Endogenous expression of ASLV viral proteins in specific pathogen free chicken embryos: relevance for the developmental biology research field

We demonstrate that certified SPF chicken embryos have transcriptionally active endogenous ASLV loci (ev loci) capable of expressing ASLV viral proteins, such as p19 and p27, even when those loci are not capable of producing viral particles. We also show that the extent of viral protein expression in embryonic tissues varies not only among flocks but also between embryos of the same flock. In addition, our genetic screening revealed significant heterogeneity in ev loci composition even among embryos of the same flock.These observations have critical implications for the developmental biology research field, since they strongly suggest that the current standard methodology used in experimental studies using the chick embryo and RCAS vectors may lead to inaccurate interpretation of results. Retrospectively, our observations suggest that studies in which infected cells have been identified simply by pan-ASLV viral protein expression may need to be considered with caution. For future studies, they point to a need for careful selection and screening of the chick SPF lines to be used in combination with RCAS constructs, as well as the methodology utilized for qualitative analysis of experimental results. A series of practical guidelines to ensure research quality animals and accuracy of the interpretation of results is recommended and discussed.The chicken embryo has historically been the animal model par excellence to study vertebrate development, and its potential was significantly enhanced with the generation of the RCAS retrovirus vectors [1,2]. Among the main characteristics that make these vectors so useful for developmental studies are: i) ability to efficiently infect proliferating cells; ii) integration in the genome of infected cells; and iii) ability to efficiently replicate [3]. As a result, RCAS vectors offer a powerful system ensuring long-term expression of the gene of interest and its transmission to daughter cells, coupled to the production of new replica