Complementation for replication by unrelated animal viruses containing DNA genomes

Coinfection in the host can result in functional complementation between live vaccines and virulent virus

One of the greatest achievements of the last century is the development of vaccines against viral diseases. Vaccines are essential for battling infectious diseases and many different formulations are available, including live attenuated vaccines. However, the use of live attenuated vaccines has the potential for adverse effects, including reversion of pathogenicity, recombination, and functional complementation in the host. Marek’s disease is a serious disease in poultry controlled by live attenuated vaccines that has resulted in increased virulence over the decades. Recombination between circulating field viruses or vaccines is a proposed mechanism for the increase in virulence, however, complementation between vaccines and field strains has not been demonstrated in chickens. Here, we describe functional complementation of vaccines with virulent virus to functionally complement transmission and spread in the host. Using the natural virus-host model of Marek’s disease in chickens, our results show dual infection of target cells in chickens with vaccine and virulent virus providing the opportunity for recombination or complementation to transpire. Interestingly, our controlled results showed no evidence of recombination between vaccine and virulent virus, but functional complementation occurred in two independent experiments providing proof for complementation during natural infection in vaccinated individuals. These results suggest complementation as a potential mechanism for vaccine-mediated viral evolution and the potential for complementation should be taken into consideration when developing novel vaccines.

Mutations at U2555, a tRNA-protected base in 23S rRNA, affect translational fidelity

A plasmid carrying a mutation in the highly conserved base U2555 in Escherichia coli 23S rRNA was isolated by selecting for suppression of the -1 frameshift mutation trpE91. U2555 is normally protected in chemical footprinting experiments by the aminoacyl residue of A-site-bound tRNA. Substitution of U2555 by adenine or guanine (but not by cytosine) increased readthrough of all three stop codons and +1 and -1 frameshifting. These effects on translational fidelity demonstrate the importance of U2555 for selection of the correct tRNA at the ribosomal A site.

Pestivirus is a common contaminant in maedi-visna and caprine arthritis-encephalitis virus stocks

Eight different laboratory stocks of maedi-visna or caprine arthritis-encephalitis virus were examined for the presence of pestiviruses by a fixed-cell immunoperoxidase assay with polyclonal and monoclonal antibodies. All of the viral stocks examined were found to contain noncytopathic pestivirus contaminants. The panel of monoclonal antibodies could not type the isolates as being more related to bovine virus diarrhea virus or border disease virus. However, the results did indicate that all isolates were not the same, except for two from the same laboratory where the source of pestivirus contamination may have been common.

Mechanisms associated with the generation of biologically active human immunodeficiency virus type 1 particles from defective proviruses

The human immunodeficiency virus (HIV) is the etiological agent of acquired immunodeficiency syndrome (AIDS). HIV exhibits extensive genetic diversity and it is apparent that an infected individual contains different populations of distinct viral strains, a large proportion of which has been found surprisingly to be defective for replication. A similar phenomenon has also been observed with some cell lines that are known to produce infectious viral particles but harbor defective proviral genomes. Here, we investigated the molecular basis of this phenomenon by cloning proviral genomes of HIV from a cell line that was capable of producing high titers of biologically active HIV particles that readily induced syncytia with CD4+ cell lines and peripheral blood lymphocytes. This cell line was found to contain five proviral genomes, all of which, when tested individually, failed to produce replication-competent viruses upon transfection into human cells. However, when a specific combination of two proviral genomes was used in such transfection studies, it was possible to obtain biologically active, replication-competent viral particles that infected and replicated in CD4+ cell lines and induced syncytia characteristic of HIV. Such a result may be due to homologous recombination between proviral DNAs occurring in cells after transfection and/or complementation of replication-defective proviral DNAs. The diploid nature of the viral RNA genome present in the viral particle may enable the persistence of defective HIV genomes.