Biological and molecular characterization of naturally occurring recombinant avian leukosis virus isolated from commercial layers affected with myeloid leukosis

Avian leukosis virus (ALV) is an economically important poultry pathogen resulting in low productivity and tumor mortality in chickens. Limited information is currently available about natural recombination among different ALV subgroups and pathogenesis in chicken. Several biological and molecular studies were conducted to characterize seven isolates (AF 115-1, AF 115-5, AF 115-7, AF 115-10, AF 115-13, AF 115-14 and AF 115-16) of a naturally occurring recombinant ALV from commercial layers affected with myeloid leukosis.;The first aim was to characterize seven isolates (AF 115-1, AF 115-5, AF 115-7, AF 115-10, AF 115-13, AF 115-14 and AF 115-16) of a naturally occurring recombinant ALV by biological assays and polymerase chain reaction. Closely and distinctly related strains of naturally occurring recombinant ALVs were identified using various diagnostic techniques. The host range and antigenic relationships of the seven AF 115 isolates were comparable with those of the first characterized naturally occurring recombinant ALVB/J (AF 115-4). However, the PCR amplification results, using different primer sets, indicated the presence of recombinant ALV-B/J sequences in proviral DNA of all AF 115 isolates. Moreover, three isolates (AF 115-5, AF 115-7 and AF 115-10) demonstrated different patterns of PCR amplification from the recombinant ALV-B/J (AF 115-4) and the rest of the isolates, suggesting the presence of mutations and rearrangements in different genomic regions.;The second aim was to molecularly characterize these seven AF 115 isolates by DNA sequence analyses of various genomic regions. All AF 115 isolates were confirmed to be recombinant ALVs with different genomic regions representing different ALV subgroups. Five isolates (AF 115-1, AF 115-7, AF 115-13, AF 115-14 & AF 115-16) were closely related to the naturally occurring recombinant ALV-B/J, containing a subgroup B gp85, subgroup E gp37 and subgroup J 3' UTR (including DR1 and E element) and LTR. On the other hand, AF 115-5 had a subgroup A gp85 and is believed to be contain a mixture of two recombinant ALVs (ALV-A/J & ALV-B/J). The 3' UTR and LTR of AF 115-10 contained base deletions and insertions and were highly associated with a recombinant ALV isolate (ALV TymS_90). In addition, all AF 115 isolates contained important potential sites for protein structure, stability and function in the envelope glycoproteins as well as transcription regulatory elements in the 3' UTR and LTR. It is believed that recombination hotspots exist between SU and TM domains of the envelope, between the gp37 and the 3' UTR upstream of the DR1 region as well as 3' UTR and LTR.;Therefore, these research studies have added new information on ALV diversity (mutations, rearrangement and recombination) and should aid in improving the diagnosis and control of ALV infection in field flocks.