Two Distinct Polymorphisms in the Basic Region of Meq Protein of Marek’s Disease Virus Alter Pathological Progression and Clinical Manifestations

Marek’s disease virus (MDV) causes Marek’s disease (MD) in chickens, which is characterized by malignant lymphomas and neurological disorders. Although MD is currently controlled using live vaccines, the virulence of field strains has continuously increased in recent decades. Polymorphisms in the MDV-encoded oncoprotein Meq are shared among field strains according to their virulence. In particular, very virulent MDV strains harbor characteristic amino acid changes in the basic region of Meq at positions 77 and 80; however, the contribution of these polymorphisms to virulence remains unclear. To assess the impact of these polymorphisms on MDV virulence, we generated recombinant MDV (rMDV) based on the very virulent RB-1B strain, harboring K77E and D80Y substitutions in Meq found in low-virulent strains (rRB-1B_Meq77/80). Chickens were challenged with rMDVs, and survival rates and tumor incidence were evaluated. Viral loads in major organs were quantified by quantitative PCR, and the dynamics of MDV-infected cells and T cells were analyzed using flow cytometry. In addition, histopathological analysis was performed to further examine differences in pathogenesis in detail. To elucidate the mechanisms underlying pathogenesis, we conducted reporter assays to assess the effect of these polymorphisms in the basic region on its transcriptional regulatory activity. rRB-1B_Meq77/80 exhibited a reduced virulence but unexpectedly caused other clinical signs, including open-mouth breathing, in infected chickens. Quantitative PCR analysis showed consistently lower viral loads across all examined organs in rRB-1B_Meq77/80-infected chickens. Flow cytometric analysis revealed a reduction in MDV-infected cells, accompanied by a notable increase in CD8⁺ T cell populations. Histopathological analysis showed bronchus-associated lymphoid tissue hyperplasia in the lungs. Reporter assays revealed that most amino acid substitutions in the basic region in low-virulence strains reduced transcriptional regulatory activity. Our data indicate that polymorphisms at positions 77 and 80 in the Meq of low-virulence strains reduce MDV virulence and Meq-mediated transcription and possibly alter pathogenesis. This study improves our understanding of the mechanisms underlying MDV virulence.