Selective shedding and congenital transmission of endogenous avian leukosis viruses

Development and Application of a Reverse-transcription Recombinase-Aided Amplification Assay for Subgroup J Avian Leukosis Virus

Subgroup J Avian leukosis virus (ALV-J) is an important pathogen of poultry tumor diseases. Since its discovery, it has caused significant economic losses to the poultry industry. Thus, the rapid detection of molecular level with strong specificity is particularly important whether poultry are infected with ALV-J. In this study, we designed primers and probe for real-time fluorescent reverse-transcription recombinase-aided amplification assay (RT-RAA) based on the ALV-J gp85 sequence. We had established a real-time fluorescent RT-RAA method and confirmed this system by verifying the specificity and sensitivity of the primers and probe. In addition, repeatability tests and clinical sample regression tests were used for preliminary evaluation of this detection method. The sensitivity of established method was about 10¹ copies/μL, and the repeatability of the CV of the C_T value is 4%, indicating repeatability is good. Moreover, there was no cross-reactivity with NDV, IBV, IBDV, H9N2, MDV, and REV, and other avian leukosis virus subgroups, such as subgroups A, B, C, D, K and E. Importantly, the real-time fluorescent RT-RAA completed the test within 30 min at a constant temperature of 41°C. Forty-two clinical samples with known background were tested, and the test results were coincided with 100%. Overall, these results suggested that the real-time fluorescent RT-RAA developed in this study had strong specificity, high sensitivity, and good feasibility. The method is simple, easy, and portable, that is suitable for clinical and laboratory diagnosis, and provides technical support for the prevention and control of ALV-J.

Isolation and identification of avian leukosis viruses: a review

Avian leukosis virus (ALV) is the most common naturally occurring avian retrovirus that can cause a variety of neoplastic disease conditions in chickens. In addition to causing neoplasia, ALV is known to be associated with reduced productivity and other production problems in affected flocks. Biological and molecular assays for the detection of ALV are very useful in identification and classification of new isolates, safety testing of vaccines and in testing pathogen-free and other breeder flocks for freedom of virus infection. However, such assays are not particularly helpful in the diagnosis of virus-induced neoplastic disease of poultry, as avian oncogenic viruses are widespread and infection in the absence of tumor formation is common. Current technology based on molecular and antigenic characteristics of the virus is being used to develop more sensitive and specific procedures for isolation and identification of ALV. This review is primarily focused on the discussion of current technology most commonly used in isolation and identification of ALV associated with clinical disease in chickens.

Proviral integrations and expression of endogenous avian leucosis virus during long term selection for high and low body weight in two chicken lines

Background

Long-term selection (> 45 generations) for low or high juvenile body weight from a common founder population of White Plymouth Rock chickens has generated two extremely divergent lines, the LWS and HWS lines. In addition to a > 9-fold difference between lines for the selected trait, large behavioural and metabolic differences between the two lines evolved during the course of the selection. We recently compared gene expression in brain tissue from birds representing these lines using a global cDNA array analysis and the results showed multiple but small expression differences in protein coding genes. The main differentially expressed transcripts were endogenous retroviral sequences identified as avian leucosis virus subgroup-E (ALVE).

Results

In this work we confirm the differential ALVE expression and analysed expression and number of proviral integrations in the two parental lines as well as in F₉ individuals from an advanced intercross of the lines. Correlation analysis between expression, proviral integrations and body weight showed that high ALVE levels in the LWS line were inherited and that more ALVE integrations were detected in LWS than HWS birds.

Conclusion

We conclude that only a few of the integrations contribute to the high expression levels seen in the LWS line and that high ALVE expression was significantly correlated with lower body weights for the females but not males. The conserved correlation between high expression and low body weight in females after 9 generations of intercrosses, indicated that ALVE loci conferring high expression directly affects growth or are very closely linked to loci regulating growth.

