Heat stress (HS) is a growing concern in broiler production due to increasing environmental temperatures. Little is known of the overall effect of HS on innate immunity. Lipopolysaccharide (LPS), an outer membrane component of Gram-negative bacteria, is commonly used to study the inflammatory response. In avian species, the local tissue and systemic inflammatory activities in response to LPS may be determined concurrently, over time, in an individual, using the growing feather (GF) pulp dermal test along with blood measurements (dual-window approach). To examine the effect of cyclic HS on the local and systemic acute inflammatory responses, Cobb 500 male broiler chicks were reared under thermoneutral (TN) or cyclic HS conditions. Eight environmental chambers were used, four TN and four HS, with each chamber split into two pens. Beginning at 4 d of age, cyclic HS birds were subjected to 35°C from 8 am to 10 pm and TN temperatures from 10 pm to 8 am. At 37 d of age, four groups of broilers were formed: TN-LPS (n = 8 broilers), TN-PBS (phosphate-buffered saline (vehicle) control; n = 4), HS-LPS (n = 8) and HS-PBS (n = 4). The pulps of 12 GF per broiler were each intradermally (i.d). injected with 10 μL of LPS (100 μg/mL) or 10 μL of PBS. Blood and GF were collected before (0 h) and at 6- and 24-h after GF pulp injection. Blood and GF pulp cell suspensions were immunofluorescently stained and leukocyte population profiles analyzed by flow cytometry. Blood also was used to prepare Wright-stained blood smears for differential leukocyte counts. Data were analyzed by ANOVA (P ≤ 0.05) and Fisher’s multiple means comparison tests to determine differences (P ≤ 0.05) between means. Locally, HS-LPS broilers exhibited lower levels (% pulp cells) of GF pulp infiltrating heterophils at 6- and 24-h, and lower macrophage levels at 24-h post-injection, compared to TN-LPS birds. In the blood, TN and HS broilers had similar baseline line (0 h) concentrations (cells/μL) of heterophils, monocytes, eosinophils, and basophils, but HS broilers had lower (P ≤ 0.05) total WBC and T- and B-lymphocyte levels. Concentrations of circulating heterophils and monocytes were greatly elevated (P ≤ 0.05) at 6- and 24-h, respectively, only in TN-LPS broilers, although a minimal increase (P = 0.091) in heterophils also was observed in HS-LPS broilers at 6 h. By 24 h, blood heterophils returned to pre-injection levels in both HS-LPS and TN-LPS broilers. Overall, results indicated that cyclic HS reduced both the local and systemic acute inflammatory response to LPS in broilers, likely impairing their innate defense against microbial infection. With growing concern regarding HS in the poultry industry, further research should be pursued to elucidate the mechanisms by which HS affects the innate immune system of broilers. Application of this approach may be utilized to select individuals expressing greater innate immune robustness while experiencing HS
