Our findings provide new insights into the role of IL-17 based on the following results: (i) in vivo treatment with IL-17 at 4-dpi resulted in a reduction in MD lesion severity but not tumor incidence; (ii) in vivo treatment with IL-17 at 4-dpi led to a reduction in CD4+ αβ T-cells and an increase in CD8+ αβ T-cells with the highest proportion peaking at 21-dpi; and (iii) in vivo inoculation with IL-17 at 4-dpi resulted in an increase in MDV viral load in the chicken spleen and lung tissues at 10-dpi and 21-dpi. This evidence concerns the gene CD4 and neoplasm.