Response of white leghorn chickens of various genetic lines to infection with avian leukosis virus subgroup J

In Experiment 1, chickens from various white leghorn experimental lines were inoculated with strain ADOL-Hcl of subgroup J avian leukosis virus (ALV-J) either as embryos or at 1 day of age. At various ages, chickens were tested for ALV-J induced viremia, antibody, and packed cell volume (PCV). Also, at 4 and 10 wk of age, bursal tissues were examined for avian leukosis virus (ALV)-induced preneoplastic lesions with the methyl green-pyronine (MGP) stain. In Experiment 2, chickens harboring or lacking endogenous virus 21 (EV21) were inoculated with strain ADOL-Hcl of ALV-J at hatch. All embryo-inoculated chickens in Experiment 1 tested positive for ALV-J and lacked antibody throughout the experimental period of 30 wk and were considered viremic tolerant, regardless of line of chickens. By 10 wk of age, the incidence of ALV-J viremia in chickens inoculated with virus at hatch varied from 0 (line 0 chickens) to 97% (line 1515); no influence of ALV-J infection was noted on PCV. Results from microscopic examination of MGP-stained bursal tissues indicate that ALV-J can induce typical ALV-induced transformation in bursal follicles of white leghorn chickens. Lymphoid leukosis and hemangiomas were the most common ALV-J-induced tumors noted in chickens in Experiment 1. At termination of Experiment 2 (31 wk of age), 54% of chickens harboring EV21 were viremic tolerant compared with 5% of chickens lacking EV21 after inoculation with ALV-J at hatch. The data indicate that genetic differences among lines of white leghorn chickens, including the presence or absence of EV21, can influence response of chickens to infection with ALV-J.

Role of contact and genetic transmission of endogenous virus-21 in the susceptibility of chickens to avian leukosis virus infection and tumors

The role of contact and genetic transmission of endogenous virus-21 (EV21) on response of chickens to avian leukosis virus (ALV) infection and tumors was studied. F1 progeny of a cross between RPRL late-feathering (LF) line EV21+ males and RPRL early feathering (EF) line 15B1 females harboring or lacking EV21 were used. The EF chicks lacking EV21 were inoculated with a field strain of subgroup A ALV at hatch and contact exposed to LF, EV21+ hatchmates for various time intervals. In a second experiment, EV21 contact-exposed and unexposed EF chicks as well as LF, EV21+ hatchmates were inoculated with ALV at various ages. Chickens were tested for ALV-induced viremia and antibody and were observed for tumors until 24 wk of age. Antibody to EV21 in EF chickens contact-exposed to LF, EV21+ hatchmates varied from 10 to 65%, and was detected by 10 wk of age. By 24 wk of age, ALV-induced viremia and tumors in EF chickens varied from 5 to 30%, and from 15 to 32%, respectively, regardless of exposure to EV21. The incidence of ALV-induced tumors was significantly higher in LF chickens genetically infected with EV21 than in EV21 contact-exposed or unexposed EF chickens, but only in chickens inoculated with ALV at hatch. The data suggest that contact infection with EV21 has no influence on ALV infection and tumors. The data also suggest that genetic transmission of EV21 may increase susceptibility of chickens to ALV infection and tumors following infection with ALV at hatch, but not at 4 wk of age or older.

Avian retroviruses

Avian leukosis virus (ALV) and reticuloendotheliosis virus (REV) are the most common naturally occurring avian retroviruses associated with neoplastic disease conditions in domesticated poultry. Avian leukosis virus infects primarily chickens, whereas REV infects chickens, turkeys, and other avian species. In addition to causing tumors, both ALV and REV can reduce productivity and induce immunosuppression and other production problems in affected flocks.

Lymphoid leukosis viruses, their recognition as 'persistent' viruses and comparisons with certain other retroviruses of veterinary importance

Diseases caused by lymphoid leukosis virus (LLV), a retrovirus, take a long time after infection to develop and have a wide variety of pathological manifestations. This long latent period is characteristic of 'persistent virus infections'. Disease produced by LLV infection and its underlying mechanisms is compared with 'persistent' infections caused by other retroviruses in birds and mammals of veterinary importance. The diseases considered for comparison are those caused by reticuloendotheliosis, feline leukaemia, bovine leukosis and equine infectious anaemia viruses. There are significant changes in the immunological status in all diseases caused by these viruses. LLV infections follow this trend with, in manifestations of neoplastic disease, a perturbation of the normal switch that occurs from IgM to IgG synthesis. There are also indications of other immunological disturbances. Factors other than immunological disturbances may contribute to the length of time after infection required for the many forms of LLV infection to appear. Such additional factors may include the operation of 'biological clocks', such as the arrival of sexual maturity, and also the very nature of retroviruses. These factors, like the immunological changes, play major roles in the maintenance and progression of persistent retrovirus infections.

