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This thesis aims to explore options and targets for immunomodulation in horses, particularly in foals, which are prone to bacterial infections during the first months of life. The thesis particularly focuses on the possibilities to modulate Toll-like receptors (TLRs), which are a crucial link between innate and adaptive immune responses, regulating both tolerance to commensal bacterial flora as well as defensive immune responses to potentially harmful pathogens. Several methods to modulate inflammatory responses in equine immune cells were investigated ex vivo, inducing inflammation by means of a standardised lipopolysaccharide (LPS) challenge, hence activating TLR-4. Concurrent administration of the synthetic TLR-2 ligand Pam3-Cys-Ser-Lys4 (PCSK), which previously has been shown to mitigate intestinal inflammatory processes in rodent species, did not cause significant changes in the ex vivo induced inflammatory response, neither in immune cells of adult horses nor in cells obtained from neonatal foals, indicating the need to test potential immune-modulators in the target animal species. Aside from this, we performed several studies using specific carbohydrate fractions in comparable ex vivo models of inflammation. Previously performed studies in humans and rodent species have indicated that non-digestible carbohydrates, particularly natural oligosaccharides from different sources, seemed to modulate TLR signalling in the intestinal tract by acting as prebiotic. In addition, there is increasing evidence for a direct impact of oligosaccharides on TLR signalling. The in vitro studies in this thesis support the latter hypothesis. For instance, we demonstrated that the carbohydrate fraction of equine colostrum dose-dependently suppressed LPS-induced inflammatory responses in equine peripheral blood mononuclear cells (PBMCs). In addition, we tested several commercially produced oligosaccharide products, derived from cow’s milk and botanicals. We found distinct effects of several specific oligosaccharide fractions, either enhancing or suppressing the ex vivo induced inflammation. Both scenarios can be useful for the future development and optimisation of oligosaccharide supplements, depending on the clinical context in which they are applied. In the final part of the thesis we describe an in vivo pilot study concerning oral supplementation of galacto-oligosaccharides in foals. During the period of investigation, no significant effects of the treatment were observed on the apparent clinical and blood parameters for health and immune status. However, in PBMCs derived from these foals after four weeks of treatment, a standardised LPS challenge resulted in significantly lower relative mRNA expression levels of pro-inflammatory cytokines in the treated foals compared with the control group. In addition to these in vivo experiments, the first results of the characterisation of the oligosaccharides in equine colostrum are presented in the thesis. These preliminary results reveal distinct oligosaccharide patterns in horse colostrum and identify several unique oligosaccharides, specific for the horse. In conclusion, our data provide the first evidence for efficacy of specific immunomodulatory oligosaccharides in the horse and hence serve as starting point for the development of methods to improve immunocompetence in young foals, in particular by modulating TLR signalling

Gastrointestinal defence in the new-born is limited in comparison to adults, due to an immature epithelial barrier function and deficits in both innate and adaptive immune responses. Consequently, neonates (including foals) are at increased risk of disturbance to mucosal homeostasis during initial intestinal colonisation that may lead to excessive inflammation and bacterial translocation into the bloodstream, resulting in septicaemia. Bacterial recognition by Pattern Recognition Receptors (PRRs) and their downstream regulation of cytokine release have been shown to be pivotal for gastrointestinal mucosal homeostasis and the development of a functional intestinal barrier. Evidence suggests that selective PRR agonists limit the inflammatory responses and improve epithelial barrier function. Milk, and in particular colostrum, contain a broad array of oligosaccharides which seem to act as PRR agonists. This class of compounds forms a source for new dietary formulas that may orchestrate gut colonisation by the commensal flora in the early phase of life and so reduce the risks of inflammation and pathogen invasion.