Reticuloendotheliosis Virus (REV) Long Terminal Repeats Incorporated in the Genomes of Commercial Fowl Poxvirus Vaccines and Pigeon Poxviruses without Indication of the Presence of Infectious REV

Abstract

Because of reticuloendotheliosis virus (REV) contamination in commercial poultry vaccines, polymerase chain reaction (PCR) assays have been described to increase the sensitivity of biological assays used to detect REV in vaccines. The PCR assay designed to amplify the long terminal repeat (LTR) region of REV identified REV LTRs in many of the commercial fowl poxvirus (FPV) vaccines evaluated. These commercial vaccines were not thought to be contaminated with replicating REV because of the lack of REV outbreaks, the lack of in vitro amplification, and lack of a serologic response to REV. As previously described, the FPV S vaccine strain is known to carry infectious integrated proviral REV, whereas FPV M vaccine strain and its derivatives carry integrated LTRs or remnants of REV proviral DNA inserted into the FPV genome. Another PCR assay designed to amplify the envelope gene of REV was used to verify that the envelope proviral gene was not present in REV LTR PCR-positive samples. Southern blot analysis with REV LTR probes hybridized to the 9-kb EcoRI genomic fragment of all FPV and pigeon poxviruses evaluated, whereas the envelope probe did not hybridize to any poxvirus genome. Sequence analysis of the 9-kb EcoRI fragment indicated that an integrated REV LTR exists in the 9-kb EcoRI of some poxvirus genomes. A new PCR assay designed to amplify integrated REV LTRs in the 9-kb EcoRI fragment identified complete and incomplete integrated REV LTRs in all FPV and pigeon poxvirus genomes evaluated.

Reticuloendotheliosis Virus Sequences within the Genomes of Field Strains of Fowlpox Virus Display Variability

“…Moreover, short nucleotide stretches exhibiting Ͼ70% homology with the R and U3 regions of the long terminal repeat (LTR) of the avian retrovirus reticuloendotheliosis virus (REV) have been found in the DNA of two serotype I oncogenic strains of Marek's disease virus (10). Likewise, remnants of the REV LTR have been retained in the genomes of all FPV vaccine strains so far examined (9,15,22).…”

“…Whether there was only one progenitor or others resulting from independent insertions is also a point of conjecture. However, it should be noted that this association with a retrovirus may be novel in avipoxviruses, since no REV sequences have been found in the DNAs of either canarypox virus, quailpox virus, or a field isolate of pigeonpox virus (13,15). Of equal interest is the origin of the attenuated strains currently used as vaccines.…”

“…Of equal interest is the origin of the attenuated strains currently used as vaccines. Presumably, these have arisen from FPV field strains that have lost the provirus, as their genomes contain only a partial and rearranged LTR (9) or individual LTR (15). Although such deletion mutants have also been detected in field virus populations (9,13,22), the continuous presence of an intact, integrated retrovirus implies that REV expression may be advantageous to their survival and ability to successfully infect poultry.…”

The Knowledge that Human Tumor Virology can Gain from Studies on Avian Tumor Viruses

“…[62][63][64][65] These studies demonstrated that REV-inserted fragments into wild type field FPV isolates were variable in content, although they included the REV long terminal repeats (LTR) of various lengths, and also additional sections of the proviral genome. Although the REV genomic integrations into the FPV genome has been demonstrated only in the last 10 years, the analysis of previous field isolates indicated that such events occurred even 50 years ago.…”

“…63,65 The inserted REV fragment into the FPV differed in composition between wild type FWPV and FWPV vaccine strains. While all the field isolates contained complete REV provirus or various fragments of the 3' and 5' REV-LTRs, in addition to other REV fragments, including the REV env gene, recent FPV vaccine strains included only the REV LTR.…”