Oncogenicity of AKR endogenous leukemia viruses.

Four biologically distinct groups of endogenous murine leukemia virus (MuLV) have been isolated from AKR mice. These viruses included (i) ecotopic XC+ MuLV that occur in high titer in normal tissues and serum of AKR mice throughout their life span, (ii) ecotropic XC- MuLV that are produced in high titers by leukemia cells, (iii) xenotropic MuLV that are readily demonstrable only in aged mice, and (iv) polytropic MuLV thatarise in the thymuses of aged mice as a consequence of genetic recombination between ecotropic and xenotropic MuLV. Virus of each of these biological classes were assayed in AKR mice for their ability to accelerate the occurrence of spontaneous leukemia. Certain isolates of ecotropic XC- MuLV and polytropic MuLV were found to have high oncogenic activity. These viruses induced 100% leukemias within 90 days of inoculation. In contrast, ecotropic XC+ MuLV that were obtained from AKR embryo fibroblasts and xenotropic MuLV that were obtained from the lymphoid tissues of aged AKR mice did not demonstrate oncogenic activity. These findings demonstrate fundamental differences between XC- and XC+ ecotropic MuLV that are found in leukemic and normal tissues, respectively. Furthermore, these findings point to the role of ecotropic XC- and polytropic MuLV in the spontaneous leukemogenesis of AKR mice.     

Lack of Significant Oncogenicity of Biological Products in Hamsters

A variety of biological products which included live and inactivated viral vaccines, inactivated rickettsial and bacterial vaccines, toxoids, and a multiple bacterial antigen product did not appear to be oncogenic for newborn hamsters following a single subcutaneous inoculation. The incidence of spontaneous tumors was approximately 4.7%, and this figure was not significantly altered by the inoculation of any of the test materials.     

Cancer selective adenoviruses

Ten years ago Frank McCormick proposed dl1520 as an oncolytic adenovirus. Although great as an inspiration for better oncolytic viruses it was far from a good product. As Onyx-015, it underwent a wish-fulfilling clinical development program seizing the opportunity left by its p53-targeted non-replicative counterpart Ad-p53. Now, facing a skeptical environment, more selective and potent oncolytic adenoviruses await their clinical opportunity. However, advance in key issues remains elusive, such as, selectivity or retargeting at the level of cell receptors to improve pharmacokinetics. Preclinical models and a few clinical data on biodistribution show that only a minimal proportion of the injected dose reaches the tumors after systemic administration. Once in the tumor, the virus must overcome barriers to efficient spread imposed by stroma and immune responses. Arming the oncolytic virus with transgenes is a natural combination of virotherapy and gene therapy strategies. Transgenes that increase virus production or cellular spread may help to overcome these barriers. Cytotoxic transgenes can help to eliminate tumor cells but need to be compatible with efficient virus replication. These challenges require a careful approach to clinical development and a great deal of collaboration to launch clinical tests with a virus backbone that contains intellectual property from multiple sources.     

STP and Tip Are Essential for Herpesvirus Saimiri Oncogenicity

Mutant forms of herpesvirus saimiri (HVS) subgroup C strain 488 with deletions in either STP-C488 or Tip were constructed. The transforming potentials of the HVS mutants were tested in cell culture and in common marmosets. Parental HVS subgroup C strain 488 immortalized common marmoset T lymphocytes in vitro to interleukin-2-independent growth, but neither of the deletion mutants produced such growth transformation. Wild-type HVS produced fatal lymphoma within 19 to 20 days of experimental infection of common marmosets, while HVS ΔSTP-C488 and HVS ΔTip were nononcogenic. Virus was repeatedly isolated from the peripheral blood of marmosets infected with mutant virus for more than 5 months. These results demonstrate that STP-C488 and Tip are not required for replication or persistence, but each is essential for transformation in cell culture and for lymphoma induction in common marmosets.     

Transformation by human adenoviruses

When, approximately 10 years ago, it was shown that the functions essential for cell transformation were localized in a small region of the adenovirus genome, a DNA segment which at that time was thought to be capable of encoding two or three average-sized proteins at most, it seemed reasonable to hope that an understanding of the mechanisms by which adenoviruses transform cells might be quickly achieved. While such optimism might be forgiven, it was quite clearly naive in the extreme. As a consequence of mRNA splicing and the use of overlapping reading frames the number of proteins encoded within E1 is 2-3-times greater than would have been predicted a decade ago, and post-translational modifications may add another dimension of complexity. In fact it has taken nearly all of the past decade just to identify the proteins encoded in E1 and to characterize them in the most rudimentary way. However, we have now entered a period in which new information is accumulating at an extremely rapid rate as a result of several major technical and fundamental advances. Chief among these are the use of recombinant DNA techniques, particularly site-directed mutagenesis, which combined with methods for introducing mutations made in cloned sequences back into infectious virus, clearly represents a powerful approach to studying the functions of transforming proteins. In addition, the ability to express transforming proteins in bacteria and to produce large amounts of highly purified proteins which previously were only just detectable in infected and transformed cells is a major breakthrough. Advances in immunological techniques, particularly the development of monoclonal antibodies and antisera against synthetic peptides, have enormously simplified the task of detecting and characterizing E1 proteins. Finally, recent results suggesting that adenovirus transforming proteins may be functionally and structurally similar to other oncogenes brings a new perspective to the study of oncogenic transformation. Have all the proteins involved in transformation by adenoviruses been identified? It seems probable that all those virally coded proteins which play a major role are now known but of course minor players in the cast could still be waiting in the wings. We have pointed out that viral functions encoded outside region E1 may have some importance at least in initiation of transformation by virions and have speculated on the possibility that one or more of these may be involved in the integration of viral DNA into the host cell chromosome.(ABSTRACT TRUNCATED AT 400 WORDS)     

Replication of ovine adenoviruses.

