Divergent ghrelin expression patterns in sheep genetically resistant or susceptible to gastrointestinal nematodes

Gastrointestinal nematodes are a major problem for pastoral ruminant production systems. This problem could be reduced by the application of breeding strategies that select for nematode resistant sheep, but no suitable molecular markers are available. Research selection flocks containing lines that are resistant (R) or susceptible (S) to gastrointestinal nematodes provide an excellent resource for discovering selectable markers, and for studying the underlying mechanisms of an effective anti-nematode response. In this study we have used a combination of quantitative real time PCR assays and ELISA to determine if nematode challenge impacts on the expression of the satiety-regulating hormone ghrelin. The expression responses were then compared between the selection flock R and S lines. The results show that the basal levels of ghrelin in plasma were greater than 2-fold higher in nematode naïve S line sheep. Three days after a primary nematode challenge divergent ghrelin expression patterns were observed between the selection lines, with levels increasing in R sheep while decreasing in S sheep. After a secondary challenge this trend was repeated, but following a third challenge ghrelin expression levels rose in both R and S sheep, by which time the S animals had acquired an effective immune response to the nematodes, as measured by a significant reduction in faecal egg output. Importantly, this phenomenon was observed in gene expression studies in gut tissues and also in ELISA measurements of ghrelin peptide levels in plasma. A regression analysis showed that ghrelin transcript expression in the gut accounted for >40% of the variation in faecal egg count measured following Haemonchus or Trichostrongylus infection. We therefore hypothesise that the direction of ghrelin expression (up or down) immediately following nematode exposure may play an important role in regulating the differing anti-nematode responses that occur in the R and S lines. Such differences identify ghrelin as a previously unrecognized factor influencing the acquisition of immunity to nematodes.