Effect of In-vivo heat challenge on physiological parameters and function of peripheral blood mononuclear cells in immune phenotyped dairy cattle

The frequency of heat waves and hot days are increasing due to climate change, which leads to an increase in the occurrence of heat stress in dairy cattle. Previous studies have shown that dairy cattle identified as high immune responders have a reduced incidence of disease and improved vaccine response compared to average and low responders. Additionally, it has been observed that when cells from immune phenotyped cattle are exposed to in-vitro heat challenge, high immune responders exhibit increased heat tolerance compared to average and low immune responders. Therefore, the objective of this study was to evaluate physiological parameters and the function of blood mononuclear cells of immune phenotyped dairy cattle exposed to in-vivo heat challenge. A total of 24 immune phenotyped lactating dairy cattle (8 high, 8 average and 8 low) were housed in the tie-stall area of the barn and exposed to an in-vivo heat challenge for 4 h on 2 subsequent days, where the temperature was set at 29 ℃. Blood samples were taken both pre- and post-challenge each day and manual respiration rates and rectal temperatures were recorded pre challenge and every 30 min during the challenge. Temperature and humidity measurements were taken in correspondence with all respiration rate and rectal temperature measurements to calculate the temperature humidity index pre heat challenge and at 30-minute intervals during the heat challenge. Blood mononuclear cells were isolated from blood collected pre and post challenge and the concentration of heat shock protein 70 and cell proliferation were assessed. Results showed that average and low responders had significantly greater respiration rates compared to high responders at a temperature humidity index of 77 and above. No significant difference was observed between phenotypes for rectal temperature. High responders had a higher heat shock protein 70 concentration and greater cell proliferation after in-vivo heat challenges compared to average and low responders. These results paralleled those found during in-vitro heat challenge adding further credence to the concept that high responders may be more resilient to heat stress compared average and low responders.