Immunotolerance of dairy heifers in response to repeated exposure to bacterial lipopolysaccharide endotoxin

Dairy cattle face a variety of stressful events on a daily basis. More specifically, climate change has resulted in more frequent heat stress events that increase the incidence of chronic bacterial infections by inducing conditions like leaky gut syndrome, whereby the integrity of the intestinal epithelium is compromised allowing for luminal bacterial lipopolysaccharide (LPS) endotoxin to infiltrate the host's bloodstream resulting in acute or chronic systemic stimulation of the innate immune system. Repeated exposure to LPS over a short period of time is reported to induce immunotolerance within the host. This LPS tolerance is an essential immunohomeostatic response that can protect against over activation of the inflammatory response during subsequent exposure to LPS. In the present study, Holstein calves (n = 20) were initially stress challenged with either saline, or 100, 200 or 400 ng/kg of LPS administered intramuscular, and again re-challenged with 200 ng/kg of LPS 2-weeks later. Serum was collected every 2 hr for 6 hr to profile changes in circulatory stress biomarkers after the repeated LPS exposures. Heifers that were initially challenged with 100, 200 and 400 ng/kg of LPS demonstrated significantly attenuated cortisol responses in the second challenge (p < 0.01, 0.01 and 0.05, respectively), whereas control animals who previously received saline demonstrated a strong cortisol response at 2 hr after receiving 200 ng/kg of LPS (p < 0.05). The cytokine/chemokine (IL-6, CCL2, CCL3 and CCL4) responses were also attenuated during the LPS rechallenge (p < 0.05). Finally, microRNA expression profiles were determined to assess the epigenetic response to repeated LPS exposure. Interestingly, miR-31 and miR-223 were downregulated in response to the second LPS challenge. The present study demonstrates the dynamic nature of the stress response in dairy cattle as it relates to the development of LPS tolerance. Understanding the roles of various stress biomarkers in the context of innate immune cell tolerance is essential for evaluating their impact on immune system homeostasis.