Transcriptomic analysis reveals physiological adaptations and immune system modulation in Romanian Holstein cattle during heat stress exposure.

Heat stress represents a significant impairment to productivity, health, and reproductive efficiency of dairy cattle globally. The present study aimed to investigate genes and molecular mechanisms involved in physiological response to heat stress in Romanian Holstein dairy cows, by conducting a whole-blood transcriptomic and bioinformatics analysis. RNA sequencing (RNA-Seq) was performed on samples collected from seven multiparous Romanian Holstein cows under both heat stress (HS, average temperature-humidity index -THI of 77.5) and thermoneutral (TN, average THI of 48.3) conditions. A total of 487 (447 downregulated and 40 upregulated) significantly (|log2FC| ≥ 1 and adjusted p-value ≤ 0.05) differentially expressed genes (DEGs) were identified, during HS exposure compared to TN conditions. Downregulated genes were primarily associated with crucial physiological functions, including immune responses, inflammation, and metabolic pathways, all of which directly influence milk production, health and reproductive efficiency. Upregulated genes revealed active cellular protection mechanisms, notably heat shock proteins (HSPs), and a pronounced activation of the innate immune and antiviral defense responses, alongside metabolic adjustments and complex reproductive responses. Significant enrichment in pathways in response to heat stress included viral defense and lysosomal activity, suggesting an intensified cellular catabolism under thermal stress. In conclusion, Romanian Holstein cattle exhibit a complex, integrated molecular response to heat stress, characterized by a survival-oriented shift in gene expression. These findings provide detailed molecular insights into the physiological challenges faced by dairy cattle during heat stress exposure.