Abstract
Dairy cows exposed to heat stress (HS) show decreased performance and immunity, but increased heat shock protein expressions and apoptosis. Zymosan, an extract from yeast cell walls, has been shown to modulate immune responses and defense against oxidative stress. However, few literatures are available about the effects of zymosan on immune responses and other parameters of the dairy cows under HS. Here, both primary peripheral blood mononuclear cell (PBMC) and dairy cow models were established to assess the effects of zymosan on performance, immunity, heat shock protein, and apoptosis-related gene expressions of dairy cows under HS. In vitro study showed that proliferation, IL-2 production, and Bcl-2/Bax-α ratio of cow primary PBMC were reduced, whereas hsp70 mRNA and protein expressions, as well as Annexin V-bing, were increased when PBMCs were exposed to heat. In contrast, zymosan significantly reversed these above changes induced by the HS. In the in vivo study, 40 Holstein dairy cows were randomly selected and assigned into zymosan group (supplemental zymosan; n = 20) and control group (no supplemental zymosan; n = 20). The results showed that zymosan improved significantly the dry matter intake and milk yield, increased IgA, IL-2, and tumor necrosis factor-α (TNF-α) contents in sera, as well as hepatic Bcl-2/Bax-α ratio, but decreased respiration rate and hepatic hsp70 expressions in the dairy cows under HS. Taken together, zymosan could alleviate HS-induced immunosuppression and apoptosis and improve significantly the productive performance and immunity of dairy cows under HS.