Activation of endogenous type C virus in BALB/c mouse cells by herpesvirus DNA

Reprint of: cancer "causation" by infections--individual contributions and synergistic networks

The search for infectious agents playing a role in human carcinogenesis and their identification remain important issues. This could provide clues for a broader spectrum of cancers preventable by vaccination and accessible to specific therapeutic regimens. Yet, the various ways of interacting among different factors functioning synergistically and their different modes of affecting individual cells should bring to question the validity of the term "causation". It also should put a word of caution into all attempts to summarize criteria for "causality" of infectious agents in cancer development. At least in the opinion of these authors, we would be much better off avoiding these terms, replacing "causal factor" by "risk factor" and grading them according to their contribution to an individual's cancer risk.

Cancer "causation" by infections--individual contributions and synergistic networks

The search for infectious agents playing a role in human carcinogenesis and their identification remain important issues. This could provide clues for a broader spectrum of cancers preventable by vaccination and accessible to specific therapeutic regimens. Yet, the various ways of interacting among different factors functioning synergistically and their different modes of affecting individual cells should bring to question the validity of the term "causation". It also should put a word of caution into all attempts to summarize criteria for "causality" of infectious agents in cancer development. At least in the opinion of these authors, we would be much better off avoiding these terms, replacing "causal factor" by "risk factor" and grading them according to their contribution to an individual's cancer risk.

Chemical induction of endogenous retrovirus particles from the vero cell line of African green monkeys

Endogenous retroviral sequences are present in high copy numbers in the genomes of all species and may be expressed as RNAs; however, the majority are defective for virus production. Although virus has been isolated from various Old World monkey and New World monkey species, there has been no report of endogenous retroviruses produced from African green monkey (AGM) tissues or cell lines. We have recently developed a stepwise approach for evaluating the presence of latent viruses by chemical induction (Khan et al., Biologicals 37:196-201, 2009). Based upon this strategy, optimum conditions were determined for investigating the presence of inducible, endogenous retroviruses in the AGM-derived Vero cell line. Low-level reverse transcriptase activity was produced with 5-azacytidine (AzaC) and with 5'-iodo-2'-deoxyuridine (IUdR); none was detected with sodium butyrate. Nucleotide sequence analysis of PCR-amplified fragments from the gag, pol, and env regions of RNAs, prepared from ultracentrifuged pellets of filtered supernatants, indicated that endogenous retrovirus particles related to simian endogenous type D betaretrovirus (SERV) sequences and baboon endogenous virus type C gammaretrovirus (BaEV) sequences were induced by AzaC, whereas SERV sequences were also induced by IUdR. Additionally, sequence heterogeneity was seen in the RNAs of SERV- and BaEV-related particles. Infectivity analysis of drug-treated AGM Vero cells showed no virus replication in cell lines known to be susceptible to type D simian retroviruses (SRVs) and to BaEV. The results indicated that multiple, inducible endogenous retrovirus loci are present in the AGM genome that can encode noninfectious, viruslike particles.

Alterations in the cellular phenotype induced by herpes simplex viruses

Numerous studies have shown that herpes simplex virus types 1 and 2 (HSV-1, HSV-2) are able to transform the morphological phenotype of rodent cells. Unlike other DNA tumor viruses the transformed cells did not consistently retain or express a given set of viral genes. In fact, transformation could be obtained using fragments of viral DNA that did not wholly encode viral proteins. Of interest within the transforming fragments were sequences which could assume a secondary structure like that of insertion elements. The failure to detect viral DNA in transformed cells led to the hit-and-run hypothesis of HSV transformation. The mechanism by which HSV induces transformation is not understood. Various lines of investigation have shown that HSV is able to cause mutations--both point mutations and gene rearrangements. HSV is also able to induce gene amplification, particularly of sequences harboring an origin of replication such as SV40 or papillomaviruses. Other experiments have shown that HSV can activate the expression of endogenous type C retroviruses. More broadly, HSV has been shown to activate cellular transcription or to switch on the synthesis of host cell proteins not normally expressed in untransformed cells. The failure to detect viral DNA in a high proportion of human anogenital tumors made it difficult to implicate HSV in the etiology of those neoplasias, but it is consistent, however, with the observations on the mode of HSV transformation in vitro, and suggests that HSV could be involved in a multistage process of oncogenic transformation.

Molecular cloning of DNA sequences from cervical intraepithelial neoplasia that hybridize to human cytomegalovirus DNA

We have prepared EMBL3 libraries of DNA extracted from the cervix of a patient with cervical intraepithelial neoplasia (CIN) and isolated seven recombinant clones containing sequences that hybridize to human cytomegalovirus (HCMV) DNA. Restriction analysis of one clone with BamHI and SalI endonucleases revealed that the insert DNA showed a high degree of homology to the HCMV Ad169 genome over the region between the HindIII K/E site and the SalI site located within the BamHI P fragment. The HCMV insert in the CIN clone is integrated and flanked by cellular sequences. The major immediate early gene that encodes a polypeptide of approximately 69 kD was found to be conserved in the CIN clone. Transfection of clones encoding the immediate early region of HCMV resulted in cells that were positive in immunofluorescence studies with two monoclonal antibodies directed against the HCMV 69 kD immediate early polypeptide. Infection of human ectocervical cells with HCMV Ad169 revealed that they could express the 69 kD polypeptide encoded by the immediate early gene but could not replicate the virus, whereas HCMV was able to replicate productively in cultured endocervical cells. HCMV has been shown to activate endogenous retroviruses and also to transcriptionally activate the long terminal repeat of human immunodeficiency virus. Activation of virus and cellular genes by HCMV may be a means by which this virus is involved in the multistage process of oncogenesis and/or the activation of latent infections.

