Contribution of leukocytes to the induction and resolution of the acute inflammatory response in chickens

A successful immune response against invading pathogens relies on the efficient activation of host defense mechanisms and a timely return to immune homeostasis. Despite their importance, these mechanisms remain ill-defined in most animal groups. This study focuses on the acute inflammatory response of chickens, important both as an avian model with a unique position in evolution as well as an increasingly notable target of infectious zoonotic diseases. We took advantage of an in vivo self-resolving intra-abdominal challenge model to provide an integrative view of leukocyte responses during the induction and resolution phases of acute inflammation. Our results showed rapid leukocyte infiltration into the abdominal cavity post zymosan challenge (significant increase as early as 4 h), which was dominated by heterophils. Peak leukocyte infiltration and ROS production reached maximum levels at 12 h post challenge, which was significantly earlier than comparative studies in teleost fish and mice. Both heterophils and monocyte/macrophages contributed to ROS production. Local leukocyte infiltration was preceded by an increase in peripheral leukocytes and a drop in the number of bone marrow leukocytes. The proportion of apoptotic leukocytes increased following peak of acute inflammation, rising to significant levels within the abdominal cavity by 48 h, consistent with other indicators for the resolution of inflammation. Importantly, comparison of chicken phagocytic responses with those previously shown in agnathan, teleost and murine models suggested a progressive evolutionary shift towards an increased sensitivity to pro-inflammatory pathogen-derived particles and decreased sensitivity towards homeostatic stimuli. Thus, while significant conservation can be noted across the immune systems of endotherms, this study highlights additional unique features that govern the induction and resolution of acute inflammation in the avian system, which may be relevant to disease susceptibility and performance.