Attenuation of a recombinant Marek's disease virus lacking the meq oncogene and evaluation on its immune efficacy against Marek's disease virus

SC9-2 is a recombinant Marek's disease virus (MDV) strain lacking the meq oncogene. Previous study demonstrated that SC9-2 virus provides good protection against challenge with a very virulent MDV rMd5, but it induces immunosuppressive effects in specific pathogen-free (SPF) chickens. In the present study, SC9-2 was serially passaged on chicken embryo fibroblast (CEF) cell cultures. The pathogenicity and immune efficacy of SC9-2/10ᵗʰ and SC9-2/40ᵗʰ against rMd5 were evaluated. Animal experimental results showed that SC9-2/10ᵗʰ and SC9-2/40ᵗʰ showed no lethality or tumorigenicity in SPF chickens. Body weight of chickens inoculated with SC9-2/40ᵗʰ were significantly higher than that of the chickens inoculated with SC9-2/10ᵗʰ but lower than that of the uninoculated controls. The severity of bursa and thymus atrophy (BTA) and spleen enlargement in SC9-2/40ᵗʰ-inoculated chickens were also weaker than the SC9-2/10ᵗʰ-inoculated ones but stronger than the uninoculated controls. Chickens inoculated with SC9-2/40ᵗʰ and SC9-2/10ᵗʰ showed similar antibody levels induced by H9N2 subtype avian influenza virus/Newcastle disease virus inactivated vaccines, both of which were lower than the uninoculated controls. Replication of SC9-2/40ᵗʰ was significantly lower than SC9-2/10ᵗʰ in feather follicle epithelium (FFE) of infected chickens. The immune protection index of SC9-2/40ᵗʰ was also lower than that of SC9-2/10ᵗʰ, but the difference was not significantly, and both of which were significant higher than that of the commercial MDV vaccine CVI988/Rispens. The results of our studies demonstrated that SC9-2/40ᵗʰ showed weaker severity of BTA, spleen enlargement, and body weight loss and lower replication level in FFE than SC9-2/10ᵗʰ in SPF chickens. However, SC9-2/40ᵗʰ was able to confer better immune protection as compared with CVI988/Rispens vaccination in SPF chickens. In conclusion, serially attenuation of SC9-2 in CEFs reduced the lymphoid organ atrophy and replication in SPF chickens, and the immune protective efficacy of attenuated viruses was still superior than CVI988/Rispens.