Interferon induced by UV-irradiated murine cytomegalovirus decreases type C virus expression in BALB/c cells treated by 5-iodo-deoxyuridine or cycloheximide

UV-irradiated mouse cytomegalovirus (MCMV) activates at low incidence (2.0 X 10(-4) - 8.0 X 10(-4)) a xenotropic type C virus in Kirsten sarcoma virus-transformed nonproducer BALB 3T3 (KBALB) cells. When KBALB cells are treated with UV-MCMV and subsequently with potent chemical inducers, such as 5-Iododeoxyuridine (IdUrd) or cycloheximide, the activation frequencies of type C virus are significantly lower than in controls exposed to the chemical inducers alone: 60% for IdUrd and 44% for cycloheximide. This decrease of activation appears to be mediated by endogenous interferon (IFN) induced by UV-MCMV in KBALB cells, as could be proven by neutralization of the inhibitory activity. Indeed, after exposure of UV-MCMV-infected cells to anti-IFN gamma globulin, the level of C type virus increases, reaching values obtained with IdUrd or cycloheximide in the absence of UV-MCMV. The inhibitory activity associated with UV-MCMV is dose-dependent and the xenotropic type C virus appears to be more sensitive than the ecotropic (N-tropic) retrovirus of BALB/c cells.

Activation of an endogenous mouse type C virus by UV-irradiated murine cytomegalovirus

Infection of Kirsten sarcoma virus-transformed non-producer BALB-3T3 mouse cells with UV-irradiated mouse cytomegalovirus (MCMV) resulted in activation of a xenotropic type C virus detected by infectious center formation in permissive rat or mink cells. The levels of type C virus activated by MCMV were related to the UV dose applied. Under optimal conditions the frequencies of activation varied from 3.0 to 8.0 x 10(-4). The capacity of MCMV to activate type C virus was abolished by heat inactivation and by neutralization with specific antiserum against MCMV. Virus induction decreased under conditions of cell exposure to hydroxyurea or actinomycin D, inhibitors of DNA and RNA synthesis, respectively, with actinomycin D having a greater inhibitory effect. This suggests that both DNA and RNA synthesis are required for UV-MCMV induction of murine xenotropic virus.

Interaction between cellular and viral genes in the expression of the RSV-induced transformation-specific cell-surface antigen VCSA

Transformation of BHK hamster fibroblasts by an env- strain of Rous sarcoma virus (RSV) leads to the appearance at the cell surface of a virus-induced nonvirion antigen (VCSA), specific for transformation, whose expression is controlled by the transforming src gene. Previous work has shown that a rabbit anti-VCSA serum lyses specifically, in the presence of complement, 51Cr-labelled RSV-transformed cells from different animal species. Now, by competition experiments with a panel of different unlabelled cells we show that the VCSA expressed on RSV-transformed hamster fibroblasts is a complex of at least three distinct antigenic specificities: (1) one expressed on all RSV-transformed fibroblasts, regardless their species and the subgroup or strain of the transforming virus; (2) one cross-reacting with a cell-surface antigen (CSA) expressed at various degrees on untransformed avian fibroblasts, but not on mammalian fibroblasts; (3) one species-specific, present only on RSV-transformed hamster fibroblasts. It is concluded that VCSA is a complex of several antigenic determinants, and that some of these differ in different cells transformed by RSV. This observation indicates that VCSA expression at the cell surface is likely to be the result of the interaction between the viral src gene product pp60src with host cell gene(s) or gene product(s), rather than the simple expression of this molecule at the cell surface.

The role of natural killer cells and antibody-dependent cell-mediated cytotoxicity during murine cytomegalovirus infection

Definition of the functions by which the cellular immune system contributes to control of cytomegalovirus (CMV) infection should permit determination of the specific defects which result in the increased susceptibility to infection of immunosuppressed individuals. Using a murine model, we studied the cytotoxic lymphocyte response to murine CMV infection. This response was found to be biphasic. The initial phase extended from the 3rd to the 6th d after infection, was not genetically restricted, and correlated to a rise in numbers of natural killer (NK) and antibody-dependent killer (K) cells in spleens. The NK- and K-cell responses were preceded, by 24 h, by a rise in serum interferon levels, and occurred before the time when antibody could be measured in serum by neutralization. NK and K cells appear to develop the capacity for specific recognition of CMV-infected cells and the potential to contribute to control of the acute phase of CMV infection.

Reactivation of herpes simplex virus type 2 from a quiescent state by human cytomegalovirus

The ability of human cytomegalovirus to stimulate replication of herpes simplex virus type 2 (HSV-2) was examined. The system used involved HSV-2-infected human embryonic lung cells under conditions (39.5-40 degrees C) in which HSV-2 remains undetectable. Reactivation of HSV-2 was maximal and persisted for the longest duration when cultures were superinfected with 0.02 plaque-forming unit of human cytomegalovirus per cell. Infectious HSV-2 appeared 2 days after superinfection with human cytomegalovirus and ranged from 10(2) to 10(6) plaque-forming units per culture. Virus reactivated from these cultures was neutralized by rabbit immune serum produced against HSV-2. The specificity of this interaction was demonstrated by various criteria: production of HSV-2 was not observed in cultures treated with mock infecting fluid, and inactivation of human cytomegalovirus by heat, ultraviolet irradiation, or immune serum prior to superinfection eliminated its ability to induce HSV-2 replication. These results sugges that interaction between these two human herpesviruses may be of importance in herpesvirus latency in vivo.