Role of Macrophages in the Pathogenesis of Genotype VII Newcastle Disease Virus in Chickens

Simple Summary Genotype VII Newcastle disease virus (NDV) is renowned for its high tropism to the immune organs of chickens, and macrophages are a critical component in the antiviral innate immune system. The objective of this study is to investigate the role of macrophages in the pathogenesis of genotype VII NDV in chickens. Genotype VII NDV has significantly higher infectivity in macrophages compared to genotype IV NDV. Macrophage depletion resulted in no changes in virulence and virus replication but a significant deterioration of tissue damage in the spleen. Therefore, macrophages play an important role in alleviating tissue lesions caused by genotype VII NDV infection. Long-term evolution of Newcastle disease virus (NDV) results in substantial alteration in viral pathogenesis. NDVs of genotype VII, a late genotype, show marked tropism to lymphoid tissues, especially to macrophages in chickens. However, the role of macrophages in the pathogenesis of genotype VII NDV is still unclear. Herein, NDV infectivity in macrophages and the role of macrophages in the pathogenesis of genotype VII NDV in chickens were investigated. We reported that NDV strains of genotype VII (JS5/05) and IV (Herts/33) can replicate in the adherent (predominantly macrophages) and non-adherent cells (predominantly lymphocytes) derived from chicken peripheral blood mononuclear cells (PBMCs), and significantly higher virus gene copy was detected in the adherent cells. In addition, JS5/05 had significantly higher infectivity in PBMC-derived adherent cells than Herts/33, correlating with its enhanced tropism to macrophages in the spleen of chickens. Interestingly, the depletion of 68% of macrophages exerted no significant impact on clinical signs, mortality and the systematic replication of JS5/05 in chickens, which may be associated with the contribution of non-depleted macrophages and other virus-supportive cells to virus replication. Macrophage depletion resulted in a marked exacerbation of tissue damage and apoptosis in the spleen caused by JS5/05. These findings indicated that macrophages play a critical role in alleviating tissue damage caused by genotype VII NDV in chickens. Our results unveiled new roles of macrophages in NDV pathogenesis in chickens.