Heat stress has detrimental effects on livestock via diverse immune and physiological changes; heat-stressed animals are rendered susceptible to diverse diseases. However, there is relatively little information available regarding the altered immune responses of domestic animals in heat stress environments, particularly in cattle steers. This study aimed to determine the changes in the immune responses of Holstein and Jersey steers under heat stress. We assessed blood immune cells and their functions in the steers of two breeds under normal and heat stress conditions and found that immune cell proportions and functions were altered in response to different environmental conditions. Heat stress notably reduced the proportions of CD21⁺MHCII⁺ B cell populations in both breeds. We also observed breed-specific differences. Under heat stress, in Holstein steers, the expression of myeloperoxidase was reduced in the polymorphonuclear cells, whereas heat stress reduced the WC1⁺ γδ T cell populations in Jersey steers. Breed-specific changes were also detected based on gene expression. In response to heat stress, the expression of IL-10 and IL-17A increased in Holstein steers alone, whereas that of IL-6 increased in Jersey steers. Moreover, the mRNA expression pattern of heat shock protein genes such as Hsp70 and Hsp90 was significantly increased in only Holstein steers. Collectively, these results indicate that altered blood immunological profiles may provide a potential explanation for the enhanced susceptibility of heat-stressed steers to disease. The findings of this study provide important information that will contribute to developing new strategies to alleviate the detrimental effects of heat stress on steers.

热应激通过多种免疫和生理变化对牲畜产生有害影响；热应激的动物容易感染多种疾病。然而，关于热应激环境下家畜免疫反应改变的信息相对较少，尤其是阉牛。本研究旨在确定荷斯坦牛和泽西牛在热应激下免疫反应的变化。我们评估了两个品种的阉牛在正常和热应激条件下的血液免疫细胞及其功能，发现免疫细胞的比例和功能会随着不同的环境条件而改变。热应激显着降低了两个品种中 CD21 ⁺ MHCII ⁺ B 细胞群的比例。我们还观察到品种特定的差异。在热应激下，荷斯坦公牛的多形核细胞中髓过氧化物酶的表达减少，而泽西公牛的 WC1 ⁺ γδ T 细胞群减少。还根据基因表达检测到品种特异性的变化。为了应对热应激，荷斯坦公牛中 IL-10 和 IL-17A 的表达增加，而泽西公牛中 IL-6 的表达增加。此外，热休克蛋白基因（例如Hsp70和Hsp90）的mRNA表达模式仅在荷斯坦阉牛中显着增加。总的来说，这些结果表明，血液免疫学特征的改变可能为热应激的阉牛对疾病的易感性增强提供了潜在的解释。这项研究的结果提供了重要的信息，将有助于制定新的策略，以减轻热应激对阉牛的不利影响。