The Immuno-Suppressive Effects of Cyclic, Environmental Heat Stress in Broiler Chickens: Local and Systemic Inflammatory Responses to an Intradermal Injection of Lipopolysaccharide

Simple Summary Heat stress is a growing concern in broiler production due to increasing environmental temperatures. Little is known about the effects of heat stress on the natural immune system defenses and health of broiler chickens. Using an environmental heat stress model in combination with minimally invasive tissue sampling techniques, the local and systemic, acute inflammatory responses to intradermal injection of bacterial component lipopolysaccharide or sterile saline (injection control) were examined in broilers reared in cyclic heat-stress- or thermoneutral-temperature conditions. This study revealed suppressive effects of heat stress on inflammatory responses initiated by lipopolysaccharide and by the sterile saline injection. Specifically, at the site of LPS injection, heat stressed broilers had lower infiltration of inflammatory leukocytes, reactive oxygen species generation, and expression of inflammatory factors than thermoneutral control broilers. Additionally, in the blood, concentrations of inflammatory leukocytes and plasma acute phase proteins did not increase in heat-stressed broilers like in thermoneutral broilers. Moreover, heat-stressed broilers had lower baseline concentrations of circulating lymphocytes than thermoneutral broilers. Hence, broilers reared in cyclic heat stress conditions have impaired ability to respond to bacterial challenges and associated tissue damage.Abstract To assess effects of environmental heat stress (HS) on the local and systemic inflammatory responses to lipopolysaccharide (LPS), broilers were reared under thermoneutral (TN) or cyclic HS conditions. Thermoneutral temperatures followed commercial production settings, with HS broilers exposed to 35 degrees C for 14 h/day from 4 days onward. At 37 days, HS- and TN-broilers were assigned to either LPS (100 mu g/mL) or endotoxin-free phosphate-buffered saline (PBS; vehicle) treatments, eight each to HS- and TN-LPS, four each to HS- and TN-PBS. Treatments were administered by intradermal injection of growing feather (GF) pulps; 10 mu L/GF; 12 GF/broiler. Blood and GF were collected before and at 6 and 24 h post-injection to assess leukocyte population changes in GF-pulps and blood, reactive oxygen species (ROS) generation and cytokine expression in GF-pulps, and plasma concentrations of alpha-1 acid glycoprotein (AGP-1). HS-LPS broilers had lower (p <= 0.05) infiltration of heterophils and macrophages, ROS generation, and inflammatory cytokine expression in GF-pulps, and lacked the increases in heterophil, monocyte, and plasma AGP-1 concentrations observed in TN-LPS broilers. HS-broilers had similar or greater drops in blood lymphocytes 6 h post-LPS or -PBS injection, respectively, and lower baseline levels (p <= 0.05) of circulating T- and B-lymphocytes than TN-broilers. Results indicated that cyclic HS reduced the local and systemic acute inflammatory responses to LPS in broilers, likely impairing their innate defense against microbial infection.