Transcriptome changes in leukocytes of dairy calves exposed to heat stress.

Heat stress impacts the health and immune function of dairy calves. Our objective was to conduct a longitudinal study to assess temporal alterations in the transcriptome of white blood cells from dairy calves exposed to heat stress. Eight Holstein bull calves (68.5 ± 1.37 kg of BW, aged 3.5 ± 0.5 weeks) were raised in individual pens following current commercial management practices. Calves were exposed to cyclic heat stress treatment (40°C ambient temperature from 0800 to 1900 h daily followed by 27°C) for 5 days. Blood samples were collected at 0, 1, and 5 days relative to treatment to conduct RNA extractions from white blood cells for RNA sequencing. Comparisons between control on day 0 against day 1 and day 5 of differentially expressed genes (DEGs) were considered using an adjusted P-value < 0.05. A total of 5 DEGs were detected when comparing control day 0 against day 1 (i.e., 3 downregulated and 2 upregulated). A total of 1,133 DEGs were detected when comparing control day 0 against day 5 (i.e., 722 downregulated and 411 upregulated). Differentially expressed genes were screened to fit into functional categories of pathways or ontologies associated with known impacts on functions. The functional annotation of differentially expressed genes showed upregulation of pathways associated with the heat shock response and changes in lipid metabolism possibly to support the activation of immune cells on day 1 compared with day 0. Transcriptional changes showed downregulation of "Spliceosome," "Neutrophil extracellular trap formation," and "ATP-dependent chromatin remodeling" on day 5 compared with day 0 due to the inhibition of most of spliceosomal RNA-related genes and several clustered histone variants respectively. In general, on day 1 heat stress was associated with preservation of cellular homeostasis and changes in metabolism. On day 5 heat stress was associated with the inhibition of various pathways, possibly to tightly control inflammation and prevent dysregulation of leukocytes' function.