  The warming climate is causing livestock to experience heat stress at an
increasing frequency. Holstein cows are particularly susceptible to heat stress
because of their high metabolic rate. Heat stress negatively affects immune
function, particularly with respect to the cell-mediated immune response, which
leads to increased susceptibility to disease. Cattle identified as having
enhanced immune response have lower incidence of disease. Therefore, the
objective of this study was to evaluate the impact of in vitro heat challenge
on blood mononuclear cells from dairy cattle, that had previously been ranked
for immune response, in terms of heat shock protein 70 concentration, nitric
oxide production, and cell proliferation. Bovine blood mononuclear cells, from
Holstein dairy cattle previously ranked for immune response based on their
estimated breeding values, were subjected to three heat treatments:
thermoneutral, heat stress 1 and heat stress 2. Cells of each treatment were
evaluated for heat shock protein 70, cell proliferation and nitric oxide
production. Blood mononuclear cells from dairy cattle classified as high immune
responders, based on their estimated breeding values for antibody and
cell-mediated responses, produced a significantly greater concentration of heat
shock protein 70 under most heat stress treatments compared to average and low
responders, and greater cell-proliferation across all treatments. Similarly, a
trend was observed where high responders displayed greater nitric oxide
production compared to average and low responders across heat treatments.
Overall, these results suggest that blood mononuclear cells from high immune
responder dairy cows are more thermotolerant compared to average and low immune
responders