The replication of ovine adenoviruses shows intertypic and intratypic variations, e.g. the extracellular and intracellular virus yield of ORT/111, a strain related to bovine adenovirus type 2, reached its peak 40 to 46 hr postinfection, in contrast to GY/14, a strain classified as ovine adenovirus type 1, which required 52 to 58 hr to reach the highest yield. The replication cycle was not appreciably influenced either by rolling of the tube cultures or the age (between 0 and 7 days) of the supporting ovine cell culture. All the strains under study replicated at 40 degree C more rapidly than at 34 degree C or 37 degree C. There was some intratypic variation in the replication of strains at different temperatures. Ovine adenoviruses replicated well after three consecutive passages in cultured lamb testicle or secondary heterologous cells such as calf kidney, calf testicle or pig kidney cell cultures as well as the MDBK cell line and a cell line from the calf trachea. The tissue spectra of strains showed intratypic variations. As examined by direct immunofluorescence, fluorescing adenovirus antigen appeared first at 8 hr postinfection and wharacter of fluorescence are described.     

Replicative adenoviruses for cancer therapy

Rapid advances are being made in the engineering of replication-competent viruses to treat cancer. Adenovirus is a mildly pathogenic human virus that propagates prolifically in epithelial cells, the origin of most human cancers. While virologists have revealed many details about its molecular interactions with the cell, applied scientists have developed powerful technologies to genetically modify or regulate every viral protein. In tandem, the limited success of nonreplicative adenoviral vectors in cancer gene therapy has brought the old concept of adenovirus oncolysis back into the spotlight. Major efforts have been directed toward achieving selective replication by the deletion of viral functions dispensable in tumor cells or by the regulation of viral genes with tumor-specific promoters. However, the predicted replication selectivity has not been realized because of incomplete knowledge of the complex virus-cell interactions and the leakiness of cellular promoters in the viral genome. Capsid modifications are being developed to achieve tumor targeting and enhance infectivity. Cellular and viral functions that confer greater oncolytic potency are also being elucidated. Ultimately, the interplay of the virus with the immune system will likely dictate the success of this approach as a cancer therapy.     

Cell receptors for human adenoviruses

During the early stage of the adenovirus infection, the virion binds to a "primary receptor" on the host cell plasma membrane via the fibre projection jetting out of the penton base capsomers located at the twelve apices of the icosahedral capsid. The second step consists of a receptor-mediated endocytosis which involves membrane integrin molecules (the "secondary receptors") and the RGD and/or LDV motifs of penton base. The latter step is inhibited at low temperature, whereas virus attachment to its primary receptor is temperature-independent. Two different primary receptors with a high affinity for the Adenovirus have been recently identified. One is common to Coxsackievirus B3 and adenovirus (CAR), the other one corresponds to a conserved region of the alpha-2 domain of the heavy chain of the major histocompatibility complex class I molecules (MHC-I-alpha 2), overlapping tryptophane-167. The receptor usage by the virus is governed by both cellular and viral parameters. On the cellular side, the relative abundance of one versus the other type of primary receptors would theoretically determine the virus choice: CAR receptor has been mainly found in tissues from mesodermic origin, whereas MHC-I-alpha 2 is ubiquitous. On the virus side, the molecular determinants of the receptor usage have been mapped to the terminal knob of the fiber projection, and have been found to be different for CAR and MHC-I-alpha 2. CAR recognizes linear motifs in fiber knobs in a subgroup-dependent manner, as it binds to all Adenovirus serotypes except for the subgroup B members. MHC-I-alpha 2 however recognizes conformational epitopes carried by fiber knobs from all serotypes tested including subgroup B members. These results should have significant implications in the cell targeting of adenoviral vectors used in gene therapy.     

Induction of cell death by adenoviruses

Adenoviruses have proved to be excellent tools for gaining insight into the regulation, and deregulation, of the mammalian cell cycle. With the widespread clinical use of gene therapy fast approaching, there comes a need for a better understanding of how the cell death process is regulated. A greater understanding will allow the development of therapeutic approaches that both maximise transgene expression while minimising cytotoxicity to the target cell. Consequently, much adenovirus research has centered on understanding the mechanisms governing adenovirus induced cell death or apoptosis. This review discusses recent advances in the field of adenovirus cell death regulation and evaluates the roles of implicated gene products and their respective data. The data suggest the existence of multiple virus gene products involved in cell death regulation and point towards several distinct, yet related, cell death pathways. A discussion of the shortcomings of current adenoviral research, along with a proposed model based upon the data is also given.     

Oncogenicity of virulent Marek's disease virus cloned as bacterial artificial chromosomes

Marek's disease virus (MDV) is an oncogenic alphaherpesvirus that induces T-cell lymphomas in poultry. We report the construction of bacterial artificial chromosome (BAC) clones of the highly oncogenic RB-1B strain by inserting mini-F vector sequences into the U(S)2 locus. MDV reconstituted from two BAC clones induced rapid-onset lymphomas similar to those induced by the wild-type virus. Virus reconstituted from another BAC clone that showed a 7.7-kbp deletion in the internal and terminal unique long repeat regions was nononcogenic, suggesting that the deleted region may be associated with oncogenicity. The generation of the oncogenic BAC clones of MDV is a significant step in unraveling the oncogenic determinants of this virus.