Amino Acid Polymorphisms in the Basic Region of Meq of Vaccine Strain CVI988 Drastically Diminish the Virulence of Marek's Disease Virus

Marek's disease virus (MDV) is the etiological agent of Marek's disease (MD), a lymphoproliferative disorder in chickens. Polymorphisms in the MDV-encoded oncoprotein Meq are shared among field strains and correlate with their virulence. The attenuated vaccine strain CVI988 harbors unique amino acid polymorphisms in Meq, particularly at positions 71, 77, and 326. In this study, we investigated the impact of these polymorphisms on Meq protein function and MDV virulence. Reporter assays revealed that the substitutions, particularly A71S and K77E, markedly impaired the transcriptional regulatory activity of Meq. To evaluate their effect on virulence, we generated a recombinant MDV based on the very virulent RB-1B strain, encoding Meq with A71S and K77E substitutions (rRB-1B_Meq71/77). Chickens infected with rRB-1B_Meq71/77 developed neither clinical signs nor lymphomas. Flow cytometry revealed no expansion of infected cells in this group, but a marked increase in CD8+ T and gamma delta T cells during early infection. Histopathological analysis also confirmed the absence of MD-associated lesions. These findings demonstrate that the polymorphisms at positions 71 and 77 in the CVI988 strain are sufficient to abolish MDV virulence. This study provides insight into the molecular basis of MDV virulence and informs the strategy for the design of more effective vaccines.