Reflections on the pathogenesis of diseases caused by the acute avian leukosis/sarcoma viruses with special reference to avian erythroblastosis

The various diseases that follow experimental infection with the acute and non-acute avian oncoviruses are discussed with special reference to the pathogenesis of avian erythroblastosis. One view, based on in vitro studies, sees erythroblastosis as the product of a failure in the differentiation of virus-infected stem cells to mature erythrocytes, as a result of cell 'transformation'. The results of some in vivo studies, however, point to a resemblance of the disease to a haemolytic anaemia, where cellular death is an important component. It seems probable that the disease is the result of transformation of cells of the erythroblastic series followed by the death of many of these cells due to influences that have not yet been determined. Determination of the causes of this cellular death may prove to be as important for our understanding of the problem of leukaemia as the work that has already been accomplished in explaining the causes of cell transformation. It is also suggested that the tendency of gs amino acid sequences of the avian leukosis viruses and mouse leukaemia viruses to form fusion proteins with a variety of proto-oncogenes may be part of a wider phenomenon, and that these sequences may fuse with other proteins, altering their properties. More work is required on the possibility that there is an undiscovered immunological component in the progression of the L/S diseases.

Phylogenetic distribution of the novel avian endogenous provirus family EAV-0

A new family of related endogenous proviruses, existing at 50 to 100 copies per haploid genome and distinguishable by remarkably short long terminal repeats, has been described for domestic chickens (Gallus gallus subsp domesticus). In this communication, by using Southern blot analysis and probes derived from both internal viral sequences and locus-specific, cellular flanking sequences, we studied the genetic distribution of this family of moderately repetitive avian endogenous retroviruses within the genomes of four Gallus species. Eight inbred lines of domestic chickens, the evolutionary progenitor to the domestic chicken (red jungle fowl), and two more distantly related species (grey and green jungle fowl) were studied. All Gallus species harbored this class of elements, although the different lines of domestic chickens and different species of jungle fowl bore distinguishable complements of the proviral loci. Jungle fowl appeared to have fewer copies than domestic chickens. For three randomly isolated proviral loci, domestic chickens (G. gallus subsp. domesticus) and red jungle fowl (G. gallus subsp. gallus) showed only a proviral state, whereas the most primitive and divergent of the jungle fowl, the green jungle fowl (G. varius), consistently demonstrated only preintegration states or disparate alleles. The presence of this family in all Gallus species and of related sequences in other genera suggests that a primordial founding integration event occurred prior to the evolutionary separation of Gallus species and possibly related genera. Additionally, at least one proviral locus has been acquired subsequent to speciation, indicating that this family was actively infectious after the primary founding event. This conserved, repetitive proviral family appears to represent the vestigial remnant of an avian retrovirus class related to and evolutionarily more ancient than the Rous-associated virus-0 family of avian endogenous retroviruses.

Influence of avian leukosis viral sequences on transmission to the egg and embryo

Rous associated virus type-0 (RAV-0), a subgroup E replication-competent endogenous virus of chickens, is associated with a low efficiency of virus shedding into the egg albumen and failure to establish congenital transmission. In contrast, RAV-1, a subgroup A virus of exogenous origin, is efficiently shed into the albumen and readily infects the embryo. Among a series of in vitro constructed recombinants between RAV-0 and RAV-1, we have identified subgroup E recombinants that efficiently shed virus into the egg albumen but do not undergo efficient congenital transmission. The LTR region, subgroup-determining sequences in env, and sequences within a 375 bp Sacl-Xhol fragment at the 5' end of the genome each influenced the efficiency of virus shedding into the albumen. Egg inoculations with viruses differing only in env were used to confirm the low rate of congenital transmission of subgroup E viruses. These studies revealed that subgroup A envelope antigens are at least 100-fold more effective for the establishment of embryonic infection than subgroup E.

