soluble cytoplasmic fraction, and that the nuclear Cys-66 protein was markedly reduced. The mutant proteins in COS-1 cells appeared to be less stable than the wild type, because the immunofluorescent cells were fewer and because the E6 bands in autoradiograms were less dense. The substitution mutants lost their capacity to enhance HPV 16 E7 transformation of rat 3Y1 cells. The data indicate that Cys-66 plays a crucial role for zinc binding and nuclear localization of E6 and that both Cys-66 and Cys-136 are required for a stable or functional structure of E6.

Independent association of antibodies against human papillomavirus type 16 E1/E4 and E7 proteins with cervical cancer

The E4 open reading frame (ORF) of human papillomaviruses (HPVs) is transcribed in abundant mRNAs encoding an E1/E4 fusion gene during the productive infection, and the HPV 16 E7 ORF encodes an oncoprotein detectable in the cell lines derived from cervical carcinoma. We examined 421 human sera, which included 108 samples from the patients with cervical carcinoma, for the presence of IgG antibodies against the HPV 16 E4 and E7 proteins by enzyme-linked immunosorbent assay. Bacterially expressed fusion protein lac-E1/E4 and nonfusion protein E7 were purified and used as antigens. All of the 22 serum samples positive for anti-E7 antibody and the 11 out of 15 samples positive for anti-E1/E4 antibody were from the patients with cervical carcinoma, but only one sample was found to contain both anti-E1/E4 and anti-E7 antibodies. These findings show specific and independent association of these antibodies with cervical carcinoma.

Expression of human papillomavirus type 16 E7 gene induces DNA synthesis of rat 3Y1 cells

Human papillomavirus type 16 (HPV 16) open reading frames (ORF) E6, E7, and E6E7, placed under the control of dexamethasone-inducible mouse mammary tumor virus long terminal repeat, were introduced into rat 3Y1 cells, an immortalized fibroblast line, with the aid of neomycin-selection. The cell clones containing inducible HPV 16 ORFs were selected and examined for DNA synthesis. Following induction of HPV mRNA synthesis by the hormone, DNA synthesis was stimulated in the cells containing E7 or E6E7 ORFs. The data indicate that expression of the HPV 16 E7 gene is mitogenic for rat 3Y1 cells.

Inhibition of replication of HIV-1 at both early and late stages of the viral life cycle by single-chain antibody against viral integrase.

Retroviruses including HIV-1 integrates a DNA copy of their RNA genome into cellular DNA of the infected cell. This reaction, essential and unique to replication of retroviruses, is mediated by the viral enzyme, integrase (IN). We constructed a recombinant gene encoding a single-chain, antigen-binding peptide (scAb2-19), which interacted with a carboxyl terminal part of HIV-1 IN. HeLa CD4 cells expressing scAb2-19 localized in either cytoplasmic or nuclear compartment were resistant to HIV-1 infection at an multiplicity of infection (MOI) of 0.25 or 0.063, but the resistance was overcome when MOI was increased to 1. To determine whether this resistance was due to inhibition of the early events, transduction experiments were performed with a replication-incompetent HIV-1 vector carrying bacterial lacZ driven by an internal Tat-independent cytomegalovirus immediate early promoter. Both cytoplasmic and nuclear expressions of scAb2-19 resulted in decrease in the transduction efficiency on HeLa CD4 cells. This implies that an early step of replication--before or during integration--was affected by the scAb2-19. Furthermore, cytoplasmic expression of scAb2-19 did not affect the viral amount released from the cells transfected with HIV-1 infectious clone DNA (pLAI). However, infectivity relative to reverse transcriptase activity was lower for virions released from the 293T cells cotransfected with pLAI and the cytoplasmic scAb2-19 expression plasmid than for those released from the 293T cells transfected with pLAI alone. This implies that scAb2-19 reduced infectivity of released virions by interfering a late step of the viral replication. The single-chain, antigen-binding peptide molecule may prove useful not only for studies of the functions of IN and its role in the viral life cycle but also for developing a gene therapy strategy against AIDS.