Host-type specific immune responses in broiler and layer chickens following experimental infection with avian...

In our previous study, broiler-type chicks exhibited greater susceptibility to infection with avian pathogenic Escherichia coli (APEC) than layer-type chicks under identical experimental conditions. To better understand the underlying mechanism, most likely attributed to the immune response, the present work aimed to investigate the humoral and cellular immune parameters in specific pathogen-free broiler-type and layer-type chicks using samples collected from that experiment. Briefly, thirty-two chicks of each bird type were equally divided into two groups. At 14 days of life, one group of broilers and layers were aerosolized with bioluminescent APEC, while the remaining groups received phosphate-buffered saline as negative controls. Birds were necropsied at 3, 7, 10, and 14 days post inoculation (dpi). Blood samples were collected prior to necropsy for serum IgY quantification using an in-house enzyme linked immunosorbent assay (ELISA) based upon the homologous strain. Lung and spleen samples were collected, and the frequencies of mononuclear cell subsets (CD45, CD4, CD8α, TCR-γδ, Bu-1, monocytes/macrophages) were determined by flow cytometry. Both infected groups demonstrated a significant rise in IgY levels post-infection, with antibody titers rising in infected broilers from 7 dpi and in infected layers from 10 dpi onward. In infected broilers, a significant increase of CD8α⁺ cells in the lung and spleen was observed. Additionally, a significant decrease in CD4⁺ cells in the lung and TCR-γδ⁺ cells as well as Bu-1⁺ cells in the spleen was detected. Infected layers showed a reduction of monocytes/macrophages in the lung compared to non-infected birds. Overall, the present investigation revealed an earlier onset of the humoral immune response in broilers being more severely affected than layers. Furthermore, APEC infection also induced marked alterations in cellular immune cell populations, particularly in broilers whereas layers responded with much less cellular changes.