Down-Regulation in the Chicken Thymus Transcriptome in Response to Heat Stress and Lipopolysaccharide

Exposure to high temperatures can suppress innate and adaptive immune responses. In chickens, heat stress can reduce phagocytic activity of macrophages, decrease circulating antibody levels and lead to atrophy of immune tissues, like the thymus. Understanding thymic responses to heat stress, and the interaction of heat with the pro-inflammatory stimulus lipopolysaccharide (LPS), can provide insights on how to maintain T lymphocyte-mediated disease resistance in chickens. In this study, transcriptome responses were characterized by RNA-sequencing (RNA-seq) of thymus samples collected from 22-day-old chickens exposed to acute heat stress and/or LPS. Four treatments (thermoneutral + saline, heat + saline, thermoneutral + LPS, and heat + LPS) were examined using two genetic lines (an inbred Fayoumi line and an outbred broiler line). Indexed cDNA libraries (n = 3-4 libraries/treatment/line) were sequenced on the HiSeq 2500. Approximately 500 million of the resulting single-end reads were mapped to the chicken genome (Galgal 4). Differential expression analyses identified over 1,300 genes with significant differences between treatments or genetic lines. Although expression changes in the Fayoumi were minimal, all treatments had a predominantly down-regulatory effect on the broiler transcriptome. Functional analyses identified reduced expression of genes involved in Wnt signaling, lymphogenesis, and cell movement, potentially indicating an inflammatory response and a T lymphocyte decline in all treatments. These thymic transcriptome responses could contribute to the immunosuppressive effects of heat stress and provide targets for future investigations of disease resistance. USDA-NIFA-AFRI Climate Change Award #2011-67003-30228; the USDA National Institute of Food and Agriculture, Hatch project #5358.