A comparative evaluation of the gastrointestinal immune response of the modern and heritage broiler chicken
Date
2020
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Publisher
University of Delaware
Abstract
The modern broiler chicken is the result of 60 years of selective breeding for rapid rates of growth and muscle deposition. These birds are highly efficient, require relatively small quantities of feed, and are ready for market in as little as six weeks. However, selecting for the rapid growth phenotype has seemingly come at the expense of other biological processes, namely the immune system. Modern broiler chickens exhibit damaging over-inflammatory responses to certain disease challenges or have insufficient responses to clear other pathogenic challenges. In the past, growth promoting antibiotics protected broilers from disease challenges, so in a post-antimicrobial industry it is imperative to understand why they are vulnerable to safeguard animal welfare and human food safety. Heritage broiler breeds that have not been selectively bred since the 1950s provide a standard to which the modern broiler’s immune responses can be compared and the impacts of rapid growth on immune capacity examined. Chickens lack encapsulated lymph nodes, so the gut associated lymphoid tissues (GALT) and intestinal immune cells contribute significantly to the immune response. Furthermore, the gut is the point of intersection of metabolism, immune responses, and exposure to external antigens. Therefore, the GALT and intestinal tissues were studied. To assess the differences in modern and heritage birds’ immune systems, both breeds were injected with an immunostimulatory synthetic oligonucleotide (CpG) or a control injection that was administered just once on the day of hatch, or multiple times throughout the experimental time period up to day 35 post-hatch. Tissue samples from the birds that received the treatment were subjected to kinome array analysis and compared to the same tissues from birds that received the control, a non-stimulatory oligonucleotide. The kinome array provides a real-time snapshot of kinase activity ongoing within the tissues in immune and metabolic signaling pathways. qPCR was also performed to assess the mRNA expression of inflammatory cytokines in the treatment and control birds. Luminal gut content samples were collected for microbiome sequencing and analysis, as the gut microbiome shapes host immune responses and vice versa. Overall, the modern broilers exhibited a more varied response to the single-dose of CpG, with an initial increase in immune signaling and cytokine expression, which dropped off by day 15 post-hatch and lead to inhibition of immune activation, mTOR inhibition and decreased cytokine expression. The heritage birds, conversely, demonstrated a consistent level of activation and response to the CpG treatment throughout the experimental time period. The modern broilers’ gut microbiome composition was more varied than the heritage birds, which was consistent despite different measures from different sampling time points and different treatments. In the second trial, when birds received multiple injections of the treatment or control throughout the experimental time period, the modern broiler’s initial response to the injection was indicative of more innate signaling when compared to the heritage broiler, whereas the heritage broilers’ response showed more adaptive signaling. At the day 16 post-hatch time point, the signaling patterns in response to the injection in the modern broilers’ GALT indicated more metabolic signaling changes, and the signaling patterns in their intestinal tissues indicated more immune signaling. The inverse pattern was evident in the heritage broilers. The data from both trials suggest that the energetic needs of developing GALT and mounting successful immune responses may not be met in the modern broilers, and this may play a role in their immune limitations. The role of the gut microbiome in the energy available to the birds’ GALT remains to be elucidated, though the changes in microbiome composition in the modern birds and stability in the heritage birds suggest the microbiome may contribute to the variations in immune response throughout the trial period.
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Keywords
Broiler chickens, Gut health, Immunology, Immunometabolism, Metabolism