Analysis of immune responses to fowl adenovirus serotype 4 infection in Korean native compared to specific pathogen free chickens

Fowl Adenovirus serotype 4 (FAdV-4) is a major avian virus responsible for economic losses in the poultry industry. Although frequent virus outbreaks have been reported in Korean native chickens (KNCs), their immunological responses to FAdV-4 infection remain insufficiently characterized. Therefore, this study aimed to characterize the clinical and immune responses of KNCs following FAdV-4 infection and compare them with those observed in SPF chickens. Each breed was divided into control (KNC, n = 21; SPF, n = 20) and infected groups (KNC, n = 35; SPF, n = 33). Chickens in the infected groups were challenged with FAdV-4 at 8 days of age, whereas those in the control groups were injected with PBS. At 4 days post-infection (dpi), liver and spleen tissues were collected. Clinical manifestations, hepatic FAdV-4 hexon DNA detection, immune cell composition, and inflammation-related gene expression were evaluated and compared between the two breeds. KNCs showed milder clinical manifestations than SPF chickens and maintained monocytes/macrophages and T cell populations. In addition, expansion of B cells and γδ T cells was also observed following FAdV-4 infection. In contrast, SPF chickens showed a marked accumulation of monocytes/macrophages together with reductions in hepatic γδ T cells and B cells. These findings demonstrate different clinical and immune response patterns between KNCs and SPF chickens during acute FAdV-4 infection. Overall, KNCs and SPF chickens exhibited distinct immune cells and transcriptional response patterns following FAdV-4 challenge. These findings provide insights into the acute host response to FAdV-4 infection in KNCs and may serve as a basis for subsequent studies investigating factors contributing to other poultry diseases, especially in native chickens.