Effects of heat stress on leukocyte function and gene expression in lactating dairy cows
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] The effects of heat stress in animal agriculture are expected to become more serious as global temperatures continue to increase. Negative effects on production, reproduction, behavior, and health lead to greater costs for producers and decreased welfare for animals. In terms of health, the current literature does not provide conclusive evidence of the effects of heat stress on the immune system of dairy cows. To elucidate a potential cellular mechanism linking the effects of increased thermal stress on the regulation and function of immune cells, a ubiquitous cellular stress response pathway, termed the unfolded protein response (UPR), was examined. Additionally, a pair-feeding experiment was conducted in order to differentiate between heat stress (HS) and nutrient restriction effects within the cow. Three HS experiments were conducted in order to observe the effects of elevated environmental temperature on leukocyte function and response in dairy cows. We hypothesized that HS would invoke the UPR in blood leukocytes, and prolonged HS would cause suppressive effects on immune system factors. Additionally, we hypothesized that HS would lead to enhanced effects, beyond those observed in nutrient restricted cows, on UPR activation and immune suppression. ... While HS appeared to decrease expression of the same genes on d 5 of HS in the 15-d and pair-feeding experiments, the lack of treatment differences in the latter study indicates that this change cannot be attributed to HS effects. Thus, HS or nutrient restriction did not invoke the UPR nor lead to decreased expression of immune genes in blood leukocytes. However, increased secretion of TNFa in the HS and PF groups indicates that heat stress and feed restriction may enhance inflammatory cytokine secretion from leukocytes.
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