Genetic cellular resistance to subgroup E avian leukosis virus in slow-feathering dams reduces congenital transmission of an endogenous retrovirus encoded at locus ev21

The gene ev21, which encodes an infectious endogenous subgroup E avian leukosis virus (ALV-E), designated EV21, is closely linked to the sex-linked, slow-feathering (SF) gene K. To address the relationship between congenital transmission of EV21 and host susceptibility to ALV-E infection, SF roosters that were heterozygous for the dominant gene for susceptibility to ALV-E were mated with ev-negative rapid-feathering (RF), subgroup E-resistant dams to produce SF and RF progeny. The SF female progeny that were heterozygous ALV-E susceptible were viremic at hatch and, when mature, consistently shed the ALV group-specific antigen, p27, in the egg albumen they produced. In contrast, SF females that were homozygous ALV-E resistant were neither viremic at hatch nor shedders of ALV p27. Infectious ALV-E assays of progeny from susceptible and resistant SF dams confirmed that maternal cellular resistance to ALV-E limited congenital transmission of EV21. It is proposed that, in progeny, immunologic tolerance toward exogenous ALV infection due to congenital transmission of antigenically related endogenous viruses may be abrogated by selection for cellular resistance to ALV-E in SF lines used as female parents of feather-sexed crosses.

Influence of congenital transmission of endogenous virus-21 on the immune response to avian leukosis virus infection and the incidence of tumors in chickens

After contact exposure to Strain RPL-40 avian leukosis virus-infected hatchmates, a dilatory neutralizing antibody response and prolonged RPL-40 viremia was found among most pullets that were congenitally infected with endogenous virus 21 (EV21). Conversely, most of the hatchmates that were not congenitally infected seroconverted within 10 wk after exposure to Strain RPL-40 virus. Compared with noncongenitally infected hatchmates, EV-21 infection-induced tolerance to pathogenic avian leukosis viruses was reflected in a significantly higher incidence of lymphomas in congenitally infected hens. The rate of seroconversion and the incidence of RPL-40 virus-induced tumors among noncongenitally infected daughters from slow-feathering dams homozygous resistant to EV were similar to those found among daughters of rapid-feathering dams that lacked genetic locus ev21. Results suggest that selection for resistance to EV may eliminate tolerance toward oncogenic field strains of avian leukosis viruses and may improve the performance of progeny from a feather-sex cross.

Association of the slow feathering (K) and an endogenous viral (ev21) gene on the Z chromosome of chickens

A dominant sex-linked gene, K, regulates slow feathering (SF), whereas a recessive allele, k+, determines rapid feathering (RF) in chickens. This trait provides a convenient and inexpensive approach to gender identification of White Leghorn (WL) chicks at hatch, i.e., in a sex-linked mating using k+/k+ males mated with K/- females, the K/k+ male chicks are SF, and the k+/- females are RF. However, in many WL strains, female progeny of SF dams produce fewer eggs and have higher mortality than progeny of RF dams. This loss in productivity has been attributed to higher infection and shedding rates for leukosis viruses (ALV) in SF than in RF dam lines. Because infectious endogenous viruses (EV) can induce immunological tolerance to ALV, we examined the expression and distribution of ev genes in SF and RF siblings from heterozygous K/k+ sires and k+/- dams. Infectious ALV and EV were detected by cocultivation of frozen heparinized blood cells on selected chick embryo fibroblasts and culture supernatants were tested for viral antigen by enzyme-linked immunosorbent assay tests. Specific ev genes were identified as restriction fragment length polymorphisms after hybridization with a recombinant plasmid containing the complete genome of a Rous-associated virus. It was concluded that ev21 and K genes are tightly linked because, in different WL crosses, all SF chicks inherited ev21 but RF siblings uniformly lacked ev21. Alternatively, the K gene in WL may be a mutation resulting from the insertion of ev21 in the k+ gene. The SF chicks which harbor ev21 expressed infectious EV21; evidence that EV21 may influence susceptibility to ALV is presented.

Embryonic infection with the endogenous avian leukosis virus Rous-associated virus-0 alters responses to exogenous avian leukosis virus infection

We inoculated susceptible chicken embryos with the endogenous avian leukosis virus Rous-associated virus-0 (RAV-0) on day 6 of incubation. At 1 week after hatching, RAV-0-infected and control chickens were inoculated with either RAV-1 or RAV-2, exogenous viruses belonging to subgroups A and B, respectively. The chickens injected with RAV-0 as embryos remained viremic with exogenous virus longer and either failed to develop type-specific humoral immunity to exogenous virus or developed it later than the control chickens not inoculated with RAV-0. The RAV-0-injected chickens also developed neoplasms at a much higher frequency than did the control chickens. We suggest that the lower immune responses of the RAV-0-injected chickens were due to an immunological tolerance to envelope group-specific glycoproteins shared among endogenous and exogenous viruses.